D2 dopaminergic receptor activation enhances the spontaneous release of 3H-GABA in the prefrontal cortex of rats, in vitro. The facilitating role of D1 dopaminergic receptors

The effects of three D2 dopaminergic agonists on the spontaneous release of 3H-GABA have been studied on rat prefrontal cortical slices. LY171555 (10(-9) M), RU24926 (3 x 10(-8) M) and lisuride (10(-7) M) respectively enhanced the spontaneous release by 25, 20.5 and 23%. These effects were totally reversed by the D2 antagonist sulpiride (10(-5) M). Furthermore, subliminar concentration of RU24926 (10(-9) M) and of the D1 agonist SKF38393 (10(-6) M) induced a clear enhancement of the spontaneous release of 3H-GABA when they were superfused simultaneously. Our results suggest that in the prefrontal cortex, the spontaneous release of 3H-GABA is under an activatory D2 dopaminergic control. The activation of D1 receptors seems to have an enabling effect on this regulation.     

Histamine H3 receptors regulate acetylcholine release from the guinea pig ileum myenteric plexus.

The effect of selective histamine H3-receptor agonists and antagonists on the acetylcholine release from peripheral nerves was evaluated in the guinea pig longitudinal muscle-myenteric plexus preparations, preloaded with (3H)choline. In the presence of H1 and H2 blockade, histamine (10(-7)-10(-4) M) and (R)-alpha-methylhistamine (10(-8)-10(-6) M) inhibited the electrically-evoked acetylcholine release, being (R)-alpha-methylhistamine more active than histamine, but behaving as a partial agonist. The effect of histamine was completely reversed by selective H3-blocking drugs, thioperamide and impromidine, while only submaximal doses of (R)-alpha-methylhistamine were antagonized. Furthermore, thioperamide and impromidine enhanced the electrically-evoked acetylcholine release. On the contrary, the new H3-blocker, HST-7, was found substantially ineffective, both as histamine antagonist and as acetylcholine overflow enhancer. These data suggest that histamine exerts an inhibitory control on the acetylcholine release from intestinal cholinergic nerves through the activation of H3 receptors.     

An antagonistic interaction between A2B adenosine and D2 dopamine receptors modulates the function of rat carotid body chemoreceptor cells

We have previously demonstrated that adenosine controls the release of catecholamines (CA) from carotid body (CB) acting on A2B receptors. Here, we have tested the hypothesis that the control is exerted via an interaction between adenosine A2B and dopamine D2 receptors present in chemoreceptor cells. Experiments were performed in vitro in CB from 3 months rats. The effect of A2B adenosine and D2 dopamine agonists and antagonists applied alone or in combination were studied on basal (20%O2) and hypoxia (10%O2)-evoked release of CA and cAMP content of CB. We have found that adenosine A2 agonists and D2 antagonists dose-dependently increased basal and evoked release CA from the CB while A2 antagonists and D2 agonists had an inhibitory action. The existence of A2B-D2 receptor interaction was established because the inhibitory action of A2 antagonists was abolished by D2 antagonists, and the stimulatory action of A2 agonists was abolished by D2 agonists. Further, A2 agonists increased and D2 agonist decreased cAMP content in the CB; their co-application eliminated the response. The present results provide direct pharmacological evidence that an antagonistic interaction between A2B adenosine and D2 dopamine receptors exist in rat CB and would explain the dopamine-adenosine interactions on ventilation previously observed.     

Excitatory Transmitter Amino Acid Release from the Ischemic Rat Cerebral Cortex: Effects of Adenosine Receptor Agonists and Antagonists

The effects of selective adenosine receptor agonists [N6-cyclopentyladenosine (CPA) and N-ethylcarboxamidoadenosine (NECA)] and antagonists [8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazoline-5-im ine (CGS-15943A)] on aspartate and glutamate release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (for 20 min) was elicited by four-vessel occlusion. Excitatory amino acid releases were compared from control ischemic rats and drug-treated rats. Basal levels of aspartate and glutamate release were not greatly affected by pretreatment with the adenosine receptor agonists or antagonists. However, CPA (10(-10) M) and NECA (10(-9) M) significantly inhibited the ischemia-evoked release of aspartate and glutamate into cortical superfusates. The ability to block ischemia-evoked release of excitatory amino acids was not evident at higher concentrations of CPA (10(-6) M) or NECA (10(-5) M). The selective A1 receptor antagonist DPCPX also had no effect on release when administered at a low dosage (0.01 mg/kg, i.p.) but blocked the ischemia-evoked release of aspartate and glutamate at a higher dosage (0.1 mg/kg). Evoked release was inhibited by the selective A2 receptor antagonist CGS-15943A (0.1 mg/kg, i.p.). Thus, adenosine and its analogs may suppress ischemia-evoked release of excitatory neurotransmitter amino acids via high-affinity A1 receptors, whereas coactivation of lower-affinity A2 receptors may block (or reverse) the A1-mediated response.     

