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

Calcium signaling dysfunction in schizophrenia: a unifying approach

The present paper demonstrates a remarkable pervasiveness of underlying Ca(2+) signaling motifs among the available biochemical findings in schizophrenic patients and among the major molecular hypotheses of this disease. In addition, the paper reviews the findings suggesting that Ca(2+) is capable of inducing structural and cognitive deficits seen in schizophrenia. The evidence of the ability of antipsychotic drugs to affect Ca(2+) signaling is also presented. Based on these data, it is proposed that altered Ca(2+) signaling may constitute the central unifying molecular pathology in schizophrenia. According to this hypothesis schizophrenia can result from alterations in multiple proteins and other molecules as long as these alterations lead to abnormalities in certain key aspects of intracellular Ca(2+) signaling cascades.

Clozapine-induced dopamine levels in the rat striatum and nucleus accumbens are not affected by muscarinic antagonism

The effect of the muscarinic antagonist, scopolamine, was examined for a change in the increase in extracellular dopamine, dihydroxyphenyl acetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA), induced by haloperidol or clozapine in the striatum and nucleus accumbens of anaesthetised and awake rats, monitored using in vivo cerebral microdialysis. Rats received scopolamine (1 mg kg(-1); s.c.) or vehicle followed by haloperidol (1 mg kg(-1); s.c.) or clozapine (20 mg kg(-1); s.c.). Dopamine, DOPAC, HVA and 5-HIAA overflow into striatal or accumbens perfusates was determined using high performance liquid chromatography with electrochemical detection (HPLC-ECD). Scopolamine failed to modify the clozapine- or haloperidol-induced efflux of dopamine or its metabolites in either the striatum or nucleus accumbens following systemic administration in anaesthetised or awake rats. Although pretreatment with scopolamine tended to produce a smaller increase in the clozapine-induced efflux of DOPAC in striatal perfusates than following clozapine treatment alone, this was not statistically significant. Furthermore, local infusion of scopolamine (100 microM) with clozapine (1 mM) via the microdialysis probe did not attenuate the elevated efflux of dopamine observed following clozapine alone, in either the striatum or nucleus accumbens, in anaesthetised rats. This treatment did prevent the clozapine-induced increase in DOPAC and HVA in the striatum but not the nucleus accumbens. Carbachol (50 microM) infused into the dorsolateral striatum or nucleus accumbens raised extracellular dopamine levels 200% and 150%, respectively above baseline. Our data suggest that the increased efflux of dopamine and its metabolites in the rat basal ganglia following clozapine administration is not significantly dependent upon the interaction of clozapine with muscarinic receptors.

Muscimol increases acetylcholine release by directly stimulating adult striatal cholinergic interneurons

Because GabaA ligands increase acetylcholine (ACh) release from adult striatal slices, we hypothesized that activation of GabaA receptors on striatal cholinergic interneurons directly stimulates ACh secretion. Fractional [3H]ACh release was recorded during perifusion of acutely dissociated, [3H]choline-labeled, adult male rat striata. The GabaA agonist, muscimol, immediately stimulated release maximally approximately 300% with EC50 = approximately 1 microM. This action was enhanced by the allosteric GabaA receptor modulators, diazepam and secobarbital, and inhibited by the GabaA antagonist, bicuculline, by ligands for D2 or muscarinic cholinergic receptors or by low calcium buffer, tetrodotoxin or vesamicol. Membrane depolarization inversely regulated muscimol-stimulated secretion. Release of endogenous and newly synthesized ACh was stimulated in parallel by muscimol without changing choline release. Muscimol pretreatment inhibited release evoked by K+ depolarization or by receptor-mediated stimulation with glutamate. Thus, GabaA receptors on adult striatal cholinergic interneurons directly stimulate voltage- and calcium-dependent exocytosis of ACh stored in vesamicol-sensitive synaptic vesicles. The action depends on the state of membrane polarization and apparently depolarizes the membrane in turn. This functional assay demonstrates that excitatory GabaA actions are not limited to neonatal tissues. GabaA-stimulated ACh release may be prevented in situ by normal tonic dopaminergic and muscarinic input to cholinergic neurons.

Modulation of dopamine release from rat striatum by protein kinase C: interaction with presynaptic D2-dopamine-autoreceptors

1. Interactions between dopamine receptors and protein kinase C (PKC) have been proposed from biochemical studies. The aim of the present study was to investigate the hypothesis that there is an interaction between protein kinase C and inhibitory D2-dopamine receptors in the modulation of stimulation-induced (S-I) dopamine release from rat striatal slices incubated with [3H]-dopamine. Dopamine release can be modulated by protein kinase C and inhibitory presynaptic D2 receptors since phorbol dibutyrate (PDB) and (-)-sulpiride, respectively, elevated S-I dopamine release. 2. The protein kinase C inhibitors polymyxin B (21 microM) and chelerythrine (3 microM) had no effect on stimulation-induced (S-I) dopamine release. However, when presynaptic dopamine D2 receptors were blocked by sulpiride (1 microM), an inhibitory effect of both PKC inhibitors on S-I dopamine release was revealed. Thus, sulpiride unmasks an endogenous PKC effect on dopamine release which suggests that presynaptic D2 receptors normally suppress endogenous PKC activity. This is supported by results in striatal slices which were pretreated with PDB to down-regulate PKC. In this case the facilitatory effect of sulpiride was completely abolished. 3. The inhibitory effect of the dopamine D2/D3 agonist quinpirole on S-I dopamine release was partially attenuated by PKC down-regulation. Since the effect of sulpiride was completely abolished under the same conditions, this suggests that exogenous agonists may target a PKC-dependent as well as a PKC-independent pathway. The inhibitory effect of apomorphine was not affected by either polymyxin B or PKC down-regulation, suggesting that it operated exclusively through a PKC-independent mechanism. 4. These results suggest that there are at least two pathways involved in the inhibition of dopamine release through dopamine receptors. One pathway involves dopamine receptor suppression of protein kinase C activity, perhaps through inhibition of phospholipase C activity and this is preferentially utilized by neuronally-released dopamine. The other pathway which seems to be utilized by exogenous agonists does not involve PKC.

