Quantitative autoradiography of dopamine D2 sites in rat caudate-putamen: localization to intrinsic neurons and not to neocortical afferents

Effect of amygdaloid kindling on [3H]dopamine and [14C]acetylcholine release from rat prefrontal cortex and striatal slices

The involvement of the dopaminergic (DA) systems in the control of limbic kindled seizures is ill defined. The effects of kindling on DA activity may have been overlooked in the past, because of its subtle unilateral occurrence and/or the variance of the endogenous imbalance of DA activity in normal animals. In the present study rats were screened for their endogenous DA imbalance using amphetamine-induced rotational behaviour. Electrical or sham kindling was applied in the hemisphere with the higher endogenous DA activity. Sections of the bilateral prefrontal cortex and dorsal and ventral striatum were dissected either 2 hours or 21 days after the final seizure and the electrically stimulated release of [3H]DA and [14C]acetylcholine (ACh) determined. Release was also measured in the presence of quinpirole or sulpiride to assess the activity of pre- and postsynaptic DA D2-receptors. Long-term effects of kindling consisted of facilitation of ACh release in the ventral striatum contralateral to the kindled amygdala and bilateral depression of DA release in the prefrontal cortex. Kindling therefore produced area specific changes in neurotransmitter systems giving rise to increased pro-convulsive cholinergic activity in the ventral striatum and decreased anti-convulsive dopaminergic activity in the prefrontal cortex.

Adaptive changes of dopamine-D2 receptors in rat brain following ethanol withdrawal: a quantitative autoradiographic investigation

The effect of subchronic treatment with two doses of ethanol (5 and 10 vol% drinking fluid) on the density of dopamine-D2 receptors was investigated at two different phases of withdrawal, namely 24 h and 5 days after the cessation of the ethanol application. The number of dopamine-D2 receptors was affected in regions receiving projections from both the substantia nigra as well as the ventral tegmentum. Twenty-four hours after the replacement of the ethanol solution by water, a dose-dependent decrease of D2 receptors was found in all regions (N. caudatus dorsalis, medialis and ventralis, N. accumbens lateralis and medialis, tuberculum olfactorium) and most of the analyzed planes [interaural 7.7-10.2 according to the atlas of Paxinos and Watson (35)]. At day 5 of withdrawal, the number of dopamine-D2 receptors of the animals treated with 5 vol% ethanol reached the level of water controls in most planes. In contrast, two- to three-fold higher numbers were detected in animals treated with the higher dose. Only in the most caudal parts of the investigated regions, was the number of receptors decreased with the higher dose. The mesocorticolimbic system seems to be less sensitive to the effects of ethanol than the nigrostriatal neurones. The findings of the present study suggest an increased activity of dopaminergic neurons with an adaptive reduction of dopamine-D2 receptors during the subchronic treatment with ethanol during the first day(s) of withdrawal. This phase is followed by a reduced turnover rate for up to 7 days (21). The reduced activity of dopaminergic neurones induces a compensatory increase of the number of receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of NMDA antagonists induces D2 receptor synthesis in rat striatum

Dopamine D2 receptor gene expression was examined in rat striatum after chronic treatment with N-methyl-D-aspartate (NMDA) receptor antagonists (ketamine at 15 mg/kg/day or MK-801 at 0.1, 0.2 and 0.4 mg/kg/day per os, for 50 days). The long-isoform mRNA, as well as the total D2 mRNA expression were induced. No change was noticed in striatal dopamine release or turnover. D2 binding studies carried out in MK-801 chronically treated (0.3 mg/kg/day per os, for 50 days) and control rats revealed an increased receptor density in treated animals without a significant change in receptor affinity. These results suggest that the synthesis of both striatal D2 receptor isoforms is postsynaptically regulated at the transcriptional level, by events triggered by glutamate through the NMDA-type receptor.

NMDA receptors mediate the behavioral effects of amphetamine infused into the nucleus accumbens

The present experiments examined glutamate-dopamine interactions within the nucleus accumbens in rats. It has been hypothesized that dopaminergic nerve terminals exert a modulatory influence on glutamate-mediated signals from corticolimbic areas. In the present studies, the effect of the selective NMDA (n-methyl-d-aspartate) antagonist AP5 (2-amino-5-phosphonopentanoic acid) on amphetamine-mediated behaviors was observed. In two behavioral paradigms, AP5 (0, 0.05, 0.5, 1.0 micrograms bilaterally) was microinjected immediately prior to amphetamine (5 micrograms bilaterally) in the nucleus accumbens. In the first experiment, the influence of AP5 on amphetamine-induced motor activity was examined. AP5 dose-dependently reduced the effectiveness of amphetamine in stimulating motor behavior. AP5 alone, paradoxically, tended to increase motor activity. In the second experiment, the effects of AP5 on amphetamine-potentiated responding (lever pressing) for conditioned reward (CR) were investigated. Normally, when amphetamine is infused into the nucleus accumbens, a marked potentiation of CR responding occurs. Prior infusion of AP5 also attenuated this behavioral effect of amphetamine. The results demonstrate that NMDA receptors within the nucleus accumbens mediate the behavioral consequences of increased dopamine release. They provide additional evidence for the involvement of limbic-striatal connections in the activating and reinforcing effects of psychostimulant drugs.

Effects of chronic catecholamine depletions on muscarinic M1-receptor and its mRNA in rat brain

In order to compare the effects of total catecholamine (CA) or noradrenaline (NA) depletions on cholinergic systems, and the mechanisms of receptor regulation in various brain regions, the regional changes in the levels of acetylcholine (ACh), M1-receptor (M1-R) binding, and M1-R messenger RNA (mRNA) were mainly examined in rats which had received either repeated reserpine treatment or a single injection of the selective noradrenergic neurotoxin N-2-chloroethyl-N-ethyl-2-bromo-benzylamine (DSP-4). The levels of dopamine (DA), its metabolites, NA, binding to both D1 and D2 sites, and the mRNA encoding the D2 receptor were also measured. Administration of reserpine (0.5 mg/kg/day, s.c.) for 2, 7 and 14 days depleted DA and NA in virtually all brain regions, while the short-term treatment increased DA metabolites in the striatum (at 2 days) and basal forebrain (at both 2 and 7 days). Administration of DSP-4 (50 mg/kg, i.p.) resulted in a specific loss of NA in the brain 10 days after the injection. These DSP-4 treated rats showed no change in the levels of ACh or M1-R except for an increase in ACh in the frontal cortex. In contrast, numerous changes in cholinergic indices were seen in the reserpine treated groups, and these changes varied from region to region of brain and with the length of drug treatment. In the striatum, ACh levels were increased in rats treated for 2 or 7 days but were normal after 14 days. M1-Rs were decreased at 14 days. These changes suggest that striatal DA, initially released by reserpine, inhibits the release of ACh from striatal cholinergic interneurons, while prolonged depletion of DA relieves this inhibition, leading to a subsequent down-regulation of M1-Rs. In the frontal cortex, ACh and M1-R levels were all decreased by reserpine treatment for 2 or 7 days, and the M1-Rs remained depressed at 14 days. In the basal forebrain, which contains the cholinergic cells that project to the cortex, DA metabolism was increased by 2 or 7 day reserpine treatment. This increased DAergic activity in the basal forebrain may facilitate cholinergic neurons, causing increased release of ACh in the frontal cortex. This, in turn, may lead to a down-regulation of the M1-Rs in that region. The levels of mRNAs encoding M1-Rs were increased in the striatum and frontal cortex by reserpine treatment, despite the decreases in the M1-Rs themselves.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis

Glutamate and aspartate are the primary neurotransmitters of projections from motor and premotor cortices to the striatum. Release of glutamate may be modulated by dopamine receptors located on corticostriatal terminals. The present study used microdialysis to investigate the dopaminergic modulation of in vivo striatal glutamate and aspartate release in the striatum of awake-behaving rats. Local perfusion with a depolarizing concentration of K+ through a dialysis probe into the rat striatum produced a significant increase in the release of glutamate, aspartate, and taurine. The D2 agonist LY171555 blocked the K(+)-induced release of glutamate and aspartate, but not taurine, in a concentration-dependent manner. The D1 agonist SKF 38393 did not alter K(+)-induced release of glutamate and taurine, but did significantly decrease aspartate release. Neither agonist had any effect on basal amino acid release. The D2 antagonist (-)-sulpiride reversed the inhibitory effects of LY 171555 on K(+)-induced glutamate release. These results provide in vivo evidence for a functional interaction between dopamine, the D2 receptor, and striatal glutamate release.

