Effects of Thryptophan Hydroxylase Blockade by P-Chlorophenylalanine on Contextual Memory Reconsolidation after Training of Different Intensity

The processes of memory formation and its storage are extremely dynamic. Therefore, the determination of the nature and temporal evolution of the changes that underlie the molecular mechanisms of retrieval and cause reconsolidation of memory is the key to understanding memory formation. Retrieval induces the plasticity, which may result in reconsolidation of the original memory and needs critical molecular events to stabilize the memory or its extinction. 4-Chloro-DL-phenylalanine (P-chlorophenylalanine-PCPA) depresses the most limiting enzyme of serotonin synthesis the tryptophan hydroxylase. It is known that PCPA reduces the serotonin content in the brain up to 10 times in rats (see Methods). We hypothesized that the PCPA could behave the similar way in snails and could reduce the content of serotonin in snails. Therefore, we investigated the effect of PCPA injection on contextual memory reconsolidation using a protein synthesis blocker in snails after training according to two protocols of different intensities. The results obtained in training according to the first protocol using five electrical stimuli per day for 5 days showed that reminding the training environment against the background of injection of PCPA led to a significant decrease in contextual memory. At the same time, the results obtained in training according to the second protocol using three electrical stimuli per day for 5 days showed that reminding the training environment against the injection of PCPA did not result in a significant change in contextual memory. The obtain results allowed us to conclude that the mechanisms of processes developed during the reconsolidation of contextual memory after a reminding depend both on the intensity of learning and on the state of the serotonergic system.

Responses of Withdrawal Interneurons to Serotonin Applications in Naïve and Learned Snails Are Different

Long-term changes in membrane potential after associative training were described previously in identified premotor interneurons for withdrawal of the terrestrial snail Helix. Serotonin was shown to be a major transmitter involved in triggering the long-term changes in mollusks. In the present study we compared the changes in electrophysiological characteristics of identifiable premotor interneurons for withdrawal in response to bath applications of serotonin (5-HT) or serotonin precursor 5-hydroxytryptophan (5-HTP) in preparations from naïve, neurotoxin-injected or associatively trained snails. It was found that 5-HT or 5-HTP applications caused a significant decrease of membrane potential in premotor interneurons of naïve snails, associatively trained snails and snails with impaired serotonergic system by injection of a selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) 1 week before the experiments. Applications of 5-HT or 5-HTP did not cause significant changes in the action potential (AP) threshold potential of these neurons in naïve snails. Conversely, applications of 5-HT or 5-HTP to the premotor interneurons of previously trained or 5,7-DHT-injected snails caused a significant increase in the firing threshold potential in spite of a depolarizing shift of the resting membrane potential. Results demonstrate that responsiveness of premotor interneurons to extracellularly applied 5-HT or 5-HTP changes for days after the associative training or serotonin depletion. Similarity of the effects in trained and 5,7-DHT-injected animals may be due to massive release of serotonin elicited by 5,7-DHT injection. Our results suggest that serotonin release due to aversive conditionining or elicited by the neurotoxin administration triggers similar changes in resting membrane potential and AP threshold in response to bath applications of 5-HT or its precursor 5-HTP.