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D1 receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K(+)-evoked [3H]ACh release while SCH 23390, a dopamine D1 receptor antagonist, had no effect. [3H]ACh release was decreased by the dopamine D2 receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D2 receptor antagonist haloperidol. The selective neurokinin NK1 receptor agonist [Sar9, met(O2)11]SP also potentiated K(+)-evoked release of [3H]ACh. GR 82334, a NK1 receptor antagonist, blocked not only the effect of [Sar9, met(O2)11]SP but also the release of ACh induced by the D1 receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D1 receptor stimulation. Mesulergine, a more selective 5-HT2C antagonist, showed an intrinsic releasing effect but did not affect K(+)-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT1/2 antagonists, as well as ondansetron, a 5-HT3 receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT2 receptor agonists were ineffective. However, the 5-HT2 agonist DOI was able to prevent the antagonism by ketanserin of the increased [3H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK1 agonist. The results suggest that 5-HT2 receptors, probably of the 5-HT2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D1 receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK1 receptors located on cholinergic interneurons.

Influence of dopamine on GABA release in striatum: evidence for D1-D2 interactions and non-synaptic influences

Striatal slices from the rat were preincubated with [3H]GABA and superfused in the presence of nipecotic acid and aminooxyacetic acid, inhibitors of high-affinity GABA transport and GABA aminotransferase, respectively. GABA efflux was estimated by monitoring tritium efflux, 98% of which was in the form of [3H]GABA. The following three major observations were made: (1) The overflow of GABA evoked by electrical field stimulation (8 Hz) was increased two-fold by SKF-38393 (10 microM), an agonist at the D1 family of dopamine receptors. This increase was completely blocked by the D1 receptor antagonist SCH-23390 (10 microM). However, SCH-23390 had no effect on GABA overflow when given alone. Thus, dopamine agonists appear to exert an excitatory influence on GABA release; however, this effect was not elicited by endogenous dopamine under the conditions of this experiment. (2) Electrically evoked GABA overflow was reduced 50% by quinpirole (10 microM), an agonist at the D2 family of dopamine receptors, and this effect was blocked by the D2 antagonist sulpiride (10 microM). Moreover, exposure to sulpiride alone caused a 60% increase in GABA overflow, and this effect was abolished by 3-iodotyrosine (2 mM), a dopamine synthesis inhibitor. Thus, D2 agonists appear to exert an inhibitory influence on dopamine release, an effect that can be exerted by endogenous stores of dopamine. (3) The stimulatory effect of SKF-38393 was attenuated by quinpirole, whereas the sulpiride-induced increase in GABA efflux was attenuated by SCH-23390. Sulpiride also increased [3H]GABA efflux during KCl-induced depolarization, an effect that was antagonized by SCH-23390 as in the case of electrical stimulation. However, although tetrodotoxin did not alter the stimulatory effect of sulpiride, it did block the ability of SCH-23390 to antagonize the sulpiride-induced increase in GABA overflow. These latter results suggest that there is an interaction between D1 and D2 receptors whereby the effects of dopamine mediated via D1 sites are inhibited by an action on D2 sites. In conclusion, our results suggest that (i) dopamine agonists can exert an excitatory influence on depolarization-induced GABA release within neostriatum via D1 receptors and an inhibitory influence via D2 receptors; (ii) under the conditions of these experiments, endogenous dopamine fails to act on D1 sites but does exert an inhibitory influence via D2 sites; and (iii) there is an interaction between D1 and D2 receptors such that the actions of dopamine mediated via D1 sites are inhibited as a result of the concomitant actions exerted via D2 sites.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Adenylate cyclase in striatal cholinergic interneurons regulates acetylcholine release

Fractional [3H]ACH efflux from dissociated rat striata tested whether tonic inhibition prevents stimulation of acetylcholine (ACH) release by adenylate cyclase. Forskolin stimulated release from the dissociated cells (threshold at 300 nM; EC50 > or = 1 MicroM). Release was also stimulated by 3-isobutyl-1-methylxanthine and was additive with forskolin. The 1,9-dideoxy forskolin analog that lacks cyclase-stimulating activity was ineffective. Thus, stimulation of adenylate cyclase within striatal cholinergic interneurons increases ACH secretion but is tonically inhibited by endogenous striatal transmitters. Disinhibition of the excitatory cyclase by denervation of striatal cholinergic interneurons in situ could contribute to supersensitivity without receptor upregulation.