Neonatal 6-OHDA Lesions differentially affect striatal D1 and D2 receptors

Lesions to the dopamine (DA) system in early postnatal development have different behavioral consequences compared to lesions made in adulthood. Intrastriatal injections of the neurotoxin 6-hydroxydopamine (6-OHDA) on the day of birth (PO) or postnatal day 1 (P1) produce a selective supersensitivity to D1 receptor agonists and a subsensitivity to D1 antagonists (Neal and Joyce, 1991a). In this paper, we describe the long-term effects of early DA loss on DA receptor regulation. Pups received bilateral intrastriatal injections of the neurotoxin 6-OHDA (4 micrograms per striatum) on PO or P1. Adult rats were killed at 90 days of age and the brains were processed for quantitative autoradiography (QAR) or tyrosine hydroxylase (TH) immunocytochemistry. Cohorts were tested for the behavioral responses to the selective D1 receptor agonist SKF38393 (10 mg/kg). Neonatally lesioned rats exhibited increases in abnormal perioral movements in response to D1 receptor stimulation. There was a heterogenous and patchy loss (40-50%) of [3H]mazindol binding to high-affinity DA uptake sites (a marker of DA terminal density) and a similar loss of TH-like immunoreactivity within the striata of the neonatally lesioned rats. There was also a reduction in the number of mu-opioid receptor patches (labelled with [3H]naloxone), a marker for the striatal patch compartment, and a similar patchy loss of D1 binding sites (labeled with [3H]SCH23390). The binding of [3H]spiroperidol to D2 sites was not altered. This is in contrast to the changes observed following adult 6-OHDA lesions, wherein there is a significant increase in the number of D2 binding sites (Joyce, 1991a,b). The results are discussed with respect to the behavioral consequences of neonatal lesions and the differences between neonatal and adult lesions.

Localization of dopamine D2 receptor protein in rat brain using polyclonal antibody

The precise distribution of the dopamine type D2 receptor has been mapped for the first time in rat brain using an antibody to D2 receptor protein. Polyclonal antisera were collected from rabbits inoculated with an undecapeptide identical to residues 24-34 of the D2 protein sequence. Rat brain slices, 40 microns in thickness, were incubated with either primary antiserum, the antiserum plus free peptide antigen, or pre-immune serum. Antibody binding was visualized by peroxidase-antiperoxidase (PAP) reaction followed by light microscopy. PAP complex bound moderately-to-densely throughout the medial forebrain bundle, and was seen in more discrete regions in the midbrain, consistent with the binding of D2 radioligands. There were some unexpected results, namely in the cerebral cortex and nucleus accumbens, there were unexpectedly steep gradients in binding density, decreasing caudally; no binding was detected in the hippocampus or the substantia nigra pars reticulata. In all positive-staining regions examined, the antibody was highly localized to neuronal cell bodies, except in the frontal cortex where antibody was also evident on basilar dendrites. These data confirm that the polyclonal antibody recognized dopamine D2 receptor protein throughout the rat brain, and suggest that the D2 receptor is distributed more abundantly on somata than on cellular processes.

Chronic neuroleptic treatment: D2 dopamine receptor supersensitivity and striatal glutamatergic transmission

We studied the in vitro electrical activity of rat neostriatal neurons following chronic neuroleptic treatment. In haloperidol-treated rats, unlike naive animals, activation of neostriatal D2 dopamine receptors induced a potent presynaptic inhibition of glutamate-mediated excitatory synaptic potentials. Haloperidol treatment did not affect the intrinsic membrane properties of the neostriatal neurons. Pre- and postsynaptic physiological responses to direct and indirect gamma-aminobutyric acid (GABA)-ergic and cholinergic agonists were not affected by chronic haloperidol treatment. These findings suggest that movement disorders induced by chronic neuroleptic treatment may result, at least in part, from a hypersensitivity of presynaptic D2 dopamine receptors regulating the release of glutamate.

Effect of raclopride on dopamine D2 receptor mRNA expression in rat brain

Prolonged treatment with dopamine D2 receptor antagonists is known to elevate the density of dopamine D2 receptor binding sites in caudate-putamen and nucleus accumbens in rat and human brain. In this study we used the dopamine D2 receptor antagonist raclopride (3 mumol/kg, s.c.) to determine if a single injection or daily administration of this drug for up to 18 days changed the expression of dopamine D2 receptor mRNA in rat caudate-putamen and accumbens as measured by in situ hybridization. A single injection of raclopride did not significantly change the numerical density of dopamine D2 receptor mRNA-expressing neurons in any of the regions examined. A daily administration of raclopride for 18 days resulted in a 31% increase in the number of cells expressing detectable amounts of dopamine D2 receptor mRNA in dorsolateral caudate-putamen and in a 20% increase in the area of silver grains over individual hybridization-positive neurons in this brain region measured on emulsion-dipped slides. The region-specific increase in the D2 receptor mRNA level in dorsolateral caudate-putamen was confirmed by measurement of the hybridization signal on X-ray film autoradiograms. The levels of D2 receptor mRNA remained unchanged in medial caudate-putamen and accumbens after 18 days' treatment. The region-selective increase in dopamine D2 receptor mRNA expression in dorsolateral caudate-putamen indicates a differential regulation of dopamine D2 receptor mRNA expression in a subpopulation of caudate-putamen neurons by this neuroleptic. We suggest that the increase in dopamine D2 receptor density in caudate-putamen known to follow prolonged dopamine D2 receptor blockade to some extent is regulated at the level of gene expression.

Does impairment of energy metabolism result in excitotoxic neuronal death in neurodegenerative illnesses?

The etiology of nerve cell death in neuronal degenerative disease is unknown, but it has been hypothesized that excitotoxic mechanisms may play a role. Such mechanisms may play a role in diseases such as Huntington's disease, Parkinson's disease, amyotropic lateral sclerosis, and Alzheimer's disease. In these illnesses, the slowly evolving neuronal death is unlikely to be due to a sudden release of glutamate, such as occurs in ischemia. One possibility, however, is that a defect in mitochondrial energy metabolism could secondarily lead to slow excitotoxic neuronal death, by making neurons more vulnerable to endogenous glutamate. With reduced oxidative metabolism and partial cell membrane depolarization, voltage-dependent N-methyl-D-aspartate (NMDA) receptor ion channels would be more easily activated. In addition, several other processes involved in buffering intracellular calcium may be impaired. Recent studies in experimental animals showed that mitochondrial toxins can result in a pattern of neuronal degeneration closely resembling that seen in Huntington's disease, which can be blocked with NMDA antagonists. NMDA antagonists also block neuronal degeneration induced by 1-methyl-4-phenylpyridium, which has been implicated in experimental models of Parkinson's disease. The delayed onset of neurodegenerative illnesses could be related to the progressive impairment of mitochondrial oxidative phosphorylation, which accompanies normal aging. If defective mitochondrial energy metabolism plays a role in cell death in neurodegenerative disorders, potential therapeutic strategies would be to use excitatory amino acid antagonists or agents to bypass bioenergetic defects.

