Sequential changes of cholinergic and dopaminergic receptors in brains after 6-hydroxydopamine lesions of the medial forebrain bundle in rats

Summary. We studied sequential changes in muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors in the brain after 6-hydroxydopamine lesions of the medial forebrain bundle in rats. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks postlesion. [³H]Quinuclidinylbenzilate (QNB), [³H]hemicholinum-3 (HC-3) and [³H]raclopride were used to label muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors, respectively. The degeneration of nigrostriatal pathway produced a transient decrease in [³H]QNB binding in the parietal cortex of both ipislateral and contralateral sides at 2 and 8 weeks postlesion. [³H] QNB binding also showed a mild but insignificant decrease in the ipsilateral striatum throughout the postlesion periods. No significant change was observed in the substantia nigra (SN) of both ipsilateral and contralateral sides throughout the postlesion periods. In contrast, [³H]HC-3 binding showed no significant change in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. However, [³H]HC-3 binding was upregulated in the ipsilateral dorsolateral striatum throughout the postlesion periods. The ventromedial striatum also showed a significant increase in [³H]HC-3 binding at 1 week and 2 weeks postlesion. On the other hand, no significant change in [³H]raclopride binding was found in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. [³H]Raclopride binding showed a conspicuous increase in the ipsilateral striatum (35–52% of the sham-operated values in the lateral part and 39–54% in the medial part) throughout the postlesion periods. In the contralateral side, a mild increase in [³H]raclopride binding was also found in the striatum (10–15% of the sham-operated values in the lateral part and 22% in the medial part) after lesioning. However, a significant decline in [³H]raclopride binding was observed in the ipsilateral SN and ventral tegmental area during the postlesion. The present study indicates that 6-hydroxydopamine injection of medial forebrain bundle in rats can cause functional changes in high-affinity choline uptake site in the striatum, as compared with muscarinic cholinergic receptors. Furthermore, our studies demonstrate an upregulation in dopamine D₂ receptors in the striatum and a decrease in the receptors in the SN and ventral tegmental area after the 6-hydroxydopamine injection. Thus, these findings provide further support for neurodegeneration of the nigrostriatal pathway that occurs in Parkinson's disease. Received April 26, 1999; accepted November 12, 1999     

Long-term behavioral and biochemical effects of 6-hydroxydopamine injections in rat caudate-putamen

Unilateral injections of 6-hydroxydopamine into the rat striatum result in amphetamine-induced circling behavior. This rotational behavior was associated with an almost complete disappearance of desmethylimipramine-insensitive [3H]mazindol binding sites--which represent dopamine uptake sites-in the ipsilateral caudate-putamen (CPu), the substantia nigra pars compacta (SNpc), and in the ventral tegmental area (VTA). There were significant increases in [3H]spiperone-labeled dopamine (DA) D2 receptors in specific subdivisions of the ipsilateral CPu, with the dorsolateral (DL) and ventrolateral (VL) regions showing significant increases in DA D2 receptors. There were nonsignificant increases in the dorsomedial (DM) aspects of the ipsilateral CPu whereas there were no changes in the ventromedial (VM) aspects of that structure. In contrast, there were no significant changes in [3H]SCH 23390-labeled DA D1 receptors in any of the subdivisions of the CPu ipsilateral to the 6-OHDA-induced lesions. These results provide evidence that intrastriatal injections of 6-OHDA result in biochemical changes in rat brain which are almost identical to those observed after 6-OHDA-induced lesions of the substantia nigra. These long-term biochemical effects caused by intrastriatal 6-OHDA injections provide further support for the idea that the nigral DA cell loss observed in the brains of parkinsonian patients could be secondary to retrograde changes due to oxyradicals generated during the metabolism of catecholamines within the caudate-putamen.     

Effect of 6-hydroxydopamine lesions of the medial forebrain bundle on the distribution of cholecystokinin in rat forebrain

The quantitative contribution of ascending dopaminergic fibers of the medial forebrain bundle to the cholecystokinin-like immunoreactivity (CCK-LI) in rat forebrain areas has been studied using unilateral 6-hydroxydopamine lesions of this pathway. The lesions produced depletions of CCK-LI in only two of the 18 areas studied. The results suggest that the terminal areas of the mesolimbic dopamine neurons also contain substantial CCK-LI innervations from other non-dopaminergic neurons.     

