Stereoselectivity of presynaptic autoreceptors modulating dopamine release

Efficacy of Vesicular Monoamine Transporter 2 Inhibition and Synergy with Antipsychotics in Animal Models of Schizophrenia

Antipsychotic medications function by blocking postsynaptic dopaminergic signaling in the central nervous system. Dopamine transmission can also be modulated presynaptically by inhibitors of vesicular monoamine transporter 2 (VMAT2), which inhibit loading of dopamine into presynaptic vesicles. Here we investigated the combination of these mechanisms in animal models of schizophrenia and weight gain (a primary side effect of antipsychotics). When dosed alone, the highly selective VMAT2 inhibitor RRR-dihydrotetrabenazine (RRR-DHTBZ, also known as [+]-α-HTBZ) elicited efficacy comparable to conventional antipsychotics in prepulse inhibition and conditioned avoidance models without eliciting weight gain. In combination experiments, synergy was observed: subthreshold doses of RRR-DHTBZ and risperidone or olanzapine produced robust efficacy, and in dose response experiments, RRR-DHTBZ increased the antipsychotic potency in the efficacy models but did not affect weight gain. The combinations did not affect plasma compound concentrations. The synergy is consistent with VMAT2 inhibition blocking the counterproductive presynaptic stimulation of dopamine by antipsychotics. These results suggest a therapeutic strategy of adding a VMAT2 inhibitor to lower the antipsychotic dose and reduce the side effect burden of the antipsychotic while maintaining and potentially enhancing its therapeutic effects. SIGNIFICANCE STATEMENT: Antipsychotics are often necessary and life-changing medications that reduce psychotic symptoms; however, these benefits come with a high side effect burden. This study shows that combining these postsynaptic dopaminergic modulators with a presynaptic dopamine modulator (vesicular monoamine transporter 2 [VMAT2] inhibitor) potentiates efficacy synergistically in animal models of schizophrenia without potentiating weight gain. Our data suggest that adding a VMAT2 inhibitor may be a viable therapeutic strategy for reducing antipsychotic side effects by lowering antipsychotic dose while maintaining therapeutic efficacy.

Synaptopathic mechanisms of neurodegeneration and dementia: Insights from Huntington's disease

Dementia encapsulates a set of symptoms that include loss of mental abilities such as memory, problem solving or language, and reduces a person's ability to perform daily activities. Alzheimer's disease is the most common form of dementia, however dementia can also occur in other neurological disorders such as Huntington's disease (HD). Many studies have demonstrated that loss of neuronal cell function manifests pre-symptomatically and thus is a relevant therapeutic target to alleviate symptoms. Synaptopathy, the physiological dysfunction of synapses, is now being approached as the target for many neurological and psychiatric disorders, including HD. HD is an autosomal dominant and progressive degenerative disorder, with clinical manifestations that encompass movement, cognition, mood and behaviour. HD is one of the most common tandem repeat disorders and is caused by a trinucleotide (CAG) repeat expansion, encoding an extended polyglutamine tract in the huntingtin protein. Animal models as well as human studies have provided detailed, although not exhaustive, evidence of synaptic dysfunction in HD. In this review, we discuss the neuropathology of HD and how the changes in synaptic signalling in the diseased brain lead to its symptoms, which include dementia. Here, we review and discuss the mechanisms by which the 'molecular orchestras' and their 'synaptic symphonies' are disrupted in neurodegeneration and dementia, focusing on HD as a model disease. We also explore the therapeutic strategies currently in pre-clinical and clinical testing that are targeted towards improving synaptic function in HD.

Dopamine and glutamate in Huntington's disease: A balancing act

Huntington's disease (HD) is caused by a CAG repeat expansion in exon 1 of the HD gene resulting in a long polyglutamine tract in the N-terminus of the protein huntingtin. Patients carrying the mutation display chorea in early stages followed by akinesia and sometimes dystonia in late stages. Other major symptoms include depression, anxiety, irritability or aggressive behavior, and apathy. Although many neuronal systems are affected, dysfunction and subsequent neurodegeneration in the basal ganglia and cortex are the most apparent pathologies. In HD, the primary hypothesis has been that there is an initial overactivity of glutamate neurotransmission that produces excitotoxicity followed by a series of complex changes that are different in the striatum and in the cortex. This review will focus on evidence for alterations in dopamine (DA)-glutamate interactions in HD, concentrating on the striatum and cortex. The most recent evidence points to decreases in DA and glutamate neurotransmission as the HD phenotype develops. However, there is some evidence for increased DA and glutamate functions that could be responsible for some of the early HD phenotype. Significant evidence indicates that glutamate and dopamine neurotransmission is affected in HD, compromising the fine balance in which DA modulates glutamate-induced excitation in the basal ganglia and cortex. Restoring the balance between glutamate and dopamine could be helpful to treat HD symptoms.

Neurochemical changes in the entopeduncular nucleus and increased oral behavior in rats treated subchronically with clozapine or haloperidol

The purpose of the present experiment was to test the possibility that atypical antipsychotics and classical antipsychotics differentially regulate specific neurochemical processes within the entopeduncular nucleus. For these experiments, rats were administered clozapine (25 mg/kg), haloperidol (1 mg/kg), or Tween-80 (control) daily for 21 days. Dopamine D(1)-receptor binding was assessed with in vitro receptor autoradiographic methods and the mRNAs corresponding to the two forms of glutamate decarboxylase (glutamate decarboxylase-65 and glutamate decarboxylase-67) were analyzed using in situ hybridization histochemical methods. In addition, vacuous chewing movements (VCM) were measured throughout the drug administration period as a functional indicator of drug action and changes in striatal dopamine D(2)-receptor binding were measured as a positive control for D(2)-receptor antagonist properties of haloperidol and clozapine. In agreement with previous reports, haloperidol increased D(2)-receptor binding throughout the striatum while clozapine had a more limited impact on D(2)-receptors. Behavioral analysis revealed that both haloperidol and clozapine enhanced the display of vacuous chewing movements to a similar extent but with a different postinjection latency. In the entopeduncular nucleus, clozapine increased D(1)-receptor binding compared to controls while haloperidol was without effect. With respect to the regulation of GAD mRNAs, haloperidol increased glutamate decarboxylase-65 and glutamate decarboxylase-67 mRNA levels throughout the entopeduncular nucleus. The effects of clozapine were restricted to increases in glutamate decarboxylase-65 mRNA. These studies show that clozapine and haloperidol, both of which increase the occurrence of VCM, differentially modulate the neurochemistry of the entopeduncular nucleus.

Neurons exhibiting dopamine D2 receptor immunoreactivity in the substantia nigra of the mutant weaver mouse

Neurons exhibiting D2 receptor-like immunoreactivity were investigated in the substantia nigra pars compacta of weaver mice at the light and electron microscope levels using immunocytochemical techniques. At the light microscope level, there was significant loss of D2-like immunoreactive cells in weaver mice and the remaining labeled cells exhibited less intense immunoreactivity. At the ultrastructural level, there was a decrease in the number of immunoreactive profiles and fewer synapses were observed abutting labeled dendritic profiles. In addition, degenerative changes were noted in some of the D2 receptor-like immunoreactive profiles. Double labeling with D2 and tyrosine hydroxylase indicated that the majority of the labeled profiles were double labeled. Eight-week-old homozygous weavers were paired with wild-type littermates as controls and perfused with a buffered solution of acrolein/paraformadehyde. Midbrain sections were reacted immunocytochemically either with an antiserum to D2 or with antisera to D2 and tyrosine hydroxylase, using a double-labeling technique. Sections were processed for light and electron microscopy by standard methods. The results of this study confirm the autoreceptor-like activity of D2 receptors on nigral dopamine neurons. The cell degeneration, down-regulation of D2 receptors, and decreased dendritic and synaptic components in the neuropil suggest that the synaptic integrity of the substantia nigra has been compromised, which in turn would affect the functional efficacy of the basal ganglia circuitry. This altered circuity is expressed in the Parkinson-like symptoms displayed by this mutant mouse.

