C.D. Barnes and 0. Pompeiano (Eds.1 Progress in Brain Research, Val. 88 0 1991 Elsevier Science Publishers B.V.

381 CHAPTER 29

Descending noradrenergic influences on pain S.L. Jones Department of Pharmacology, College of Medicine, University of Oklahoma, Oklahoma City, OK, U.S.A.

Multiple separate and distinct supraspinally organized descending inhibitory systems have been identified which are capable of powerfully modulating spinal nociceptive transmission. Until recently, brainstem sites known to be involved in the centrifugal modulation of spinal nociceptive transmission were few in number, being limited to midline structures in the midbrain and medulla (e.g., periaqueductal gray and nucleus raphe magnus). However, with continued investigation, that number has increased and brainstem sites previously thought to be primarily involved in cardiovascular function and autonomic regulation (e.g., nucleus tractus solitarius; locus coeruleus/subcoeruleus (LC/SC); A5 cell group; lateral reticular nucleus) also have been demonstrated to play a role in the modulation of spinal nociceptive transmission. Spinal monoamines (norepinephrine (NE) and serotonin) have been shown to mediate stimulation-produced descending inhibition of nociceptive transmission from these brainstem sites. The majority of NE-containing fibers and terminations in the spinal cord arise from supraspinal sources; thus, the

LC/SC, the parabrachial nuclei, the Kolliker-Fuse nucleus and the A5 cell group have all been suggested as possible sources of the spinal noradrenergic (NA) innervation involved in the centrifugal modulation of spinal nociceptive transmission. Several lines of evidence suggest that the LC/SC plays a significant role in a functionally important descending inhibitory NA system. Focal electrical stimulation in the LC produces an antinociception and increases significantly the spinal content of NA metabolites. The inhibition of the nociceptive tail-flick withdrawal reflex produced by electrical stimulation in the LC/SC has been demonstrated to be mediated by postsynaptic a,-adrenoceptors in the lumbar spinal cord. Similarly, electrical or chemical stimulation given in the LC/SC inhibits noxious-evoked dorsal horn neuronal activity. Thus, results reported in electrophysiological experiments confirm those reported in functional studies and the NA coeruleospinal system appears to play a significant role in spinal nociceptive processing.

Key words: antinociception, descending inhibition, analgesia, electrical stimulation, dorsal horn, spinal cord

Introduction Multiple separate and distinct supraspinally organized descending inhibitory systems have been identified which, when activated either by electrical stimulation or by the microinjection of drugs (e.g., morphine or glutamate) into selected regions of the brain (see Yaksh and Rudy, 1978; Gebhart, 1982 and Hammond, 1986 for reviews),

powerfully modulate spinal nociceptive transmission (Besson et al., 1975; Handwerker et al., 1975; Duggan et aZ., 1977 and Soja and Sinclair, 1983). The relevance of these descending inhibitory systems to antinociception and analgesia became clear with the demonstration that both spinal nociceptive reflexes and complex behaviors elicited by nociceptive stimuli are inhibited in rats, cats and monkeys (e.g., see Mayer, 1979 for

382

review) and analgesia is produced in man (e.g., Hosobuchi ef al., 1977) by stimulation or opioids given into these same brain regions. Until recently, brainstem sites known to be involved in the centrifugal modulation of spinal nociceptive transmission were few in number, being limited to midline structures in the midbrain and medulla (e.g., the periaqueductal gray and nucleus raphe magnus (NRM)). However, with continued investigation, that number has increased and brainstem sites previously thought to be primarily involved in cardiovascular function and autonomic regulation, (e.g., nucleus tractus solitarius; locus coeruleus (LC); A5 cell group; lateral reticular nucleus) also have been shown to play a role in the modulation of spinal nociceptive transmission. Spinal monoamines (norepinephrine (NE) and serotonin) have been demonstrated to mediate stimulation-produced descending inhibition of nociceptive transmission from these brainstem sites (see Proudfit, 1988 for review). This chapter will review the role that monoamines play in spinal nociceptive processing and will focus on the descending coeruleospinal system. Adrenoceptor agonists and antinociception It has been known since the 1940s that monoamines are involved in the modulation of pain and analgesia when it was demonstrated that systemically administered sympathomimetic agents (e.g., amphetamine) produced an analgesia in man (Burill et al., 1944). It became clear, however, that not all sympathomimetic agents produce an antinociception (e.g., oxymetazoline) when administered systemically. It was soon recognized that the failure of such agents to produce antinociceptive effects was due to their inability to cross the blood-brain barrier and enter the central nervous system; when administered intracerebrally, such agents also produce antinociceptive effects (see Yaksh, 1985 for review). That monoamines produce their antinociceptive effects via spinal sites of action has been

