Generalized post-tetanic changes in excitatory postsynaptic and acetylcholine-evoked currents in neurons in the common snail.

Changes in excitatory postsynaptic currents (EPSC) and influx currents induced by local iontophoretic application of acetylcholine (ACh currents) to the surfaces of the bodies of identified neurons LPa3 and RPa3 in the common snail after heteroreceptor tetanic stimulation were studied. The effects of rhythmic application of acetylcholine (0.5-1.0 times per sec for 10-40 sec) on the magnitude of EPSC were analyzed, along with the effects of tetanic stimulation of the nerves (n. intestinalis, n. pallialis dexter, n. pallialis sinister; 1-5 Hz for 1-2 min) on the amplitudes of the ACh current and the magnitudes of EPSC evoked by stimulation of another nerve. The following changes in conditioning responses were found over 1-1.5 h after tetanization: rhythmic application of ACh to the dorsal surface of the cell body resulted in potentiation of EPSC, and tetanic orthodromic stimulation potentiated the ACh current and induced heterosynaptic depression of EPSC. These post-tetanic changes were not local--they developed in those chemoreceptor zones of the membrane which were not activated by the preceding conditioning rhythmic stimulation. It is concluded that generalized post-tetanic changes in neuron responses are induced by activation of its subsynaptic and extrasynaptic mediator chemoreceptors.