[3H]SCH 23390 binding to D1 dopamine receptors in the basal ganglia of the cat and primate: delineation of striosomal compartments and pallidal and nigral subdivisions

Distribution of messenger RNAs for D1 dopamine receptors and DARPP-32 in striatum and cerebral cortex of the cynomolgus monkey: relationship to D1 dopamine receptors

Messenger RNAs for the D1 dopamine receptor and dopamine- and cyclic AMP-regulated phosphoprotein of relative mass 32,000 (DARPP-32) were examined by in situ hybridization in the cynomolgus monkey brain. The messenger RNA distribution was compared to the distribution of D1 dopamine receptors using [3H]SCH 23390 autoradiography. In the caudate nucleus and putamen, D1 dopamine receptor messenger RNA-positive cells were unevenly distributed. Clusters of cells with an approximately three-fold higher intensity of labeling, as compared to surrounding regions, were found. Some of these D1 dopamine receptor messenger RNA intensive cell clusters in the caudate nucleus appeared to some extent to be matched to regions of higher intensity of [3H]SCH 23390 binding. The distribution of cells expressing DARPP-32 messenger RNA in the caudate nucleus and putamen was found to be non-clustered. In neocortical regions, cells of different sizes expressing D1 dopamine receptor messenger RNA were present in layers II-VI. D1 dopamine receptor messenger RNA-positive cells were most abundant in layer V. Unexpectedly, no DARPP-32 messenger RNA signal was detected in neocortex. Chronic SCH 23390 administration did not change the relative levels of messenger RNAs for the D1 dopamine receptor and DARPP-32 or [3H]SCH 23390 binding as measured by quantitative image analysis. The clustered distribution of D1 dopamine receptor messenger RNA is in contrast to that of DARPP-32 messenger RNA. This suggests that D1 dopamine receptors may play a more significant role in regulating DARPP-32 function in patch regions as compared to matrix regions. D1 dopamine receptor messenger RNA-expressing cells could also be visualized in several layers of the primate neocortex, implying that dopamine acts through D1 dopamine receptors within functionally different neuronal circuits of the neocortex.

Distribution of dopamine D1, D2, and D5 receptor mRNAs in the monkey brain: ribonuclease protection assay analysis

The distribution of the mRNAs encoding the dopamine D1, D2 and D5 receptors was determined in brain tissues obtained from intact female rhesus monkeys, using a ribonuclease protection assay. Tissue blocks from the frontal cortex, striatum, thalamus, hippocampus and substantia nigra were dissected and total RNA was extracted. Dopamine D2 and D5 receptor DNA fragments were generated from rhesus monkey genomic DNA using polymerase chain reaction. To generate dopamine receptor subtype-specific cRNA probes, DNA fragments corresponding to the carboxy terminus of the rhesus monkey D1 and D2 receptor genes and to the putative transmembrane domain regions (IV-VI) of the D5 receptor gene, were subcloned into the pGEM3Z/4Z vectors. Expression of D1 receptor mRNA exhibited significant regional differences: striatum > > > cerebral cortex > or = hippocampus > or = lateral thalamus. D1 receptor mRNA was found in low quantities in the medial thalamus, but was not consistently expressed in the substantia nigra area. In contrast, D2 receptor mRNA was detected in all regions that were studied: striatum > > > substantia nigra > > hippocampus > or = cerebral cortex > or = medial thalamus > or = lateral thalamus. D5 receptor mRNA was also expressed in all regions, with highest levels in the cerebral cortex, striatum and lateral thalamus, and moderate levels in the substantia nigra, medial thalamus and the hippocampus. The D5 receptor mRNA appears to be widely distributed in the monkey brain. Most interesting is the expression of D5 receptor mRNA in tissues of the substantia nigra area.

Heterogeneous and compartmental distribution of zinc in the striatum and globus pallidus of the rat

The distribution of vesicular or chelatable zinc was analysed in the dorsal and ventral subdivisions of the striatum and globus pallidus of the rat with Danscher's selenium method. Acetylcholinesterase and Calbindin-D28k were used as striatal and pallidal markers in order to analyse the possible compartmentalization of the distribution of zinc in the striatum and globus pallidus. The main findings of this study are the following: (1) The distribution of vesicular zinc in the dorsal striatum was heterogeneous. A peripheral rim of tissue heavily stained for zinc was detected in the medial, dorsal and lateral striatal areas, along most of the rostrocaudal extent of the striatum. addition, patch-like zones intensely stained for zinc were prominent in the rostral half of the caudate-putamen complex. (2) In some regions of the rostral half of the caudate-putamen complex, the staining for zinc appeared to follow the well-known striatal patches (striosomes)/matrix organization. However, in other regions of the rostral half of the striatum such a relation was not detected. (3) The ventral striatum also showed a heterogeneous staining for zinc. Thus, in the most ventral part of the caudate-putamen complex, both subdivisions of the nucleus accumbens and parts of the olfactory tubercle displayed different patterns of compartmentalized distribution of zinc. In the dorsal half of the shell of the nucleus accumbens, some patches with an intense reaction for zinc seemed to overlap with acetylcholinesterase-poor patches. (4) There was a remarkable absence of staining for zinc in the globus pallidus. This histochemical study illustrates, on the one hand, the high content of vesicular zinc in the dorsal and ventral subdivisions of the striatum, which was distributed following different patterns of chemical compartmentalization, and on the other hand, the absence of vesicular zinc in the globus pallidus of the rat.

Immunocytochemical localization of D1 and D2 dopamine receptors in the basal ganglia of the rat: light and electron microscopy

The modulatory actions of dopamine on the flow of cortical information through the basal ganglia are mediated mainly through two subtypes of receptors, the D1 and D2 receptors. In order to examine the precise cellular and subcellular location of these receptors, immunocytochemistry using subtype specific antibodies was performed on sections of rat basal ganglia at both the light and electron microscopic levels. Both peroxidase and pre-embedding immunogold methods were utilized. Immunoreactivity for both D1 and D2 receptors was most abundant in the neostriatum where it was mainly contained within spiny dendrites and in perikarya. Although some of the immunoreactive perikarya had characteristics of interneurons, most were identified as medium-sized spiny neurons. Immunoreactivity for D1 receptor but not D2 receptor was associated with the axons of the striatonigral pathway and axons and terminals in the substantia nigra pars reticulata and the entopeduncular nucleus. In contrast, D2 immunoreactivity but not D1 immunoreactivity was present in the dopaminergic neurons in the substantia nigra pars compacta and ventral pars reticulata. In the globus pallidus, little immunoreactivity for either D1 or D2 receptor was detected. At the subcellular level, D1 and D2 receptor immunoreactivity was found to be mainly associated with the internal surface of cell membranes. In dendrites and spines immunoreactivity was seen in contact with the membranes postsynaptic to terminals forming symmetrical synapses and less commonly, asymmetrical synapses. The morphological features and membrane specializations of the terminals forming symmetrical synapses are similar to those of dopaminergic terminals previously identified by immunocytochemistry for tyrosine hydroxylase. In addition to immunoreactivity associated with synapses, a high proportion of the immunoreactivity was also on membranes at non-synaptic sites. It is concluded that dopamine receptor immunoreactivity is mainly associated with spiny output neurons of the neostriatum and that there is a selective association of D1 receptors with the so-called direct pathway of information flow through the basal ganglia, i.e. the striatoentopeduncular and striatonigral pathways. Although there is an association of receptor immunoreactivity with afferent synaptic inputs a high proportion is located at extrasynaptic sites.

Ontogeny of the D1 dopamine receptor in the rat striatonigral system: an immunohistochemical study

Antibodies were raised against a recombinant protein to analyse the pre- and postnatal ontogeny of the neurons expressing the D1 dopamine receptor in the striatum by immunohistochemistry. We report that D1 immunoreactivity is detectable from gestational day (G) 15 and is distributed homogeneously throughout the striatum from G15 to G18. From G19-20 to postnatal day (P) 3, D1 immunoreactivity becomes heterogeneous and predominates in cell bodies of the patch compartment while very limited immunoreactivity is detectable in the matricial compartment. The differential intensity between patches and matrix reaches its peak around P0. From P2, the pattern of D1 immunoreactivity progressively assumes the homogeneous distribution characteristic of the adult striatum. The expression of D1 mRNA in striatal neurons, as investigated by in situ hybridization, displays a similar pattern during this period. Substance P mRNA is also preferentially expressed in the patch compartment during the same period. D1 immunoreactivity appears at G17 in the substantia nigra as clusters of fibres and increases subsequently until reaching its adult form during the first postnatal week. These results demonstrate that the two compartments of the developing striatum display differential transcriptional and translational activity for the D1 gene and consequently two different and successive patterns of expression of D1 protein: patch neurons first express D1 receptor intensely while matrix neurons express it later and in smaller amounts so that D1 receptor appears transiently during the perinatal period as a marker of the patch compartment.

