Oxytocin secretion is inhibited by opioids, and oxytocin is important in parturition. The effects on parturition of morphine, a relatively selective mu-opioid receptor agonist, were studied in the rat. Morphine or vehicle with or without the opiate antagonist naloxone were administered immediately after the birth of the second pup and the subsequent course of parturition was recorded in a total of 80 rats. Both s.c. morphine (10 mg/kg) and intracerebroventricular (i.c.v.) morphine (18 micrograms through a previously implanted cannula) interrupted parturition, delaying the birth of the sixth pup after treatment to 187.3 +/- 35.9 (S.E.M.) min and 195.4 +/- 19.5 min respectively, compared with 46.4 +/- 3.7 and 66.1 +/- 17.5 min after vehicle alone. The dose of morphine given i.c.v. had no effect when given s.c. Naloxone given concurrently prevented the effects of morphine. Eventually the rate of parturition in the morphine-treated groups recovered. Perinatal pup mortality rate was not increased when morphine was given to the mothers, but it did inhibit the expression of normal intrapartum maternal behaviour. Pup mortality was increased 48 h post partum by morphine given during parturition, and it reduced the proportion of rats with normal maternal behaviour 24 h post partum. Morphine did not affect spontaneous or oxytocin-stimulated contractile activity of the parturient uterus in vitro. The concentration of oxytocin in trunk blood plasma was decreased 40 min after i.c.v. morphine (24.3 +/- 3.9 vs 39.3 +/- 6.5 pmol/l in controls), as was vasopressin (7.2 +/- 1.5 vs 19.7 +/- 4.5 pmol/l in controls). Intravenous infusion of oxytocin (2-5 mU/min for 144.3 +/- 8.2 min; total infused 448.5 +/- 61.9 mU) after i.c.v. morphine re-started parturition; all pups were born to these rats (mean time to pup 6, 110.3 +/- 12.7 min) before the i.v. vehicle-infused rats given i.c.v. morphine re-started (mean time to pup 6, 406.3 +/- 125.2 min). It is concluded that morphine given during parturition acts centrally through opioid receptors to inhibit oxytocin secretion, and impairs the expression of maternal behaviour. Reversal of the effects of morphine on parturition by i.v. oxytocin demonstrates the important role of oxytocin in fetus ejection and expulsion.

Gepirone (BMY 13805), a buspirone analog, was used to determine the antianxiety mechanism of the arylpiperazine class of drugs. Because of the weak effects of these drugs on conflict behavior, isolation-induced aggressive mice were used as the antianxiety model. Gepirone, like buspirone, potently inhibited attacks against group housed intruder mice (ED50 = 4.5 mg/kg i.p.) without causing sedation or ataxia. Inhibition of aggression was potentiated by co-administration of 0.25 mg/kg methiothepin or 2.5 mg/kg methysergide. Gepirone had variable effects on dopamine metabolism and reduced 5-hydroxytryptamine (5HT) metabolism about one third after a dose of 2.5 mg/kg. In contrast to buspirone, which markedly increased dopaminergic impulse flow, gepirone inhibited the firing of most cells recorded from the substantia nigra zona compacta in doses of 2.3-10 mg/kg i.v. and the effects were reversible by administration of haloperidol. The common metabolite of buspirone and gepirone, 1-(2-pyrimidinyl)-piperazine, caused increased firing rates only. Gepirone potently inhibited serotonergic impulse flow recorded from the dorsal raphe nucleus (88.3% after 0.04 mg/kg) and this effect was partially reversed by serotonergic antagonists. Both buspirone and gepirone displaced [3H]-5HT from the 5HT1a binding site in the hippocampus with IC50 values of 10 and 58 nM, respectively. Non-alkyl substituted aryl-piperazines displaced [3H]-5HT from both 5HT1a and 5HT1b binding sites. Thus, although gepirone may be a weak postsynaptic 5HT agonist, its primary effect is to decrease 5HT neurotransmission. In support of this conclusion was the observed potentiation of antiaggressive effects by blocking 5HT receptors wit small doses of methiothepin or methysergide, which would exacerbate the decreased release of 5HT caused by gepirone. These results are in harmony with reports that decreased serotonergic activity has anxiolytic-like effects in animal models of anxiety.

The novel anxiolytic drug, buspirone, reverses catalepsy induced by haloperidol. A series of aryl-piperazine analogues of buspirone and other 5-hydroxytryptaminergic agonists were tested for their ability to reverse haloperidol induced catalepsy. Those drugs with strong affinity for 5-hydroxytryptamine1a receptors were able to reverse catalepsy. Drugs with affinity for other 5-HT receptors or weak affinity were ineffective. However, inhibition of postsynaptic 5-HT receptors neither inhibited nor potentiated reversal of catalepsy and leaves open the question as to the site or mechanism for this effect.

McMillen, B.A., E.A. DaVanzo, S.M. Scott, and A.H. Song: N-alkyl-substituted arylpiperazine drugs: Relationship between affinity for serotonin receptors and inhibition of aggression. Drug Dev. Res. 12:53-62, 1988. Buspirone is the first of several N-alkyl-substituted aryl-piperazine-containing drugs to be approved for use as an anxiolytic agent. These drugs do not interact with the benzodiazepine-GABA receptor complex and therefore do not share the sedating and anticonvulsant effects of benzodiazepines. Buspirone and its analogs, gepirone and ipsapirone, act as agonists both pre-and postsynaptically at 5HT1, receptor sites. A series of piperazinecontaining drugs was tested for their affinity at the 5HTI, (postsynaptic) binding sites in hippocampus and compared with their ability to inhibit isolation-induced aggression by mice. The potent 5HTI, agonist, 8-OH-DPAT, was the most potent drug in both tests: lC50 = 1 nM, and ED5,, = 2.4 pmollkg for inhibition of [3H]-5HT binding and aggression, respectively. There was a good correlation between ability to inhibit aggression and ability to inhibit [3H]-5HT binding to hippocampal membranes. Fluprazine (DU 27716) was the major exception, but results with SKF 525-A pretreatment to inhibit drug oxidation suggest that both fluprazine and metabolites have antiaggressive activity. Like other aryl-piperazine drugs, fluprazine is potentiated by coadministration of a postsynaptic 5HT receptor antagonist, methysergide. Since antagonists of postsynaptic 5HT receptors potentiate gepirone and 8-OH-DPAT inhibition of aggression, these data suggest that affinity for 5HTI, recep- tors at presynaptic autoreceptor sites is critical for the antiaggressive activity of these drugs and possibly for their anxiolytic effects.

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