Age-related changes in the turnover rates of D1-dopamine receptors in the retina and in distinct areas of the rat brain.

The steady-state density and the turnover rates of D1-dopamine receptors were investigated in the striatum, nucleus accumbens, substantia nigra, and retina of adult (3-month-old) and aged (23-month-old) rats. The turnover rates were measured by monitoring the repopulation kinetics of D1-dopamine receptors labeled with [3H]-SCH 23390 after the irreversible inactivation induced by a single dose of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 10 mg/kg, s.c.). In all the neural tissues examined, the repopulation of D1 dopamine receptors could be adequately described by a theoretical model that assumes a constant rate of receptor production (i.e. zero order) and a rate of degradation that is dependent on the receptor density at any time (i.e. first order). The results obtained indicate that the reduction in the density of D1-dopamine receptors in the striatum, nucleus accumbens and substantia nigra of aged rats is the result of a larger decrease in the receptor production rate (-44 to -60%) than in the receptor degradation rate (-21 to -46%). By contrast, the production rate of D1-dopamine receptors in the retina of aged rats remains unchanged, whilst the degradation rate is reduced by 25%. This results in an age-related increase in the density of D1-dopamine receptors in the rat retina.     

Human dopamine receptor subtypes—in vitro binding analysis using3H-SCH 23390 and3H-raclopride

Summary Affinities and regional densities of the D₁- and D₂-dopamine receptor subtypes were studied in the human post-mortem brain in vitro using the two selective radioligands³H-SCH23390 and³H-raclopride.³H-Raclopride binding was confined to the caudate nucleus, the putamen and the substantia nigra, while³H-SCH23390 bound to cortical regions as well. The binding of³H-SCH 23390 was reduced by a low concentration of ketanserin, indicating binding to 5-HT₂ receptors in addition to the D₁dopamine receptors. The endogenous neurotransmitter dopamine interacted potently both with the D₁-dopamine receptor and the D₂-dopamine receptor, displaying two affinity states for each subtype. The distribution of the dopamine receptor subtypes obtained in the present in vitro investigation is in agreement with data obtained with¹¹C-SCH 23390 and¹¹C-raclopride in positron emission tomographic studies in human volunteers.     

A single (−)-nicotine injection causes change with a time delay in the affinity of striatal D2 receptors for antagonist, but not for agonist, nor in the D2 receptor mRNA levels in the rat substantia nigra

The in vitro and in vivo effects of (−)-nicotine on dopamine D₂ receptors in the rat neostriatum have been studied using biochemical binding, in situ hybridization and immunocytochemistry. A single i.p. injection (1 mg/kg) of (−)-nicotine resulted in a reduction of theK_D value of the D₂ antagonist [³H]raclopride binding sites in rat neostriatal membrane preparations at 12 h without any significant change in theB_max value. This action of (−)-nicotine was counteracted by pretreatment 15 min earlier with the nicotine antagonist mecamylamine (1 mg/kg, i.p.). However, theK_D and theB_max values of the D₂ agonist [³H]NPA binding sites in the rat neostriatal membrane preparations were not significantly affected 0.5–48 h after a single i.p. injection with 1 mg/kg of (−)-nicotine. No significant change in neostriatal D₂ receptor mRNA levels was observed at any time interval after the (−)-nicotine injection. No significant change was observed in tyrosine hydroxylase (TH) immunoreactivity in either the substantia nigra or the neostriatum, nor in nigral TH mRNA levels during the time interval studied (4–24 h posttreatment). Furthermore, addition of low (10 nM) or high (1 μM) concentrations of (−)-nicotine in vitro to rat neostriatal membranes did not alter the characteristics of [³H]raclopride or [³H]NPA binding. These results indicate that a single (−)-nicotine injection can produce a selective and delayed increase in the affinity of D₂ receptors for the antagonist, but not for the agonist without modifying the levels of D₂ receptor mRNA, probably via the activation of central nicotinic receptors.     