Antagonistic effects of beta-phenylethylamine on quinpirole- and (-)-sulpiride-induced changes in evoked dopamine release from rat striatal slices

To assess the role of beta-phenylethylamine in aspects of dopamine release, we measured the level of beta-phenylethylamine in the rat striatum after killing the rats by microwave irradiation. We then investigated the effect of beta-phenylethylamine on electrically evoked dopamine release from rat striatal slices in vitro. The striatal beta-phenylethylamine level was 46.5 +/- 3.5 ng/g wet tissue, equivalent to 0.3 micromol/l. Superfusion with low concentrations of beta-phenylethylamine up to 1 micromol/l had no effect on spontaneous or electrically evoked dopamine release from striatal slices. Quinpirole reduced the evoked dopamine release from slices in a concentration-dependent manner. The quinpirole-induced reduction of evoked dopamine release was attenuated 30% by superfusion with 0.3 micromol/l beta-phenylethylamine. Moreover, the (-)-sulpiride (0.1 micromol/l)-induced increase in evoked dopamine release was also attenuated by superfusion with 0.3 micromol/l beta-phenylethylamine. These data indicate that submicromolar levels of beta-phenylethylamine could modify the dopamine autoreceptor mediated changes in evoked dopamine release from rat striatal slices.

Antagonistic effects of OPC-14597, a novel antipsychotic drug, on quinpirole- and (-)-sulpiride-induced changes in evoked dopamine release in rat striatal slices

The effects of a newly synthesized quinolinone derivative, 7-[4-(4-(2,3-dichlorophenyl)-1-piperazinyl) butoxy)-3,4-dihydro-2-(1H)-quinolinone (OPC-14597), a novel antipsychotic drug, on electrically evoked dopamine release in rat striatal slices were investigated. OPC-14597 (0.1-10 microM) had no effect on the dopamine release evoked in the striatal slices. The decrease induced by quinpirole, a dopamine receptor agonist, in evoked dopamine release was attenuated by superfusion with OPC-14597 (1 and 10 microM) which by itself had no effect on evoked dopamine release. The increase induced by (-)-sulpiride, a dopamine receptor antagonist, in evoked dopamine release was, moreover, also attenuated by 1 and 10 microM OPC-14597. These findings indicate that OPC-14597 antagonizes both dopamine agonist- and antagonist-induced changes in evoked dopamine release in striatal slices in rats.

In vivo evidence for preferential role of dopamine D3 receptor in the presynaptic regulation of dopamine release but not synthesis

Brain microdialysis was used to investigate the effects of the putative dopamine D3 receptor agonist (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on dopamine release, metabolism and synthesis in the dorsal striatum and nucleus accumbens of awake rats. The drug administered i.p. dose dependently decreased the release, metabolism and synthesis of dopamine in both brain areas. The potency of 7-OH-DPAT to decrease dopamine release was found to be higher in the nucleus accumbens than in the dorsal striatum (ED50 for nucleus accumbens 0.0096 mg/kg, i.p.; for dorsal striatum 0.068 mg/kg, i.p.). Dopamine metabolism, assessed by measuring 3,4-dihydroxyphenylacetic acid extracellular levels, and dopamine synthesis, determined as 3,4-dihydroxyphenylalanine output following perfusion with the L-aromatic acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (10(-5) M), were decreased at higher dose ranges of 7-OH-DPAT (ED50 for decrease of 3,4-dihydroxyphenylalanine output in nucleus accumbens 0.124 mg/kg, i.p.; in dorsal striatum 0.101 mg/kg, i.p.). The hypomotility of rats induced by 7-OH-DPAT in doses of 0.002-0.25 mg/kg, i.p., was shown to correlate with the decreased dopamine release in the nucleus accumbens. Pretreatment of animals with 7-OH-DPAT at the putative dopamine D3 receptor 'selective' dose of 0.05 mg/kg, i.p., was found to prevent the increase of dopamine release but not the increase in metabolism in the dorsal striatum of freely moving rats induced by (+)-AJ76, cis (+)-(1S,2R)-5-methoxy-1-methyl-1-2-(n-propylamino)tetralin HCI (7 mg/kg, i.p.) and haloperidol (0.1 mg/kg, i.p.). Local application of 7-OH-DPAT by addition into the perfusing medium also resulted in a preferential decrease of dopamine release in the nucleus accumbens as compared with the dorsal striatum (EC50 for nucleus accumbens 1.9 nM; for dorsal striatum 11.3 nM). The present results give further support to the hypothesis that the dopamine D3 autoreceptor is preferentially involved in the presynaptic regulation of dopamine release, while the D2 autoreceptor controls dopamine synthesis.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Regulation of dopamine release and metabolism in rat striatum in vivo: effects of dopamine receptor antagonists

1. The acute effects of some of typical and atypical antipsychotic drugs on the dopamine release and metabolism in the dorsal striatum of freely moving rats were studied using transcerebral microdialysis technique. 2. Classical neuroleptic drugs haloperidol (0.05, 0.1 and 0.2 mg/kg), thioproperazine (0.1, 0.2 and 0.4 mg/kg) and spiperone (0.02, 0.04 and 0.07 mg/kg) administered i.p. induced pronounced elevation of extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) up to 250-300% to basal level while producing less increase in that of dopamine (DA) (up to 150-170%). 3. Atypical neuroleptics clozapine and thioridazine (both 2, 5 and 20 mg/kg) increased striatal DA release and DOPAC level approximately at the same degree (maximally up to 200% and 160%, respectively). 4. Dopamine D3 receptor and autoreceptor preferring antagonists (+)-UH232 and (+)-AJ76 (both 4, 7 and 14 mg/kg) more potently increased DA release in comparison with DOPAC dialysate level (+)-AJ76 elevated DA level maximally up to 330%, DOPAC-up to 250%). 5. The features of typical and atypical neuroleptics in preferential action on DA release or DOPAC output were observed in all doses of the drugs studied. 6. The ability of the drugs to affect preferentially DA release or DOPAC extracellular level in rat striatum correlates to their relative affinities at D3 and D2 DA receptors. 7. It is concluded that typical and atypical antipsychotic drugs might be clearly distinguished on the basis of their ability to affect preferentially DA synthesis/metabolism or release in rat dorsal striatum in vivo.

The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--II. Effects of dopamine D1 and D2 receptor agonists

In vivo microdialysis was used to study the effect of intracerebral administration of dopamine agonists on dynorphin B release in the striatum and substantia nigra of rats. The release of dopamine and GABA was also investigated. Administration of the dopamine D1 agonist SKF 38393 (10-100 microM) into the striatum increased extracellular dynorphin B and GABA levels in the ipsilateral substantia nigra, in a concentration-dependent manner. After a short-lasting increase, nigral dopamine levels were significantly decreased after the highest concentration of striatal SKF 38393. An increase in striatal dynorphin B, GABA and dopamine levels was also observed. When SKF 38393 (10 microM) was administered into the substantia nigra, nigral dynorphin B and GABA, but not dopamine levels increased. No significant effects were observed on striatal levels. Administration of the dopamine D2 agonist, quinpirole (100 microM), into the striatum decreased dopamine levels in both striatum and substantia nigra, while no effect was observed on striatal or nigral dynorphin B and GABA levels. Quinpirole (10-100 microM) given into the substantia nigra, decreased striatal dopamine levels in a concentration manner. In the nigra, a short-lasting increase in dopamine levels was observed following the highest concentration of nigral quinpirole (100 microM). The effect was followed by a decrease in dopamine levels. No significant effects were observed on striatal or nigral dynorphin B and GABA levels. The results show that stimulation of D1 receptors in striatum and substantia nigra leads to activation of the striatonigral dynorphin pathway. A parallel effect could also be seen on nigral GABA release.(ABSTRACT TRUNCATED AT 250 WORDS)

The weaver mutant mouse as a model of nigrostriatal dysfunction

The weaver mutant mouse has a genetic defect that results in the loss of dopamine neurons in the nigrostriatal pathway. Striatal tyrosine hydroxylase and dopamine content are reduced by 60-70%, and dopamine uptake is reduced by as much as 95%. Deficits in all three of these striatal dopamine markers are seen as early as postnatal d 3. The striatal dopamine systems in the weaver apparently have the ability to compensate for this dopamine deficit. Thus, in the weaver, in vitro resting release, as well as amphetamine-evoked fractional release of endogenous dopamine are increased. An additional change seen in the weaver striatum is an elevated serotonin content. These alterations may play an adaptive role in attempting to compensate for the dopamine loss. In summary, the weaver mutant mouse has dramatic deficits in the nigrostriatal pathway, but also seems to develop certain adaptive mechanisms in dopaminergic and other transmitter systems that may compensate functionally for the dopamine deficit. Thus, the weaver mouse provides a unique animal model for studying naturally induced neuronal degeneration that complements those models using surgical and pharmacological protocols.