established. Monoaminergic agonists, including serotonin (Yaksh and Wilson, 1979) and NE (Kuraishi et al., 1979; Reddy et al., 19801, when administered intrathecally directly into the spinal subarachnoid space produce powerful antinociceptive effects. Systematic studies have revealed that the structure-activity series for intrathecally administered adrenoceptor agonists for the hot plate and tail-flick (TF) analgesiometric tests is: ST-91 (2-[2,6-diethyl-phenylamine]-2-imidazoline; (a,)= NE > methoxamine (a,) > > isoproterenol ( p ) = 0 (Yaksh, 1985). In the primate, using a shock titration task in which the animal defines a nociceptive threshold, similar results have been reported (Yaksh, 1985). Thus, the ability of intrathecally administered adrenoceptor agonists, including NE, to elevate nociceptive thresholds appears to be mediated by a adrenoceptors. To assess the relative role of spinal a,-versus spinal a,-adrenoceptors in antinociception, dose-response relationships have been generated examining the ability of selective a,-and qadrenoceptor antagonists to alter the antinociceptive effects produced by the intrathecal administration of a adrenoceptor agonists. The rank order of potency for antagonizing the antinociceptive effects of the a,-adrenoceptor agonist ST-91 is: yohimbine (a,),rauwolscine (a,),prazosin (a,), phentolamine (a, and a2),corynanthine (a,), propranolol ( p ) = 0 (Yaksh, 1985). Thus, a population of spinal a,-adrenoceptors specifically appears to mediate a-adrenoceptor agonist-produced antinociception. Modulation of spinal nociceptive transmission by a-adrenoceptor agonists Behavioral studies suggest that intrathecally administered NE is selective with regard to its antinociceptive effects; doses of NE required for maximal antinociceptive effects produce no significant signs of motor dysfunction (e.g., Reddy et al., 1980; Howe et al., 1983; Yaksh, 1985). NE also has been demonstrated to modulate sensory transmission in the dorsal horn; however, the

383

N E applied via microiontophoresis has been demonstrated to inhibit selectively unit activity of nociceptive-specific neurons in deep laminae of the cat dorsal horn (Belcher et al., 1978; Headley

effects of NE on evoked dorsal horn neuronal activity, and the selectivity of those effects for spinal nociceptive transmission, is less clear (see Table 1). TABLE 1

Effects of iontophoretically administered norepinephrine o n spinal cord dorsal horn neurons Lami- Neuron nae' Type

n

Activity/stimulus

Inhibition

Engberg & Ryall, 1966 Cat/Decerebrate

-

204

Elec. Stim.

96

Weight & Salmoiraghi. Cat/Decerebrate Cat/Ether 1966

-

92 36

Reference

Species/Anesthetic

Cat /Fluothane/ a-Chl

Belcher er al., 1978

a

-

HT

9 6

25 Spontaneous 10 Bradykinin ',heat 14 D,L-homocysteic acid 45 Spontaneous 12 Brush, movement 18 u,L-homocysteic acid

LT

Headley er ul., 1978

Cat/a-Chl

IV-v

MULTI

23

Satoh et al., 1979

Rabbit/Ether

V

MULTI

20 Bradykinin 11 Bradykinin, brush

Todd & Millar, 1983

Cat/Pentobarbitone

1-111

47

Willcockson et ul.,

Primate/Hal/N20/ a-Chl

I-v

19

1984

Flee twood-Walker

Cat/Hal/a-Chl

-

Rat/Hal/Urethane

1-11

et al. 19x5 ~

Howe & Zieglgansberger, 1987

MULTI LT

46 10

deep ~

~

~

~

~

~

Pinch Brush Glutamate

~

No Effect 108

30 6

53 24 5 1 2

2 1 2

35 10 14

4 19 12

8 4

7 5

17

25

3 2

2

43 Heat Spontaneous, brush, DL-H 2

3 8

25 Pinch Pinch, Brush Brush 39 Pinch Pinch, Brush Brush 30 Proprioceptive Deep Pinch

111

~

Heat Heat, brush

20 9 12 8 1 2

Excitation

12

5

1 1

3

4 1

2 '

