D-1 and D-2 dopamine receptor blockade: interactive effects in vitro and in vivo

Haloperidol, at low concentrations that block D-2 dopamine (DA) receptors but not D-1 DA receptors (less than 10 microM), potentiated the enhancement of adenylate cyclase activity produced by the D-1 agonist SKF 38393. Low concentrations of haloperidol (less than or equal to 5 microM) also potentiated the K+-evoked release of [3H]acetylcholine from superfused striatal tissue slices. Both of these effects of haloperidol were blocked by nanomolar concentrations of SCH 23390, a D-1 receptor antagonist. In addition, SCH 23390 reduced the ability of haloperidol to antagonize the inhibition of [3H]acetylcholine release produced by the DA agonist apomorphine. By itself, SCH 23390 did not alter either basal adenylate cyclase activity or the K+-evoked release of [3H]acetylcholine. These findings suggest that SCH 23390 can attenuate in vitro responses to D-2 receptor blockade. Likewise, in vivo, very low doses (less than 1 microgram/kg) of SCH 23390 reduced the ability of haloperidol to elevate striatal DA metabolite concentrations and plasma prolactin concentrations. Thus, D-1 receptor blockade may attenuate the effects of D-2 DA receptor blockade both in vitro and in vivo.     

The discriminative stimulus effects of clozapine in pigeons: involvement of 5-hydroxytryptamine1C and 5-hydroxytryptamine2 receptors.

Pigeons were trained to discriminate i.m. injections of the atypical antipsychotic clozapine (1.0 mg/kg) from saline in a two-key operant procedure. In substitution tests, compounds that shared antagonistic action at 5-hydroxytryptamine (5-HT)1C and 5-HT2 receptors produced discriminative stimulus effects similar to clozapine: cyproheptadine, metergoline, mianserin, pizotifen and fluperlapine. 5-HT antagonists selective for 5-HT2 vs. 5-HT1C receptors (e.g., ketanserin, pirenperone, risperidone and methiothepin) failed to produce substantial clozapine-appropriate responding. Other serotonergic compounds failed to produce substantial clozapine-appropriate responding: the 5-HT3 antagonist, ondansetron; the 5-HT1A agonists, (+-)-8-hydroxy-2-(di-n-propylamino)tetralin and BMY 14802; the 5-HT1A/1B agonist, RU24969; the 5-HT1A partial agonist, NAN190; the 5-HT1C/2 antagonist, mesulergine; the 5-HT1 agonist, I-5-hydroxytryptophane; and the 5-HT1C/2 agonist, quipazine. Other reference compounds such as the typical antipsychotics, chlorpromazine and thioridazine; the selective dopamine D-2 antagonists, droperidol and sulpiride; the dopamine D-1 antagonist, SCH 23390; the antimuscarinics, atropine and scopolamine; the antihistamines, pyrilamine and diphenhydramine; the alpha-1 antagonist, prazosin; and the antidepressants, imipramine and chloromipramine also failed to produce clozapine-appropriate responding. Promethazine, cinanserin and amitriptyline produced only partial generalization to the clozapine cue. The results suggest that blockade of both 5-HT2 and/or 5-HT1C receptors is important in the pharmacological mediation of the discriminative stimulus effects of clozapine. Blockade of 5-HT2 receptors appears not to be sufficient to produce clozapine-like discriminative stimulus effects. The precise role of 5-HT1C receptors in the clozapine discriminative stimulus is unclear due to the lack of compounds selective for this receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ICI 169,369, a selective serotonin2 antagonist, in electrophysiological tests predictive of antipsychotic activity

