Some central effects of opioid antagonists. Part I

This study concerned the influence of opioid antagonists: naloxone, naltrexone and diprenorphine on amphetamine hyperactivity in mice and rats, apomorphine hyperactivity in rats, amphetamine and apomorphine stereotypy in rats, and stereotyped gnawing induced by methylphenidate in mice. Naloxone, naltrexone and, in higher doses, diprenorphine attenuated the amphetamine hyperactivity in rats. In mice in the same test all antagonists at some doses produced attenuation. However they did not affect the apomorphine hyperactivity in rats. Both naloxone and naltrexone attenuated amphetamine and apomorphine stereotypy, while the effect of diprenorphine was different: it slightly attenuated the amphetamine stereotypy, but slightly potentiated the stereotypy induced by apomorphine. The influence of various antagonists on methylphenidate-induced stereotyped gnawing varied: naloxone had no effect, while naltrexone and diprenorphine showed a tendency to potentiate the response. The results suggest that naloxone and naltrexone show some actions resembling hose of antipsychotics, but of a mechanism different to that characteristic of typical neuroleptics.