Degeneration of the nigral dopamine neurons after 6-hydroxydopamine injection into the rat striatum

6-Hydroxydopamine (6-OHDA) was injected into the rat striatum unilaterally. After 2-4 weeks, a marked decrease in the number of tyrosine hydroxylase-immunoreactive neuronal perikarya and dendrites was observed in the substantia nigra (SN) ipsilateral to the injection. Nissl staining showed a severe cell loss in the same region and electron microscopy revealed neuronal perikarya under degenerating process in the SN. The results showed a retrograde cytotoxic effect of 6-OHDA from the striatal terminals to their dopaminergic neuronal perikarya in the SN, and suggest the possibility that the striatum may be a primary locus in the degeneration process in Parkinson's disease.     

Glutamate receptors in striatum and substantia nigra: Effects of medial forebrain bundle lesions

We examined NMDA-sensitive [³H]glutamate, [³H]AMPA, [³H]kainate and metabotropic-sensitive [³H]glutamate binding sites in neostriatum and substantia nigra pars reticulata (SNr) in rats after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. One week after the lesion, NMDA, AMPA, kainate and metabotropic receptors were decreased in the ipsilateral neostriatum, whereas at three months NMDA receptors were increased while AMPA, kainate and metabotropic receptors were not changed. In the SNr at one week, only AMPA and metabotropic receptors were significantly decreased whereas three months after the lesion NMDA, AMPA and kainate binding sites were decreased. The early decrease of excitatory amino acid receptors in the striatum is likely to reflect degeneration of dopaminergic fibers, suggesting that specific subpopulations of excitatory amino acid binding sites are located on dopaminergic terminals.     

Gamma-aminobutyric acidA and benzodiazepine receptor alterations in the rat brain after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle.

Gamma-aminobutyric acidA (GABA(A)) and benzodiazepine (BZ) receptors and dopamine uptake sites in 6-hydroxydopamine-treated rat brains were studied by receptor autoradiography using [3H]muscimol, [3H]flunitrazepam and [3H]mazindol binding, respectively. The rats were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks post-lesion. Degeneration of the nigrostriatal pathway after 6-hydroxydopamine treatment caused a significant loss of dopamine uptake sites in the ipsilateral striatum and substantia nigra (SN) in the lesioned animals. In the contralateral side, however, dopamine uptake sites showed no significant changes in the brain throughout the experiments. On the other hand, no significant changes in GABA(A) receptors were observed in the brain of both the ipsilateral and contralateral sides during post-lesion. In contrast, BZ receptors were observed significantly increased in the ventromedial part of striatum of the ipsilateral side from 2 to 4 weeks post-lesion. Furthermore, a transient increase in BZ receptors was found in the ipsilateral SN only at 2 weeks post-lesion. In contralateral side, most regions examined showed no significant changes in BZ receptors throughout the experiments except for a transient increase in the SN at 1 week post-lesion. These results demonstrate that 6-hydroxydopamine can cause severe functional damage in dopamine uptake sites in the nigrostriatal pathway. Our results also suggest that the change in BZ receptors is more pronounced than that in GABA(A) receptors in the brain after 6-hydroxydopamine treatment. Furthermore, our findings suggest that the increase in BZ receptors in the brain of 6-hydroxydopamine-treated model may be due to the additional disruption of the nigrostriatal dopamine system. Thus, investigations into possible changes in neurotransmitter receptors other than dopaminergic receptors appear to be important for the elucidation of pathogenesis of Parkinsons disease.     

Supersensitivity to the reinforcing effects of cocaine following 6-hydroxydopamine lesions to the medial prefrontal cortex in rats