Long-term D1-dopamine receptor sensitization in neonatal 6-OHDA-lesioned rats is blocked by an NMDA antagonist

Repeated administration of the D₁-dopamine agonist SKF-38393 to adult rats having had dopaminergic neurons destroyed early in development results in an increasing enhancement of the behavioral response to SKF-38393 with each dose until a maximum is reached. This increased sensitivity lasts for at least 6 months. In the present study, this long-lasting change in behavioral responsiveness to repeated treatment with SKF-38393, referred to as D₁-dopamine receptor priming, was shown to be dose dependent with smaller doses requiring an increased number of administrations to produce a maximal response when compared to higher doses. In addition, priming occurred equally well when treatment intervals ranged from 1 day to 14 days. These latter data reinforced the view that activation of D₁-dopamine receptors results in a prolonged change in neural function. In subsequent experiments D₁-dopamine receptor priming was blocked by pretreatment with the NMDA-receptor antagonist MK-801. This antagonism of priming could not be attributed to a blockade of D₁-dopamine receptors by MK-801 or to the induction of interfering behaviors. Because an NMDA antagonist interfered with D₁-dopamine priming as it does with other long-term neural messages, a common requirement for these diverse forms of neuronal plasticity appears to involve activation of the NMDA receptor. This functional link between NMDA receptors and dopaminergic function and its relationship to neuronal palsticity could have relevance to the biochemical mechanism involved in learning and to symptons in central disorders during development that worsen over time, particularly those proposed to involved malfunctioning dopaminergic mechanisms.     

Modifications of head turning and circling movement following sulpiride microinjections into nucleus accumbens in the rat

The aim of this study was to determine whether a relationship exists between nucleus accumbens D₂ receptors, circling behavior, and its first stage, the head turning. Rats were unilaterally lesioned in the substantia nigra with 6-hydroxydopamine and afterward treated with d-amphetamine IP Following bilateral intraaccumbens microinjections (1, 5, 10 μg/0.5 μl) of sulpiride, a D₂ receptor antagonist. Computer-assisted video analysis allowed the study of some parameters (number of turns, type of turn, head turning duration, degree and speed) characterizing rotatory activity. Sulpiride microinfusion resulted in a dose-dependent decrease of the number of turns and head rotation speed and in a dose-dependent increase of head-turning duration. Two turn types were observed in relation to the animal's position: a large head-to-tail position with a short-diameter turn type following sulpiride microinjection, and a close head-to-tail position in relation to a wide diameter turn type in the control condition (saline). The results show a relationship between head turning parameters, circling behavior, and D₂ receptors in nucleus accumbens, which may be also involved in the regulation of some mechanisms related to sensory-motor integration in the rat.     

Blunted brain metabolic response to ketamine in mice lacking D1A dopamine receptors

The interaction of glutamatergic and dopamine neurotransmission is thought to have relevance to both the pathophysiology and pharmacotherapy of schizophrenia. For example, subanesthetic doses of the N-methyl- -aspartate receptor (NMDA-R) antagonist ketamine induce schizophrenia-like behavioral effects in humans and both behavioral and brain metabolic activation in rodents. Blockade of NMDA-R results in dopamine release, and antipsychotic drugs that block dopamine neurotransmission decrease NMDA-R antagonist-induced behavioral activation. The involvement of dopamine receptors in brain metabolic activation induced by ketamine is, however, unknown. The present study used D_1A knockout mice to determine the role of dopamine D_1A receptors in the effects of subanesthetic doses of ketamine on both behavioral responses and on alterations in regional [¹⁴C]2-deoxyglucose (2-DG) uptake. There was less ketamine-induced behavioral activation in D_1A knockout mice than in wild-type mice. In wild-type mice, ketamine (30 mg/kg) induced dramatic increases in 2-DG uptake in limbic cortical regions, hippocampal formation, nucleus accumbens, basolateral amygdala, and caudal parts of the substantia nigra pars reticulata. D_1A knockout mice exhibited blunted metabolic activation in response to ketamine in a neuroanatomically specific manner. The selective D₁ antagonist, SCH23390 (0.3 mg/kg), inhibited both ketamine-induced brain metabolic activation and behavioral responses in the wild-type mice, with a similar neuroanatomical specificity observed in the D_1A knockout mice. Thus, the neuroanatomically selective role that D_1A receptors play in ketamine-induced behavior and regional brain metabolic activation in mice provides a useful model for further studies of how the D_1A receptor function may be altered in schizophrenia.     

