Calcium Conductance in An Identified Cholinergic Synaptic Terminal in the Central Nervous System of the Cockroach

Abstract

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. l μmoll−1 tetrodotoxin (TTX) abolished LFHSN spikes but the addition of 0.1 mmoll−1 4-aminopyridine (4-AP) enabled regenerative depolarizations to be evoked which were followed by large EPSPs in GI3. Addition of 20mmol−1 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 amplitude by increased [Ca2+]o; (2) accompanied by an increase in conductance; (3) not abolished by replacement of external Na+ with Tris+ or choline+; (4) blocked by 1 mmoll−1 Cd2+ and 10 mmoll−1 Co2+; (5) not supported by substitution of Mg2+ for Ca2+; and (6) supported by Ba2+ substitution. They are thus considered to be Ca2+ spikes. The Ca2+ spikes were blocked by organic Ca2+ channel blockers at 0.5-1 mmoll−1. The putative Ca2+ spike was followed by a hyperpolarizing afterpotential (HAP), the duration of which was proportional to the amplitude and duration of the Ca2+ spike. The HAP was (1) accompanied by a conductance increase; (2) reversed at potentials 30mV more negative than resting potential; (3) not supported by substituting Ba2+ for Ca2+; and (4) partially blocked by 150 mmoll−1 TEA+. The HAP is considered to result from an increase in Ca2+-dependent K+ conductance. It is concluded that, in addition to Na+ channels and delayed rectifying K+ channels, Ca2+ channels and Ca2+-dependent K+ channels are present in the axonal membrane of LFHSN, in a region which forms many output synapses.