Robert Neale

Biochemical and pharmacological characterization of CGS 12066B, a selective serotonin-1B agonist

CGS 12066B is a novel pyrroloquinoxaline with selectivity for the serotonin-1B (5HT1B) recognition site as assessed by binding, biochemical and electrophysiological studies. The compound had an IC50 value of 51 nM at the 5HT1B recognition site as determined using the binding of [3H]5HT in the presence of 1 microM spiperone. At the 5HT1A receptor the compound had an IC50 value of 876 nM, providing a 5HT1A/5HT1B ratio of 17 in contrast to the putative 5HT1B selective agent trifluoromethylphenylpiperazine (TFMPP) which had a corresponding ratio of 3.6. The compound had minimal affinity for alpha 1-, alpha 2- and beta-adrenoceptors and for dopamine D-1 and D-2 receptors. CGS 12066B, in contrast to TFMPP, which was inactive, was found to inhibit dorsal raphe cell firing with an ED50 value of 358 nmol/kg i.v. The corresponding values for the 5HT1A selective agonists 8-OH-DPAT and ipsapirone were 1.3 and 33 nmol/kg. CGS 12066B was also effective in decreasing rat brain 5-HTP concentrations and inhibiting in vitro 5HT release. The data obtained indicate that CGS 12066B is a reasonably active 5HT1B site agonist, which due to its selectivity as compared to compounds such as TFMPP, will be a useful tool for evaluating the physiological role of such receptors in the mammalian CNS.

Neuroleptic-induced acute dyskinesias in squirrel monkeys: Correlation with propensity to cause extrapyramidal side effects

In squirrel monkeys that had undergone repeated treatment with haloperidol at intervals of 7–14 days, subsequent acute administration of haloperidol induced dystonia and dyskinesias. This acute effect of haloperidol was dose-related and occurred at the same doses that impaired Sidman avoidance performance. Chlorpromazine, fluphenazine, metoclopramide, tetrabenazine, and Su-23397, all of which have been associated with extrapyramidal side effects, reliably elicited dyskinesias in these monkeys. Dyskinesias were less marked after thioridazine and absent after clozapine, corresponding to the reported lower incidence of extrapyramidal side effects in the clinic. The non-neuroleptics, baclofen, and diazepam, failed to elicit dyskinesias. In contrast to the dyskinetic syndrome, the incidence of catalepsy or tremor did not accurately predict propensity to elicit extrapyramidal symptomatology. The acute dyskinetic syndrome in squirrel monkeys may therefore serve as an animal model for predicting the ability of antipsychotics to cause extrapyramidal dysfunction, and may yield insight into the mechanisms of these drug-induced motor disorders.

Effects of dopamine agonists, catecholamine depletors, and cholinergic and GABAergic drugs on acute dyskinesias in squirrel monkeys

It has been suggested that the neuroleptic-induced acute dyskinetic syndrome in monkeys may be a useful model of extrapyramidal dysfunction. Various drugs that have well-characterized effects on clinical extrapyramidal syndromes and on catecholaminergic, cholinergic, or GABAergic neurotransmission were assessed in dyskinesia-susceptible squirrel monkeys. Catecholamine depletors (α-methyl-p-tyrosine, tetrabenazine) induced the syndrome, as do dopamine (DA) receptor antagonists, and d-amphetamine reversed the effects of tetrabenazine. The haloperidol-induced syndrome was reversed by the indirectly acting DA agonists amantadine and L-dopa. Neither of the DA autoreceptor agonists TL-99 or 3-PPP elicited this syndrome, suggesting that these agents lack extrapyramidal involvement. Anticholinergics reversed haloperidol-induced dyskinesias and the cholinomimetic arecoline was capable of inducing dyskinesias. When coadministered repeatedly with haloperidol, benztropine suppressed the emergence of susceptibility to neuroleptic-induced dyskinesias. These results confirm that the acute dyskinetic syndrome in the monkey is characterized by DA deficiency and acetylcholine excess. Diazepam and baclofen, which have been reported to have some clinical benefit in tardive dyskinesia, suppressed haloperidol-induced acute dyskinesias without causing gross motor depression. Pharmacological manipulation of GABAergic pathways from striatum may constitute a fruitful approach to the treatment of dyskinetic motor disorders.

Interactions of isamoltane (CGP 361A), an anxiolytic phenoxypropanolamine derivative, with 5-HT1 receptor subtypes in the rat brain.