The effects of neurotoxic lesions to the medial prefrontal cortex on both the acquisition and maintenance of intravenous cocaine self-administration were examined. In one experiment, acquisition of intravenous cocaine self-administration (0.25, 0.5 or 1.0 mg/kg/infusion) was measured in separate groups of rats 14 days following either a sham or 6-hydroxydopamine lesion to the medial prefrontal cortex. For sham rats, the 1.0 and 0.5 mg/kg dose supported reliable self-administration as indicated by discriminative responding. These rats reliably chose a lever that resulted in the delivery of these doses of cocaine over an inactive lever. Reinforced response rates were reduced when 0.25 mg/kg was the available dose and there was a loss of discriminative responding for some of the rats suggesting that it was close to threshold for self-administration. For rats that sustained a 70% depletion of dopamine in the medial prefrontal cortex, the dose-response curve was an inverse function across the entire dose range tested. In contrast to the data from the control rats, lesioned rats had a high rate of reinforced responses and demonstrated good discrimination for all doses including 0.25 mg/kg/infusion, suggesting a supersensitive response to the initial reward effect of cocaine. Another group of rats was first screened for reliable cocaine self-administration (0.5 mg/kg/infusion) and then subjected to either the prefrontal cortical 6-hydroxydopamine or sham lesion. Dose-response curves for cocaine self-administration were compared 14 days following the infusions. The lesioned rats responded reliably for low doses of cocaine that were unable to maintain responding in sham rats. These data support the hypothesis that the medial prefrontal cortex plays an important role in cocaine self-administration.     

Reduction of insulin binding in the arcuate nucleus of the rat hypothalamus after 6-hydroxydopamine treatment

Insulin receptors are present in the hypothalamus, but the cell types bearing them are unknown. In order to test the hypothesis that some insulin receptors in the hypothalamus are associated with catecholamine terminals, rats were injected with 50 μg or 75 μg doses (intracerebroventricular) of 6-hydroxydopamine (6-OHDA). Control rats received vehicle only. The animals were sacrificed 7 days after injection, and catecholamine and indolamine levels in the hypothalamus were measured by high performance liquid chromatography with electrochemical detection. Localization of specific binding sites for [¹²⁵I]-insulin in the arcuate (ARC), dorsomedial (DMN) and ventromedial (VMN) nuclei were determined by quantitative film autoradiography. Treatment with 6-OHDA resulted in a 70% reduction in hypothalamic norepinephrine content as compared to vehicle-treated controls (P < 0.01). A slight depletion of epinephrine, dopamine and indolamines was also detected. Computerized image analysis of the autoradiograms was used to determine radioactivity bound (DPM/mm²) in each nucleus. Highest binding was in the ARC and DMN, with much lower binding in the VMN. Insulin binding in the ARC of the 6-OHDA-treated group was decreased by 25% compared to controls (P < 0.01). No significant change in insulin binding was observed in the DMN or VMN. The 6-OHDA treatment had no significant effect on weight gain or on plasma insulin levels. The reduction of insulin binding in the ARC after 6-OHDA treatment supports the hypothesis that some insulin binding sites are located on catecholamine terminals in the arcuate nucleus.     

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatment with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.     

Impact of the lesion procedure on the profiles of motor impairment and molecular responsiveness to L-DOPA in the 6-hydroxydopamine mouse model of Parkinson's disease

6-Hydroxydopamine (6-OHDA) lesions are being used in the mouse for basic research on Parkinson's disease and L-DOPA-induced dyskinesia. We set out to compare unilateral lesion models produced by intrastriatal or intramesencephalic injections of a fixed 6-OHDA concentration (3.2 μg/μl) in C57BL/6 mice. In the first experiment, toxin injections were performed either at two striatal coordinates (1 or 2 μl per site, termed "striatum(2 × 1 μl)" and "striatum(2 × 2 μl)" models), in the medial forebrain bundle (MFB), or in the substantia nigra pars compacta (SN) (1 μl per site). All the four lesion models produced significant forelimb use asymmetry, but spontaneous turning asymmetry only occurred in the MFB and striatum(2 × 2 μl) models. After the behavioral studies, the induction of phosphorylated extracellular signal-regulated kinases 1 and 2 (pERK1/2) by acute L-DOPA (30 mg/kg) was used as a marker of post-synaptic supersensitivity. Striatal pERK1/2 expression was sparse in the SN and striatum(2 × 1 μl) groups, but pronounced in the striatum(2 × 2 μl) and MFB-lesioned mice. In further experiments, mice with MFB and striatal(2 × 2 μl) lesions were used to compare behavioral and molecular responses to chronic L-DOPA treatment (12 days at 3 and 6 mg/kg/day). Maximally severe abnormal involuntary movements (AIMs) occurred in all MFB-lesioned mice, whereas only 35% of the mice with striatal lesions developed dyskinesia. Striatal tissue levels of dopamine were significantly lower in the dyskinetic animals (both MFB and striatum(2 × 2 μl) groups) in comparison with the non-dyskinetic ones. Noradrenaline levels were significantly reduced only in MFB lesioned animals and did not differ among the dyskinetic and non-dyskinetic cases with striatal lesions. In all groups, the L-DOPA-induced AIM scores correlated closely with the number of cells immunoreactive for tyrosine hydroxylase or FosB/?FosB in the striatum. In conclusion, among the four lesion procedures examined here, only the MFB and striatum(2 × 2 μl) models yielded a degree of dopamine denervation sufficient to produce spontaneous postural asymmetry and molecular supersensitivity to L-DOPA. Both lesion models are suitable to reproduce L-DOPA-induced dyskinesia, although only MFB lesions yield a pronounced and widespread expression of post-synaptic supersensitivity markers in the striatum.     

