Effect of morphine on self-stimulation in rats and its modification by chloramphenicol

The effect of morphine was studied on self-stimulation (SS) behavior in rats implanted with bipolar electrodes in the posterior hypothalamus. A single dose (10 mg/kg) of morphine decreased SS responding within 10-20 min, reaching a minimum level between 20-40 min after which the responding gradually returned to normal. The SS responding then increased above the control level at 120-180 min postdrug, then slowly returned to normal, thus showing a rebound effect. The combination treatment with morphine (10 mg/kg) and chloramphenicol (50 mg/kg) on SS behavior produced an accentuation of the initial decrease in responding, which was prolonged before gradually returning to the control levels without showing any rebound effect. The data suggest that alterations in protein synthesis may underlie the suppressed excitatory effect of a high dose of morphine on SS behavior.