SummaryIsamoltane (CGP 361A; (1-(2-(1-pyrrolyl)phenoxy)-3-isopropylamino-2-propanol hydrochloride), β-adrenoceptor ligand (IC50 = 8.4 nmol/l) which has reported activity as an anxiolytic in man was found to be a reasonably active inhibitor of the binding of [125I]ICYP to 5-HT1B recognition sites in rat brain membranes with 27-fold selectivity (IC50 = 39 nmol/l) as compared to the inhibition of binding of [3H]8-OH-DPAT to 5-HT1A receptors (IC50 = 1070 nmol/l). This selectivity was considerably greater than that observed for other β-adrenoceptor ligands including propranolol (5-HT1A/5-HT1B ratio = 2), oxprenolol (3.5) and cyanopindolol (8.7). The 5-HT1B activity of the compound resided in the (−)-enantiomer. (−)Isamoltane had weak activity (IC50 3–10 μmol/l) at 5-HT2 and α1-adrenoceptors. The compound was devoid of activity at a number of other central neurotransmitter recognition sites including the 5-HT1C site. Isamoltane increased the electrically evoked release of [3H]5-HT from prelabeled rat cortical slices in a manner similar to that of cyanopindolol. While both compounds were similar in potency to methiothepin, they had lower efficacy. Oxprenolol was less potent that both isamoltane and cyanopindolol while propranolol was essentially inactive. The effects of the compounds on 5-HT release appeared to be correlated with their 5-HT1B rather than 5-HT1A activity. In vivo, isamoltane increased 5-HTP accumulation in rat cortex following central decarboxylase inhibition at doses of 1 and 3 mg/kg i. p. At higher doses this effect was gradually diminished. Similar, but less clearcut results were obtained with cyanopindolol and oxprenolol, but propranolol was ineffective. No changes in brain tryptophan levels were associated with the isamoltaneevoked changes in brain 5-HTP levels. In reserpinized animals, isamoltane reduced 5-HTP accumulation even at doses which enhanced accumulation of this metabolite when given alone. The effects of the putative 5-HT1B agonist, m-trifluoromethylphenylpiperazine (TFMPP), the mixed 5-HT autoreceptor agonist/antagonist/ β-adrenoceptor antagonist, pindolol, the 5-HT uptake inhibitor, CGP 6085A and the MAO-A inhibitor, brofaromine, were not antagonized by pretreatment with isamoltane. The possibility that isamoltane and the other β-adrenoceptor antagonists are antagonists at 5-HT1B receptors and that their effect on 5-GT synthesis in vivo is the net result of their agonist/antagonist effects at 5-HT1A and 5-HT1B receptors is discussed in relation to the potential mechanism of the anxiolytic activity of isamoltane.

Antagonism of L-5-hydroxytryptophan-induced head twitching in rats by lisuride: a mixed 5-hydroxytryptamine agonist-antagonist?

Lisuride antagonized L-5-hydroxytryptophan (5-HTP)-induced head twitches at doses lower than those sufficient to induce the serotonin (5-HT) syndrome. Among several other 5-HT agonists tested, only LSD and 1-(m-trifluoromethylphenyl)-piperazine (TFMPP) shared this paradoxical profile. Assessment of various dopamine (DA) agonists revealed a lack of correlation between DA-mediated stereotyped behavior (indicative of postsynaptic DA agonism) and blockade of 5-HTP-induced head twitches. Lisuride displaced specific ligand binding from putative S1a, S1b and S2 receptors at nanomolar concentrations, and other drugs that blocked 5-HTP-induced head twitches also displaced binding at S2 sites. It is proposed that lisuride may have agonist properties at S1a receptors mediating the 5-HT syndrome but antagonist properties at S2 receptors mediating 5-HTP-induced head twitching.

Progressive changes in the acute dyskinetic syndrome as a function of repeated elicitation in squirrel monkeys