5 10 6' 14 4 1 9

1

9k 5'

4

~

Abbrer,iutions: a-Chl, a-chloralose; Hal, halothane; N,O, nitrous oxide. Laminae in which neurons were recorded and norepinephrine iontophoresed; - indicates that laminae were not described. Neuron Type: HT, high threshold; LT, low threshold; MULTI, multireceptive. n , number of spinal units studied and number in which responses to stimuli were inhibited, excited or unaffected by iontophoretically applied norepinephrine. Electrical stimulation of dorsal roots and cutaneous, muscle and joint afferent nerves. Bradykinin (5-15 f i g ) was injected into the blood supply of the peripheral receptive fields. Noxious heat, usually radiant (45-50°C). The inhibition of heat-evoked unit activity produced by iontophoretically applied norepinephrine was selective for nociceptiveevoked activity; spontaneous and non-noxious-evoked unit activity was unaffected for these same 43 units. ' One unit exhibited biphasic inhibition/excitation. I Six units exhibited biphasic inhibition/excitation. One unit exhibited biphasic inhibition/excitation. I One unit exhibited biphasic inhibition/excitation.

384

et al., 1978). Fleetwood-Walker and colleagues (1985) have established further that the selective inhibition of nociceptive-evoked unit activity produced by the iontophoresis of NE onto spinocervical tract or dorsal column neurons in the cat is mediated by a,-adrenoceptors. The q a d r e n o c eptor agonists, clonidine and metaraminol, mimicked the selective inhibition of nociceptiveevoked unit activity produced by NE; the a l adrenoceptor agonist, phenylephrine, and the p adrenoceptor agonist, isoprenaline, did not. Additionally, the a,-adrenoceptor antagonists, yohimbine and idazoxan, either reversed or reduced the potency of the inhibition of nociceptiveevoked unit activity produced by NE. In the primate, iontophoretically applied NE has been reported to inhibit glutamate-evoked unit activity of dorsal horn neurons specifically characterized as having ascending projections to the thalamus. The inhibition, however, was not selective for nociceptive-specific neurons; both nociceptive- and nonnociceptive-evoked unit activity was inhibited by NE (Willcockson et al., 1984). In contrast, Todd and Millar (1983) reported that iontophoretically applied NE excited approximately 50% of the neurons examined in laminae I and 11, but had no effect on units in laminae 111; no correlation was found between effects on unit activity and neuron modality. Howe and Zieglgansberger (1987) similarly reported that iontophoresed NE inhibited or excited dorsal horn neuronal activity; NE had exclusively inhibitory effects on low threshold neurons in laminae I and 11, and multireceptive neurons in lamina 111. In an in vitro intracellular study in the rat spinal cord slice preparation, North and Yoshimura (1984) reported that NE, applied either by superfusion or pressure ejection, hyperpolarized 80% of the neurons examined in lamina 11. The hyperpolarization was associated with an increase in potassium conductance, and was blocked by phentolamine or yohimbine but not by propranolol or prazosin. However, due to limitations of the in uitro preparation, it was not possible to determine whether the neurons influenced by NE received nociceptive afferent input.

Thus, electrophysiological evidence supports behavioral data which suggests that NE modulates sensory transmission in the spinal cord dorsal horn. The bulk of evidence suggests that iontophoretically applied NE has predominantly inhibitory effects on dorsal horn neuronal activity; however, excitatory effects also have been reported. From the data currently available, it is difficult to conclude that NE has selective inhibitory effects on spinal nociceptive transmission; the physiological characterization of the neurons influenced by NE (i.e., low threshold, multireceptive or high threshold) often is unclear, as is the dorsaI horn laminae in which they were located (see Table 1). The coeruleospinal projection

