Modulation of defensive reflex conditioning in snails by serotonin

Abstract

Highlights Daily injection of serotonin before a training session accelerated defensive reflex conditioning in snails.Daily injection of 5-hydroxytryptophan before a training session in snails with a deficiency of serotonin induced by the “neurotoxic” analog of serotonin 5,7-dihydroxytryptamine, restored the ability of snails to learn.After injection of the “neurotoxic” analogs of serotonin 5,6- and 5,7-dihydroxytryptamine as well as serotonin, depolarization of the membrane and decrease of the threshold potential of premotor interneurons was observed.We studied the role of serotonin in the mechanisms of learning in terrestrial snails. To produce a serotonin deficit, the “neurotoxic” analogs of serotonin, 5,6- or 5,7-dihydroxytryptamine (5,6/5,7-DHT) were used. Injection of 5,6/5,7-DHT was found to disrupt defensive reflex conditioning. Within 2 weeks of neurotoxin application, the ability to learn had recovered. Daily injection of serotonin before a training session accelerated defensive reflex conditioning and daily injections of 5-HTP in snails with a deficiency of serotonin induced by 5,7-DHT restored the snail's ability to learn. We discovered that injections of the neurotoxins 5,6/5,7-DHT as well as serotonin, caused a decrease in the resting and threshold potentials of the premotor interneurons LPa3 and RPa3.

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

“…Previously we found that PCPA at doses mentioned above induced the impairment of the formation of defensive conditioning in terrestrial snails as neurotoxins 5,7-DHT and 5,6-DHT [3], which occurred in the condition of the serotonin depletion in the nerve system [1][2][3]7,27,28,77,114,115].…”

“…Serotonin (5-HT) is the main neurotransmitter that plays primary roles in learning based on defensive reflexes in mollusks [1][2][3][4][5][6][7]. Serotonin is one of the most widely distributed and well-researched neurotransmitters of the nervous system.…”

“…It was shown that electrophysiological correlates of plasticity can be reproduced by applying 5-HT applications to the solution surrounding the central nervous system of mollusks [13][14][15][16][17][18][19][20][21]. On the other hand, the neurotoxic analogues of serotonin 5,6-and 5,7-dihydroxytryptamine (5, leading to the depletion of serotonin is widely used to study the role of the serotoninergic system in the formation of behavior and learning [1,7,[22][23][24][25][26][27][28].…”

“…It was shown that the administration of neurotoxins 5,7-DHT and 5,6-DHT led to a disruption in the formation of a conditioned reflex of food aversion [1] and conditioned defensive reflex of pneumostome closure in snails [3,7]. It was found that neurotoxin 5,7-DHT blocked heterosynaptic facilitation in Aplysia [2], impaired the formation of the long-term sensitization in snail [3,32,33].…”

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

“…In addition, we carried out studies of the changes of the excitability of the premotor interneurons in response to applications of 5-HT or its precursor 5-hydroxytryptophan (5-HTP) in preparations from snails with an impaired 5-HT system by injection of the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Previously it was shown that snails with impaired serotonergic system cannot acquire aversive memory (Balaban et al, 1987 ; Glanzman et al, 1989 ; Gainutdinov et al, 1999 ; Andrianov et al, 2015 ).…”