Effect of morphine on acquisition of the classically conditioned nictitating membrane response of the rabbit.

The nictitating membrane response of the rabbit was classically conditioned to tone and light-conditioned stimuli presented for 800 msec before delivery of the unconditioned stimulus, consisting of a 100-msec electrical shock to the skin over the paraorbital region of the head. Morphine significantly retarded (1 and 5 mg/kg) or completely blocked (10 mg/kg) the acquisition of conditioned responses. The retarded or blocked acquisition of conditioned responses produced by morphine could still be detected when the rabbits were tested 5 days after cessation of drug injections, suggesting that morphine was affecting acquisition and not performance of conditioned responses. Separate groups of rabbits received explicitly unpaired presentations of tone, light and shock stimuli. The frequency of base-line responding or responding to the tone and light stimuli was low for vehicle controls (1-3%) and was not affected by any dose of morphine. Morphine also had no effect on the latency or amplitude of the unconditioned response to the shock stimulus. These results indicate that the acquisition of conditioned responses was associative, i.e., only occurred during the paired presentations of the conditioned and unconditioned stimuli. Consequently, the retarded acquisition of conditioned responses produced by morphine would appear to be due to an effect on associative learning.