Differential effects of dopamine agonists on evoked dopamine release from slices of striatum and nucleus accumbens in rats

The effects of dopamine receptor agonists on electrically evoked dopamine release from slices of nucleus accumbens were compared with the effects on release from striatal slices in rats. Apomorphine, which has equal potency at the dopamine D2 and D3 receptors, reduced the evoked dopamine release from both regions to the same extent (ED50, 0.42 microM for nucleus accumbens; ED50, 0.46 microM for striatum). Quinpirole or 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT), which are much more potent at the D3 receptor than at the D2 receptor, reduced the evoked dopamine release from the nucleus accumbens (ED50, 0.12 microM for quinpirole; 0.02 microM for 7-OHDPAT) much more than the release from the striatum (ED50, 1.6 microM for quinpirole; 0.55 microM for 7-OHDPAT). These results suggest that the contribution of D3 receptors in nucleus accumbens to regulate dopamine release from dopamine nerve terminals is much greater than in striatum.

Remoxipride and raclopride differ from metoclopramide by their effects on striatal dopamine release and biosynthesis in rats

Effects of new neuroleptics remoxipride (2.4 mg/kg, i.p.) and raclopride (1.2 mg/kg, i.p.) in comparison to metoclopramide (5 mg/kg, i.p.) on the extracellular levels of DA and its metabolites using microdialysis technique in freely moving rats were studied. The effects of these drugs as well as that of cis- and trans-carbidine (25 and 1 mg/kg, i.p., respectively), sulpride (50 mg/kg, i.p.) and haloperidol (1 mg/kg, i.p.) on the DA biosynthesis rate by accumulation of L-DOPA after inhibition of L-DOPA decarboxylase (NSD-1015 model) in rat striatum were also investigated. All the drugs studied increased significantly DA biosynthesis rate. The order of potency of drugs was: metoclopramide > haloperidol > raclopride > remoxipride > sulpride > cis-carbidine > trans-carbidine. In microdialysis study metoclopramide was shown to produce much greater increase in HVA and DOPAC and only modest rise in DA levels. Meanwhile remoxipride and raclopride were found to differ from the former drug being approximately equally effective in increasing both DA and its metabolite extracellular levels. It is suggested that typical and atypical neuroleptics may differentially affect dopamine release and synthesis in rat striatum.

The dopamine D3 receptor and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232; a microdialysis study

The in vivo neurochemical profiles of haloperidol, raclopride and the dopamine D3 and autoreceptor preferring dopamine receptor antagonists (+)-UH232 and (+)-AJ76 on dopamine release and metabolism in the dorsal striatum and in the nucleus accumbens are described. It is shown that both (+)-UH232 and especially (+)-AJ76 have different effects on brain dialysate dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) as compared to haloperidol or raclopride. It is suggested that the relative increase in dialysate dopamine over the relative increase in DOPAC is a neurochemical fingerprint, unique for different dopamine receptor antagonists. As a consequence the increased release and metabolism of dopamine after systemic administration of dopamine receptor antagonists may be controlled by different receptors and different dopamine antagonists can partly distinguish between these receptors. This may be due to their different interactions with different dopamine D2 type receptors. It is finally concluded that (+)-UH232 and especially (+)-AJ76 seem to prefer release regulating autoreceptors at the level of the axon terminals.

YM-09151-2 but not l-sulpiride induces transient dopamine release in rat striatum via a tetrodotoxin-insensitive mechanism

The effects of the selective dopamine D2 receptor antagonists YM-09151-2 and l-sulpiride on the in vivo release of dopamine (DA), L-3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in rat striatum were investigated. The drugs were injected into the striatum through a microinjection needle attached to a dialysis probe. YM-09151-2 (0.1 or 1.0 microgram/0.5 microliter) injected into the striatum produced a dramatic rapid-onset transient increase in striatal DA release in a dose-dependent manner. However, the DA increase induced by l-sulpiride (15 or 75 ng/0.5 microliter) was small and of slower onset. An increase of DOPAC levels by YM-09151-2 was biphasic: The first peak occurred at 40 min, followed by a delayed-onset gradual increase. Slower-onset gradual increases were also found in DOPAC levels after l-sulpiride injection and in HVA levels after injections of both YM-09151-2 and l-sulpiride. The infusion of tetrodotoxin (TTX; 2 microM) revealed two different types of DA release mechanisms: The rapid-onset transient DA release induced by YM-09151-2 was TTX insensitive, whereas the slower-onset DA release induced by l-sulpiride was TTX sensitive. Moreover, the rapid-onset transient DA release was Ca2+ independent and was not affected by pretreatment with l-sulpiride or nomifensine. Therefore, it is concluded that YM-09151-2 injected into the striatum produced a transient striatal DA release that is independent of D2 receptors and the action potential.

Modulation of (-)-sulpiride-induced increase in electrically-evoked release of dopamine from rat striatal slices

(-)-Sulpiride (10 nM-10 microM) in the superfusate, dose-dependently increased the electrically-evoked release of dopamine from rat striatal slices. (+)-Sulpiride had little effect on evoked release of dopamine up to 10 microM. Apomorphine inhibited electrically evoked release of dopamine, and this effect of apomorphine was antagonized by (-)-sulpiride. SCH23390 and forskolin had no effect on the (-)-sulpiride-induced increase in evoked release of dopamine. Treatment with the irreversible dopamine-receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline caused a significant increase in evoked release of dopamine and prevented the (-)-sulpiride-induced increase in the evoked release of dopamine. These results indicate that the (-)-sulpiride-induced increase in evoked release of dopamine is due to antagonism of the activation of dopamine autoreceptors by endogenously released dopamine.

In vivo assessment of release and metabolism of dopamine in the ventrolateral striatum of awake rats following administration of dopamine D1 and D2 receptor agonists and antagonists

The ability of specific dopamine (DA) receptor agonists and antagonists to modify the release and metabolism of DA in the ventrolateral striatum of awake rats was assessed using in vivo microdialysis. The specific DA D2 receptor antagonist, raclopride (0.1, 0.5 and 2.0 mg/kg, i.p.), dose-dependently increased release of DA and levels of the metabolites DOPAC and HVA, while the D2 receptor agonist, quinpirole (0.03, 0.1 and 0.3 mg/kg), decreased levels of DA, DOPAC and HVA. The DA D1 receptor antagonist, SCH23390 [(R + (+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl- 1H-3-benzazepin-7-ol) (0.01, 0.05 and 0.25 mg/kg), produced an increase in DA, DOPAC and HVA but of a lesser magnitude than raclopride. The D1 agonist SKF38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (1.0, 3.0 and 10.0 mg/kg) failed to affect the release of metabolism of DA at any dose. These results support previous findings that activation of D2 receptors has greater control over in vivo DA function, than drugs specifically affecting D1 receptors.