Transection studies have revealed that spinal cord NE and serotonin content are depleted significantly caudal, but not rostral, to the level of a complete transection of the spinal cord, indicating that the source of spinal cord monoaminergic innervation is organized supraspinally (e.g., Carlsson et al., 1963; Magnusson and Rosengren, 1963). In the rat, pontine NA cell groups have been demonstrated to be the primary source of NA nerve terminals in the spinal cord (Nygren and Olson, 1977; Moore and Bloom, 1979; Bjorklund and Skagerberg, 1982; Westlund et al., 1983). Neurons immunohistochemically labeled with retrogradely transported dopamine-P-hydroxylase (DPH) antiserum from the spinal cord have been localized in the nucleus LC, the nucleus subcoeruleus (SC), the parabrachial nuclei, the Kolliker-Fuse nucleus and the region of the superior olivary nucleus (the A5 cell group) (Westlund et al., 1983). Numerous studies, utilizing a variety of techniques, have demonstrated that a direct coeruleospinal projection exists in the rat, cat and primate (Olson and Fuxe, 1972; Amaral and Sinnamon, 1977; Nygren and Olson, 1977; Commissiong et al., 1978; Ader et al., 1979; Guyenet, 1980; Westlund and Coulter, 1980; Foote et al., 1983; Westlund et al., 1983; Carlton et al., 1985;

385

Loughlin et al., 1986a,b; Fritschy et al., 1987). Westlund et al. (1983) reported that 86% of neurons retrogradely labeled with D P H antibody from the spinal cord in the rat are located in the LC/SC. Bilateral lesions of the LC/SC produce a 30-40% reduction in spinal cord NE content in the rat, and an 80% reduction in spinal cord NE content in the ventral horn of the spinal cord in the cat (see Bjorklund and Skagerberg, 1982 for review). NA coeruleospinal efferents originate primarily from ventral portions of the LC and the SC (e.g., Westlund et aL, 1983 Loughlin et al., 1986a,b); they can be traced the full length of the spinal cord and decussate at all levels of the neuraxis (Nygren and Olson, 1977; Commissiong et al., 1978; Westlund et al., 1983; Carlton et al., 1985; Jones and Yang, 1985). Coeruleospinal fibers descend the spinal cord predominantly in the ipsilateral, ventrolateral quadrant; however, axons and terminals also have been identified which extend into the dorsolateral quadrant of the spinal cord (Nygren and Olson, 1977; Commissiong et al., 1978; Bjorklund and Skagerberg, 1982; Westlund et al., 1983; Jones and Yang, 1985). It should be noted that significant anatomical differences exist between the rat and cat with regard to the organization and spinopetal efferents of the NE-containing dorsolateral pontine nuclei. The rat and cat are the two most commonly used species in pharmacological and electrophysiological studies of descending inhibition. In contrast to the rat, it has been reported that in the cat, although catecholamine-containing neurons in the LC do project to the spinal cord (Jones and Moore, 1974; Kuypers and Maisky, 19751, the primary source of spinal NE-containing nerve terminals is not the LC, but rather the Kolliker-Fuse nucleus. Many cells that do project to the spinal cord from the LC do not contain NE (Stevens et al., 1982, 1985); serotonin-containing cells in the LC have been shown to contribute to the monoaminergic content of the ventral horn of the spinal cord (Lai and Barnes, 1985). These anatomical differences between species have

added confusion to the literature and, as a result, the extrapolation of data obtained in pharmacological, neurochemical and behavioral studies in the rat, to data obtained in electrophysiological studies in the cat, may not necessarily be valid. Termination patterns in the spinal cord High densities of DpH-labeled fibers have been identified in the superficial laminae of the dorsal horn, in the ventral horn around large motoneurons, around the central gray and in the intermediolateral spinal gray matter in the thoracic and sacral spinal cord (e.g., Westlund et aL, 1983). Both a l - and a,-adrenoceptors are present in the spinal gray matter of the rat spinal cord (Seybold and Elde, 1984; Unnerstall et al., 1984). Autoradiographic studies have revealed dense distributions of p-[3H]aminoclonidine binding sites (i.e. , a,) in lamina I1 of the rat spinal cord, around the central canal and in the intermediolateral cell column (see Seybold, 1986 for review). Nociceptive-specific primary afferents also have been demonstrated to terminate primarily in the superficial laminae of the spinal cord dorsal horn (Light and Perl, 1979a,b); thus, NE-containing nerve terminals and NA binding sites are ideally situated to contribute significantly to the modulation of spinal nociceptive processing. A recent study examined the termination patterns of NA coeruleospinal fibers within the rat dorsal horn by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with DPH immunohistochemistry (Fritschy et al., 1987). Contrary to what previous studies have reported utilizing retrograde-tracing techniques (e.g., Jones and Yang, 1983, dense projections of coeruleospinal axons were not observed in the ventral horn and deep laminae of the spinal cord dorsal horn. Rather, dense coeruleospinal projections were observed in the superficial laminae of the dorsal horn, particularly in the substantia gelatinosa ( i e . , lamina 11; however, see Proudfit and Clark, this volume). More than 80% of the PHA-L labeled fibers in

