Two dopamine receptors: biochemistry, physiology and pharmacology

Dopaminergic modulation of sodium current in hippocampal neurons via cAMP-dependent phosphorylation of specific sites in the sodium channel alpha subunit

Phosphorylation of brain Na+ channel alpha subunits by cAMP-dependent protein kinase (PKA) decreases peak Na+ current in cultured brain neurons and in mammalian cells and Xenopus oocytes expressing cloned brain Na+ channels. We have studied PKA regulation of Na+ channel function by activation of D1-like dopamine receptors in acutely isolated hippocampal neurons using whole-cell voltage-clamp recording techniques. The D1 agonist SKF 81297 reversibly reduced peak Na+ current in a concentration-dependent manner. No changes in the voltage dependence or kinetics of activation or inactivation were observed. This effect was mediated by PKA, as it was mimicked by application of the PKA activator Sp-5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-monophosphorothioate(cBIMPS) and was inhibited by the specific PKA inhibitor peptide PKAI5-24. cBIMPS had similar effects on type IIA brain Na+ channel alpha subunits expressed in tsA-201 cells, but no effect was observed on a mutant Na+ channel alpha subunit in which serine residues in five PKA phosphorylation sites in the intracellular loop connecting domains I and II (LI-II) had been replaced by alanine. A single mutation, S573A, similarly eliminated cBIMPS modulation. Thus, activation of D1-like dopamine receptors results in PKA-dependent phosphorylation of specific sites in LI-II of the Na+ channel alpha subunit, causing a reduction in Na+ current. Such modulation is expected to exert a profound influence on overall neuronal excitability. Dopaminergic input to the hippocampus from the mesocorticolimbic system may exert this influence in vivo.

On the mechanism(s) of cholecystokinin (CCK): receptor stimulation attenuates morphine dependence in mice

In the present study, effect of cholecystokinin (CCK) agonists and on dependence to morphine in mice has been investigated. The influence of dopaminergic, adrenergic, cholinergic and serotonergic on attenuation of naloxone-induced jumping in morphine-dependent mice by CCK agonists were also considered. Mice were treated subcutaneously with morphine (50, 50 and 75 mg/kg) three times daily (10 a.m. 1 p.m. and 4 p.m.) for 3 days, and a last dose of morphine (50 mg/kg) was administered on the 4th day. Withdrawal syndrome (jumping) was precipitated by naloxone (5 mg/kg) which was administered intraperitoneally 2 hr after the last dose of morphine. To study effects of CCK receptor agonists, 10 injection of morphine (3 administrations each day) for dependence and a dose of 5 mg/kg of naloxone for withdrawal induction were employed. The CCK agonists CCK-8 (0.001-0.1 mg/kg), unsulfated CCK-8 (CCK-8U; 0.001-0.1 mg/kg) and caerulein (0.00001-0.01 mg/kg) were able to prevent withdrawal signs precipitated by naloxone (5 mg/kg). Sulpiride and pimozide increased response induced by CCK-8 agonists. The dopamine antagonists also attenuates jumping by themselves. SCH 23390 did not alter the CCK-8 effect, but decreased the jumping by itself. Phenoxybenzamine, propranolol, methysergide and atropine did not change the caerulein effect significantly. However, single administration of atropine increased and methysergide decreased jumping. It is concluded that CCK mechanism(s) may be involved in morphine dependence, and dopaminergic mechanism(s) may interact with CCK in attenuation of naloxone-induced jumping.

Effects of lead exposure on bromocriptine-induced penile erection in rats

In the present work we have studied the effects of lead exposure on penile erection induced by bromocriptine. Intraperitoneal injection of bromocriptine (2, 3, 4 and 8 mg/kg) induced dose-related penile erection in rats. Maximum response was observed with 4 mg/kg of the drug. Lead exposure (as Pb-acetate in drinking water) for periods of 7, 14, 21 and 28 days decreased the bromocriptine-induced penile erection response. Higher concentrations of lead (0.05%) were shown to cause a more prominent decrease of penile erection. The same procedure for lead administration did not significantly alter penile erection induced by physostigmine (0.1 and 0.3 mg/kg, intraperitoneally). In a series of experiments, blood lead concentrations were measured 7 and 21 days after lead exposure. Significant increases of lead concentrations were found after lead exposure. It is concluded that lead can influence bromocriptine-induced penile erection.

DAD1- and DAD2-like agonist effects on motor activity of C57 mice: differences compared to rats

Studies on rats indicate that DAD1- and DAD2-like agonists produce a biphasic action on motor activity, with low doses reducing activity below control levels, and higher doses initially reducing, then elevating, activity for a prolonged period. Although some of the reported effects of DAD1- and DAD2-like receptor agonists on motor activity of mice are consistent with their effects on rats, the possibility of species differences is also apparent. In the current study the effects of DAD1- and DAD2-like agonists on motor activity of C57BL/6 (C57) mice were determined to establish species consistencies and differences with respect to their effects on rats. The partial DAD1-like agonist SKF 38393 reduced the activity of C57 mice at low doses and elevated activity above control levels at higher doses, if the mice were thoroughly habituated to the test chamber. The full DAD1 agonist SKF 82958 also increased the activity of C57 mice, and along with the SKF 38393 results indicates a response to DAD1 receptor stimulation similar to that reported for rats. In contrast to the species similarity in response to DAD1 stimulation, the DAD2-like agonist quinpirole produced only a dose-responsive monotonic reduction in the activity of C57 mice, whether the animals were nonhabituated or well-habituated to the testing environment, male or female, young or mid-aged, injected intraperitoneally (i.p.) or subcutaneously (s.c.), and with either low or high doses. This apparent species difference in response to quinpirole might reflect distinguishable functional properties of the DA subreceptor systems.

Differential influence of D1 and D2 dopamine receptors on acute opiate withdrawal in guinea-pig isolated ileum

1. The effects exerted by D1 and D2 dopamine agonists and antagonists on the acute opiate withdrawal induced by mu- and kappa-receptor agonists were investigated in vitro. 2. Following a 4 min in vitro exposure to morphine (moderately selective mu-agonist), [D-Ala2, Me-Phe4, Gly-ol5]enkephalin (DAMGO, highly selective mu-agonist) or U-50488H (highly selective kappa-agonist) the guinea-pig isolated ileum exhibited a strong contracture after the addition of naloxone. 3. The non-selective dopamine receptor antagonist haloperidol when added before or after the opioid agonists, was able dose-dependently to prevent or to reverse the naloxone-induced contracture after exposure to mu- (morphine and DAMGO) and kappa- (U-50488H) opioid agonists. The non-selective dopamine receptor agonist, apomorphine, was able to exert the same effects only at the highest concentration used. 4. The selective D2 dopamine receptor antagonist, sulpiride, was also able to reduce dose-dependently both mu- and kappa-opioid withdrawal, whereas the D1-receptor selective antagonist SCH 23390 did not affect either mu- or kappa-opioid withdrawal. 5. Bromocriptine, a D2 selective dopamine receptor agonist was able to increase significantly, and in a concentration-dependent manner, the naloxone-induced contracture by mu- and kappa-opioid agonists, whereas SKF 38393, a D1 selective dopamine receptor agonist, increased only the withdrawal after morphine or U50-488H. 6. Our data indicate that both D1 and D2 dopamine agonists and antagonists are able to influence opiate withdrawal in vitro, suggesting an important functional interaction between the dopaminergic system and opioid withdrawal at both the mu- and kappa-receptor level. 7. Furthermore, the ability of sulpiride to block strongly opiate withdrawal when compared to SCH 23390, as well as the effect of bromocriptine to increase opiate withdrawal suggest that D2 dopamine receptors may be primarily involved in the control of opiate withdrawal.