A subset of striatopallidal neurons are Fos-immunopositive following acute monoamine depletion in the rat

Experiments were conducted to characterise the Fos-immunopositive neurons that are observed in the dorsal rim of the striatum following monoamine depletion by the systemic administration of reserpine. Using a retrograde tract-tracer, some of these neurons could be shown to project to the globus pallidus but none were seen to project to the entopeduncular nucleus. In addition, these neurons were located in a region of both poor calbindin immunoreactivity and cholinesterase activity. It can be concluded that Fos levels are increased only in a subset of striatopallidal neurons following monoamine depletion. This subset of neurons is located in the dorsal region of the striatum where it has previously been shown that neurons can preferentially be induced to undergo apoptosis upon monoamine depletion.

Heterogeneous topographical distribution of the striatonigral and striatopallidal neurons in the matrix compartment of the cat caudate nucleus

The topographical organization of the striatonigral projection was investigated in the cat by comparing the localization and the intensity of labelling of retrogradely labelled cells in the caudate nucleus following one or multiple injections of horseradish peroxidase-wheat germ agglutinin into the center or along the rostrocaudal axis of the substantia nigra pars reticulata. Second, the localizations of retrogradely labelled striatopallidal neurons and of clusters of aggregated striatonigral neurons (as outlined by the transport of 14C-material) were compared in cats that received four horseradish peroxidase-wheat germ agglutinin injections into the internal segment of the globus pallidus and three nigral injections of 14C-amino acids into the substantia nigra pars reticulata. Two types of striatonigral neurons located predominantly within the matrix compartment were identified: poorly collateralized aggregated cells distributed in clusters and more numerous collateralized cells distributed outside the clusters. In addition, two cell types were distinguished within each cluster of aggregated neurons. Those innervating the center of the substantia nigra pars reticulata were observed after a single nigral injection of the tracer, whereas those projecting to distinct sites of the substantia nigra pars reticulata along a rostrocaudal axis were observed only after multiple injections. Striatal neurons innervating the internal segment of the globus pallidus were heterogeneously distributed predominantly within the matrix but outside the clusters of aggregated striatonigral neurons. Together, these results provide further evidence for the heterogeneity of the matrix and for the complexity of matrix striatonigral connections that send both diverging and converging signals to the substantia nigra pars reticulata.

Quantitative autoradiography of dopamine-D1 receptors, D2 receptors, and dopamine uptake sites in postmortem striatal specimens from schizophrenic patients

A number of previously published homogenate receptor binding studies have postulated that dopaminergic dysfunction in schizophrenia may be related to abnormalities in dopamine receptors. In this study, postmortem striatal specimens from patients with schizophrenia, normal controls, and psychiatric controls that had received neuroleptics were studied with quantitative autoradiography for dopamine receptors. Autoradiography with single concentrations of [3H]-SCH 23390 for D1 receptors, [3H]-raclopride for D2 receptors, and [3H]-CFT for dopamine uptake sites failed to define significant differences between the study groups. [3H]-CFT bound in a patchy distribution in the striatum that is believed to correspond to striosomal and matrix striatal compartments. There were no differences between groups when [3H]-CFT binding density was examined in the striosomal and matrix compartments. There were also no differences between groups in the percentage of striatal area occupied by striosomal or matrix compartments as defined by [3H]-CFT binding. We conclude that abnormalities of these dopamine receptor subtypes are probably not primary features of the schizophrenic syndrome in the brain collection examined. Previous reports of elevated D2 receptor binding in schizophrenia may have been related to drug treatment effects. Alternatively, the relatively high affinity of ligands used in previous studies for D4 receptors may explain the discrepancy in our findings. Unchanged [3H]-CFT binding in the schizophrenic group also suggests that the density of mesostriatal neuronal terminals is not altered in schizophrenia.

Localization of dopamine D3 receptors to mesolimbic and D2 receptors to mesostriatal regions of human forebrain

We characterized the binding of [125I]epidepride to dopamine D2-like and D3-like receptors in tissue sections of human striatum. The competition for binding of [125I]epidepride by domperidone, quinpirole, and 7-hydroxy-N,N-di(1-propyl)-2-aminotetralin (7-OH-DPAT) was best fit by assuming one site in the caudate but two sites in nucleus accumbens. Guanosine 5'-[beta, gamma-imido]triphosphate showed a large modulatory influence in agonist inhibition of [125I]epidepride binding in caudate but not in nucleus accumbens. The binding of [125I]epidepride in the presence of 7-OH-DPAT (1000-fold selective for D3-like versus D2-like sites) and domperidone (20-fold selective for D2-like versus D3-like sites) was used to quantify the numbers of D2-like and D3-like receptors in areas of human brain. The distribution of D2-like and D3-like receptors was largely nonoverlapping. Binding of [125I]epidepride to D3-like receptors was negligible in the dorsal striatum but was concentrated in islands of dense binding in the nucleus accumbens and ventral putamen that aligned with acetylcholinesterase-poor striosomes. Binding to D3-like receptors was also enriched in the internal globus pallidus, ventral pallidum, septum, islands of Calleja, nucleus basalis, amygdalostriatal transition nucleus of the amygdala, central nucleus of the amygdala, and ventral tegmental area. Binding of [125I]epidepride to D2 but not D3 receptors was detected in cortex and hippocampus.

The distribution of cholinergic perikarya with respect to enkephalin-rich patches in the caudate nucleus of the adult cat

The distribution of cholinergic interneurons with respect to enkephalin-rich patches in the caudate nucleus of the cat was examined using both computer-assisted 3-D reconstruction and immunocytochemical techniques. Examination of the 3-D distribution of perikarya staining for choline acetyltransferase (ChAT) revealed that these cells were not evenly distributed within the caudate nucleus but exhibited areas of increased and decreased density. Comparison of the 3-D distribution of cholinergic perikarya to that of the enkephalin-rich patches indicated that areas of increased ChAT+ cell density often corresponded to the positions of enkephalin-rich patches within the dorsal-lateral caudate nucleus. At more ventral regions, there was no clear correspondence between areas of increased ChAT+ cell density and enkephalin-rich patches. In agreement with these observations, a quantitative analysis of sections double-labeled for ChAT and enkephalin revealed that the density of cholinergic neurons within enkephalin-rich patches was twice that in the surrounding tissue in the dorsal region of the caudate nucleus. In contrast at more ventral levels, the difference in the density of ChAT+ cells in enkephalin-rich patches did not significantly differ from that in the surrounding striatal tissue. Both the results of the 3-D and the double-labeling analysis suggest that cholinergic neurons are not evenly distributed within the caudate nucleus of the cat but form loose clusters which are associated dorsally with the enkephalin-rich patches. These results also provide further evidence of heterogeneity within the striosomal compartment in the cat.

Parallel changes in dopamine D2 receptor binding in limbic forebrain associated with chronic mild stress-induced anhedonia and its reversal by imipramine

Chronic sequential exposure to a variety of mild stressors has previously been found to cause an antidepressant-reversible decrease in the consumption of palatable sweet solutions, associated with abnormalities of dopaminergic neurotransmission in the nucleus accumbens. In the present study, 5 weeks of treatment with imipramine (10 mg/kg b.i.d.) reversed the decreased sucrose intake of rats exposed to chronic mild stress. Stress also caused a decrease in D2-receptor binding in the limbic forebrain (but not the striatum), which was completely reversed by imipramine. In nonstressed animals, imipramine decreased D1-receptor binding in both regions. However, in stressed animals, imipramine did not significantly alter D1-receptor binding in either area. Stress alone slightly increased D1-receptor binding, in striatum only. Scatchard analysis showed that all changes in receptor binding resulted from changes in receptor number (Bmax) rather than receptor affinity (KD). The results support the hypothesis that changes in D2-receptor function in the nucleus accumbens are responsible for chronic mild stress-induced anhedonia and its reversal by antidepressant drugs. They do not support the hypothesis that the sensitization of D2-receptors seen following chronic antidepressant treatment is caused by a down-regulation of D1-receptors.

Binding sites for 5-hydroxytryptamine-2 receptor agonists are predominantly located in striosomes in the human basal ganglia

Previous autoradiographic studies have shown that serotonin 5-HT2 receptors are homogeneously distributed in the human striatum. While these studies were done using antagonist radioligands such as [3H]ketanserin, we describe here a heterogeneous distribution of 5-HT2 binding sites in the human striatum, using [3H]LSD and [125I]DOI as ligands. Beside their agonist properties, these compounds belong to the family of psychedelic drugs. The localization of their binding sites in the dorsal striatum is very similar to that of striosomes, as visualized by acetylcholinesterase histochemistry or [3H]flunitrazepam labelling. This heterogeneous distribution seems to be a peculiarity of the human and guinea-pig brain, for it is not found in the monkey, cat, pig, and cow. In the rat striatum, a weak patchniness was seen, but corresponded to 5-HT1C binding sites. The density of [125I]DOI binding sites over striosomes presents large variations, which can neither be correlated with parameters such as age, gender and post-mortem delay nor with the effects of neurodegenerative disorders, with the exception of Huntington's disease, at late stages of the disease. The drug binding profile of [125I]DOI binding sites in the striosomes is identical to that of matrix binding sites. It is also fully comparable to the pharmacological profile of cortical 5-HT2 sites reported using [3H]ketanserin as a radioligand, with the exception of the higher affinity displayed by agonists for [125I]DOI binding sites. Interestingly, biphasic displacement curves yield a better fit for spiperone, cinanserin and ketanserin competitions. This biphasic profile can probably neither be accounted for by the presence of 5-HT1C sites nor by the existence of multiple affinity states. Taken together, these data suggest that a heterogeneous population of 5-HT2 receptors is present on the cell bodies or dendrites of striosomal neurons. These receptors provide an additional anatomical substrate to explain the psychedelic action of indoleamine (LSD) and phenylethylamine (DOI, DOM) drugs.