Dopaminergic regulation of serotonin release in the substantia nigra of the freely moving rat using microdialysis

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. A D₁- and D₂-receptor-mediated inhibition of nigral 5-HT release was demonstrated in this study. Continuous administration of the D₁-receptor agonist CY 208243 (10 μM) through the probe did not alter extracellular DA nor 5-HT from the SN, whereas intranigral administration of the D₁-receptor antagonist SCH-23390 HCl (10 μM) significantly increased both DA (to 214%) and 5-HT release (to 168%) from the SN. Co-perfusion of the D₁-receptor agonist and antagonist did not change nigral DA nor 5-HT release compared to perfusion of the antagonist alone. The continuous intranigral perfusion of the D₂-receptor agonist, (−)-quinpirole HCl (1 μM) significantly decreased both DA and 5-HT release to 71% and 78%, respectively. These decreases were abolished when the D₂-receptor antagonist S(−)-sulpiride (10 μM) and the D₂-receptor agonist (−)-quinpirole HCl (1 μM) were co-perfused. In contrast, the intranigral perfusion of the DA precursor, -DOPA (5 μM; 1 h), significantly increased nigral and striatal 5-HT release to 202% and 155%, respectively. This enhanced nigral 5-HT release might not be receptor-mediated. The results of the present study suggest a D₁ and D₂ regulation of nigral 5-HT release, either directly mediated by DA receptors on nigral 5-HT terminals or indirectly by nigral GABA, Glu or Asp. Alternatively, the observed DA–5HT-interaction in the SN might not reflect a local interaction but might involve an interaction at the level of the serotonin cell body region, the dorsal raphe nuclei (DRN).     

Differential effect of aging on3H-SCH 23390 binding sites in the retina and in distinct areas of the rat brain

Summary The effects of age on the binding parameters of the selective D-1 dopamine (DA) receptor antagonist³H-SCH 23390 were studied in membrane preparations from rat striatum, substantia nigra, olfactory tubercle, prefrontal cortex and retina. When compared with 3-month-old animals, there was a significant decrease in the density of³H-SCH 23390 binding sites in the striatum (–25%), substantia nigra (–24%), and olfactory tubercle (–23%), but not in the prefrontal cortex of senescent (23-month-old) rats. The affinity of³H-SCH 23390 for D-1 DA receptors did not change with age in any of the brain areas analyzed. In contrast, the density of³H-SCH 23390 binding sites was higher (+53%) in the retina of aged rats that in 3-month-old controls. Confirming previous studies, we observed that light deprivation induced a significant increment in the density of³H-SCH 23390 binding sites in the retina of adult rats (+31%) but not in the retina of aged animals. The ability of light exposure to activate DAergic neurons in the rat retina was not altered by normal aging. In fact, a similar increase in the concentration of DOPAC was observed in the retina of light-adapted adult and senescent rats when compared to their respective dark-adapted controls (+94% and +95%, respectively).The results indicate that aging has a differential effect on D-1 DA receptors in the retina and different areas of the rat brain.Finally, the age-related increment in the density of retinal D-1 DA receptors does not appear to depend on presynaptic mechanisms, since DA metabolism is increased by light to the same extent in young and aged rats.     

Topographical distributions of 32K and 48K cAMP-regulated phosphoproteins: Relationships to dopamine and serotonin innervations in striatum, substantia nigra and cerebral cortex

The topographical distribution of a cAMP-regulated phosphoprotein of an apparent molecular weight (M_r) of 32,000 (32K) was examined in homogenates prepared from microdiscs punched out from serial frozen slices of the striatum. The amount of this phosphoprotein progressively diminished from the rostral to the caudal part of the striatum as did both the dopamine-innervation and the dopamine (D₁)-sensitive adenylate cyclase. After kainic acid lesion of the rostral part of the striatum, the 32K phosphoprotein disappeared in this area and we observed a 48% decrease in the amount of 32K phosphoprotein found in the substantia nigra. 6-Hydroxydopamine lesions of the nigro-striatal dopaminergic pathway did not affect the 32K phosphoprotein either in striatum or substantia nigra. These results suggest that in the nigro-striatal pathway, the 32K phosphoprotein is closely associated with dopaminoceptive neurons containing D₁ receptors. In the cerebral cortex the association of 32K phosphoprotein with dopaminoceptive neurons is more questionable since we did not find a higher density of this phosphoprotein in areas containing a high amount of D₁ receptor (frontal cerebral cortex) than in areas containing a low amount of D₁ receptor (parietal cerebral cortex). In the course of this study we found another cAMP-regulated phosphoprotein of an M_r of 48,000 (48K). The amount of this phosphoprotein increased progressively from the rostral to the caudal part of the striatum, a pattern of distribution close to that of serotonin terminals. This protein was also present in the substantia nigra. Kainic acid lesioning of the rostral part of the striatum did not affect the amount of the 48K phosphoprotein within the substantia nigra. 6-Hydroxydopamine lesions of the substantia nigra also did not alter the amount of 48K phosphoprotein in this brain region.     