Opposing Roles of Dopamine D1 and D2 Receptors in Nigral γ-[3H]Aminobutyric Acid Release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.     

Role of Dendritic Dopamine of the Substantia Nigra in the Modulation of Nigrocollicular γ-Aminobutyric Acid Release: In Vivo Studies in the Rat

The release of gamma-[3H]aminobutyric acid ([3H]GABA) newly synthesized from [3H]glutamine was estimated in the superior colliculus of ketamine-anesthetized rats superfused via a push-pull cannula. A significant amount of [3H]GABA was spontaneously released in the superior colliculus (582 +/- 49 pCi/10 min). A major part of the large K(+)-evoked increase of the [3H]GABA release was Ca2+ dependent. When neuronal activity of the substantia nigra was enhanced by nigral application of K+ (30 mM) or bicuculline (10(-4) M), a persistent increase of the collicular [3H]GABA release was observed (60 and 80%, respectively). Conversely, when nigral activity was reduced by nigral application of GABA (10(-4) M) or superfusion with a Ca(2+)-free medium, a sustained decrease of the collicular [3H]GABA release was observed (-30 and -40%, respectively). Following the nigral application of a selective D2-receptor agonist. RU 24926 (10(-6) M), for 30 min in 6-hydroxydopamine-lesioned rats, a phasic increase (60%) of the collicular [3H]GABA release was detected. This effect could result from an activation of nigrocollicular GABAergic neurons by D2-receptor stimulation, because nigral activity and collicular release of [3H]GABA changed in a parallel direction.     

Selective D2 dopamine receptor agonists prevent catalepsy induced by SCH 23390, a selective D1 antagonist

SCH 23390, an apparently selective antagonist of central D1 dopamine receptors, produced profound catalepsy at low doses (0.1 mg/kg, s.c.). Pretreatment with the selective D2 receptor agonists LY 141865, RU 24213 or LY 171555, the active (-) enantiomer of LY 141865, elicited a dose-dependent inhibition of the cataleptic response. Pergolide and apomorphine were also effective. This effect was not due to altered disposition or penetration of SCH 23390 into the brain since pretreatment with a dose of LY 171555 which completely blocked catalepsy had no effect on the ID50 of SCH 23390 to inhibit 3H-cis-piflutixol binding to D1 receptors measured ex vivo. Alternative mechanisms are considered to explain the results, which offer new insights into striatal dopaminergic regulation of motor activity.     

GABA-ergic effects of harman independent of its influence on benzodiazepine receptors

It has been shown in experiments on rat cortex slices preincubated with 3H-GABA that chlorodiazepoxide (10(-6), 3.10(-5) M) does not change basal and electric stimulation-induced release of the label. It has been also shown that it does not eliminate the autoinhibitory effect of GABA on electric stimulation-induced release of 3H-GABA. However, harmane and some other (but not all) derivatives given at the same concentrations increase 3H-GABA release induced by electric stimulation and abolish the inhibitory effect of GABA without changing or slightly raising spontaneous release of 3H-GABA. It is concluded that harmane enhances the electrically stimulated release of the transmitter by GABAergic axons whatever the effect on benzodiazepine-binding areas of GABA receptors.     

Dopamine induces hyperpolarization of locust salivary gland acinar cells via D(1)-like receptors