In vitro release of endogenous dopamine from the striatum of the weaver mutant mouse

The weaver mutant mouse has a genetically determined defect in the nigrostriatal dopaminergic system. The present study was undertaken to test the hypothesis that in the weaver mutant mouse, striatal nerve terminals undergo compensatory changes in response to this deficiency. To test this hypothesis, we studied the basal and stimulated release of dopamine from striatal slices of weaver mutant mice and matched controls. By using a superfusion system and concentrating the superfusate by passage over alumina, resting dopamine release could be determined in the weaver mutant despite the fact that striatal tissue content of dopamine in these mice is reduced by greater than 75% compared with control mice. Fractional resting release of dopamine in weaver striatal slices was significantly elevated compared with that in controls, suggesting that the release mechanisms in the weaver may be adapting to overcome the dopamine deficit. Potassium-evoked release (24 and 48 mM potassium) was not significantly different between the two genotypes. In contrast, amphetamine-evoked release (1 microM) was significantly greater in the weaver mice than in controls. In both genotypes, release evoked by amphetamine was completely inhibited by cocaine, implicating the dopamine uptake carrier in this release process. These findings suggest that fundamental differences in dopamine release mechanisms exist between weaver and control mice and support the hypothesis that compensatory mechanisms may develop in neurons in response to dopamine deficits.

Increased midbrain dopaminergic cell activity following 2'CH3-MPTP-induced dopaminergic cell loss: an in vitro electrophysiological study

Several days after the administration of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) to the BALB/cJ mouse there is a loss of midbrain dopaminergic neurons, a reduction of forebrain dopamine (DA) content, and an elevation in forebrain DA turnover. The purpose of the present study was to determine whether the increase in forebrain DA turnover is related to an increase in dopaminergic neuronal activity. In vitro extracellular single unit recordings were made from midbrain dopaminergic neurons in the substantia nigra pars compacta (nucleus A9) and ventral tegmental area (nucleus A10) of BALB/cJ mice. The experimental animals were treated intraperitoneally with 40, 50 or 55 mg/kg 2'CH3-MPTP and killed 7-15 days later. Forebrain DA concentrations were decreased below control values by the two higher toxin doses in the caudate-putamen (67% and 78%, respectively), but not in the nucleus accumbens. DA turnover increased more than 2-fold in the caudate-putamen, but was unchanged in the nucleus accumbens. Nucleus A9 cells, in the 2'CH3-MPTP-treated animals, exhibited a 3-fold increase in the number of spontaneously active cells, and an 84% increase in basal firing rates. There was also a positive correlation between the A9 cell firing rates, and the DA turnover in the striatum of the toxin-treated mice. Nucleus A10 cells, in the 2'CH3-MPTP-treated animals, exhibited neither changes in number of spontaneously active cells nor changes in firing rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic SCH 23390 treatment on dopamine autoreceptor function in rat brain

The effects of chronic treatment with SCH 23390 (0.5 mg/kg per day, 18 days), a selective (DA) D-1 receptor antagonist, on dopamine autoreceptor function in rats were investigated. The decrease of DA metabolism in mesolimbic and nigrostriatal systems and inhibition of spontaneous explorative locomotor activity by low doses of the selective D-2 receptor agonist, quinpirole, were used as behavioral and biochemical indices of DA autoreceptor function. The DA metabolism decreasing effect of quinpirole (20 and 50 micrograms/kg) in mesolimbic or nigrostriatal DA systems was similar in SCH 23390- and vehicle-treated rats. Two-way analysis of variance showed that rats treated chronically with SCH 23390 were significantly more active than vehicle-treated rats. However, there were no statistically significant differences in the quinpirole-induced hypomotility response between these groups. These results indicate that chronic D-1 blockade, unlike classical antipsychotic drugs, does not modulate the biochemical and behavioral indices of DA autoreceptor activation, suggesting a lack of interaction between DA autoreceptors and D-1 receptors in the central nervous system.

The actions of (-)N-n-propylnorapomorphine and selective dopamine D1 and D2 receptor agonists to modify the release of [3H]dopamine from the rat nucleus accumbens

The ability of (-)N-n-propylnorapomorphine and selective D1 and D2 dopamine receptor agonists and antagonists to modify the release of [3H]dopamine, induced by potassium from the nucleus accumbens, was studied using an in vitro superfusion technique. (-)N-n-Propylnorapomorphine, in picomolar concentrations, inhibited the release of [3H]dopamine, the inhibition being antagonised by fluphenazine and the selective D2 receptor antagonist sulpiride; the selective D1 receptor antagonist SCH 23390 was ineffective. The selective D1 receptor agonist SKF 38393 and the selective D2 agonist quinpirole, both inhibited the potassium-induced release of [3H]dopamine; no synergistic effect was observed to a combined treatment with SKF 38393 and quinpirole. The effects of SKF 38393 and quinpirole were selectively antagonised by SCH 23390 and sulpiride, respectively, although both antagonists failed to modify the release of [3H]dopamine when administered alone. Receptor antagonists for other transmitter sites, e.g. noradrenaline, 5-hydroxytryptamine and acetylcholine, failed to modify potassium-induced release of [3H]dopamine, when administered alone or to prevent the inhibition of the release caused by (-)N-n-propylnorapomorphine. It is concluded that the action of dopamine agonists on both dopamine D1 and D2 receptors in the nucleus accumbens can reduce the release of [3H]dopamine in the in vitro system. Comparable actions in vivo may contribute to the ability of dopamine agonists to moderate locomotor responding.

Interactions of 7-[3-(4-[2,3-dimethylphenyl]piperazinyl)-propoxy]-2(1H)-quinolinone binding in rat striatum: effects of lesions

7-[3-(4-[2,3-Dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), a presynaptic dopamine autoreceptor agonist and postsynaptic D-2 receptor antagonist (Yasuda et al., Life Sci. 42: 1941-1954, 1988), was studied for its binding characteristics at 3H-SCH 23390-labeled dopamine D-1 receptors and 3H-spiperone-labeled dopamine D-2 receptors in rat striatum. The binding affinity of OPC-4392 for 3H-spiperone-labeled D-2 receptors was 500 times higher than for 3H-SCH 23390-labeled D-1 receptors. 6-Hydroxydopamine lesions of striatum and high-frequency current lesions of medial forebrain bundle did not affect the competition of OPC-4392 for 3H-spiperone binding. Kainic acid lesions of striatum significantly changed the one-site model fit to a two-site model fit of the competition curve of OPC-4392 for 3H-spiperone binding, suggesting that OPC-4392 competed with 3H-spiperone binding differently for postsynaptic dopamine D-2 receptors and for presynaptic dopamine autoreceptors.

Antagonism of presynaptic dopamine receptors by phenothiazine drug metabolites

Electrically evoked release of dopamine from the caudate nucleus is reduced by the dopamine receptor agonists, apomorphine and bromocriptine, and facilitated by neuroleptic drugs, which act as dopamine autoreceptor antagonists. The potencies of chlorpromazine, fluphenazine, levomepromazine and their hydroxy-metabolites in modulating electrically evoked release of dopamine were examined by superfusion of rabbit caudate nucleus slices pre-incubated with 3H-dopamine. O-Desmethyl levomepromazine, 3-hydroxy- and 7-hydroxy metabolites of chlorpromazine and levomepromazine facilitated electrically evoked release of 3H-dopamine, having potencies similar to that of the parent compounds. 7-Hydroxy fluphenazine was less active than fluphenazine in this system. These results indicate that phenolic metabolites of chlorpromazine and levomepromazine, but not of fluphenazine, may contribute to effects of the drugs mediated by presynaptic dopamine receptors.

On the mechanism of neuroleptic induced increase in striatal dopamine release: brain dialysis provides direct evidence for mediation by autoreceptors localized on nerve terminals

The influence of sulpiride on the in vivo release of dopamine and DOPAC from the rat striatum was investigated by microdialysis. Racemic sulpiride was administered systemically (i.p.) to control rats and to rats in which a striatum was pretreated by kainic acid. In addition various concentrations (10(-8) to 10(-5) M) of the two enantiomers of sulpiride were infused into the striatum and the effects on the release of dopamine were recorded. Infusion as well as systemic administration of sulpiride caused a maximal increase in the release of dopamine of about 180-190% of basal values. A similar increase was seen in kainic acid-pretreated rats. The rises in dopamine seen after systemic administration or infusion of sulpiride were not additive, suggesting that similar mechanisms were involved. DOPAC dialysate levels also increased during infusion of the neuroleptic but the rise was significantly less when sulpiride was administered i.p. to kainic acid pretreated rats. It is concluded that the rise in dopamine release seen after neuroleptics is mediated by autoreceptors localized on nerve terminals. This implies that the well-known increase in electrical activity of dopaminergic neurons during neuroleptic treatment, is not responsible for the increased release of the transmitter.