A D1 dopamine agonist stimulates acetylcholine release from dissociated striatal cholinergic neurons

We tested the hypothesis that a D1 dopamine agonist could stimulate acetylcholine release directly from striatal cholinergic neurons. A suspension of viable dissociated striatal cells was made enzymatically and mechanically from normal adult male rats. The heterogeneous suspension was incubated in [3H]choline to allow synthesis of [3H]acetylcholine selectively by cholinergic neurons. Fractional [3H]acetylcholine release from the cholinergic cells in the suspension was recorded during continuous dynamic perifusion. The D1 agonist, 50 microM (+/-) SKF 38393, increased the basal rate of release from the cholinergic cells by 50% and the action was inhibited by the D1 antagonist, SKF 83566. Stimulation of [3H]acetylcholine secretion was recorded as low as 500 nM SKF 38393. The (S, -) SKF 38393 stereoisomer was significantly less effective than the (R, +) isomer in stimulating release. The D1-mediated stimulation of acetylcholine secretion was abolished in a low-calcium environment that also inhibited basal release. The data suggest that striatal cholinergic cells express D1 receptors functionally coupled to the regulation of acetylcholine release. These D1 actions in the absence of synaptic circuitry imply that such circuitry is not required in situ. In vivo however, indirectly mediated D1 actions and those of other transmitters may modify the manifestations of this direct cholinergic stimulation.

Acetylcholine release from dissociated striatal cells

To study the regulation of striatal acetylcholine (ACH) release, adult male rat striata were dissociated and incubated with 3H-choline to synthesize 3H-ACH. Fractional 3H-ACH efflux per min during continuous perifusion was: (1) tightly regulated; (2) dependent on calcium influx; (3) stimulated by 10 mM K+ and 1 mM glutamate; and (4) comparable to ACH release detected by HPLC. Thus, acutely dissociated striata exhibit calcium-sensitive, voltage-dependent secretion of 3H-ACH and direct receptor-mediated stimulation of release through the glutamate receptor family. This new approach toward cholinergic secretory physiology will help clarify complex striatal circuitry.

Acute dopamine depletion potentiates independent stimulatory and inhibitory D1 DA receptor-mediated control of striatal acetylcholine release in vitro

Fractional release of [3H]ACh was evaluated under basal and evoked conditions in striatal slices from normal and acutely dopamine-depleted adult rats for the influence of D1- and D2-DA receptor agonists. The D1 ligand had no effect on normal slices but DA depletion unmasked two independent but simultaneous supersensitive responses: augmentation of K(+)-evoked and inhibition of glutamate-evoked release. The D2 ligand inhibited evoked release in normal slices and this effect was not potentiated. This is a new cholinergic model of acute D1 receptor supersensitivity.

Dopaminergic agonists have both presynaptic and postsynaptic effects on the guinea-pig's medial vestibular nucleus neurons

A number of studies have indicated a possible interaction between dopamine and the vestibular system. Using intracellular recordings in brainstem slices, we have tested the effects of dopamine and other dopaminergic compounds on guinea-pig medial vestibular nucleus (MVN) neurons. In normal medium, MVN neurons were depolarized by dopamine as well as by (-)quinpirole and piribedil, which are selective D2 dopaminergic agonists. The dependence of this effect on the presence of D2 receptors was confirmed by using (-)sulpiride, a D2 antagonist which blocked the depolarizing effect of dopamine. Dopaminergic D1 receptors were apparently not involved in this effect since a selective D1 agonist, SKF-38393, had no effect on MVN neurons and the D1 antagonist (+)SCH-23390 could not block the effect of dopamine. These depolarizing responses to dopamine must be due to a presynaptic action on terminals that normally release GABA spontaneously on MVN neurons, and tonically maintain them in a state of hyperpolarization. Indeed, such a spontaneous release was demonstrated to occur in the slice since application of bicuculline, a GABAA antagonist, depolarized MVN neurons in normal saline, but not in a high Mg2+/low Ca2+ solution known to block synaptic transmission. When dopamine was applied in conditions in which no GABAA-dependent transmission could occur (either in the presence of bicuculline or in a high Mg2+/low Ca2+ solution) only a hyperpolarizing, most probably postsynaptic, effect occurred. These results indicate that dopamine might exert in vivo a significant modulatory action on the vestibular system, either by a direct action on the vestibular neurons or by modulation of GABAergic transmission.

Increases in striatal acetylcholine by SKF-38393 are mediated through D1 dopamine receptors in striatum and not the frontal cortex

It has been proposed by some that the D1 receptor effects are mediated through striatal actions while others have suggested that they are determined indirectly through the frontal cortex. The experiments reported here represent a further attempt to resolve this controversy. It was found that focal applications of the inactive and active enantiomers of SKF-38393 (a D1 dopamine receptor agonist) to the rat striatum via reverse dialysis increased extracellular acetylcholine (ACh) in a stereoselective manner. Infusions of SKF-38393 into the frontal cortex, on the other hand, were ineffective in altering striatal ACh. Furthermore, partial hemisections caudal to the frontal cortex did not alter the ability of systemically administrated SKF-38393 to increase striatal ACh. Taken together, these results suggest that at least some of the effects of D1 receptor agonists on striatal cholinergic function are mediated through actions in the striatum and not the frontal cortex.

