Voltage clamp studies of ionic currents in identified snail neurones.

This thesis describes work on two components of the ionic current in neurones of the snail Helix aspersa under voltage clamp conditions. Most of the work is a study of the inward calcium current (ICa), produced by depolarization, under conditions where the potassium (K) permeability was reduced by the K channel blockers tetraethylammonium (TEA) ions and 4-aminopyridine (4-AP). Under these conditions Ica activated then inactivated more slowly to a level which was still inward at the end of long depolarizations to potentials +20mV. Above this potential Ica was masked by a TEA and 4-AP resistant component of the outward current. The activation of the inward current, over a limited voltage range, was fitted in Hodgkin-Huxley terms with m2. Particular attention was paid to the mechanism of Ica decline, for which a number of possibilities were considered including; voltage- and calcium-dependent inactivation, depletion or accumulation of the permeant ion and outward current activation. Evidence from measurements of Ica decline, and 2 pulse experiments to measure the voltage-dep-endence of inactivation suggested that decline was dependent on Ca-entry and was unlikely to be caused by activation of a Ca-dependent K current. Injection of the Ca chelator EGTA caused an increase in the inward current and also slowed decline. Ca/EGTA buffers could either increase or decrease the peak inward current depending on the free [Ca] in the buffer, supporting evidence of the existence of a Ca binding site sensitive to [Ca]1. Experiments on the early outward current (IA) suggested that this current is not Ca-activated, in this tissue, and that 4-AP blocks the current in a time-dependent manner, from the inside of the cell, on depolarization