Localization of molecular correlates of memory consolidation to buccal ganglia mechanoafferent neurons after learning that food is inedible in Aplysia

Abstract

Training paradigms affecting Aplysia withdrawal reflexes cause changes in gene expression leading to long-term memory formation in primary mechanoafferents that initiate withdrawal. Similar mechanoafferents are also found in the buccal ganglia that control feeding behavior, raising the possibility that these mechanoafferents are a locus of memory formation after a training paradigm affecting feeding. Buccal ganglia mechanoafferent neurons expressed increases in mRNA expression for the transcription factor ApC/EBP, and for the growth factor sensorin-A, within the first 2 h after training with an inedible food. No increases in expression were detected in the rest of the buccal ganglia. Increased ApC/EBP expression was not elicited by food and feeding responses not causing long-term memory. Increased ApC/EBP expression was directly related to a measure of the efficacy of training in causing long-term memory, suggesting that ApC/EBP expression is necessary for the expression of aspects of long-term memory. In behaving animals, memory is expressed as a decrease in the likelihood to respond to food, and a decrease in the amplitude of protraction, the first phase of consummatory feeding behaviors. To determine how changes in the properties of mechanoafferents could cause learned changes in feeding behavior, synaptic contacts were mapped from the mechanoafferents to the B31/B32 neurons, which have a key role in initiating consummatory behaviors and also control protractions. Many mechanoafferents monosynaptically and polysynaptically connect with B31/B32. Monosynaptic connections were complex combinations of fast and slow excitation and/or inhibition. Changes in the response of B31/B32 to stimuli sensed by the mechanoafferent could underlie aspects of long-term memory expression.