The effects of chronic continuous versus intermittent levodopa treatments on striatal and extrastriatal D1 and D2 dopamine receptors and dopamine uptake sites in the 6-hydroxydopamine lesioned rat — an autoradiographic study

The effects of chronic ‘continuous’ infusion and ‘intermittent’ modes of levodopa/carbidopa administration on apomorphine induced circling behaviour, DA uptake sites (labelled with [³H]mazindol) and D₁ and D₂ DA receptor binding (labelled with [³H]SCH 23390 and [³H]sulpiride, respectively) were investigated in rats with unilateral 6-OHDA lesions of the medial forebrain bundle. The circling behaviour in response to apomorphine was greatly enhanced following chronic ‘intermittent’ but not ‘continuous’ levodopa treatments. Following the ‘intermittent’ regime, the lower dose of apomorphine induced a period of intense circling with delayed onset and rapid offset, than in rats given either ‘continuous’ infusion of levodopa or saline. The 6-OHDA lesion itself induced gross depletion of [³H]mazindol binding in all striatal subregions, NAc and OT, but not frontal cortex. [³H]Sulpiride binding in the ventrolateral striatal quadrant was increased on the denervated side and this correlated with the peak contralateral turns in response to 0.5 mg/kg apomorphine challenge. This asymmetry in striatal [³H]sulpiride binding was reduced in both groups of rats receiving levodopa. [³H]sulpiride binding in the NAc and OT and [³H]SCH 23390 binding in the striatum, NAc, OT and SNr were unaffected by DA denervation or either regime of levodopa treatments. ‘Continuous’ infusion and not ‘intermittent’ injections of levodopa reduced [³H]mazindol binding in the striatal subregions and the frontal cortex on both the denervated and intact sides. The potentiation of the behavioural response to apomorphine by chronic ‘intermittent’ levodopa treatment does not correspond with the levodopa induced alterations in striatal or extrastriatal DA receptors. In the same group of animals the narrowing of the duration of response to the lower dose of apomorphine may mimic the fluctuations in response to levodopa, seen clinically in long-term levodopa treated parkinsonian patients.     

Neocortical movement representations are reduced and reorganized following bilateral intrastriatal 6-hydroxydopamine infusion and dopamine type-2 receptor antagonism

The neurophysiologic model of Parkinson's disease predicts nigrostriatal dopamine depletion leads to increased inhibitory basal ganglia output resulting in frontal neocortical hypoactivity. The nature of this hypoactivation is not well understood and modeled predominantly by a unilateral representation. Intracortical microstimulation (ICMS) was used to probe topographic movement representations of the left forelimb motor area 2 weeks following sham, unilateral left hemisphere or bilateral intrastriatal 6-hydroxydopamine (6-OHDA) infusion and under acute dopamine receptor antagonism with haloperidol in non-lesioned rats. 6-OHDA infusions induced a significant loss of substantia nigra pars compacta (SNc) dopamine neurons. Bilateral SNc lesions and haloperidol significantly reduced map area which was preserved in unilateral lesions. All lesion conditions and haloperidol induced significant map reorganization, characterized by increased representation of distal forelimb movements. Results suggest basal ganglia dopamine deficiency can affect the topographic organization of sensorimotor neocortex and lead to significant reduction in the size of motor representations. We conclude that the neurophysiologic model is supported but that bilateral loss of dopamine is required to see a reduction in the size of motor maps.     

