Specific alterations in local cerebral glucose utilization following striatal lesions

Brain Research, 233 (1982) 157-172

157

Elsevier BiomedicalPress

SPECIFIC ALTERATIONS IN LOCAL CEREBRAL GLUCOSE UTILIZATION FOLLOWING STRIATAL LESIONS

PAUL A. T. KELLY, DAVID I. GRAHAMand JAMES McCULLOCH Wellcome Surgical Institute and (D.LG.) Department of Neuropathology, University of Glasgow. Glasgow ( U.K.)

(Accepted July 16th, 1981) Key words: kainic acid -- 2-deoxyglucose -- striatum -- extrapyramidal system -- bilateral

interaction

SUMMARY Regional cerebral glucose utilization was measured in conscious, lightly restrained rats, using the 2-deoxyglucose autoradiographic technique, l0 days after the unilateral injection of kainic acid into the striatum. The stereotactic infusion of kainic acid (2 #g in 2 / 4 of mock CSF) resulted in lesions localized to the caudate nucleus with no involvement of surrounding brain areas, such as septal nucleus and nucleus accumbens. Only mechanical damage around the needle tract was observed in CSF injected control animals. Local cerebral glucose use was most markedly affected ipsilateral to the infusion site in areas which normally receive input from the caudate nucleus. In globus pallidus and substantia nigra pars reticulata, increases in glucose use of 82 ~ and 74 ~, respectively, were measured when compared with CSF injected controls. However, significant increases were also measured in contralateral pallidus and substantia nigra reticulata (16 ~o and 20 ~o, respectively). Of the brain structures examined, significant unilateral increases from control were observed in ipsilateral habenula (23 ~) and ventrolateral thalamus (13 ~o), and contralateral substantia nigra pars compacta (14~) and sensory-motor cortex (15~). However, the side-to-side difference in response from control was not large. Symmetrical, bilateral increases in glucose use were found in the nucleus accumbens (15~), ventral tegmental area (24 ~), and red nucleus (17 ~o). The only area in which the measured rate of glucose use was decreased was the ipsilateral caudate nucleus. However, these changes were invariably associated with histologically definable tissue damage. Caution must therefore be exercised in the interpretation of this result and that from other areas where damage was apparent. The increases of functional activity, as measured by glucose utilization within certain regions in the absence of cellular damage, provide an insight into the 0006-8993/82/00004)000/$02.75 © ElsevierBiomedicalPress

158 mechanisms by which overt motor behavior returns to normal a short time after the removal of striatal interneurons and efferent perikarya by the neurotoxic action of kainic acid. Of particular interest are the responses observed contralateral to the affected striato-nigral system in view of the proposed functional interaction between the two sides of the brain in the absence of direct neuronal pathways.

INTRODUCTION The neurochemical consequences which result from lesioning perikarya in the corpus striatum have been documented extensively. Intrastriatal injections of kainic acid result in decreases in levels of various biochemical markers of cholinergic and GABAergic systems within the area of injection2,7,15,25,27,a6. In addition, alterations in GABAergic systems are also found in those regions of the CNS known to receive projections from the caudate nucleus, namely the ipsilateral globus pallidus and substantia nigra25,27. Although a wealth of information is available for the neurochemical alterations which occur ipsilateral to the site of kainate-induced striatal lesions, much less attention has been paid to the contralateral hemisphere which, indeed, has been employed in many studies as the control against which the changes were measured 2,7,15,25,27. The nigrostriatal systems in the two cerebral hemispheres appear to function in an interdependent manner to effect symmetry of behavior z~,al. Interaction between the two sides occurs in spite of the apparent paucity of direct neuronal connections between them 21,a°. Moreover, the behavioral effects associated with unilateral intrastriatal injection of kainic acid which have been widely reported (i.e. spontaneous contraversive turning) are manifest only in the initial post-operative period 5. Several days after striatal lesions, when the spontaneous asymmetry in motor activity has been largely resolved, the dopaminergic nigrostriatal system in the contralateral hemisphere appears to have adapted functionally to counteract the unilateral loss of striatal interneurons and efferent fibers 1. However, the neuroanatomical basis for this compensatory adaptation remains obscure. In the present studies, we have employed the 2deoxyglucose (2-DG) technique to monitor local functional activity in an attempt to characterize the local functional alterations which occur chronically following striatal lesions, particularly those present in specific regions of the cerebral hemisphere contralateral to the site of kainic acid injections. MATERIALS AND METHODS The experiments were performed on adult male Sprague-Dawley rats weighing between 350 and 400 g. Food and water were available ad libitum until the start of the experiments. The animals were anesthetized with halothane/nitrous oxide gas mixture and mounted in a Trent-Wells small mammal stereotaxic frame. A midline scalp incision was made to expose the skull. The position of bregma was determined and, using this as a reference point, a small burr hole was drilled at 1.5 A, 2.2 L. After

