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E-4424 (2-(4-[4-(4-chloro-1-pyrazolyl)butyl]-1-piperazinyl)pyrimidine) was shown to be a 5-hydroxytryptamine1A receptor ligand in radioligand binding assays and in an in vitro guinea pig ileum preparation had both 5-hydroxytryptamine1A antagonist and agonist effects. The antagonist/agonist ratio of E-4424 was greater than in the case of buspirone and ipsapirone. E-4424 was compared to diazepam, buspirone and ipsapirone to inhibit the behavioral response to an aversive situation in the mouse black and white test box, the rat social interaction test and a marmoset human threat test. The acute administration of E-4424 (0.0001-0.5 mg/kg, i.p.) to the mouse decreased aversion to the white area of the test box and was as effective as diazepam (0.125-1.0 mg/kg, i.p.) and much more potent than buspirone (0.25-1.0 mg/kg, i.p.) or ipsapirone (0.5-5.0 mg/kg, i.p.). E-4424 was also effective in enhancing rat social interaction and reducing anxiety-related behaviors in the marmoset and was again more potent than diazepam, buspirone or ipsapirone. Withdrawal from a 14-day administration of diazepam, cocaine, nicotine or alcohol exacerbated the response to the aversive situation in the mouse test. This was not observed after withdrawal from a chronic treatment with E-4424, buspirone or ipsapirone. However, E-4424 administered during drug withdrawal prevented the response caused by withdrawal from cocaine, alcohol, nicotine and diazepam: buspirone was ineffective and ipsapirone only attenuated that syndrome after alcohol withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence is reviewed that 5-HT (5-hydroxytryptamine, serotonin) acting through the 5-HT3 receptor subtype can influence behaviour relevant to anxiety, schizophrenia and cognitive disorders, and that 5-HT3 receptor antagonists such as ondansetron (CAS 116002-70-1) can correct behavioural disturbance in the absence of effect on normal behaviour. The 5-HT3 receptor antagonists exert a breadth of action over a wide dose range in rodent and primate models to inhibit aversive behaviour in animal models of anxiety and certain symptoms of withdrawal from drugs of abuse, alcohol, nicotine, diazepam and cocaine, to antagonise increased locomotor activity caused by mesolimbic dopamine excess, and facilitate performance in cognitive tests. The studies reveal an important role for 5-HT3 receptors in the regulation of limbic-cortical functioning, and a critical role for 5-HT3 receptor antagonists to establish the role of 5-HT3 receptors in schizophrenia, anxiety, drug withdrawal phenomena and cognitive disturbance. Preliminary clinical trials indicate a positive effect of ondansetron in anxiety, schizophrenia, alcohol withdrawal and age associated memory impairment.

5-HT3 receptors or recognition sites have been located in the brain of many animal species and man, and are considered to mediate an increase in cation conductance. The functional relevance of the central 5-HT3 receptors is being established using selective 5-HT3 receptor antagonist ligands, which have been shown to modify behaviour in animal tests that is predictive of an action to reduce anxiety, schizophrenia and cognitive disorders. A remarkable feature of the effect of 5-HT3 receptor antagonists is their ability and potency to control a disturbed behaviour in the absence of effect on normal behaviour. This reveals an important role for 5-HT in different behaviours: indeed, the breadth of action of the 5-HT3 receptor antagonists may be indicative of the critical importance of 5-HT in the control of catecholaminergic, cholinergic and peptidergic systems throughout the forebrain. Precisely how such effects are achieved remains a significant investigative challenge, with the potential for novel therapeutic developments being a major goal.

Evidence is presented that 5-HT3 receptor antagonists such as ondansetron are highly effective in influencing behavioural disturbance in the absence of effect on normal behaviour. The 5-HT3 receptor antagonists demonstrate a remarkable potency and breadth of action over a wide dose range in rodent and primates to inhibit aversive behaviour, antagonise the hyperactivity caused by mesolimbic dopamine excess and facilitate performance in cognitive tests. These effects are achieved in the absence of withdrawal phenomana following cessation of chronic treatment. The studies indicate a widespread role for 5-HT3 receptors in the regulation of mesolimbic-cortical functioning, and the importance of the 5-HT3 receptor antagonists as investigative tools to establish the role of 5-HT3 receptors in anxiety, schizophrenia and cognitive dysfunction, and their treatment.