Extracellular single unit recording techniques were used to compare the effects of ICI 169,369, a selective serotonin2 receptor antagonist, with the reference antipsychotic (AP) agents clozapine and haloperidol, in electrophysiological tests that may predict AP activity. ICI 169,369 was found to reverse the inhibitory actions of amphetamine on A9 and A10 dopamine (DA) neurons, a common property shared by other AP drugs, and was comparable in potency to clozapine. In cell population studies, acute treatment with ICI 169,369 (at a low dose only) and clozapine selectively increased the number of spontaneously active A10 DA cells, which was found to correlate with the ability of both these drugs to cause depolarization inactivation (DI) of A10 DA cells after chronic administration. Interestingly, chronic treatment with ICI 169,369 also caused a significant increase in the number of actively discharging A9 DA cells, an effect not predicted on the basis of the acute data. A similar effect was noted for clozapine, although the magnitude did not reach statistical significance. This profile of activity was unlike that of haloperidol, which acutely caused a nonselective increase in the number of active A9 and A10 DA cells, associated with the ability of this agent to cause DI of both A9 and A10 DA cells after chronic treatment. Inasmuch as DI of A10 DA cells may be correlated with AP efficacy whereas DI of A9 DA cells may predict the ability of an AP to cause extrapyramidal side effects, ICI 169,369, like clozapine, may be a potential AP with a reduced likelihood for producing extrapyramidal side effects.     

ICI 169,369 is both a competitive antagonist and an allosteric activator of the arterial 5-hydroxytryptamine2 receptor system

The mode of action of ICI 169,369, a novel 5-hydroxytryptamine2 (5-HT2) receptor antagonist, was investigated in arterial muscle. Isolated preparations from calf coronary artery and from rat tail artery with the endothelium rubbed off were set up to contract isometrically with 5-HT. ICI 169,369 (1-3000 nM) antagonized surmountably and competitively the contractile effects of 5-HT in coronary artery (pKB, 9.1) and tail artery (pKB, 8.8). Methysergide antagonized unsurmountably 5-HT-induced contractions by reducing maximum effects to 25% (coronary artery: pIC50, 9.8) and 60% (tail artery: pIC80, 9.0). ICI 169,369 (100-300 nM) restored the maximum effects of 5-HT that had been depressed by methysergide (20 nM coronary artery, 100 nM tail artery). Preincubation with ICI 169,369 also prevented the methysergide-induced depression of the maximum effects of 5-HT. The protective effect of ICI 169,369 was overcome by high methysergide concentrations (up to 3 microM), suggesting competition between the two drugs for a common site. The data are consistent with an allosterically modulated interconversion of the 5-HT2 receptor between two states (R in equilibrium R'). ICI 169,369 competes with 5-HT for the 5-HT2 receptor. ICI 169,369 and methysergide also compete for an allosteric site of the 5-HT2 receptor system, thereby facilitating the highly active R-state and low active R'-state, respectively.     

Dimethylation of the activator ICI 169,369 results in a high-affinity partial deactivator, ICI 170,809, of the arterial 5-hydroxytryptamine2 receptor system

We investigated the mode of action of the potent antagonist ICI 170,809 in the 5-hydroxytryptamine (5-HT)2 receptor system of arterial smooth muscle. We used isolated preparations from rat tail artery and calf coronary artery with the endothelium rubbed off. In tail artery ICI 170,809 (0.3-30 nM) antagonized surmountably and nearly competitively the contractile effects of 5-HT (pKB = 10.0) and partially prevented the depression of 5-HT-induced contractions caused by methysergide. Increasing methysergide concentrations gradually prevented the protective effect of ICI 170,809. The combination of 30 nM ICI 170,809 with 300 nM of its demethylated analog ICI 169,369 (pKB = 8.8) caused surmountable blockade of the effects of 5-HT as expected from competition of the three drugs for the same receptor. In calf coronary artery ICI 170,809 (1-100 nM) reduced the maximum contractile response to 5-HT by 35% and caused competitive antagonism (pKB = 10.4) of the remaining 65% of the responses to 5-HT. ICI 169,369 (100 nM) completely prevented the depression of the maximum response to 5-HT caused by ICI 170,809. Methysergide (3 nM) depressed the maximum response to 5-HT by 65 and 30% in the absence and presence of ICI 170,809. The results are consistent with the existence of two interconvertible states R in equilibrium R' of the 5-HT2 receptor. The equilibrium of R in equilibrium R' is shifted toward R' by methysergide greater than ICI 170,809 much greater than ICI 169,369.     

