0306452392 SS.00 + 0.00 Pergamon Pi-w L&i t I992 IBRO
NEURONAL FIRING IN THE NUCLEUS ACCUMBENS ASSOCIATED WITH THE LEVEL OF CORTICAL AROUSAL
IS
C. W. CALLAWAY and S. J. HENRIKSEN* Research
Institute
of Scripps
Clinic,
10666 North
Torrey
Pines Road,
La Jolla.
CA 93030,
USA
Abstract--Because of evidence that the nucleus accumbens mediates the activating effects of many drugs, this study examined the hypothesis that the firing rates of individual nucleus accumbens neurons are positively correlated with spontaneous changes in behavioral arousal that occur during the sleep- wake cycle. The present report examined the firing patterns of 80 neurons in the nucleus accumbens 01 unanesthetized. unrestrained rats during various electrographically determined levels of arousal. Synaptic responses to stimulation of hippocampal and pallidal nucleus accumbens afferents indicated that the present sample of neurons was similar to a large population of nucleus accumbens neurons previously recorded in anesthetized rats. Confirming the participation of the nucleus accumbens in behavioral arousal,the firing rates of nucleus accumbens neurons were greatest during wakefulness and rapid eye movement sleep and lowest during non-rapid eye movement sleep. Furthermore, the induction of halothane anesthesia decreased behavioral and electrocorticographic arousal concurrent with a suppression of the spontaneous nucleus accumbens unit discharge. These data support the hypothesis that the firing of nucleus accumbens neurons is closely related to ~NXF.231
Behavioral studies suggest that the nucleus accumbens septi (NAS) is important for the control of motivated behaviors as well as for the expression of pharmacologically stimulated motor activity.‘“.” In an attempt to understand the normal physiology of the NAS. numerous studies have examined diverse measur’cs of NAS function. including electrophysiological measures. These stud& have confirmed that NAS neurons receive excitatory afferents from regions including the llippocampus,~ hasolateral amygdala’~” and parafasicuiar thalamic nuclei.” Furthermore. projections from midbrain catecholamincrgic nuclei have been shown to modify the effects of these excitatory afferents.i.‘5.‘K While these studies provided information about the effects of afferent stimulation and of pharmacological manipulations on NAS neuronal firing, speculations about the parti~ipation of NAS neurons in behavior have been limited by the use of anesthetized animals in which correlations between behavioral activity and electrophysiological measures are impossible. In order to more directly characterize NAS neuronal firing under physiological conditions. the prcsent study identified a population of NAS neurons in unanesthetized rats. Spontaneous variations in the discharge of NAS neurons wras observed during the transitions between stages of wakefulness and sleep. In order to relate the findings of these studies to ______-._.
*To whom correspondence
should be addressed. .4~~r~,~~1~~~~~~~.~: EEG, el~troencephalogram; EMG, electromyographic activity; NAS, nucleus accumbens semi: NREM, non-rapid eye movement: REM, rapid eye movement; SWS, slow wave sleep.
studies in anesthetized rats. separate expcrimcnts also examined the effect of halothanc anesthesia induction on NAS neuronal discharge. These data indicate that the clcctrophysiological characteristics of NAS neurons in unanesthetized animals resemble the patterns observed in anesthetized animals. and that the rate of NAS ncuronal discharge is related to behavioral state. previous
EXPERIMENTAl.
PROC‘EI)I’RES
Male SpragueDawley rats (Charles River) were housed individually under regular lighting (lights on: Oh.00 1X.00). One week after arrival (body weight 300 -350 g). animals were surgically prepared for chronic elcctrophysiologic;ll recordings. Each rat was anesthetized with 2.54;1 halothanc in air, and mounted in a Kopf stereotaxrc devtce. Anesthesia was maintained during the procedure with 1.5% halorhanc in air. Body tem~~ture was maintained using an abdominal heating pad thermostat&thy controled by a rectal thermometer. Hippocampal afferents to the NAS in the hmbria-fornix were stimulated via a bipolar twisted-wire electrode with a 0.5.mm tip separation implanted a~ coordinates 1.3 mm anterior (A) from bregma, 1.3 mm lateral (L) from midline, 335 mm ventral (V) from skull surface. The ventral pallidurn, a structure having both afferent and elferent connections with the NAS. was stimulated via an electrode implanted at coordinates A-2.0 mm, L I .Smm, V 667 mm. In addition, wires were attached to stainless-steel skull screws in the frontal and parietal bones for grounding and to allow recording of electroencephulographic activity @EC). Wires with 2 3 mm of exposed conductor at their ends were implanted bilaterally in the neck muscles to record electromyog~phic activity (EMG). All wire leads were gathered into a plastic “hat” connector. and the entire assembly was fixed to the skull using acrylic dental cement. For single unit recording, a removable microdrive system (Bield Engineering. Irvine, CA) was employed that has
548
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W.
C‘ALI.AW.ZY
previously been described.3 Craniotomies were made in the skull overlying the NAS (A f 1.3 1.7. L 1.2 -1.5 mm). A threaded stainless-steel hub was placed over the craniotomy and permanently fixed to the skull using dental cement. A removable cap protected the brain between recording sessions. Immed~at~iy prior to recording sessions, the cap was replaced with a mechanical microdrive assembly carrying a tungsten microelectrode. Commercially fabricated tungsten microelectrodes (Frederick Haer. Brunswick, ME) with impedances of 9- 10 Mn at 1000 Hz were used in all experiments. For NAS recordings. the microelectrode was advanced to between 6.0 and 8.5 mm below the skull surface. Animals were tested in a 28 x 2X x 38 cm Plexiglas box with a wire mesh floor. The animal was tethered by an electrical commutator/cable assembly. that allowed free movement within the box while promdmg remote access to recording and stimulating electrodes. Poiygraphic records of EEG and EMG were collected using a Grass Model 79D polygraph (Grass Instrument, Quincy, MA). Stimulation (0.1-1.5 mA, 0.15-ms pulses, 0.1 Hz maximum frequency) was provided via a Grass S-88 stimulator and stimulation isolation unit. The signal from the microelectrode was amplified at the headstage by three operational amplifiers configured as an instrumentation amplifier with gain of IO. This signal was displayed unfiltered or filtered on oscilloscopes. Units were islolated by slowly advancing the microelectrode while providing auditory and tactile stimulation to the animal (in order to detect sensorimotor-related units) and stimulating NAS afferents (to detect otherwise silent stimulation-evoked units). Single unit activity was discriminated from the filtered signal based on amplitude and spike width using a window discriminator (Fintronics, Orange. CN). The discriminated output of this spike detector was relayed to the polygraph and to a computer for analysis (Apple, Cupertino, CA). Experiments were performed during both the light and dark phases of the light-dark cycle. Other than the dccreased frequency of sleep during the dark phase, no differences were noted between light-phase and dark-phase experiments, and the results have been combined. Behavioral state was determined from the polygraph recordings. Wakefulness usas recognized by a desynchronized, lowvoltage EEG and direct visual observation (animal maintains an upright posture with eyes open and orients to sound or touch). Slow wave sleep (SWS) was characterized by the presence of high-amplitude (greater than 75 p V) slow waves in the EEG, behaviorai signs of sleep and continued EMG activity. Rapid eye movement (REM) sleep was characterized by a low-voltage EEG, continued behavioral signs of sleep and a decrease in EMG activity to the level of background noise. Firing rates for NAS neurons were dete~ined during each arousal state by counting the number of spikes occurring within a continuous 180-s period of each state. When 180 s of data were not available (as was typical during REM sleep episodes lasting no longer than 120 s), the largest available continuous record was used. In order to control for the possible influence of motor activity on neuronal firing and to provide a suitable comparison with other states of arousal, firing rates for wakefulness were determined during periods when the animal exhibited no gross motor activity. After experiments were completed. animals were kilied by an overdose of halothane vapors. The locations of each stimulating electrode and of a representative position of the recording electrode were marked by passing a continuous current (I mA, 10 s) through the electrode, allowing subsequent histological verification. Additional experiments examined the influence of motor activity on NAS neuronal firing. Units were isolated while the animal was held in an opaque 20 x 20 cm plastic box. When a NAS unit was identified, the animal was gently transferred into a 30.5 x 61 .O cm Plexiglas chamber. Three equally spaced 2.5-cm holes were placed 2.5 cm above the floor in each long wall and in the midline of the floor. A
md
S.
