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In this paper, we present a study of the effects of reducing serotonin levels by para-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, the limiting enzyme in serotonin biosynthesis, on two forms of behavioral plasticity in the terrestrial snail, Helix lucorum: the conditioned defensive reflex food aversion and long-term sensitization. We found that the snails injected with PCPA did not learn the conditioned reflex food aversion, while saline solution injection did not affect the learning process. The latency of consuming one kind of food in these snails was significantly reduced compared to the active control. We also found that PCPA injection prevents the formation of long-term sensitization.

It was previously shown that both associative learning and the formation of long-term sensitization led to the increase in excitability of premotor interneurons of the defensive behavior of terrestrial snail Helix lucorum. In the present study, we analyzed the role of intracellular calcium ions in the maintenance of increased excitability in premotor interneurons of terrestrial snail after the formation of a conditioned defensive reflex. It was shown that the increase of the intracellular Ca2+ concentration after adding caffeine to the solution washing the nervous system of the mollusk led to a decrease of the threshold of action potential and to an increase of the critical level of depolarization without a change of the membrane potential of premotor interneurons in both intact and trained snails. The decrease of the intracellular Ca2+ concentration in premotor interneurons by the intracellular injection of (ethylene glycol-bis (2-aminoethylether)-N, N, N, N-tetraacetic acid) (EGTA) resulted in a significant increase of the threshold of generation of the action potential in intact snails. But the values of threshold of generation of the action potential in trained snails after injection of EGTA did not significantly differ from the values of studied parameters before injection. After application of the membrane-penetrating chelator, BAPTA-AM, the changes in the membrane and threshold potentials of premotor interneurons of intact and trained snails were not observed. Our results demonstrated that both the increase and decrease of intracellular Ca2+ concentration were not involved in maintaining the changes of membrane characteristics of premotor interneurons observed after associative learning.

It is shown that after the elaboration of a conditioned reflex in snails, a reliable decrease can be observed in the membrane potential (Vm) of the premotor interneurons at 4 mV, daily injection of serotonin (5-HT) causes a decrease in Vm at 4.5 mV, the same change is observed for Vm in the snails trained after the injection of 5-HT. A single injection of 5-HT causes a depolarization shift of Vm at 5 mV. After the initial stage of training (10–12 pairs) the snails, injected by 5-HT, there is a depolarization at 4.5 mV.

It has been shown that after 5 h of spinal cord injury, there is a decrease of nitric oxide (NO) production in the spinal cord. Seventy-two hours after the spinal cord injury, the level of NO production in the spinal cord and the heart increases by 2.5 times, and in the liver, it increases threefold. In the chronic period of a traumatic spinal cord disease in the spinal cord tissue, the level of NO production was significantly higher than at the control level.

Experimentally, it was shown that the action potential threshold in neurons of the trained snails increases in response to serotonin (5-HT) application, in contrast to intact snails. This result demonstrates a decrease of excitability of the premotor interneurons in response to extracellular 5-HT after the training.

It has been found that application of a nitric oxide donor—the sodium nitroprusside—causes the hyperpolarization shift of the membrane potential of the premotor interneurons of defensive behavior of terrestrial snails. It is assumed that the response of a neuron to NO depends on its location in the neural network.