Dopamine agonist-mediated inhibition of acetylcholine release in rat striatum is modified by thyroid hormone status

K(+)-evoked acetyl[3H]choline ([3H]ACh) release was inhibited in a concentration-dependent manner by apomorphine and the D2 agonist quinpirole in striatal slices prepared from euthyroid and hypothyroid rats. However, there was a significant increase in the maximum inhibition observed with both agonists in the hypothyroid compared with the euthyroid group, which paralleled the increased D2 agonist sensitivity reported for stereotyped behavior. The D2 antagonist raclopride decreased, and the D1 antagonist SCH 23390 increased, the inhibition of [3H]ACh release by apomorphine, confirming an inhibitory role for D2 receptors and an opposing role for D1 receptors. Because there is no difference in D1 or D2 receptor concentration between the euthyroid and hypothyroid groups, it is suggested that thyroid hormone modulation of D2 receptor sensitivity affects a receptor-mediated event. Following intrastriatal injection of pertussis toxin (PTX), apomorphine no longer inhibited [3H]ACh release. In fact, increased [3H]-ACh release was observed, an effect reduced by SCH 23390, providing evidence that D1 receptors enhance [3H]-ACh release, and confirming that a PTX-sensitive G protein mediates the D2 response. As it has been reported that thyroid hormones modulate G protein expression, this mechanism may underlie their effect on dopamine agonist-mediated inhibition of ACh.

Reduction of dopamine synthesis inhibition by dopamine autoreceptor activation in striatal synaptosomes with in vivo reserpine administration

In an attempt to clarify the mechanisms by which dopamine (DA) autoreceptor activation inhibits DA synthesis, the efficacy and potency of the D2 DA agonists bromocriptine, lisuride, and pergolide, and the D1-D2 DA agonist apomorphine were studied in rat striatal synaptosomes, in which the rate of DA synthesis (formation of 14CO2 from L-[1-14C]tyrosine) was increased 103% by treating the animals from which the synaptosomes were obtained with reserpine (5 mg/kg i.p. twice, 24 and 2 h before they were killed), using the striatal total homogenate as the standard synaptosomal preparation. The increase in DA synthesis evoked by reserpine was additive with that produced by treatment of the synaptosomes with dibutyryl cyclic AMP, suggesting that, not a cyclic AMP-dependent, but possibly a Ca(2+)-dependent mechanism was involved. The DA agonists showed a concentration-dependent inhibition of DA synthesis in the control synaptosomes, which was antagonized by the selective D2 DA antagonist (-)-sulpiride. In the synaptosomes with increased rate of DA synthesis obtained from the rats treated with reserpine, the concentration-response curves of DA synthesis inhibition for the other DA agonists were shifted to the right, and the effect of bromocriptine was completely eliminated, whereas bromocriptine antagonized the effect of apomorphine. The increased rate of DA synthesis was not preserved in the striatal P1 + P2 fraction obtained from the reserpine-treated rats, but the effects of the DA agonists were still reduced to the same degree as those in the total homogenate. (-)-Sulpiride did not enhance DA synthesis in synaptosomes from the reserpine-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of release of acetylcholine in the striatum in mice following continuous exposure to selective D1 and D2 dopaminergic agonists

The effect of continuously infusing the selective D1 and D2 dopamine receptor agonists, SKF 38393 and quinpirole, on the release of [3H]acetylcholine from prelabeled striatal slices was investigated. These biochemical parameters were correlated with the behavioral effects of these agonists. Acute injections of SKF 38393 or quinpirole did not affect either K(+)-stimulated or spontaneous release of [3H]acetylcholine. Chronic exposure to quinpirole reduced the K(+)-evoked release of [3H]acetylcholine by 25.7%; long-term treatment with SKF 38393 did not alter the release of [3H]acetylcholine, induced by K+ stimulation. Added in vitro, SKF 38393 increased the release of [3H]acetylcholine from striatal slices. The effect of the D1 dopamine receptor agonist, SKF 38393 was reduced after 7-days of infusion of SKF 38393 but was enhanced by 7-days of infusion of quinpirole. Activation of D2 dopamine receptors with quinpirole or of muscarinic receptors with carbachol induced an inhibition of release of [3H]acetylcholine. Chronic treatment with quinpirole diminished the response to the in vitro addition of quinpirole. The ability of carbachol to inhibit release of acetylcholine was not altered by continuous treatment with either SKF 38393 or quinpirole. Continuous infusion of SKF 38393 produced an initial grooming behavior; this behavior disappeared by 2 hr and remained absent during the 7 days of infusion of SKF 38393. Similarly, continuous administration of quinpirole produced stereotyped behavior, which peaked at 1 hr and disappeared by 4 hr and remained absent for the duration of the infusion. These findings demonstrate that continuous exposure to D1 or D2 agonists caused receptor-selective functional desensitization of D1 or D2 dopamine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphetamine-induced changes in behavior and caudate extracellular acetylcholine

In vivo microdialysis was used to study the effects of amphetamine on caudate extracellular acetylcholine and to compare these effects to the drug-induced behavioral response profile. Consistent with an inhibitory dopamine/acetylcholine interaction, the dopamine receptor agonist, apomorphine, decreased acetylcholine concentrations, while the dopamine receptor antagonist, haloperidol, increased acetylcholine. In contrast, an intermediate dose of amphetamine (1.75 mg/kg), did not significantly alter acetylcholine levels. Furthermore, a higher dose of amphetamine (5.0 mg/kg) promoted a two-fold increase in acetylcholine levels, and the increase paralleled the appearance of oral stereotypies in these animals. These results suggest that the effects of amphetamine on caudate acetylcholine, which may be implicated in the appearance of stereotyped behaviors, are not strictly dependent on caudate dopamine receptor activation.