The effect of dopamine on the salivary gland acinar cells of the locust was examined using conventional intracellular recording techniques. Application of dopamine induced a reversible, dose-dependent hyperpolarization of the acinar cells, with an EC(50) of 0.1 &mgr;M dopamine. We investigated the pharmacology of the dopamine receptor mediating hyperpolarization of the acinar cells using a range of dopaminergic agonists and antagonists. The effect of dopamine could be mimicked by the selective D(1) receptor agonist SKF82958, whilst the D(2) receptor agonists PPHT-HCl and TNPA-HBr were far less potent at inducing hyperpolarization. The receptor also showed selectivity to certain synthetic D(1)-like agonists. SKF82958 was much more effective at inducing a hyperpolarization than SKF81297. The dopamine-induced hyperpolarization of locust acinar cells could be blocked using the selective D(1) receptor antagonist SCH23390 whilst the D(2) receptor antagonists sulpiride and spiperone were inactive. The rank order of potency of several dopaminergic agonists and antagonists was obtained and suggests that the dopamine receptor mediating the hyperpolarization in locust salivary gland acinar cells is similar to a mammalian D(1) receptor. Stimulation of the salivary nerve mimicked the effect of dopamine on the acinar cells, inducing a rapid reversible hyperpolarization. This neurally-evoked hyperpolarization of the locust acinar cells was suppressed using 1.0 &mgr;M SCH23390, whilst 10 &mgr;M sulpiride was inactive. This demonstrated that both exogenously applied dopamine and endogenously released dopamine are probably acting on the same receptor.     

Specific Inhibition of Dopamine D-1-Mediated Cyclic AMP Formation by Dopamine D-2, Muscarinic Cholinergic, and Opiate Receptor Stimulation in Rat Striatal Slices

The ability of different receptors to mediate inhibition of cyclic AMP accumulation due to a variety of agonists was examined in rat striatal slices. In the presence of 1 mM 3-isobutyl-1-methylxanthine, dopamine D-2, muscarinic cholinergic, and opiate receptor stimulation by RU 24926, carbachol, and morphine (all at 10(-8)-10(-5) M), respectively, inhibited the increase in cyclic AMP accumulation in slices of rat striatum due to dopamine D-1 receptor stimulation by 1 microM SKF 38393. In contrast, these inhibitory agents were unable to reduce the ability of a number of other agonists, including isoprenaline, prostaglandin E1, 2-chloroadenosine, vasoactive intestinal polypeptide, and cholera toxin, to increase cyclic AMP levels in striatal slices. These results suggest that in rat striatum either dopamine D-2, muscarinic cholinergic, and opiate receptors are only functionally linked to dopamine D-1 receptors or that the D-1 and D-2 receptors linked to adenylate cyclase lie on the cells, distinct from other receptors capable of elevating striatal cyclic AMP levels.     

The Effects of Cholinergic and Dopaminergic Antagonists on Nicotine-Induced Cerebral Neurotransmitter Changes

In a continuing study of nicotine-induced mechanisms in brain areas associated with cognitive processes, the effects of cholinergic and dopaminergic antagonists on nicotine-induced changes in dopamine, norepinephrine, and serotonin were examined. These effects were measured via in vivo microdialysis in the dorsal and ventral hippocampus and in the prefrontal and medial temporal cortex of conscious, freely moving, adult male rats. Nicotine (0.3 mg/kg, free base) was administered subcutaneously and the antagonists were infused locally via the microdialysis probe. Nicotine alone induced an increase of dopamine and its metabolites in all areas, an increase of norepinephrine in the cortex, and an increase of the norepinephrine metabolite 4–hydroxy-3-methoxy-phenylglycol in all areas. Serotonin was decreased in the hippocampus and increased in the cortex. Nicotine-induced dopamine increases were inhibited by nicotinic (mecamylamine 100 μM, methyllycaconitine 500 μM), muscarinic (atropine 100 μM), and dopaminergic D1 (SCH23390 100 μM) and D2 (eticlopride 100 μM) antagonists, in the hippocampal and cortical areas. In the hippocampal areas, these antagonists had less significant effect on norepinephrine and serotonin. However, in the cortical areas, all antagonists inhibited the nicotine-induced increase of serotonin to varying degrees; and some, primarily nicotinic and dopamine D1 antagonists, inhibited the induced increase of norepinephrine. In the hippocampal and cortical areas, the mechanisms of nicotine-induced dopamine increase seem to be similar, but the mechanisms seem to be different for noradrenergic and serotonergic systems, as shown by the fact that nicotine induces no change in norepinephrine and a decrease in serotonin in the hippocampus, while it induces an increase in both in the cortex. Nicotine-induced dopamine release seems to be mediated, in part locally, by nicotinic and muscarinic receptors on dopaminergic cells. In contrast, nicotine’s effect on norepinephrine and serotonin is at least partially mediated by initial changes at other than local sites, and through different receptors. Thus, the effects of nicotine and the mechanisms involved differ for different neurotransmitters and in different brain areas.     