Effects of DA-1- and DA-2-dopamine antagonists on apomorphine-induced inhibition of peripheral sympathetic neurotransmission

1. Apomorphine, a dopamine receptor agonist, caused a dose-dependent inhibition of the electrically induced increase in diastolic blood pressure in the pithed rat. The anti-apomorphine effects of a range of dopamine antagonists with different DA-1/DA-2 selectivities were evaluated to characterize the dopamine receptor subtype(s) involved in this response. 2. Selective DA-2 antagonists domperidone and (-)-sulpiride and combined DA-2/DA-1 antagonists haloperidol, fluphenazine and (+)-sulpiride were active in this model, whereas the DA-1 antagonist SCH 23390 and the alpha-2 adrenoceptor antagonist idazoxan (RX-781094) were inactive. The rank order of potency of the dopamine antagonists was domperidone greater than fluphenazine greater than or equal to (-)-sulpiride greater than or equal to haloperidol greater than (+)-sulpiride much greater than SCH 23390. (-)-Sulpiride was about 25 times more potent than the (+)-isomer. 3. The observed antagonist profile suggests that the apomorphine-induced inhibition of neurogenic vasoconstriction in the pithed rat is mediated via dopamine receptors of the DA-2 subtype. The sympatho-inhibitory dopamine receptors in the cat heart have similar pharmacological characteristics. 4. The dopamine antagonists were devoid of influences on the vasoconstrictor response to electrical stimulation in the absence of apomorphine, whereas idazoxan potentiated it. This provides evidence against the participation of dopamine receptors in the modulation of the vasoconstrictor response to sympathetic stimulation in the pithed rat model.

Effects of locally applied D-1 and D-2 receptor agonists and antagonists studied with brain dialysis

The effect on dopamine (DA) release of D-1 and D-2 receptor agonists and antagonists applied locally in the caudate through a trans-striatal dialysis probe was studied in freely moving rats. D-2 agonists (LY 171555 and BHT 920) reduced DA release in a concentration-dependent manner. The same local application of haloperidol abolished the effect of 10 microM LY 171555 and BHT 920. A specific D-1 agonist, the catechol benzazepine SKF 38393, reduced DA release and this effect was not modified by systemic or local administration of the D-1 antagonist, SCH 23390, nor by the D-2 antagonist, haloperidol. In contrast, the non-catechol D-1 agonist, CY 208243, failed to modify DA release. Local application of the D-1 antagonist, SCH 23390, or of the D-2 antagonist, (-)-sulpiride, stimulated DA release in a concentration-dependent manner. The D-1 agonist, CY 20843, reversed the stimulatory effect of SCH 23390 but not that of (-)-sulpiride. It is concluded that D-1 and D-2 receptors located in the caudate control DA release separately in this area.

Actions of dopamine antagonists on stimulated striatal and limbic dopamine release: an in vivo voltammetric study

1. Fast cyclic voltammetry at carbon fibre microelectrodes was used to study the effects of several dopamine antagonists upon stimulated dopamine release in the rat striatum and nucleus accumbens. 2. In both nuclei, stimulated dopamine release was increased by D2-receptor-selective and mixed D1/D2-receptor antagonists. The D1-selective antagonist SCH 23390 had no effect. 3. Striatal and limbic dopamine release were elevated by cis- but not trans-flupenthixol. 4. The 'atypical' neuroleptics (clozapine and thioridazine) did not cause a selective elevation of dopamine release in the limbic terminal region, whereas the non-antipsychotic drug metoclopramide increased dopamine release more in striatum than nucleus accumbens. 5. We conclude from this study that striatal and limbic dopamine release are under the control of a stereoselective dopamine D2-autoreceptor on the nerve terminal and that atypical neuroleptics do not show a limbic-selective effect at this receptor after acute administration.

Comparison of neuroleptic binding characteristics in rat striatum and renal cortex

The binding characteristics of the dopaminergic ligand, 3H-spiperone, were compared in renal cortical and striatal membrane homogenates of the rat. This ligand labelled a single class of high affinity binding sites in striatum with an apparent dissociation constant (Kd) of 0.13 nM and a maximal number of binding sites (Bmax) of 890 fmol/mg protein representing D-2 receptors. In the renal cortex, 3H-spiperone identified a population of binding sites with a Bmax and a Kd of 310 fmol/mg protein and 5.1 nM, respectively. The antagonist displacing profile suggests the dopaminergic nature of the renal binding site. The affinities of dopamine antagonists for the peripheral 3H-spiperone binding site were in general in the micromolar range while the affinities of D-2 or D-2/D-1 dopamine antagonists in striatum were in the nanomolar range. Moreover, these sites showed differential stereoselectivity for (+)- and (-)-isomers of sulpiride. In conclusion, the presence of a D-2/DA-2 dopamine receptor population in renal cortex could not be confirmed. The pharmacological properties of the peripheral 3H-spiperone binding site are also different from the DA-1 receptor but seem to resemble those previously reported for dopamine receptors in sympathetic ganglia and adrenal medulla.

Repeated cocaine administration results in supersensitivity of striatal D-2 dopamine autoreceptors to pergolide

Groups of rats administered cocaine-HC1 (10 mg/kg, i.p.) or saline either acutely or once daily for 8 or 14 days were killed 24 hrs after the last dose. In striatal slices prelabelled with [3H]DA, modulation of [3H]-overflow by pergolide was used to measure D-2 autoreceptor activity. Compared to the contemporaneous control group pergolide produced a greater inhibition only in striatal slices from rats treated repeatedly with cocaine. In radioligand binding studies using striatal membranes from control rats, pergolide had a 500-fold greater affinity for the D-2, as opposed to the D-1, dopamine (DA) receptor subtype. These results indicate that repeated treatment with cocaine produces supersensitive striatal D-2 release-modulating autoreceptors consistent with a compensatory change to diminish the effect of elevated synaptic concentrations of DA produced by cocaine. In contrast, supersensitivity of D-2 receptors was not detected in [3H]spiperone binding assays.

Mechanisms contributing to the recovery of striatal releasable dopamine following MFB stimulation

In vivo voltammetry was used to monitor the concentration of dopamine released into striatal extracellular fluid during electrical stimulation of the medial forebrain bundle of anesthetized rats. During stimulation the dopamine concentration increases rapidly to micromolar levels. After cessation of the stimulus, uptake mechanisms rapidly clear the released dopamine. That dopamine is the compound detected during stimulation is confirmed by voltammetry and pharmacology. The interval between stimulations influences the amount of dopamine released. When 10 s stimulations are spaced by 30 s, only 50% of the amount of dopamine released by the first stimulation is released by the second stimulation. Twenty minutes are required for full recovery of the stimulation response. Full recovery requires dopamine synthesis but during the initial 2 min following stimulation the recovery exceeds synthesis rates. Possible mechanisms that contribute to the rapid initial recovery are the mobilization of intraneuronally stored dopamine and the recycling of the released dopamine via presynaptic uptake mechanisms. Dopaminergic receptors are able to modulate the rapid recovery.

6-Hydroxydopamine lesions reduce specific [3H]sulpiride binding in the rat substantia nigra: direct evidence for the existence of nigral D-2 autoreceptors

[3H]Sulpiride bound to substantia nigra homogenates in a saturable manner and with a pharmacological profile typical of a specific D-2 ligand. Unilateral 6-OH-dopamine (DA) lesions of the nigrostriatal DA neurons reduced by 27-46% the Bmax of the specific [3H]sulpiride binding in substantia nigra homogenates depending on the time after 6-OHDA lesion and significantly increased the Bmax in the caudate. KDs remained unchanged in both areas. The localization of specific [3H]sulpiride binding and its reduction in the substantia nigra of the 6-OHDA-lesioned side were confirmed by quantitative autoradiography. The results provide evidence for the existence of D-2 autoreceptors in substantia nigra.