159 careful removal of the dura, a fine needle was lowered to 5.5 V (modified from ref. 11). Two microliters of mock CSF containing 2 #g of kainic acid and adjusted to pH 7.2 was injected at a constant rate into the caudate nucleus over 5 min. In one animal the kainic acid injection was made slightly more medially, although otherwise at the same level, thus injecting the neurotoxin into the septal nucleus. Mock CSF alone, pH 7.2, was injected into 7 animals which formed the control group. The needle was left in situ for 10 min before being slowly withdrawn. The skull was reconstituted with bone cement, the wound held closed with sutures and local anesthetic applied to the scalp before the animals were left to recover. Four kainic acid-injected rats were taken at 7 days after the lesioning procedure, and cannulae were inserted into one femoral vein and both femoral arteries under light halothane anesthesia. Local anesthetic was applied to the incision sites. The rats were restrained within a loose-fitting plaster cast applied around the lower body before being allowed to recover from the anesthesia. At least 2 h elapsed before the experiments proceeded. Various physiological parameters were measured from these conscious animals; blood gas tensions, arterial blood pressure and heart rate, hematocrit, and plasma glucose levels. Ten days after the intracerebral injections, rats of the control group, together with 7 lesioned animals (including one septal lesion) were prepared as detailed above, and local cerebral glucose utilization was determined using the 2-DG method. A detailed account of the 2-DG quantitative autoradiographic technique has been published previously38, and in the present study, experiments were conducted in the manner described. Briefly, 14C-labeled 2-DG (125 #Ci/kg) was injected i.v. over 30 s, and timed samples of arterial blood were drawn off over the subsequent 45 min. Arterial plasma was separated by centrifugation, and levels of 14C (liquid scintillation analysis) and glucose (glucose oxidase assay) were determined. At 45 min the animals were killed by decapitation, and the brains quickly dissected out and frozen by rapid immersion in 2-methylbutane at --45 °C. Brain sections (20/~m thick) were cut in a cryostat at --22 °C, and 3 in every 10 sections were mounted on glass cover-slips and dried on a hotplate. In selected areas of the brain a fourth section was taken, fixed and stained with cresyl violet for subsequent histological examination. Autoradiographs were prepared from the sections which, together with a set of plastic standards of known 14C concentrations (44-1175 mCi/g), were applied to X-ray film (Kodak SB-5) for 10 days. Optical densities (OD) of the image resulting from the brain sections were measured using a computer-based densitometer (Quantimet 500, Cambridge Instruments) and, by referring these to the OD from the plastic standards, tissue 14C concentrations were determined. Tissue OD measurements were taken from at least 6 sections in which the structures were anatomically definable. Local glucose utilization was calculated using the operational equation for the technique from the plasma history of 14C and glucose levels. Glucose use was quantified for a number of structures both contralateral and ipsilateral to the site of striatal injection. Statistical analysis was performed using the tmethod for grouped comparisons. Comparisons were made between the respective sides of lesioned and control animals.