Effects of monoamine receptor antagonists on nicotine-induced hypophagia in the rat

(--)-Nicotine, in doses of 0.2-0.6 mg/kg intraperitoneally (i.p.), induced a dose-dependent anorexia 1 h, 2 h and 4 h after food presentation in 20-h food-restricted male rats. The anorectic response of nicotine (0.4 mg/kg, 30 min before the test) was prevented by pretreatment with the central nicotine receptor antagonist mecamylamine (0.5 and 1 mg/kg). The peripheral nicotine receptor antagonist hexamethonium (5 and 10 mg/kg), the muscarinic receptor antagonist atropine (5 and 10 mg/kg), the dopamine D2 receptor antagonist pimozide (0.5 and 1 mg/kg), the dopamine D1 receptor antagonist SCH23390 (R-(+)-8-chloro-2, 3, 4, 5-tetrahydro-3-methyl-5-phenyl-1 H-3-benzazepine-7ol maleate; 0.05 and 0.1 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (5 and 10 mg/kg), and the beta-adrenoceptor antagonist propranolol (5 and 10 mg/kg) amplified the nicotine response while promoting anorexia by themselves. The dopamine D2 receptor antagonist sulpiride (25, 50 and 100 mg/kg) increased food intake and amplified the anorectic effect of nicotine. The 5-HT receptor antagonists metergoline (0.5 and 1 mg/kg) and mianserin (1 and 2 mg/kg) increased the nicotine effect. When the antagonists were used alone, metergoline did not change food intake, while mianserin increased food intake. It can be concluded that part of nicotine-induced anorexia is mediated through central nicotinic receptors.

D2 dopamine receptors reduce N-type Ca2+ currents in rat neostriatal cholinergic interneurons through a membrane-delimited, protein-kinase-C-insensitive pathway

Dopamine has long been known to regulate the activity of striatal cholinergic interneurons and the release of acetylcholine. Yet, the cellular mechanisms by which this regulation occurs have not been elucidated. One way in which dopamine might act is by modulating voltage-dependent Ca2+ channels. To test this hypothesis, the impact of dopaminergic agonists on Ca2+ channels in neostriatal cholinergic interneurons was studied by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reactions. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly, all interneurons tested (90%, n = 17) coexpressed D2 (short and long isoforms) and D1b (D5) dopamine receptor mRNAs. D1a receptor mRNA was found in only a small subset (20%) of the sample and D3 and D4 receptor mRNAs were undetectable. D2 receptor agonists rapidly and reversibly reduced N-type Ca2+ currents. D1b/D1a receptor activation had little or no effect on Ca2+ currents. The D2 receptor antagonist sulpiride blocked the effect of D2 agonists. Dialysis with guanosine-5'-O-(2-thiodiphosphate) or brief exposure to the G protein (Gi/o) alkylating agent N-ethylmaleimide also blocked the D2 modulation. The reduction in N-type currents was neither accompanied by kinetic slowing nor significantly reversed by depolarizing prepulses. The D2 receptor effects were mediated by a membrane-delimited pathway, because the modulation was not seen in cell-attached patches when agonist was applied to the bath and was not disrupted by perturbations in cytosolic signaling pathways known to be linked to D2 receptors. Activation of M2 muscarinic receptors occluded the D2 modulation, suggesting a shared signaling element. However, activation of protein kinase C attenuated the M2 modulation without significantly affecting the D2 modulation. Taken together, our results suggest that activation of D2 dopamine receptors in cholinergic interneurons reduces N-type Ca2+ currents via a membrane-delimited, Gi/o class G protein pathway that is not regulated by protein kinase C. This signaling pathway may underlie the ability of D2 receptors to reduce striatal acetylcholine release.

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(-4) 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(-4) 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(-5) 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 micrograms/10 microliters)-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.

Evolution of dopamine receptors in the rat after neonatal hypoxia-ischemia: autoradiographic studies

The aim of this work was to follow the evolution of striatal dopamine D1 and D2 receptors after hypoxic-ischemic (H/I) insult in immature rats. SPET imaging of these receptors could be used as an index to assess brain dysfunctions after perinatal H/I without change in cerebral blood flow or neuronal loss. We submitted 1-week-old rat pups to unilateral ligation of the left carotid artery plus 2h exposure to 8% O2. After recovery periods of 1, 2 or 9 weeks, ex vivo and in vitro autoradiographic studies of dopamine receptors were performed on normal appearing brains using specific ligands usable in human SPET imaging. Striatal dopamine D2 receptors tended to decrease bilaterally after one week's recovery. The decrease then reached 40% at 3 weeks of age and at 10 weeks of age the level of receptors had returned to normal values. By contrast, no change in dopamine D1 receptors was seen, whatever the age studied. In conclusion, changes in dopamine D2 receptors could be a valuable index for SPET imaging to evaluate H/I brain damage in the absence of anatomical lesions.

Bupropion induces sniffing: a possible dopaminergic mechanism

The effect of bupropion, an antidepressant drug, on sniffing behaviour was examined in rats. Animals treated intraperitoneally (i.p.) with different doses of bupropion (5-40 mg/kg) showed sniffing behaviour in a dose-related manner. Pretreatment of animals (i.p.) with the dopamine antagonists SCH 23390 (0.025-0.1 mg/kg) or sulpiride (12.5-50 mg/kg) decreased the sniffing induced by bupropion. Reserpine pretreatment (2.5 mg/kg, i.p., 18 h) alone and in combination with alpha-methyl-p-tyrosine (AMPT; 250 mg/kg, i.p.) also reduced the behaviour produced by the drug. The alpha-adrenoceptor phenoxybenzamine (5 and 10 mg/kg, i.p.) and the beta-adrenoceptor antagonist propranolol (5 and 10 mg/kg, i.p.) administered 60 min prior to bupropion did not affect the drug's effect on sniffing. Propranolol alone, however, induced sniffing. The antimuscarinic atropine (5 and 10 mg/kg, i.p.) administered 30 min prior to bupropion increased the bupropion response. Atropine alone induced vigorous sniffing. It is concluded that bupropion induced sniffing through a dopaminergic mechanism.

Endogenous dopamine limits the binding of antipsychotic drugs to D3 receptors in the rat brain: a quantitative autoradiographic study

[3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin was used as a radioligand for the autoradiographic measurements of dopamine D3 receptors in rat and human brain. Preincubation of the brain sections was necessary to obtain binding of the radioligand in the islands of Calleja and in the nucleus accumbens, but not in cerebellar lobules 9/10 of the rat. D3 receptors were also totally occluded in unwashed sections of the human striatum. The radioligand binding to D3 receptors was maximal after preincubating the sections for at least 10 min. Pretreatment of the animals with reserpine or tetrabenazine, which results in a severe depletion of endogeneous monoamines, strongly reduces the occlusion of D3 receptors in unwashed brain sections. The occlusion of dopamine D3 receptors in brain sections suggests that the in vivo access to D3 receptors may be locally inhibited by endogenous dopamine. The in vitro binding affinities of 12 antipsychotic drugs for D2 and D3 receptors were evaluated in competition binding experiments, using both rat and cloned human receptors. Most of the compounds showed only a slightly lower affinity for D3 than for D2 receptors in vitro. Affinities of the antipsychotic drugs for cloned human D21 and D3 receptors were very close to their affinities for the rat receptors. In vivo occupancy of these receptors in the rat brain was measured ex vivo by quantitative autoradiography, 2 hours after subcutaneous drug administration. For most compounds, occupancy of D3 receptors, as compared to D2 receptor occupancy, was lower than expected from the corresponding in vivo affinity ratios. For the new antipsychotic risperidone, in vivo occupancy of D3 receptors was measured both in the islands of Calleja and in the cerebellar lobules 9/10. This compound was three times less potent for the occupancy of D3 receptors in the islands of Calleja than in the cerebellum, an area lacking endogenous dopamine (ED50 = 28 and 10 mg kg-1, respectively). Based on the observations in the rat brain, it may reasonably be supposed that therapeutic dosages of antipsychotic drugs will induce in patients only a minor occupancy of D3 receptors in brain areas containing high dopamine concentrations. The role of dopamine D3 receptors as a target of antipsychotic drugs may therefore be less important than previously thought.

