Studies on bursting pacemaker potential activity in molluscan neurons. I. Membrane properties and ionic contributions

Bursting pacemaker potential (BPP) activity of identified molluscan neurons has been studied using cells from Aplysia and Otala. The results presented in this paper indicate that (1) a potassium conductance mediates the hyperpolarizing phase of the BPP; (2) the BPP amplitude is directly dependent on [Na+]0; (3) BPP activity requires the presence of divalent cations and is prevented by C02+ and La3+, but not D-600; (4) the apparent increase in membrane resistance during the depolarizing phase of the Bd can be accounted for by the movement of the membrane potential along the non-linear portion of the I-V curve; and (5) non-linear I-V relations and a minimal effective membrane resistance are pre-requisite to BPP generation. Coupled with recent observations on the presence of an inward current in these cells, the results suggest that the mechanisms underlying the BPP are similar to those proposed to describe the myocardial pacemaker potential: the hyperpolarizing phase is due to activation of a potassium conductance which slowly inactivates, resulting in a gradula deplorization until a voltage-dependent inward current is activated which then leads to an increasingly rapid deplorization and initiation of the burst of spikes. It would appear that Na+ may play the major role in carrying the inward current, although a secondary role for divalent cations cannot be discounted.