160 In a parallel study, 5 animals which had been lesioned and 5 which had received intrastriatal CSF were perfusion-fixed at 10 days with formaldehyde/acetic acid/ methanol (1:1:8) and the brains subjected to conventional light microscopy. The cryostat sections from the 2-DG experimental animals allowed a direct comparison to be made between the autoradiograms and the extent of damage. RESULTS

General During the first 48 h after the injection of kainic acid, lesioned animals displayed spontaneous, predominantly contraversive turning, and were both aphagic and adipsic over the same time period. The disruption of feeding behavior together with increased motor activity resulted in the rats suffering a substantial weight loss (up to 50 g in 48 h) accompanied by a marked reduction in plasma glucose levels. By 7 days, weight loss had been almost restored, but plasma glucose concentrations remained depressed. Ten days after the injection of kainic acid the weights of all the animals were greater than the pre-injection values, and plasma glucose levels were similar to those observed in normal, well nourished rats. Animals which had received intrastriatal injections of CSF displayed no overt abnormalities of motor or of feeding behavior at any time over the 10 days. Histology Histological examination of the sections revealed a pattern of damage which was generally restricted to the caudate nucleus (approximately two-thirds of total volume affected) and the overlying cortex through which the needle had passed. However, in two of the animals, kainic acid-associated damage was observed in the ipsilateral hippocampus corresponding to terminal field CA3. No damage was observed in any other area spatially removed from the injection site, or in the contralateral hemisphere. In animals subjected to intrastriatal injections of CSF, damage was restricted to the needle tract. Although the quality of frozen sections precludes the detection of subtle neuropathological changes in cellular morphology, the pattern of damage observed in sections taken from the 2-DG experimental animals closely resembled that found in the parallel histological study. The greatest damage observed was in the caudate nucleus and overlying cortex, but, in 3 of the animals, a limited degree of damage was found in ipsilateral hippocampus. In CSF-injected animals, mechanical damage was evident around the needle tract. Frozen sections from the septal lesioned animal revealed extensive damage in the injected septum and bilaterally in hippocampus, but with minimal involvement of ipsilateral caudate or contralateral septal nuclei. Local cerebral glucose utilization Alterations in local cerebral glucose utilization were measured in structures both ipsi- and contralateral to the striatal lesion site when compared with the corresponding side of CSF-injected controls. The more pronounced changes were immediately

161

B

Fig. 1. Autoradiograms of coronal brain sections at the level of the caudate nucleus, slightly caudal to the point of injection. A: control, CSF injected brain. No asymmetry in OD. B: kainic acid lesioned brain. Marked asymmetry of O D in caudate with reductions on the lesioned side (left side).

A

Fig. 2. Autoradiograms of coronal brain sections at the level of the globus pallidus following striatal injection of kainic acid. A : control, CSF injected brain. No asymmetry in OD. B: kainic acid lesioned brain. Marked asymmetry of OD in globus pallidus with increases ipsilateral to the lesion of caudate nucleus (left side).

163

A

B

Fig. 3. Autoradiograms of coronal brain sections at the level of the substantia nigra following striatal injection of kainic acid. A: control, CSF injected brain. N o asymmetry in OD. B: kainic acid lesioned brain. Marked asymmetry of OD in substantia nigra, pars reticulata, ipsilateral to the lesion of caudate nucleus (left side).

164 TABLE 1

Local cerebral glucose utilization Jbllowing unilateral intrastriatal injections of kainic acid Values are expressed as mean glucose utilization (/~mol/100 g/min) E S.E.M. Sham animals (n 7) had been subjected to intrastriatal injections of 2 t~1 o f C S F 10 days prior to measurement of local cerebral glucose utilization. Lesioned animals had been subjected to intrastriatal injections of 2/~g o f kainic acid 10 days prior to the measurement o f local cerebral glucose utilization.

Structure

Caudate nucleus Globus pallidus Substantia nigra pars reticulata parscompacta Nucleus accumbens Ventral tegmental area Red nucleus Lateral habenula Thalamus mediodorsal nucleus ventrolateral nucleus Subthalamic nucleus Sensorimotor cortex Hippocampus CA3 molecular layer dentategyrus Corpus callosum Internal capsule * ** *** §

Sham

Lesioned

Ipsi-

Contra-

Ipsi-

Contra-

82 ± 5 44 :u I

86 ± 5 45 i 1

67 ± 3*§ 80 ± 4***

92 t_- 5 52 ± 2**

46 56 68 42 61 95

:~: ± i ± :~ i

1 I 2 I I 5

45 57 67 42 62 95

± ± ± ± :~ i

1 1 2 1 I 5

80 62 78 52 71 117

_£ i ~ £ i: +:

5*** 3 2** 2** 3** 5*

54 65 76 52 74 102

± ± i i ± ~:

2** 3* 3* 2** 3** 7

89 68 73 96

± ± :~i

3 2 2 5

89 68 76 93

± i ± £

3 2 3 2

88 77 79 101

~ ± ~,: i

3 2* 3 4

93 76 76 107

± i ± .-~

5 3 3 3**

55 70 58 32 31

± ± i :~: i

1 1 2 l 1

57 71 59 31 30

:~: :~ i ~ ±

2 2 2 1 1

79 73 64 34 31

~ :~ ± £ -~:

7**§ 4 3 2 2

60 72 63 35 32

± ~ ± :L ±

1 2 2 2 2

P < 0.05 for the comparison between the same hemisphere in the two groups. P < 0.01. P < 0.001. See discussion in relation to the accuracy o f these determinations in view o f local structural alterations.

apparent from a visual inspection of the autoradiograms, and were confirmed by quantification. In the ipsilateral caudate nucleus, a large area of decreased optical density (OD) (Fig. 1) corresponded directly to the location of histologically defined damage. However, within this region of generalized depression there were small, punctate areas of increased OD. Large increases in OD were observed in the globus pallidus (GP) and substantia nigra pars reticulata (SNR) ipsilateral to the lesioned caudate (Figs. 2 and 3) which, upon quantification, corresponded to increases in glucose utilization of 82 and 7 4 ~ , respectively (Table I). Increased glucose use was also measured in contralateral GP and SNR, but, although these metabolic alterations were highly significant (Table I), they were quantitatively less marked than those in the same regions of the ipsilateral hemisphere. Of the brain structures examined, significant ipsilateral increases in the absence

165 of structural alterations were observed in only the lateral habenula and ventrolateral nucleus of the thalamus (Table I). However, the increase in glucose use was much greater in habenula (23 ~o) than in thalamus (13 ~o), as was the side-to-side difference in response (A 16 ~ in habenula and A 1 ~ in thalamus). A further unilateral increase in glucose use was measured in specific regions of the dorsal hippocampus (i.e. terminal field CA3) ipsilateral to the lesioned caudate (Table I). However, these foci corresponded closely to areas of damage clearly definable on the stained, frozen sections. Within the substantia nigra pars compacta (SNC) and sensory-motor cortex, glucose use was elevated significantly only contralateral to the lesion (Table I), although once again the side-to-side difference in response was quite small. The change observed in the SNC is in marked contrast to the bilateral, but highly asymmetric response in SNR with which it is intimately related in functional terms. Other statistically significant increases were observed bilaterally in the nucleus accumbens, ventral tegmental area, and red nucleus. The responses in these areas were numerically similar in both hemispheres (Table I). The pattern of response to intraseptal injection of kainic acid involved an increase in OD in the lesioned septal nucleus and also bilaterally in the hippocampal formation (Fig. 4). Within both of these areas, microscopy of the frozen sections revealed marked morphological changes associated with damage. However, around the injection site in ipsilateral caudate and contralateral septal nucleus, where damage was apparently minimal, OD changes were also observed (Fig. 4A). In caudate OD was decreased, but was increased in septum. DISCUSSION

2-Deoxyglucose autoradiography in conjunction with kainic acid The development of the 2-deoxyglucose technique to map function-related glucose use a7 and the introduction of kainic acid to effect selective lesions of neuronal perikarya 6 represent two major advances for neuroscience research in the recent past. The autoradiographic 2-deoxyglucose technique permits, if certain constraints are met, quantitative determination of the rate of glucose utilization in discrete regions of the CNS. The energy requirements of cerebral tissue are met almost exclusively from glucose catabolism in well nourished animals 2~ and, as a consequence, the rate of glucose utilization can provide considerable insight into functional events in discrete brain nuclei. The stereotaxic injection of kainic acid, a neurotoxic analogue of glutamate, results in the selective destruction of nerve cell bodies within the injected region, whilst afferents and fibers of passage are left largely intact 6. Although the selectivity of action of kainic acid constitutes a major advantage over other lesioning techniques, such as electrolytic lesioning, the frequency with which structural damage occurs in areas outwith the injection site~,a5 emphasizes the need for careful histological monitoring whenever this method is employed. The combination of the two experimental approaches, whether with systemic4 or intracerebral administration of kainic acid 28,41, results in spectacular alterations in the appearance of the autoradiographic pattern of deoxyglucose uptake. However,

166

A

B

Fig. 4. Autoradiograms of coronal brain sections at the level of: A, septal nucleus; and B, hippocampal formation following intraseptal injection of kainic acid. A: marked increases in OD in septal nuclei, both ipsilateral (left-hand side) and contralateral. Slight decreases in OD in ipsilateral caudate nucleus (cf. Fig. 1). B: marked bilateral increases in OD within areas of the hippocampal formation.