Differential effects of clozapine and haloperidol on dopamine receptor mRNA expression in rat striatum and cortex

The regulation of the dopamine (DA) receptors is of considerable interest, in part because treatment with antipsychotic drugs is known to upregulate striatal D2-like receptors. While previous studies have focused on the regulation of striatal DA receptors, less is known about the pharmacological regulation of cortical DA receptors. The purpose of this study was to examine the regulation of DA mRNA receptor expression in the cortex compared to the striatum following treatment with antipsychotic agents. Adult male Sprague-Dawley rats were injected daily with haloperidol (2 mg/kg/day), clozapine (20 mg/kg/day) or a control vehicle for a period of 14 days. Following treatment, brains were subjected to in situ hybridization for the mRNAs encoding the five dopamine receptors; only D1, D2, and D3 receptor mRNAs were detected in these regions. Haloperidol tended to either modestly upregulate or have no effect on dopamine receptor mRNAs detected in striatal structures, while clozapine generally downregulated these mRNAs. On the other hand, in the cortex, both drugs had striking effects on D1 and D2 mRNA levels. Cortical D1 mRNA was upregulated by haloperidol, but this effect was primarily restricted to cingulate cortex; clozapine also upregulated D1 mRNA, but primarily in parietal regions. Haloperidol downregulated D2 mRNA in the majority of cortical regions, but most dramatically in frontal and cingulate regions; clozapine typically upregulated this mRNA, but primarily in regions other than frontal and cingulate cortex. These results indicate that clozapine and haloperidol each have regionally-specific effects, and differentially regulate dopamine receptor mRNA expression in striatal and cortical regions of the rat brain.

A randomized crossover trial of levodopa in congestive heart failure

BACKGROUND

The short- and long-term effects of levodopa (L-dopa), an oral dopaminergic prodrug, were assessed in patients with severe left ventricular dysfunction.

METHODS AND RESULTS

Initially, 26 patients were included in the study group. After clinical, radiographic, and radionuclide examination, each patient underwent right heart catheterization (Swan-Ganz thermodilution catheter). Plasma noradrenaline levels were measured. In two patients, a favorable hemodynamic response to L-dopa was not observed, another two required permanent pacemaker implantation. These four patients were excluded from the study. Two patients required permanent pacemaker implantation. The remaining 22 patients with favorable hemodynamic response to L-dopa (increase in cardiac index, stroke volume index, reduction in total systemic resistance) were randomized in a nonblinded fashion to the conventional (11 patients) or conventional plus L-dopa (11 patients) treatment groups. During the study period, two patients, one from each group, died. They were excluded from the analysis. The final analyzed study group consisted of 20 men, aged 33-69, in New York Heart Association functional class IV (9 patients) and III (11 patients). The cause of congestive heart failure was primary dilated cardiomyopathy in 11 patients and ischemic heart disease in 9 patients. After 3 months' treatment, all patients were crossed over. Clinical, radiographic, radionuclide, and hemodynamic evaluation was repeated at the end of the 3-month treatment period. After 3 months of therapy with L-dopa in each group (covariance analysis), there was improvement in clinical, radiographic (relative heart volume, -128 mL/m2), radionuclide (left ventricular ejection fraction, +4.6; right ventricular ejection fraction, +4.8%), hemodynamic (mean pulmonary wedge pressure, -8 mmHg; total systemic resistance, -1.8 Wu; total pulmonary resistance, -3.5 Wu), and neurohumoral (noradrenaline, -218 pg/mL) measures.

CONCLUSIONS

The addition of L-dopa to conventional therapy has beneficial short- and long-term effects in patients with severe left ventricular dysfunction.

Behavioural and neurochemical sensitization of morphine-withdrawn rats to quinpirole

The sensitivity of dopamine D2-like receptors in morphine-withdrawn rats was studied using the selective agonist quinpirole. Morphine was administered twice daily increasing the daily dose from 20 to 50 mg/kg during 7 days. Twenty-four hours after the last morphine administration the rats were given quinpirole (0.01-1 mg/kg) and their behavior was assessed. Withdrawal from repeated morphine treatment enhanced yawning behavior and penile erections induced by small doses (0.01-0.1 mg/kg) as well as the intensity of stereotypy induced by a large dose (1.0 mg/kg) of quinpirole. In the morphine-withdrawn rats the dose of 1 mg/kg of quinpirole caused less yawning than in the control rats, whereas the number of erections induced by this dose was enhanced as compared with the control animals. In the control rats, the striatal and limbic concentrations of dopamine metabolites, 3,4-dihydroxphenylacetic acid (DOPAC), and homovanillic acid (HVA), were not clearly affected by the smallest dose of quinpirole. However, the small dose of quinpirole (0.01 mg/kg) significantly reduced the levels of DOPAC and HVA in the striatum and limbic forebrain of the rats withdrawn from morphine either for 24 or 48 h. These findings indicate that withdrawal from repeated morphine treatment enhances the sensitivity of dopamine D2-like receptors.

Cloning and sequence analysis of a hypothalamic cDNA encoding a D1c dopamine receptor in tilapia

Physiological and pharmacological studies have indicated that during acid stress a D1-like dopamine receptor becomes functional on intermediate pituitary melanocyte-stimulating hormone cells of tilapia (Oreochromis mossambicus). As a first step towards physiological expression studies we isolated a D1-like dopamine receptor from a tilapia hypothalamus cDNA library. Construction of a phylogenetic tree of most of the D1-like receptors known in human, rat, Xenopus, goldfish and Drosophila revealed that the here presented clone is most likely the tilapia equivalent of the Xenopus D1c dopamine receptor.

Is renal dose dopamine protective or therapeutic? Yes

In conclusion, dopamine has the unique ability, compared with other catecholamines, to improve renal blood flow, glomerular filtration rate, sodium excretion, and creatinine clearance, independent of its cardiac effects. In addition, low-dose dopamine can decrease renal and systemic vascular resistance, suppress aldosterone secretion, and interact with atrial natriuretic factor. Because of these clinically significant properties, dopamine has been used successfully to improve and treat acute oliguric renal failure in a variety of clinical situations as just described. In addition, there were no adverse or toxic cardiac effects, such as tachyarrhythmias or hypertension, detected with low-dose dopamine in studies reviewed for this publication. By increasing renal and mesenteric vasodilation, dopamine has been shown to be beneficial in preserving renal function in cardiac surgery, vascular surgery, liver transplantation, contrast-induced nephropathy, hypertension, and pediatric patients. A therapeutic renal effect has been observed in patients with hepatorenal syndrome or severe ovarian hyperstimulation syndrome, in patients requiring vasopressors and IABP, and in selected cases of acute oliguric renal failure and shock. Furthermore, the combination of low-dose dopamine with furosemide or prostaglandin results in enhanced renal effects. Further investigation is necessary to evaluate the important and specific therapeutic role of low-dose dopamine through prospective, randomized, double-blind studies. Until those data are available, the plethora of clinical evidence supporting the ability of low-dose dopamine to augment renal function continues to grow. For those who are skeptical, we offer the following suggestion: "The obscure we see eventually, the obvious takes a little longer"--E.R. Murrow.