Antagonist functional selectivity: 5-HT2A serotonin receptor antagonists differentially regulate 5-HT2A receptor protein level in vivo

Dysregulation of the 5-HT(2A) receptor is implicated in both the etiology and treatment of schizophrenia. Although the essential role of 5-HT(2A) receptors in atypical antipsychotic drug actions is widely accepted, the contribution of 5-HT(2A) down-regulation to their efficacy is not known. We hypothesized that down-regulation of cortical 5-HT(2A) receptors contributes to the therapeutic action of atypical antipsychotic drugs. To test this hypothesis, we assessed the effect of chronically administered antipsychotics (clozapine, olanzapine, and haloperidol) and several 5-HT(2A) antagonists [ketanserin, altanserin, α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol (M100907), α-phenyl-1-(2-phenylethyl)-4-piperidinemethano (M11939), 4-[(2Z)-3-{[2-(dimethylamino)ethoxy]amino}-3-(2-fluorophenyl)prop-2-en-1-ylidene]cyclohexa-2,5-dien-1-one (SR46349B), and pimavanserin], on the phencyclidine (PCP)-induced hyperlocomotor response and cortical 5-HT(2A) receptor levels in C57BL/6J mice. Clozapine and olanzapine, but not haloperidol, induced receptor down-regulation and attenuated PCP-induced locomotor responses. Of the selective 5-HT(2A) antagonists tested, only ketanserin caused significant receptor protein down-regulation, whereas SR46349B up-regulated 5-HT(2A) receptors and potentiated PCP-hyperlocomotion; the other 5-HT(2A) receptor antagonists were without effect. The significance of these findings with respect to atypical antipsychotic drug action is discussed.     

Improving the treatment of schizophrenia: focus on serotonin (5-HT)(1A) receptors

Although antagonism of mesolimbic dopamine D(2) receptors by neuroleptics such as haloperidol attenuates positive symptoms of schizophrenia, a significant population of "resistant" patients fails to respond while negative and cognitive symptoms are little modified. Furthermore, concomitant blockade of striatal D(2) receptors is associated with extrapyramidal motor side effects. The superior "atypical" antipsychotic profile of clozapine appears to reside in its broad pattern of interaction with D(2) receptors and a diversity of other monoaminergic sites. In this regard, serotonergic mechanisms are of particular relevance both in view of their modulation of dopaminergic transmission and their key role in the control of mood, cognition, and motor behavior. While most attention has focused on potential advantages of preferential 5-HT(2A) versus D(2) receptor blockade, 5-HT(1A) receptors likewise represent a valid target for improved antipsychotic agents. In this regard, rather than selective agents, ligands interacting with both 5-HT(1A) and D(2) receptors appear of interest. A modest level of efficacy appears optimal, that is, sufficient to engage highly sensitive 5-HT(1A) autoreceptors while blocking their low-sensitivity postsynaptic counterparts. Such a profile may counter negative and cognitive symptoms, improve mood, diminish extrapyramidal 5-HT(1A) motor side effects, and, perhaps, enhance efficacy in refractory patients. Notably, "partial agonist" properties of clozapine at 5-HT(1A) receptors may contribute to its distinctive functional profile. However, notwithstanding this compelling body of experimental data, clinical studies of antipsychotics interacting with 5-HT(1A) receptors are required to establish their genuine pertinence to the-hopefully improved-treatment of schizophrenia.     

Pharmacological characterization of AC-90179 [2-(4-methoxyphenyl)-N-(4-methyl-benzyl)-N-(1-methyl-piperidin-4-yl)-acetamide hydrochloride]: a selective serotonin 2A receptor inverse agonist

The primary purpose of the present series of experiments was to characterize the in vitro and in vivo pharmacology profile of 2-(4-methoxy-phenyl)-N-(4-methyl-benzyl)-N-(1-methyl-piperidin-4-yl)-acetamide hydrochloride (AC-90179), a selective serotonin (5-HT2A) receptor inverse agonist, in comparison with the antipsychotics haloperidol and clozapine. The secondary purpose was to characterize the pharmacokinetic profile of AC-90179. Like all atypical antipsychotics, AC-90179 shows high potency as an inverse agonist and competitive antagonist at 5HT2A receptors. In addition, AC-90179 exhibits antagonism at 5HT2C receptors. In contrast, AC-90179 does not have significant potency for D2 and H1 receptors that have been implicated in the dose-limiting side effects of other antipsychotic drugs. The ability of AC-90179 to block 5-HT2A receptor signaling in vivo was demonstrated by its blockade of the rate-decreasing effects of the 5-HT2A agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride, under a fixed ratio schedule of reinforcement. Similar to clozapine and haloperidol, AC-90179 attenuated phencyclidine-induced hyperactivity. Although haloperidol impaired acquisition of a simple autoshaped response and induced cataleptic-like effects at behaviorally efficacious doses, AC-90179 and clozapine did not. Furthermore, unlike haloperidol and clozapine, AC-90179 did not decrease spontaneous locomotor behavior at efficacious doses. Limited oral bioavailability of AC-90179 likely reflects rapid metabolism rather than poor absorption. Taken together, a compound with a similar pharmacological profile as AC-90179 and with increased oral bioavailability may have potential for the treatment of psychosis.     