167 meaningful interpretation of these observations can present considerable difficulties. Strong doubt exists as to whether deoxyglucose uptake into damaged CNS tissue truly represents the rate of glucose phosphorylation, particularly in view of the large, dynamic alterations in the value of the 'lumped constant' (a crucial determinant of deoxyglucose uptake) which have been reported to occur in damaged tissue 16. It is tempting to conclude from the present study that the measured reduction (18 ~) of glucose utilization in the striatum into which kainic acid was injected 10 days previously represents a loss of the contribution made by intraneurons and efferent perikarya to the overall energy generation in the striatum. However, it is doubtful whether the measured rate of glucose phosphorylation can be accurately determined using rate constants and a 'lumped constant' derived originally from normal, intact animals. Even if the measurement was correct, it is unlikely that the rate of glucose utilization in damaged tissue would bear a direct relationship to energy production, as it does in intact CNS tissue, in view of the accumulation of lactate which almost invariably occurs in damaged tissuelL In a recent report 41 widespread, pronounced alterations in deoxyglucose uptake were reported in the acute and sub-acute period following intrastriatal injections of kainic acid. However, profound neuronal necrosis and glial reaction paralleled the disordered uptake of deoxyglucose. Although others have reported neuropathological changes in distant regions following intrastriatal application of kainic acid 6,a5,42, they have rarely been as extensive as those reported in this previous 2-deoxyglucose study where damage involved hippocampus, nucleus accumbens, globus pallidus, amygdala, frontal and entorhinal cortices, etc. In view of the widespread occurrence of structural damage, interpretation of the measured deoxyglucose uptake in terms of glucose utilization is fraught with difficulties. In the present studies, discrete lesions restricted to the caudate nucleus resulted from intrastriatal injections of kainic acid. Damage was absent from adjacent areas such as the septal nuclei, globus pallidus and nucleus accumbens. Within a discrete region of the ipsilateral hippocampus, the dorsal aspect of terminal field CA3, a limited degree of structural damage was evident in 3 of the 6 lesioned animals. As with the caudate nucleus where damage was present, it is uncertain whether the measured alteration in deoxyglucose uptake accurately reflects altered glucose utilization. The limited extent of damage to the hippocampus and the consistency in the pattern of altered glucose use in the extrapyramidal system, irrespective of the presence or absence of hippocampal damage (half of the experimental group in each category), allow a greater credibility to be attached to the measured changes in glucose use found to be significant in these animals. However, the marked differences which have been reported previously in the distribution of neuropathological alterations following intrastriatal injections of kainic acid emphasize the necessity for careful histological verification of the extent of the lesion and involvement of areas outside the injection site. Differences in the extent of the lesion produced are likely to result from technical considerations such as the volume and rate of injection, dose, period during which the needle remains in situ, and the choice of anesthesia. However, unexpected variables --- impurities in the kainic acid itself, for example - may also contribute to the degree of resultant damage.