Basal ganglia: functional anatomy and physiology. Part 1

Advances in knowledge about basal ganglia structure and connectivity from 1925 to date are reviewed. Current concepts about neuronal populations, transmitters, and input and output of each of the basal ganglia nuclei are presented. The portrayal by Wilson, in 1925, of the striatum as a simple homogeneous structure has been replaced by the recognition, based on staining characteristics, connectivity, and function, that the neostriatum is compartmentalized into striosomes, matrisomes, and matrix compartments. Electrophysiologic studies have further shown the existence, in the neostriatum, of neuronal clusters that represent basic functional units much like the functional columns described much earlier for the cerebral cortex. Whereas the neostriatum is considered the major receiving area of the basal ganglia, the globus pallidus and substantia nigra pars reticulata constitute the major output nuclei. Combined neuroanatomic and neurophysiologic studies have revealed precise somatotopic organization throughout the basal ganglia system such that the leg, arm, and face areas of the cerebral cortex related to respective topographic areas within the striatum, pallidum, substantia nigra, and subthalamus. The previous concept of an inhibitory role for dopamine on striatal neurons has been modified. It is now acknowledged that dopamine exerts an inhibitory effect on striatal neurons that project to the external pallidum and a facilitatory effect on striatal neurons that project to the internal pallidum and substantia nigra pars reticulata. The previous concept of serial connectivity of the neostriatum (funnel concept) has been replaced by the concept of parallel connectivity. Within the internal connectivity of the basal ganglia, there is a fast system in which the neurotransmitter is gamma-aminobutyric acid (GABA) and a slow system modulated by neuropeptides. The slow system is believed to give identity to an otherwise homogenous GABAergic system.

NMDA and carbachol but not AMPA affect differently the release of [3H]GABA in striosome- and matrix-enriched areas of the rat striatum

The effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 10(-3) M), N-methyl-D-aspartate (10(-3) M, in the absence of magnesium or presence of AMPA) and carbachol (10(-3) M) on the release of preloaded [3H]gamma-aminobutyric acid ([3H]GABA) from microdiscs of tissue punched out from sagittal brain slices in striosome- or matrix-enriched areas of the rat striatum have been compared. Although AMPA stimulated similarly the release of [3H]GABA in both striatal compartments, the release of [3H]GABA evoked by either N-methyl-D-aspartate (in the presence of AMPA) or carbachol was more pronounced in matrix- than in striosome-enriched areas. AMPA- and N-methyl-D-aspartate- (in the absence of magnesium) evoked responses were reduced but not abolished in the presence of tetrodotoxin (10(-6) M) in both compartments while the carbachol-evoked release of [3H]GABA was decreased by tetrodotoxin only in the matrix. The interruption of cholinergic transmission by the combined application of atropine (10(-5) M) and pempidine (10(-4) M) was without effect on the AMPA-evoked release of [3H]GABA, but it reduced the N-methyl-D-aspartate- (in the absence of magnesium or presence of AMPA) evoked release of [3H]GABA in both compartments, these reductions being of similar amplitude than those observed with tetrodotoxin.

Intranigral injections of SCH 23390 inhibit SKF 82958-induced rotational behavior

The selective D1 receptor antagonist, SCH 23390, was injected into the pars reticulata region of the lesioned substantia nigra at various concentrations (3.0, 1.5, 1.0, 0.6 or 0.3 mM) just before a s.c. injection of either the selective D1 agonist, SKF 82958; the selective D2 agonist, quinpirole; or the mixed D1-D2 receptor agonist, apomorphine. SCH 23390 pretreatment (1) had no significant effect on quinpirole rotational behavior, (2) attenuated apomorphine rotational behavior and (3) dose-dependently inhibited SKF 82958 rotational behavior with the highest SCH 23390 doses completely blocking SKF 82958 rotational behavior in some animals. These data provide further evidence that dopamine release in the midbrain may act as a neuromodulator of motor behavior, and that D1 receptors play a functional role in this process.

Levodopa-induced dyskinesias in Parkinson's disease phenomenology and pathophysiology

The aim of this study was to provide further insight into the phenomenology and pathophysiology of monophasic and biphasic dyskinesias induced by levodopa in Parkinson's disease. For this purpose, the type, localization, severity, and timing of dyskinesias were evaluated in 15 parkinsonian patients in relation to motor disability after administration of levodopa using a video-electromyographic recording device. Foot-dystonia, myoclonus, and akathisia were observed in most patients. The dyskinesias started in the foot, usually on the side most affected by the disease, and spread in an "ascending wave" to the contralateral side, the trunk, and upper extremities. In a few patients, onset was axial, spreading almost instantaneously to all limbs. The dyskinesias were dystonic and ballistic at the start, and became increasingly choreic as they attained the upper limbs. Their intensity was maximal in the lower limbs, then progressively decreased, while increasing in upper limbs and head. The results indicate that there is no strict dichotomy between biphasic and monophasic dyskinesias. In other words, there is a "continuum" between the first dyskinesias and those observed during the period of maximal clinical improvement. These dyskinesias can also appear in reverse order, as if there were an "oscillator" determining a sequence of alternating patterns.

Molecular aspects of neuropeptide regulation and function in the corpus striatum and nucleus accumbens

In the corpus striatum and nucleus accumbens, neuropeptides participate along with conventional neurotransmitters such as dopamine, gamma-aminobutyric acid (GABA), acetylcholine and glutamate in the regulation of locomotor activity, stereotyped motor behaviors and neural events related to reward and affective state. The present review concerns itself with four major neuropeptide systems--enkephalin, dynorphin, tachykinins and neurotensin--and it summarizes neuroanatomical and functional studies as well as emphasizing regulatory interactions between neurotransmitters and neuropeptides at the level of neuropeptide gene expression. Dopaminergic transmission emanating from midbrain dopaminergic cell bodies of the substantia nigra and the ventral tegmentum regulates striatal and accumbens neuropeptide levels and their mRNAs. Evidence is presented for D1 or D2 receptor involvement as well as D1-D2 interactions that modulate neuropeptide and mRNA levels in striatum and accumbens neurons. Regulatory influences by GABAergic, serotonergic and cortical (glutamatergic) neurotransmission and via sigma receptors and circulating adrenal steroids are also described. The evidence gathered in many laboratories thus far indicates that these major basal ganglia peptidergic systems are modulated dynamically and sometimes in opposing ways by various neurochemical inputs which alter neuropeptide and neuropeptide mRNA levels over both short- and long-term. Neuropeptide systems are involved in the regulation and execution of motor programs and may also be involved in the control of mood and affect as well as self-administration behavior and behavioral sensitization, especially via the nucleus accumbens and its reciprocal connections with the midbrain, hippocampus and frontal cortex. Glucocorticoids modulate mood as well as self-administration behavior and influence locomotor activity and certain forms of stereotypy. The modulation of striatal proenkephalin and protachykinin mRNA levels by adrenal steroids is described along with distribution of adrenal steroid receptor subtypes. Adrenal steroid regulation of neuropeptide gene expression in striatum, accumbens and midbrain suggests that there may be a wider role for glucocorticoids and for other neuropeptide systems in environmental and drug influences on normal and abnormal behaviors involving the nigrostriatal and mesolimic systems.

Spatial organization of patch and matrix compartments in the rat striatum

The visualization of mu opiate receptors by [3H]naloxone binding was used to determine precisely the spatial organization of the patch compartment in the rat striatum and its reproducibility in different animals. Three-dimensional reconstruction of the patch network was made using maps of autoradiographic data obtained from successive coronal, sagittal or horizontal sections. The extreme rostral pole of the striatum (A 11) was characterized by a large patch territory exhibiting complex and tortuous fields with several extensions. In the intermediate part of the structure (A 9.0-10.0), about 20 serial parallel continuous patch channels running in a mediolateral axis, obliquely oriented and displaying in some cases connecting branches, could be observed. However, no channels could be distinguished in the rostrocaudal direction. More caudally, patches were rare and of small size. In addition, the laterocaudal region of the striatum was almost exclusively represented by a large matrix field. Finally, a fine discontinuous band of [3H]naloxone binding was seen in all sections, bordering and limiting the dorsolateral part of the striatum. The topographical and spatial distribution of the patch compartment was similar in all animals investigated. However, due to the tortuous shape and the labyrinthine organization of the patches, the precise degree of reproducibility from one animal to another could not be established. Nevertheless, the prominent patch compartment observed in the rostral pole of the striatum, the patch channels, oriented in the mediolateral axis as well as the large laterocaudal matrix field were observed in all cases. These results were compared with previous data obtained in the cat in which patch (striosome) channels oriented along a rostrocaudal axis are also observed.

