Dose-dependent dual effect of morphine on electrophysiologic correlates of positive reinforcement (Reward Contingent Positive Variation: RCPV) in the cat

Abstract

In cats trained to press a lever for milk reward, the postreinforcement EEG synchronization (PRS) and the associated epicortical steady potential shift, known as “Reward Contingent Positive Variation (RCPV) restricted to the occipital cortex, were studied prior to and after administration of morphine sulfate. Contrary to what has ordinarily been described as a typical feline response to morphine, such as restlessness, aggressiveness, rage, and exaggerated startle reaction to environmental stimuli, associated with an increased tonus of the brainstem-hypothalamic arousal system and desynchronized EEG patterns, doses of 0.1–0.4 mg/kg, IM, caused a strong monophasic enhancement of the PRS-RCPV phenomenon. Doses of 0.6–1.0 mg/kg, IM, had clearly a biphasic action: the initial enhancement of the PRS-RCPV responses was followed by their strong suppression. Chlorpromazine (0.1–1.6 mg/kg, i.e.) promptly restored the EEG responses. During the peak effect of the enhancing doses of morphine, the reward-related EEG phenomena also occurred prior to or after the nonrewarded bar press when the animals licked the empty cup. This dissociation of the PRS-RCPV from consumption was much more conspicuous in animals whose control frequency of the PRS oscillations was higher, and after morphine showed more significant slowing. Despite the strong facilitation of the PRS-RCPV in the presence of light, morphine, in contrast to LSD-25, was not able to restore the reward-induced phenomena in the dark.