Regional differences in the regulation of acetylcholine release upon D2 dopamine and N-methyl-D-aspartate receptor activation in rat nucleus accumbens and neostriatum

The effect of D2 dopamine receptor activation on either the electrically, or N-methyl-D-aspartate induced release of radiolabeled acetylcholine (ACh) was investigated in different areas of the nucleus accumbens and the neostriatum of rats, by using a superfusion technique. Sequential slices of 100 microns were chopped along either a rostrocaudal, mediolateral or dorsoventral axis. In every slice the effect of a supramaximal concentration of the selective D2 receptor agonist quinpirole on the release of ACh was measured. In the entire neostriatum the release of ACh was reduced by approximately 70% in the presence of quinpirole. By contrast, in the nucleus accumbens, a gradual decrease in the inhibitory effect of quinpirole on the release of ACh was observed along both the rostral-to-caudal and the lateral-to-medial axes. Whereas in the rostrolateral part a 50% inhibition could be observed, in the caudomedial part no significant inhibition could be detected. Also the N-methyl-D-aspartate induced release of ACh was smaller in the caudomedial part as compared to the rostrolateral part of the nucleus accumbens. It is concluded that the nucleus accumbens is a very heterogeneous structure with respect to the regulation of the release of ACh by D2 dopamine and N-methyl-D-aspartate receptor activation.

Effect of unilateral perinatal hypoxic-ischemic brain injury in the rat on dopamine D1 and D2 receptors and uptake sites: a quantitative autoradiographic study

The effect of a unilateral perinatal hypoxic-ischemic brain injury on dopamine D1 and D2 receptors and uptake sites was investigated in rats by using in vitro quantitative binding autoradiography, 2-3 weeks after the insult. We observed significant decreases in the Bmax and KD for [3H]SCH 23390-labeled D1 and in the Bmax for [3H]spiperone-labeled D2 receptors in the lesioned caudate-putamen in rats with moderate brain injury (visible loss in hemispheric volume ipsilateral to the injury) compared with the nonlesioned contralateral caudate-putamen or with control rats. Changes in [3H]SCH 23390 and [3H]spiperone binding predominated in the dorsolateral part of the lesioned caudate-putamen. Pronounced reduction in [3H]SCH 23390 binding was also observed in the substantia nigra pars reticulata on the side of the lesion. In contrast, we did not observe any significant change in Bmax or KD for [3H]mazindol-labeled dopamine uptake sites. Similarly, no significant changes in the levels of dopamine or its metabolites were found on the side of the lesion. The observed reductions in striatal dopamine D1 and D2 receptors are a reflection of striatal cell loss induced by the hypoxic-ischemic injury. The absence of changes in [3H]mazindol binding or dopamine levels in the lesioned caudate-putamen indicates that the dopaminergic presynaptic structures are preserved.

Heterogeneous distribution of dopamine D2 receptor mRNA in the rat striatum: a quantitative analysis with in situ hybridization histochemistry

Dopamine D2 receptor mRNAs have recently been cloned and their gross distribution in the central nervous system described. Quantitative in situ hybridization histochemistry with a cRNA probe complementary to the mRNAs encoding approximately 70% of the third intracellular loop of the rat D2 receptor was performed on sections of rat brain to determine whether differences previously observed in the density of ligand binding sites in subregions of the striatum were related to differences in mRNA levels. Film autoradiographic analysis demonstrated 30% more hybridization signal in the lateral compared to the medial caudate-putamen, a distribution parallel to that of binding of ligands specific for the D2 receptor. Inspection at the cellular level using emulsion autoradiography also indicated a differential distribution of the D2 receptor mRNA. Fewer positively labelled cells, as well as fewer silver grains per cell, were seen in the medial compared to the lateral half of the striatum. This suggests that the gradient seen in autoradiographic studies of the distribution of D2 receptors is related both to regional differences in D2 mRNA levels and to the density of cells expressing the receptor. In addition, the distribution of cells expressing D2 receptor mRNA in the extrastriosomal matrix was compared to that in striosomes identified by the presence of a high density of 3H-naloxone binding sites. Labelled cells were mainly found in the matrix (3H-naloxone binding-poor) but were also seen in striosomes (3H-naloxone binding-rich). The results suggest that differences in levels of D2 binding sites in subregions of the striatum are related to differences in the level of expression of this receptor in intrinsic striatal neurons, suggesting differential regulation of dopamine D2 receptor gene expression in topographically distinct striatal neurons.

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. I. Effects of intranigral or intracerebroventricular 6-hydroxydopamine lesions of the mesostriatal dopamine system

Quantitative autoradiography was utilized to examine the response of the dopamine (DA) and muscarinic cholinergic system within the striatum to lesions of the mesostriatal DA system following intranigral 6-hydroxydopamine (6-OHDA) injections. In addition, the response of DA system was examined in the striatum of animals treated with low, medium, or high doses of 6-OHDA made intracerebroventricularly (icv). Three weeks following removal of the mesostriatal DA fibers with intranigral 6-OHDA, there was an almost complete depletion of DA and [3H]mazindol binding throughout the striatum. The resulting increase in D2 receptors labeled with [3H]spiroperidol (27%) was most evident in the lateral striatum and topographically correlated with an increase in choline uptake sites labeled with [3H]hemicholinium-3 (20%). There was a smaller but significant decrease in D1 receptors labeled with [3H]SCH 23390 (15-18%) that was not topographically related to changes in [3H]spiroperidol or [3H]hemicholinium-3 binding. All doses of icv 6-OHDA produced a significant loss of DA and of [3H]mazindol binding as compared to vehicle injections that was more pronounced in the medial than in the lateral striatum. No increase in D1 receptors was observed with any dose of 6-OHDA and greater than 90% loss of DA and [3H]mazindol resulted in an increase in D2 receptors in the lateral striatum and a reduction in D1 receptors in the dorsal striatum. These data are consistent with the evidence that there is independent regulation of the two subtypes of the DA receptor. Moreover, the distribution and regulation of the subtypes of the muscarinic receptor were independent. Muscarinic M2 receptors ([3H]N-methylscopolamine in presence of excess pirenzepine) showed a lateral to medial gradient (highest laterally) that was related to the pattern of choline uptake sites and D2 receptors. Loss of DA resulted in a reduction in M2 receptors (24-30%) that was correlated with the increase in choline uptake sites. In contrast, M1 ([3H]pirenzepine) receptors showed a reverse gradient from the M2 receptor and a smaller reduction following loss of DA.

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. II. Effects of 6-hydroxydopamine or colchicine microinjections into the VTA or reserpine treatment

In the previous paper it was demonstrated that striatal dopamine (DA) D1 and D2 receptor subtypes and muscarinic M1 and M2 receptor subtypes show differing responses to lesions of the mesostriatal DA system. To examine this differential regulation further rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) or colchicine into the ventral tegmental area (VTA), or treated chronically with reserpine or saline. Two weeks later the animals were tested for their behavioral response to a subthreshold dose of apomorphine and 24 h later their brains were removed and processed for quantitative autoradiography or for analysis of DA levels by high-performance liquid chromatography. The 6-OHDA-lesioned animals showed a supersensitive rotational response to apomorphine. The loss of DA, loss of DA uptake sites, regulation of DA D1 and D2 receptors and regulation of the muscarinic cholinergic system was similar to the previous paper. Injection of colchicine in the VTA resulted in incomplete loss of striatal DA (50%), [3H]mazindol binding (50%), and no behavioral supersensitivity to apomorphine. There was a small loss of presynaptically located D2 receptors (13%). Similar to the 6-OHDA lesions there was a loss of D1 (12%) and M1 receptors. Reserpine treatment produced an 86% decrease in DA levels, an enhanced stereotyped responsiveness to apomorphine, and an increase of both D2 (28%) and D1 receptors (26%). There was a loss of muscarinic M1 but not M2 receptors. Thus removal of DA terminals or blockade of transport of proteins in the mesostriatal axons can lead to a reduction in D1 receptor density in the striatum. In contrast, loss of DA without removal of DA terminals leads to a significant up-regulation of the D1 receptor. D2 receptors show increases following removal of DA or of DA terminals. Alteration in the muscarinic cholinergic system following damage to the mesostriatal DA system is a complex response not mimicked by either reserpine or colchicine treatment.