Local GABAergic regulation of the N-methyl-D-aspartate-evoked release of dopamine is more prominent in striosomes than in matrix of the rat striatum

Using an in vitro microsuperfusion device we have previously demonstrated that in the absence of magnesium, the N-methyl-D-aspartate-evoked release of [3H]dopamine (continuously synthesized from [3H]tyrosine) is more prominent in matrix- than in striosome-enriched areas of the rat striatum and that in the matrix, the response is partially tetrodotoxin-sensitive. Since the medium-sized GABAergic neurons are the main targets of the corticostriatal glutamatergic fibers, the involvement of local GABAergic regulation in the N-methyl-D-aspartate-evoked release of [3H]dopamine was investigated in both striatal compartments using the same experimental approach. Firstly, bicuculline alone (5 microM, 25-min application) was shown to enhance the release of [3H]dopamine similarly in both compartments revealing the existence of a tonic GABAergic control of the spontaneous release of [3H]dopamine. Secondly, the N-methyl-D-aspartate (50 microM, 25-min application)-evoked release of [3H]dopamine was markedly amplified in the presence of bicuculline (5 microM, continuous delivery). This effect being more important in striosome- than in matrix-enriched areas (5.5- and two-times the N-methyl-D-aspartate-evoked response observed in the absence of the GABAA antagonist, respectively). Thirdly, the tetrodotoxin (1 microM, continuous delivery)-resistant N-methyl-D-aspartate-evoked responses were also enhanced in the presence of bicuculline, but in this case, the amplification of the N-methyl-D-aspartate-evoked release of [3H]dopamine was less marked than in the absence of tetrodotoxin and identical in both compartments (about two-times the tetrodotoxin-resistant N-methyl-D-aspartate-evoked responses observed in the absence of bicuculline). Altogether, these results indicate that GABAergic neurons exert locally an important inhibitory regulation of the N-methyl-D-aspartate-evoked release of dopamine and that this effect is more prominent in the striosome-enriched area. Both tetrodotoxin-sensitive (striosome) and tetrodotoxin-resistant (striosome and matrix) processes intervene in this inhibitory GABAergic presynaptic regulation of dopamine release.

Intranigral injections of SCH 23390 inhibit amphetamine-induced rotational behavior

Rats were given unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway and permanent indwelling cannula were surgically implanted into the non-lesioned side of the brain; cannula were used for direct injections of dopamine antagonists into the pars reticulata region of the non-lesioned substantia nigra. The selective D1 receptor antagonist, SCH 23390, was injected intranigrally at various concentrations (3.0, 1.5, 1.0, 0.6, or 0.3 mM) just prior to an intraperitoneal injection of amphetamine. SCH 23390 dose-dependently inhibited amphetamine-induced rotational behavior with the highest doses completely blocking rotational behavior in some animals. An intranigral injection of the selective D2 receptor antagonist, (-)-sulpiride (1.0 mM), did not produce a significant reduction in amphetamine-induced rotational behavior whereas an equivalent molar concentration of SCH 23390 (1.0 mM) produced a significant 62% reduction in amphetamine-induced rotational behavior. A concentration of SCH 23390 that produced a 50% reduction in rotational behavior when injected directly into the substantia nigra was unable to produce a significant reduction in rotational behavior when injected directly into the striatum. The effects of intranigral injections of SCH 23390 on apomorphine-induced rotational behavior were directly opposite to that observed for amphetamine-induced rotational behavior; contralateral rotational behavior increased relative to baseline measures. These data support the hypothesis that dopamine release in the midbrain may act as a neuromodulator of motor behavior, and that D1 receptors play a functional role in this process.

Heterogeneous distribution of D1, D2 and D5 receptor mRNAs in monkey striatum

The primate striatum has a compartmental organization reflected both in the topography of its afferent projections and in the segregation of its morphologically similar but neurochemically distinct efferent neurons. Discretely projecting mesostriatal neurons release dopamine (DA) which modulates the responses of striatal neurons to other afferent inputs. Multiple DA receptor (DAR) subtypes have been cloned and characterized and mapping their cellular expression is crucial for understanding the influence of DA on striatal function. We report the distribution of mRNAs for D1, D2 and D5 DAR subtypes (D2R, D2R and D5R) in the striatum of cynomolgus monkeys (Macaca fascicularis) studied by in situ hybridization histochemistry (ISH) using monkey-specific cRNA probes. Adjacent sections were stained for calbindin immunoreactivity to distinguish striosomal and matrix compartments for comparison with the patterns obtained with ISH. In the caudate nucleus, D1R mRNA was concentrated in calbindin-poor striosomes where dense grain clusters were seen overlying the majority of medium-sized neurons (diameter approximately 15 microns). D1R mRNA localization was relatively homogeneous in the putamen. By contrast, the distributions of D2R and D5R mRNAs showed no clear preference for the striosomal or matrix compartments of either caudate nucleus or putamen. In the ventral striatum (nucleus accumbens, olfactory tubercle and ventral portions of caudate nucleus and putamen), expression of D1R and D2R mRNA was sparse relative to dorsal striatum, while D5R mRNA expression was roughly equal in ventral and dorsal striatum. Circumscribed zones of hybridization associated with islands of tightly packed small cells occurred with all three DAR mRNA subtypes in the ventral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in striatal and extrastriatal D-1 and D-2 dopamine receptors in the MPTP-treated common marmoset: an autoradiographic study

In adult common marmosets (Callithrix jacchus), MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) treatment induced almost total depletion of cells in the substantia nigra pars compacts (SNc) but partial cell loss in the ventral tegmental area (VTA). There was severe depletion of [3H]-mazindol binding to dopamine (DA) uptake sites in the caudate, putamen, and SNc. The loss of [3H]-mazindol binding in the nucleus accumbens (NAc) and olfactory tubercle (OT) was less marked. [3H]-mazindol binding in the body of caudate nucleus showed a small but significant recovery with increasing post-lesion survival times. The specific binding of [3H]-SCH 23390 to D-1 DA receptor sites was increased after MPTP treatment in all subregions of both caudate and putamen but was unaltered in the NAc and OT. Substantia nigra pars reticulata (SNr), frontal cortex, and medial segment of globus pallidus (GPm) all demonstrated moderate levels of [3H]-SCH 23390 binding in control animals, which were unaffected by MPTP treatment. Specific [3H]-spiperone binding to D-2 DA receptor sites was not altered by MPTP treatment in the subregions of caudate-putamen. Moderate levels of [3H]-spiperone binding were observed in control animals in the NAc, OT, SNc, and the lateral segment of globus pallidus (GP1). [3H]-spiperone binding in the SNc and OT was partially decreased in MPTP-treated animals. The changes in specific [3H]-spiperone and [3H]-SCH 23390 binding induced by MPTP-treatment did not alter with post-lesion survival times. These results demonstrate that MPTP treatment causes greater dopaminergic denervation of the caudate-putamen than in NAc/OT. This resulted in an increase in postsynaptic D-1 DA receptor sites in the caudate-putamen but not in the NAc/OT. Also, there appeared to be loss of presynaptic D-2 DA receptor sites in the SNc and OT. In the caudate-putamen, the loss of presynaptic D-2 DA receptor sites may have masked postsynaptic D-2 DA receptor upregulation.

Functional heterogeneity of the caudate-putamen as revealed by c-fos induction in response to D1 receptor activation

SKF 38,393, a dopamine receptor agonist which stimulates the D1 receptor, has been shown to activate c-fos to a detectable level in rats only when the striatum has been sensitized by a previous 6-OHDA lesion of the nigrostriatal tract. In the present study, we reexamine the effect of SKF 38,393 by in situ hybridization in non-lesioned mice and show an induction of c-fos mRNA which is restricted to the ventricular edge and to the caudal part of the caudoputamen (CP) even with large doses of SKF 38,393. These results suggest a functional heterogeneity in the CP in response to dopaminergic drugs, which could reflect the anatomical or neurochemical heterogeneity of the striatum itself and its neuroanatomical connections.

