[Divergent effect of acetylcholine on processes of habituation and disinhibition]

The elaboration of habituation to repeated tactile stimuli was studied in snail (Helix pomatia) by means of intracellular recording of the neuronal activity and microionophoresis of acetylcholine (ACh). Light or a stronger tactile stimulation served as extrastimulus. The elaboration of habituation was accompanied by a rise in the threshold of action potentials (APs) generation, evoked by the habitual stimulus, and by a lowering of the threshold of APs elicited by the extrastimulus. Prior to the habituation neuronal responses to tactile or photic stimuli did not change against the ACh background. After habituation was elaborated the ACh administration led to disinhibition of responses to the habitual stimulus (in particular, the AP threshold became lower), and to a decrease in responses to the extrastimulus (AP threshold rose). It is assumed that hibituation changes the chemical responses caused in the neurone by the activation of both the pool of synapses, corresponding to the habitual stimulus, and the pool of synapses corresponding to the extrastimulus. These changes, however, are of a qualitatively different character.