Alterations in the dopaminergic receptor system after chronic administration of cocaine

Several studies suggest that one of the most important factors contributing to cocaine dependence is an alteration in the actions of the neurotransmitter dopamine in the central nervous system. In order to understand some of the neuroreceptor consequences of cocaine administration, groups of rats were injected with cocaine (2 daily doses of 15 mg/kg) for 1 to 21 days. Binding of [³H]cocaine, [³H]SCH23390, [³H]raclopride, and [³H]BTCP in striatal and cortical tissue from the treated animals was compared to controls. [³H]Cocaine binding was increased by the drug in the striatum and cortex at days 14 and 21, respectively. The binding of [³H]SCH23390 to D₁ dopamine receptors was significantly increased at day 3 of cocaine exposure. In striatal membranes, [³H]BTCP binding to dopamine uptake sites was significantly increased after day 7, whereas binding in cortical membranes was increased from day 1. [³H]Raclopride binding to D₂ dopamine receptors remained unchanged throughout the study in both cortical and striatal tissues. These results indicate that repeated exposure to cocaine produces an upregulation (possible supersensitivity) in cortical D₁, cocaine, and DA-uptake sites which occurs in a time-dependent manner. These increases are coupled with an upregulation in striatal D₁, cocaine, and DA-uptake sites, without simultaneous changes in D₂ receptors. Thus, cocaine's effects are not uniformly distributed across all brain regions, but rather are focused within areas of the dopamine system. © 1993 Wiley-Liss, Inc.     

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague-Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.     

The in vivo microdialysis recovery of dopamine is altered independently of basal level by 6-hydroxydopamine lesions to the nucleus accumbens

The present study was designed to test the hypothesis that the active neurotransmitter processes of release and uptake affect the in vivo microdialysis recovery of dopamine (DA) in the nucleus accumbens (N ACC) of the rat. The in vivo recovery for DA was established for rats which had received either unilateral infusions of the neurotoxin 6-hydroxydopamine (6-OHDA, 8 μg) or vehicle (0.2 μg ascorbate). In the quantitative dialysis method used (point of no net flux method), DA is added to the perfusate at concentrations above and below the expected extracellular concentration (0, 5, 10 and 20 nM) and DA is measured in the dialysate from the brain to generate a series of points. A linear fit is performed, the slope of which is the in vivo recovery of the dialysis probe. The in vivo recovery of the 6-OHDA group was 30 ± 3% which was significantly lower (P < 0.002) than the in vivo recovery of the control group which was 60 ± 3% (mean ± SEM; n = 6/group). The zero intercept of this regression is the point of no net flux, which is the extracellular concentration of DA independent of the probe sampling characteristics. The extracellular DA concentration for the 6-OHDA group was 7.8 ± 1.1nM, which was not significantly different than the control group which was 6.9 ± 0.7nM. The tissue DOPAC/DA ratios of the 6-OHDA lesioned hemispheres were significantly higher than the contralateral hemispheres of the same animals (0.62 ± 0.1vs.0.27 ± 0.1; P < 0.02) while the DOPAC/DA ratios in the control group were not significantly different (0.24 ± 0.1vs.0.27 ± 0.1). The fractional DA efflux from the terminals in the 6-OHDA group was significantly higher than the fractional DA efflux of the control group (0.52 ± 0.08vs.0.03 ± 0.003; P < 0.0001), indicating that the remaining terminals have increased turnover of DA. Despite the increased turnover, however, the number of remaining release and uptake sites are not sufficient to maintain the high in vivo recovery observed in the control group.     

Spontaneous and graft-induced behavioral recovery after 6-hydroxydopamine lesion of the nucleus accumbens in the rat

In the present study the long-term evolution of behavioral deficits following a local lesion of the dopaminergic innervation of the nucleus accumbens with 6-hydroxydopamine (6-OHDA) was compared in two groups of rats: lesioned animals and animals bearing a dopaminergic implant in the nucleus accumbens. Lesioned animals gradually recovered on various behavioral tests (amphetamine-induced locomotion, exploration, hoarding) and were indistinguishable from the control group on most parameters by 10 months postlesion. The deficits were, however, reinstated by a second intra-accumbens 6-OHDA lesion, a finding which suggests a role for dopaminergic reinnervation in the observed recovery. Conversely, grafted animals still displayed marked deficits even 10 months after grafting, although the lesioned areas were well reinnervated by the graft. These results indicate that the graft, while being unable on its own to compensate for part of the deficits, can nevertheless impair and compete with endogenous processes leading to behavioral recovery following a local lesion.     

