Localization of the dopamine uptake complex using [3H]N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) in rat brain

Behavioral responses to injections of muscimol into the subthalamic nucleus: temporal changes after nigrostriatal lesions

Changes in cellular activity in the subthalamic nucleus are a cardinal feature of Parkinson's disease and occur in rodents after lesions of the nigrostriatal pathway, a model of Parkinson's disease. GABA-ergic neurons from the globus pallidus provide a major input to the subthalamic nucleus. Previous electrophysiological studies revealed temporal changes in the activity of pallidal neurons after nigrostriatal lesions in rats. However, little is known about the impact of these changes on GABAergic transmission in the subthalamic nucleus. We have examined the behavioral responses to a local administration of the GABA A agonist muscimol into the subthalamic nucleus. Muscimol (0.01 and 0.1 microg) induced orofacial dyskinesia in normal rats; this response was blunted 2 weeks but enhanced 2 months after a unilateral lesion of the nigrostriatal pathway. The early decrease in the behavioral response occurred at a time when increased expression of mRNA for glutamic acid decarboxylase, the enzyme of GABA synthesis, and burst firing have been reported in the globus pallidus, suggesting an adaptive post-synaptic response to increased GABAergic transmission in the subthalamic nucleus. In contrast, we now show that glutamic acid decarboxylase mRNA is unchanged in the globus pallidus at the later time point, when electrophysiological changes also subside in this region. The increased behavioral response at this later time point may reflect a decreased activity in GABAergic inputs to the subthalamic nucleus. The results show time-dependent changes in behavioral responses to GABA A receptor stimulation in the subthalamic nucleus which may reflect adaptive changes in postsynaptic inhibitory responses after dopaminergic lesions.

Effect of GABA(A) receptor stimulation in the subthalamic nucleus on motor deficits induced by nigrostriatal lesions in the rat

Inhibition of the subthalamic nucleus by lesions or GABAergic agonists improves motor symptoms in monkeys or humans with a loss of nigrostriatal dopaminergic neurons, a characteristic of Parkinson's disease. In rats, nigrostriatal lesions induce deficits in a variety of motor tests that are ameliorated by dopaminergic agonists. However, the validity of these tests to predict the beneficial effects of subthalamic inhibition is not known. We have examined the effects of an intrasubthalamic injection of the GABA(A) receptor agonist muscimol (0.1 microg/0.1 microL) in intact rats and in rats with a unilateral nigrostriatal lesion. Muscimol induced a mild ipsiversive rotation in sham-operated (control) rats and blocked contraversive rotations induced by apomorphine in lesioned rats. In addition, in the cylinder test of limb use asymmetry, muscimol decreased the ipsilateral bias after lesion without inducing any significant effect in sham-operated controls. In the forced-step test, however, 0.1 microg (but not 0.01 microg) of muscimol into the subthalamic nucleus induced a behavioral bias by markedly decreasing the number of adjusting steps of the contralateral limb in control rats, similar to the effect of a nigrostriatal lesion. Neither dose improved performance in this test in rats with lesions, and the higher dose exacerbated the deficit. The data support a beneficial role of stimulating subthalamic GABA(A) receptors for akinesia but also reveal negative behavioral effects of this treatment and suggest that the cylinder and forced-step tests measure different aspects of behavioral deficits after dopaminergic lesions.

Time dependent changes in DA uptake sites, D1 and D2 receptor binding and mRNA after 6-OHDA lesions of the medial forebrain bundle in the rat brain

Quantitative receptor autoradiography and in situ hybridization techniques were used to examine the temporal pattern of changes in dopamine uptake sites, D1 and D2 receptors and their transcripts in the striata of animals lesioned with 6-hydroxydopamine. Animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4, 6, 8, and 16 weeks postlesion. Degeneration of the nigrostriatal pathway induced a significant loss of dopamine uptake sites in the ipsilateral caudate putamen of all lesioned animals. D1 receptor binding was significantly increased in the caudate putamen on the lesioned side from 1 week to 16 weeks postlesion, whereas the expression of D1 receptor mRNA did not show any change during this period. There was a significant upregulation of D2 receptor binding as well as D2 mRNA from 2 weeks to 8 weeks postlesion. However, at 16 weeks postlesion, D2 receptor binding continued to increase, whereas the mRNA appeared to compensate. These studies show that a different regulatory mechanism may exist between these two DA receptor subtypes. D1 receptor changes occur at the post-transcriptional or translational level, whereas D2 alterations occur by both transcriptional and translational processes. These studies also indicate that the postsynaptic supersensitivity observed in D1 receptors may not be accompanied by a corresponding increase in D1 receptor mRNA.

