Antagonism of footshock stress-induced inhibition of intracranial self-stimulation by naloxone or methamphetamine.

Rats were trained to lever-press for intracranial self-stimulation (ICSS) with electrodes implanted in the ventral tegmental area (VTA). The effect of inescapable footshock on response rates to ICSS was examined in the present study. Markedly decreased response rates to ICSS were observed 15 min to 24 h following inescapable footshock. Naloxone (10.0 mg/kg) itself was without effect on response rates to ICSS, but completely antagonized the decreased response rates by the stressor treatment. A relatively low dose of methamphetamine (0.5 mg/kg), which showed no effect on ICSS rates in naive rats, also antagonized the decreased response rates to ICSS. The present results suggest that inescapable footshock may release endorphin in the mesolimbic or mesocortical area; the released endorphin may act on dopaminergic nerve endings and interrupt dopaminergic transmission. The decreased activity of dopaminergic neurons may cause the decreased response rates to ICSS.