J. exp. Biol. 129, 347-364 (1987) 347 Printed in Great Britain © The Company of Biologists Limited 1987 CALCIUM CONDUCTANCE IN AN IDENTIFIED CHOLINERGIC SYNAPTIC TERMINAL IN THE CENTRAL NERVOUS SYSTEM OF THE COCKROACH BY JONATHAN M. BLAGBURN AND DAVID B. SATTELLE AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK Accepted 20 January 1987 SUMMARY 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 [Ca 2+ ] o . ljumoir 1 tetrodotoxin (TTX) abolished LFHSN spikes but the addition of 0-1 m m o i r 1 4-aminopyridine (4-AP) enabled regenerative depolariz- ations to be evoked which were followed by large EPSPs in GI3. Addition of 20mmoir 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 ampli- tude by increased [Ca z+ ] o ; (2) accompanied by an increase in conductance; (3) not abolished by replacement of external Na + with Tris + or choline + ; (4) blocked by 1 mmoir 1 Cd 2+ and lOmmolT 1 Co 2+ ; (5) not supported by substitution of Mg 2+ for Ca 2+ ; and (6) supported by Ba z+ substitution. They are thus considered to be Ca z+ spikes. The Ca 2+ spikes were blocked by organic Ca 2+ channel blockers at 0-5-1 mmoir 1 . The putative Ca 2+ spike was followed by a hyperpolarizing afterpotential (HAP), the duration of which was proportional to the amplitude and duration of the Ca 2+ 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 Ba 2+ for Ca 2+ ; and (4) partially blocked by 150mmoir 1 TEA + . The HAP is considered to result from an increase in Ca 2+ -dependent K + conductance. It is concluded that, in addition to Na + channels and delayed rectifying K + channels, Ca 2+ channels and Ca 2+ -dependent K + channels are present in the axonal membrane of LFHSN, in a region which forms many output synapses. Key words: calcium spike, calcium-activated potassium conductance, synaptic terminal, identified insect neurone.