J.
HENKIKSEN
single 2.5-cm hole was placed in one short wali. ln~ruduclrl~ the animal into this open field reliably increased locomotor activity and rearing, as well as promoting directed investigation of the holes. The exploration of the open field was recorded on a videotape that was synchromzed uith J sequential record of the unit discharge acquired by the computer. This procedure allowed subsequent nir-line corn-. parisons between unit firing and videotaped behavior A subset of experiments assessed the influence of halothane anesthesia on the discharge characteristics of NAS neurons. For these studies. anesthesia was induced quickly by introducing air saturated with halothanc vapors into the recording chamber, typically resulting in cessation of motor activity and the appearance of high-voltage slowwaves in the EEG within 1min. Administration of halothane continued until the animal exhibited only a slow hindlimb withdrawal in response to foot-pinch (1 2 min), at which time anesthesia was discontinued. Although steadystate levels of anesthesia probably were not achieved, this procedure minimized the period of agitation prior to cessation of motor activity during which units were likely to be lost, and provided a shorter period of recovery (3 4 min) for the animal. Furthermore, the level of anesthesia as assessed by foot-pinch-withdrawal was comparable for several minutes to the level of anesthesia employed in acute electrophysiological experiments using a continuoLls exposure to 0.8% halothane in air.‘,’
RESULTS The studied
characteristics were
determined
of
the using
neuronal a total
popuiat~o~ of 80 neurons
that were recorded from NAS of 11 animals. A minimum signal-to-noise ratio of 2: 1 was required for discrimination of a spike, and all spikes were required to have similar amplitudes and waveforms to be considered a single unit. Neurons (mean 2S.D. of 1.9 f 1.4 cells/track) were encountered with firing rates ranging from 0.1 to 18 Hz (mean 2S.D. = 4.08 & 3.86 Hz) during wakefulness. The distribution of firing rates did not suggest any subgroups of neurons. The interspike intervals of representative units were found to be unimodally distributed, indicating the absence of bursting activity. An additional nine units isolated at ventral recording sites with firing rates between 20 and 50 Hz were considered to be cells of the olfactory tubercle. Similar fast-firing neurons recorded previously in anesthetized animals have been localized to the border between the NAS and olfactory tubercle.7 Because the location of individual neurons could not be determined using the present methods, it is impossible to determine whether the NAS units were recorded from the core or shell of the NAS.” However, the coordinates employed, were more likely to sample from the core of the NAS, and none of the final lesion marks were located medial to the lateral ventricle. In addition, anatomical studies indicate that the core region of the NAS resembles the overlying caudate-putamen.’ Neurons encountered as electrodes were passing through the caudate-putamen exhibited discharge patterns grossly similar to those of NAS neurons, further suggesting that the present population of NAS neurons were derived primarily from the core region.
Nucleus
accumbens
5ms
and cortical
549
arousal
_I
5 ms
I IrnV
D.
B. I-
\
“r”
0.5 mV
-
i t
5 ms
5 ms -
t
100 ms
Fig. I. Responses of NAS neurons to afferent stimulation. Stimulation of the fimbriaafornix (arrows indicate shock artifact) evokes a short-latency excitatory response in a single NAS neuron (A, unfiltered; B, filtered 0.4--16 kHz). Stimulation of the ventral pallidurn evoked similar orthodromic responses that were visible in both unfiltered (C) and filtered (D) signals. In the same unit, ventral pallidal stimulation also produced a complete cessation of spontaneous activity that lasted approximately I50 ms (after the excitatory response (E, 10 sweeps superimposed).
Respornes to gferent stimulation The overall responses of NAS neurons to stimulation of afferents are very similar in unanesthetized rats (Fig. 1) compared to anesthetized rats.’ A total of 55 neurons were tested for responsiveness to stimulation of the fimbria-fornix (Table I). Of these neurons. 27 (49%) fired single spikes at relatively short latencies in response to the test shock (Fig. IA. B). As in the anesthetized animal,’ the latencies of NAS neuronal firing were distributed bimodally with 21 neurons discharging between 5 and 12 ms (7.3 + 2.1 ms) and five neurons discharging between Table Stimulation
1. Responses
of nucleus
site
Fimbriaafornix (tr) (“h) Mean rate in Hz (+S.D.) Ventral I;) Mean
accumbens
18 and 20 ms (19.6 k 0.9 ms) after fimbria-fornix stimulation. The stimulation-evoked spikes did not follow high-frequency (100 Hz) stimulation. and the latency for an individual unit exhibited some variation that typically did not exceed I ms. In nine of these “fimbria driven” units, a 100-150 ms cessation of spontaneous activity was observed after the single evoked spike, and this period of inhibition was often evident when the stimulation current was reduced to levels insufficient to evoke the short-latency spike. A separate six units (11%) were tested that showed only an inhibitory response to fimbria-fornix stimulation. These units exhibited no short-latency excitatory neurons
to afferent
Excitation
Excitation inhibition
Inhibition
3.21 i- 3.15
(KY%) 4.69 k 2.37
(2&) 8.49 f 6.47
stimulation
in awake
rats
No effect
Total
5.45 f 3.54
2.92 k 2.43
(IO& 3.62 + 2.87
(Qg,) 3.44
(4lL) 3.08 + 3.12
(lOlO&, 6.20 k 5.89
pallidum (24;) rate in Hz ( f SD.)
