Dopamine denervation leads to an increase in the intramembrane interaction between adenosine A2 and dopamine D2 receptors in the neostriatum

We have previously found, in striatal membrane preparations from young (2 months old) rats, that stimulation of adenosine A₂ receptors (with the selective adenosine A₂ agonist CGS 21680) increases the dissociation constants of high- (K_h) and low-affinity (K_l) dopamine D₂ binding sites (labelled with the selective dopamine D₂ antagonist [³H]raclopride) without changing the proportion of high affinity binding sites (R_h). In the present study in striatal preparations from adult (6 months old) rats, it was found that in addition to the increase in both K_h and K_l values, stimulation of adenosine A₂ receptors is associated with an increase in R_h. These result suggest that, in the adult rat, adenosine A₂ stimulation may inhibit the behavioural effects induced by dopamine D₂ stimulation both by decreasing the affinity and the transduction of dopamine D₂ receptors. We have also studied the intramembrane A₂-D₂ receptor interaction in an experimental model of Parkinson's disease, namely in rats with a unilateral 6-OH-dopamine-induced lesion of the nigro-striatal dopamine pathway. It was found that a unilateral dopamine denervation is associated with a higher density of striatal dopamine D₂ receptors in the order of 20%, without any change in their affinity compared with the unlesioned neostriatum. Furthermore, the density (B_max values) of dopamine D₂ receptors in the contralateral neostriatum was significantly higher (about 20%) than in the striatum from native animals. This finding suggests that an unilateral dopamine denervation also induces compensatory long-lasting changes of dopamine D₂ receptors in the contralateral neostriatum. In addition to the hightened sensitivity to dopamine agonists, it is known that the dopamine denervated striatum is more sensitive to adenosine antagonists like methylxanthines. If the adenosine A₂-dopamine D₂ interaction is the main mechanism of action mediating the central effects of methylxanthines, the dopamine denervation might also potentiate this interaction, i.e., dopamine D₂ receptors could be not only more sensitive to dopamine but also to adenosine A₂ receptor activation. Our results support this hypothesis, since membrane preparations from the denervated neostriatum are more sensitive to the effect of CGS 21680 on dopamine D₂ receptors. Thus a low dose of CGS 21680 (3 nM), which is not effective in membrane preparations from the neostriatum of naive animals, is still effective in membranes from the denervated neostriatum. These results underline the potential antiparkinsonian activity of adenosine A₂ antagonists.     

The role of dopamine in the maintenance and breakdown of D1/D2 synergism

The neurochemical factors involved in the maintenance and breakdown of dopamine D₁/D₂ receptor synergism were investigated by giving rats various pharmacological treatments that diminish the ability of dopamine to interact with its D₁ and/or D₂ receptors. Following these treatments, rats were observed for the expression of stereotyped motor behavior in response to independent stimulation of D₁ or D₂ receptors. Independent D₂-mediated responses were observed: (a) 2 h after the last of three daily reserpine (1 mg/kg) injections, (b) 48 h after bilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathways, (c) 24 h after a concentrated 48-h regimen (one injection/6 h) of eticlopride (0.5 mg/kg) or eticlopride + SCH 23390 (0.5 mg each), and (d) 2 h after a concentrated 48-h regimen (one injection/6 h) of α-methyl-p-tyrosine (αMPT; 100 mg/kg), but not after control treatments or a concentrated regimen of SCH 23390 alone. By contrast, independent D₁-mediated responses were observed only after three daily reserpine injections or 48 h after bilateral 6-OHDA lesions. Independent D₁-mediated stereotypy was not observed under control conditions or following a concentrated 48-h regimen of (a) SCH 23390 or eticlopride (0.5 mg/kg each) alone or in combination, (b) a high dose of SCH 23390 (1.0 mg/kg), (c) αMPT (100 mg/kg), or (d) αMPT (100 mg/kg)+SCH 23390 (1.0 mg/kg). Reserpine, bilateral 6-OHDA, and αMPT treatments produced striatal dopamine depletions of 96%, 92%, and 71%, respectively. These data indicate that the breakdown in D₁/D₂ synergism consists of two components: (a) D₁ independence from the controlling influence of D₂ receptors, and (b) D₂ independence from the controlling influence of D₁ receptors. The interaction of synaptic DA with its D₂ receptors plays a major role in determining whether these receptors can function independently of D₁ receptors, whereas reduced DA-D₁ activity alone appears insufficient to elicit D₁ independence.     