386

J )loo/*

P 3.0

I

... &&:::m.

\ - 25-50

I

MOO/*

P 3.5

Fig. 1. Diagrammatic summaries of stimulation thresholds for inhibition of the tail-flick (TF) reflex drawn on representative coronal brain sections. The thresholds (PA) for inhibition of the TF reflex are indicated: > loo/*, indicates no inhibition of the TF reflex at stimulation intensities of > 100 p A and up to 200 p A or non-antinociceptive effects of stimulation at stimulation intensities ranging between 6.25 and 200 p A . These summary diagrams were constructed from 14 electrode tracks through the pons (v). (From Jones and Gebhart, 1986a, with permission.)

the spinal cord also exhibited positive immunostaining for DPH, indicating that the majority of fibers projecting to the spinal cord from the LC in the rat utilize NE as their neurotransmitter (Fritschy et al., 1987). Modulation of spinal nociceptive transmission by coeruleospinal efferents

As discussed above, NE has been shown to be involved in both antinociception and inhibition of spinal nociceptive transmission. Since the majority of NE-containing fibers and terminations arise from supraspinal sources, the LC/SC, the parabrachial nuclei, the Kolliker-Fuse nucleus and the A5 cell group have all been suggested as

possible sources of the spinal NA nerve terminals involved in the centrifugal modulation of spinal nociceptive transmission. Several lines of evidence suggest that the LC/SC plays a significant role in a functionally important descending inhibitory NA system. In the rat, focal electrical stimulation in the LC produces an antinociception (Segal and Sandberg, 1977; Margalit and Segal, 1979) and increases significantly the spinal content of NE metabolites (Crawley et af.,1979). Recent studies have examined systematically the role of the LC/SC in the modulation of spinal nociceptive reflexes and spinal nociceptive transmission in the rat and cat. Systematic mapping studies have revealed that, in the lightly pentobarbital-anesthetized rat, inhi-

387 Phentolornine

Yohirnbine

Prazosin

Naloxone

Methysergide

100

40

20

0

Fig. 2. Summary of the effects of intrathecally administered phentolamine ( n = 6), yohirnbine ( n = 101, prazosin ( n = 61, naloxone ( n = 4) and methysergide ( n = 6 ) on the stimulation thresholds in the locus coeruleus/subcoeruleus (LC/SC) required to inhibit the TF reflex. Mean % increases (kS.E.M.) in the stimulation threshold following cumulative intrathecal doses of antagonists. *, P 2 0.01 VS. pre-treatment LC/SC stimulation threshold (paired t-test).

bition of the nociceptive T F withdrawal reflex is produced by electrical stimulation throughout a wide region of the dorsolateral pons; however, stimulation sites requiring the lowest intensities of stimulation to inhibit the T F reflex (12.5-25 pA) are in the LC/SC (Fig. 1). S-glutamate microinjections and stimulation strength-duration determinations indicated that inhibition of the T F reflex produced by stimulation in the LC/SC results from the activation of presumably NA cell bodies (Jones and Gebhart, 1986a). The intrathecal administration of pharmacological antagonists (phentolamine, yohimbine, prazosin, naloxone, methysergide, atropine or bicuculline) revealed that only the non-selective a-adrenoceptor antagonist, phentolamine, and the selective a,-adrenoceptor antagonist, yohimbine, significantly increased stimulation thresholds in the LC/SC to inhibit the T F reflex (Fig. 2). A cumulative intrathecal dose of 30 p g of phentolamine produced a mean 83.1 16.3% increase in the inhibitory threshold; a cumulative 30 p g dose of yohimbine produced a mean 93.9 t- 13.2% increase in the stimulation threshold to inhibit the TF reflex (Jones and Gebhart, 1986a). Thus, inhibition of the spinal nociceptive TF reflex produced by electrical stimulation in the LC/SC appears to be mediated by postsynaptic a,-adrenoceptors in the lumbar spinal cord.