D2-dopamine receptors regulate the release of [3H]dopamine in rat basal hypothalamus and neurointermediate lobe of the pituitary gland

Homogenates of the rat basal hypothalamus and the neurointermediate lobe of the pituitary gland contained relatively high levels of dopamine as was estimated by high-performance liquid chromatography (HPLC). The release of [3H]dopamine was studied in these regions and desipramine was used to prevent uptake of [3H]dopamine in noradrenergic nerve terminals. The release of radioactivity could be stimulated electrically and was calcium-dependent. It appeared that the release of radioactivity could be inhibited by drugs stimulating D2-dopamine receptors in both regions. The radioactivity released during electrical stimulation was analysed by cation exchange chromatography and appeared to consist predominantly of [3H]dopamine. It is our conclusion that D2-receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals in the basal hypothalamus and in the neurointermediate lobe.

D2-dopamine receptor-mediated inhibition of intracellular Ca2+ mobilization and release of acetylcholine from guinea-pig neostriatal slices

The effect of dopamine receptor activation on electrically- or high K+ (30 mM)-evoked neurotransmitter release and rise in intracellular Ca2+ concentration was investigated using slices of guinea-pig neostriatum. A specific D2-dopamine receptor agonist, LY-171555 (a laevorotatory enantiomer of LY-141865: N-propyl tricyclic pyrazole) at 10(-6) M inhibited electrical stimulation- and high K+-evoked release of [3H]-acetylcholine ([3H]-ACh) to 47.7 +/- 6.0% and 54.1 +/- 5.0% of control, respectively. The maximal inhibition by LY-171555 at 10(-5) M was 54.8 +/- 5.1% reduction of the control. The half-maximal effective concentration (EC50) of LY-171555 for the inhibition of [3H]-ACh release was 2.3 X 10(-7) M. A specific D2-dopamine receptor antagonist, (-)-sulpiride (10(-7) M) reversed the inhibition of [3H]-ACh release induced by LY-171555. A specific D1-dopamine receptor agonist, SK&F 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-benzazepine) (10(-5) M) had no effect on the release of [3H]-ACh. LY-171555 (10(-6) M) also inhibited the high K+-evoked endogenous glutamate release, by 47% of control. This inhibitory effect was reversed by (-)-sulpiride (10(-7) M). We used a fluorescent, highly selective Ca2+ indicator, 'quin 2' to measure intracellular free Ca2+ concentrations ([Ca2+]i). Electrical stimulation of slices preloaded with quin 2 led to an elevation of relative fluorescence intensity and this response was reduced by the removal of Ca2+ from the bathing medium. These results indicate that the enhanced elevation in fluorescence intensity in the quin 2-loaded slices reflects the increase of intracellular free Ca2+ concentration, [Ca2+]i. The mixed D1- and D2-receptor agonist, apomorphine and LY-171555 inhibited the increase of [Ca2+]i induced by electrical stimulation or high K+ medium, in a concentration-dependent manner, while SK&F 38393 did not affect the increase of [Ca2+]i. The maximal inhibitory effect of LY-171555 at 3 X 10(-5) M was 35 +/- 3% reduction in control values. The inhibitory effect of LY-171555 was antagonized by (-)-sulpiride (10(-7) M). There was a high correlation (r = 0.997, P less than 0.05) between the D 2-receptor-mediated inhibition of the stimulated rise of [Ca2+]i and [3H]-ACh release. When the slices were superfused with the Ca2+-free medium containing EGTA (10(-4) M) for 5 min, the rise in [Ca2+]i was markedly suppressed to 18.0% of control by LY-171555 (10(-6) M). These data indicate that activation of the D2-dopamine receptor suppresses the elevation of [Ca2+]i induced by depolarizing stimuli. This may be due to inhibition of mobilization of Ca2+ from the intracellullar store. We propose that the D2-receptor-mediated inhibition of transmitter release is probably due to a reduction in intracellular Ca2+ mobilization.

Presynaptic control of dopamine metabolism in the nucleus accumbens. Lack of effect of buspirone as demonstrated using in vivo voltammetry

Buspirone is a non-benzodiazepine drug with anxiolytic properties. It has been reported to induce a marked increase in the metabolism of dopamine in the striatum and the nucleus accumbens which is similar to that induced by neuroleptics. It has been suggested that the effect observed in the striatum reflects an action of buspirone on dopaminergic autoreceptors in both terminals and cell bodies. In the present study, presynaptic effects of buspirone on dopaminergic metabolism in the nucleus accumbens were investigated, and they were compared to the effects of the classical neuroleptic, haloperidol. Dopaminergic terminals were isolated by infusion of tetrodotoxin into the median forebrain bundle in order to evaluate the effects of buspirone and haloperidol on presynaptic receptors. Changes in dopamine metabolism were determined by in vivo voltammetry. Buspirone administered after interruption of the impulse flow did not affect dopamine metabolism. In contrast haloperidol treatment led to an increase in metabolism of dopamine. It is concluded that buspirone did not act at the presynaptic level and furthermore on dopaminergic autoreceptors.

D-2 dopamine-receptors regulate the release of [3H]dopamine in rat cortical regions showing dopamine immunoreactive fibers

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.

Inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release by the putative dopamine autoreceptor agonist, B-HT 920

The inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release from preloaded rat striatal slices was used to examine the presynaptic selectivity of the putative dopamine autoreceptor agonist, B-HT 920. In the micromolar range, B-HT 920 caused a concentration-dependent inhibition of the release of both labeled neurotransmitters as evoked by 20 mM K+. The effect of B-HT 920 on both [3H]dopamine and [14C]acetylcholine release was completely blocked by (+) butaclamol but not by (-) butaclamol. Sulpiride, a selective D2 antagonist, similarly blocked the inhibitory effect of B-HT 920 on the release of both labeled neurotransmitters indicating both responses were mediated by D2 receptors. (+) Butaclamol alone elevated stimulated [3H]dopamine release suggesting a significant amount of autoreceptor occupancy by endogenously released dopamine. Experiments with tolazoline and the alpha 2 agonist, B-HT 933, did not suggest any involvement of alpha-adrenoceptor activity in the inhibitory effects of B-HT 920 on the release of either transmitter. Inhibition of release was a selective effect of B-HT 920 as the drug was without effect on the K+-stimulated release of [3H]serotonin. The results indicate that in vitro B-HT 920 is active of both pre- and postsynaptic dopamine receptors in contrast to the pattern of effects observed after its in vivo administration.

Effect of dopamine D-1 and D-2 receptor selective drugs on dopamine release and metabolism in rat striatum in vivo

The effect of the dopamine (DA) D-1 agonist SKF 38393, the D-2 agonist LY 171555 and the mixed D-1/D-2 agonist apomorphine on striatal DA release and metabolism was tested in vivo using an intracerebral dialysis method in halothane-anaesthetized rats. The specificity of responses to these agonists was tested using the selective DA antagonists SCH 23390 (D-1) and sulpiride (D-2). Both LY 171555, 0.01 mg/kg, and SKF 38393, 10 mg/kg, reduced levels of DA in striatal perfusates. Neither SCH 23390, 0.5 and 5 mg/kg, nor sulpiride, 10 mg/kg, affected levels of DA in striatal perfusates, but 250 mg/kg sulpiride caused a DA increase. The decrease of DA levels induced by LY 171555 (0.01 mg/kg) was prevented by pretreatment with sulpiride (10 mg/kg) but not SCH 23390 (0.5 mg/kg). In comparison, pretreatment with SCH 23390 (0.5 mg/kg) completely inhibited the reduction of DA induced by SKF 38393 (10 mg/kg) while sulpiride (10 mg/kg) was without effect. Apomorphine (0.05 mg/kg) also decreased DA in striatal perfusates and this action was partially inhibited by both SCH 23390 (0.5 mg/kg) and sulpiride (10 mg/kg). Levels of the DA metabolite DOPAC in striatal perfusates also significantly decreased following LY 171555 (0.01 mg/kg) and apomorphine (0.05 mg/kg) but not SKF 38393 (10 mg/kg). The antagonist SCH 23390, in a dose, 0.5 mg/kg, that alone did not increase levels of DOPAC, inhibited the reduction of DOPAC induced by both LY 171555 and apomorphine. Sulpiride, 10 mg/kg, caused a marked increase in striatal DOPAC and this was not affected by a subsequent injection of LY 171555, SKF 38393 or apomorphine.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of dopamine content, synthesis, and release in the rabbit retina in vitro: II. Effects of high potassium, adenylate cyclase activators, and N-n-propyl-3-(3-hydroxyphenyl) piperidine

