Brain Research, 541 (1991) 29-40 Elsevier
29
BRES 16299
The effects of excitotoxin lesions of the lateral hypothalamus on self-stimulation reward James R. Stellar 1, Frank S. Hall 1 and Meg Waraczynski 2'* 1Department of Psychology, Northeastern University, Boston, MA 02215 (U.S.A.) and eDepartment of Psychology, University of Pennsylvania, Philadelphia, PA 19104 (U.S.A.)
(Accepted 28 August 1990) Key words: Self-stimulation; Reward; Excitotoxin; Lateral hypothalamus, Medial forebrain bundle; Demyelination
Unilateral microinjection into rat lateral hypothalamus (LH) of the excitotoxins ibotenic acid (IBO) and N-methyl-D-asparticacid (NMDA) produced a local zone of neuronal death but also produced a zone of demyelination. The size of this demyelination zone was related to excitotoxin dose and was smaller than the zone of neuron killing. In behavioral testing, MFB self-stimulation reward and performance were measured with a rate-frequency curve-shift method before and after IBO or NMDA lesions of the LH. Excitotoxin lesions were made anterior or posterior to the LH electrode so that the zone of neuronal death, but not demyelination, extended to the electrode tip. These lesions produced small, temporary LH stimulation reward deficits, leading to the conclusion that intrinsic LH neurons are not a major substrate of MFB stimulation reward. INTRODUCTION Electrical stimulation of the medial forebrain bundle (MFB), particularly in the lateral hypothalamus (LH), produces a powerful reward effect, the functional neuroanatomy of which has long interested neuroscientists 25' 30,34,41,47 m more precise neuroanatomical question concerns which of the perhaps 50 or so axonal pathways traversing the MFB 28 are directly activated by the stimulating electrode and carry the reward signal. A number of approaches have been taken to answer this question (see refs. 45 and 59 for review). Regional metabolic studies (e.g. ref. 12) indicate that rewarding LH stimulation activates neurons along the MFB from the diagonal band of Broca to the ventral tegmental area (VTA). Psychophysical experiments have specified ranges for the axonal refractory period and conduction velocity of reward-relevant neurons 39'6°, the direction of conduction of at least part of the reward signal 2, and other properties of the directly excited reward-relevant neurons 13'4°. In addition, there appears to be a direct axonal link between sites of self-stimulation in the LH and the VTA 39. Unit recording studies have found neurons in the septal region with conduction properties matching those determined by psychophysical experiments, and which can be considered candidate
sources of axons carrying the reward signal 36"43. Such data have led to a hypothesis that the directly activated substrate is comprised of medium-to-small myelinated axons that descend from somata in the forebrain toward the VTA 14'39'41'47"58. Before accepting any group of cells as candidate sources of the directly stimulated substrate, it is important to investigate the effects of their destruction on MFB stimulation reward. Early lesion work directed towards this end often used behavioral measurement techniques that failed to discriminate between lesion effects on the reward signal, and effects on the subjects' performance abilities (cf. refs. 6, 21, 45, 47, 52). More recent lesion studies have employed measurement techniques such as the rate-frequency curve shift paradigm, which allows for independent assessment of changes in reward and performance ability (see refs. 3, 10 and 49 for discussion of this paradigm and presentation of validational data). Such studies have shown large MFB stimulation reward impairments following posterior MFB electrolytic lesions, and somewhat smaller effects of anterior MFB lesions 46. Transections of the anterior MFB by coronal plane knife-cuts produce little consistent reduction of MFB stimulation reward, except in some cases where the transection was placed in the lateral preoptic area (LPO) or in the rostral LH 1s'55. Several subjects showed as much
* Present address: Department of Psychology,Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Quebec, Canada H3G 1M8. Correspondence: J.R. Stellar, Department of Psychology, 125 NI Building, Northeastern University, 360 Huntington Avenue, Boston, MA 02215, U.S.A.
0006-8993/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division)
30 as a 50% loss of reward efficacy after transection, although some subjects with similarly placed cuts showed little or no permanent decrements. Murray and Shizga126 have shown that electrolytic lesions in the LPO and anterolaterai MFB can reduce LH stimulation reward by 20-30%, and appear to remove reward-relevant axons directly linking the LH and VTA 42. One explanation of reward decrement in these studies is that intrinsic LH neurons, often termed the 'path neurons' of the MFB, may have been damaged, perhaps through destruction of distal dendrites or axon collaterals, local bleeding, etc. These path neurons have been
20x
suggested on the basis of their anatomy and physiology, as well as their proximity to sites of avid self-stimulation, to play some role in MFB stimulation reward 23-25' 29,46,47,51. Their destruction reduces self-stimulation behavior 2°'27'44'53'54, but the measurement techniques, or data analysis, used in these studies cannot determine whether the behavioral changes resulted from reward degradation or loss of performance capacity. This report examines the role of the LH intrinsic neurons in MFB self-stimulation by using excitotoxin injections to kill these neurons. It improves on previous studies in two ways. First, we use the rate-frequency curve-shift paradigm to obtain quantitative estimates of MFB stimulation reward changes following excitotoxin treatment. Second, we take into account the potential demyelinating effects of excitotoxins. Previous studies reported sparing of axonal transport and/or normal transmitter release from axons passing through an excitotoxin lesion (e.g. refs. 4, 16, 17, 31,33, 38, 50), but did not directly examine affected axons. However, our own analysis of excitotoxin lesions 48'56, as well as a similar study of thalamic lesions 22 and electronmicroscopic examination of lesioned tissue 7, have revealed that axons
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29
BRES 16299
The effects of excitotoxin lesions of the lateral hypothalamus on self-stimulation reward James R. Stellar 1, Frank S. Hall 1 and Meg Waraczynski 2'* 1Department of Psychology, Northeastern University, Boston, MA 02215 (U.S.A.) and eDepartment of Psychology, University of Pennsylvania, Philadelphia, PA 19104 (U.S.A.)