Antiparkinsonian dopamine agonists: a review of the pharmacokinetics and neuropharmacology in animals and humans

With the intention of compensating for the deficit of endogenous dopamine (DA) in the basal ganglia of Parkinsonian patients by substitution with agents which directly stimulate central DA receptors, synthetic DA agonists have been introduced almost 20 years ago for the symptomatic treatment of Parkinson's disease. The original expectation that DA agonists would be able to completely restore extrapyramidal motor function in Parkinsonian patients has turned out as too mechanistic and simplicative. However, undoubtedly DA agonists have improved therapeutic possibilities in Parkinson's disease. Thus, clinical evidence from controlled chronic studies in patients indicates that the therapeutic results following the early application of DA agonists in combination with L-DOPA on a long-term base are superior to the respective monotherapy. However, none of the DA agonists currently employed for antiparkinsonian treatment i.e. apomorphine and the ergoline derivatives bromocriptine, lisuride and pergolide, is optimal with respect to pharmacokinetic properties (poor oral bioavailability with considerable intra- and interindividual variation) or pharmacological profiles (low selectivity for DA receptors in case of the ergot agonists). The pathophysiology underlying Parkinson's disease which turned out more complex than initially expected might provide another explanation for the limited therapeutic potential of DA agonists. Therefore, apart from summarizing the pharmacokinetics, biotransformation, neuropharmacology and neurobiochemistry of the DA agonists employed clinically, the present article also reviews physiological aspects of (a) central dopaminergic neurotransmission including the topographical distribution of DA receptor subtypes and their functional significance, (b) the intracellular signal processing in striatal output neurons and (c) the intraneuronal mechanisms which integrate the various neurotransmitter signals converging on the striatal output neuron to a demand-adjusted effector cell response via the cross-talk between the different second messenger systems. Based on these considerations, potential pharmacological approaches for the development of improved antiparkinsonian drugs are outlined. There is a therapeutic demand for more selective and better bioavailable DA agonists. In particular, selective D-1 receptor agonists are highly desirable to provide a more specific probe than SKF 38 393 for clarifying the current controversy on the disparate findings in nonprimate species and monkeys or Parkinsonian patients, respectively, regarding the functional significance of D-1 receptors for the antiparkinsonian action of DA agonists or L-DOPA. The therapeutic importance of D-2 receptor activation is generally accepted; whether DA agonists combining a balanced affinity to both D-1 and D-2 receptors within one molecule (to some extent a property of apomorphine) might be superior to subtype-specific DA agonists remains to be tested clinically.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal localization and modulation of the D2 dopamine receptor mRNA in brain of normal mice and mice lesioned with 6-hydroxydopamine

A novel oligonucleotide probe was designed, characterized and utilized to study the distribution and modulation of the mRNA encoding the D2 dopamine receptor in the brain of the mouse. Using in situ hybridization histochemistry, the highest levels of the D2 receptor mRNA were found in regions of the brain containing the cell bodies and the terminal projection fields of the nigrostriatal, mesolimbic and mesocortical dopaminergic systems. Particularly high levels of the D2 receptor mRNA were found in substantia nigra pars compacta, ventral tegmental area, caudate-putamen and olfactory tubercle. This distribution generally paralleled that of the D2 dopamine receptor. Some areas, not usually associated with dopaminergic systems, also contained significant levels of the D2 receptor mRNA signal. These areas included the hippocampus, certain thalamic nuclei, the inferior colliculus and the spinal trigeminal nucleus of the medulla and spinal cord. Lesioning the corpus striatum with 6-hydroxydopamine had little effect on the level of the D2 receptor mRNA in the striatum but greatly reduced the hybridization signal in the substantia nigra pars compacta and ventral tegmental area. Similarly, lesioning the substantia nigra, nearly abolished the signal in the pars compacta but failed to substantially alter the D2 receptor mRNA signal in the striatum. These results suggest that the D2 receptor mRNA in the substantia nigra pars compacta was localized largely to dopaminergic cell bodies, the terminal projections of which lie in the striatum and codes for D2 autoreceptors and that the D2 receptor mRNA of the striatum is in non-dopaminergic cell bodies that are intrinsic to the striatum and probably codes for post-synaptic D2 receptors. Further, the evidence that lesions of striatum and substantia nigra induced with 6-hydroxydopamine greatly reduced the D2 receptor mRNA signal in the substantia nigra, without concomitantly increasing the D2 receptor mRNA in the striatum, suggests that the increase in dopamine receptor binding in the striatum that is ipsilateral to the lesion with 6-hydroxydopamine and the enhanced behavioral sensitivity to dopaminergic agonists, cannot be accounted for solely by an increase in D2 receptor mRNA.

Effect of unilateral perinatal hypoxic-ischemic brain injury on striatal dopamine uptake sites and D1 and D2 receptors in adult rats

The effect of unilateral perinatal hypoxic-ischemic brain injury on striatal dopamine uptake sites and on D1 and D2 receptors was investigated in rat by using in vitro quantitative receptor binding autoradiography, 9-11 weeks after the insult. Saturation experiments revealed a significant 20% decrease in maximal binding capacity (Bmax) for [3H]spiperone-labeled D2 receptors on the side of the lesion in comparison to the non-lesioned contralateral side or to either side of control animals. There was no significant change in [3H]mazindol-labeled dopamine uptake sites or in [3H]SCH 23390-labeled D1 receptor characteristics (Bmax and Kd) on the lesioned side. We conclude that the decrease in D2 receptor binding previously observed in immature animals is persistent, whereas the decrease in D1 binding is not.

Dopaminergic modulation of excitotoxicity in rat striatum: evidence from nigrostriatal lesions

Considerable evidence indicates that dopamine may, under certain circumstances, play a role in the mediation of central nervous system tissue damage. Furthermore, recent studies suggest a synergistic role between the neurotoxic effects of excitatory amino acids and dopamine. To address this issue, rats received a unilateral injection of 6-hydroxydopamine or vehicle into the medial forebrain bundle. After recovery (18 days), both groups of animals received an ibotenic acid injection of the ipsilateral striatum. Seven days later the brains were removed and the size of the striatal lesion was assessed histologically and by means of receptor autoradiography. Regional analysis of profound D1 receptor loss was determined using [3H]SCH 23390, and extent of astrocytic proliferation was examined using autoradiography with the peripheral benzodiazepine receptor ligand [3H]R05-4864. Prior interruption of the nigrostriatal pathway (resulting in dopaminergic denervation of the ipsilateral striatum) partially protected this latter structure from subsequent injection of ibotenic acid (the extent of the lesion was reduced by 28%, P less than .05). The findings indicate that endogenous dopamine release may modulate (and intensify) the excitotoxic effects of ibotenic acid.

Ontogeny of dopamine D1 and D2 receptor subtypes in rat basal ganglia: a quantitative autoradiographic study