Theophylline-induced grooming: possible indirect dopaminergic mechanism

The ability of theophylline, an adenosine antagonist and phosphodiesterase inhibitor, to induce grooming was studied in rats. Grooming was induced by intraperitoneal (i.p.) injection of different doses (6-25 mg/kg) of theophylline to rats. The effect was dose-dependent. However, the response was decreased with increasing doses of the drug from 25-75 mg/kg. Administration of the dopamine D1 receptor agonist SKF 38393 (1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrochloride; 16 mg/kg i.p.) also caused grooming in a dose-dependent manner. The response induced by SKF 38393 (1-4 mg/kg i.p.) was decreased by the high doses of theophylline (50 and 75 mg/kg i.p.). The dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1 H-benzazepine-7-ol maleate) decreased the theophylline and SKF 38393 response. Pretreatment of animals with reserpine (2.5 mg/kg i.p., 24 h) reduced the effect of theophylline (12.5 and 25 mg/kg i.p.) but not that of SKF 38393 (1 and 4 mg/kg i.p.). It is concluded that theophylline elicits grooming through an indirect D1 dopaminergic mechanism.

Effects of cerebral ischemia on dopamine receptors in the gerbil striatum

Dopamine D1 and D2 receptors and uptake sites were studied in the gerbil striatum and frontal cortex 1 h to 7 days after 10 min of cerebral ischemia caused by occlusion of the bilateral common carotid arteries. [3H]SCH23390 ([N-methyl-3H]R[+]-8-chloro-2, 3,4,5-tetrahydro-3-methyl-5-phenyl-7-ol-benzazepine), [3H]nemonapride and [3H]mazindol were used as markers of dopamine D1 receptors, D2 receptors and uptake sites, respectively. A significant reduction in [3H] SCH23390 binding was found in the striatum from 48 h after ischemia. In contrast, during the recirculation periods, [3H]nemonapride and [3H]mazindol binding was mostly unaffected in this region which was the most vulnerable to ischemia. The frontal cortex, where ischemic neuronal damage was mild, also showed no significant changes in [3H]SCH23390, [3H]nemonapride and [3H]mazindol binding after ischemia. Thus, cerebral ischemia that was associated with cell loss in the striatum resulted in a selective reduction of dopamine D1 receptors and not D2 receptors. No changes in dopamine D1 or D2 receptors were observed in frontal cortex. If massive dopamine release occurs with cerebral ischemia, it is not reflected by modification in the number of uptake sites located on dopamine terminals.

Stimulation of adenosine A1 receptors prevents the EEG arousal due to dopamine D1 receptor activation in rabbits

The influence of adenosine A1 (N6-cyclopentyladenosine, CPA) and A2 (2-[4-(2-carboxylethyl)phenethylamino]-5'-N-ethylcarboxamido -adenosine hydrochloride, CGS 21680) receptor agonists on SKF 38393-induced electroencephalographic (EEG) arousal was studied in rabbits. While CPA (0.1 mg/kg i.v.) significantly prevented the EEG effects of SKF 38393, CGS 21680 (0.2 mg/kg i.v.) did not affect them. These results demonstrate that adenosine A1 receptors can modulate dopamine D1 receptor-induced EEG arousal and show, for the first time, that adenosine-dopamine interactions are involved in brain functions other than motor activity.

Effects of ceruletide on perioral movements and the dopamine receptor-adenylate cyclase system in rats chronically treated with fluphenazine

The effects of repeated administration of ceruletide (100 micrograms/kg/perday, i.p. for 3 days) on perioral movements and the striatal dopamine receptor adenylate cyclase system were examined in rats chronically treated with fluphenazine enanthate (FPZ) (25 mg/kg i.m. every 3 weeks for 30 weeks) and sesame oil-treated (control) rats. After the tenth injection of fluphenazine, the rats started to display five types of perioral movements (teeth chattering, chewing, tongue protrusion, mouth opening and perioral tremors). Moreover, increases in SCH23390 binding and spiperone binding to striatal membranes, were found in the FPZ-treated rats. Furthermore, dopamine receptor-coupled adenylate cyclase activity was potentiated in striatal membranes. High amplitude EMG discharges (8-10 Hz), recorded from the masseter in the FPZ-treated rats occurred concurrently with perioral tremors. Repeated ceruletide (CLT) injections abolished perioral movements, and reversed both the elevated SCH23390 binding and the dopamine stimulated adenylate cyclase (AC) activity to the control level. The effect of CLT on perioral movements, D1 receptors and dopamine-stimulated AC activity continued for 6 days after the final CLT injection. These findings suggest that systemically administered CLT affects the D1 receptor adenylate cyclase system and that an increase of the D1 receptor mechanism may play an important role in the pathogenesis of tardive dyskinesia.

Effect of picrotoxin on antinociception in the formalin test

Subcutaneous injection of diluted formalin (0.25 microliter of 0.5%) caused a biphasic pain response in mice. The first phase of pain was observed during the first 5 min., while the second phase occurred 10-30 min. after formalin administration. With the formalin test, it was found that the antinociception produced by the GABA-A antagonist, picrotoxin, and the GABA-B antagonist, phaclofen, was abolished when employed in combination. The opioid antagonist naloxone and antimuscarinic atropine also decreased the picrotoxin response. However, sulpiride, SCH 23390, phenoxybenzamine and propranolol did not alter the picrotoxin response. Administration of naloxone, sulpiride and propranolol showed a pain response. The data indicate that dopaminergic and adrenergic mechanisms may not be involved in the picrotoxin antinociceptive effect. However, postsynaptic GABA-A and GABA-B may be involved in the drug effect, and involvement of opioid or cholinergic systems can not be excluded.

Nicotine potentiates morphine antinociception: a possible cholinergic mechanism

The effect of nicotine on morphine induced antinociception was studied using the tail-flick test. A low dose of nicotine (0.0001 mg/kg) which did not induce any antinociception, potentiated the morphine antinociceptive response dose-dependently. The opioid antagonist naloxone decreased the response of morphine and morphine plus nicotine. The muscarinic receptor antagonist atropine decreased the nicotine-induced potentiation of the morphine response. Sulpiride, pimozide, SCH 23390, phenoxybenzamine, propranolol, methysergide, metergoline, hexamethonium and mecamylamine did not alter the antinociception induced by nicotine and morphine. It is concluded that the potentiation of morphine-induced antinociception by nicotine may be mediated through a cholinergic mechanism.