Various neuroleptic-induced motor disorders that appear in primates previously treated with neuroleptics are collectively designated the acute dyskinetic syndrome. The relative incidence of these motor disorders was examined as the syndrome was repeatedly elicited by haloperidol and other dopamine antagonists in individual monkeys. After several weekly or biweekly treatments with haloperidol (1.25 mg/kg orally), catalepsy began to appear, which was then accompanied by athetoid movements (writhing and limb extensions) as intermittent neuroleptic treatment continued. Other dyskinetic movements (‘duck walk’, oral dyskinesias, pushing of the head into a cage corner, and perseverative circling) that were suggestive of hyperkinesia subsequently began to be clicited by haloperidol and other neuroleptics after additional treatments with these drugs had intervened. As intermittent treatments continued, tolerance to the athetoid movements gradually developed and, eventually, only circling and pushing could be consistently elicited by haloperidol. In monkeys that had reached this phase, the athetoid movements were not again induced by higher doses of haloperidol (up to 5 mg/kg), chlorpromazine (3 mg/kg), or metoclopramide (3 mg/kg). In these tolerant monkeys, haloperidol impaired Sidman avoidance performance less and benztropine more than in drugnaive monkeys. Neither pharmacokinetic changes nor behavioral tolerance could readily account for these results. It is hypothesized that they reflect progressive functional alterations in dopaminergic or cholinergic neurotransmission.

CGS 27830, a potent nonpeptide endothelin receptor antagonist

Abstract Treatment of the dihydropyridine carboxylic acid 1 with EDCI produced a diastereomeric mixture of chemically stable anhydrides. The meso-isomer, CGS 27830, was isolated by crystallization and found to be a potent (16 nM) endothelin receptor antagonist for the ETA receptor subtype.

Acute dyskinesias in monkeys elicited by halopemide, mezilamine and the “antidyskinetic” drugs, oxiperomide and tiapride

Oxiperomide and tiapride are dopamine receptor antagonists claimed to have “antidyskinetic” properties in animal models and in the clinic. Halopemide and mezilamine are other dopamine antagonists predicted to lack extrapyramidal side effects in man on the basis of animal studies. Acute dyskinesias, a neuroleptic-induced acute extrapyramidal syndrome, were elicited in squirrel monkeys by oxiperomide (1 mg/kg), tiapride (30 mg/kg), and halopemide (10 mg/kg). The dyskinesias were virtually indistinguishable from those caused by a standard behaviorally equivalent dose of haloperidol (1.25 mg/kg PO) in the same individual monkeys. Mezilamine (0.3 mg/kg) also induced dyskinesias, which appeared to be less pronounced than those following haloperidol. The antidyskinetic properties of oxiperomide and tiapride evidently do not confer protection against dyskinetic movements induced by dopamine antagonism.

Down-regulation of tryptamine receptors following chronic administration of clorgyline.

Chronic treatment with clorgyline, a type A monoamine oxidase (MAO) inhibitor (1 mg/kg/day for 11 days), reduced the number (Bmax) but not the affinity (Kd) of [3H]tryptamine binding sites in rat frontal/parietal cortical membranes. Binding was reduced for at least 36 days following the last injection. The reduction in [3H]tryptamine binding was dose-related and appeared to be maximal following a daily dose of 3 mg/kg. Chronic treatment with deprenyl, a type B MAO inhibitor (1 mg/kg/day for 11 days), did not affect [3H]tryptamine binding. Acute clorgyline administration (11 mg/kg) also had no effect. These data suggest that [3H]tryptamine binds to neurotransmitter receptors for tryptamine since mere chemical recognition sites would not be expected to be modulated by chronic drug treatment. Also, since [3H]tryptamine binding was down-regulated by a type A, but not a type B, MAO inhibitor, tryptamine may be selectively metabolized by type A MAO in vivo.

Differential reversal of various dopamine antagonists by anticholinergics in sidman avoidance: possible relationship to adrenergic blockade

Various dopamine receptor antagonists have divergent clinical and neurochemical properties. The relative ability of anticholinergics (benztropine and scopolamine) to reverse these drugs was assessed in squirrel monkeys and rats performing a Sidman avoidance task. In monkeys, benztropine markedly attenuated the effects of oxiperomide, metoclopramide, halopemide, tiapride and mezilamine as well as haloperidol. Chlorpromazine and fluphenazine were antagonized to a moderate extent; thioridazine and perlapine were not antagonized; and clozapine was actually potentiated by benztropine. In the rat, benztropine antagonized haloperidol strongly but reversed fluphenazine, thioridazine or clozapine only weakly or not at al. The overall effects of scopolamine in both species were similar to those of benztropine. The dopamine receptor antagonists that were most completely reversed by benztropine were found to inhibit 3H-spiroperidol more strongly than 3H-WB-4101 binding in calf caudate, while the reverse was true for drugs that were antagonized only moderately or not at all by benztropine. These results support a previous suggestion that anticholinergic reversal is less marked aginst dopamine antagonists with alpha-adrenergic blocking properties. Benztropine reversal of experimental dopamine receptor antagonists in the squirrel monkey Sidman avoidance test may contribute to their preclinical characterization.