Effect of haloperidol on cyclic AMP and inositol trisphosphate in rat striatum in vivo

To investigate the effect of haloperidol (HAL) on second messengers in the brain striatum, the concentrations of cAMP and inositol trisphosphate (IP-3) were measured in the striatum of rats in vivo after intravenous administration of HAL, and their concentrations were compared with the severity of catalepsy and changes in dopamine (DA) metabolism in the striatum. Catalepsy developed both in the animals treated with 5 mg/kg and those with 0.5 mg/kg of HAL, but it appeared earlier, and the period of severe catalepsy was longer in the former than in the latter. In the animals treated with 5 mg/kg of HAL, DOPAC and HVA began to increase at 20 min after administration, and their percent increases were correlated with the severity of catalepsy. In the 5 mg/kg animals, both cAMP and IP-3 increased. The IP-3 showed a delayed peak but a greater increase as compared with the cAMP. In the 0.5 mg/kg animals, only IP-3 increased. These findings suggest that HAL might affect not only the adenylate cyclase system but also the phosphoinositide response in the striatum. Moreover, the changes in the phosphoinositide response might be secondarily induced by the blocking of D-2 receptors by HAL.

Simultaneous measurement of oxygen and dopamine: coupling of oxygen consumption and neurotransmission

Fast-scan cyclic voltammetry was used to simultaneously measure increases in dopamine concentration and decreases in O2 concentration evoked by brief electrical stimulation (two pulses at 10 Hz) in slices of rat caudate nucleus. Dopamine concentration began increasing immediately after the first pulse and reached a maximum within 200 ms of stimulation. The O2 concentration began to decrease 300-700 ms after onset of stimulus. Responses for both dopamine and O2 were dependent on external Ca2+ and were Cd2+ and tetrodotoxin sensitive. Only the O2 response was sensitive to CN- (0.15 mM). At short times after exposure to 50 microM ouabain, electrically stimulated dopamine overflow was increased by 150% and electrically stimulated changes in O2 concentration were unaffected. Maximum dopamine concentration was increased 28% by sulpiride (2 microM), 78% by L-DOPA (60 microM), 105% by nomifensine (10 microM) and unaffected by nialamide (10 microM). Maximum decrease in O2 concentration was increased by 25% by sulpiride and unaffected by nialamide, L-DOPA, or nomifensine. The decreases in O2 concentration are indicative of increased O2 consumption and are a measure of oxidative energy production evoked by electrical stimulation. The increase in dopamine is due to the release of dopamine balanced by uptake and serves as an indication of neurotransmitter activity. The results indicate that increases in oxidative energy production following electrical stimulation are dependent on external Ca2+ entry through Cd(2+)-sensitive channels. Possible mechanisms for this coupling are discussed.

Mechanism for the gastrokinetic action of domperidone. In vitro studies in guinea pigs

To test the hypothesis that domperidone stimulates gastric muscle contraction by antagonizing the inhibitory effects of dopamine on postsynaptic cholinergic neurons in the myenteric plexus, the effects of dopamine on circular muscle from the body of the guinea pig stomach were examined. Dopamine inhibited circular muscle contraction evoked by electric field stimulation in a dose-related manner. The threshold dose was 10(-6) mol/L and half-maximal inhibition occurred at 10(-5) mol/L. Preincubation of muscle contraction with atropine or tetrodotoxin abolished the contractile response to electric field stimulation, indicating mediation via a cholinergic pathway. The adrenergic antagonists phentolamine and propranolol and the DA1 antagonist SCH 23390 were ineffective in antagonizing the action of dopamine. In contrast, the DA2 antagonist domperidone blocked the inhibitory effect of dopamine on electric field stimulation-mediated contractions. Schild analysis showed a Ki of 3 x 10(-8) mol/L and a slope of unity. In addition, it was shown that dopamine inhibited veratridine-evoked release of [3H]acetylcholine from the gastric myenteric plexus in a dose-related manner (median effective dose, 5.2 x 10(-5) mol/L). Tetrodotoxin abolished [3H]acetylcholine release evoked by veratridine, but hexamethonium had no effect. Domperidone, but not SCH 23390, antagonized the inhibitory action of dopamine. Pretreatment with pertussis toxin blocked the action of dopamine to inhibit evoked release of [3H]acetylcholine. These observations indicate that dopamine inhibits gastric muscle contraction evoked by electric field stimulation by inhibiting cholinergic transmission. This is mediated by DA2 receptors located on the postganglionic cholinergic neurons, and the pathway involves a pertussis toxin-sensitive G protein. The DA2-receptor antagonist domperidone antagonizes the inhibitory effect of dopamine, resulting in stimulation of gastric muscle contraction. This provides a mechanism for the gastrokinetic effect of domperidone.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

Dopamine D1 receptor stimulation increases striatal acetylcholine release in the rat

The effect of selective D1 receptor agonists on acetylcholine (ACh) release in the striatum was investigated using in vivo microdialysis. Administration of the reactive enantiomer, (+)-SKF 38393 (2, 10 mg/kg s.c.), doses which elevate grooming and sniffing behaviour, increased ACh release by 40 and 75%, respectively. Another D1 receptor agonist CY 204-283 (1 mg/kg s.c.) also produced a 75% increase in ACh output. The racemate (+/-)-SFK 38393 (20 mg/kg s.c.) increased ACh output by 60% and this was completely blocked by the D1 receptor antagonist SCH 23390 (0.3 mg/kg s.c.). In contrast, administration of the D2 receptor antagonist raclopride (1 mg/kg s.c.), 60 min after (+/-)-SKF 38393 (20 mg/kg s.c.), further increased ACh release. These results suggest that activation of D1 receptors increases ACh release in vivo and that D1 and D2 receptors have opposing roles in the regulation of striatal ACh release.