I I .25 k 9.50
responses with stimulation currents of up to 1.5 mA. Only 22 units (40%) that were tested failed to respond to fimbria-fornix stimulation. The firing rates of neurons during wakefulness appeared to be unrelated to the type of response (excitatory or inhibitory) evoked by fimbria-fornix stimulation (Table 1). A total of 17 neurons were tested for responsiveness to stimulation of the ventral pallidal region (Table 1). Of these neurons. nine (53%) exhibited short-latency, orthodromic excitatory responses (Fig. lC, D) with five units also showing an inhibitory response (Fig. IE). The latencies for excitatory responses were distributed unimodally with a mean *SD. of 5.9 _t 2.8 ms. One unit (6%) exhibited only inhibition, and seven units (41%) were unaffected by ventral pallidal stimulation. The mean firing rates of these neurons during wakefulness did not differ significantly according to the type of response evoked by ventral pallidal stimulation. The apparently high mean firing rates of neurons showing excitatory responses is explained by the fact that this small sample contained two cells with firing rates 18.0Hz (the highest rates in the entire population). The remainder of the cells that were excited by ventral pallidal stimulation had a mean firing rate (5.4 i_ 3.5 Hz) similar to that of the other groups. Influence of‘ sleep-wake cycle
The firing rates of NAS neurons varied across electrographically determined states of arousal (Fig. 2). Firing rates were determined for 61 neurons during quiet wakefulness (4.08 + 3.86 Hz), for 27 neurons during SWS (1.77 + 1.68 Hz), and for five neurons during REM sleep (4.22 + 4.54 Hz). Behavioral state significantly influenced the firing rate of NAS neurons (F = 4.458; d.f. = 2,90; P = 0.0142). Specifically, the mean firing rate of 25 NAS neurons recorded during both wakefulness and SWS was reduced by 44% during the transition from wakefulness (3.22 k 2.80 Hz) to SWS (1.87 + 1.70 Hz; t = 3.595; P = 0.0015; paired t-test; Fig. 3). In contrast, the mean firing rate was not significantly differ-
Waking EMG
EEG
NREM
ent between wakefulness and REM sleep tt~g. 3). 01 five neurons recorded during both wakefulness anti REM sleep, three actually discharged faster during REM sleep than during wakefulness. Relationship to motor uctirlt I‘
In a separate series of experiments, six neurons were recorded during wakefulness as animals were introduced into the open fieldiholeboard arena. Signilicant positive correlations (Pearson product-moment correlation coefficient, r = 0.773. 0.488) between sector entries and unit discharge rate per I-nun epoch were observed for two units, and one of these units is depicted in Fig. 3. The firing rate of two units displayed negative correlations with locomotor activity (r = -0.469; -0.377; Fig. 3) and two units were not significantly related to locomotor activity. In addition, spike discharge rate was not correlated to the number of rearings or nose-pokes exhibited by the animal. Ir$luence oj’ halothane anesthesia
Halothane reduced the firing rate of NAS neurons. In six units tested, the mean firing rate observed during wakefulness (2.97 k 3.61 Hz) was reduced by 47% after the induction of halothane anesthesia (1.58 k 2.31 Hz; t = 2.578; P = 0.0496; paired t-test). In one of these units, the excitatory response to stimulation of the fimbria-fornix was observed during the induction of anesthesia. During wakefulness, evoked firing of this neuron followed 100% of test stimulations at a current of 0.6 mA. After the induction of anesthesia, no evoked spikes were observed at this stimulation current. Moreover. evoked spikes followed only 40% of test stimulations at a much higher current (I .2 mA). This decreased excitability to afferent stimulation reversed as the animal recovered from anesthesia. DISCUSSJON
These studies illustrate both similarities and differences between the populations of NAS neurons
REM I100
w
llv
150 fiv
Fig. 2. Variation of NAS neuronal firing across behavioral states. Waking was recognized by a desynchronized EEG with EMG activity. Non-REM (NREM) sleep was recognized by EEG synchrony and continued EMG activity. Firing rate of a NAS neuron was decreased during NREM sleep relative to the firing rate during waking or REM sleep.
Nucleus accumbens and cortical arousal
-O-
Crossings
--t-
Firing Rate (Hz)
Minutes
-0
10
20
30
Minutes
in Box
40
50
60
70-
in Box
Fig. 3. Firing rates of NAS neurons were both positively and negatively correlated with locomotor activity. Each panel depicts the firing rate of an individual NAS neuron during I-min epochs after introduction into an open field, along with the number of sector entries (Crossings) per 1-min epoch made by the animal. The cell represented in the top figure increases firing during periods of activity, while the cell in the bottom figure decreases activity during periods of activity.
recorded from the awake and the anesthetized rat. For example, the patterns of responses to afferent stimulation observed in the present study resemble the response patterns of NAS neurons in anesthetized animals. The most prominent effect of fimbria-fornix stimulation in both preparations is a short-latency excitatory response that is consistent with monosynaptic activation of NAS neurons by hippocampal efferents.5.9 The proportion of NAS neurons that exhibited short-latency excitatory responses in the present study (49%) was similar to the proportion of “fimbria-driven” cells previously reported in halothane-anesthetized animals (45%).’ Likewise, a subset of NAS neurons exhibit longer duration inhibitory responses to afferent stimulation in both awake and anesthetized animals, with some neurons exhibiting combinations of the short-latency excitation and the long-lasting period of inhibition. Similar response patterns after ventral pallidal stimulation are also observed both in the awake (Table 1) and in the anesthetized preparation.6 These different patterns of responses suggest a phenomenological heterogeneity of NAS neurons. However, intracellular recordings in vitro and in vivo indicate that NAS neurons may respond to afferent inputs with a de-
polarization-hyperpolarization sequence, and these two components may be differentially regulated.3.‘4.‘8 Thus, the apparently diverse responses of NAS neurons to afferent stimulation observed in extracellular recordings may actually be a result of different resting states of the NAS neurons rather than different patterns of afferent input. The discharge rate of NAS neurons in awake rats was much greater than previously reported in anesthetized rats. In fact, many neurons in the anesthetized preparation can only be detected by stimulation-evoked firing*,’ or by iontophoresis of excitatory substances. I6 This study directly demonstrated that halothane anesthesia decreases NAS neuronal discharge. The fact that a decreased response to afferent stimulation accompanied the decreased discharge rate of one cell strongly suggests that halothane reduced the excitability of NAS neurons. Direct effects of halothane on neuronal excitability have been demonstrated in vitro.” However, these data cannot exclude the possibility that halothane acted primarily to reduce the quantity of excitatory input into the NAS, thereby indirectly reducing the excitability of NAS neurons. These issues might be resolved by intracellular recordings
552
C‘. W.