*

Similarly, electrical stimulation in the dorsolatera1 pons has been demonstrated to inhibit, significantly, spinal nociceptive transmission. Systematic tracking studies in pentobarbital-anesthetized rats revealed that the site of maximal inhibition of noxious heat-evoked (50°C) dorsal horn neuronal activity produced by 100 p A of electrical stimulation was in the ventral LC and the SC (Fig. 3). Electrical stimulation in either the ipsilateral or contralateral LC/SC was equally effective in inhibiting heat-evoked dorsal horn unit activity (Fig. 3). The inhibition was intensity-, pulse duration-, and frequency-dependent. Inhibition of heat-evoked unit activity to 50% of control was produced at a significantly lower intensity of stimulation using a pulse duration of 400 ps (54.0 k 8.8 FA) compared with stimulation using a pulse duration of 100 ps (82.1 11.2 PA); electrical stimulation at a frequency of 100 Hz resulted in maximal inhibition of heat-evoked dorsal horn neuronal activity (Jones and Gebhart, 1986b). Microinjections of S-glutamate or kainic acid into the LC/SC suggest that the inhibition of dorsal horn neuronal responses to noxious heating of the skin produced by electrical stimulation in the LC/SC is the result of the excitation of cell bodies in the LC/SC, which are presumably NA (Jones and Gebhart, 1986b). Antibodies to DPH have been demonstrated

+

388

A

B

CONTRALATERAL rsrpanrs to skin healing

depth. mm

105

P3 0

20

,so

m

0

d

.

-

b

C

Llyu

o m % control respanse lo skin heating

Fig. 3. Histological reconstructions of electrode tracks (vertical lines) through the ipsilateral and contralateral LC/SC. A. The units’ responses to skin heating are shown as a percentage of control to the right of each track. At each stimulation site ( b ) the effects of 100 p A stimulation on heat-evoked activity was determined. Two example peristimulus time histograms are shown for each track at sites indicated by the matching letters; onset and termination of brain stimulation are indicated by upward arrows and downward arrows, respectively. Beneath the histograms the black bar illustrates the duration of skin heating. In both the ipsilateral and contralateral tracks the site of maximal inhibition by 100 F A stimulation was in the SC where heat-evoked activity was inhibited to 19.7 and 22.8% of control, respectively. B. Three additional tracks lateral to, through, and medial to the LC/SC; at each stimulation site ( b ) the effects of 100 p A stimulation on heat-evoked spinal unit activity was determined. The unit’s responses to skin heating are shown as a percentage of control below each track corresponding to the matching letters (a, b, and c). (From Jones and Gebhart, 1986b, with permission.)

to be selectively incorporated and retrogradely transported by NA nerve terminals (Fillenz et al., 1976; Ziegler et aL, 1976; Westlund et al., 1983). Microinjections of DOH into the lumbar spinal cord into the same region in which dorsal horn neurons were recorded in the above studies resulted in DpH-labeled cells in the LC/SC (Fig. 4). Thus, NA, coeruleospinal terminals are located in the same region of the spinal cord in which dorsal horn unit recordings were made, supporting the supposition that LC/SC stimula-

tion-produced inhibition of noxious-evoked dorsal horn unit activity is an NA-mediated effect. Mic,roinjections of the local anesthetic, lidocaine, and transection techniques to interrupt neuronal transmission have been used to examine the funicular trajectories of the coeruleospinal fibers functionally important in the modulation of spinal nociceptive transmission (Jones and Gebhart, 1987). Microinjections (0.5 pl) of lidocaine to reversibly interrupt neuronal transmission were made into the ipsilateral and/or contralateral