Compartmental origin of the striato-entopeduncular projection in the rat

The mammalian neostriatum is divisible into neurochemically and cytoarchitectonically distinct striosome and matrix compartments. This compartmentalization is respected by many afferent and efferent projections of the striatum. The distribution of distinct types of neuroactive substances and receptors and the unique connections of the striosome and matrix suggest a functional segregation between these compartments. The present study examines the organization of efferent projections from each of the striatal compartments to the entopeduncular nucleus (EPN), a major output center of the basal ganglia. The fluorescent retrograde tracer fluorogold, or rhodamine-conjugated dextran, was injected into the lateral habenula or the ventrolateral nucleus of the thalamus of adult Wistar rats to identify the topographical organization of EPN-habenular and EPN-thalamic neurons. Fluorogold was then placed into the rostral or caudal parts of the EPN, identified from the previous experiment as areas containing predominantly EPN-habenular or EPN-thalamic neurons, respectively. Sections containing retrogradely labeled neurons in the neostriatum were simultaneously immunolabeled for calbindin-D28kDa, a calcium-binding protein found exclusively in the projection neurons of the matrix. The results indicate that the striatal projection to the EPN-habenular and EPN-thalamic parts of the EPN originates from striosome and matrix neurons, respectively. The duality of striatal outflow involving the EPN suggests a mechanism whereby the striosome is integrated into subcortical pathways that modulate the activity of the basal ganglia via the ascending serotoninergic projection from the dorsal raphe nucleus, whereas the matrix is involved in a loop that includes the thalamus and the cerebral cortex.

Compartmentalization of parvalbumin immunoreactivity in the human striatum

The distribution of parvalbumin in the basal ganglia was studied in eight human brains using immunohistochemical techniques and the pattern of staining was compared to the distribution of enkephalin immunoreactivity in adjacent sections. The results showed a heterogeneous pattern of parvalbumin immunoreactivity in the caudate nucleus and putamen; this pattern of staining was characterized by irregularly shaped patches of low parvalbumin immunoreactivity dispersed against a background matrix of moderate-to-high immunoreactive staining. These parvalbumin-poor patches in the caudate nucleus and putamen aligned with the enkephalin-rich striosomes. These results show that parvalbumin immunoreactivity in the human striatum has the same compartmental mosaic organization as other neurochemical markers in the mammalian striatum.

Binding of [3H]SCH 39166 to human post mortem brain tissue

The dopamine D1 antagonist SCH 39166 was labelled with tritium and used for in vitro binding and autoradiography using human post mortem brain tissue. Competition studies on tissue from human nucleus caudatus showed that SCH 23390 inhibited the binding of [3H]SCH 39166 biphasically. The non-specific binding of [3H]SCH 39166 in both nucleus caudatus and cerebellum was lower after the addition of SCH 23390 or SCH 39166 than after flupentixol (10 microM). Autoradiography showed specific [3H]SCH 39166 binding in the caudate nucleus and putamen in the brain sections. The binding of [3H]SCH 39166 in the medial part of the caudate nucleus was very dense and similar to that obtained with [3H]SCH 23390, which was used as a reference ligand. Dense binding of [3H]SCH 39166 was also found in cortical regions, and binding was also obtained in the cerebellum and in the hippocampus. Addition of flupentixol (10 microM) abolished some but not all the binding of [3H]SCH 39166. The binding of [3H]SCH 39166 to caudate and putamen was not totally abolished by the addition of excess SCH 23390, while excess SCH 39166 diminished the binding of [3H]SCH 23390 in all regions. The present study indicates that [3H]SCH 39166, similar to [3H]SCH 23390, binds to dopamine D1 receptors in the human brain. It is concluded that [3H]SCH 39166 has a slightly different binding pattern than [3H]SCH 23390, which can be due to labelling of one or two additional binding site(s) pharmacologically unrelated to dopamine D1 receptors.

Heterogeneous distribution of dopamine D1 and D2 receptors in the human ventral striatum

The distribution of dopamine D1 and D2 receptors was examined in the human ventral striatum using in vitro quantitative autoradiography. Both D1 and D2 receptors have a markedly heterogeneous distribution, that includes regional differences in binding for the D1 receptor and a pattern of smaller heterogeneities for both receptors. The latter heterogeneities in D1 and D2 binding are interrelated and, in addition, appear to be related to inhomogeneities in the acetylcholinesterase histochemistry and cellular density of the ventral striatum. The present data indicate that in the human ventral striatum the ratio between D1 and D2 receptors varies widely from one area to another.

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. III. Results in Parkinson's disease cases

The distribution and number of DA uptake sites, DA receptors (D1 and D2) and cholinergic muscarinic receptors (M1 and M2) were examined by autoradiography in the striatal complex of Parkinson's and age-matched control cases. The greatest loss of DA uptake sites occurred in the dorsolateral striatum which was the only region showing an increase in D2 receptors. The number of M2 receptors was reduced in the dorsolateral striatum and M1 receptors were reduced in most regions of the striatum. The anatomical pattern of changes in DA uptake sites, D2 receptors and M2 receptors suggests a coordinated change in the dopaminergic-cholinergic interneuron synapse in Parkinson's disease.

The dopamine hypothesis of schizophrenia: limbic interactions with serotonin and norepinephrine

The "dopamine hypothesis" of schizophrenia has been the predominant guiding theoretical construct for driving studies of the neurobiology of schizophrenia. There has, however, been much interest in the contributions of non-dopamine systems to the clinical symptoms of schizophrenia, in particular, norepinephrine and serotonin. However, direct evidence for altered transmission in monoamine systems has been quite limited. In part this reflects a focus on specific brain regions for different transmitters, in contrast to a "neural systems" approach. Thus, evidence for the dopamine hypothesis has been derived from studies of the basal ganglia in schizophrenic cases and infrequently from other (e.g. cortical) regions. Recent studies have suggested that disturbances in the organization or development of the temporal lobe may underlie certain aspects of the symptoms of schizophrenia In particular, the hippocampus may show cellular loss or disturbances in cell orientation. These results are supported by the work that has identified neuropsychological and in vivo brain disturbances in schizophrenia specific to the medial temporal lobe. However, not all cases show such pathology and it is likely that these disorders could, in addition, involve an important afferent and/or efferent system associated with the temporal lobe. This model is based on the currently held view that parallel cortico-striatal-pallidal-thalamo circuits form an important basis for information processing in the brain. One such circuit involves the primary efferent of the hippocampus, the subiculum, and associated cortical regions that project onto the ventral striatum. Many of the cortical regions that project directly to the ventral striatum also project to the hippocampus via the entorhinal cortex. These include the anterior cingulate, posterior cingulate, superior temporal cortex, and inferior temporal cortex. The ventral striatum, made up of the nucleus accumbens, olfactory tubercle, and ventral putamen, has as its target the ventral pallidum. The ventral pallidum projects to the medial dorsal nuclei of the thalamus, which, in turn, projects to the anterior prefrontal cortical area. This loop has been referred to as the limbic loop. The patterns of innervation and expression of monoamine receptors in the brain have been delineated for the non-human primate and are being unraveled in the human. We, and other, have described the patterns of receptor expression in the limbic circuit. However, few studies have been published to date that detail what the neurochemical counterparts of the neuronal and neuropsychological disturbances in the limbic circuit might be. We have explored the possibility that monoamine systems are altered at more than one synaptic station in this circuit.(ABSTRACT TRUNCATED AT 400 WORDS)

Day/night differences in D1 but not D2 DA receptor protection from EEDQ denaturation in rats treated with continuous cocaine

The effect of chronic cocaine administration on the in vivo occupation of dopamine (DA) receptor subtypes was examined using the irreversible receptor blocker N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Rats were given continuous infusions of cocaine (vehicle, 2.5, 7.5, or 22.5 mg/day) via subcutaneous implants of Alzet osmotic minipumps for 14 days. Some groups were also given the D1 antagonist SCH 23390 and/or the D2 antagonist raclopride for this same time period. DA receptor binding techniques were used 24 hours post-EEDQ injection (Day 15, 5 mg/kg, intraperitoneally [ip]) in order to examine changes in D1 and D2 receptor densities in the striatum. Half of the rats were killed in the day with the other half killed at night in order to examine day/night differences in the effects of cocaine treatment. Results showed that chronic cocaine increased the protection of D1 receptors from EEDQ inactivation in a dose-dependent fashion during the day, and decreased D1 protection from EEDQ at night. Since EEDQ has a low affinity for the DA receptor relative to endogenous DA or the exogenous ligands in this study, only receptors that are vacant are inactivated thereby allowing for an estimate of DA receptor occupation in vivo. Cocaine can therefore be said to increase D1 receptor occupation by DA in vivo during the day and decrease it at night. Coadministration of the DA antagonists eliminated this cocaine-induced day/night difference and, in the case of the D1 antagonist, produced opposite D1 receptor effects when administered alone. Chronic SCH 23390 treatment protected D1 receptors from EEDQ denaturation while D2 receptors were protected by chronic raclopride. In addition, raclopride was found to affect the affinity of both the D1 and the D2 receptors to the [3H] SCH 23390 and [3H] spiperone ligands, respectively. Since no day/night differences were found in D2 receptor density with respect to chronic cocaine treatment these findings have implications for a phasic D1/tonic D2 receptor hypothesis such that cocaine treatment selectively alters the level of DA at sites containing D1 receptors with differential effects depending on the day/night cycle.