CALLAM
4~
in viw to determine whether induction of anesthesia hyperpolarizes the neuron, decreases the number of spontaneous postsynaptic potentials or both. These experiments found that the discharge of NAS units varied in relation to the electrographically determined behavioral state. Although NAS neurons may increase their firing rate during wakefulness in response to sensory stimuli or motor activity. NAS units were also activated during REM sleep, a state characterized by inhibition of EMG activity and decreased responsiveness to external stimuli.” This observation indicates that it is possible for the discharge of NAS units to increase independently of motor activity or sensory input. Furthermore, changes in gross motor activity exhibited both positive and negative correlations with NAS neuronal firing rate, suggesting that NAS neuronal activity does not merely reflect changes in motor output. Although NAS unit discharge slowed both during SWS and during halothane anesthesia. this study cannot distinguish whether this decreased discharge of NAS units resulted from decreased afferent input or from intrinsic decreases in the excitability of NAS neurons. The state-dependency of neuronal discharge in the NAS of rats observed here was similar to the discharge pattern of “waking-active” neurons observed previously in the basal forebrain of cats.” Taken together, these data support the hypothesis that increased activity of neurons in the region of the
and S. J.
HtNtuKsEh
NAS is associated with behavioral activation. Houever, these data are only correlative. and further studies are required to establish causal or mechanistic connections between NAS neuronal firing and behalioral state. CONCLUSIONS
In summary, this study reports that a population of NAS neurons that is characterized by responses to afferent stimulation can be recorded in awake rats and that this population is very similar to NAS neurons previously recorded in anesthetized rats. However, the discharge rate of NAS neurons is influenced by behavioral state, increasing during states associated with EEG desynchrony. The discharge of NAS neurons during wakefulness varies with locomotor activity, suggesting some participation of NAS neurons in the motor component of behavioral activation. Future studies should examine whether NAS neuronal firing during wakefulness is correlated with particular motor, sensory or electrographic events under more controlled conditions. Acknowledgements-Supported by ADAMMA and TRDRP awards DA03665, DAOO131,AA07365 and RT183 to S.J.H. The NIH Medical Scientist Training Program (M07198) supported C.W.C. Special thanks to Dr Scott C. Steffensen for invaluable technical advice. This is manuscript NP 6870 from the Research Institutes of Scripps Clinic.
REFERENCES
1. Alheid G. F. and Heimer L. (1988) New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid, and corticopetal components of substantia innominata. Neuroscience 27, l-39. 2. Callaway C. W., Hakan R. L. and Henriksen S. J. (1991) Distribution of amygdala input to the nucleus accumbens septi: an electrophysiological investigation. J. Neural Trunsm. 83, 215-225. 3. Chang H. T. and Kitai T. (1986) Intracellular recordings from rat nucleus accumbens neurons in vitro. Bruin Res. 366, 392-396. 4. Deadwyler S. A., Biela J., Rose G., West M. and Lynch G. (1978) A microdrive assembly for use with glass or metal microelectrodes in recording from freely-moving rats. Electroenceph. din. Neurophysiology 47, 752-754. 5. DeFrance J. F.. Sikes R. W. and Chronisfer R. B. (1985) Dopamine action in the nuleus accumbens J. Neurophysiol. 54, 1568-l 557. 6. Hakan R. L., Berg G. I. and Henriksen S. J. (1992) Electrophysiological evidence for reciprocal connectivity between the nucleus accumbens septi and ventral pallidal region. Bruin Res. (in press).
7. Hakan R. L. and Henriksen S. J. (1987) Systemic opiate administration has heterogeneous effects on activity recorded from nucleus accumbens neuron in vivo. keurosci.-Left. 83, 307-312. 8. Hara M., Akaike A.. Sasa M. and Takaori S. (1987) Acute effects of methamphetamine applied microiontophoretically to nucleus accumbens neurons in rats. Neurosci. Res. 4, 279-290. 9. Kelley A. E. and Domesick V. B. (1982) The distribution of the projection from the hippocampal formation to the nucleus accumbens in the rat: an anterograde- and retrograde-horseradish peroxidase study. Neuroscience 7,2321-2335. IO. Kelly P. H., Seviour P. W. and Iversen S. D. (1975) Amphetamine and apomorphine responses in the rat following 6-OHDA lesions of the nucleus accumbens septi and corpus striatum. Brain Res. 95, 507-522. 11. Nicoll R. A. and Madison D. V. (1982) General anesthetics hyperpolarize neurons in the vertebrate central nervous system. Science 217, 1055- 1057. 12. Roberts D. C. S., Koob G. F., Klonoff P. and Fibiger H. C. (1980) Extinction and recovery of cocaine self-administration following 6-OHDA lesions of the nucleus accumbens. Pharmac. Biochem. Behav. 12, 781-787. 13. Szymusiak R. and McGinty D. (1986) Sleep-related neuronal discharge in the basal forebrain of cats. Bruin Rex 370, 82-92. 14. Uchimura N., Higashi H. and Nishi S. (1989) Membrane properties and synaptic responses of the guinea pig nucleus accumbens neurons in vitro. J. Neurophysiol. 61, 769-779. 15. Unemoto H., Sasa M. and Takaori S. (1989) A noradrenaline-induced inhibition from locus coeruleus of the nucleus accumbens neuron receiving input from the hippocampus. Jap. J. Pharmuc. 39, 233-239. 16. White F. J. and Wang R. Y. (1986) Electrophysiological evidence for the existence of both D-l and D-2 dopamine receptors in the rat nucleus accumbens. J. Neurosci. 6, 274-280.
Nucleus
accumbens
and cortical
arousal
553
17. Wu F., Mallick B. D. and Siegel J. M. (1989) Lateral geniculate spikes, muscle atonia and startle response elicited by auditory stimuli as a function of stimulus parameters and arousal state. Bruin Res. 499, 7 17. I8 Yim C. Y. and Mogenson G. J. (1988) Neuromodulatory action of dopamine in the nucleus accumbens: an in r,irw intracellular study. Neuroscience 26, 4033415. 19. Zaborsky L., Alheid G. F.. Beinfeld M. L.. Eiden L. E., Heimer L. and Palkovits M. (1985) Cholecystokinin innervation of the ventral striatum; a morphological and radioimmunological study. Nrurostimce 14, 427 453. (Acw/m/
I6 Jutw 1992)
NEURONAL FIRING IN THE NUCLEUS ACCUMBENS ASSOCIATED WITH THE LEVEL OF CORTICAL AROUSAL
IS
C. W. CALLAWAY and S. J. HENRIKSEN* Research
Institute
of Scripps
Clinic,
10666 North
Torrey
Pines Road,
La Jolla.