Functional regulation by dopamine receptors of serotonin release from the rat hippocampus: in vivo microdialysis study

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat hippocampus was investigated by use of in vivo microdialysis. Dialysate 5-HT levels were reduced by co-perfusion of 10 microM tetrodotoxin (TTX) and were elicited by K+ (60 and 120 mM) stimulation in a concentration-dependent manner. Local perfusion (10 microM) and peripheral administration (20 mg/kg, i.p.) of fluoxetine produced increases in 5-HT levels. These results indicate that the spontaneous 5-HT levels in the rat hippocampus can be used as indices of neuronal origin from the serotonergic nerve terminals. The nonselective dopamine (DA) receptor agonist apomorphine (1, 10 and 100 microM), when perfused through the probe over a period of 40 min, increased 5-HT release in a concentration-dependent manner. Apomorphine-induced (100 microM) increases in 5-HT release was abolished by pretreatment with the selective D2 receptor antagonist, S(-)-sulpiride (1 and 10 microM), but not prevented by pretreatment with the selective D1 receptor antagonist, R(+)-SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2, 3, 4, 5-tetrahydro-1H-3-benzazepine) (1 microM). S(-)-Sulpiride and R(+)-SCH-23390 by themselves did not alter the spontaneous 5-HT levels. The 5-HT release was elevated by perfusion of the selective DA reuptake inhibitor GBR 12909 (1-[2-[bis(4-fluorophenyl) methoxy]ethyl]-4-[3-phenyl-propyl]piperazine) (1, 10 and 100 microM), indicating the possibility of not only exogenous but also endogenous DA-mediated facilitatory effects on 5-HT release in vivo. The 5-HT release was also elevated by perfused (+/-)-PPHT ((+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin) (1, 10 and 100 microM), the selective D2 receptor agonist, in a concentration-dependent manner. On the other hand, (+/-)-PPHT (100 microM) failed to increase 5-HT release in catecholamine (CA)-lesioned rats pretreated with 6-hydroxydopamine (6-OHDA)(200 micrograms/rat, i.c.v.). The (+/-)-PPHT-induced (100 microM) increase in 5-HT release was prevented not only by pretreatment with 10 microM S(-)-sulpiride but also by pretreatment with the alpha 2-adrenoceptor antagonist idazoxan (10 microM). These findings suggest that the functional regulation of 5-HT release via D2 receptors exists in the rat hippocampus. Furthermore our results indicate that the facilitatory effect of 5-HT release via D2 receptors may be mediated indirectly by noradrenergic neurons, but not mediated directly through D2 receptors located on serotonergic nerve terminals.

In vivo binding of [11C]SKF 75670 and [11C]SKF 82957 in rat brain: two dopamine D-1 receptor agonist ligands

The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 labeled with C-11 were evaluated in vivo in rats as potential radioligands for imaging dopamine D1 receptors with positron emission tomography (PET). Their in vivo pharmacological profile revealed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen. The more lipophilic [11C]SKF 82957 (6-chloro-[11C]SKF 75670) showed a higher brain uptake (more than 2-fold up to 30 min), higher specific uptake in the striatum and higher signal-to-noise ratio (striatum-to-cerebellum = 3.2 +/- 0.4 for [11C]SKF 75670 and 9.7 +/- 2.5 for [11C]SKF 82957 at 60 min post-injection) as compared to [11C]SKF 75670. Both radiotracers exhibited high specificity and selectivity for D1 receptors, since only D1 competitors but not the D2 antagonist sulpiride or the 5-HT2 antagonist ritanserin reduced significantly their binding the striatum with [11C]SKF 75670 or the striatum and olfactory tubercles with [11C]SKF 82957. Previous reports have shown that only D1 agonists can recognize the functional high-affinity state from the low-affinity state of D1 receptors. [11C]SKF 75670 and especially [11C]SKF 82957 are D1 agonist radioligands that can potentially be used to study in vivo the functional high-affinity state of D1 receptors using PET.

Synthesis and autoradiographic localization of the dopamine D-1 agonists [11C]SKF 75670 and [11C]SKF 82957 as potential PET radioligands

The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 were labeled with 11C by N-[11C]methylation of SKF 38393 and SKF 81297, respectively, using [11C]methyl iodide in the presence of N-ethyldiisopropylamine. Both radiotracers were purified using a semi-preparative cation exchange HPLC column. Radiochemical yields of 20-75% were obtained (from [11C]methyl iodide, decay-corrected) with a synthesis time of 30-35 min from EOB. The specific activities were 700-2500 Ci/mmol (25.9-92.5 GBq/mumol) at EOS, and the radiochemical purities were > 99%. Autoradiographic studies showed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen, nucleus accumbens, olfactory tubercles and substantia nigra. [11C]SKF 75670 and [11C]SKF 82957 are thus potential PET radioligands for the functional high-affinity state of D1 receptors.

Nicotine attenuates naloxone-induced jumping behaviour in morphine-dependent mice

In the present study the effect of nicotine on naloxone-induced jumping behaviour in morphine-dependent mice was examined. In addition, the modulatory role of dopaminergic, adrenergic and cholinergic mechanisms upon the effect of nicotine were investigated. Animals were rendered dependent on morphine by subcutaneous (s.c.) injections of morphine sulfate 3 times a day for 3 days, and jumping behaviour was induced by intraperitoneal (i.p.) administration of naloxone 2 h after the tenth injection of morphine sulfate on day 4. Nicotine (0.001-2 mg/kg s.c.) caused a significant decrease in withdrawal jumping behaviour in morphine-dependent mice. The effect of nicotine was blocked by the central nicotinic antagonist mecamylamine (0.01-0.1 mg/kg i.p.) but not by the peripheral nicotinic antagonist hexamethonium (0.01 and 0.1 mg/kg i.p.) nor the muscarinic receptor antagonist atropine (2.5-10 mg/kg i.p.). The dopamine receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5- phenyl-1H-3-benzazepine-7-ol maleate) (0.01-0.5 mg/kg s.c.) reduced the response induced by nicotine. The dopamine receptor antagonist sulpiride (25 and 50 mg/kg s.c.) and the adrenoceptor antagonists phenoxybenzamine (5 and 10 mg/kg i.p.) and propranolol (5 and 10 mg/kg i.p.) were without an effect. The results indicate that the effect of nicotine on naloxone-induced jumping is mediated by central nicotinic receptors.

Characterisation of dopamine receptors in insect (Apis mellifera) brain

In vitro binding experiments using the vertebrate D1 dopamine receptor ligand [3H]SCH23390 and the vertebrate D2 dopamine receptor ligand [3H]spiperone were conducted on membrane preparations of honey bee (Apis mellifera) brain. Specific binding of [3H]SCH23390 was saturable and reversible. Analysis of saturation data gave an apparent Kd of 6.3 +/- 1.0 nM and Bmax of 1.9 +/- 0.2 pmol/mg protein for a single class of binding sites. The specificity of high affinity [3H]SCH23390 binding was confirmed in displacement experiments using a range of dopaminergic antagonists and agonists. The rank order of potency for antagonists was: R(+)-SCH23390 > cis-(Z)-flupentixol > or = chlorpromazine > fluphenazine > S(+)-butaclamol > spiperone. R(+/-)-SKF38393 and dopamine were the most effective agonists tested. [3H]SCH23390 labels a site in bee brain that is similar, but not identical to the vertebrate D1 dopamine receptor subtype. [3H]Spiperone also bound with high affinity to bee brain homogenates. Scatchard analysis of [3H]spiperone saturation data revealed a curvilinear plot suggesting binding site heterogeneity. The high affinity site had a apparent Kd of 0.11 +/- 0.02 nM and Bmax of 9.2 +/- 0.5 fmol/mg protein. The calculated values for the low affinity site were a Kd of 19.9 nM and Bmax of 862 fmol/mg protein. Kinetic analyses also indicated that [3H]spiperone recognises a heterogeneous population of sites in bee brain. Furthermore, agonist competition studies revealed a phenolaminergic as well as a dopaminergic component to [3H]spiperone binding in bee brain. The rank order of potency of dopaminergic antagonists in competing for [3H]spiperone binding was: spiperone > fluphenazine > S(+)-butaclamol > domperidone > R(+)-SCH23390 > S(-)-sulpiride.