The ontogeny of D1 and D2 dopamine (DA) receptors in rat basal ganglia was examined by quantitative autoradiography using the iodinated ligands [125I]SCH 23982 and [125I]iodobenzamide [( 125I]IBZM), respectively. Temporal and spatial differences in the development of the receptor subtypes were observed. Scatchard transformation of saturation isotherms conducted at postnatal day 10 (P10) and P60, showed that there was no age-related change in the affinity of [125I]SCH 23982 binding to D1 receptors (Kd = 2.6 nM) but there was a significant increase in the Bmax (771 compared to 2032 fmol/mg protein, P = 0.002). A statistically significant difference in Kd was noted between ages P10 and P60 for [125I]IBZM labelling of D2 receptors (0.62 vs 1.00 nM, respectively, P less than 0.01). A significant increase in the Bmax (211 and 721 fmol/mg protein, P less than 0.01) was also observed. D1 receptors were visible as distinct patches at P1. The highest density was found in the ventrolateral caudate-putamen (CPu). By P5 the patches were found in all subregions of the CPu and nucleus accumbens. Between P7 and P10 the binding became distinctly less patchy due to a marked increase in the density of D1 receptors in non-patch (matrix) regions. Adult levels of receptor were seen by P30. The concentration of DA (measured by HPLC) and binding of [3H]mazindol to DA uptake sites in whole striatum showed similar and nonlinear increases with age. The age-related change in the topography of binding sites for [3H]mazindol was similar to that of D1 receptors at the same ages. Both D2 receptors and [3H]hemicholinium-3 (HC-3) binding to high affinity transport sites for choline developed initially in the dorso-lateral CPu. Their topography was largely overlapping but distinct from that of the D1 receptor. D2 receptors were not consistently observed until P3 in the CPu, and zones of enriched binding were aligned with zones of low density for D1 receptors. The density of D2 receptors reached adult levels by P30. The differential development of the DA receptors was also evident in the substantia nigra (SN) and globus pallidus (GP). D1 receptors were found in SN prior to the appearance of D2 receptors and throughout development the density was greater in pars reticulata than in pars compacta, whereas the density of D2 receptors was higher in the pars compacta. At all ages the density of D1 receptors was greater than the density of D2 receptors in the GP and reached adult levels before reaching it in the CPu or SN.(ABSTRACT TRUNCATED AT 400 WORDS)

Terminal excitability of the corticostriatal pathway. I. Regulation by dopamine receptor stimulation

Glutamatergic cortical and dopaminergic nigral afferents converge onto neurons of the neostriatum forming synapses in close proximity. Studies, mainly using pharmacological methods, suggest presynaptic interactions between these afferents. The influence of dopaminergic transmission on the cortical terminal fields in the striatum was assessed electrophysiologically using the terminal excitability method. Antidromic action potentials recorded from neurons in the prefrontal cortex were elicited by bipolar electrical stimulation (250 microns wire, 0.5 mm tip separation) of the cortical terminal field in the contralateral dorsomedial neostriatum. Threshold excitability was defined as the minimum current sufficient to elicit 95-100% antidromic response on non-collision trials. Under control conditions, the mean threshold current was 1.7 +/- 0.2 mA. Drugs were applied in a volume of 312 nl delivered over 5 min to the striatal stimulation site. Following local striatal administration of amphetamine (10 microM) or electrical stimulation of the nigrostriatal pathway (1-2 pulses, 1.5 mA/0.5 ms/1 Hz) an increase in striatal stimulating current was required in order to reinstate threshold levels of antidromic response. This decrease in the excitability of corticostriatal afferents could be reversed by local infusion of haloperidol (1 microM) or L-sulpiride (10 nM) and did not occur following depletion of dopamine stores with alpha-methylparatyrosine and reserpine. The possible participation of postsynaptic dopamine receptor stimulation was ruled out as these effects were still seen in animals with kainic acid induced lesions of the striatum. In addition, terminal excitability was not modified by the muscarinic agonist carbachol (10 microM). Striatal administration of apomorphine (10 microM) decreased terminal excitability similar to amphetamine. The specific D-2 agonist, quinpirole (10-20 microM) did not affect excitability. These results indicate that manipulations which have been shown to increase the release of endogenous dopamine decrease the excitability of prefrontal corticostriatal afferents by stimulation of presynaptic dopamine receptors which are insensitive to low doses of quinpirole but sensitive to L-sulpiride and apomorphine. The mechanisms underlying dopamine-induced changes in terminal excitability are likely to be similar to those which have been shown to alter conductance at postsynaptic sites.

D2 dopamine receptor gene expression in the rat striatum during ontogeny: an in situ hybridization study

D2 dopamine receptor (D2R) gene expression in the rat striatum was studied by in situ hybridization throughout the pre- and the postnatal period from gestational day 12 to postnatal day 8. D2R mRNA was detected with 35S-labelled oligonucleotide probes, one that hybridized equally to the two isoforms of the D2R mRNA (D2(415) and D2(444)) and the other that hybridized specifically to the large isoform (D2(444)). D2R mRNA was first detected in the striatal primordium at day 14 of gestation with the probe that recognizes indifferently the two isoforms and with the probe specific for the D2(444) mRNA. At day 16, D2R mRNA was present in the lateral part of the striatum and in the germinal ventricular zone lining the lateral ventricle. At day 18, D2R mRNA was found in neurons of the caudate-putamen, the nucleus accumbens, the olfactory tubercle and the subependymal zone lining the lateral ventricle. The microautoradiographic analysis demonstrated that the labelled cells have a neuroblastic and immature aspect before birth. After birth the topography and aspect of labelled cells was similar to the one observed in the adult animals. D2R mRNA was present in neurons of the caudate-putamen, the nucleus accumbens and the olfactory tubercle. In the caudate-putamen there was a latero-medial gradient of labelling. From postnatal day 2 onward the D2R gene was expressed in two striatal cell types, small neurons probably enkephalinergic, and large-sized neurons with prominent cytoplasm, most probably cholinergic.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease in mRNA levels but not in the density of D2 dopamine receptors in rat striatum after transient forebrain ischemia

D2 dopamine receptor mRNA was analyzed by in situ hybridization histochemistry in rat striatum 7 days after transient forebrain ischemia. A patchy disappearance of the D2 receptor mRNA was observed in the dorsolateral striatum. In the same area, a disappearance of D1 binding sites occurred in the absence of significant changes in D2 receptor density. These results suggest that, although D2 receptors seem to be apparently unaffected after forebrain ischemia, a long-lasting impairment of their neosynthesis may be present in striatal D2 dopaminoceptive neurons.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

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.

Changes in striatal mu and delta opioid receptors after transient forebrain ischemia: a quantitative autoradiographic study

Transient forebrain ischemia induces specific changes in several neurochemical markers in the dorsolateral striatum. In the present paper, the density and distribution of mu and delta opioid receptors were analyzed in rat striatum 7 days after 30 min forebrain ischemia using the 4-vessel occlusion model. A marked (about 70%) decrease in the density of both opioid receptor subtypes was found in the dorsolateral striatum overlapping the areas of histological damage and of D1 dopamine receptor disappearance. Moreover, the density of delta opioid receptors and of the diffuse mu opioid receptors was also affected (30% decrease) in the ventromedial striatum, an area which is substantially spared by the ischemic lesion. In contrast, the striatal patches of mu opioid receptors were not affected in the ventro-medial striatum and were preserved to a large extent in the area of lesion, although their area and receptor density resulted markedly reduced. The impairment of both opioid receptor subtypes suggests that opiate systems, like dopaminergic systems, are involved in the neurochemical changes observed in the striatum after transient forebrain ischemia.

Quantitative image analysis with densitometry for immunohistochemistry and autoradiography of receptor binding sites--methodological considerations

Major technical progress in the development of computer-based image analysis has made possible the entry of autoradiography and immunohistochemistry into a new era where quantification by densitometry has become easily accessible. Autoradiography could become quantitative and displayed adequate reproducibility with the help of emulsion-coated films and the use of scales of standards of known radioactivity exposed and analyzed in parallel to the tissue sections. Immunohistochemistry after revelation by a color-based enzymatic technique can also become quantitative, providing that standardization of the crucial steps of the procedure and calibration through a parallel treatment of a scale of antigen standards can be ensured. Such an approach is described here in the rat with reference to tyrosine hydroxylase (TH), the main synthesizing enzyme for catecholamines, and with dopamine (DA) itself, a catecholaminergic neurotransmitter. The different parts of the procedure, which can influence the results, such as the fixation of the animals by perfusion and the evaluation of the fluctuations via the calibration curve, are discussed in detail. Biological validation of the proposed procedure is described by reference to experiments already well documented biochemically, such as the induction effect of reserpine on TH in the rat locus coeruleus and the depleting effect of alpha-methyltyrosine (AMPT), a well-known blocker of TH activity, on rat striatal DA content. Finally the importance of restricting the measurements to the (pseudo)linear portion of the calibration curve is illustrated by the autoradiographic identification of the differential intrastriatal repartition of the dopaminergic D1 and D2 receptor sites, particularly the dual patch-matrix compartments.