Spiny neurons lacking choline acetyltransferase immunoreactivity are major targets of cholinergic and catecholaminergic terminals in rat striatum

The ultrastructural substrate for functional interactions between intrinsic cholinergic neurons and catecholaminergic afferents to the caudate-putamen nucleus and nucleus accumbens septi (NAS) was investigated immunocytochemically. Single sections of glutaraldehyde-fixed rat brain were processed 1) for the immunoperoxidase labeling of a rat monoclonal antibody against the acetylcholine-synthesizing enzyme choline acetyltransferase (CAT) and 2) for the immunoautoradiographic localization of a rabbit polyclonal antiserum against the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). The ultrastructural morphology and cellular associations did not significantly differ in the caudate-putamen versus NAS. Immunoperoxidase reaction for CAT versus NAS. Immunoperoxidase reaction for CAT was seen in perikarya, dendrites, and terminals, whereas immunoautoradiography for TH was in terminals. The perikarya and dendrites immunolabeled for CAT were large, sparsely spiny, and postsynaptic mainly to unlabeled axon terminals. Only 2-3% of the CAT-labeled terminals (n = 136) and less than 1% of the TH-labeled terminals (n = 86) were apposed to, or formed synapses with, perikarya or dendrites immunoreactive for CAT. Most unlabeled and all labeled terminals formed symmetric synapses. In the same sample, 18% of the CAT and 16% of the TH-labeled terminals were directly apposed to each other. Unlabeled dendritic shafts received the major (40% for CAT versus 23% for TH) synaptic input from cholinergic terminals, while unlabeled spines received the major (47% for TH versus 23% for CAT) synaptic input from catecholaminergic terminals. Neither the unlabeled dendrites or spines received detectable convergent input from CAT and TH-labeled terminals. Thirteen percent of the CAT-labeled and 14% of TH-labeled terminals were in apposition to unlabeled terminals forming asymmetric, presumably excitatory, synapses with unlabeled dendritic spines. We conclude that in both the caudate-putamen and NAS cholinergic and catecholaminergic terminals 1) form symmetric, most likely inhibitory, synapses primarily with non-cholinergic neurons, 2) differentially synapse on shafts or spines of separate dendrites, and 3) have axonal appositions suggesting the possibility of presynaptic physiological interactions. These results support the hypothesis that the cholinergic-dopaminergic balance in striatal function may be mediated through inhibition of separate sets of spiny projection neurons with opposing excitatory and inhibitory functions.

Striatal dopamine in motor activation and reward-mediated learning: steps towards a unifying model

On the basis of behavioural evidence, dopamine is found to be involved in two higher-level functions of the brain: reward-mediated learning and motor activation. In these functions dopamine appears to mediate synaptic enhancement in the corticostriatal pathway. However, in electrophysiological studies, dopamine is often reported to inhibit corticostriatal transmission. These two effects of dopamine seem incompatible. The existence of separate populations of dopamine receptors, differentially modulating cholinergic and glutamatergic synapses, suggests a possible resolution to this paradox. The synaptic enhancement which occurs in reward-mediated learning may also be involved in dopamine-mediated motor activation. The logical form of reward-mediated learning imposes constraints on which mechanisms can be considered possible. Dopamine D1 receptors may mediate enhancement of corticostriatal synapses. On the other hand, dopamine D2 receptors on cholinergic terminals may mediate indirect, inhibitory effects of dopamine on striatal neurons.

Factors influencing the calculation of results from studies of the release of tritiated neurotransmitters from superfused slices of guinea pig striata

Slices of guinea pig striata were incubated with tritiated choline, dopamine, or serotonin and the release of radioactive transmitter was studied in a superfusion system. Some experiments were also done on the release of [3H]acetylcholine from rat striatal slices. For analysis of the results, various parameters of the system were determined in order to establish the most reliable method of assessing drug effects on transmitter release. Peaks of radioactivity (S1 and S2) above basal release of radioactivity (B1 and B2) were observed after two depolarizations of the nerve endings with high K+ buffer. Each stimulation was for 2 min, and S2 occurred 20 min after S1. There was a highly significant correlation between S1 and the protein content of the slices for acetylcholine release from guinea pig striata. Basal release of radioactivity, and the ratio S2/S1, were not sensitive to minor changes in the experimental conditions. It was concluded that S2/S1, rather than S1 or S1/B1, should be used as a measure of drug effects on release. The experiments also demonstrated that re-uptake of released neurotransmitter is operative in the superfusion system for dopamine, but not for serotonin. Differences were observed between the rat and the guinea pig with respect to the release of [3H]acetylcholine.

Investigations of the roles of dihydropyridine and omega-conotoxin-sensitive calcium channels in mediating depolarisation-evoked endogenous dopamine release from striatal slices

The relative roles of L- and N-type voltage-sensitive calcium channels (VSCC) in mediating endogenous dopamine release have been investigated by examining the effects of the dihydropyridine (DHP) agonist BAY K 8644 and the antagonist PN 200-110, as well as the VSCC-blocking peptide omega-conotoxin GVIA, on depolarisation-evoked dopamine release from superfused rat striatal slices. Dopamine release evoked by electrical field stimulation was virtually unaffected by either of the DHP drugs, but release evoked by raising the K+ concentration to 25 mmol/l was significantly increased by BAY K 8644 and reduced stereospecifically by PN 200-110. Quantitative differences between electrically-evoked and K+-evoked dopamine release with respect to their dependence on extracellular calcium concentration were also observed, with electrically-evoked release requiring higher calcium concentrations. The adenylate cyclase activator forskolin itself increased dopamine release, but did not appear to influence the effectiveness of either DHP drug in altering dopamine release. In contrast to the relatively small effects of the DHP drugs, omega-conotoxin produced a major reduction in electrically-evoked dopamine release as well as a substantial decrease in K+-evoked release. Since omega-conotoxin is thought to block both L- and N-type neuronal VSCC whereas DHP drugs affect only L-type VSCC, these findings suggest that electrically-evoked dopamine release is mediated mainly by calcium influx through N-type VSCC, accounting for the reported lack of effect of many organic calcium antagonists on this process. In contrast, K+-evoked dopamine release appears to involve both L- and N-type VSCC, and can occur at lower extracellular calcium concentrations.