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.

Consequences of an intrastriatal injection of kainic acid on the dopaminergic neuronal and vesicular uptake systems

Intrastriatal kainic acid injection destroys the neurons originating from the striatum, including those on which terminals of the nigro- and meso-striatal dopaminergic neurons project. We have studied at various times the consequences of this lesion on dopamine metabolism and on the neuronal and vesicular transporters. Two and 15 days after kainic acid injection, whereas dopamine turnover was increased and the dopamine content unchanged, there was no modification in the binding of [3H]GBR 12783, a marker of the neuronal uptake complex, but the binding of [3H]dihydrotetrabenazine, a marker of the vesicular transporter, was significantly decreased. At later times (30 and 60 days) when the dopamine turnover was decreased as well as the dopamine content, the binding of [3H]dihydrotetrabenazine was more dramatically decreased (about 30% of controls) than that of [3H]GBR 12783. In addition autoradiography showed an increase in the density of [3H]dihydrotetrabenazine binding sites in the substantia nigra. Thus it appears that the long-term (60 days) repercussions of a kainic acid lesion affect simultaneously the dopamine turnover (which is decreased) and the vesicular transporter whereas the dopamine uptake complex is little affected.

Regional differences in the effect of N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) on extracellular dopamine levels: an in vivo microdialysis study

N-[1-(2-Benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) is a phencyclidine derivative highly selective for the dopamine (DA) uptake complex. Its effect on extracellular DA levels was studied by in vivo microdialysis on freely moving rats. In the striatum, BTCP induced a dose-dependent increase in DA levels, without affecting DA metabolites. In the nucleus accumbens, a lower increase in DA was observed, but with concomitant decreases in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). These experiments show that the effects of BTCP on extracellular DA levels are significantly different on extrapyramidal and mesolimbic dopaminergic terminals.

Effects of hypoxic-ischemic brain damage on dopaminergic markers in the neonatal rat: a regional autoradiographic analysis

Dopamine has been implicated as an endogenous substance that may mediate neuronal death after hypoxic-ischemic insult. Using semiquantitative autoradiography, we studied the effect of perinatal hypoxic-ischemic injury on dopamine binding sites in rat brain. Experimental injury resulted in a substantial decrease in dopamine type-1 (D1) and forskolin (adenylate cyclase) binding sites. In contrast, markers for dopamine type-2 (D2) sites and for dopamine uptake were unaffected in lesioned animals. Changes within dopaminergic pathways were variable, with reduction in binding being encountered mainly in components of the extrapyramidal motor system: caudate-putamen, -61%; globus pallidus, -64%; entopeduncular nucleus, -60%; and substantia nigra, -69%. Furthermore, the topography of D1 receptor loss within the caudate-putamen was not uniform, with the greatest decrement in dorsolateral regions. Reduced D1 versus D2 receptor activation may underlie extrapyramidal movement disorders that appear as a consequence of perinatal hypoxic-ischemic insult.

CNS distribution of D1 receptors: Use of a new specific D1 receptor antagonist, [3H]SCH39166

D1 dopamine receptors have been localized using a radioactive form of a new specific antagonist, [3H]SCH39166. This compound has been shown, in in vitro binding studies, to be highly selective for the D1 receptor subtype; more so than its predecessor, [3H]SCH23390. These ligand binds saturably, reversibly and with high affinity. Use of appropriate conditions produces a high signal to noise binding ratio to D1 receptors in slide-mounted tissue sections. Autoradiographic localization of radiolabeled receptors shows high densities of the D1 receptor subtype in such brain structures as the caudate-putamen, nucleus accumbens, entopeduncular nucleus, and the substantia nigra pars reticulata. A lower density of receptors is found in a few other areas including lamina VI of the cerebral cortex. A distinct paucity of binding was apparent in lamina IV of the cerebral cortex and in the choroid plexus, two areas thought to have D1 receptors. SCH39166 thus represents a superior ligand for obtaining selective labeling of D1 receptors in autoradiographic and binding studies.