The modulation of 3,4-dihydroxyphenylethylamine (dopamine, DA) synthesis and release in rabbit retina in vitro by high K+; adenylate cyclase activators such as forskolin, 2-chloroadenosine, vasoactive intestinal polypeptide (VIP); and the putative DA autoreceptor agonist N-n-propyl-3-(3-hydroxyphenyl) piperidine (3-PPP) has been investigated. Incubation of retinas in 50 mM K+ resulted in the activation of tyrosine hydroxylase (TH). Activation did not require the presence of extracellular Ca2+. K+ 50 mM also induced a Ca2+-dependent release of DA. Forskolin 50 microM stimulated TH but 100 microM 2-chloroadenosine and 650 nM VIP did not. Individually, (+)-3-PPP, (-)-3-PPP, and (+/-)-3-PPP reduced DA synthesis and increased its release. The effects of (+/-)-3-PPP were dose-dependent and did not require the presence of extracellular Ca2+. The activation of TH induced by 50 mM K+, but not that induced by 50 microM forskolin, was abolished by 100 microM (+/-)-3-PPP.

Modulation of [3H]-dopamine released by different frequencies of stimulation from rabbit retina

Rabbit retinal pieces were incubated with [3H]-dopamine and superfused with Krebs solution. Calcium-dependent tritium release was elicited twice within each experiment by electrical field stimulation (360 pulses, 2 ms, 20 mA) at frequencies of 1 Hz, 3 Hz, and 6 Hz. The evoked release of [3H]-dopamine for the first period of stimulation (S1) was 2.09 +/- 0.23% (n = 5) at 1 Hz and was of similar magnitude at all other frequencies of stimulation employed. The D2-dopamine receptor agonist, LY 171555 (quinpirole HCl; 0.01-1 microM) added to the superfusion medium before the second period of stimulation, inhibited the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, and was more potent the lower the stimulation frequency. The isomer of quinpirole, LY 181990 (0.01-1 microM) did not inhibit the stimulation-evoked overflow of tritium, regardless of concentration or stimulation frequency. The stereoisomers of the D2-dopamine receptor antagonist sulpiride (0.01-3 microM) increased the calcium-dependent release of [3H]-dopamine in a concentration-dependent manner, being more potent the higher the frequency of stimulation. S-sulpiride was more potent than R-sulpiride at all stimulation frequencies. The inhibitory effect of quinpirole was stereoselectively antagonized by sulpiride, but not by LY 181990. The alpha-adrenoceptor antagonist phentolamine (1 microM) did not modify the quinpirole-induced inhibition of [3H]-dopamine release. When the synaptic concentration of dopamine was increased by the presence of the dopamine uptake inhibitor nomifensine (3 microM), the potency of the agonist quinpirole in inhibiting the release of [3H]-dopamine elicited by field stimulation at 1 Hz (180 pulses) was decreased. In contrast, the potency of S-sulpiride in enhancing the evoked release of [3H]-dopamine was increased when nomifensine was present in the superfusion medium. Picomolar concentrations of the hormone melatonin (0.1-10 nM) inhibited the calcium-dependent release of [3H]-dopamine from rabbit retina with the same potency regardless of the frequency of stimulation applied (1 Hz, 3 Hz or 6 Hz). The potency of D2-dopamine receptor agonists and antagonists in modifying [3H]-dopamine release from rabbit retina appears to depend on the synaptic concentration of dopamine which is altered by the frequency of stimulation, and by the dopamine uptake inhibitor nomifensine.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic dopamine autoreceptors control tyrosine hydroxylase activation in depolarized striatal dopaminergic terminals

The possible control of tyrosine hydroxylase (TH) activity by dopaminergic receptor-dependent mechanisms was investigated using rat striatal slices or synaptosomes incubated in the presence of various 3,4-dihydroxyphenylethylamine (dopamine or DA) agonists and antagonists. Under "normal" conditions (4.8 mM K+ in the incubating medium), the DA agonists apomorphine, 6,7-dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99), 7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT), Trans-(-)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-3,4- quinoline, and 3-(3-hydroxyphenyl)-N-n-propylpiperidine decreased TH activity in soluble extracts of incubated tissues. In the case of the catechol-containing drugs apomorphine and TL-99, this effect was partly due to a direct inhibition of the enzyme, but in all other cases it appeared to depend on the stimulation of presynaptic DA autoreceptors. No effect of DA antagonists was detected on TH activity under "normal" conditions. In contrast, when tissues were incubated in a K+ -enriched (60 mM) medium, (-)-sulpiride and other DA antagonists enhanced TH activation due to depolarization whereas DA agonists were ineffective. Because (-)-sulpiride also increased the enzyme activity in striatal slices exposed to drugs inducing release of DA, such as veratridine and d-amphetamine, it is concluded that the stimulating effect of the DA antagonist resulted in fact from the blockade of the negative control of TH normally triggered by endogenous DA acting on presynaptic autoreceptors. In contrast to TH activation due to K+ -induced depolarization, the activation evoked by tissue incubation with dibutyryl cyclic AMP was unaffected by the typical agonist 7-OH-DPAT or the antagonist (-)-sulpiride. This would suggest that TH control via presynaptic DA autoreceptors normally concerns possible modulations of the cyclic AMP-dependent phosphorylation of the enzyme.

Ascorbic acid does not modulate stimulated dopamine release: in vivo voltammetric data in the rat

Electrical stimulation of the nigrostriatal pathway released dopamine (DA) in the striatum of the anaesthetized rat. The level of DA released by 10-s stimulus trains was measured by high-speed cyclic voltammetry. Metoclopramide (10 mg/kg) increased DA release by approximately 20%. Apomorphine (1.76 mg/kg) caused a approximately 40% decrease in release which was blocked by metoclopramide. Ascorbate (1.76 g/kg) had no effect on stimulated DA release. Furthermore, pretreatment of rats with ascorbate trebled the striatal extracellular ascorbate level, but failed to modify the effects of metoclopramide and apomorphine on DA release. We conclude that ascorbate has no effect on the presynaptic autoreceptors that modulate striatal DA release in vivo.

Inhibition of protein carboxylmethylation and dopamine autoreceptor functioning

S-Adenosyl-L-homocysteine (SAH, 2-100 microM) greatly inhibited protein carboxymethylation (PCM) in rat striatal synaptosomes, but did not alter the ability of apomorphine and other DA agonists to inhibit DA synthesis. SAH (10 microM) also did not significantly alter the ability of either 0.5 or 1.0 microM apomorphine to inhibit DA release from superfused rat striatal tissue slices, but it did antagonize the response to 5.0 microM apomorphine. The former two concentrations of apomorphine predominantly affected only DA release, whereas the latter concentration suppressed both DA and acetylcholine release. These findings are discussed with regard to the possible relationship between DA autoreceptor functioning and PCM activity.