Repeated exposure to methamphetamine, cocaine or morphine induces augmentation of dopamine release in rat mesocorticolimbic slice co-cultures

Repeated intermittent exposure to psychostimulants and morphine leads to progressive augmentation of its locomotor activating effects in rodents. Accumulating evidence suggests the critical involvement of the mesocorticolimbic dopaminergic neurons, which project from the ventral tegmental area to the nucleus accumbens and the medial prefrontal cortex, in the behavioral sensitization. Here, we examined the acute and chronic effects of psychostimulants and morphine on dopamine release in a reconstructed mesocorticolimbic system comprised of a rat triple organotypic slice co-culture of the ventral tegmental area, nucleus accumbens and medial prefrontal cortex regions. Tyrosine hydroxylase-positive cell bodies were localized in the ventral tegmental area, and their neurites projected to the nucleus accumbens and medial prefrontal cortex regions. Acute treatment with methamphetamine (0.1-1000 μM), cocaine (0.1-300 μM) or morphine (0.1-100 μM) for 30 min increased extracellular dopamine levels in a concentration-dependent manner, while 3,4-methylenedioxyamphetamine (0.1-1000 μM) had little effect. Following repeated exposure to methamphetamine (10 μM) for 30 min every day for 6 days, the dopamine release gradually increased during the 30-min treatment. The augmentation of dopamine release was maintained even after the withdrawal of methamphetamine for 7 days. Similar augmentation was observed by repeated exposure to cocaine (1-300 μM) or morphine (10 and 100 μM). Furthermore, methamphetamine-induced augmentation of dopamine release was prevented by an NMDA receptor antagonist, MK-801 (10 μM), and was not observed in double slice co-cultures that excluded the medial prefrontal cortex slice. These results suggest that repeated psychostimulant- or morphine-induced augmentation of dopamine release, i.e. dopaminergic sensitization, was reproduced in a rat triple organotypic slice co-cultures. In addition, the slice co-culture system revealed that the NMDA receptors and the medial prefrontal cortex play an essential role in the dopaminergic sensitization. This in vitro sensitization model provides a unique approach for studying mechanisms underlying behavioral sensitization to drugs of abuse.     

Presynaptic Glutamate Receptors Modulate Dopamine Release from Striatal Synaptosomes

The wide-ranging neuronal actions of glutamate are thought to be mediated by postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. The present report demonstrates the existence of presynaptic glutamate receptors in isolated striatal dopaminergic nerve terminals (synaptosomes). Activation of these receptors, by NMDA in the absence of Mg2+ and presence of glycine and by non-NMDA agonists in the presence of Mg2+, results in Ca(2+)-dependent release of dopamine from striatal synaptosomes. The release stimulated by NMDA is blocked by Mg2+ and by selective NMDA antagonists, whereas the release stimulated by selective non-NMDA agonists is blocked by a non-NMDA antagonist but not by Mg2+ or NMDA antagonists. Thus, these presynaptic glutamate receptors, localized on dopaminergic terminals in the striatum, appear to be pharmacologically similar to both the NMDA and the non-NMDA postsynaptic receptors. By modulating the release of dopamine, these presynaptic receptors may play an important role in transmitter interactions in the striatum.     

Stimulation of 5-hydroxytryptamine (5-HT(2C) ) receptors in the ventrotegmental area inhibits stress-induced but not basal dopamine release in the rat prefrontal cortex

The present study investigated whether 5-HT(2C) receptors in the ventrotegmental area and prefrontal cortex regulate basal and stimulus-evoked dopamine release in the prefrontal cortex. Using the in vivo microdialysis technique in conscious rats, we studied the effect of a selective 5-HT(2C) receptor agonist, Ro60-0175, on basal and immobilization stress-induced dopamine release in the prefrontal cortex. Ro60-0175 intraperitoneally (2.5 mg/kg) and into the ventrotegmental area (10 microg/0.5 microL) completely antagonized the effect of stress on extracellular dopamine without altering basal levels. Infusion of 10 microm Ro60-0175 through the cortical probe had no significant effect on basal and stress-induced dopamine release. SB242084 (10 mg/kg), a selective antagonist of 5-HT(2C) receptors, significantly increased basal extracellular dopamine and completely prevented the effect of intraperitoneal and intraventrotegmental Ro60-0175 on the stress-induced rise of extracellular dopamine, but had no effect itself in stressed rats. The results show that Ro60-0175 suppresses cortical dopamine release induced by immobilization stress through the stimulation of 5-HT(2C) receptors in the ventrotegmental area. While confirming that endogenous 5-HT acting on 5-HT(2C) receptors tonically inhibit basal dopamine release in the prefrontal cortex, the present findings suggest that the stimulation of 5-HT(2C) receptors with an exogenous agonist preferentially inhibit stimulated release.     