Effects of nicotine on memory retrieval in mice

The effect of nicotine was tested on retrieval 24 h after training on a passive avoidance task. Intraperitoneal (i.p.) injection of nicotine (0.25-1.5 mg/kg) increased the step-down latency in mice dose dependently. Pretreatment with the nicotinic receptor antagonist mecamylamine (0.5-1 mg/kg) decreased, whereas pretreatment with the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol maleate) (0.01, 0.05 and 0.1 mg/kg) and the beta-adrenoreceptor antagonist propranolol (10 mg/kg) increased the nicotine response. The dopamine receptor D2 receptor antagonist sulpiride (5-10 mg/kg), the anti-muscarinic agent atropine (2.5-10 mg/kg), the peripheral nicotinic receptor antagonist hexamethonium (0.01-0.5 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (1 and 10 mg/kg) and the peripheral dopamine D2 receptor antagonist domperidone (5 and 10 mg/kg) did not change the response induced by nicotine. Single administration of the antagonists did not cause response; however, a high dose of domperidone (10 mg/kg) and propranolol alone increased the step-down latencies. It may be concluded that a nicotinic receptor mechanism is involved in the nicotine-induced improvement of memory retrieval.

Alterations of bromocriptine-induced penile erection by chronic lithium in rats

In the present study, the effects of chronic lithium pre-treatment (30 days) on penile erection (PE) induced by bromocriptine were investigated in rats. Intraperitoneal administration of the dopamine receptor agonist, bromocriptine (4-32 mg/kg) induced PE in a biphasic manner. The maximum response was obtained with 8 mg/kg of bromocriptine and the effect was decreased with increasing doses of the drug from 8 to 32 mg/kg. When animals were pre-treated with different doses of the D-1 dopamine receptor antagonist, SCH 23390, or the D-2 dopamine receptor antagonist, sulpiride, the PE response was decreased. The response induced by bromocriptine (4-32mg/kg) was reduced in animals pre-treated with chronic lithium. SCH 23390 did not produce a larger inhibitory effect on the bromocriptine response in animals pre-treated with chronic lithium, but the inhibitory effect of sulpiride was increased in this condition. It is concluded that chronic lithium treatment may alter the D-1/D-2 receptor activity and inhibit bromocriptine-induced PE.

Effects of high doses of theophylline on memory acquisition

Intraperitoneal (IP) injection of high doses of theophylline decreased memory acquisition dose dependently. Low but not high doses of the dopamine receptor D2 antagonist sulpiride and the dopamine D1 receptor antagonist SCH 23390 decreased the theophylline response. Pimozide and the beta-adrenoreceptor propranolol also decreased the drug response. The anti-muscarinic agent atropine, the alpha-adrenoceptor phenoxybenzamine and the 5-HT antagonist metergoline did not affect the response induced by theophylline. It is concluded that dopamine and beta-adrenergic receptor mechanisms may be involved in the theophylline-induced attenuation of memory acquisition.

Effects of adenosine analogues on apomorphine-induced penile erection in rats

1. In the present work, the effect of adenosine agonists and antagonists on apomorphine-induced penile erection (PE) has been studied. 2. Subcutaneous (s.c.) injection of the nonselective D1/D2 dopamine receptor agonist apomorphine (0.05-0.5 mg/kg) induced PE in a biphasic manner. The maximum effect was obtained with 0.1 mg/kg of the drug. The response decreased with increasing doses of apomorphine, from 0.1 to 0.5 mg/kg. 3. Intraperitoneal (i.p.) injections of adenosine agonists 5'-N-ethylcarboxamidoadenosine (NECA) and N6-cyclohexyladenosine (CHA) decreased the response of apomorphine. Apomorphine-induced PE was increased by low doses (25, 50 mg/kg, i.p.) and decreased by high doses (75, 100 mg/kg, i.p.) of the adenosine antagonist theophylline, respectively. Inhibition of PE induced by NECA and CHA was antagonized by 8-PT pretreatment. 4. Intracerebroventricular (i.c.v.) administration of CHA, NECA, and theophylline produced the same effects as i.p. injections of these agents on PE responses. It is concluded that A-1 and A-2 adenosine receptor activation may inhibit PE induced by dopaminergic mechanism(s), which can be prevented by 8-PT pretreatment.

Effects of putative dopamine D3 receptor agonists, 7-OH-DPAT, and quinpirole, on yawning, stereotypy, and body temperature in rats

7-OH-DPAT ((+/-)-2-(dipropylamino)-7-hydroxy-1,2,3,4-tetrahydronaphthalene) was recently identified as a dopamine receptor agonist having a > 100-, 1,000- and > 10,000-fold higher affinity for dopamine D3 than for D2, D4 and D1 receptors, respectively. Quinpirole (LY 171555) has also been reported to have a 113-fold greater affinity for dopamine D3 receptors than for D2 receptors. Therefore, we investigated the effects of these putative dopamine D3 receptor agonists on yawning, stereotypy and rectal temperature in rats (N = 424). 7-OH-DPAT and quinpirole administered subcutaneously (SC) at respective low doses of 10-250 micrograms/kg and 25-500 micrograms/kg elicited yawning behavior. The yawning induced by these agents was blocked by spiperone (0.5 mg/kg, SC) and scopolamine (0.5 mg/kg, SC) but was increased by intraperitoneal (IP) administration of pindolol (20 mg/kg). The yawning was also potentiated after treatment with reserpine. 7-OH-DPAT and quinpirole at respective high doses of 0.25 mg/kg (SC) and 0.5 mg/kg (SC) evoked slight stereotypy such as sniffing and licking, and this effect was enhanced by a selective dopamine D1 receptor agonist, SK&F 38393 (1-phenyl-2,3,4,5,-tetrahydro-(1H)-3-benzazepine-7,8-diol). 7-OH-DPAT (0.5 mg/kg, SC) and quinpirole (0.5 mg/kg, SC) decreased, but SK&F 38393 (10 mg/kg, SC) increased body temperature. However, the hyperthermia induced by SK&F 38393 was interestingly enhanced by 7-OH-DPAT and quinpirole.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic lithium pretreatment on apomorphine-induced penile erection

1. The effects of chronic lithium pretreatment (600 mg/l in drinking rats, 30 days) on penile erection (PE) induced by apomorphine were investigated in rats. This treatment resulted in a serum Li concentration after 30 days of 0.31 +/- 0.01 mmol/l. 2. Subcutaneous (s.c.) administration of mixed D1/D2 dopamine receptor agonist apomorphine (0.05-0.5 mg/kg) induced PE in a biphasic manner. The maximum effect was obtained with 0.1 mg/kg of the drug while the response decreased with increasing doses of apomorphine from 0.1 to 0.5 mg/kg. 3. Pretreatment of animals with 0.0125-0.1 mg/kg of D1 dopamine receptor antagonist SCH 23390 or D2 dopamine receptor antagonist sulpiride (12.5-100 mg/kg) decreased apomorphine-induced PE. Combination of SCH 23390 (0.025 mg/ kg) with sulpiride (12.5 mg/kg) caused a stronger inhibitory effect on apomorphine response. This indicates that both D1 and D2 dopamine receptors may be involved in PE induced by apomorphine. 4. The response induced by apomorphine (0.05-0.05 mg/kg) was decreased in animals pretreated with chronic lithium. The inhibitory effect of sulpiride on apomorphine response, increased in animals pretreated with lithium, in contrast the inhibitory effect of SCH 23390 did not change in this condition. However, a combination of SCH 23390 with sulpiride increased inhibitory effect on apomorphine response in lithium pretreated rats. 5. It is concluded that chronic lithium inhibits PE induced by dopaminergic mechanism(s).