CI-943, a potential antipsychotic agent. II. Neurochemical effects

The effects of CI-943 (a novel 8-ethyl-7,8-dihydro-1,3,5-trimethyl-1H-imidazo[1,2-c]pyrazolo[3,4- e]pyrimidine compound exhibiting a favorable antipsychotic profile in animal tests) on neurochemical parameters related to biogenic amine neurons have been studied in rat brain. CI-943 (1-40 mg/kg p.o. and 20 mg/kg i.p.) accelerated the turnover of dopamine (DA) in rat brain as demonstrated by the enhancement of levels of the DA metabolites homovanillic acid, 3,4-dihydroxyphenylacetic acid or 3-methoxytyramine and by the enhancement rate of DA synthesis in either striatum or mesolimbic regions. These increases in DA turnover induced by CI-943 are not due to DA receptor blockade as CI-943, unlike known antipsychotics, did not exhibit affinity for DA receptors either in vitro or in vivo and did not affect rat serum basal prolactin levels. Amfonelic acid enhanced the action of haloperidol in increasing striatal homovanillic acid with no effect on that of CI-943 and clozapine, suggesting that CI-943, like clozapine, would be predicted to have a low risk of extrapyramidal side effects as compared to haloperidol. Chronic administration of CI-943 (40 mg/kg i.p.) to rats for 28 days did not affect the affinity or number of striatal DA receptors; in comparison haloperidol (0.5 mg/kg i.p.) caused an increase in number of DA receptors with no change in affinity. Measures of serotonergic function were increased; noradrenergic function was not affected by CI-943, nor did it exhibit affinity for a number of central nervous system receptors in vitro. The molecular mechanism by which CI-943 increases brain DA turnover is not known at this time but appears to be unique in comparison to known antipsychotic agents.     

LY215840, a potent 5-hydroxytryptamine (5-HT)2 receptor antagonist, blocks vascular and platelet 5-HT2 receptors and delays occlusion in a rabbit model of thrombosis.

Certain ergolines are potent and selective 5-hydroxytryptamine (5-HT)2 receptor antagonists. Previous studies with two ergoline esters, LY53857 and sergolexole, documented their potency as 5-HT2 receptor antagonists and their metabolism in rats to a less active metabolite, 1-isopropyl dihydrolysergic acid. LY215840, an ergoline amide, has been identified as a potent 5-HT2 receptor antagonist that is not hydrolyzed to 1-isopropyl dihydrolysergic acid. In the rat jugular vein, LY215840 (3 x 109-10) to 10(-8) M) blocked 5-HT2 receptors mediating contraction to 5-HT in vitro. After i.v. and p.o. administration to rats, LY215840 was a potent 5-HT2 receptor antagonist, documented by its ability to block the pressor response to 5-HT administered i.v. Furthermore, after i.v. and p.o. administration of LY215840, blockade of vascular 5-HT2 receptors persisted in excess of 2 and 6 hr, respectively. LY215840 also blocked vascular 5-HT2 receptors in doses that did not affect alpha-1, beta-1 receptors or angiotensin II pressor responses, documenting the selectivity of LY215840 as an inhibitor of 5-HT2 and not other vascular receptors that modulate vasoconstriction. In addition to inhibiting vascular 5-HT2 receptors, LY215840 also inhibited 5-HT-amplified, ADP-induced aggregation (another 5-HT2 receptor-mediated response) in both rabbit and human platelets. Because of its ability to block both platelet and vascular 5-HT2 receptors, we studied the effectiveness of LY215840 in the rabbit carotid artery model of vascular occlusion. Low i.v. doses of LY215840 markedly prolonged time to vascular occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin 5-HT2A receptors underlie increased motor behaviors induced in dopamine-depleted rats by intrastriatal 5-HT2A/2C agonism