Retrograde degeneration of nigrostriatal neurons induced by intrastriatal 6-hydroxydopamine injection in rats

Quantitative receptor autoradiography was used to assess the effects of unilateral intrastriatal injections of 6-hydroxydopamine (6-OHDA) on the distribution of D1 and D2 dopamine (DA) receptors and of DA uptake sites in the mesostriatal pathway. [3H]Mazindol-labeled DA uptake sites were reduced both in the striatum (-97%) and in the substantia nigra pars compacta (SNpc) (-88%) on the injected side. There were also significant decreases of dopamine uptake sites in the nucleus accumbens (NAc) (-73%) and in the ventral tegmental area (VTA) (-70%). Changes in [3H]mazindol binding were also found within the contralateral VTA (-30%) and SNpc (-13%) but not in the contralateral-striatum. [3H]SCH23390-labeled D1 receptors were significantly reduced in the dorsomedial (-18%) and ventromedial (-14%) aspects of the striatum ipsilateral to the side of the lesions. In contrast, the concentration of [3H]spiperone-labeled D2 receptors was not altered. There were also significant decreases in D1 (-18%) and of D2 (-27%) receptors in the SNpc and of D1 (-10%) in the SN pars reticulata (SNpr). These results suggest that oxyradical-induced damage in striatal DA terminals could lead to retrograde changes in the SNpc. In addition, the data indicate that unilateral striatal damage can result in bilateral changes in the SNpc, thus confirming the interdependence of the two nigrostriatal pathways in rats.

Autoradiographic localization of dopamine D1 and D2 receptors in rat nucleus accumbens: resistance to differential rearing conditions

The radioligands [3H]SCH 23390 and [3H]spiroperidol were used to label dopamine D1 and D2 receptors, respectively, in rat brain slices. Striatal sections were incubated in one of various concentrations of radioligand in the presence or absence of (+)-butaclamol and the resulting labeling was determined by liquid scintillation spectrometry. Scatchard analyses of the data revealed KD values of 1.18 nM for D1 receptors and 0.33 nM for D2 receptors. Tissue sections taken from the entire rostrocaudal extent of the nucleus accumbens, as well as other brain regions, were then processed for autoradiographic analysis of D1 and D2 receptors using a radioligand concentration equal to 1.5 X KD. After apposing the slices to 3H-sensitive film, topographical differences among brain areas were analysed using a quantitative densitometry system which determined the absolute amount of ligand binding relative to calibrated optical density standards. The nucleus accumbens exhibited a rostral-to-caudal density gradient for both D1 and D2 receptors. For D1 receptors, the density was similar across most of the nucleus accumbens, although the most caudal portion examined had a lower density than rostral portions. In contrast, the density of D2 receptors exhibited a more gradual gradient across the entire rostrocaudal extent of the nucleus accumbens. There was no significant rostrocaudal density gradient of either D1 or D2 receptors in either the olfactory tubercle or caudate-putamen in the same tissue sections. A lateral-to-medial gradient of D2 receptors was also present in the nucleus accumbens. That is, while there was no difference in the density of D1 receptors between the lateral core and medial shell subdivisions, the shell had a lower density of D2 receptors than did the core. The density of D1 and D2 receptors in the mesolimbic and nigrostriatal systems was compared in groups of animals raised from 30 to 60 days of age in an impoverished condition, a group-caged condition or an enriched condition. While the brain weight of enriched condition animals was higher than impoverished condition animals, there were no significant differences in the density of D1 or D2 receptors among the different groups. Apparently, the densities of D1 and D2 receptors in the brain are resistant to differential rearing conditions.

Quinolinic acid lesion of nucleus accumbens reduces D1 but not D2 dopamine receptors: an autoradiographic study

Information concerning the cellular localization of dopamine receptor subtypes in the nucleus accumbens (NAcc) was obtained using receptor autoradiographic analysis. Unilateral, stereotaxic injection of the axon-sparing neurotoxin, quinolinic acid, into the NAcc resulted in a prominent loss of dopamine D1 receptors (as labeled by [3H]SCH 23390). Contrarily, no appreciable decrement in D2 receptors (labeled by [3H]raclopride) could be identified within the same region of the NAcc. The findings support the view that accumbens D1 receptors are located postsynaptically on neurons or their processes, while D2 receptors within this nucleus are primarily located on afferent terminals.

Decrease of behavioral and biochemical denervation supersensitivity of rat striatum by nigral transplants

The effect of fetal mesencephalic transplants on dopamine receptor supersensitivity has been studied behaviorally and biochemically in rats with a unilateral lesion of the nigrostriatal pathway. Female rats were lesioned with 6-hydroxydopamine in the left substantia nigra. At least one month later they were tested with apomorphine (0.25 mg/kg, s.c.), amphetamine (5 mg/kg, s.c.), LY 171555 (D2 agonist) (0.5 mg/kg, i.p.) and CY 208243 (D1 agonist) (0.5 mg/kg, s.c.). A suspension containing approximately 1.5 x 10(6) cells from the ventral mesencephalon of rat embryos was distributed in three sites in a triangular fashion in the center of the denervated striatum. Six months later, grafted dopamine neurons reinnervated the medial part of the dorsal striatum, increased the dopamine level and reversed the rotational asymmetry evoked by amphetamine. Apomorphine given four months post-transplant still elicited contraversive circling but the number of turns was reduced. Circling evoked six months post-transplant by CY 208243 or LY 171555 was significantly less in grafted rats than in lesioned non-grafted rats. The density of dopaminergic receptors in the striatum of grafted and lesioned rats was examined by autoradiography by means of in vitro binding with [3H]SCH 23390 for D1 receptors and [3H] spiperone for D2 receptors. The results show that intrastriatal nigral transplants decrease the supersensitivity of the D2 receptors and to a lesser extent of the D1 receptors. Normalization of D2 receptors may explain the decrease of behavioral supersensitivity following administration of apomorphine and D2 agonist in grafted rats. D1 receptors were less affected by the lesion and also less normalized than D2 receptors by the transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneous distribution of the limbic system-associated membrane protein in the caudate nucleus and substantia nigra of the cat

The limbic system-associated membrane protein is a glycoprotein selectively associated, in the adult, with dendrites and cell bodies of neurons of the limbic system and related brain regions. In the present study, the distribution of the limbic system-associated membrane protein was studied by immunohistochemistry in the caudate nucleus and substantia nigra of the cat to determine how its expression relates to the compartmentalization of these areas. In all areas of the caudate nucleus, the pattern of limbic system-associated membrane protein immunoreactivity was highly heterogeneous, labeling zones that were in register with areas expressing neurochemical markers that classically identify striosomes. The extrastriosomal matrix exhibited low levels of staining. The results show that the limbic system-associated membrane protein is expressed by neurons within the target areas (striosomes) of subsets of limbic afferents (originating mainly from the basolateral nucleus of the amygdala and the prefrontal cortex), whereas regions of the caudate nucleus (extrastriosomal matrix) receiving inputs from other subdivisions of the limbic system, such as the cingulate cortex and the ventral tegmental area, contain relatively low levels of limbic system-associated membrane protein immunoreactivity. Thus the expression of this antigen may reflect the targeting of specific groups of limbic afferents to regions that are intimately associated with distinct components of the limbic system. The presence of limbic system-associated membrane protein in neurons of the substantia nigra pars compacta does not appear to be related to the presence or absence of the protein in their striatal target areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant effect of striatal dopamine D2 receptor stimulation: dependence on cortical circuits?