The putative dopamine D3 agonist, 7-OH-DPAT, reduces dopamine release in the nucleus accumbens and electrical self-stimulation to the ventral tegmentum

The present experiments were designed to test further the idea that 7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin), a putative dopamine (DA) D3 agonist, has effects at DA autoreceptors to reduce intracranial DA levels and to reduce behaviours that are DA-dependent. Rats were trained to respond on a self-stimulation protocol for electrical stimulation to the ventral tegmental area (VTA). Each press of a lever delivered a 0.5 s train of square wave, 1.5 ms duration, 100 Hz, 90-120 mA stimulation. Systemic administration of 7-OH-DPAT at 0.01-0.3 mg/kg i.p., quickly dose-dependently reduced responding. Electrical stimulation using similar parameters to those that supported self-stimulation were then applied to the VTA of anaesthetized rats. Fast cyclic voltammetry (FCV) revealed that this stimulation released DA in the nucleus accumbens (NAC). 7-OH-DPAT i.p. (0.1-3.0 mg/kg) quickly and potently reduced the size of the DA-generated voltammetric signal. This effect of 0.3 mg/kg 7-OH-DPAT was not blocked by sulpiride (60 mg/kg, i.p.) a D2-specific antagonist that may preferentially block D2 autoreceptors. These data are discussed with reference to the possibility that 7-OH-DPAT reduces the release of dopamine in the NAC, at D3, but not at D2, autoreceptors and that this in turn may reduce the rewarding effect of VTA stimulation.

Autoradiographic localization of dopamine D2-like receptors in the rat adrenal gland

The pharmacological profile and the anatomical localization of dopamine D2-like receptors were studied in sections of the rat adrenal gland using combined radioligand binding and autoradiographic techniques with [3H]-spiroperidol as a ligand. [3H]-Spiroperidol was bound to sections of the rat adrenal gland in a manner consistent with the labelling of dopamine D2-like receptor sites. The binding was time-, temperature- and concentration-dependent and of high affinity with a dissociation constant (Kd) value of 1.6 +/- 0.04 nM and a maximum density of binding sites (Bmax) of 60 +/- 3.6 fmol/mg tissue. Experiments on the pharmacological specificity of [3H]-spiroperidol binding to sections of the rat adrenal gland suggest the labelling of dopamine D3 and/or D4 receptors. The presence of dopamine D3 and D4 receptors in the rat adrenal gland was confirmed by the demonstration of a specific binding for the D3 radioligand [3H]-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (DPAT) and for the D4 radioligand [3H]-clozapine. Light microscope autoradiography showed the highest accumulation of silver grains which correspond to [3H]-spiroperidol binding sites in the rat adrenal medulla. In the adrenal cortex, where density of silver grains is about 40% lower than in the medulla, the radioligand is accumulated primarily in the zona glomerulosa and to a lesser extent in the zona reticularis. These findings suggest that dopamine D2-like receptor sites in the rat adrenal gland cortex are primarily involved in the modulation of catecholamine secretion from the medulla and of aldosterone secretion from the cortex. The possible relevance of the occurrence of dopamine D3 and D4 receptor subtypes in the adrenal gland is discussed.

The discriminative properties of the D1 dopamine agonist dihydrexidine in the rat

The objective of this study was to train rats to discriminate the interoceptive stimuli produced by a selective dopamine D1 agonist. Fourteen male Sprague-Dawley rats acquired the discrimination of the fully effective, high potency, D1 agonist dihydrexidine (DHX) within 20 sessions using a training dose of 3.0 mg/kg. DHX (0.75-4.5 mg/kg) dose-dependently increased DHX-appropriate responding with an ED50 = 1.44 mg/kg. The selective D1 agonist SKF 38398 (2.0-8.0 mg/kg) dose-responsively generalized with an ED50 = 3.54 mg/kg; significantly less potent than DHX. The selective D1 antagonist SCH 23390 (0.06-0.12 mg/kg) dose-responsively decreased DHX-appropriate discriminative performance. These data would indicate that DHX is a selective D1 agonist that may allow for testing of the selectivity of other putative D1 agonists in this experimental procedure. Administration of non-selective dopaminergically active drugs, including apomorphine, amphetamine and cocaine, were each shown to produce intermediate DHX-appropriate discriminative performance.

Characterization of subtype-specific antibodies to the human D5 dopamine receptor: studies in primate brain and transfected mammalian cells

To achieve a better understanding of how D5 dopamine receptors mediate the actions of dopamine in brain, we have developed antibodies specific for the D5 receptor. D5 antibodies reacted with recombinant baculovirus-infected Sf9 cells expressing the D5 receptor but not with the D1 receptor or a variety of other catecholaminergic and muscarinic receptors. Epitope-tagged D5 receptors expressed in mammalian cells were reactive with both D5 antibodies and an epitope-specific probe. A mixture of N-linked glycosylated polypeptides and higher molecular-mass species was detected on immunoblots of membrane fractions of D5-transfected cells and also of primate brain. D5 receptor antibodies intensely labeled pyramidal neurons in the prefrontal cortex, whereas spiny medium-sized neurons and aspiny large interneurons of the caudate nucleus were relatively lightly labeled. Antibodies to the D5 dopamine receptor should prove important in experimentally determining specific roles for the D5 and D1 receptors in cortical processes and diseases.

Morphine and naltrexone modulate D2 but not D1 receptor induced motor behavior in MPTP-lesioned monkeys

Interactions at the behavioral level between dopamine (DA) and opioid receptors in the mammalian brain have been amply demonstrated. Considering the pivotal role for DA receptors in the pharmacotherapy of Parkinson's disease (PD), these interactions might be clinically relevant. Therefore, in the present study the effects of the opioid antagonist naltrexone and agonist morphine on D1 and D2 receptor induced stimulation of motor behavior in the unilateral MPTP monkey model (n = 5) of PD were investigated. The results show that both naltrexone and morphine [0.1-1.0 mg/kg; intramuscular injection (IM)] inhibited D2 receptor stimulated contralateral rotational behavior and hand use induced by administration of quinpirole (LY 171555; 0.01 mg/kg, IM) in a dose-related way. However, no effects of these opioid drugs were observed on D1 receptor stimulated contralateral rotational behavior and hand use induced by administration of SKF 81297 (0.3 mg/kg, IM). Interestingly, the action of the alleged preferential mu-receptor antagonist naltrexone was mimicked by the selective delta-opioid antagonist naltrindole (0.5 mg/kg, IM). From this study it is concluded that in a non-human primate model of PD, alteration of opioid tonus leads to modulation of D2 receptor but not D1 receptor controlled motor behavior. The possible underlying mechanisms and clinical relevance of these findings are discussed.