CA 93030,
USA
Abstract--Because of evidence that the nucleus accumbens mediates the activating effects of many drugs, this study examined the hypothesis that the firing rates of individual nucleus accumbens neurons are positively correlated with spontaneous changes in behavioral arousal that occur during the sleep- wake cycle. The present report examined the firing patterns of 80 neurons in the nucleus accumbens 01 unanesthetized. unrestrained rats during various electrographically determined levels of arousal. Synaptic responses to stimulation of hippocampal and pallidal nucleus accumbens afferents indicated that the present sample of neurons was similar to a large population of nucleus accumbens neurons previously recorded in anesthetized rats. Confirming the participation of the nucleus accumbens in behavioral arousal,the firing rates of nucleus accumbens neurons were greatest during wakefulness and rapid eye movement sleep and lowest during non-rapid eye movement sleep. Furthermore, the induction of halothane anesthesia decreased behavioral and electrocorticographic arousal concurrent with a suppression of the spontaneous nucleus accumbens unit discharge. These data support the hypothesis that the firing of nucleus accumbens neurons is closely related to ~NXF.231
Behavioral studies suggest that the nucleus accumbens septi (NAS) is important for the control of motivated behaviors as well as for the expression of pharmacologically stimulated motor activity.‘“.” In an attempt to understand the normal physiology of the NAS. numerous studies have examined diverse measur’cs of NAS function. including electrophysiological measures. These stud& have confirmed that NAS neurons receive excitatory afferents from regions including the llippocampus,~ hasolateral amygdala’~” and parafasicuiar thalamic nuclei.” Furthermore. projections from midbrain catecholamincrgic nuclei have been shown to modify the effects of these excitatory afferents.i.‘5.‘K While these studies provided information about the effects of afferent stimulation and of pharmacological manipulations on NAS neuronal firing, speculations about the parti~ipation of NAS neurons in behavior have been limited by the use of anesthetized animals in which correlations between behavioral activity and electrophysiological measures are impossible. In order to more directly characterize NAS neuronal firing under physiological conditions. the prcsent study identified a population of NAS neurons in unanesthetized rats. Spontaneous variations in the discharge of NAS neurons wras observed during the transitions between stages of wakefulness and sleep. In order to relate the findings of these studies to ______-._.
*To whom correspondence
should be addressed. .4~~r~,~~1~~~~~~~.~: EEG, el~troencephalogram; EMG, electromyographic activity; NAS, nucleus accumbens semi: NREM, non-rapid eye movement: REM, rapid eye movement; SWS, slow wave sleep.
studies in anesthetized rats. separate expcrimcnts also examined the effect of halothanc anesthesia induction on NAS neuronal discharge. These data indicate that the clcctrophysiological characteristics of NAS neurons in unanesthetized animals resemble the patterns observed in anesthetized animals. and that the rate of NAS ncuronal discharge is related to behavioral state. previous
EXPERIMENTAl.
PROC‘EI)I’RES
Male SpragueDawley rats (Charles River) were housed individually under regular lighting (lights on: Oh.00 1X.00). One week after arrival (body weight 300 -350 g). animals were surgically prepared for chronic elcctrophysiologic;ll recordings. Each rat was anesthetized with 2.54;1 halothanc in air, and mounted in a Kopf stereotaxrc devtce. Anesthesia was maintained during the procedure with 1.5% halorhanc in air. Body tem~~ture was maintained using an abdominal heating pad thermostat&thy controled by a rectal thermometer. Hippocampal afferents to the NAS in the hmbria-fornix were stimulated via a bipolar twisted-wire electrode with a 0.5.mm tip separation implanted a~ coordinates 1.3 mm anterior (A) from bregma, 1.3 mm lateral (L) from midline, 335 mm ventral (V) from skull surface. The ventral pallidurn, a structure having both afferent and elferent connections with the NAS. was stimulated via an electrode implanted at coordinates A-2.0 mm, L I .Smm, V 667 mm. In addition, wires were attached to stainless-steel skull screws in the frontal and parietal bones for grounding and to allow recording of electroencephulographic activity @EC). Wires with 2 3 mm of exposed conductor at their ends were implanted bilaterally in the neck muscles to record electromyog~phic activity (EMG). All wire leads were gathered into a plastic “hat” connector. and the entire assembly was fixed to the skull using acrylic dental cement. For single unit recording, a removable microdrive system (Bield Engineering. Irvine, CA) was employed that has
548
(
W.
C‘ALI.AW.ZY
previously been described.3 Craniotomies were made in the skull overlying the NAS (A f 1.3 1.7. L 1.2 -1.5 mm). A threaded stainless-steel hub was placed over the craniotomy and permanently fixed to the skull using dental cement. A removable cap protected the brain between recording sessions. Immed~at~iy prior to recording sessions, the cap was replaced with a mechanical microdrive assembly carrying a tungsten microelectrode. Commercially fabricated tungsten microelectrodes (Frederick Haer. Brunswick, ME) with impedances of 9- 10 Mn at 1000 Hz were used in all experiments. For NAS recordings. the microelectrode was advanced to between 6.0 and 8.5 mm below the skull surface. Animals were tested in a 28 x 2X x 38 cm Plexiglas box with a wire mesh floor. The animal was tethered by an electrical commutator/cable assembly. that allowed free movement within the box while promdmg remote access to recording and stimulating electrodes. Poiygraphic records of EEG and EMG were collected using a Grass Model 79D polygraph (Grass Instrument, Quincy, MA). Stimulation (0.1-1.5 mA, 0.15-ms pulses, 0.1 Hz maximum frequency) was provided via a Grass S-88 stimulator and stimulation isolation unit. The signal from the microelectrode was amplified at the headstage by three operational amplifiers configured as an instrumentation amplifier with gain of IO. This signal was displayed unfiltered or filtered on oscilloscopes. Units were islolated by slowly advancing the microelectrode while providing auditory and tactile stimulation to the animal (in order to detect sensorimotor-related units) and stimulating NAS afferents (to detect otherwise silent stimulation-evoked units). Single unit activity was discriminated from the filtered signal based on amplitude and spike width using a window discriminator (Fintronics, Orange. CN). The discriminated output of this spike detector was relayed to the polygraph and to a computer for analysis (Apple, Cupertino, CA). Experiments were performed during both the light and dark phases of the light-dark cycle. Other than the dccreased frequency of sleep during the dark phase, no differences were noted between light-phase and dark-phase experiments, and the results have been combined. Behavioral state was determined from the polygraph recordings. Wakefulness usas recognized by a desynchronized, lowvoltage EEG and direct visual observation (animal maintains an upright posture with eyes open and orients to sound or touch). Slow wave sleep (SWS) was characterized by the presence of high-amplitude (greater than 75 p V) slow waves in the EEG, behaviorai signs of sleep and continued EMG activity. Rapid eye movement (REM) sleep was characterized by a low-voltage EEG, continued behavioral signs of sleep and a decrease in EMG activity to the level of background noise. Firing rates for NAS neurons were dete~ined during each arousal state by counting the number of spikes occurring within a continuous 180-s period of each state. When 180 s of data were not available (as was typical during REM sleep episodes lasting no longer than 120 s), the largest available continuous record was used. In order to control for the possible influence of motor activity on neuronal firing and to provide a suitable comparison with other states of arousal, firing rates for wakefulness were determined during periods when the animal exhibited no gross motor activity. After experiments were completed. animals were kilied by an overdose of halothane vapors. The locations of each stimulating electrode and of a representative position of the recording electrode were marked by passing a continuous current (I mA, 10 s) through the electrode, allowing subsequent histological verification. Additional experiments examined the influence of motor activity on NAS neuronal firing. Units were isolated while the animal was held in an opaque 20 x 20 cm plastic box. When a NAS unit was identified, the animal was gently transferred into a 30.5 x 61 .O cm Plexiglas chamber. Three equally spaced 2.5-cm holes were placed 2.5 cm above the floor in each long wall and in the midline of the floor. A
md
S.
