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Journal of Experimental Biology 129,347-364 (1987)
Published by Company of Biologists 1987

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

JOHATHAN M. BLAGBURN ¹ and DAVID B. SATTELLE ¹

¹ AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK

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.

Key words: calcium spike, calcium-activated potassium conductance, synaptic terminal, identified, insect neurone

Accepted on January 20, 1987