The role of membrane properties, synaptic input and electrical junction in determination of spike output of a pair of peptidergic neurons in the mollusc Lymnaea stagnalis.

The two electrotonically coupled peptidergic neurons, VD1 and RPD2 show in the isolated central nervous system (CNS) a patterned activity. The cells fire in almost perfect synchrony in CNS's derived from animals of moderate age, while in old animals disturbances in synchrony are observed. The firing pattern varies from beating to bursting. Isolated VD1's show a beating firing pattern, indicating that the cell possesses pacemaker properties, while the isolated RPD2 is almost always silent. Hybrid current/voltage clamp experiments show that in the intact CNS spike generation in the two cells is due to VD1, indicating that the pacemaker properties of VD1 constitute the main driving force for the firing rhythm of the two cell system. In low calcium/high magnesium solutions the rhythm is a beating one, while in solutions which do not suppress synaptic input, sometimes a bursting pattern is observed. The idea that the bursting pattern is due to synaptic modulation of these cells is corroborated by the observation that application of certain transmitters induces a very prominent burst of activity in these neurons. These bursts are very reminiscent of the bursts, observed in semi-intact preparations, which are accompanied by penumostome movements. Increase in coupling resistance, which occurs at older age, is accompanied by irregularities in firing rhythm. Apparently the electrotonic junction is essential for the patterned output of the two cell system.