In the pilocarpine model of epilepsy, dopamine can either inhibit (via D2 receptors) or facilitate (via D1 receptors) the spread of limbic motor seizures. The anticonvulsant action of D2 receptor activation has been localized to the anterior striatum, but disappears if excessive damage is caused to the overlying cerebral cortex. This study examines the possibility that the corticostriatal projection is involved in the anticonvulsant response to striatal D2 receptor stimulation, by comparing the seizure-protecting efficacy of intrastriatal trans-(+)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-(3,4-g)quinol ine hydrochloride (LY 171555) in control rats, and in rats bearing discrete bilateral kainic acid lesions of the cerebral cortex. The results show that neurotoxin injection induces a punctate lesion of the primary motor area of the cortex in each hemisphere, with no injury to the underlying caudate-putamen, or to more distant structures such as the hippocampus. The lesion, however, was sufficient to abolish the protective effect of intrastriatal LY 171555 against pilocarpine challenge. To explain these findings, an interplay between nigrostriatal dopaminergic and corticostriatal glutamatergic neurons is proposed, in which the anticonvulsant tendency of the excitatory amino acid is accentuated by dopamine, probably by acting on D2 receptors which facilitate the release of glutamate from axon terminals.

Ischemic damage in the striatum of adult gerbils: relative sparing of somatostatinergic and cholinergic interneurons contrasts with loss of efferent neurons

The pattern of ischemia-induced cell death was examined with histochemical methods in the striatum of adult gerbils 4 and 7 days after transient forebrain ischemia. The results showed a massive loss of immunoreactivity to enkephalin and tachykinins, peptides present in striatal efferent neurons. In contrast, neurons expressing acetylcholinesterase activity, or choline acetyltransferase immunoreactivity, as well as neurons immunoreactive for somatostatin, were relatively preserved in areas of severe neuronal loss. The selective vulnerability of subpopulations of striatal neurons to transient ischemia in the adult is similar to that observed in the neonate and after local injections of agonists of N-methyl-D-aspartate receptors, but not of agonists of other glutamate receptor subtypes. It also presents striking similarities to the pattern of neuronal death observed in Huntington's disease. The results further support a role for overstimulation of a subtype of excitatory amino acid receptor in ischemia-induced cell death and show that the selective sparing of subpopulations of striatal interneurons after ischemic injury is not related to immaturity of these neurons but also occurs in the adult.

Selective localization of striatal D1 receptors to striatonigral neurons

A new technique for producing anatomically selective lesions within the brain was used to investigate the cellular localization of the D1 and D2 receptor. The cytotoxic lectin, volkensin, is taken up by nerve terminals and retrogradely transported, killing those neurons projecting to the site of injection. Comparison of D1 and D2 binding following a unilateral volkensin injection into the substantia nigra has demonstrated that striatal D1 binding sites are selectively localized to striatonigral projection neurons.

Sex differences in regulatory changes following quinolinic acid-induced striatal lesions

The effects of sex differences and hormonal factors on the weight loss following intrastriatal injections of quinolinic acid (QA) were examined. Male, female or ovariectomized female rats were weighed each day for 30 days following bilateral injections of 150 nmol of QA or vehicle into the striatum. Although all rats injected with QA exhibited an equivalent initial loss of body weight, female lesioned rats rapidly regained weight and did not differ from controls after the first postsurgical day. On the other hand, male lesioned and female ovariectomized lesioned rats exhibited a significant but transient loss of body weight. By 19 days postsurgery all rats had recovered and exhibited comparable body weights. Histological analysis did not reveal any sex-related differences in the extent of striatal pathology following QA. These results suggest that sex and hormonal variables such as estrogen levels play an important role in the regulatory changes following excitotoxic-induced striatal damage.

D2 dopamine receptor gene expression by cholinergic neurons in the rat striatum

In situ hybridization with D2 receptor probe and immunohistochemistry with choline acetyltransferase (ChAT) antibody performed on adjacent sections demonstrate dopamine D2 receptor gene expression in cholinergic neurons of the rat caudate-putamen and nucleus accumbens. Eighty per cent of cholinergic neurons in the striatum contain detectable D2 receptor mRNA. The other neurons without detectable D2 mRNA do not display specific localization or aspect in the caudate-putamen and nucleus accumbens as compared to the other cholinergic neurons. The absence of detectable D2 mRNA in certain cholinergic neurons can be due to the limited sensitivity of the procedure that would not detect low mRNA levels, or alternatively can reflect the existence of two cholinergic cell populations in the striatum, one of which would not express the D2 receptor gene. The other forebrain cholinergic neurons do not contain D2 mRNA.

In the rat medial nucleus accumbens, hippocampal and catecholaminergic terminals converge on spiny neurons and are in apposition to each other

The nucleus accumbens septi (Acb) represents an interface between limbic and motor systems and a site for modulation of these integrative functions by ascending catecholaminergic, principally dopaminergic, axons. This modulatory regulation is most likely attributed to pre- or postsynaptic associations between limbic telencephalic and brainstem afferents. In the present investigation, we examined the ultrastructure and synaptic associations of hippocampal afferents, as well as their relation to catecholaminergic terminals, in the medial Acb of adult rats. Hippocampal afferents were identified by anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injected in the ventral subiculum, and by anterograde degeneration seen 2-3 days following lesion of the fimbria. Specific comparisons between these methods were made (1) to determine whether similar populations of terminals were labeled and (2) to assess the feasibility of combining degeneration with immunoperoxidase labeling for the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). Hippocampal afferents labeled with HRP were finely myelinated or unmyelinated and gave rise to small terminals (mean diameter 0.58 micron) containing mostly clear, round vesicles. Of the HRP-labeled terminals which made recognizable junctions, 85% (104/122) formed asymmetric synapses with the heads of dendritic spines. The remainder either formed asymmetric axodendritic synapses or symmetric junctions. Degenerating terminals were significantly smaller (mean diameter 0.35 micron) than terminals labeled with HRP. However, these also formed principally asymmetric axospinous synapses (89/102, 87%). Whether identified by HRP transport or anterograde degeneration, the hippocampal afferents comprised approximately 10% of all terminals and 30% of all asymmetric axospinous synapses in the medial Acb. In contrast to hippocampal afferents, TH-labeled terminals formed primarily symmetric contacts with dendritic shafts and the heads and necks of spines. Quantitative analysis of sections containing both anterograde degeneration and TH-immunoreactivity showed that 25% (26/104) of associations formed by degenerating hippocampal terminals involved convergent inputs with TH-labeled terminals on the same postsynaptic structure. These included dual input either to the same spine head or to different parts of the same dendrite. In addition, the plasma membranes of hippocampal and TH-labeled terminals were often directly apposed to each other (10/58, 17% of axo-axonal associations formed by degenerating terminals), without recognizable synaptic specializations.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine stimulates [3H]noradrenaline release by activating a strychnine-sensitive receptor present in rat hippocampus

Rat hippocampus slices were prelabeled with [3H]noradrenaline ([3H]NA) and depolarized by superfusion with KCl. The release evoked by 12 mM K+ was totally calcium-dependent and more than 90% tetrodotoxin (TTX)-sensitive. Glycine (0.1-1 mM) increased the K(+)-evoked [3H]NA overflow in a concentration-dependent manner. The effect of 1 mM glycine reached 300%. Strychnine (0.3 microM) shifted to the right the concentration-response curve for glycine. The effect of glycine (0.1 or 1 mM) was totally abolished by 3 microM strychnine but was unaffected by the GABAA receptor antagonist, bicuculline (10 microM), or by 100 microM of 1-hydroxy-3-aminopyrrolidone-2 (HA-966), a proposed antagonist of glycine at the strychnine-insensitive site located on the N-methyl-D-aspartate (NMDA) receptor. The effect of glycine was mimicked by L-serine, although less potently; the release of [3H]NA was enhanced by 200% in presence of 3 mM L-serine. At this concentration D-serine was ineffective. Strychnine shifted to the right the concentration-response curve for L-serine. Glycine (1 mM) had only a minor effect (less than 20% potentiation) on the release of [3H]NA evoked by 12 mM KCl in hippocampal synaptosomes. While the effect of glycine in slices was increased by decreasing the depolarizing concentration of K+ (about 500% potentiation at 9 mM K+), the response of synaptosomes remained minimal, even in presence of 9 mM KCl. Hippocampal synaptosomes prelabeled with [3H]glycine released the radiolabeled amino acid when exposed to superfusion with 12 mM KCl. The release of [3H]glycine was more than 75% calcium-dependent. The results suggest that the release of NA in rat hippocampus may be enhanced by glycine through the activation of a strychnine-sensitive receptor. This receptor does not seem to be located on noradrenergic terminals.