J.
HENKIKSEN
single 2.5-cm hole was placed in one short wali. ln~ruduclrl~ the animal into this open field reliably increased locomotor activity and rearing, as well as promoting directed investigation of the holes. The exploration of the open field was recorded on a videotape that was synchromzed uith J sequential record of the unit discharge acquired by the computer. This procedure allowed subsequent nir-line corn-. parisons between unit firing and videotaped behavior A subset of experiments assessed the influence of halothane anesthesia on the discharge characteristics of NAS neurons. For these studies. anesthesia was induced quickly by introducing air saturated with halothanc vapors into the recording chamber, typically resulting in cessation of motor activity and the appearance of high-voltage slowwaves in the EEG within 1min. Administration of halothane continued until the animal exhibited only a slow hindlimb withdrawal in response to foot-pinch (1 2 min), at which time anesthesia was discontinued. Although steadystate levels of anesthesia probably were not achieved, this procedure minimized the period of agitation prior to cessation of motor activity during which units were likely to be lost, and provided a shorter period of recovery (3 4 min) for the animal. Furthermore, the level of anesthesia as assessed by foot-pinch-withdrawal was comparable for several minutes to the level of anesthesia employed in acute electrophysiological experiments using a continuoLls exposure to 0.8% halothane in air.‘,’
RESULTS The studied
characteristics were
determined
of
the using
neuronal a total
popuiat~o~ of 80 neurons
that were recorded from NAS of 11 animals. A minimum signal-to-noise ratio of 2: 1 was required for discrimination of a spike, and all spikes were required to have similar amplitudes and waveforms to be considered a single unit. Neurons (mean 2S.D. of 1.9 f 1.4 cells/track) were encountered with firing rates ranging from 0.1 to 18 Hz (mean 2S.D. = 4.08 & 3.86 Hz) during wakefulness. The distribution of firing rates did not suggest any subgroups of neurons. The interspike intervals of representative units were found to be unimodally distributed, indicating the absence of bursting activity. An additional nine units isolated at ventral recording sites with firing rates between 20 and 50 Hz were considered to be cells of the olfactory tubercle. Similar fast-firing neurons recorded previously in anesthetized animals have been localized to the border between the NAS and olfactory tubercle.7 Because the location of individual neurons could not be determined using the present methods, it is impossible to determine whether the NAS units were recorded from the core or shell of the NAS.” However, the coordinates employed, were more likely to sample from the core of the NAS, and none of the final lesion marks were located medial to the lateral ventricle. In addition, anatomical studies indicate that the core region of the NAS resembles the overlying caudate-putamen.’ Neurons encountered as electrodes were passing through the caudate-putamen exhibited discharge patterns grossly similar to those of NAS neurons, further suggesting that the present population of NAS neurons were derived primarily from the core region.
Nucleus
accumbens
5ms
and cortical
549
arousal
_I
5 ms
I IrnV
D.
B. I-
\
“r”
0.5 mV
-
i t
5 ms
5 ms -
t
100 ms
Fig. I. Responses of NAS neurons to afferent stimulation. Stimulation of the fimbriaafornix (arrows indicate shock artifact) evokes a short-latency excitatory response in a single NAS neuron (A, unfiltered; B, filtered 0.4--16 kHz). Stimulation of the ventral pallidurn evoked similar orthodromic responses that were visible in both unfiltered (C) and filtered (D) signals. In the same unit, ventral pallidal stimulation also produced a complete cessation of spontaneous activity that lasted approximately I50 ms (after the excitatory response (E, 10 sweeps superimposed).
Respornes to gferent stimulation The overall responses of NAS neurons to stimulation of afferents are very similar in unanesthetized rats (Fig. 1) compared to anesthetized rats.’ A total of 55 neurons were tested for responsiveness to stimulation of the fimbria-fornix (Table I). Of these neurons. 27 (49%) fired single spikes at relatively short latencies in response to the test shock (Fig. IA. B). As in the anesthetized animal,’ the latencies of NAS neuronal firing were distributed bimodally with 21 neurons discharging between 5 and 12 ms (7.3 + 2.1 ms) and five neurons discharging between Table Stimulation
1. Responses
of nucleus
site
Fimbriaafornix (tr) (“h) Mean rate in Hz (+S.D.) Ventral I;) Mean
accumbens
18 and 20 ms (19.6 k 0.9 ms) after fimbria-fornix stimulation. The stimulation-evoked spikes did not follow high-frequency (100 Hz) stimulation. and the latency for an individual unit exhibited some variation that typically did not exceed I ms. In nine of these “fimbria driven” units, a 100-150 ms cessation of spontaneous activity was observed after the single evoked spike, and this period of inhibition was often evident when the stimulation current was reduced to levels insufficient to evoke the short-latency spike. A separate six units (11%) were tested that showed only an inhibitory response to fimbria-fornix stimulation. These units exhibited no short-latency excitatory neurons
to afferent
Excitation
Excitation inhibition
Inhibition
3.21 i- 3.15
(KY%) 4.69 k 2.37
(2&) 8.49 f 6.47
stimulation
in awake
rats
No effect
Total
5.45 f 3.54
2.92 k 2.43
(IO& 3.62 + 2.87
(Qg,) 3.44
(4lL) 3.08 + 3.12
(lOlO&, 6.20 k 5.89
pallidum (24;) rate in Hz ( f SD.)