Gene expression studies have suggested that dopamine (DA) depletion increases the sensitivity of striatal direct pathway neurons to the effects of serotonin (5-HT) via the 5-HT(2) receptor. The present study examined the possible influence(s) of 5-HT(2A) or 5-HT(2C) receptor-mediated signaling locally within the striatum on motor behavior triggered by 5-HT(2) receptor agonism in the neonatal DA-depleted rat. Male Sprague-Dawley rats were treated with 6-hydroxydopamine (6-OHDA; 60 microg in 5 microl per lateral ventricle) on postnatal day 3 to achieve near-total DA depletion bilaterally. Sixty days later, sham-operated (saline-injected) or 6-OHDA-treated rats were challenged with the 5-HT(2A/2C) agonist DOI [(+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane] or saline either by systemic treatment or bilateral intrastriatal infusion. Motor behavior was quantified for 60 min after agonist injection using computerized activity monitors. Systemic DOI treatment (0.2 or 2.0 mg/kg i.p.) was more effective in inducing motor activity in the DA-depleted group compared with intact controls. Intrastriatal DOI infusion (1.0 or 10.0 microg/side) also produced a significant rise in motor activity in the DA-depleted group during the 30- to 60-min period of behavioral analysis but did not influence behavior in intact animals. The effects of intrastriatal DOI infusion were blocked by intrastriatal coinfusion of the 5-HT(2) antagonist ketanserin (1.0 microg) and the 5-HT(2A)-preferring antagonist M100907 [(R)(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol; 1.0 microg] but not the 5-HT(2C)-preferring antagonist RS102221 [8-[5-(2,4-dimethoxy-5-(4-trifluoromethylsulfo-amido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione; 1.0 microg]. Such results support the hypothesis that 5-HT(2A) receptor-mediated signaling events are strengthened within the striatum under conditions of DA depletion to provide a more potent regulation of motor activity.     

Contrasting contribution of 5-hydroxytryptamine 1A receptor activation to neurochemical profile of novel antipsychotics: frontocortical dopamine and hippocampal serotonin release in rat brain

Several novel antipsychotics, such as aripiprazole, bifeprunox, SSR181507 [(3-exo)-8-benzoyl-N-(((2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl)methyl)-8-azabicyclo(3.2.1)octane-3-methanamine], and SLV313 [1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine], activate serotonin 5-hydroxytryptamine (5-HT)1A receptors. Such activity is associated with enhanced treatment of negative symptoms and cognitive deficits, which may be mediated by modulation of cerebral dopamine and serotonin levels. We employed microdialysis coupled to high pressure liquid chromatography with electrochemical detection to examine 5-HT1A receptor activation in the modulation of extracellular dopamine in medial prefrontal cortex and serotonin in hippocampus of freely moving rats. The above compounds were compared with drugs that have less interaction with 5-HT1A receptors (clozapine, nemonapride, ziprasidone, olanzapine, risperidone, and haloperidol). Hippocampal 5-HT was decreased by bifeprunox, SSR181507, SLV313, sarizotan, and nemonapride, effects similar to those seen with the 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)8-OH-DPAT], consistent with activation of 5-HT1A autoreceptors. These decreases were reversed by the selective 5-HT1A antagonist, WAY100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide]. In contrast, haloperidol, risperidone, clozapine, olanzapine, ziprasidone, and aripiprazole did not significantly modify hippocampal serotonin levels. In medial prefrontal cortex, dopamine levels were increased by SSR181507, SLV313, sarizotan, and (+)8-OH-DPAT. These effects were reversed by WAY100635, indicating mediation by 5-HT1A receptors. In contrast, the increases in dopamine levels induced by clozapine, risperidone, olanzapine, and ziprasidone were not blocked by WAY100635, consistent with predominant influence of other mechanisms in the actions of these drugs. Haloperidol, nemonapride, and the D2 partial agonists, aripiprazole and bifeprunox, did not significantly alter dopamine release. Taken together, these data demonstrate the diverse contribution of 5-HT1A receptor activation to the profile of antipsychotics and suggest that novel drugs selectively targeting D2 and 5-HT1A receptors may present distinctive therapeutic properties.     