The selective 5-HT3 receptor antagonist, onansetron, has been assessed in three tests of cognition in the mouse, rat and marmoset. In a habituation test in the mouse, ondansetron facilitated performance in young adult and aged animals, and inhibited an impairment in habituation induced by scopolamine, electrolesions or ibotenic acid lesions of the nucleus basalis magnocellularis. Arecoline failed to improve basal performance in young adult mice but inhibited the impairment caused by scopolamine and lesions of the nucleus basalis magnocellularis. In the T-maze reinforced alternation task in rats, ondansetron and arecoline antagonised a scopolamine-induced impairment. In an object discrimination and reversal learning task in the marmoset, assessed using a Wisconsin General Test Apparatus, ondansetron improved performance in a reversal learning task. We conclude that ondansetron potently improves basal performance in rodent and primate tests of cognition and inhibits the impairments in performance caused by cholinergic deficits.

The abilities of prostaglandin E1 (PGE1), PGE2, PGD2 and PGF2 alpha to antagonize striatal dopamine function were assessed following bilateral and unilateral injections into the striata of the rat and guinea-pig. Three tests were used to assess the effects of the bilateral injections, ability to antagonize dyskinetic biting induced by 2-di-n-propylamino-5,6-dihydroxytetralin (0.025 mg kg-1 s.c.), ability to antagonize stereotyped behaviour induced by apomorphine (0.5 or 2 mg kg-1 s.c.) and ability to induce catalepsy. Asymmetry/circling behaviour revealed on challenge with apomorphine (0.25 mg kg-1 s.c.) was measured following unilateral injection into the striatum. In the rat, dyskinetic biting induced by 2-di-n-propylamino-5,6-dihydroxytetralin was antagonized by PGE1 (0.001-1 micrograms) and PGE2 (0.00001-1 micrograms) but not by PGD2 or PGF2 alpha (1 microgram). Stereotyped behaviour induced by apomorphine was not antagonized by any of the prostaglandins. A weak catalepsy was induced by PGE1 (1 microgram only), PGE2 (0.001-1 micrograms) and PGD2 (0.001-1 micrograms) but not by PGF2 alpha. Asymmetry and circling behaviour was only observed following the unilateral injection into the striatum of PGE1 and PGD2 (0.01-1 microgram) and challenge with apomorphine. In the guinea-pig the actions of PGE1 and E2 were compared with those of PGF2 alpha. Dyskinetic biting induced by 2-di-n-propylamino-5,6-dihydroxytetralin was antagonized by bilateral injections into the striatum of PGE2 (0.001-1 microgram), but not PGE1 (0.5 micrograms) and PGF2 alpha (1 microgram) but not PGE, (0.5 micrograms) and PGF2 alpha (1 microgram). Similar injections of PGE1, E2 and F2 alpha, all failed to antagonize apomorphine-induced stereotyped behaviour, or to induce catalepsy. PGE, (0.01-0.5 fig) and PGE2 (0.002-1 pg), but not PGF2 alpha, caused asymmetry following unilateral injection into the striatum and peripheral challenge with apomorphine. 5 It is concluded that the major effect in the striatum of the prostaglandins of the E series is to antagonize dyskinetic biting; this action is not shared by other prostaglandins tested, and does not reflect a generalised ability to antagonize striatal dopamine function. It is suggested that the actions of the prostaglandins to modify differentially dopamine-dependent behaviours from the striatum may reflect activity at a site subsequent to the dopamine receptor.

The present study evaluated whether 2-amino-5,6-dihydroxy-1,2,3,4,-tetrahydronaphthalene (A-5,6 DTN) and its 6,7-dihydroxy isomer (A-6,7 DTN) had effects on striatal dopaminergic terminals in addition to their well known action as agonists on postsynaptic dopamine (DA)-receptors. The presynaptic effects of 2-aminotetralins were evaluated on rat striatal tissue slices prelabeled with [3H]DA. Both, A-5,6 and A-6,7 DTN enhanced the basal efflux of radioactive products in a concentration-dependent fashion. Benztropine (1 microM) antagonized the releasing effects of low concentrations of both drugs (0.1-3.0 microM). Apomorphine and A-5,6 DTN (alpha rotamers, trans conformation) were 3 to 5 times more potent than A-6,7 DTN (beta rotamer, trans conformation) in inhibiting rat striatal monamine oxidase. This effect could account for the 50-fold greater potency of A-6,7 than A-5,6 DTN in enhancing the efflux of [3H]dihydroxyphenylacetic acid. A-5,6 DTN, A-6,7 DTN and apomorphine (0.3 microM each), reduced by 30% the depolarization elicited overflow of radioactive products. In summary, A-5,6 and A-6,7 DTN seem to enter DA-terminals and storage vesicles to accelerate the efflux of [3H]DA and 3H-dihydroxyphenylacetic acid and act upon presynapatic receptors to inhibit transmitter release. A greater selectivity for monamine oxidase inhibition was found with the alpha rotameric agents. The present study indicates that A-5,6 and A-6,7 DTN have marked presynaptic effects on rat striatal DA-containing neurons in addition to their well known effects as agonists on postsynaptic DA receptors.

