Behavioral effects of morphine on free-operant avoidance under hyperbaric pressure

Abstract

Morphine sulfate was tested under hyperbaric pressure to assess its effects on behavior. Four male hooded rats were trained to avoid brief electric shocks under a free-operant unsignalled avoidance procedure. Using an individual organism design, we injected each rat subcutaneously with morphine sulfate (2.0, 4.0, 6.0, 8.0 mg/kg body wt.) or saline (0.1 ml/100 g body wt.). Rats were tested at 1.0 and 7.1 atmospheres absolute (ATA) in a dry hyperbaric chamber while breathing a mixture of helium and oxygen. Each session lasted 60 min. Overall, the analgesic effects of morphine at 1.0 and 7.1 ATA were found to be similar. Shock avoidance by a rat was found to be a monotonic function of the drug dose; the fewest shocks were associated with the 2.0 mg/kg dose. Increases pressure did not significantly affect the number of shocks received by a rat across doses. Total responding remained stable throughout the study, but the temporal pattern of responding was differently influenced by drug dose.