Identification of Dopamine "D3'' and "D4'' Binding Sites, Labelled with [3H]2-Amino-6,7-Dihydroxy- 1,2,3,4- Tetrahydronaphthalene, as High Agonist Affinity States of the D1 and D2 Dopamine Receptors, Respectively

On the basis of affinity differences for spiperone, two binding sites for [3H](+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene ([3H]ADTN) in the rat brain could be distinguished: "D3" with a low and "D4" with a high affinity for spiperone. Evidence is provided that D3 and D4 sites are related to high agonist affinity states of the D1 and D2 dopamine receptors, respectively. Various well-known selective D1 and D2 agonists and antagonists showed potencies at these sites in agreement with this hypothesis. A comparison of the Bmax values for [3H]ADTN binding to D3 and D4 sites with the numbers of D1 receptors (labelled by [3H]SCH 23390) and of D2 receptors (labelled by [3H]spiperone), both in the striatum and in the mesolimbic system, indicated that under the conditions used for 3H-agonist binding experiments, both populations of D1 and D2 receptors were converted to their high agonist affinity states to a considerable, although different extent. In fact, when competition experiments with [3H]spiperone were performed under the conditions otherwise used for [3H]ADTN binding experiments (instead of the conditions usually used for antagonist binding), substantial shifts of the displacement curves of 3,4-dihydroxyphenylethylamine (dopamine) and ADTN toward higher affinities were observed. A comparison of the effects of various agonists and antagonists in the [3H]ADTN binding experiments and in functional tests revealed a significant correlation between their potencies at D4 binding sites and at D2 receptors modulating the release of [3H]acetylcholine from striatal slices. However, in the situation of the D1/D3 pair, when the measurement of adenylate cyclase activity was taken as a functional test for D1 receptors, agonists were more active in the binding than in the functional test, whereas for many antagonists the opposite was found. The results are discussed with regard to the classification and functional aspects of brain dopamine receptors.     

Expression of muscarinic acetylcholine receptors (M1-, M2-, M3- and M4-type) in the neuromuscular junction of the newborn and adult rat

Using intracellular recording and immunohistochemistry, we studied the presynaptic muscarinic autoreceptor subtypes controlling ACh release in the neuromuscular junctions of the newborn (3-6 days postnatal) and adult (30-40 days) rat. In the Levator auris longus muscles of both newborn and adult rats, acetylcholine release was modified by the M1-receptor selective antagonists pirenzepine (10 microM) and MT-7 (100 nM) and by the M2-receptor selective antagonists methoctramine (1 microM) and AF-DX 116 (10 microM). The M4-receptor selective antagonists tropicamide (1 microM) and MT-3 (100 nM) can also modify the neurotransmitter release in certain synapses of the newborn muscles. The neurotransmitter release was not altered by the M3-receptor selective antagonist 4-DAMP (1 microM) in the adult or newborn rats. However, we directly demonstrate by immunocytochemistry the presence of these receptors in the motor endplates and conclude that M1-, M2-, M3- and M4-type muscarinic receptors are present in all the neuromuscular junctions of the rat muscle both in newborn and adult animals. These receptors may be located in the perisynaptic glial cell as well as at the nerve terminals.     

Striatal cholinergic function reflects differences in D-2 dopaminergic receptor activation

The ergot derivatives, bromocriptine, lisuride and quinpirole (Ly-171555), activators of D-2 receptors, increased striatal acetylcholine (ACh) content by about 40% and induced a 30% inhibition of ACh evoked release from striatal slices, similar to the effects of the dopaminergic agonist apomorphine. These actions were a consequence of dopaminergic activation since they were antagonized by pretreatment with the neuroleptic agent, pimozide. In contrast, pretreatment with L-sulpiride (100 mg/kg), a specific antagonist for the D-2 dopaminergic receptor only, prevented the rise of ACh levels induced by apomorphine or quinpirole but did not interfere with the lisuride- or bromocriptine- induced ACh increases. Similarly, inhibition of the ACh evoked release produced by lisuride (3ωM) was prevented by pimozide (1mg/kg) but not by pretreatment with L-sulpiride. Addition of L-sulpiride (5ωM) to the Krebs solution had no effect on the inhibition of ACh-evoked release induced by lisuride, but a lower concentration (1ωM) antagonized the inhibition induced by quinpirole. Lisuride and bromocriptine responses were both insensitive to sulpiride. These results are discussed in terms of different interaction with the dopaminergic D-2 receptors by the drugs studied.     

