Scopolamine Enhances Generalization between Odor Representations in Rat Olfactory Cortex

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Abstract
Acetylcholine (ACh) has a critical, modulatory role in plasticity in many sensory systems. In the rat olfactory system, both behavioral and physiological data indicate that ACh may be required for normal odor memory and synaptic plasticity. Based on these data, neural network models have hypothesized that ACh muscarinic receptors reduce interference between learned cortical representations of odors within the piriform cortex. In this study, odor receptive fields of rat anterior piriform cortex (aPCX) single-units for alkane odors were mapped before and after either a systemic injection of the muscarinic receptor antagonist scopolamine (0.5 mg/kg) or aPCX surface application of 500 μM scopolamine (or saline/ACSF controls). Cross-habituation between alkanes differing by two to four carbons was then examined following a 50-sec habituating stimulus. The results demonstrate that neither aPCX spontaneous activity nor odor-evoked activity (receptive field) was affected by scopolamine, but that cross-habituation in aPCX neurons was enhanced significantly by either systemic or cortical scopolamine. These results indicate that scopolamine selectively enhances generalization between odor representations in aPCX in a simple memory task. Given that ACh primarily affects intracortical association fibers in the aPCX, the results support a role for the association system in odor memory and discrimination and indicate an important ACh modulatory control over this basic sensory process.