D3 dopamine receptor mRNA is widely expressed in the human brain

Considerable attention has been given to the association of the D₃ dopamine receptor subtype and limbic function based on the abundant localization of D₃ receptor sites and mRNA expression in the islands of Calleja and nucleus accumbens in experimental animals. Though most human anatomical studies have focused on the role of D₃ receptors in limited brain structures, detailed information about the overall anatomical organization of the D₃ receptor in the human brain is still, however, not available. In the current study, we examined the anatomical distribution of D₃ receptor mRNA expression at different levels of the human brain in whole hemisphere horizontal cryosections using in situ hybridization. This approach made it possible to establish for the first time the wide and heterogenous expression of the D₃ receptor gene throughout the human brain. As expected, the most abundant D₃ mRNA expression levels were found in the islands of Calleja and discrete cell cluster populations within the ventral striatum/nucleus accumbens region. High levels were also evident within the dentate gyrus and striate cortex. Low to moderate D₃ mRNA expression levels were apparent in most brain areas including all other cortical regions (highest in the anterior cingulate/subcallosal gyrus), caudate nucleus, putamen, anterior and medial thalamic nucleus, mammillary body, amygdala, hippocampal CA region, lateral geniculate body, substantia nigra pars compacta, locus coeruleus, and raphe nuclei. While the current anatomical map of D₃ receptor mRNA expression in the human brain does confirm previous reports that D₃ receptors may play important roles in limbic-related functions such as emotion and cognition, the findings also suggest other non-limbic functions for D₃ mRNA-expressing cell populations such as processing of motor and sensory information.     

Intranigral GR-113808, a selective 5-HT4 receptor antagonist, attenuates morphine-stimulated dopamine release in the rat striatum

GR-113808, a potent and selective 5-HT₄ receptor antagonist, was infused through a microdialysis probe into the striatum and nucleus accumbens of awake rats, and basal and morphine-stimulated extracellular concentrations of dopamine (DA) were measured in these regions. At 1 and 10 μM GR-113808 did not effect the extracellular concentrations of DA in either region and 100 μM significantly reduced dialysate DA only in the striatum. A subcutaneous dose of 5 mg/kg morphine significantly raised extracellular concentrations of DA in the striatum and nucleus accumbens from 60 to 120 min after injection and the effect was not modified by 10 μM GR-113808 infused through the probe 20 min before and for 60 min after morphine. Bilateral injections of GR-113808 (1, 2.5 and 10 μg/0.5 μl) in the substantia nigra pars compacta did not affect dialysate DA in the striatum, except for a significant increase 120 min after the injection of 10 μg but the highest dose of GR-113808 prevented the increase of striatal DA caused by 5 mg/kg morphine s.c. The results suggest that 5-HT₄ receptors in the substantia nigra modulate the activity of the dopaminergic nigrostriatal system only when the neurons are activated.     

Differential regulation, by MK-801, of dopamine receptor gene expression in rat nigrostriatal and mesocorticolimbic systems

Glutamate agonists have been shown to stimulate striatal dopamine release, but less is known about dopamine-glutamate interactions at the receptor level. We treated rats with 0.3, 1.0, or 3.0 mg/kg of MK-801, an NMDA antagonist, daily for 1 week and, using in situ hybridization, measured dopamine receptor rnRNA levels in cortical and subcortical structures. MK-801 caused a significant increase of D₁ and D₂ mRNA in the dorsal and ventral striatum, a significant decrease of D₃ mRNA in the nucleus accumbens, and a significant decrease of D₁ mRNA in the limbic cortex. Dopamine autoreceptor expression, reflected by D₂ mRNA in the midbrain, was increased in the ventral tegmental area, but not in the substantia nigra. Thus, MK-801 appears to differentially regulate the mesocorticolimbic and nigrostriatal dopamine systems.     