Stimulation of D-2 dopamine receptors decreases the evoked in vitro release of [3H]acetylcholine from rat neostriatum: role of K+ and Ca2+

Reportedly, stimulation of D-2 dopamine receptors inhibits the depolarization-induced release of acetylcholine from the neostriatum in a cyclic AMP-independent manner. In the present study, we investigated the role of K+ and Ca2+ in the D-2 receptor-mediated inhibition of evoked [3H]acetylcholine release from rat striatal tissue slices. It is shown that the D-2 receptor-mediated decrease of K+-evoked [3H]acetylcholine release is not influenced by the extracellular Ca2+ concentration. However, increasing extracellular K+, in the presence and absence of Ca2+, markedly attenuates the effect of D-2 stimulation on the K+-evoked [3H]acetylcholine release. Furthermore, it is shown that activation of D-2 receptors in the absence of Ca2+ also inhibits the veratrine-evoked release of [3H]acetylcholine from rat striatum. These results suggest that the D-2 dopamine receptor mediates the decrease of depolarization-induced [3H]acetylcholine release from rat striatum primarily by stimulation of K+ efflux (opening of K+ channels) and inhibition of intracellular Ca2+ mobilization.

Mechanisms of signal transduction at the dopamine D2 receptor

D2 dopamine receptor activation induces inhibition of adenylate cyclase, with a rapid decrease of cAMP levels, and an ensuing blockade of IP3-dependent release of Ca2+ from intracellular stores. K+ channels are concomitantly activated and Ca2+ channels are possibly also inhibited. The increased K+ conductance causes hyperpolarization, which may be responsible for the abolition of Ca2+ action potentials and [Ca2+]i fluctuations occurring both at rest and after activation of receptors coupled to PIP2 hydrolysis. Lucia Vallar and Jacopo Meldolesi analyse this spectrum of intracellular signals which might be sufficient to sustain inhibition of secretion in pituitary lactotroph cells and possibly the other effects of D2 receptors in other cell systems.

Localization of striatal dopamine receptor function by central injection of an irreversible receptor antagonist

The stereotypic head-down sniffing response to systemically administered apomorphine (0.65 mumol/kg) was assessed in rats 48 h after the bilateral injection of 0.2-0.5 microliters of the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (60 micrograms/microliters) into the caudate-putamen and nucleus accumbens. This response was significantly attenuated in animals that had received injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into the anterior/ventral part of the caudate-putamen but not in those that received injections into regions more dorsal/posterior. Animals were killed after apomorphine challenge and the region of dopamine D1 or D2 receptor reduction due to N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline mapped and quantified. This analysis revealed that the dopamine receptors involved in the apomorphine-induced stereotyped head-down sniffing response were located in a discrete region of the ventrolateral caudate-putamen and the dorsolateral nucleus accumbens. Animals that were pretreated with the selective dopamine D2 receptor antagonist raclopride (0 20 mumol/kg, i.p.) 20 min prior to central injection of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into this area showed a dose-dependent protection of the stereotyped sniffing response to systemic apomorphine 48 h later. This combination of techniques constitutes a novel way to investigate striatal function and the results obtained support the concept of a functional subdivision of both the caudate-putamen and the nucleus accumbens.

Stimulation of D2 dopamine receptors decreases intracellular calcium levels in rat anterior pituitary cells but not striatal synaptosomes: a flow cytometric study using indo-1

An important question is whether all D2 dopamine (DA) receptors employ the same signal transduction mechanisms. Anterior pituitary cells and striatal synaptosomes, which possess pharmacologically similar D2 DA receptors, were compared with respect to the effect of D2 DA receptor stimulation on free intracellular Ca2+ levels [( Ca2+]i). Flow cytometry, in combination with either the fluorescent calcium indicator indo-1 or fluorescent voltage-sensitive dyes, was used to measure [Ca2+]i and to detect changes in membrane potential. In subpopulations of anterior pituitary cells, increases in [Ca2+]i were produced by elevated K+, veratridine, thyrotropin-releasing hormone, and BAY K 8644. These increases were blocked by nifedipine, suggesting the involvement of L-type voltage-sensitive calcium channels (VSCC's). In 10-15% of the cells, D2 agonists decreased resting [Ca2+]i, reversed stimulus-induced increases in [Ca2+]i, and caused a hyperpolarization. In striatal synaptosomes, elevated K+ and veratridine also increased [Ca2+]i. However, the K+-induced increase was eliminated if choline was substituted for Na+ in the medium, suggesting that Ca2+ entry in response to sustained K+ depolarization resulted from reversal of Na+/Ca2+ exchange. Nifedipine and verapamil inhibited K+-induced increases in [Ca2+]i only at concentrations greater than 10 microM, while omega-conotoxin had no effect. D2 agonists had no effect on resting or stimulated [Ca2+]i but did hyperpolarize 10-20% of the synaptosomes, indicating that D2 DA receptors are functional in this preparation. The ability of pituitary but not striatal D2 DA receptors to modulate [Ca2+]i may reflect the fact that the two systems differ with respect to pathways for Ca2+ influx.

