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Effect of morphine on acquisition of the classically conditioned nictitating membrane response of the rabbit
Abstract:
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.
Insights
Morphine impairs associative learning in rabbits by hindering the acquisition of conditioned responses, even days after drug administration. This effect on learning, not performance, suggests a direct impact on how the brain forms new associations.
Area of Science:
- Neuroscience
- Behavioral Psychology
- Pharmacology
Background:
- Classical conditioning, specifically the nictitating membrane response (NMR) in rabbits, is a well-established model for studying associative learning.
- Opioids, such as morphine, are known to have complex effects on cognitive functions, including learning and memory.
Purpose of the Study:
- To investigate the effects of morphine on the acquisition of classically conditioned nictitating membrane responses in rabbits.
- To determine if morphine affects associative learning or response performance.
Main Methods:
- Rabbits underwent classical conditioning with auditory (tone) and visual (light) conditioned stimuli preceding an unconditioned stimulus (electrical shock).
- Morphine was administered at doses of 1, 5, and 10 mg/kg, and its effects on conditioned response acquisition were assessed.
- Control groups received explicitly unpaired stimuli, and the unconditioned response to shock was also measured.
Main Results:
- Morphine significantly retarded or blocked the acquisition of conditioned responses in a dose-dependent manner.
- These effects persisted for at least 5 days after drug cessation, indicating an impact on acquisition rather than performance.
- Morphine did not affect baseline responding to stimuli or the unconditioned response to the shock.
Conclusions:
- Morphine impairs the associative learning process underlying conditioned response acquisition in rabbits.
- The findings suggest that morphine interferes with the neural mechanisms critical for forming stimulus-stimulus associations.
- The study highlights morphine's specific detrimental effect on associative learning, independent of motor performance or sensory processing.