168 A major difference between the present study and previous investigations into the consequences of local kainic acid injection upon glucose use and neuronal damage lies in the minimal, unilateral changes observed in this series, and the marked bilateral increase in deoxyglucose uptake in the hippocampus observed formerly. In one additional animal we aimed the kainic acid injection at the septal nucleus, which resulted in a discrete area of damage with minimal involvement of either the contralateral septum or the ipsilateral striatum. In this animal, 10 days later, the bilateral pattern of increased deoxyglucose uptake and structural damage observed in hippocampus is similar to those changes reported previously following putative local intrastriatal injections of kainic acid (Fig. 4). Two additional features should be emphasized in relation to the investigation of glucose utilization using stereotaxic lesions within the extrapyramidal system. Whilst glucose utilization constitutes, almost exclusively, the energy generating pathway in cerebral tissues of well-nourished animals, kainic acid lesions of the striatum disrupt feeding behaviora4, resulting, probably, in substrates other than glucose being used to meet the functional demands of brain cells ~s. The aphagia and the metabolic demands of increased motor activity, both associated with intrastriatal kainic acid injectionsa4, resulted in severe weight loss and hypoglycemia for up to 7 days afterwards. Functional activity reflects meaningfully upon glucose utilization only when normal feeding has returned and normal glycemic conditions prevail, generally about 9-10 days after the striatal lesion. In addition to the nutritional problems following kainic acid, the sham intervention (i.e. the injection of mock cerebrospinal fluid into the striatum) was associated with subtle functional alterations in the CNS, and consequently altered glucose utilization. Glucose use was reduced in all primary auditory areas of the CNS (inferior colliculus, medial geniculate body, auditory cortex, etc.), possibly as a direct result of mechanical damage during placement of the ear bars for stereotaxic surgery. Moreover, small (10-15 ~o) reductions in glucose use in sham animals were observed in regions of the CNS not primarily concerned with processing auditory information (e.g. mediodorsal thalamus, subthalamic nucleus and hippocampus) when compared with unoperated animals used in this laboratory. Irrespective of whether this subtle depression in local cerebral glucose utilization is a consequence of the neurosurgical intervention (craniotomy, needle tract damage, etc.), or to the extensive connections auditory areas have with non-auditory regions in the CNS, they emphasize the importance of performing rigorous control experiments, particularly when an investigative tool as powerful as 2deoxyglucose autoradiographic mapping is employed. Function-related alterations following striatal lesions Both the afferent and efferent connections of the caudate nucleus have been characterized in some considerable detail. In the rat, the caudate nucleus receives fibers from the cerebral cortex, the posterior thalamus, the rapM nucleus and pars compacta of the substantia nigra3,2~,24,a°,39. In contrast, the efferent fibers have a restricted distribution projecting to only two ipsilateral structures, the globus pallidus and pars reticulata of the substantia nigra14,zl,2s,3°,33. In the present study, marked

169 increases in glucose utilization were observed in these two areas 10 days after the selective destruction of striatal efferents with kainic acid. Striatal efferents appear to exert an inhibitory influence upon pallidal and nigral neuronesS, 12 and, therefore, the increased glucose use in the two areas is consistent with the effective removal of an inhibitory influence upon metabolic activity within these regions. The derangements in glucose use (and, presumably, functional activity) following striatal lesions are not only persistent, a similar pattern being evident 5 weeks after the lesion, but they are also susceptible to pharmacological manipulation with dopaminergic and GABAergic agonists (unpublished observations). Previous observations in a limited qualitative investigation failed to reveal any increases 7 days after striatal lesions with kainic acid; indeed, the tendency was toward decreases in glucose use3L The reasons for this obvious dichotomy are difficult to ascertain, other than as a result of the difficulties associated with the use of kainic acid and deoxyglucose in conjunction, and the necessity for appropriate controls (see previous section). However, a recent preliminary report, using a qualitative approach to deoxyglucose autoradiography 23, has demonstrated relative increases in the pallidus and pars reticulata at approximately the same time after striatal kainic acid injection. Unlike the increased glucose utilization in the ipsilateral pallidus and pars reticulata following chronic striatal lesions, the neuroanatomical basis for the subtle, significant alterations in glucose utilization in specific areas of the hemisphere contralateral to the lesion site is more difficult to identify. There exists considerable biochemical evidence suggestive of a functional interplay between the two nigrostriatal systems 1,2a,31,32. Behavioral observations on the re-establishment of symmetrical motor activity, after the compulsive turning behavior displayed acutely following striatal lesions 5, suggest adaptive compensatory changes within the contralateral nigrostriatal system. In this context, the increased glucose utilization in the contralateral pars compacta of the substantia nigra is of considerable interest in view of the compensatory contralateral alterations in dopaminergic systems following chronic, unilateral, kainic acid-induced striatal lesions 1. Moreover, the observation of bilateral elevations in the pars reticulata of the substantia nigra, a region implicated in the regulation of activity within the pars compacta, highlights further the extent of the functional adjustment in the two nigrostriatal systems following a unilateral striatal lesion. Although there is no evidence for direct neuronal pathways linking the nigrostriatal systems, a number of complex neuroanatomical interconnections have been described which could provide the necessary circuitry linking the two systems 21,3°. From a knowledge of the known anatomical connections, the ventral nuclear complex of the thalamus and lateral habenular nucleus of the epithalamus constitute two important relays in the conveyance of information from one nigrostriatal system to the other. The ventral thalamus, which has extensive reciprocal connections with the neocortex 9, receives input from the globus pallidus (to ventrolateral nucleus) 29 and from the substantia nigra pars reticulata (to ventromedial nucleus) 1°. The callosal connections of the neocortex and its extensive efferent projections 17 could provide the final pathway via which functional processes in the two