A comparison of behaviour following stimulation of the anterior substantia nigra by direct cholinergic agonists and anticholinesterases

Microinjections of carbachol, a muscarinic cholinergic receptor agonist, into the anterior substantia nigra increase feeding, drinking and sexual behaviour if there is a pre-existing tendency to respond and a low baseline rate of behaviour. The present experiment was undertaken to compare the effects of carbachol with other cholinergic stimulants. Groups of 6-12 satiated rats received 0.5 microliter microinjections into the anterior substantia nigra of 0.1-5.0 micrograms carbachol, 0.1-5.0 micrograms nicotine, 2.5-10.0 micrograms eserine, and 1.25-5.0 micrograms or 0.1-1.0 microgram neostigmine (each dissolved in sterile saline) and the effects on feeding, drinking, locomotion, grooming, rearing and sniffing were examined. Carbachol, nicotine and low doses of neostigmine stimulated eating in a dose-dependent manner. The increased feeding following neostigmine was over a shorter time-period than following carbachol or nicotine. Neither carbachol nor nicotine had any significant effect on behaviour other than eating. The higher doses of neostigmine increased the frequency of sniffing and rearing, but not eating, and no dose of eserine had a clear effect on behaviour. These data are discussed in terms of their relationship to the cholinergic input to substantia nigra which excites pars compacta dopamine-containing neurones.

Functional heterogeneity of the matrix compartment in the cat caudate nucleus as demonstrated by the cholinergic presynaptic regulation of dopamine release

Previously, using a new in vitro microsuperfusion procedure, we have demonstrated marked differences in the cholinergic presynaptic regulation of the release of [3H]dopamine continuously synthesized from [3H]tyrosine in two close striosomal- and matrix-enriched areas of the cat caudate nucleus. A tetrodotoxin-resistant stimulatory effect of acetylcholine mediated by muscarinic receptors was observed in both compartments. However, in addition, two opposing types of tetrodotoxin-sensitive acetylcholine-evoked regulation of [3H]dopamine release were only seen in the matrix: one facilitatory, involving nicotinic receptors located on as yet unidentified neurons, and the other inhibitory, mediated by muscarinic receptors located on dynorphin-containing neurons. In the present study, using the same approach, a functional heterogeneity was demonstrated in the matrix. Indeed, in various conditions the effects of acetylcholine (50 microM) on the release of [3H]dopamine were different in a matrix-enriched area (matrix 2) distinct from that previously investigated (matrix 1); these areas being characterized by the presence or absence of islands of striatonigral cells, respectively. As in matrix 1, acetylcholine induced a short-lasting stimulation of [3H]dopamine release in matrix 2 but, in contrast to that observed in matrix 1, the acetylcholine-evoked response in matrix 2 was not modified in the presence of tetrodotoxin (1 microM). Experiments made in the presence of the tetrodotoxin and atropine (1 microM) indicated that both muscarinic and nicotinic receptors are located on dopaminergic nerve terminals in matrix 2 while muscarinic receptors are only present in matrix 1. In the absence of tetrodotoxin, the short-lasting stimulation of [3H]dopamine release was transformed into a long-lasting response in the presence of pempidine (50 microM), in matrix 2 but not in matrix 1 while prolonged responses were seen in both matrix areas in the presence of atropine. Finally, the acetylcholine short stimulatory effect on [3H]dopamine release was transformed into a long stimulatory response in the presence of bicuculline (50 microM) but not naloxone (1 microM) in matrix 2 while the reverse was observed in matrix 1. By providing further evidence for a functional heterogeneity of the matrix, our results suggest that depending on the matrix area investigated, dynorphin- or GABA-containing neurons are involved in the indirect cholinergic inhibitory control of dopamine release.

5'-nucleotidase: a new marker for striosomal organization in the rat caudoputamen

The distribution of the adenosine-producing ectoenzyme 5'-nucleotidase was studied by means of a histochemical lead technique in the caudoputamen of normal adult rats and of rats in which injections either of 6-hydroxydopamine in the medial forebrain bundle or of ibotenic acid in the caudoputamen had been made 1-3 weeks previously. The patterns of striatal 5'-nucleotidase activity in these animals were compared in serial sections to the patterns of calbindin-D28k immunoreactivity and of 3H-naloxone ligand binding, which respectively mark the known matrix and striosome (patch) compartments of the caudoputamen. In the normal rats, 5'-nucleotidase activity was differentially concentrated in striosomes, where it produced a dense staining of the neuropil. The enzymatic staining followed a striosomal distribution in all but the caudal caudoputamen. Within the striatal matrix, 5'-nucleotidase staining also observed a lateromedial density gradient. Depletion of the dopamine-containing nigrostriatal innervation of the caudoputamen with 6-hydroxydopamine did not alter the striosomal selectivity of 5'-nucleotidase activity. Destruction of intrastriatal neurons by ibotenic acid led to a strongly 5'-nucleotidase-positive gliosis within the resulting necrotic region. Elsewhere in the caudoputamen, the enzyme's striosomal distribution was not detectably altered. We conclude that 5'-nucleotidase histochemistry provides an advantageous tool for detecting the striosomal architecture of the rat's caudoputamen. Moreover, 5'-nucleotidase is prominently associated with glial membranes in the central nervous system, so that the concentration of this enzyme in striosomes could mark these as sites of selective glial populations within striatum. These properties and actions of 5'-nucleotidase in purinergic neurotransmission and in neuroadhesion may contribute to the specialized functions of striosomes and matrix.

Compartmentalization of excitatory amino acid receptors in human striatum

Division of the mammalian neostriatum into two intermingled compartments called striosomes and matrix has been established by analysis of enzyme activity, neuropeptide distribution, nucleic acid hybridization, and neurotransmitter receptor binding. Striosomes and matrix are distinct with respect to afferent and efferent connections, and these regions provide the potential for modulation and integration of information flow within basal ganglia circuitry. The primary neurotransmitters of corticostriatal afferents are excitatory amino acids, but to date no correlation of excitatory amino acid receptors and striatal compartments has been described. We examined binding to the three pharmacologically distinct ionotropic excitatory amino acid receptors, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate, in human striatum using in vitro receptor autoradiography and compared the binding to striosomes and matrix histochemically defined by acetylcholinesterase activity. Our findings reveal increased binding to N-methyl-D-aspartate receptors and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in matrix relative to striosomes and increased kainate receptor binding in striosomes relative to matrix. These results suggest that afferent input to the two striatal compartments may be mediated by pharmacologically distinct excitatory amino acid receptor subtypes.

Efferent connections of the centromedian and parafascicular thalamic nuclei in the squirrel monkey: a light and electron microscopic study of the thalamostriatal projection in relation to striatal heterogeneity

The organization of the thalamostriatal projections arising from the centromedian (CM) and parafascicular (Pf) thalamic nuclei in the squirrel monkey (Saimiri sciureus) was studied at both light and electron microscopic levels. Following selective injections of the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into the CM or Pf, patterns of terminal arborization within the striatum were compared to the biochemical heterogeneity of the striatum as revealed by immunohistochemical staining for the calcium-binding protein calbindin D-28k (CaBP), and histochemical staining for the enzymes acetylcholinesterase (AChE) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase). The PHA-L-labeled axon terminals within the striatum were further analyzed at the ultrastructural level to characterize their pattern of synaptic organization. Dense and heterogeneous terminal fields occur in the "sensorimotor" territory of the striatum after CM injections, or in the "associative" striatal territory following Pf injections. In the associative territory labeled axons arborize in a diffuse manner predominantly within areas enriched with CaBP, AChE, or NADPH-diaphorase, representing the matrix compartment, and tend to avoid areas poor in these substances, corresponding to the patch/striosome compartment. In the sensorimotor territory labeled axons form bands that occupy a subregion of the NADPH-diaphorase-rich zone in the putamen. The terminal pattern of the CM-striatal projection suggests the existence of a more complex mosaic organization within the sensorimotor territory. Ultrastructural analysis of PHA-L-labeled elements within the striatum reveals that both CM and Pf projections form asymmetric synapses upon dendrites and spines of striatal cells. A total of 339 PHA-L-labeled boutons were examined after CM injections and compared to 293 boutons following Pf injections. After CM injections, 29% of PHA-L-labeled terminals form synapses on dendritic spines and 66% on dendritic shafts, whereas after Pf injections only 12% of synapses occur on dendritic spines compared to 81% on dendritic shafts. Labeled terminals forming axosomatic or axoaxonic synapses were not seen within the striatum following either CM or Pf injections. It is concluded that in the squirrel monkey: 1) Pf-striatal fibers profusely arborize within the matrix compartment of the associative territory, 2) CM-striatal fibers form bands that occupy a subregion of the NADPH-diaphorase-rich zone within the sensorimotor territory, and 3) that both Pf- and CM-striatal projections establish asymmetric synapses with dendrites and spines of medium-sized spiny cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Within-subject sensitization of striatal dopamine release after a single injection of cocaine: an in vivo microdialysis study