The 5-hydroxytryptamine (5-HT2) receptor stimulates inositol phosphate formation in intact and broken WRK1 cells: determination of occupancy-response relationships for 5-HT agonists

5-Hydroxytryptamine (5-HT) stimulates the accumulation of inositol-trisphosphate in WRK1 cells, a cell line originating from a rat mammary tumor. 5-HT acts via a single receptor type for which it has an affinity constant estimated to be 1.27 microM. A series of agonists known to act at 5-HT2 receptors are partial agonists in this system and have a rank order of relative intrinsic efficacies corresponding to that seen in other systems possessing 5-HT2 receptors. There is an essentially linear occupancy-response relationship for 5-HT and other agonists indicating the absence of a strong amplification mechanism between receptor activation and inositol phosphate formation. The selective blockade of the 5-HT response by nanomolar concentrations of 5-HT2 selective antagonists but not by drugs acting at other 5-HT receptor subtypes suggest that the receptor in WRK1 cells is of the 5-HT2 type. Additionally, we demonstrate that in WRK1 membranes 5-HT acts via the 5-HT2 receptor to elicit a GTP dependent increase in the production of inositol-bisphosphate and inositol-trisphosphate. These properties of the WRK1 cell line indicate that it is a useful model with which to study the nature of 5-HT receptor coupling to the putative second messenger(s), the inositol phosphates.     

Binding of typical and atypical antipsychotic agents to transiently expressed 5-HT1C receptors.

We determined the affinities of clozapine and 21 other typical and atypical antipsychotic agents for the cloned 5-hydroxytryptamine-1C (5-HT1C) receptor. For these studies, 5-HT1C receptors were transiently expressed in COS-7 cells using the vector pSVK3-5HT1C. We discovered that clozapine and several other putative typical and atypical antipsychotic agents (loxapine greater than tiosperone greater than SCH23390 greater than fluperlapine greater than rilapine greater than chlorpromazine) had relatively high affinities (7-30 nM) for the cloned 5-HT1C receptor. Other antipsychotic agents (risperidone greater than tenilapine greater than mesoridazine greater than thioridazine greater than cis-fluphenthixol) had intermediate affinities (30-100 nM), whereas many other antipsychotics (fluphenazine greater than spiperone greater than amperozide greater than melperone greater than thiothixene greater than haloperidol, metoclopramide, pimozide, domperidone, sulpiride) had low affinities (greater than 500 nM) for the cloned 5-HT1C receptor. The results indicate that although several putative atypical antipsychotic agents have high affinities for the cloned rat 5-HT1C receptor, the spectrum of drug binding does not correlate with the atypical nature of these compounds.     

Effects of acute and chronic clozapine and haloperidol on in vitro release of acetylcholine and dopamine from striatum and nucleus accumbens

The purpose of this investigation was to determine if striatal or nucleus accumbens dopamine (DA) release, ACh release or DA receptor function were altered by acute and chronic haloperidol or clozapine treatment in a manner consistent with the reported pharmacological effects of each drug on A9 and A10 DA cell bodies and projection areas, when experiments were performed without a drug-free, or washout, period after drug treatment. The release of neurotransmitters reported here was evaluated using a slice-superfusion assay system. Transmitter release was induced either by an electrical field (for DA and ACh) or by application of either amphetamine or amfonelic acid (DA only). Dopaminergic receptor function was assessed by inhibiting electrically stimulated ACh release with in vitro TL-99 (a dopaminergic agonist) and by reversing that inhibition with in vitro neuroleptics or with ex vivo experimental paradigms (the in vitro analysis of transmitter release subsequent to in vivo drug administration). These data suggest that although there are differences between haloperidol and clozapine, there is no difference between the degree of postsynaptic DA receptor blockade produced that can be attributed to the duration of neuroleptic treatment. Chronic clozapine (20 mg/kg x 21 days) reversed TL-99-induced inhibition of ACh release in the nucleus accumbens only, whereas chronic haloperidol (0.5 mg/kg x 21 days) produced a similar reversal in both brain areas. One possible explanation for the lack of effect of chronic clozapine treatment in the striatum is that carrier-mediated (amphetamine-stimulated) DA release is enhanced in the striatum but not in the nucleus accumbens, suggesting that the potential DA receptor block in the striatum may be compromised by enhanced striatal DA levels. Acute haloperidol (0.5 mg/kg) was found to increase electrically stimulated ACh release in the striatum and DA release in the nucleus accumbens. Tolerance developed in the striatum, but not the nucleus accumbens, with repeated administration. However, acute clozapine had no effect on ACh release in either area, but it was found to enhance DA release in the striatum, an effect to which tolerance developed with chronic administration. Further, comparison of these data with data obtained using haloperidol and clozapine in vitro suggests that it is unlikely that these effects are due to residual drug still present in these tissues at the time of experimentation. These data are discussed with regard to electrophysiological and pharmacological differences observed between clozapine and haloperidol on the activity of A9 and A10 DA cells after chronic neuroleptic treatment.     

