Effects of injected or orally ingested opiate drugs on conditioned avoidance response learning in rats

Differential effect of morphine on trigeminal nucleus versus reticular aversive stimulation: independence of negative effects from stimulation parameters

Electrodes were implanted in mesencephalic, pontine, and bulbar reticular formation, and in spinal trigeminal nucleus and tract of rats. Central and peripheral aversive response thresholds were studied under normal conditions and with morphine. Peripherally elicited aversive reactions were assessed with tail-flick, hot-plate, and footshock responses. Centrally elicited aversive reaction thresholds were in all cases based on unconditioned behavioral distress signs (non-stereotyped, escape-like movements, vocalization, freezing, excretion, etc.) and confirmed in some cases with avoidance learning. Morphine (10 mg/kg) elevated the unconditioned aversive reaction threshold for brain stimulation in the trigeminal complex and for peripheral aversive stimulation, but failed to affect the thresholds for reticular brain stimulation. The failure to affect reticular thresholds was independent of stimulation frequency. Thresholds for 5 and 200 Hz sinusoidal stimulation were both unaffected as were previously reported thresholds with 333 Hz pulsatile stimulation. Trigeminal nucleus and tract stimulation were affected in similar degrees. The data were discussed as supporting descending inhibitory models of opiate analgesia.

Acute effects of morphine and chlorpromazine on the acquisition of shuttle box conditioned avoidance response

Morphine sulfate, 0.25-24.0 mg/kg, or chlorpromazine hydrochloride, 0.0625-4.0mg/kg were administered subcutaneously to naive rats 30 min prior to the start of massed-trials conditioned avoidance response (CAR) testing. The graded doses of both drugs were applied in each of three CAR task difficulty levels created by manipulation of the duration of conditioned and unconditioned stimuli, intertrial interval and shock intensity. Chlorpromazine, in a dose-related manner, caused a decrement in CAR acquisition in all tasks. Morphine, in comparison, produced a biphasic dose response. For a given task difficulty, low doses of morphine enhanced acquisition, whereas higher doses inhibited acquisition. With increasing task difficulty, relatively larger doses of morphine were required to inhibit or facilitate acquisition of CAR. These results emphasize the need to consider not only drug dosage levels, but also the interaction of task difficulty in the application of drugs in learning paradigms.