Dopamine D1- and D2-dependent catalepsy in the rat requires functional NMDA receptors in the corpus striatum, nucleus accumbens and substantia nigra pars reticulata

This study investigated the anticataleptic activity of MK-801 versus the D₁ antagonist SCH 23390 and the D₂ antagonist raclopride, using the horizontal bar test in the rat. MK-801, 0.2 mg/kg i.p., strongly opposed the cataleptogenic actions of SCH 23390 and raclopride administered systemically (1 and 3 mg/kg i.p., respectively), or directly into the corpus striatum (CS) or nucleus accumbens (NAc; 1 and 10 μg, respectively). Conversely, intraCS and intraNAc pretreatment with MK-801 (10 μg) markedly attenuated the cataleptic response to a systemic injection of SCH 23390 or raclopride. In the latter experiments the anticataleptic effect of MK-801 was pronounced and sustained (>2 h), except with intraCS MK-801 versus raclopride, where it was initially profound but only short-lived (15 min). Stereotaxic injection of MK-801 (1 μg) into the substantia nigra pars reticulata (SNr) prevented catalepsy developing to either dopamine D₁ or D₂ receptor antagonism. These results indicate there must be unimpeded glutamate neurotransmission in the CS and NAc before catalepsy can develop fully to D₁ and D₂ dopamine receptor blockade in these structures. The weaker glutamate–D₂ interaction in the CS than in the NAc may be related to differences in the N-methyl-

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-aspartate receptor subpopulations in these nuclei. Finally, the ability of intranigral MK-801 to diminish both D₁- and D₂-dependent catalepsy suggests the SNr acts as a common output pathway for the expression of both forms of catalepsy in the rat.     

Regulation of Dopamine D1 and D2 Receptors on Striatal Acetylcholine Release in Rats

The effects of dopamine (DA) D₁ and D₂ receptors on striatal acetylcholine (ACh) releases were investigated by in vivo microdialysis. All drugs were applied via dialysis membrane directly to the striatum. The levels of ACh release were increased by 10⁻⁴ M SKF38393, a D₁ receptor agonist. Although 10⁻⁴ M SCH23390, a D₁ receptor antagonist, exhibited an increase in the levels of ACh release, the agonist (10⁻⁴ M) induced-increase in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁴ M). In contrast, the levels of ACh release were decreased by the D₂ receptor agonist, N-434, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M) and increased by the D₂ receptor antagonist, sulpiride, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M). The agonist (10⁻⁵ M) induced-decrease in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁶ M). Coperfusion of D₁ (10⁻⁴ M) and D₂ (10⁻⁵ M) agonists blocked both effects of respective drug alone. In order to clarify the effect of endogenous DA, two drugs with different mechanisms for enhancing DA concentration in the synaptic cleft, the DA release-inducer methamphetamine, and the DA uptake inhibitor nomifensine were perfused separately. Both (10⁻⁴ M to 10⁻⁶ M) produced a dose- and a time-dependent decrease in the levels of ACh release. Significant higher levels of ACh release were observed in the striatum of the 6-hydroxydopamine (8

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)-treated rats with significant depletion of striatal DA content. These results suggest that in striatal DA-ACh interaction ACh release, as cholinergic interneuron's activity, is tonically inhibited via the D₂ receptor, mainly by dopaminergic input, and the D₁ receptor probably modifies the effect of the D₂ receptor indirectly.     

Dopamine D1 receptor number — a sensitive PET marker for early brain degeneration in Huntington's disease

Summary D₁-dopamine receptor binding in the brain was determined by positron emission tomography (PET) in five patients with Huntington's disease, in one asymptomatic gene carrier and in five control subjects. [¹¹C] SCH 23390 was used as the radioligand. Brain morphology was recorded by MRI. The patients who all had a mild to moderate functional impairment showed an almost 50% reduction of putamen volume as well as D₁-dopamine receptor density as compared to the controls. The total D₁-dopamine receptor number in the putamen was reduced by 75% in the patient group. A similar reduction was found for the caudate nucleus. The asymptomatic gene carrier had volume and density values in the lower range of the control subjects. In the frontal neocortex there also tended to be a reduced D₁-dopamine receptor binding in the symptomatic patients. The results indicate that [¹¹C] SCH 23390 binding in combination with MRI can be used as a sensitive marker for early brain degeneration in Huntington's disease. This marker may be useful to monitor the pathophysiological effect of the disease gene and also to follow therapeutic interventions aiming at preventing the degenerative process.     

