Effects of naloxone on d-amphetamine- and apomorphine-induced behavior

The effects of acute naloxone administration on d-amphetamine- and apomorphine-induced behavior were studied. Naloxone, in doses of 0.3-10 mg/kg (s.c.), antagonized the increase in ambulation and rearing induced by 1 mg/kg of d-amphetamine. When the dose of d-amphetamine was increased to 3 mg/kg, naloxone (3 mg/kg) antagonized only the increase in rearing activity. No dose (0.3-10 mg/kg, s.c.) of naloxone significantly affected d-amphetamine- or apomorphine-induced stereotyped activity. Naloxone (3 mg/kg) significantly augmented the apomorphine (1 mg/kg. s.c.)-induced increase in ambulation but attenuated the apomorphine (0.3 mg/kg)-induced increase in rearing activity. Naloxone (3 mg/kg) or apomorphine (0.03 mg/kg) significantly decreased the ambulation and rearing induced by a novel environment. In combination and in these doses, naloxone and apomorphine produced an additive effect on these behaviors. The neurochemical mechanisms by which naloxone affects d-amphetamine- and apomorphine-induced behavior were investigated. Naloxone (10^-6M) had no significant effect on [³H]spiroperidol binding in either the caudate nucleus or nucleus accumbens except for a modest inhibition (24%) of both the K_m and B_max in the accumbens microsomal fraction. Similarly, naloxone (10^-6 M) had no significant effect on [³H]dopamine(DA) uptake into either brain region nor did naloxone alter the d-amphetamine-inhibition of uptake. Using perfused tissue slices, naloxone (10^-6-10^-5 M) significantly attenuated the increase in [³H]DA release induced by d-amphetamine (10^-5 M) in both brain regions. Naloxone (1 mg/kg) had no significant effect on DA or dihydroxyphenylacetic acid (DOPAC) levels or on the DA/DOPAC ratio in the caudate nucleus or nucleus accumbens. However, naloxone did reverse the marked increases in the DA-DOPAC ratio induced by d-amphet-amine (1 mg/kg) in both brain regions.