Effects of apomorphine and methamphetamine on a quickly-learned conditioned-suppression response in rats.

Rats exhibited a marked suppression of motor activity when placed in the same chamber where they had been given electric shocks. Administration of apomorphine-HCl (0.1, 0.5 and 1.0 mg/kg, i.p.) attenuated the conditioned suppression, dose-dependently, but did not facilitate motor activity of control (i.e. non-shocked) rats. Methamphetamine-HCl (0.1, 0.5 and 1 mg/kg, i.p.) increased motor activity of both the shocked and non-shocked rats, in a dose-related manner. Haloperidol (0.3 and 1 mg/kg, i.p.), but not chlorpromazine-HCl (5 mg/kg, i.p.), enhanced the conditioned-suppression response. Atropine-sulfate (5 and 10 mg/kg, i.p.), p-chlorophenylalanine (300 mg/kg, i.p.) and alpha-methyl-p-tyrosine (100 mg/kg, i.p.) were slightly effective in reducing the conditioned-suppression response. Apomorphine- and methamphetamine-induced reduction of the conditioned-suppression response was inhibited by pretreatment with haloperidol. When catecholamine-synthesizing processes in rats were inhibited by pretreatment with alpha-methyl-p-tyrosine, or alpha-adrenergic receptor sites were blocked by pretreatment with phenoxybenzamine, the effect of methamphetamine, but not that of apomorphine, was reduced. Therefore, enhancement of dopaminergic neurotransmission may be responsible for attenuation of the conditioned-suppression response.