Differential effects of repeated treatment with haloperidol and clozapine on dopamine release and metabolism in the striatum and the nucleus accumbens.

The differential effects of haloperidol (HAL) and clozapine (CLOZ) on dopamine (DA) release and metabolism (dihydroxyphenylacetic acid levels) in striatum and nucleus accumbens (accumbens) of freely moving rats were investigated using microdialysis. Chronic HAL (2 mg/kg/day x 21 days in drinking water) decreased basal DA release and metabolism in both regions, and produced tolerance to HAL-induced increase in DA metabolism in striatum. No modification of HAL-induced increases in DA release and metabolism were observed in accumbens. Together with D2 receptor blockade, this may produce decreased dopaminergic neurotransmission in both regions during chronic treatment. Chronic HAL (0.5 mg/kg/day x 21 days in drinking water) also decreased basal DA release and metabolism in both regions which were not reversed by 25 micrograms/kg of (-)-apomorphine, s.c. In marked contrast, chronic CLOZ (20 mg/kg/day x 21 days in drinking water) had no effect on basal DA release and metabolism in either region, whereas it produced tolerance to CLOZ-induced increase in DA release and metabolism in accumbens. Together with weak D2 receptor blockade, this may lead to slightly decreased dopaminergic neurotransmission in accumbens and slightly increased dopaminergic neurotransmission in striatum during chronic CLOZ treatment. These differences may contribute to the clinical differences between the two agents.     

Pharmacological characterization of PD 118717, a putative piperazinyl benzopyranone dopamine autoreceptor agonist.

PD 118717 (7-[3-[4-(2-pyrimidinyl)-1-piperazinyl]-propoxy]-2H-1- benzopyran-2-one sulfate) proved to be a dopamine (DA) D-2 autoreceptor agonist in biochemical and electrophysiological studies in rats and to exhibit an antipsychotic-like profile in behavioral tests in rodents and monkeys. In vitro binding studies indicated that PD 118717 bound selectively to DA D-2 vs. D-1 receptors and exhibited agonist binding properties (biphasic inhibitory curves and GTP shift) similar to DA. It also had significant affinity for serotonin-(5-HT)1A but not 5-HT1B and 5-HT2 receptors. PD 118717 was active in antagonizing the tau-butyrolactone-induced accumulation of dopa in rat striatum and mesolimbic regions. PD 118717 also depressed the firing of DA neurons in substantia nigra pars compacta of rats. In both of the latter tests the effects of PD 118717 were reversed by haloperidol. PD 118717 decreased brain DA metabolism, decreased DA utilization, decreased accumulation of dopa after inhibition of L-aromatic amino acid decarboxylase, stimulated serum corticosterone and inhibited stimulated serum prolactin levels. PD 118717 did not alter striatal acetylcholine levels; nor did it induce locomotor stimulation or stereotypy in normal animals, suggesting a lack of postsynaptic DA stimulation of normosensitive DA receptors. In tests designed to reveal even weak postsynaptic DA agonist effects, PD 118717 stimulated locomotor activity in 6-hydroxydopamine-lesioned animals and relatively higher doses induced a low degree of stereotyped behavior when combined with the DA D-1 agonist SKF 38393. PD 118717 decreased the accumulation of 5-hydroxytryptophan in brain, an effect probably due to an agonist action at 5-HT1A receptors. PD 118717 decreased spontaneous locomotor activity in rodents, antagonized amphetamine-stimulated hyperactivity in mice and inhibited Sidman avoidance in monkeys, effects seen with antipsychotic agents. Unlike DA antagonist antipsychotics, PD 118717 did not induce extrapyramidal dysfunction in monkeys. PD 118717 displayed behavioral activity after p.o. dosing and its effects did not show tolerance on repeated dosing. In conclusion, PD 118717 has the profile of a DA autoreceptor agonist in neurochemical and neurophysiological tests and produces effects suggestive of antipsychotic efficacy without neurological side effect liability in preclinical behavioral tests.     