170 nigrostriatal systems could become integrated. The results of the present study, in which glucose use was elevated in the contralateral neocortex and the ventrolateral thalamic nucleus, provide some circumstantial evidence for the involvement of such a route. Alternatively, the lateral habenular nucleus, which projects bilaterally to the substantia nigra pars compacta and receives a major projection from the globus pallidus 19,2°, could be the intermediate relay nucleus. Support for the possible involvement of this pathway is gained from the observation of increased glucose utilization in the ipsilateral portion of the habenula in kainic acid-injected rats. Moreover, the proposed involvement of the lateral habenula in integrating information from striatal and limbic forebrain areas19,2°, 40, together with its connections with the ventral tegmental area 8, may be a route through which the nucleus accumbens is functionally affected in response to unilateral lesioning of striatal efferents. Irrespective of which anatomical connections are ultimately involved, the pattern of alterations in glucose utilization, particularly within the contralateral hemisphere, emphasize the widespread extent of the functional changes which are present when the animal has adapted to the unilateral loss of a major portion of its striatal efferent fibers. The present studies demonstrate that the conjoint application of quantitative measurement of local cerebral glucose utilization with stereotaxic injections of kainic acid can, when both are employed judiciously, be used to map the complex functional alterations associated with specific lesions of neuronal perikarya within the CNS. ACKNOWLEDGEMENTS This study was supported by the Wellcome Trust. We would like to thank the staff of the Welcome Surgical Institute and Department of Neuropathology for their expert assistance. REFERENCES 1 Andersson, L., Schwartz, R. and Fuxe, K., Compensatory bilateral changes in dopamine turnover after striatal kainate lesion, Nature (Lond.), 283 (1980) 94-96. 2 Baring, M. D., Waiters, J. R. and Eng, N., Action of systemic apomorphine on dopamine cell firing after neostriatal kainic acid lesion, Brain Research, 181 (1980) 214-218. 3 Beckstead, R. M,, Domesick, V. B. and Nauta, W. J. H., Efferent connections of the substantia nigra and ventral tegmental area in the rat, Brain Research, 175 (1979) 191-217. 4 Collins, R. C., McLean, M. and Olney, J., Cerebral metabolic response to systemickainic acid: 14C-deoxyglucose studies, Life Sci., 27 (1980) 855-862. 5 Coyle, J. T., Biziere, K., Campachiaro, P., Schwarcz, R. and Zaczek, R., Kainic acid-induced lesion of the striatum as an animal model for Huntington's disease. In P. Krogsgaard-Larsen, J. Scheel-KriJger and J. Kofod (Eds.), GABA Neurotransmitters. Pharmacochemical, Biochemical and Pharmacological Aspects, Munksgaard, Copenhagen, 1979, pp. 419-431. 6 Coyle, J. T., Molliver, M. E. and Kuhar, M. J., In situ injection of kainic acid: a new method for selectively lesioning neuronal cell bodies while sparing axons of passage, J. comp. Neurol., 180 (1978) 301-324. 7 Coyle, J. T. and Schwarcz, R., Lesion of striatal neurones with kainic acid provides a model for Huntington's chorea, Nature (Lond.), 263 (1976) 244-246. 8 Crossman, A. R., Walker, R. J. and Woodruff, G. N., Picrotoxin antagonism of gamma-butyric acid inhibitory responses and synaptic inhibition in the rat substantia nigra, Brit. J. Pharmacol., 49 (1973) 696-698.

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