Alterations in local cerebral glucose utilization produced by D3 dopamine receptor-selective doses of 7-OH-DPAT and nafadotride

The D₃ dopamine receptor, localized primarily in limbic brain areas, is a potential antipsychotic site. The effects of D₃ receptor stimulation or blockade on neuronal activity were determined using the -2-deoxyglucose method. Freely-moving, adult, male, Sprague–Dawley rats were injected s.c. with saline, agonist 7-hydroxy-diphenylaminotetralin (7-OH-DPAT) (0.1 mg/kg), or antagonist l-nafadotride (1 mg/kg). These doses of 7-OH-DPAT and l-nafadotride are behaviorally active and are 10-fold lower than a dose producing significant in vivo occupancy of D₂ receptors. The -2-deoxyglucose procedure was initiated 30 min after the administration of the test compound. The rate of local cerebral glucose utilization (LCGU) was determined by quantitative autoradiography. 7-OH-DPAT produced a significant increase in LCGU in the substantia nigra. l-Nafadotride produced significant increases in LCGU in several brain areas including the lateral preoptic area, lateral habenula, caudate, septal area, entorhinal cortex, and some thalamic and hypothalamic areas. These observations indicate that stimulation or blockade of D₃ receptors alters LCGU and that produces a unique pattern of alterations in LCGU suggestive of potential antipsychotic activity.     

Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [H]GR 125743

The distribution of 5-HT_1B and 5-HT_1D receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [³H]GR 125743. [³H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [³H]GR 125743 binding was observed, reflecting low densities of 5-HT_1B/1D receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT_1D receptors and the second using SB 224289 to inhibit 5-HT_1B receptor binding. The autoradiograms indicated that in the human brain, the 5-HT_1B receptor is much more abundant than the 5-HT_1D receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [³H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT_1B receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT_1D receptor in the human brain.     

Identification of extrastriatal dopamine D2 receptors in post mortem human brain with [I]epidepride

The regional distribution of striatal and extrastriatal dopamine D₂ receptors in human brain was studied in vitro with(S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-[¹²⁵I]iodo-2,3-dimethoxybenzamide, [¹²⁵I]epidepride, using post mortem brain specimens from six subjects. Scatchard analysis of the saturation equilibrium binding in twenty-three regions of post mortem brain revealed highest levels of binding in the caudate (16.5 pmol/g tissue) and putamen (16.6 pmol/g tissue) with lower levels seen in the globus pallidus (7.0 pmol/g tissue), nucleus accumbens (7.2 pmol/g tissue), hypothalamus (1.8 pmol/g tissue), pituitary (1.3 pmol/g tissue), substantia innominata (1.0 pmol/g tissue), and amygdala (0.87 pmol/g tissue). Of note was the presence of dopamine D₂ receptors in the four thalamic nuclei studied, i.e. anterior nucleus (1.0 pmol/g tissue), dorsomedial nucleus (0.96 pmol/g tissue), ventral nuclei (0.72 pmol/g tissue), and pulvinar (0.86 pmol/g tissue), at levels comparable to the amygdala (0.87 pmol/g tissue) and considerably higher than levels seen in anterior cingulate (0.26 pmol/g tissue) or anterior hippocampus (0.36 pmol/g tissue). The frontal cortex had very low levels of dopamine D₂ receptors (0.17–0.20 pmol/g tissue) while the inferior and medial temporal cortex had relatively higher levels (0.31–0.46 pmol/g tissue). Inhibition of [¹²⁵I]epidepride binding by a variety of neurotransmitter ligands to striatal, ventral thalamic and inferior temporal cortical homogenates demonstrated that [¹²⁵I]epidepride binding was potently inhibited only by dopamine D₂ ligands. The present study demonstrates that dopamine D₂ receptors are present in basal ganglia, many limbic regions, cortex and in the thalamus. The density of thalamic D₂ receptors is comparable to many limbic regions and is considerably higher than in cortex. Very few frontal lobe D₂ receptors are present in man.     