D-16949 (anpirtoline): a novel serotonergic (5-HT1B) psychotherapeutic agent assessed by its discriminative effects in the rat.

D-16949 [6-chlor-2-(piperidyl-4-thio)-pyridine; Anpirtoline] is a novel centrally acting compound with serotonergic effects. To assess its discriminative stimulus effects, rats were trained to discriminate D-16949 (2.0 mg/kg i.p., 30 min) from no drug. D-16949 induced dose-dependent discriminative stimulus effects (ED50, 0.31 mg/kg), and did not produce sedation. The opioid analgesics codeine, pentazocine and tramadol all failed to substitute for D-16949. The opioid antagonist naltrexone did not antagonize the discriminative stimulus effects of D-16949. Phencyclidine, d-amphetamine, lysergic acid diethylamide and quipazine produced between 0 and 35% responding on the D-16949 lever. 8-Hydroxy-2-(di-n-propylamino)-tetralin substituted partially (45%) for D-16949, whereas 1-(m-trifluoromethylphenyl)-piperazine and RU 24969 completely and dose-dependently substituted for D-16949. The discriminative stimulus effects of D-16949 were not reversed by either cyproheptadine, ketanserin, pirenperone, spiperone or methylsergide. The 5-hydroxytryptamine3 (5-HT3) active antagonists ICS 205-930 and MDL 72222 were also ineffective as D-16949 antagonists. It is concluded that the discriminative stimulus effects of D-16949 are not mediated through opioid or 5-HT2 mechanisms. The present data also do not suggest the involvement of 5-HT3 mechanisms, but that D-16949 produces its discriminative stimulus effects in the rat primarily via agonistic actions at 5-HT1B receptors.     

Regulation of glutamate release by α7 nicotinic receptors: differential role in methamphetamine-induced damage to dopaminergic and serotonergic terminals

Regulation of glutamate release is an important underlying mechanism in mediating excitotoxic events such as damage to dopamine (DA) and serotonin (5-HT) neurons observed after exposure to methamphetamine (Meth). One way to regulate glutamate release may be through the modulation of α7 nicotinic acetylcholine (nACh) receptors. Meth administration is known to increase acetylcholine release; however, it is unknown whether Meth increases glutamate release and causes long-term damage to both DA and 5-HT terminals through the activation of α7 nACh receptors. To test this hypothesis, the α7 nACh receptor antagonist, methyllycaconitine (MLA), was administered before the administration of repeated doses of Meth while simultaneously monitoring extracellular striatal glutamate with in vivo microdialysis. In addition, the subsequent long-term decreases in markers of dopaminergic and serotonergic terminals, including DA reuptake transporter (DAT), serotonin reuptake transporter (SERT), vesicular monoamine transporter-2, vesicular DA, and vesicular 5-HT content in the rat striatum, were measured. The results show that MLA pretreatment prevented Meth-induced increases in striatal glutamate and protected against the subsequent long-term decreases in striatal DAT and vesicular DA content without affecting the hyperthermia produced by Meth. In contrast, the Meth-induced decreases in striatal SERT immunoreactivity and vesicular 5-HT content were not affected by MLA. This suggests that the α7 nACh receptor differentially mediates glutamate-dependent damage to DA but not 5-HT terminals in a manner that is independent of hyperthermia. Furthermore, antagonism of α7 nACh receptors may be a possible therapeutic strategy for decreasing extracellular glutamate and preventing the excitotoxic damage observed in other DA-related neurodegenerative disorders.