Autoradiographic analysis of D1 and D2 dopaminergic receptors in rat brain after paradoxical sleep deprivation

Previous work had shown that paradoxical sleep deprivation (PSD) results in potentiation of several apomorphine-induced behaviors, leading to the suggestion that PSD induces an upregulation of brain dopamine receptors. In this study, quantitative receptor autoradiography was used to verify whether PSD does, in fact, induce alterations in D₁ or D₂ receptor binding, and to investigate the regional brain specificity of such effects. After 96 h of PSD, [³H]SCH-23390 binding to D₁ receptors was examined in 30 different brain areas of 10 experimental and 10 cage control rats. [³H]Spiperone was used to label D₂ sites in adjacent tissue sections. Results revealed a 39% increase in [³H]SCH 23390 binding in the entorhinal cortex of PSD rats (p < 0.05), but no other changes in any of the remaining 29 brain areas examined. In contrast, [³H]spiperone binding was significantly elevated in the n. accumbens (+45%) and in all subrogions of the caudate-putamen (range: +13% to +23%). These results, thus, provide evidence that PSD increases D₂ but not D₁ receptor binding in brain. The present results also suggest that upregulated D₂ receptors can account for the previously reported changes in apomorphine-induced behaviors after PSD.     

Selective unilateral inactivation of striatal D1 and D2 dopamine receptor subtypes by EEDQ: turning behavior elicited by D2 dopamine receptor agonists

The unilateral intrastriatal injection of the irreversible dopamine (DA) receptor blockerN-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induces a marked decrease in the density of D₁ (-48%) and D₂ (-51%) DA receptors available for binding to [³H]SCH 23390 and [³H]raclopride, respectively. A challenge dose of the D₂ agonist LY 171555 (1 mg/kg, i.p., 24 h after EEDQ) causes intensive ipsiversive circling behavior, whereas the selective D₁ agonist SKF 38393 (20 mg/kg, i.p., 24 h after EEDQ) is unable to induce rotations. The density of D₁ and D₂ DA receptors returns to basal levels by 7 days after the intrastriatal infusion of EEDQ. This biochemical recovery is associated with a progressive decrease in the number of rotations elicited by a challenge dose of LY 171555, suggesting the EEDQ does not cause any relevant neuronal damage. A selective inactivation of striatal D₁ or D₂ DA receptors can be obtained by injecting EEDQ 30 min after the administration of the D₂ antagonist raclopride (20 mg/kg, i.p.) or of the D₁ antagonist SCH 23390 (2 mg/kg, s.c.), respectively. The intensity of the circling behavior induced by LY 171555 24 h after EEDQ in animals with a selective inactivation of D₂ DA receptors is similar to that found in rats in which both D₁ and D₂ DA receptors have been inactivated. In contrast, LY 171555 does not cause rotations when the density of D₁ DA receptors is selectively decreased by EEDQ in rats pretreated with raclopride. These results indicate that the imbalance in striatal D₂ receptors, but not in D₁ receptors, is a critical factor for the expression of the motor effects elicited by LY 171555 in EEDQ-treated rats.     