Effects of intrastriatal apomorphine on changes in switching behaviour induced by the glutamate agonist AMPA injected into the cat caudate nucleus

Bilateral intracaudate application of the glutamate agonist DL-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), viz. an agonist of quisqualate receptors, is known to produce the following effects in cats that had to climb on a small wooden bar and, subsequently, to switch to distinct patterns: it produces increases in switching from one pattern to another pattern (1) and it induces limb deficits, i.e. unilateral deficient placing of the fore- and/or hindlimb. In the present study, the effect of stimulating striatal dopamine receptors on behavioural changes induced by intracaudate injections of AMPA was investigated. Therefore, the dopamine agonist apomorphine was injected into the caudate nucleus 5 min before the striatal injection of 1.0 micrograms AMPA. AMPA-induced increases in switching behaviour were prevented by 0.6 micrograms, but not 0.3 micrograms, apomorphine. In contrast, AMPA-induced limb deficits were not prevented by pretreatment of apomorphine. In view of the notion that the dopaminergic caudate nucleus, its output station the substantia nigra, pars reticulata and the nigral output station the deeper layers of the colliculus superior are essential for switching behaviour, but not for the display of disturbances like AMPA-induced limb deficits, the present data strongly suggest that only AMPA-induced changes in switching, but not AMPA-induced limb deficits, are mediated by the caudato-nigro-collicular circuitry. The glutamate receptor-selectivity of the modulatory action of dopamine is discussed.

Age-related decline in rat striatal dopamine metabolism is regionally homogeneous

Previous research has established that the age-related decrease in rat striatal D2 sites occurs predominantly in the posterior ventral caudate-putamen, and the present work was undertaken to determine whether a corresponding preferential reduction in dopamine, its metabolites, or its synthesis rate occurs in this region. Male F344 rats 4-8 or 25-27 months old were used for regional HPLC electrochemical determinations of 1) dopamine, homovanillic acid (HVA), or dihydroxyphenylacetic acid (DOPAC) obtained from striatal micropunch samples, or 2) 3,4-dihydroxyphenylalanine (DOPA) concentrations in these same micropunch regions 30 minutes after treatment with the aromatic amino decarboxylase inhibitor, NSD-1015 (100 mg/kg, IP). Aged rats had significantly less dopamine, HVA, and DOPAC in their striatal samples than did young adult controls, as well as having less DOPA accumulation after NSD-1015. However, for none of these measures was the age x region interaction significant, suggesting that the decline in these markers of presynaptic dopaminergic function occurs uniformly throughout the striatum. The results provide evidence that the effects of aging on striatal dopamine receptors are dissociable from the influences on the dopaminergic innervation of this structure, suggesting independent control of pre- and postsynaptic elements of these synapses during the lifespan.

Striosomal organization of cholinergic and dopaminergic uptake sites and cholinergic M1 receptors in the adult human striatum: a quantitative receptor autoradiographic study

The distribution of cholinergic pre- and postsynaptic markers and dopaminergic presynaptic markers was analyzed in the adult human striatum, using quantitative receptor autoradiography. The distribution of the different binding sites indicates that cholinergic and dopaminergic uptake sites display a striosomal organization, with a higher binding site density in the striatal matrix. M1 muscarinic cholinergic receptor distribution parallels the distribution of cholinergic presynaptic markers. Our data show that cholinergic and dopaminergic terminals, and M1 receptors are enriched in the matrix of the adult human striatum.

Autoradiographic localisation of D1 and D2 dopamine receptors in primordial striatal tissue grafts in rats

Using in vitro autoradiography the study examined the binding of the dopamine (DA) D1 receptor and D2 receptor radiolabelled ligands, [125I]SCH 23982 and [125I]sulpiride, respectively, to striatal sections of control rats, those with unilateral ibotenic acid (IA) lesions of the striatum and those with grafts of primordial striatal tissue implanted into the IA-lesioned striatum. In animals with IA-lesions, there was almost total degeneration of striatal neurones and a complete disappearance of both D1 and D2 receptors in the lesioned striatum. Within the striatal grafts, dense 'patches' of both D1 and D2 receptors were observed with densities comparable to those in the intact neostriatum. These data indicate that both D1 and D2 receptors are localized on the soma of intrinsic striatal neurones. Moreover, the specific localisation of D1 and D2 receptors in the grafts provides the neurochemical basis for the mechanisms by which the host nigrostriatal DA system can regulate the functional capacity of striatal grafts.

Striatal dopamine in motor activation and reward-mediated learning: steps towards a unifying model

On the basis of behavioural evidence, dopamine is found to be involved in two higher-level functions of the brain: reward-mediated learning and motor activation. In these functions dopamine appears to mediate synaptic enhancement in the corticostriatal pathway. However, in electrophysiological studies, dopamine is often reported to inhibit corticostriatal transmission. These two effects of dopamine seem incompatible. The existence of separate populations of dopamine receptors, differentially modulating cholinergic and glutamatergic synapses, suggests a possible resolution to this paradox. The synaptic enhancement which occurs in reward-mediated learning may also be involved in dopamine-mediated motor activation. The logical form of reward-mediated learning imposes constraints on which mechanisms can be considered possible. Dopamine D1 receptors may mediate enhancement of corticostriatal synapses. On the other hand, dopamine D2 receptors on cholinergic terminals may mediate indirect, inhibitory effects of dopamine on striatal neurons.

Concentration-dependent actions of stimulated dopamine release on neuronal activity in rat striatum

Voltammetric analysis was combined with single unit recording to measure the effects of endogenous dopamine, released by electrical stimulation of the median forebrain bundle, on neuronal activity in the rat striatum in vivo. Fast differential ramp voltammetry, a more sensitive form of fast cyclic voltammetry, was used to measure extracellular dopamine levels during a 50-ms scan epoch every 500 ms. Using the same carbon fibre microelectrode, neuronal activity was recorded in between the electrochemical epochs. A steady-state electrochemical signal equivalent to about 100 nM dopamine was seen in the unstimulated striatum. The responses of 122 striatal units to stimulated dopamine release were recorded in 37 acute experiments. Ninety-one units which displayed a large spike amplitude (greater than or equal to 50 microV) were recorded during stimulated release of dopamine initially to levels of between 100 and 500 nM. The majority (49) showed a profound excitation, 23 showed inhibition, and nine units gave complex responses. Only 10 units were unresponsive. All the responses of these large units outlasted the transient increase in dopamine levels, often for more than 1 min. In contrast, all the 31 units which displayed a small spike amplitude (less than 50 microV) were powerfully activated by dopamine release within this range. Administration of alpha-methyl-para-tyrosine (250 mg/kg i.p.) abolished both dopamine release and the response of the five large units and four small units examined, indicating that the neuronal response was directly attributable to dopamine. Dopamine release was increased by increasing the stimulus duration over the range 0.25-10 s. With increasing levels of dopamine release the excitatory response of large units rose to a maximum and then decreased until it was eventually transformed entirely into an inhibition at dopamine levels above 1 microM. In contrast, the excitatory response of small units always increased in magnitude with increasing dopamine release to levels greater than 1 microM. The large units that showed inhibition at low levels of dopamine were also inhibited at high levels. Tail-pinch stimuli excited 21/23 large units and all seven small units tested, although this stimulus did not evoke a detectable rise in dopamine levels. We suggest that the fundamental action of dopamine in the striatum is excitation, whether involving D1 or D2 receptors. The small units described here could be inhibitory interneurons which convert the excitatory response of large units into inhibition. Dopamine may regulate striatal function by enhancing particular input-output pathways while also activating lateral inhibitory mechanisms serving to "gate-out" alternative outputs.