The development of antischizophrenic drugs via psychopharmacological testing procedures is historically connected with the chance observation that the drug chlorpromazine had antischizophrenic activity in the clinic. The question was raised, what effect did chlorpromazine have, as distinct from other psychoactive agents, in animals and as a result the classical tests for neuroleptic activity evolved; these included the induction of catalepsy, the antagonism of drug induced stereotyped behaviour, the antagonism of conditioned avoidance responses and brain self-stimulation behaviour, the inhibition of locomotor activity, and the inhibition of apomorphine induced emesis (Fielding and Lal, 1974). An effect in all of these behavioural tests has been associated with ability to inhibit cerebral dopamine function and, most important, there would appear to be a close correlation between activity in these animal tests and antischizophrenic activity in the clinic (see review by Costall and Naylor, 1979) Such evidence has been forwarded in support of ‘the dopamine hypothesis of schizophrenia’.

Studies have been carried out in the rodent and marmoset to assess (i) the selectivity of MPTP action to the nigrostriatal system, (ii) the possibility that melanin pigmentation may influence the actions of MPTP, (iii) whether the metabolites of MPTP may contribute to its actions on dopamine cells, (iv) the site(s) of action of MPTP in the brain, (v) the mechanism(s) of action of MPTP and whether it is possible to prevent the damaging effects of MPTP/metabolites on the brain dopamine systems. The peripheral administration of MPTP in the mouse causes depletions of dopamine and its metabolites in both the striatal and limbic systems: this action is similar in both white and pigmented mice. The MAO-B inhibitor deprenyl was shown to antagonize the actions of MPTP, and its metabolite formed via oxidation through MAO, MPP+, was shown to disrupt striatal dopamine function as determined behaviorally (as motor impairment) or biochemically (as loss of dopamine and its metabolites) when injected into the cerebral ventricles of mouse or infused into the substantia nigra of rat brain. MPTP and MPP+ act in the midbrain to interfere with dopamine cell functioning, and studies in both the rodent and marmoset are presently analyzing possible additional actions in forebrain regions. It is therefore proposed that the neurotoxic action of MPTP, effected via its metabolite MPP+, may not be selective for the nigrostriatal system. Nevertheless, the nature and profile of neurotoxicity effected by MPTP and, in particular, MPP+ should ensure continuing analyses of the relevance of such action to an understanding of the etiology of, treatment of, and even the possible prevention of Parkinson's disease in man.

The synthesis and physical properties of 8-hydroxyaporphine (3a) and 8-hydroxy-N-n-propylnoraporphine (3b) are described. The replacement of the rigid aporphine ring system by the more flexible benzyltetrahydroisoquinoline moiety, still containing all the necessary substitutents of the potent dopamine agonist N-n-propylnorapomorphine (1b) (NPA), resulted in the synthesis of 1-(3,4-dihydroxybenzyl)-2-propyl-1,2,3,4-tetrahydroisoquinoline (4). Analogous to 4,1-(4-hydroxybenzyl)-2-propyl-1,2,3,4-tetrahydroisoquinoline (5) was synthesized for a direct comparison with the biological activity of the corresponding 10-hydroxyaporphine (2). All compounds synthesized were evaluated as salts of their racemates. In animals with unilateral 6-OHDA lesion of the nigrostriatal pathway, (-)NPA and 2 caused dose-dependent contralateral circling behavior although activity was greatly reduced for the monohydroxylated aporphine 2. 3b,4, and 5 were inactive at doses of 0.25-4.0 mg/kg sc. Compounds 2 and 3b exhibited very weak activity in the stereotype tests in comparison to the response obtained with apomorphine and (-)-NPA. 4 and 5 failed to induce any sterotyped response. These compounds were also investigated for their ability to stimulate locomotor activity following direct injection into the nucleus accumbens. (-)-NPA induced a modest increase in activity but apomorphine completely failed to elicit a locomotor responses and antagonized the effect induced by dopamine. 2,3b, 4 and 5 neither enhanced locomotor activity on direct injection into the nucleus accumbens nor antagonized the hyperactivity response to intraacumbens dopamine when administered peripherally. On direct injection into the caudate-putamen only apomorphine induced stereotyped biting; (-)-NPA, 2, 3b, 4, and 5 were inactive. The differential activity of the aporphine derivatives in these tests strongly supports the possible existence of different types of dopamine receptors within the extrapyramidal and mesolimbic systems. The present studies confirm that the flexible benzylisoquinolines 4 and 5 do not adopt the active dopamine conformation and that the rigid aporphines, preferably containing hydroxyl functions at the 10 or 11 positions, are of greater importance in eliciting potent dopamine agonist activity.