Dopamine and sexual behavior

Among central neurotransmitters involved in the control of sexual behavior, dopamine is certainly one of the most extensively studied. Our attempt to review old and recent neuropharmacological, biochemical, electrophysiological, and psychobiological studies performed so far only in rats, monkeys, and humans, provides evidence that dopamine through its different neuronal systems and receptor subtypes plays different roles in the control of several aspects of sexual behavior. In fact, while the nigrostriatal system is necessary for the control of the sensory-motor coordination required for copulation, the mesolimbic-mesocortical system plays a key role in the preparatory phase of the behavior, mainly in sexual arousal, motivation and possibly reward. Conversely, the incertohypothalamic system plays a major role in the consummation of the behavior, mainly in seminal emission and erectile performance, but evidence for its involvement in sexual motivation also exists. The dopaminergic receptors playing the major role in the control of male sexual behavior belong to the D2 receptor subtype. However a D1/D2 receptor interaction is well established and an opposite role for D1 and D2 receptors in the preoptic area suggested. Despite some differences, most studies show that treatments that increase or decrease, respectively, brain dopaminergic activity improve or worsen, respectively, several parameters of copulatory activity, supporting a facilitatory role of dopamine in male sexual behavior. In contrast, no conclusion can be deduced from the available studies on the role of central dopaminergic systems in the control of proceptivity and receptivity, the two main components of female sexual behavior.

Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons

In rat neostriatal neurons, D1 dopamine receptors regulate the activity of cyclic AMP-dependent protein kinase (PKA) and protein phosphatase 1 (PP1). The influence of these signaling elements on high voltage-activated (HVA) calcium currents was studied using whole-cell voltage-clamp techniques. The application of D1 agonists or cyclic AMP analogs reversibly reduced N- and P-type Ca2+ currents. Inhibition of PKA antagonized this modulation, as did inhibition of PP1, suggesting that the D1 effect was mediated by a PKA enhancement of PP1 activity directed toward Ca2+ channels. In a subset of neurons, D1 receptor-mediated activation of PKA enhanced L-type currents. The differential regulation of HVA currents by the D1 pathway helps to explain the diversity of effects this pathway has on synaptic integration and plasticity in medium spiny neurons.

Are retinal or mesencephalic dopaminergic systems involved in monocular optokinetic nystagmus asymmetry in frog?

In monocular vision, frogs display a unidirectional optokinetic horizontal nystagmus (H-OKN) reacting only to temporal-nasal (T-N) stimulation. The N-T component is almost absent. The analysis of search coil recordings after administration of dopamine into the viewing eye, the occluded eye or directly into the pretectum, hardly modifies the H-OKN triggered by the viewing eye irrespective of the concentration used. Conversely, administration of Piribedil, a strong D2 dopamine agonist, provokes the appearance of a N-T component, suppressing the monocular H-OKN asymmetry, whether the drug is injected by intravitreal or intrapretectal route. It is suggested that Piribedil could also bind with receptors other than dopamine's.

Presynaptic dopamine autoreceptors and second messengers controlling tyrosine hydroxylase activity in rat brain

In brain areas enriched of dopaminergic nerve terminals presynaptic dopamine (DA) autoreceptors control the state of activation of tyrosine hydroxylase (TH) by regulating the extent of phosphorylation of the enzyme. Evidence is presented indicating that this autoinhibitory control may involve a decrease in the cyclic AMP-dependent activation of TH through an inhibitory coupling of presynaptic DA autoreceptors to adenylate cyclase. As indicated by the insensitivity of the DA inhibition of TH to changes in the extracellular concentrations of Ca++, to the addition of the Ca++ ionophore A 23187 and of different K+ channel blockers, a reduction of Ca++ influx and an increase in the K+ channel activity do not seem to be involved in the presynaptic regulation of TH activity by DA autoreceptors at least under basal conditions.

alpha-MSH changes cyclic AMP levels in rat brain slices by an interaction with the D1 dopamine receptor

The exposure of rat brain slices containing caudate putamen and accumbens nuclei to alpha-MSH or dopamine (DA) results in an increase in cyclic AMP (cAMP) levels. When tissues are compared with those containing both alpha-MSH and DA, a reduction in the cyclic nucleotide is observable. This study was carried out to determine whether variations in tissular cAMP levels induced by alpha-MSH might be explained by an interaction between the peptide and some dopaminergic receptors. Therefore, we measured cAMP in tissues and medium in response to alpha-MSH in the presence of haloperidol, the selective D1 (SCH 23390) or D2 (sulpiride) antagonists, or the selective D1 (SKF 38393) or D2 (bromocriptine) agonists. Haloperidol by itself induced no changes either in the cAMP content or in the cAMP efflux to the medium. When slices were exposed to alpha-MSH and haloperidol, the latter blocked the alpha-MSH effect of inducing an increase in the content of cAMP. None of the specific antagonists (at the administered doses) induced changes in the content of cAMP when compared with the control group. The presence of SCH 23390 in the incubation medium together with alpha-MSH yielded a reduction in cAMP levels compared with those incubated with alpha-MSH. A slight stimulatory effect on cAMP formation was observed when the dopaminergic agonists (SKF 38393 10 microM) were used. We conclude that alpha-MSH interacts with the D1 dopamine receptor, changing the cAMP levels in striatum and accumbens nuclei.

Nicotine-induced hypothermia through an indirect dopaminergic mechanism

The effect of nicotine on core body temperature was studied in mice. Intraperitoneal (i.p.) injection of nicotine (0.5, 1 and 2 mg/kg) induced a dose-dependent hypothermia. The response was inhibited by reserpine (5 mg/kg), the centrally active nicotinic receptor antagonist mecamylamine (0.1-1 mg/kg) and the D-2 dopamine receptor antagonist sulpiride (25-100 mg/kg). The β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) and the serotonergic blocker methysergide (5 and 10 mg/kg) did not inhibit but increased the nicotine response. The α-adrenoceptor antagonist phenoxybenzamine, the antimuscarinic agent atropine, the D-1 dopamine receptor antagonist SCH 23390, the peripheral dopamine antagonist domperidone and the peripheral nicotinic antagonist hexamethonium did not alter the nicotine-induced hypothermia. It is concluded that nicotine may cause a fall in core body temperature through a central dopaminergic mechanism.

Nicotine-induced purposeless chewing in rats: possible dopamine receptor mediation

Intraperitoneal (i.p.) administration of nicotine to rats induced purposeless chewing. The response induced by different doses of the drug (0.0001, 0.001, 0.01 and 0.1 mg/kg) seems to be dose dependent, with a maximum effect at 0.01 mg/kg and then decreasing at a higher dose (0.1 mg/kg). Pre-treatment of animals with the nicotine antagonist mecamylamine (0.01 and 0.1 mg/kg, 30 min) and the D-2 receptor antagonist sulpiride (12.5-100 mg/kg, 90 min), but not the D-1 antagonist SCH 23390 (0.01 and 0.05 mg/kg, 30 min), decreased the chewing induced by nicotine (0.01 mg/kg). When animals were pre-treated with propranolol (5 and 10 mg/kg) 60 min, reserpine (2.5 mg/kg) 18 h or α-methyl-p-tyrosine (α-MPT; 250 mg/kg) 60 min before nicotine, the effect of the drug was reduced. However, reserpine (2.5 mg/kg) at 18 h plus α-MPT (250 mg/kg) 60 min prior to nicotine completely inhibited the drug response. Pre-treatment of animals with phenoxybenzamine (2.5 and 5 mg/kg i.p., 60 min) or atropine (5 and 10 mg/kg) did not change the nicotine response significantly. It is concluded that nicotine- induced purposeless chewing is mediated through dopaminergic and nicotinic mechanisms.