[3H] beta-phenylethylamine but not [3H](+)-amphetamine is released by electrical stimulation from perfused rat striatal slices

Perfused rat striatal slices were prelabelled with either [3H](+)-amphetamine or [3H] beta-phenylethylamine. Electrical stimulation released a significant amount of radioactivity only from the slices prelabelled with [3H] beta-phenylethylamine. The electrically evoked release of radioactivity from slices labelled with [3H] beta-phenylethylamine was entirely calcium-dependent and was abolished after pretreatment with reserpine (5 mg/kg s.c., 24 h). In addition, S-sulpiride (1 microM), which facilitates the electrically evoked release of radioactivity from slices labelled with [3H]DA by blocking dopamine autoreceptors, also induced an increase of the radioactivity released by electrical stimulation from slices labelled with [3H] beta-phenylethylamine. Our results indicate that, in spite of the structural similarities between AMPH and PEA, only the latter which is the naturally occurring analog of AMPH can be released by electrical stimulation in a calcium-dependent manner.

Inhibition of the electrically evoked release of [3H]acetylcholine in rat striatal slices: an experimental model for drugs that enhance dopaminergic neurotransmission

The activation by endogenous dopamine of the inhibitory 3,4-dihydroxyphenylethylamine (dopamine) receptors modulating the electrically evoked release of [3H]acetylcholine [( 3H]ACh) and [3H]dopamine in rat striatal slices is a function of the concentration of dopamine accumulated in the synaptic cleft during electrical stimulation. When the release of 3H-neurotransmitters was elicited with a 2-min period of stimulation at a frequency of 1 Hz, neither dopamine autoreceptors nor dopamine receptors modulating [3H]ACh were activated by endogenously released dopamine. On the other hand, exposure to (S)-sulpiride facilitated the release of [3H]dopamine and [3H]ACh elicited when the 2-min stimulation was carried out at a frequency of 3 Hz but this effect was not observed at a lower frequency of stimulation (1 Hz). In the presence of amphetamine the dopamine receptors modulating the electrically evoked release of [3H]ACh can be activated by endogenous dopamine even at the lower frequency of stimulation (1 Hz). Similar effects can be obtained if the neuronal uptake of dopamine is inhibited by cocaine or nomifensine. The inhibition by amphetamine of the release of [3H]ACh elicited by electrical stimulation at 1 Hz involves dopamine receptors and can be fully antagonized by clozapine, haloperidol, chlorpromazine, or pimozide. The stereoselectivity of this antagonism can be demonstrated with the optical enantiomers of sulpiride and butaclamol. This inhibitory effect of amphetamine on cholinergic neurotransmission appears to be the result of the stimulation of dopamine receptors of the D2 subtype, as they were resistant to blockade by the preferential D1 receptor antagonist SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neuroleptic drugs on striatal dopamine release and metabolism in the awake rat studied by intracerebral dialysis

This study investigated the effect of three neuroleptic drugs, (+/-)-sulpiride, haloperidol and cis-flupenthixol, on dopamine release and metabolism in the striatum of the awake rat. Endogenous extracellular dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the 5-hydroxytryptamine (5HT) metabolite 5-hydroxyindoleacetic acid (5HIAA), were determined in striatal perfusates in awake rats by using intracerebral dialysis together with high performance liquid chromatography with electrochemical detection. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.5 and 2 mg/kg) and haloperidol (2 mg/kg) all increased the levels of dopamine in striatal perfusates. However, the time course and magnitude of these effects differed markedly depending upon the neuroleptic used. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.05, 0.5 and 2 mg/kg) and haloperidol (0.05, 0.5 and 2 mg/kg) increased extracellular levels of DOPAC and HVA while having little effect on 5HIAA. In contrast to the effect on dopamine levels the changes in DOPAC and HVA followed similar time courses and were of similar magnitude independent of the neuroleptic used. The response of the dopamine metabolites seemed to occur at lower doses of the neuroleptics than the response of dopamine release itself. Furthermore, there was no close relationship between changes in dopamine as compared to changes in DOPAC and HVA. Finally, there was no correlation between any of the neurochemical changes measured and the occurrence of catalepsy. These data suggest that neuroleptic drugs have two separate actions on the dopamine neuron in vivo, one causing an increase in dopamine release and another producing an increase in dopamine metabolism, which is probably a consequence of increased dopamine synthesis. Furthermore neither of these effects are related to catalepsy.

Dopamine synthesis in synaptosomes: relation of autoreceptor functioning to pH, membrane depolarization, and intrasynaptosomal dopamine content

Factors affecting dopamine (DA) synthesis in rat striatal synaptosomes were examined by measuring the conversion of [3H]tyrosine (Tyr) to [3H]DA. Any [3H]DA that was synthesized was extracted into a toluene-based scintillation cocktail and quantitated by liquid scintillation spectrometry. The extraction was facilitated using di-(2-ethylhexyl) phosphoric acid (DEHP), a liquid cation exchanger. DA, apomorphine, and other DA agonists were much less potent inhibitors of DA synthesis in striatal synaptosomes at pH 6.2 than at pH 7.2. 3-(3-Hydroxyphenyl)-N-n-propylpiperidine (3-PPP), a putative DA autoreceptor agonist, was inactive at pH 6.2. However, at pH 7.2, 3-PPP did inhibit DA synthesis. This inhibition was reversed by sulpiride, a DA receptor antagonist, but not by benztropine, a DA uptake blocker, suggesting that 3-PPP inhibits DA synthesis by stimulating the DA autoreceptor. DA release from synaptosomes was much greater at pH 6.2 than at pH 7.2, most probably because the synaptosomal membrane appears to be depolarized at pH 6.2, as measured by the accumulation of [3H]tetraphenylphosphonium ions. Since tyrosine hydroxylase is inhibited by DA, this finding suggested that low assay buffer pH (i.e., pH 6.2) might interfere with the ability of 3-PPP and other DA agonists to inhibit DA synthesis, by promoting DA release. Likewise, reserpine and tetrabenazine, compounds which disrupt vesicular DA storage, were much less effective inhibitors of DA synthesis at pH 6.2 (high basal DA release). Moreover, D-amphetamine and high buffer potassium concentrations, treatments which promote DA release, also interfered with the ability of 3-PPP to inhibit DA synthesis. Thus, modulation of the release of DA in equilibrium with tyrosine hydroxylase may be a mechanism by which the DA autoreceptor regulates DA synthesis.

Differential effects of the stereoisomers of 3PPP on dopaminergic and cholinergic neurotransmission in superfused slices of the corpus striatum

The two enantiomers of 3PPP were tested on the spontaneous and electrically-evoked release of 3H-dopamine from slices of the rabbit caudate nucleus and of 3H-acetylcholine (3H-ACh) from slices of the rat caudate nucleus. In caudate slices labelled with 3H-dopamine, exposure to (+)3PPP (0.1-1 microM) facilitated the spontaneous outflow of radioactivity with a concomitant inhibition of the electrically-evoked release of 3H-dopamine. In the presence of cocaine 10 microM, exposure to (+)3PPP (1 microM) inhibited the electrically evoked release of 3H-dopamine without modifying the spontaneous outflow of radioactivity. This inhibitory effect was not significantly antagonized by S-sulpiride 0.01 microM. Exposure to (+)3PPP 1 microM inhibited the electrically-evoked release of 3H-ACh, and this effect was not modified by pretreatment with reserpine alone, or in combination with alpha-methyl-p-tyrosine (alpha-MT). In contrast to the (+) enantiomer, exposure to (-)3PPP (0.1-1 microM) facilitated the electrically-evoked release of 3H-dopamine without affecting the spontaneous outflow of radioactivity. (-)3PPP antagonized the inhibitory effect of apomorphine on the electrically-evoked release of 3H-dopamine. Exposure to (-)3PPP 1 microM did not modify the spontaneous or the electrically-evoked release of 3H-ACh. Yet, this concentration of (-)3PPP antagonized significantly the inhibitory effect of 0.03 microM apomorphine, 1 microM d-amphetamine, and 1 microM (+)3PPP on the electrically-evoked release of 3H-ACh (-)3PPP (0.1-1 microM) was about 100 times less potent than S-sulpiride at antagonizing the inhibitory effect of apomorphine on the electrically-evoked release of 3H-ACh.(ABSTRACT TRUNCATED AT 250 WORDS)