Synaptosomal dopamine autoreceptors: sensitivity changes after in vitro and in vivo treatments

Dopamine (DA) synthesis in rat striatal synaptosomes was approximately doubled either by treating the animals from which the synaptosomes were obtained with reserpine, or by treating the preparations in vitro with d-amphetamine, ouabain or dibutyryl cyclic AMP. The concentration-response curve of DA synthesis inhibition by apomorphine was shifted to the right after treatment with all these compounds. The inhibitory effect of bromocriptine on DA synthesis was reduced completely after treatment with all the above compounds with the exception of dibutyryl cyclic AMP. When the inhibitory effect of bromocriptine was eliminated by treatment with reserpine or d-amphetamine, bromocriptine antagonized the inhibitory effect of apomorphine. This indicates that bromocriptine could still be bound to the DA autoreceptors and that the reduced sensitivity was due to a reduced functioning of the DA autoreceptors. The reduced sensitivity to apomorphine observed after all the above treatments was possibly due both to a reduced function of and/or to a reduced binding to the DA autoreceptors. The increase in DA synthesis produced by treatment with reserpine in vivo or with d-amphetamine or ouabain in vitro was additive to that produced by a maximally effective concentration of dibutyryl cyclic AMP in vitro, and thus mediated by a presumably non-cyclic AMP-dependent mechanism. Our results obtained with bromocriptine suggest that stimulation of the DA autoreceptors may inhibit DA synthesis by diminishing Ca2+-dependent and not cyclic AMP-dependent phosphorylation of tyrosine hydroxylase.

Inhibition by cyclic AMP of phorbol ester-potentiated norepinephrine release from guinea pig brain cortical synaptosomes

The involvement of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) and cyclic AMP-dependent protein kinase in the K+-evoked release of norepinephrine (NE) was studied using guinea pig brain cortical synaptosomes preloaded with [3H]NE. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, enhanced the K+-evoked release of [3H]NE, in a concentration-dependent manner, but with no effect on the spontaneous outflow and uptake of [3H]NE in the synaptosomes. The apparent affinity of the evoked release for added calcium but not the maximally evoked release was increased by TPA (10(-7) M). Inhibitors of PKC, polymyxin B, and a more potent inhibitor, staurosporine, counteracted the TPA-induced potentiation of the evoked release. Both forskolin and dibutyryl cyclic AMP (DBcAMP) enhanced the evoked release, but reduced the TPA-potentiated NE release. A novel inhibitor of cyclic AMP-dependent protein kinase, KT5720, blocked both the forskolin-induced increase in the evoked release and its inhibition of TPA-induced potentiation in the evoked release, thereby suggesting that forskolin or DBcAMP counteracts the Ca2+-dependent release of NE by activating cyclic AMP-dependent protein kinase. These results suggest that the activation of PKC potentiates the evoked release of NE and that the activation of cyclic AMP-dependent protein kinase acts negatively on the PKC-activated exocytotic neurotransmitter release process in brain synaptosomes of the guinea pig.

Dopamine receptor stimulation does not affect phosphoinositide hydrolysis in slices of rat striatum

The effect of dopamine receptor stimulation on the accumulation of labelled inositol phosphates in rat striatal slices under basal and stimulated conditions was examined following preincubation with [3H]inositol. Incubation of striatal slices with the selective D-1 agonist SKF 38393 or the selective D-2 agonist LY 171555 for 5 or 30 min did not affect the basal accumulation of labelled inositol mono-, bis-, tris-, and tetrakisphosphate. Resolution by HPLC of inositol trisphosphate into inositol-1,3,4-tris-phosphate and inositol-1,4,5-trisphosphate isomers revealed that under basal conditions dopamine did not influence the accumulation of inositol-1,4,5-trisphosphate. Depolarisation evoked by KCl, or addition of the muscarinic receptor agonist carbachol, produced a marked increase in the accumulation of labelled inositol phosphates in both the presence and absence of lithium. Addition of dopamine did not reduce the ability of KCl or carbachol to increase inositol phospholipid hydrolysis. In the presence of lithium, dopamine (100 microM) enhanced KCl-stimulated inositol phospholipid hydrolysis, but this effect appears to be mediated by alpha 1 adrenoceptors because it was blocked by prazosin. SKF 38393 (10 microM) or LY 171555 (10 microM) also did not affect carbachol-stimulated inositol phospholipid hydrolysis. These data, in contrast to recent reports, suggest that striatal dopamine receptors do not appear to be linked to inositol phospholipid hydrolysis.

Stimulation of D2-receptors in rat nucleus accumbens slices inhibits dopamine and acetylcholine release but not cyclic AMP formation

We compared some functional responses of D1- and D2-receptor stimulation in tissue slices of rat neostriatum with those in slices of the nucleus accumbens. In both brain regions D2-receptor stimulation inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine. In both brain regions D1-receptor stimulation and forskolin increased the cyclic AMP formation. Only in the neostriatum, stimulation of D2-receptors inhibited the formation of cyclic AMP, brought about by forskolin or by D1-receptor stimulation. It is concluded from these experiments that, although functional responses of D2-receptor stimulation can be demonstrated in the nucleus accumbens, D2-receptors in this brain region are apparently uncoupled to adenylate cyclase.