Functional antagonism of D-1 and D-2 dopaminergic mechanisms affecting striatal acetylcholine release

Rat striatal slices prelabelled with [3H]choline were superfused with dopamine D-1 and D-2 agonists and antagonists, separately and in combination, during measurement of [3H]acetylcholine (ACh) release. SKF38393 (D-1 agonist), 10(-7)-10(-4) M, and SCH23390 (D-1 antagonist), 10(-7)-10(-5) M, produced a dose-dependent increase in [3H]ACh release when given separately. The increased [3H]ACh release induced by either drug could not be attenuated by sufficient L-sulpiride to block D-2 receptors. Yet both SKF38393, 10(-6)-10(-5) M, and SCH23390, 10(-6)-10(-5) M, were able to partially or fully overcome the [3H]ACh release-depressant effect of cosuperfused LY171555 (D-2 agonist), 10(-6) M. This suggests that a functional antagonism regarding striatal ACh release exists between D-1 and D-2 dopaminergic receptor-mediated mechanisms, but that D-1 modulation of local ACh release does not occur at the level of the recognition site of the striatal D-2 receptor. Finally, although attenuation of the increased release of striatal [3H]ACh induced by 10(-5) M SCH23390 by SKF38393 was seen, it is possible that such functional antagonism is not mediated by exclusively D-1 dopaminergic means.     

Presence of dopamine D-2 receptors in human tumoral cell lines

[125I] Iodosulpride binding was examined on eight human cell lines derived from lung, breast and digestive tract carcinomas, neuroblastomas and leukemia. Specific binding was detected in five of these cell lines. In the richest cell line N417, derived from small cell lung carcinoma, [125I] iodosulpride bound with a high affinity (Kd = 1.3 nM) to an apparently homogeneous population of binding site (Bmax = 1,606 sites per cell). These sites displayed a typical D-2 specificity, established with several dopaminergic agonists and antagonists selective of either D-1 or D-2 receptor subtypes. In addition, dopamine, apomorphine and RU 24926 distinguished high- and low-affinity sites, suggesting that the binding sites are associated with a G-protein. The biological significance and the possible diagnostic implication of the presence of D-2 receptors on these cell lines are discussed.     

5-HT(2A) and D(2) receptor blockade increases cortical DA release via 5-HT(1A) receptor activation: a possible mechanism of atypical antipsychotic-induced cortical dopamine release

Atypical antipsychotic drugs (APDs), all of which are relatively more potent as serotonin (5-HT)(2A) than dopamine D(2) antagonists, may improve negative symptoms and cognitive dysfunction in schizophrenia, in part, via increasing cortical dopamine release. 5-HT(1A) agonism has been also suggested to contribute to the ability to increase cortical dopamine release. The present study tested the hypothesis that clozapine, olanzapine, risperidone, and perhaps other atypical APDs, increase dopamine release in rat medial prefrontal cortex (mPFC) via 5-HT(1A) receptor activation, as a result of the blockade of 5-HT(2A) and D(2) receptors. M100907 (0.1 mg/kg), a 5-HT(2A) antagonist, significantly increased the ability of both S:(-)-sulpiride (10 mg/kg), a D(2) antagonist devoid of 5-HT(1A) affinity, and R:(+)-8-OH-DPAT (0.05 mg/kg), a 5-HT(1A) agonist, to increase mPFC dopamine release. These effects of M100907 were abolished by WAY100635 (0.05 mg/kg), a 5-HT(1A) antagonist, which by itself has no effect on mPFC dopamine release. WAY100635 (0.2 mg/kg) also reversed the ability of clozapine (20 mg/kg), olanzapine (1 mg/kg), risperidone (1 mg/kg), and the R:(+)-8-OH-DPAT (0.2 mg/kg) to increase mPFC dopamine release. Clozapine is a direct acting 5-HT(1A) partial agonist, whereas olanzapine and risperidone are not. These results suggest that the atypical APDs via 5-HT(2A) and D(2) receptor blockade, regardless of intrinsic 5-HT(1A) affinity, may promote the ability of 5-HT(1A) receptor stimulation to increase mPFC DA release, and provide additional evidence that coadministration of 5-HT(2A) antagonists and typical APDs, which are D(2) antagonists, may facilitate 5-HT(1A) agonist activity.