Repeated microdialysis measurements, conducted 1 week apart at the same tissue site, were used to investigate the changes in basal and cocaine-stimulated extracellular dopamine (DA) levels after a single prior exposure to either saline or cocaine. Dialysis probes were placed into rats previously implanted with guide cannulas and basal levels of dopamine (DA), and its metabolites (DOPAC and HVA) were estimated in 20-min fractions of the dialysate. Basal levels in the extracellular fluid (ECF), estimated from pre-implantation calibration of the probes, were 7.9 +/- 0.7 nM DA, 4.9 +/- 0.8 microM DOPAC, and 3.6 +/- 0.6 microM HVA. After a stable baseline was obtained saline (1 ml/kg, i.p.) or cocaine (20 mg/kg, i.p.) was injected. Saline produced no significant changes in any of the neurochemical markers. A cocaine injection produced a sixfold increase in DA, while DOPAC and HVA were unchanged. One week later the same procedure was repeated except this time both groups received cocaine. In rats that had received cocaine 1 week earlier, basal DA levels in the ECF were doubled, whereas they were unchanged in rats that received saline a week earlier. Furthermore, the dopamine release in response to acute cocaine during the second week was elevated in animals which had been previously exposed to cocaine. Rotation was also measured during both weeks and, while a tendency toward behavioral sensitization was observed, it did not reach significance.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Dopamine D1 receptors in the sub-commissural part of the globus pallidus and their role in oro-facial dyskinesia in cats

The possible role of dopamine D1 receptors in the sub-commissural part of the globus pallidus in the induction of oro-facial dyskinesia was studied in cats. The present study reveals two findings. Firstly, bilateral injections of the D1 agonist (+/-)-SK& F38393 into the ventral pallidal area elicited oro-facial dyskinesia, which was quantified in terms of numbers of tongue protrusions. The results show that the dose-effect curve was bell-shaped (1.0, 1.75, 2.5, 5.0 micrograms/0.5 microliters (+/-)-SK&F38393). The oro-facial dyskinesia elicited by (+/-)-SK&F38393 was highly comparable to the oro-facial dyskinesia elicited by injections of the GABA antagonist picrotoxin or the acetylcholine agonist carbachol into the sub-commissural part of the globus pallidus. Secondly, the inactive enantiomer of SK&F38393, i.e. S(-)-SK&F38393, was found to be ineffective in eliciting oro-facial dyskinesia when injected in a dose equivalent to 50% of the most effective dose of the racemic mixture of (+/-)-SK&F38393. Furthermore, the effect elicited by 2.5 micrograms/0.5 microliters (+/-)-SK&F38393 was significantly attenuated by local injection of the D1 antagonist R(+)-SCH23390 in a dose which had no effect itself (1.0 micrograms/0.5 microliters). These findings indicate that the effects elicited by (+/-)-SK&F38393 are D1-specific. The present results thus clearly indicate that dopamine D1 receptors within the sub-commissural part of the globus pallidus are involved in mediating oro-facial dyskinesia in cats.

Expression of the immediate early gene c-fos in basal ganglia: induction by dopaminergic drugs

Expression of the immediate early gene c-fos is increased in mammalian neurons by a number of stimuli and the usefulness of this gene as a marker of neuronal activation has been demonstrated in several systems. Directly-acting dopamine agonists of the D1-type (SKF 38393, CY 208-243) and indirectly-acting dopamine agonists (amphetamine, cocaine) all produce a rapid and transient increase in Fos protein levels in varying patterns in striatum and cerebral cortex. Directly-acting dopamine agonists only produce c-fos activation in denervated (supersensitive) striatum whereas cocaine and amphetamine activate c-fos in striatum in naive animals. Remarkably, D2 selective antagonists such as haloperidol, albeit in high doses, also activate c-fos expression. Activation of c-fos and other immediate early genes may play a part in the development of such long-term dopamine-related effects as dyskinetic movements and addiction.

Striatal c-fos induction by drugs and stress in neonatally dopamine-depleted rats given nigral transplants: importance of NMDA activation and relevance to sensitization phenomena

Induction of the proto-oncogene c-fos occurred in cells of the striatum in response to stimuli that are known to release dopamine (DA). As revealed by fos immunocytochemistry, amphetamine (AMPH) produced c-fos induction in many cells of the medial two-thirds of the striatum of normal rats, with patchy labeling in the lateral third. The lateral patches were found to be coincident with patches of striatal neurons lacking calbindin immunoreactivity. In animals DA-depleted at birth, few fos-immunoreactive neurons were present in response to AMPH. In those with unilateral transplants of DA-rich mesencephalic tissue, c-fos induction was greater on the transplanted side. The DA D1 antagonist SCH 23390 completely blocked c-fos induction in all animals. The N-methyl-D-aspartate antagonist MK-801 also blocked c-fos induction by AMPH within the medial striatum, but intensified c-fos induction laterally in those animals with DA innervation. A second set of experiments examined the functional importance of c-fos induction in the AMPH sensitization of turning behavior that occurs in these animals. Both AMPH and stress produced turning, but only AMPH produced widespread c-fos induction, and stress-induced turning only occurred after exposure to AMPH. Treatment with MK-801 prior to AMPH administration blocked the subsequent development of stress-induced turning. Whereas a high dose of MK-801 (1.0 mg/kg) completely blocked c-fos induction, a lower dose (0.1 mg/kg) blocked c-fos induction in controls, but left patches of fos-immunoreactive neurons in lesioned animals given transplants. Thus the sensitization of transplant-related behaviors is NMDA dependent and associated with c-fos induction in host striatal neurons.

Pharmacologically defined M1 and M2 muscarinic cholinergic binding sites in the cat's substantia nigra: development and maturity

Muscarinic cholinergic binding in the substantia nigra of the cat was documented during development and at maturity with autoradiographic methods by labeling the pharmacologically defined M1 and M2 subtypes of muscarinic binding sites. In cats from age embryonic day 40 to postnatal day 6 and at adulthood, M1 sites were labeled with [3H]pirenzepine and M2 sites were labeled with [3H]N-methylscopolamine in competition with pirenzepine. Comparisons were made among binding site distributions, acetylcholinesterase staining and tyrosine hydroxylase-like immunoreactivity in serial or neighboring nigral tissue sections. M1 and M2 binding sites were present in the substantia nigra at all ages studied. Qualitative comparisons showed that M1 binding delineated the substantia nigra more distinctly than did M2 binding. For M1 binding sites in particular, the embryonic pars reticulata of the substantia nigra was more prominently labeled than the pars compacta. At adulthood both nigral subdivisions clearly exhibited M1 and M2 binding, with the pars compacta demonstrating some internal heterogeneity of binding density. These findings provide further evidence that the substantia nigra is a site of cholinergic transmission and suggest that the functional balance between acetylcholine and dopamine in the basal ganglia acts here as well as in the striatum.

The amphetamine conditioned place preference: differential involvement of dopamine receptor subtypes and two dopaminergic terminal areas

We investigated involvement of dopamine receptor subtypes and two dopaminergic terminal areas in the acquisition and the expression of the amphetamine conditioned place preference (CPP). When injected systemically before conditioning, both D1 and D2 dopamine antagonists blocked acquisition in a dose-dependent manner. When injected systemically before testing, the effects of the same D1 and D2 antagonists differed. The selective D1 antagonist SCH23390 dose-dependently blocked expression of the previously established conditioned behavior within the dose range that also blocked acquisition. In contrast, D2 antagonists failed to block expression of the amphetamine CPP at doses which blocked acquisition. Expression was, however, blocked by higher doses of D2 antagonists, which may have lost their selectivity for the D2 dopamine receptor. The expression of the CPP was also blocked by microinjections of SCH23390 or sulpiride into nucleus accumbens, but not into striatum. In a control experiment, sodium pentobarbital, which significantly reduced spontaneous locomotor activity in a manner similar to the higher doses of the dopamine antagonists, had no effect on the expression of the amphetamine CPP when given before testing. Finally, electrolytic lesions of the dorsal striatum potentiated the amphetamine CPP. These findings indicate that the dopamine released by amphetamine interacts with both D1 and D2 dopamine receptors to establish a CPP, but that the expression of the CPP may involve activation of the D1 dopamine receptor in the nucleus accumbens.

Dopamine D1 receptor behavioral responsitivity following selective lesions of the striatal patch compartment during development

The behavioral effects of selective destruction of the dopamine (DA) input to the patch compartment of rat striatum early in development was investigated. Rat pups were given bilateral intrastriatal (i.s.) injections of the neurotoxin 6-hydroxydopamine (6-OHDA) on day of birth (P0) or postnatal day 1 (P1), which resulted in selective behavioral alterations following DA agonist treatment in adulthood. Neonatally-lesioned rats exhibited self-biting behavior following treatment with the DA precursor L-dihydroxyphenylalanine (L-DOPA). In response to treatment with the selective D1 agonist SKF38393, there was an increased incidence of abnormal perioral movements. The cataleptogenic effects of the D1 antagonist SCH23390 and the D2 antagonist haloperidol were also studied. Neonatally-lesioned rats were significantly less cataleptic compared to control rats following D1 antagonist treatment, but not following D2 antagonist treatment. Autoradiographs of [3H]mazindol binding to DA uptake sites (a measure of DA terminal density) showed a 'patchy' loss of approx. 40-50% in striatal tissue sections derived from the i.s. lesioned rats. These data suggest that injections of 6-OHDA into the striatum during this early postnatal period cause a DA lesion that results in long-term effects on a D1 receptor system.