389

ventrolateral quadrants (VLFs) of the cervical spinal cord and the effects on LC/SC stimulation-produced inhibition of heat-evoked dorsal horn neuronal activity were determined; the results are summarized in Figure 5A. Prior to the microinjection of lidocaine, focal electrical stimulation in the LC/SC at a mean intensity of 101.4 & 5.1 pA ( n = 18) inhibited heat-evoked dorsal horn unit activity to 31.5 f 3.0% of control. Following the microinjection of lidocaine into the ipsilateral VLF, the efficacy of the same intensity of LC/SC stimulation to inhibit heat-evoked unit activity was decreased significantly to 66.5 & 4.7% of control. Subsequently blocking the contralat-

Fig. 4. LC/SC neurons containing retrogradely transported dopamine-O-hydroxylase (DPH) antibody from the lumbar spinal cord. A. Low power (4x1 photomicrograph of retrogradely filled DOH-containing neurons in the LC/SC. B. Higher magnification (16 X ) of DOH-labeled neurons in the LC/SC from another animal. Abbreviations: LC/SC, locus coeruleus/subcoeruleus; IV, fourth cerebral ventricle; PB, parabrachial nuclei.

era1 VLF, thereby producing a bilateral blockade of neuronal transmission in the VLFs of the cervical spinal cord, further decreased significantly the efficacy of LC/SC stimulation-produced inhibition to 71.1 f 6.8% of control. The mean duration of effect of lidocaine was 54.9 f 5.5 min, after which time LC/SC stimulation at the same mean intensity again inhibited heat-evoked unit activity to a mean 37.8 k 4.0% of control. Contrariwise, irreversible transections of the dorsolateral quadrants of the cervical spinal cord failed to significantly affect LC/SC stimulationproduced inhibition of heat-evoked dorsal horn neuronal activity (pre-, 40.0 f 2.8% of control; ipsi-, 51.5 f 5.6% of control; bi-, 53.7 f 8.4% of control; n = 9; Fig. 5B). Thus, these results suggest that a significant portion of coeruleospinal fibers mediating LC/SC stimulation-produced inhibition descend to the lumbar spinal cord in the ipsilateral ventrolateral quadrant; at the cervical level, coeruleospinal fibers in the dorsolateral quadrants of the spinal cord are not functionally involved in LC/SC stimulation-produced inhibition of nociceptive transmission. Thus, results reported in electrophysiological experiments confirm those reported in functional studies using the nociceptive TF withdrawal analgesiometric model. The NA coeruleospinal system appears to play a significant role in spinal nociceptive processing. In analogous studies in the cat, dorsal horn neuronal activity evoked by non-noxious and noxious cutaneous stimuli (Hodge et al., 1981; 1983) and by electrical nerve stimulation at intensities sufficient to activate A-6- and C-fibers (Mokha et al., 1985; 1986) similarly has been shown to be inhibited by electrical stimulation in the LC. A recent intracellular analysis suggests that inhibition of spinal nociceptive transmission from the LC involves both pre- and postsynaptic mechanisms (Mokha and Iggo, 1987). Transections of the spinal cord at the lower thoracic level, involving a part or whole of the ipsilateral ventral quadrant, reduced the inhibition produced by stimulation in the LC, suggesting that in the cat,

300

coeruleospinal fibers also descend the spinal cord predominantly in the ipsilateral ventrolateral funiculus (Mokha et al., 1986). In contrast to the rat, Hodge et al. (1983) have reported that in the cat LC stimulation-produced inhibition of dorsal horn neuronal activity is not mediated by spinal NE. The destruction of spinal NA terminals by the selective NA neurotoxin 6-hydroxydopamine (6-OHDA) failed to alter the efficacy of LC stimulation to inhibit dorsal horn neuronal activity evoked by noxious stimuli. How-

LC-VLF LIDO

% control

p

ever, unaccounted for in this study (and many studies involving neurotransmitter depletion) is the possible development of receptor supersensitivity. In the report by Hodge et al. (1983), cats were studied 8-10 days following the first intrathecal dose of 6-OHDA, a time likely well past the development of receptor supersensitivity. Although depIeted significantly, N E was not totally absent; thus, it is possible that released N E acting at supersensitive receptors could produce inhibitory effects on spinal neurons qualitatively