Characterization of opioid receptors in rat nucleus accumbens following mesolimbic dopaminergic lesions

The present study investigated the cellular localization of mu, delta and kappa opioid receptors in the rat nucleus accumbens in relation to dopaminergic neurons. Dopaminergic terminals were destroyed by intra-accumbens injections of the neurotoxin 6-hydroxydopamine (6-OHDA). Fourteen days after dopaminergic denervation, receptor binding assays and quantitative in vitro autoradiography with highly selective radioligands demonstrated that the density of mu opioid receptors in the nucleus accumbens was decreased by 30 +/- 6%. There was no change in delta or kappa receptors in the accumbens, a finding which indicates that the loss of mu opioid receptors was specific. A time course study demonstrated that the loss of mu receptors lagged behind the depletion of dopamine by about 5 days. Destruction of intrinsic neuronal cell bodies and dendrites by injection of ibotenic acid into the accumbens resulted in a loss of 36 +/- 3% of mu opioid receptors. Co-injection of 6-OHDA and ibotenic acid decreased mu receptors by 41 +/- 4%, only slightly more than the loss caused by ibotenic acid alone. These results suggest that only a small number of mu opioid receptors in the nucleus accumbens are located on dopaminergic terminals and are consistent with the possibility that the loss of opioid receptors following denervation of dopaminergic fibers in the accumbens is the result of transsynaptic degeneration.     

Development of brain beta-adrenergic receptors after neonatal 6-Hydroxydopamine treatment

We used [125I]-cyanopindolol in vitro autoradiography and neonatal 6-hydroxydopamine treatment to study the development of beta-adrenergic receptor subtypes in rat brain. Brain regions receiving locus coeruleus innervation, such as cerebral cortex and cerebellum, displayed low receptor densities at birth and increased in density rapidly during the second and fourth weeks postnatally. By contrast, regions which receive little innervation from the locus coeruleus, such as substantia nigra, striatum, and globus pallidus, displayed relatively high beta-receptor densities even at birth. The striatum appeared to be an exception to these generalizations. 6-Hydroxydopamine administration was associated with an increase in the densities of beta-receptor subtypes and, unexpectedly, with a change in the proportions of the two subtypes. These data support the view that innervation determines the ontogenetic patterns of some receptors in brain.     

The intrastriatal 6-hydroxydopamine model of hemiparkinsonism: quantitative receptor autoradiographic evidence of correlation between circling behavior and presynaptic as well as postsynatic nigrostriatal markers in the rat

Unilateral injections of 6-hydroxydopamine into the striatum resulted in almost immediate ipsilateral amphetamine (AMPH)- and delayed contralateral apomorphine (APO)-induced circling behavior in rats. APO-induced rotation correlated positively with that caused by AMPH. In these animals, there was al almost complete disappearance of dopamine uptake sites as well as increases in DA D₂ receptors in specific subdivision of the ipsilateral caudate-putamen CPu). Both the rate of AMPH- and APO-induced rotation correlated with the percentage of DA terminal loss in the total aspect and in various quadrants of the striatum. In contrast, AMPH- and APO-induced rotation correlated with the percentage increase in striatal D₂ receptors only in the dorsolateral (DL) aspect of the CPu. These results indicate that both AMPH- and APO-induced rotation can be sued to determine the extent of DA terminal loss in the rat basal ganglia. The positive correlation of circling behavior to only changes in DA D₂ receptors observed in the DL striatal subdivision provides further evidence for the heterogeneity of the basal ganglia. This model of hemiparkinsonism in the rat which uses a distant intrastriatal approach to the destruction of nigral DA cell bodies may be a more appropriate model to study the regenerative properties of the nigrostriatal DA system. This approach could also be used to more specifically localize peptidergic receptors on midbrain dopamine cell bodies     

Autoradiographic distribution of nicotine receptors in rat brain

The autoradiographic visualisation of 90%-specific tritiated nicotine binding to slide-mounted sections of rat brain is reported. Tritiated nicotine bound with high affinity (nanomolar K_d) and was selectively displaced by nicotinic agonists (e.gl-nicotine ACh >d-nicotine). The strikingly discrete distribution pattern obtained deviates from that of alpha-bungarotoxin, and suggests several possible roles for nicotinic transmission in the brain.