Acute monoaminergic depletion in the rat potentiates the excitatory effect of the subthalamic nucleus in the substantia nigra pars reticulata but not in the pallidal complex

Recent neurochemical evidence suggests that chemical or electrical stimulation of the subthalamic nucleus (STH) increases dopamine release in the substantia nigra (SN) with a subsequent decrease in the striatum. In a previous paper, we reported that bicuculline-induced activation of the STH increases neuronal activity in the substantia nigra pars reticulata (SNpr) and in the pallidal complex. In order to investigate the role played by the dopaminergic system in the observed activation, the neuronal responses of subthalamic nucleus target structures were studied in amine depleted rats following subthalamic stimulation. Amine depletion was accomplished by pretreating the rats with reserpine (2 mg/kg; S.C.) and with alpha-methyl-para-tyrosine (alpha-mpt; 50 mg/kg; I.P.). Following this treatment, dopamine levels were reduced by 94% in the striatum as measured by HPLC. Amine depletion significantly increased the spontaneous activity of subthalamic cells by 53%. In the SNpr, no significant changes in the spontaneous neuronal activity were observed, but the excitatory responses to bicuculline-induced stimulation of the STH were potentiated as compared to non-treated animals. In the pallidial complex (GP-EP), no potentiation was found. The data suggest that the spontaneous pattern of discharge of the STH is probably under monoaminergic control. They also suggest a reciprocal interaction between dopamine and glutamatergic afferent terminals from the STH within the SNpr, but not in the pallidal complex.

Multiple fluorescent ligands for dopamine receptors. II. Visualization in neural tissues

Selective dopamine receptor ligands, (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4, 5-tetrahydro-3-methyl-[1H]-3-benzazepin-7-ol, the 4'-amino derivative of the high affinity D1 receptor antagonist SCH 23390, the high affinity D2 receptor antagonist N-(p-aminophenethyl)-spiperone or NAPS, and the D2 selective agonist, 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin or PPHT were chemically coupled to the fluorescent compounds, Bodipy, Cascade blue, coumarin, fluorescein, rhodamine, or Texas red. The utility of the 6 fluorescent moieties linked to the 3 dopamine receptor binding ligands for anatomical study of regional and cellular distribution patterns of the two dopaminergic receptor subtypes has been assessed in frozen sections of the rat striatum and compared to our previous report using the rhodamine-labeled antagonists. The regional staining for the two dopaminergic receptor binding sites supports previous work using in vitro receptor autoradiographic analyses; the D1 receptor binding was more robust than that of D2 receptors in the caudate nucleus. The cellular element which most frequently expressed striatal D1 binding sites had a medium-diameter cell body. Medium-sized cells also exhibited fluorescence for the D2 binding site, as did a much larger diameter element; potentially the cholinergic interneuron of the caudate nucleus. The pharmacological specificity for each of the different D1 fluorescent antagonist ligands in the tissues was determined by competition with 100-fold excess of unlabeled SCH 23390 (non-specific binding), spiroperidol (binding selectivity), the stereoactive paired isomers of butaclamol, and the serotonin 5-HT2 receptor antagonist ketanserin. The same criteria were used to assess the different D2 fluorescent agonist and antagonist ligand derivatives. The anatomical efficacy of these novel ligands was determined using selective dichroic filters to stimulate the fluorescent moieties in the optimal excitation wavelength, and the amount of fluorescent dopamine receptor binding was photographically measured and contrasted for each of the newly synthesized fluoroprobes. Using the most pharmacologically specific and anatomically efficient of these novel fluoroprobes, we determined the localization pattern of the D1 and D2 dopamine receptor binding sites in tissues reported to exhibit both subtypes of the receptor. The cellular distribution of the dopamine receptor binding sites was determined concurrently using fluoroprobes in the forebrain, mesencephalon, pituitary, retina, and superior cervical ganglion of the rodent, and bovine adrenal medullary chromaffin cells were examined using the rhodamine-labeled antagonists.

Cloning of the gene for a human dopamine D5 receptor with higher affinity for dopamine than D1

Dopamine receptors belong to a superfamily of receptors that exert their biological effects through guanine nucleotide-binding (G) proteins. Two main dopamine receptor subtypes have been identified, D1 and D2, which differ in their pharmacological and biochemical characteristics. D1 stimulates adenylyl cyclase activity, whereas D2 inhibits it. Both receptors are primary targets for drugs used to treat many psychomotor diseases, including Parkinson's disease and schizophrenia. Whereas the dopamine D1 receptor has been cloned, biochemical and behavioural data indicate that dopamine D1-like receptors exist which either are not linked to adenylyl cyclase or display different pharmacological activities. We report here the cloning of a gene encoding a 477-amino-acid protein with strong homology to the cloned D1 receptor. The receptor, called D5, binds drugs with a pharmacological profile similar to that of the cloned D1 receptor, but displays a 10-fold higher affinity for the endogenous agonist, dopamine. As with D1, the dopamine D5 receptor stimulates adenylyl cyclase activity. Northern blot and in situ hybridization analyses reveal that the receptor is neuron-specific, localized primarily within limbic regions of the brain; no messenger RNA was detected in kidney, liver, heart or parathyroid gland. The existence of a dopamine D1-like receptor with these characteristics had not been predicted and may represent an alternative pathway for dopamine-mediated events and regulation of D2 receptor activity.

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.

Subpopulations of mesencephalic dopaminergic neurons express different levels of tyrosine hydroxylase messenger RNA

Subpopulations of mesencephalic dopamine containing neurons possess different electrophysiological, pharmacological, biochemical, and anatomical properties. In order to determine whether such differences are related to the regulation of tyrosine hydroxylase, the rate limiting enzyme in the synthesis of catecholamines, the regional distribution of tyrosine hydroxylase messenger RNA in these neurons was examined using in situ hybridization histochemistry. In the mouse, labelling for tyrosine hydroxylase messenger RNA associated with individual neurons was significantly less in the lateral substantia nigra pars compacta than in the medial substantia nigra pars compacta and the ventral tegmental area. A similar pattern of labelling was observed in the rat. Labelling for tyrosine hydroxylase messenger RNA was significantly less in the lateral substantia nigra pars compacta than in medial pars compacta (a densely cellular region), the area dorsal to the medial substantia nigra pars compacta (a less cell dense region), and the ventral tegmental area. Differences in levels of labelling for messenger RNA in mesencephalic dopamine neurons were not related to differences in cell size as measured in sections processed for tyrosine hydroxylase immunohistochemistry. The results suggest that tyrosine hydroxylase messenger RNA is differentially regulated in subpopulations of mesencephalic dopamine neurons, supporting the view that these neurons are physiologically distinct.

Comparative development of D1-dopamine and mu opiate receptors in normal and in 6-hydroxydopamine-lesioned neonatal rat striatum: dopaminergic fibers regulate mu but not D1 receptor distribution

The postnatal development of D1 dopaminergic receptors (D1 receptors) was investigated in the rat striatum in relation to distribution of mu opiate receptor patches and islandic tyrosine hydroxylase (TH)-immunoreactive fibers. The possible influence of dopaminergic (DA) fibers originating from the substantia nigra on the postnatal distribution of striatal D1 and mu receptors was also examined by producing an early 6-hydroxydopamine (6-OHDA) lesion of DA fibers. D1 and mu receptors were labeled with selective ligands: [3H]SCH 23390 and [3H]DAGO, respectively. During the first postnatal week, control rats showed patches of dense D1 binding sites in the entire rostro-caudal extension of the striatum. The localization of D1 receptor patches corresponded to striosomes identified by TH-immunoreactive islands. The striatal distribution of mu receptors was relatively homogeneous at postnatal day 0 (P0) but was clearly patchy at P3-P4. During the second postnatal week the striosomal pattern of D1 binding sites disappeared along a dorso-ventral gradient whereas mu binding sites remained distributed in patches. Densitometric measurements showed that there was a parallel increase of D1 binding sites in both striosomes and the surrounding matrix from P0 to P4. The disappearance of D1 receptor patches observed in the dorsal striatum at P9 was due to a faster increase of D1 binding sites in the matrix than in striosomes between P4 and P9 whereas a significant difference was still observed between these two compartments in the ventral striatum of P9 rats. During the third postnatal week, the density of D1 binding sites still increased but became progressively uniform in the whole striatum. The intrastriatal injection of 6-OHDA in 2-day-old rats produced a local disappearance of TH-immunoreactive fibers in the striatum and a distal degeneration of TH-immunoreactive cell bodies in the substantia nigra. However an early lesion of striatal DA fibers did not modify the pattern of development or the density of D1 binding sites during the postnatal period examined (1 and 3 weeks after the lesion). The distribution of mu receptors was unchanged 1 week after the lesion but showed a clear disorganization 3 weeks after the lesion. We discuss the differential influence of DA fibers on the distribution of D1 and mu receptors in the rat striatum and the possible role of DA in the regulation of the expression of mu receptors.