80

re*

*1

pre-

bi-

ipsi

recov

LC-DLF-X

% control

0

f

60

pre-

ipsi-

bi -

Fig. 5. Summary data of the effects of ipsilateral and bilateral' ventrolateral funiculus (VLF) lidocaine microinjections and dorsolateral funiculus transections (DLF-X) on locus coeruleus stimulation (LCS)-produced inhibition of heat-evoked dorsal horn unit activity. In A and B are shown mean dorsal horn unit responses to skin heating (50°C) as a percentage of the control response during LCS before lidocaine or DLF-X (pre-) and after ipsilateral (ipsi-) and bilateral (bi-) lidocaine microinjections into the VLF (A) or DLF-X (B); sites of stimulation in the LC/SC are shown to the right. In A, recov. represents the mean inhibition by LCS following dissipation of the effects of lidocaine. * represents a significant difference compared with pre-VLF lidocaine values. * * represents a significant difference compared with post-ipsilateral VLF lidocaine values. (From Jones and Gebhart, 1987, with permission.)

39 1

and quantitatively indistinguishable from that seen in intact, vehicle-treated animals. Support for this hypothesis has been provided in a study by Janss et al. (1987) in which the intrathecal administration of 6-OHDA in the rat, which depleted spinal NE content to approximately 12% of control, failed to affect the thresholds of stimulation in the LC/SC and medullary lateral reticular nucleus for inhibition of the nociceptive T F withdrawal reflex. Significant pharmacological a,-adrenoceptor supersensitivity developed within 3 days following the intrathecal administration of 6-OHDA, an observation supported by a concomitant significant increase in the number of a,-adrenoceptor binding sites in the lumbar spinal cord. Thus, the development of receptor supersensitivity is an additional factor which must be considered when estimating the contribution of spinal NE to LC/SC stimulation-produced inhibition of nociceptive transmission in both the rat and cat using neurotransmitter depletion techniques.

Physiological conclusions of LC / SC Clearly, an abundance of evidence indicates that endogenous descending inhibitory systems exist which, when activated by electrical stimulation or drugs, can modulate spinal nociceptive transmission. What peripheral stimuli activate these descending systems “naturally,” and whether they play an important physiological role in the modulation of nociceptive transmission, however, is not known. It is hypothesized that ascending nociceptive projection neurons (i.e., spinothalamic, spinoreticular and spinocervical neurons) send collaterals to brainstem nuclei involved in descending inhibition/antinociception; stimulation of peripheral nociceptors generates a positive feedback loop and neuronal activity is increased and maintained in supraspinal brainstem regions, which in turn leads to the enhancement of descending inhibition onto second order dorsal horn interneurons. The existence of a positive feedback loop between the spinal cord and the

medullary NRM has been demonstrated by Cervero and Wolstencroft (1 984). In decerebrate or decerebellate cats, extracellular recordings were made from spinal cord neurons located in or close to lamina VII of the dorsal horn; all neurons examined were excited by electrical stimulation in the NRM and/or adjacent reticular formation, had axonal projections to the NRM and/or reticular formation via spinal cord pathways in the ventrolateral quadrant, and were affected by intense (i.e., noxious) pressure applied to deep tissues of the limbs. Reciprocal excitatory connections were demonstrated between the NRM and neurons in lamina VII, establishing a positive feedback loop. That noxious peripheral stimuli can activate descending inhibitory systems also has been demonstrated. Electrical stimulation of the sciatic nerve in the cat, at intensities sufficient to activate A-6- and C-fibers, markedly increased the release of serotonin and NE in spinal cord superfusates. The increase in monoamine levels in the superfusate was attenuated significantly by a cold-block of the cervical spinal cord, suggesting activation of descending serotonergic and NA systems. The spinal release of serotonin and NE also could be evoked by stimulation of the infraorbital branch of the trigeminal nerve, indicating that small fiber afferent input from the entire body can likely drive activity in spinopetal monoaminergic systems (Tyce and Yaksh, 1981). Although the above-mentioned studies have not elucidated the supraspinal source of NE mediating the release of spinal NE and its antinociceptive effects, the LC/SC likely is involved since anatomical studies have demonstrated it to be the primary source of NE-containing nerve terminals in the lumbar spinal cord.

Acknowledgements Special thanks is extended to G.F. Gebhart for his comments regarding the manuscript. The author’s data reported here were supported by USPHS awards DA02879 and NS19912.

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