Dopamine D2 receptor agents, but not dopamine D1, modify brain glucose metabolism

The effects of recently described selective dopamine D₁ and D₂ agonists and antagonists on brain glucose metabolism were studied using the 2-[¹⁴C]deoxyglucose autoradiographic technique. The administration of LY-141865 or YM-09151-2, which behave as a specific D₂ agonist and antagonist respectively, modified brain glucose metabolism in a manner similar to that previously described for more classical dopaminergic agents, such as apomorphine and haloperidol. In contrast, the administration of SKF 38393 or SCH 23390, a specific D₁ agonist and antagonist respectively, was not followed by significant modifications of brain glucose metabolism in any of the brain regions studied. These results indicate that D₂ but not D₁ dopamine receptors are involved in the regulation of local brain glucose metabolism.     

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.     

Decreased striatal D2 dopamine receptors in obese Zucker rats: changes during aging

The specific binding of [³H]YM-09151-2 was used to investigate the possible differences in age-associated changes in striatal D₂ dopamine (DA) receptor properties in genetically obese (fa/fa) Zucker rats and their lean3(Fa/?) littermates. The maximal binding sites (B_max) of D₂ DA receptors was found to decline with age in both obese and lean rats: the rate of decline in receptor B_max was slightly higher in lean than obese rats. However. the B_max of D₂ DA receptor in 6-, 12- and 18-month-old obese rats was significantly lower compared to the age-matched lean rats. These data indicate that obesity decreases the number of striatal D₂ DA receptors without affecting the rate at which receptor number decreases with age.     

Interactive effects of stimulation of D1 and D2 dopamine receptors on fos-like immunoreactivity in the normosensitive rat striatum

Systemic administration of the selective, full, D₁ dopamine agonist A-77636 [(1R,3S)3-(1′-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyranhydrochloride] (0.36–2.9 mg/kg) led to a dose-dependent induction of Fos-like immunoreactivity (FLI) in the striatum. Quantitative analysis of the sections indicated that immunoreactive cells were more numerous in the medial than the lateral striatum and, within these regions, appeared to be randomly distributed. The staining produced by A-77636 could be abolished by pretreatment with the selective D₁ antagonist SCH-23390. The selective D₂ dopamine agonist quinpirole (3 mg/kg) had no effect on striatal FLI when given by itself, but markedly potentiated the weak striatal staining produced by low doses of A-77636. When combined with the highest dose of A-77636, which produced substantial staining by itself, quinpirole produced an increase in the number of immunoreactive cells seen in the lateral striatum but actually decreased the number present in the medial striatum. Statistical analysis of the distribution of immunoreactive cells demonstrated that, in both regions, quinpirole converted the relatively homogeneous staining seen after A-77636 alone into a markedly patchy pattern. These findings indicate that stimulation of D₂ receptors produces both stimulatory and inhibitory effects on the D₁-mediated expression of Fos in the striatum and that the interaction between D₁ and D₂ receptor stimulation must, therefore, be more complex than the simple synergism suggested by previous studies.     

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.     

Dopamine D1 and D2 receptor binding sites in brain samples from depressed suicides and controls

Dopamine D₁ and D₂ receptors were measured (by saturation binding of [³H]SCH23390 and [³H]raclopride) in caudate, putamen and nucleus accumbens, obtained at post-mortem from suicide victims with a firm retrospective diagnosis of depression and matched controls. There were no differences in the number or affinity of D₁ or D₂ receptors between suicides who had been free of antidepressants for at least three months prior to death, and controls. Increased numbers and decreased affinity of D₂ receptors were however found in each brain region of antidepressant-treated suicides. We argue that these increases are related to concurrent treatment with neuroleptics rather than a direct effect of antidepressants. Increased numbers of D₁ receptors in antidepressant-treated suicides were seen only in nucleus accumbens. This increase could not be clearly attributed to neuroleptics and may be related to antidepressant treatment.     

Modulation of dopamine metabolism in the retina via dopamine D2 receptors

When rats are placed in a lighted environment from the dark retinal DOPAC increases. There is no significant change of retinal dopamine (DA) under either lighting condition. Blockade of aromaticl-amino acid decar☐ylase results in a more rapid accumulation of DOPA in the retina of animals in the light than in the dark implying that DA synthesis and metabolism are more rapid in the light than in the dark. Retinal DOPAC increases in the dark and in the light when rats are treated with the DA D₂ antagonists sulpiride and spiperone. Treatment with the D₂ agonist, quinpirole, lowers the content of DA in the retina of rats kept in the dark or exposed to light. D₁ receptor drugs induce only limited changes in DA metabolism. We conclude that D₂ receptors play a principal role for modulating DA synthesis and metabolism in the rat retina.     

