Rhythmic electrical stimulation of the snail foot leads to sensitization of the defensive reflex. This sensitization has dynamics similar to those of posttetanic potentiation of the amplitude of the acetylcholine-evoked influx current of defensive behavior command neurons in the common snail. It is likely that an increase in the cholinosensitivity of the somatic membrane of defensive behavior command neurons in the common snail may be involved in the mechanism of sensitization of the animal's defensive response. Methiothepin, an antagonist of serotonin receptors, prevented the posttetanic potentiation of the acetylcholine-evoked influx current as well as behavioral sensitization. Serotonin, like methiothepin, also impaired posttetanic potentiation of the acetylcholine-evoked influx current. It is suggested that methiothepin-sensitive serotonin receptors are involved in the postsynaptic mechanism of behavioral sensitization.

The effects of the Na,K pump inhibitor ouabain on habituation of the common snail to tactile stimulation were identical to the ouabain-induced modification of the decrease in the cholinosensitivity of defensive behavior command neurons in the common snail in a cellular model of habituation. Studies addressed the effects of intracellularly delivered ligands of two types of Ca2+ depot receptors--inositol triphosphate (IP3) receptors and ryanodine receptors--on the action of ouabain in the cellular analog of habituation. The IP3 receptor antagonist heparin (0.1 mM), the IP3 receptor agonist inositol triphosphate (0.1 mM), and the ryanodine-dependent Ca2+ mobilization inhibitor dantrolene (0.1 mM) prevented ouabain from modifying the depression of the evoked acetylcholine current. The ryanodine agonist/antagonist ryanodine was used at two concentrations (0.1 and 1 mM) and neither had any effect on the action of ouabain. It is concluded that Ca2+ mobilized from intracellular Ca2+ depots via IP3 receptors is involved in the neuronal mechanism of regulation of the habitation of the common snail to tactile stimulation by the Na,K pump.

Local application of serotonin (5-hydroxytryptamine, 5-HT) to the soma of command neurons LPa2, LPa3, PPa3, and PPa2 of edible snail Helix lucorum reversibly decreased acetylcholine-induced inward current in these neurons. NAN-190 and methiothepin, antagonists of 5-HT1 serotonin receptors, prevented this modulatory effect of 5-HT. By contrast, LY-53.857, ICS-205.930, and SDZ-205.557, antagonists of 5-HT2, 5-HT3, and 5-HT4 serotonin receptors, respectively, produced no effect on the modulatory effect of serotonin. The data confirm the presence of modulator 5-HT1 serotonin receptor on the soma of command neurons of Helix pomatia.