Dopamine D2 receptor-mediated G-protein activation in rat striatum: functional autoradiography and influence of unilateral 6-hydroxydopamine lesions of the substantia nigra

Unilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra pars compacta (SNPC) neurons in rats induce behavioural hypersensitivity to dopaminergic agonists. However, the role of specific dopamine receptors is unclear, and potential alterations in their transduction mechanisms remain to be evaluated. The present study addressed these issues employing the dopaminergic agonist, quinelorane, which efficaciously stimulated G-protein activation (as assessed by [³⁵S]GTPγS binding) at cloned hD₂ (and hD₃) receptors. At rat striatal membranes, dopamine stimulated [³⁵S]GTPγS binding by 1.9-fold over basal, but its actions were only partially reversed by the selective D₂/D₃ receptor antagonist, raclopride, indicating the involvement of other receptor subtypes. In contrast, quinelorane-induced stimulation (48% of the effect of dopamine) was abolished by raclopride, and by the D₂ receptor antagonist, L741,626. Further, novel antagonists selective for D₃ and D₄ receptors, S33084 and S18126, respectively, blocked the actions of quinelorane at concentrations corresponding to their affinities for D₂ receptors. Quinelorane potently induced contralateral rotation in unilaterally 6-OHDA-lesioned rats, an effect abolished by raclopride and L741,626, but not by D₃ and D₄ receptor-selective doses of S33084 and S18126, respectively. In functional ([³⁵S]GTPγS) autoradiography experiments, quinelorane stimulated G-protein activation in caudate putamen and, to a lesser extent, in nucleus accumbens and cingulate cortex of naive rats. In unilaterally SNPC-lesioned rats, quinelorane-induced G-protein activation in the caudate putamen on the non-lesioned side was similar to that seen in naive animals (50% stimulation), but significantly greater on the lesioned side (80%). This increase was both pharmacologically and regionally specific since it was reversed by raclopride, and was not observed in nucleus accumbens or cingulate cortex. In conclusion, the present data indicate that, in rat striatum, the actions of quinelorane are mediated primarily by D₂ receptors, and suggest that behavioural hypersensitivity to this agonist, induced by unilateral SNPC lesions, is associated with an increase in D₂, but not D₃ or D₄, receptor-mediated G-protein activation.     

Studies on the involvement of the dopaminergic system in the 5-HT2 agonist (DOI)-induced premature responding in a five-choice serial reaction time task

The present experiments investigated whether the enhanced premature (impulsive) responding induced by DOI, [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], a 5-HT_2A/2C receptor agonist, is mediated by activation of the dopaminergic system and if this effect of DOI occurs in the nucleus accumbens. Therefore, the effects of a dopamine (D_1/2) receptor antagonist given alone or combined with DOI were examined on the performance of rats in a five-choice serial reaction time (5-CSRT) task. Secondly, the effects of DOI in nucleus accumbens core and shell were studied, in order to find the target brain area for DOI-induced premature responding. The results indicate that DOI (0.1 mg/kg, subcutaneously) increases the number of premature responses, as found previously. α-Flupenthixol (0.03 mg/kg), a D_1/2 dopamine receptor antagonist, and raclopride (0.015 mg/kg), a D₂ receptor antagonist, attenuated the DOI-induced enhancement in premature responding. SCH 23390 (0.005 mg/kg), a selective D₁ receptor antagonist with little affinity to 5-HT₂ receptors totally blocked the effect of DOI. Those doses of DA antagonists did not significantly decrease premature responding when given alone. On the other hand, higher doses of all of these dopamine antagonists increased the number of omissions and decreased the number of ITI hole responses. In contrast to subcutaneous administration, direct injections of DOI (1, 3, and 10 μg bilaterally) to the nucleus accumbens shell or core had no effect on premature responding. These results suggest that the activation of the dopamine system mediates, at least in part, the effect of a 5-HT₂ agonist on premature responding, but the nucleus accumbens is not the primary site for this action.