(Accepted 28 August 1990) Key words: Self-stimulation; Reward; Excitotoxin; Lateral hypothalamus, Medial forebrain bundle; Demyelination
Unilateral microinjection into rat lateral hypothalamus (LH) of the excitotoxins ibotenic acid (IBO) and N-methyl-D-asparticacid (NMDA) produced a local zone of neuronal death but also produced a zone of demyelination. The size of this demyelination zone was related to excitotoxin dose and was smaller than the zone of neuron killing. In behavioral testing, MFB self-stimulation reward and performance were measured with a rate-frequency curve-shift method before and after IBO or NMDA lesions of the LH. Excitotoxin lesions were made anterior or posterior to the LH electrode so that the zone of neuronal death, but not demyelination, extended to the electrode tip. These lesions produced small, temporary LH stimulation reward deficits, leading to the conclusion that intrinsic LH neurons are not a major substrate of MFB stimulation reward. INTRODUCTION Electrical stimulation of the medial forebrain bundle (MFB), particularly in the lateral hypothalamus (LH), produces a powerful reward effect, the functional neuroanatomy of which has long interested neuroscientists 25' 30,34,41,47 m more precise neuroanatomical question concerns which of the perhaps 50 or so axonal pathways traversing the MFB 28 are directly activated by the stimulating electrode and carry the reward signal. A number of approaches have been taken to answer this question (see refs. 45 and 59 for review). Regional metabolic studies (e.g. ref. 12) indicate that rewarding LH stimulation activates neurons along the MFB from the diagonal band of Broca to the ventral tegmental area (VTA). Psychophysical experiments have specified ranges for the axonal refractory period and conduction velocity of reward-relevant neurons 39'6°, the direction of conduction of at least part of the reward signal 2, and other properties of the directly excited reward-relevant neurons 13'4°. In addition, there appears to be a direct axonal link between sites of self-stimulation in the LH and the VTA 39. Unit recording studies have found neurons in the septal region with conduction properties matching those determined by psychophysical experiments, and which can be considered candidate
sources of axons carrying the reward signal 36"43. Such data have led to a hypothesis that the directly activated substrate is comprised of medium-to-small myelinated axons that descend from somata in the forebrain toward the VTA 14'39'41'47"58. Before accepting any group of cells as candidate sources of the directly stimulated substrate, it is important to investigate the effects of their destruction on MFB stimulation reward. Early lesion work directed towards this end often used behavioral measurement techniques that failed to discriminate between lesion effects on the reward signal, and effects on the subjects' performance abilities (cf. refs. 6, 21, 45, 47, 52). More recent lesion studies have employed measurement techniques such as the rate-frequency curve shift paradigm, which allows for independent assessment of changes in reward and performance ability (see refs. 3, 10 and 49 for discussion of this paradigm and presentation of validational data). Such studies have shown large MFB stimulation reward impairments following posterior MFB electrolytic lesions, and somewhat smaller effects of anterior MFB lesions 46. Transections of the anterior MFB by coronal plane knife-cuts produce little consistent reduction of MFB stimulation reward, except in some cases where the transection was placed in the lateral preoptic area (LPO) or in the rostral LH 1s'55. Several subjects showed as much
* Present address: Department of Psychology,Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Quebec, Canada H3G 1M8. Correspondence: J.R. Stellar, Department of Psychology, 125 NI Building, Northeastern University, 360 Huntington Avenue, Boston, MA 02215, U.S.A.
0006-8993/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division)
30 as a 50% loss of reward efficacy after transection, although some subjects with similarly placed cuts showed little or no permanent decrements. Murray and Shizga126 have shown that electrolytic lesions in the LPO and anterolaterai MFB can reduce LH stimulation reward by 20-30%, and appear to remove reward-relevant axons directly linking the LH and VTA 42. One explanation of reward decrement in these studies is that intrinsic LH neurons, often termed the 'path neurons' of the MFB, may have been damaged, perhaps through destruction of distal dendrites or axon collaterals, local bleeding, etc. These path neurons have been
20x
suggested on the basis of their anatomy and physiology, as well as their proximity to sites of avid self-stimulation, to play some role in MFB stimulation reward 23-25' 29,46,47,51. Their destruction reduces self-stimulation behavior 2°'27'44'53'54, but the measurement techniques, or data analysis, used in these studies cannot determine whether the behavioral changes resulted from reward degradation or loss of performance capacity. This report examines the role of the LH intrinsic neurons in MFB self-stimulation by using excitotoxin injections to kill these neurons. It improves on previous studies in two ways. First, we use the rate-frequency curve-shift paradigm to obtain quantitative estimates of MFB stimulation reward changes following excitotoxin treatment. Second, we take into account the potential demyelinating effects of excitotoxins. Previous studies reported sparing of axonal transport and/or normal transmitter release from axons passing through an excitotoxin lesion (e.g. refs. 4, 16, 17, 31,33, 38, 50), but did not directly examine affected axons. However, our own analysis of excitotoxin lesions 48'56, as well as a similar study of thalamic lesions 22 and electronmicroscopic examination of lesioned tissue 7, have revealed that axons
u •
30 25
LESION DEMYELINATION
o
20 15
uJ
