Morphine's effects on differential serial compound conditioning and reflex modification of the rabbit's (Oryctolagus...

This study sought to determine the effects of morphine (0, 2, and 5 mg/kg) on (a) differential classical conditioning of the rabbit's (Oryctolagus cuniculus) nictitating membrane response (NMR) to the serial compounds A-X-unconditioned stimulus (US) and B-X-US (Experiment 1) and (b) the reflex modification effects of the compounds and their components (Experiment 2). These experiments determined specifically morphine's effects on the distinctiveness and time course of stimulus representations by examining morphine's dose-response effect on (a) differential responding to A and B and their conditional control over responding to X within the compounds and (b) the unconditioned excitatory effects of the compounds and their components as assessed by their ability to modify the amplitude of the unconditioned NMR. The results of these experiments indicate that morphine, in a dose-dependent manner, can operate to profoundly attenuate the distinctiveness and persistence (short-term memory) of stimulus representations.