Functional recovery of supersensitive dopamine receptors after intrastriatal grafts of fetal substantia nigra

Interruption of the ascending dopamine neurons of the nigrostriatal pathway, by 6-hydroxydopamine (6-OHDA) lesion in rats, produced a significant loss of the dopamine transport complexes labeled with the phencyclidine derivative [3H]BTCP. This loss of dopamine innervation in the striatum was present at least 12 to 14 months after lesioning and was functionally manifested by ipsilateral rotation of the animals in response to amphetamine. In these same animals, in comparison to controls, there was a significant increase in the number (Bmax) of [3H]SCH 23390-labeled D-1 receptors in the striatum (36.7%) and the substantia nigra (35.1%) and a 54.4% increase in the number (Bmax) of [3H]sulpiride-labeled striatal D-2 receptors without an apparent change in affinity (Kd). Ten to twelve months after the transplantation of homologous fetal substantia nigra into the denervated striatum, there was a significant decrease in amphetamine-induced turning behavior. In these animals, there was an ingrowth of dopamine nerve terminals in the striatum as demonstrated by a return of [3H]BTCP binding. Accompanying this reinnervation was the normalization of D-1 and D-2 receptors to control values in the striatum as well as the return of D-1 receptors to prelesion densities in the substantia nigra. In a subgroup of transplanted rats, amphetamine continued to induce ipsilateral turning. In these animals both D-1 and D-2 receptors remained supersensitive. These results support the hypothesis that the functional recovery of transplanted animals is due, in part, to reinnervation of the striatum. In addition, long-term alterations in receptor density may be related to the behavioral deficits that are associated with the 6-OHDA-lesioned rat. Furthermore, dopamine receptor plasticity may play a role in the functional recovery of substantia nigra transplanted animals and graft viability seems to be a prerequisite for behavioral recovery as well as receptor normalization.

Differential interaction of phencyclidine-like drugs with the dopamine uptake complex in vivo

The phencyclidine (PCP) derivative, [3H]N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine ([3H]BTCP), was used to label in vivo the dopamine uptake complex in mouse brain. The striatum accumulated the highest level of total and specific binding. Drugs which bind to the dopamine uptake site inhibited [3H]BTCP binding on an order similar to their in vitro affinities for the high-affinity [3H]BTCP site. Drugs which label selectively other monoamine uptake complexes. PCP, or sigma recognition sites were ineffective at doses up to 40 mg/kg. PCP bound to and dissociated from the dopamine uptake complex very rapidly. N-[1-(2-Thienyl)cyclohexyl]pideridine (TCP) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) had no effect at any time or at any dose. These results imply that the pharmacological effects of PCP are due to its simultaneous interaction with the dopamine uptake complex and the PCP receptor. Conversely, TCP and MK-801, which have the same behavioral properties as PCP, exert their action only through the interaction with the PCP receptor.

Imaging the dopamine uptake site with ex vivo [18F]GBR 13119 binding autoradiography in rat brain

We studied the binding of [18F]GBR 13119 (1-[[(4-[18F]fluorophenyl) (phenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine) to rat brain with autoradiography after intravenous injection. The rank order of binding was dorsal striatum greater than nucleus accumbens = olfactory tubercle greater than substantia nigra = ventral tegmental area greater than other areas. Binding was blocked by prior injection of dopamine uptake blockers but not by injection of dopamine receptor antagonists or drugs that bind to the dialkylpiperazine site. Unilateral 6-hydroxy-dopamine lesions of dopamine neurons caused a marked decrease in striatal and nigral binding on the side of the lesion. We conclude that intravenous injection of [18F]GBR 13119 provides a useful marker of presynaptic dopamine uptake sites.

The effects of the phencyclidine analogs BTCP and TCP on nigrostriatal dopamine neuronal activity

Extracellular single unit recording techniques were used to evaluate the effects of two phencyclidine (PCP) derivatives. N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine (BTCP) and N-[1-(2-thiophenyl)cyclohexyl]piperidine (TCP) on the electrophysiological activity of antidromically identified nigrostriatal dopamine (DA) neurons in chloral hydrate-anesthetized rats. I.v. BTCP produced a dose-dependent decrease in the firing rate of identified nigrostriatal DA neurons whereas TCP elicited a dose-dependent biphasic effect which was characterized by an activation of cell firing at low doses followed by a reversal of the response with larger doses. A hemitransection of the brain anterior to the substantia nigra significantly reduced the inhibitory effect of BTCP while this surgical procedure did not affect the response to TCP. However, iontophoretic application of BTCP induced a current-dependent inhibition of the spontaneous activity of cells while local application of TCP had no effect on the firing rate of these cells. These data indicate that PCP analogs are able to interact with the nigrostriatal DAergic pathway through distinct and opposing mechanisms. The results are discussed in light of recent observations that BTCP is selective for the DA uptake site while TCP is selective for the high affinity PCP binding site.