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Microinjection of morphine into the nucleus accumbens impairs contextual learning in rats
R F Westbrook1, A J Good, M J Kiernan
1School of Psychology, University of New South Wales, Sydney, Australia. f.westbrook@unsw.edu.au
Abstract:
A unilateral microinjection of morphine into the amygdala impaired fear conditioning to both a conditioned stimulus (CS) paired with shock and the context where shock occurred, whereas a microinjection of morphine into the nucleus accumbens (NA) spared fear conditioning to the CS but impaired, in a dose-dependent and receptor-specific manner, fear conditioning to the context. Morphine in the NA also spared extinction and latent inhibition of a CS but abolished the context specificity of these effects and eliminated the increase in discriminability that results from preexposure to a to-be-shocked context. The results identify a role for the NA in the processes by which rats learn about a context and are discussed in terms of an opioid disruption of either within-context associations or of attentional processes that contribute to such associations.
Insights
Morphine in the amygdala impaired fear conditioning. However, morphine in the nucleus accumbens (NA) selectively impaired context fear conditioning, suggesting the NA
Area of Science:
- Neuroscience
- Behavioral Neuroscience
- Neuropharmacology
Background:
- Fear conditioning is a fundamental learning process involving associative memory.
- The amygdala and nucleus accumbens (NA) are key brain regions implicated in fear learning and memory.
- Opioid systems, including morphine, modulate emotional processing and learning.
Purpose of the Study:
- To investigate the differential roles of the amygdala and nucleus accumbens (NA) in fear conditioning to conditioned stimuli (CS) and context.
- To determine the effects of microinjections of morphine into these regions on various aspects of fear memory, including extinction and latent inhibition.
Main Methods:
- Unilateral microinjections of morphine into the amygdala and nucleus accumbens (NA) of rats.
- Assessment of fear conditioning, extinction, and latent inhibition to both a conditioned stimulus (CS) and the conditioning context.
- Dose-dependent and receptor-specific analyses of morphine's effects.
Main Results:
- Morphine in the amygdala impaired fear conditioning to both CS and context.
- Morphine in the NA impaired context fear conditioning but spared CS fear conditioning.
- Morphine in the NA affected extinction and latent inhibition, abolishing context specificity and discriminability.
Conclusions:
- The nucleus accumbens (NA) plays a crucial role in contextual fear learning and memory.
- Opioid modulation within the NA disrupts the formation or retrieval of within-context associations or attentional processes related to context.
- These findings highlight distinct neurobiological substrates for CS-specific and contextual fear memory.