CALCIUM CONDUCTANCE IN AN IDENTIFIED CHOLINERGIC SYNAPTIC TERMINAL IN THE CENTRAL NERVOUS SYSTEM OF THE COCKROACH

Intracellular microelectrodes were used to study a cholinergic synapse between two identified neurones: the lateral filiform hair sensory neurone (LFHSN) and giant interneurone 3 (GI 3) in the terminal ganglion of the first-instar cockroach Periplaneta americana. The presynaptic cell, LFHSN, was impaled in a region of the axon which forms large numbers of output synapses. The sign and magnitude of the LFHSN spike afterpotential were shown to depend on [Ca2+]o. ljumoir1 tetrodotoxin (TTX) abolished LFHSN spikes but the addition of 0-1 mmoir 1 4-aminopyridine (4-AP) enabled regenerative depolariz-ations to be evoked which were followed by large EPSPs in GI3. Addition of 20mmoir1 tetraethylammonium ions (TEA+) abolished the cholinergic EPSPs but resulted in long-duration LFHSN spikes. Intracellular injection of caesium ions (Cs+) into LFHSN enabled long-duration spikes to be evoked and had no effect on synaptic transmission. Long-duration LFHSN spikes were (1) increased in ampli-tude by increased [Caz+]o; (2) accompanied by an increase in conductance; (3) not abolished by replacement of external Na+ with Tris+ or choline+; (4) blocked b