I I .25 k 9.50
responses with stimulation currents of up to 1.5 mA. Only 22 units (40%) that were tested failed to respond to fimbria-fornix stimulation. The firing rates of neurons during wakefulness appeared to be unrelated to the type of response (excitatory or inhibitory) evoked by fimbria-fornix stimulation (Table 1). A total of 17 neurons were tested for responsiveness to stimulation of the ventral pallidal region (Table 1). Of these neurons. nine (53%) exhibited short-latency, orthodromic excitatory responses (Fig. lC, D) with five units also showing an inhibitory response (Fig. IE). The latencies for excitatory responses were distributed unimodally with a mean *SD. of 5.9 _t 2.8 ms. One unit (6%) exhibited only inhibition, and seven units (41%) were unaffected by ventral pallidal stimulation. The mean firing rates of these neurons during wakefulness did not differ significantly according to the type of response evoked by ventral pallidal stimulation. The apparently high mean firing rates of neurons showing excitatory responses is explained by the fact that this small sample contained two cells with firing rates 18.0Hz (the highest rates in the entire population). The remainder of the cells that were excited by ventral pallidal stimulation had a mean firing rate (5.4 i_ 3.5 Hz) similar to that of the other groups. Influence of‘ sleep-wake cycle
The firing rates of NAS neurons varied across electrographically determined states of arousal (Fig. 2). Firing rates were determined for 61 neurons during quiet wakefulness (4.08 + 3.86 Hz), for 27 neurons during SWS (1.77 + 1.68 Hz), and for five neurons during REM sleep (4.22 + 4.54 Hz). Behavioral state significantly influenced the firing rate of NAS neurons (F = 4.458; d.f. = 2,90; P = 0.0142). Specifically, the mean firing rate of 25 NAS neurons recorded during both wakefulness and SWS was reduced by 44% during the transition from wakefulness (3.22 k 2.80 Hz) to SWS (1.87 + 1.70 Hz; t = 3.595; P = 0.0015; paired t-test; Fig. 3). In contrast, the mean firing rate was not significantly differ-
Waking EMG
EEG
NREM
ent between wakefulness and REM sleep tt~g. 3). 01 five neurons recorded during both wakefulness anti REM sleep, three actually discharged faster during REM sleep than during wakefulness. Relationship to motor uctirlt I‘
In a separate series of experiments, six neurons were recorded during wakefulness as animals were introduced into the open fieldiholeboard arena. Signilicant positive correlations (Pearson product-moment correlation coefficient, r = 0.773. 0.488) between sector entries and unit discharge rate per I-nun epoch were observed for two units, and one of these units is depicted in Fig. 3. The firing rate of two units displayed negative correlations with locomotor activity (r = -0.469; -0.377; Fig. 3) and two units were not significantly related to locomotor activity. In addition, spike discharge rate was not correlated to the number of rearings or nose-pokes exhibited by the animal. Ir$luence oj’ halothane anesthesia
Halothane reduced the firing rate of NAS neurons. In six units tested, the mean firing rate observed during wakefulness (2.97 k 3.61 Hz) was reduced by 47% after the induction of halothane anesthesia (1.58 k 2.31 Hz; t = 2.578; P = 0.0496; paired t-test). In one of these units, the excitatory response to stimulation of the fimbria-fornix was observed during the induction of anesthesia. During wakefulness, evoked firing of this neuron followed 100% of test stimulations at a current of 0.6 mA. After the induction of anesthesia, no evoked spikes were observed at this stimulation current. Moreover. evoked spikes followed only 40% of test stimulations at a much higher current (I .2 mA). This decreased excitability to afferent stimulation reversed as the animal recovered from anesthesia. DISCUSSJON
These studies illustrate both similarities and differences between the populations of NAS neurons
REM I100
w
llv
150 fiv
Fig. 2. Variation of NAS neuronal firing across behavioral states. Waking was recognized by a desynchronized EEG with EMG activity. Non-REM (NREM) sleep was recognized by EEG synchrony and continued EMG activity. Firing rate of a NAS neuron was decreased during NREM sleep relative to the firing rate during waking or REM sleep.
Nucleus accumbens and cortical arousal
-O-
Crossings
--t-
Firing Rate (Hz)
Minutes
-0
10
20
30
Minutes
in Box
40
50
60
70-
in Box
Fig. 3. Firing rates of NAS neurons were both positively and negatively correlated with locomotor activity. Each panel depicts the firing rate of an individual NAS neuron during I-min epochs after introduction into an open field, along with the number of sector entries (Crossings) per 1-min epoch made by the animal. The cell represented in the top figure increases firing during periods of activity, while the cell in the bottom figure decreases activity during periods of activity.
recorded from the awake and the anesthetized rat. For example, the patterns of responses to afferent stimulation observed in the present study resemble the response patterns of NAS neurons in anesthetized animals. The most prominent effect of fimbria-fornix stimulation in both preparations is a short-latency excitatory response that is consistent with monosynaptic activation of NAS neurons by hippocampal efferents.5.9 The proportion of NAS neurons that exhibited short-latency excitatory responses in the present study (49%) was similar to the proportion of “fimbria-driven” cells previously reported in halothane-anesthetized animals (45%).’ Likewise, a subset of NAS neurons exhibit longer duration inhibitory responses to afferent stimulation in both awake and anesthetized animals, with some neurons exhibiting combinations of the short-latency excitation and the long-lasting period of inhibition. Similar response patterns after ventral pallidal stimulation are also observed both in the awake (Table 1) and in the anesthetized preparation.6 These different patterns of responses suggest a phenomenological heterogeneity of NAS neurons. However, intracellular recordings in vitro and in vivo indicate that NAS neurons may respond to afferent inputs with a de-
polarization-hyperpolarization sequence, and these two components may be differentially regulated.3.‘4.‘8 Thus, the apparently diverse responses of NAS neurons to afferent stimulation observed in extracellular recordings may actually be a result of different resting states of the NAS neurons rather than different patterns of afferent input. The discharge rate of NAS neurons in awake rats was much greater than previously reported in anesthetized rats. In fact, many neurons in the anesthetized preparation can only be detected by stimulation-evoked firing*,’ or by iontophoresis of excitatory substances. I6 This study directly demonstrated that halothane anesthesia decreases NAS neuronal discharge. The fact that a decreased response to afferent stimulation accompanied the decreased discharge rate of one cell strongly suggests that halothane reduced the excitability of NAS neurons. Direct effects of halothane on neuronal excitability have been demonstrated in vitro.” However, these data cannot exclude the possibility that halothane acted primarily to reduce the quantity of excitatory input into the NAS, thereby indirectly reducing the excitability of NAS neurons. These issues might be resolved by intracellular recordings
552
C‘. W.
