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

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.