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.     

Involvement of dopamine D1 and D2 receptors in the ethanol-associated place preference in rats exposed to conditioned fear stress

The present study was designed to investigate: (1) the involvement of dopamine D₁ and D₂ receptors, and (2) the roles of these receptors and endogenous opioid systems (endorphinergic and enkephalinergic systems) in the ethanol-induced place preference in rats exposed to conditioned fear stress using the conditioned place preference paradigm. The administration of ethanol (300 mg/kg, i.p.) induced a significant place preference. The selective D₁ receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H3-benzazepine)hydrochloride (SCH23390; 0.01 and 0.03 mg/kg, s.c.) and the selective D₂ receptor antagonist S(−)-5-(aminosulfonyl)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride; 20 and 40 mg/kg, s.c.) significantly attenuated the ethanol-induced place preference. The administration of ethanol (75 mg/kg, i.p.) tended to produce a place preference, but this effect was not significant. SCH23390 (0.03 mg/kg, s.c.) and sulpiride (40 mg/kg, s.c.) significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the μ-opioid receptor agonist morphine (0.1 mg/kg, s.c.). In addition, SCH23390 (0.03 mg/kg, s.c.) also significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the selective δ-opioid receptor agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydroquinolino[2,3,3,-g]isoquinoline (TAN-67; 20 mg/kg, s.c.). On the other hand, sulpiride (40 mg/kg) had no significant effect on the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by TAN-67. These results suggest that D₁ and D₂ receptors may be involved in the rewarding mechanism of ethanol under psychological stress. In addition, D₁ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ- and δ-opioid receptors, whereas D₂ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ-opioid receptors, but not in that modulated by the activation of δ-opioid receptors.     

Blockade of dopamine D1 receptors by SCH 23390 antagonizes discriminative stimulus effects of pentazocine

The purpose of this investigation was to test the hypothesis that the discriminative stimulus properties of pentazocine are mediated through an interaction with dopamine receptors. Rats were trained to discriminate s.c. injections of pentazocine (3.0 mg/kg) from vehicle in a two-choice discrete trial avoidance paradigm. SCH 23390 (0.003–0.056 mg/kg), a selective antagonist of dopamine D₁ receptors, inhibited the discriminative stimulus effects of pentazocine in a dose-dependent fashion, whilst the selective D₂ receptor antagonist sulpiride (20.0–80.0 mg/kg) did not antagonize them. It appears that the dopamine D₁ receptors play an important role in the discriminative stimulus effects of pentazocine.     

Blockade of in vivo binding of I-labeled 3-iodobenzamide (IBZM) to dopamine receptors by D2 antagonist and agonist

The in vivo binding of [¹²⁵I]3-iodobenzamide (IBZM), a substituted benzamide, to DA receptor binding sites in the caudate nucleus, nucleus accumbens, and olfactory tubercle was investigated by using ex vivo autoradiography. The in vivo binding of IBZM seems to be selective to D₂ dopamine receptors, since the binding was blocked by pretreatment of animals with D₂ agonist LY-171555 or antagonist YM-09151-2. Furthermore, in vitro binding assays in striatal membranes confirmed that IBZM binding was highly selective to D₂ sites. Thus, IBZM, when labeled with ¹²³I (T1/2: 13 h; 159 kev), could be a potential ligand for imaging D₂ dopamine receptors by single photon emission computerized tomography procedures.     