CALLAM
4~
in viw to determine whether induction of anesthesia hyperpolarizes the neuron, decreases the number of spontaneous postsynaptic potentials or both. These experiments found that the discharge of NAS units varied in relation to the electrographically determined behavioral state. Although NAS neurons may increase their firing rate during wakefulness in response to sensory stimuli or motor activity. NAS units were also activated during REM sleep, a state characterized by inhibition of EMG activity and decreased responsiveness to external stimuli.” This observation indicates that it is possible for the discharge of NAS units to increase independently of motor activity or sensory input. Furthermore, changes in gross motor activity exhibited both positive and negative correlations with NAS neuronal firing rate, suggesting that NAS neuronal activity does not merely reflect changes in motor output. Although NAS unit discharge slowed both during SWS and during halothane anesthesia. this study cannot distinguish whether this decreased discharge of NAS units resulted from decreased afferent input or from intrinsic decreases in the excitability of NAS neurons. The state-dependency of neuronal discharge in the NAS of rats observed here was similar to the discharge pattern of “waking-active” neurons observed previously in the basal forebrain of cats.” Taken together, these data support the hypothesis that increased activity of neurons in the region of the
and S. J.
HtNtuKsEh
NAS is associated with behavioral activation. Houever, these data are only correlative. and further studies are required to establish causal or mechanistic connections between NAS neuronal firing and behalioral state. CONCLUSIONS
In summary, this study reports that a population of NAS neurons that is characterized by responses to afferent stimulation can be recorded in awake rats and that this population is very similar to NAS neurons previously recorded in anesthetized rats. However, the discharge rate of NAS neurons is influenced by behavioral state, increasing during states associated with EEG desynchrony. The discharge of NAS neurons during wakefulness varies with locomotor activity, suggesting some participation of NAS neurons in the motor component of behavioral activation. Future studies should examine whether NAS neuronal firing during wakefulness is correlated with particular motor, sensory or electrographic events under more controlled conditions. Acknowledgements-Supported by ADAMMA and TRDRP awards DA03665, DAOO131,AA07365 and RT183 to S.J.H. The NIH Medical Scientist Training Program (M07198) supported C.W.C. Special thanks to Dr Scott C. Steffensen for invaluable technical advice. This is manuscript NP 6870 from the Research Institutes of Scripps Clinic.
REFERENCES
1. Alheid G. F. and Heimer L. (1988) New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid, and corticopetal components of substantia innominata. Neuroscience 27, l-39. 2. Callaway C. W., Hakan R. L. and Henriksen S. J. (1991) Distribution of amygdala input to the nucleus accumbens septi: an electrophysiological investigation. J. Neural Trunsm. 83, 215-225. 3. Chang H. T. and Kitai T. (1986) Intracellular recordings from rat nucleus accumbens neurons in vitro. Bruin Res. 366, 392-396. 4. Deadwyler S. A., Biela J., Rose G., West M. and Lynch G. (1978) A microdrive assembly for use with glass or metal microelectrodes in recording from freely-moving rats. Electroenceph. din. Neurophysiology 47, 752-754. 5. DeFrance J. F.. Sikes R. W. and Chronisfer R. B. (1985) Dopamine action in the nuleus accumbens J. Neurophysiol. 54, 1568-l 557. 6. Hakan R. L., Berg G. I. and Henriksen S. J. (1992) Electrophysiological evidence for reciprocal connectivity between the nucleus accumbens septi and ventral pallidal region. Bruin Res. (in press).
7. Hakan R. L. and Henriksen S. J. (1987) Systemic opiate administration has heterogeneous effects on activity recorded from nucleus accumbens neuron in vivo. keurosci.-Left. 83, 307-312. 8. Hara M., Akaike A.. Sasa M. and Takaori S. (1987) Acute effects of methamphetamine applied microiontophoretically to nucleus accumbens neurons in rats. Neurosci. Res. 4, 279-290. 9. Kelley A. E. and Domesick V. B. (1982) The distribution of the projection from the hippocampal formation to the nucleus accumbens in the rat: an anterograde- and retrograde-horseradish peroxidase study. Neuroscience 7,2321-2335. IO. Kelly P. H., Seviour P. W. and Iversen S. D. (1975) Amphetamine and apomorphine responses in the rat following 6-OHDA lesions of the nucleus accumbens septi and corpus striatum. Brain Res. 95, 507-522. 11. Nicoll R. A. and Madison D. V. (1982) General anesthetics hyperpolarize neurons in the vertebrate central nervous system. Science 217, 1055- 1057. 12. Roberts D. C. S., Koob G. F., Klonoff P. and Fibiger H. C. (1980) Extinction and recovery of cocaine self-administration following 6-OHDA lesions of the nucleus accumbens. Pharmac. Biochem. Behav. 12, 781-787. 13. Szymusiak R. and McGinty D. (1986) Sleep-related neuronal discharge in the basal forebrain of cats. Bruin Rex 370, 82-92. 14. Uchimura N., Higashi H. and Nishi S. (1989) Membrane properties and synaptic responses of the guinea pig nucleus accumbens neurons in vitro. J. Neurophysiol. 61, 769-779. 15. Unemoto H., Sasa M. and Takaori S. (1989) A noradrenaline-induced inhibition from locus coeruleus of the nucleus accumbens neuron receiving input from the hippocampus. Jap. J. Pharmuc. 39, 233-239. 16. White F. J. and Wang R. Y. (1986) Electrophysiological evidence for the existence of both D-l and D-2 dopamine receptors in the rat nucleus accumbens. J. Neurosci. 6, 274-280.
Nucleus
accumbens
and cortical
arousal
553
17. Wu F., Mallick B. D. and Siegel J. M. (1989) Lateral geniculate spikes, muscle atonia and startle response elicited by auditory stimuli as a function of stimulus parameters and arousal state. Bruin Res. 499, 7 17. I8 Yim C. Y. and Mogenson G. J. (1988) Neuromodulatory action of dopamine in the nucleus accumbens: an in r,irw intracellular study. Neuroscience 26, 4033415. 19. Zaborsky L., Alheid G. F.. Beinfeld M. L.. Eiden L. E., Heimer L. and Palkovits M. (1985) Cholecystokinin innervation of the ventral striatum; a morphological and radioimmunological study. Nrurostimce 14, 427 453. (Acw/m/
I6 Jutw 1992)