Presynaptic and postsynaptic D1 dopamine receptors in the nigrostriatal system of the rat brain: a quantitative autoradiographic study using the selective D1 antagonist [H]SCH 23390

Ibotenic acid lesions of the caudate-putamen in rat brain resulted in dramatic reductions in [³H]SCH 23390 binding in both the ipsilateral caudate-putamen and substantia nigra reticulata as assessed by quantitative autoradiography. Nigral ibotenic acid and 6-hydroxydopamine lesions did not significantly alter the binding in either structure. This indicates that D₁ receptors in the caudate-putamen are postsynaptic on striatal neurons, while those in the substantia nigra reticulata are presynaptic on nerve terminals originating in the caudate-putamen.     

Cocaine inhibits NGF-induced PC12 cells differentiation through D1-type dopamine receptors

In utero cocaine exposure can adversely affect CNS development. Previous studies showed that cocaine inhibits neuronal differentiation in a dose-dependent fashion in nerve growth factor (NGF)-stimulated PC12 cells. Cocaine binds with high affinity to several neurotransmitter transporters, resulting in elevated neurotransmitter levels in nerve endings. To determine if cocaine inhibits neurite outgrowth through the effects of these neurotransmitters, we applied dopamine, norepinephrine, serotonin, and acetylcholine to NGF-induced PC12 cells. Dopamine was the only neurotransmitter to inhibit neurite outgrowth significantly in a dose-dependent pattern without affecting cell viability. Norepinephrine and acetylcholine did not affect neurite outgrowth, while serotonin enhanced it. Furthermore, GBR 12909, a potent dopamine transporter (DAT) inhibitor, yielded similar effects. We then showed PC12 cells express D₁ and D₂ receptors and DAT proteins. Dopamine uptake measured over time was significantly blocked by cocaine and GBR 12909 which may result in elevated extracellular dopamine. The role of dopamine receptors in PC12 differentiation was further examined by using D₁ and D₂ specific receptor agonists. Only the D₁ agonist, SKF-38393, had a significant dose-dependent inhibitory effect. In addition, a D₁ antagonist produced significant recovery of neurite outgrowth in cocaine-treated cells. These findings suggest that cocaine inhibitory effects on neuronal differentiation are mediated through its binding to the dopamine transporter, resulting in increased dopamine level in the synapses. Subsequently, up regulation of D₁ receptors alters NGF signaling pathways.     

Dopamine D1and D2Receptor-Mediated Acute and Long-Lasting Behavioral Effects of Glial Cell Line-Derived Neurotrophic Factor Administered into the Striatum

To determine the differences in behavioral effects between intrastriatal and intracerebroventricular glial cell-derived neurotrophic factor (GDNF) administration, spontaneous locomotor activity was measured after intrastriatal or intracerebroventricular injection of GDNF (10 μg) in normal adult rats with implanted guide cannulae. In addition, the distribution of GDNF after intracerebral injection was studied immunohistochemically. Intrastriatal administration of GDNF significantly increased rearing behavior 3–4 h after injection. Increases in all three aspects of locomotor activity (motility, locomotion, and rearing) were most pronounced 3 days after intrastriatal injection, and they lasted for several days. This hyperactivity was blocked by the selective dopamine D₁receptor antagonist SCH22390 and by the selective D₂receptor antagonist raclopride at doses of the dopamine receptor antagonists, which by themselves did not affect spontaneous locomotor activity. These results suggest that GDNF has both acute and long-lasting pharmacological effects on dopamine neurons in adult animals and stimulates locomotor activity by activating both dopamine D₁and D₂receptors. On the other hand, intracerebroventricular administration of the same dose of GDNF failed to increase locomotor activity at any time during the test period (12 days). The immunohistochemical study demonstrated widespread distribution of GDNF in the entire body of the striatum within 24 h after intrastriatal injection. It also revealed deep penetration of GDNF from the ventricular space into the brain parenchyma after intracerebroventricular injection. GDNF-immunoreactive neuronal cell bodies were seen in the ipsilateral substantia nigra pars compacta most frequently 6 h after intrastriatal injection. The number of such cell bodies after intracerebroventricular administration, on the other hand, was much lower than that seen after intrastriatal administration. Taken together, these data suggest that intrastriatal administration of GDNF is an effective approach for affecting DA transmission. Long-lasting behavior effects are mediated via dopamine D1 and D2 receptors. Higher doses of GDNF would probably be needed using the intracerebroventricular route as compared to intraparenchymal delivery to exert effects on the nigrostriatal system in Parkinson's disease patients.     

Dopamine D2 receptors exert tonic regulation over discrete neurotensin systems of the rat brain

Blockade of dopamine D₂ receptors with either the selective antagonist, sulpiride, or the non-selective antagonist, haloperidol, induces 2- to 3-fold increases in the content of neurotensin-like immunoreactivity in the striatum and the nucleus accumbens of the rat brain. Quantitatively similar increases were also observed (a) in the striatum following selective degeneration of more than 85% of the nigrostriatal dopamine pathway with 6-hydroxydopamine and (b) in both the striatum and the nucleus accumbens after non-selective depletion of brain dopamine using reserpine plus α-methyl-p-tyrosine. Interestingly, treatment of animals with sulpiride or haloperidol, following the depletion of dopamine by either 6-hydroxydopamine or reserpine plus α-methyl-p-tyrosine, did not add to the elevation in neurotensin content of either structure caused by the dopamine depletion alone. These data suggest that an intact dopamine system is required for the neuroleptics to exert effects on individual neurotensin systems. In addition, the same mechanism appears to underlie the responses of the neurotensin pathways to treatments with the neuroleptics or dopamine-depleting drugs. A likely explanation for the effects of neuroleptics and dopamine-depleting drugs is that they eliminate tonic activity on D₂ receptors by basally released dopamine in the striatum and the nucleus accumbens. Supportive evidence for this hypothesis is that concurrent administration of the D₂ receptor agonist, LY 171555, with reserpine, completely blocked the effects of reserpine-induced dopamine depletion on neurotensin systems of the striatum and the nucleus accumbens.     

Dopamine D2 receptor-mediated modulation of the GABAergic inhibition of substantia nigra pars reticulata neurons

Neurons of the substantia nigra pars reticulata can be readily and fully inhibited by endogenously released or iontophoretically applied GABA. We have previously shown that co-application of dopamine or the D₂-like agonist quinpirole causes a current-dependent attenuation of the inhibitory response of these neurons to GABA. To determine if the modulation of GABA responsiveness was mediated by activation of D₂ receptors, effects of iontophoretic quinpirole were examined after various treatments which block or inactivate D₂ receptors, or uncouple D₂ receptors from their G-proteins. Results showed that the GABA-attenuating effect of quinpirole could be attributed to stimulation of D₂ receptors, and not a non-specific effect of the drug, since (1) co-iontophoresis of the D₂ antagonist YM 09151-2 antagonized the GABA-modulatory effect of quinpirole, (2) prior intranigral injection of the receptor inactivatorN-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; 50 nmol/0.5 ml one day before recording) prevented the response to quinpirole, and (3) prior intranigral injection of the G_i-G_o-protein inactivator pertussis toxin (1 mg/ml 0.9% NaCl 24 h before recording) completely abolished the ability of quinpirole to lessen the inhibitory response to GABA. The location of the involved D₂ receptors was examined using selective lesioning approaches. Kainic acid lesions of the striatonigral pathway did not prevent the ability of quinpirole to attenuate responses of pars reticulata neurons to GABA. Similarly, in previous studies [59], 6-hydroxydopamine lesions of the adjacent pars compacta dopamine neurons were found not to abolish the GABA-attenuating effect of dopamine. Thus, it appears that the receptors mediating the response are not localized to either striatonigral terminals nor to the adjacent dopamine neurons, leaving open the possibility that the response is mediated by D₂ receptors located on pars reticulata neurons. Collectively these results suggest that dendritically released dopamine may act via nigral D₂ receptors, perhaps located on pars reticulata neurons themselves, to regulate basal ganglia output from the substantia nigra.