Synaptic block of a calcium-activated potassium conductance in Aplysia neurones.

In the preceding paper (Kehoe, 1985) it was shown that the firing of any one of three neurones (I, II, III) presynaptic to the medial cells of the pleural ganglion of Aplysia californica causes a diminution of the cholinergically controlled K conductance in those cells. Firing of the same three presynaptic neurones was shown here to cause a similar diminution in a depolarization-induced K-dependent conductance in the same post-synaptic cells. The depolarization-induced K conductance was found to disappear when Ca ions were removed from the sea water bathing the ganglion or when the cell was injected with the Ca chelator ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetra-acetic acid (EGTA). The diminution in this Ca-activated, K-dependent current occurred even when the presynaptic neurone was fired a few seconds after the end of the depolarizing voltage step to the post-synaptic neurone, showing that the diminution in K conductance was not an indirect effect of a transmitter-induced diminution in Ca influx during the depolarizing pulse. The two K conductances affected by the 'blocking neurones' could be selectively eliminated. The cholinergic conductance could be blocked by receptor-specific cholinergic antagonists (e.g. 1 mM concentrations of phenyltrimethylammonium (PTMA), choline and tetraethylammonium (TEA]. Even at 10 mM concentrations, none of these compounds (including TEA, which is known to block certain Ca-activated K conductances) had an effect on the depolarization-induced, Ca-activated K conductance studied here. This latter conductance, on the other hand, was selectively blocked by an intracellular injection of EGTA. The three blocking neurones continued to diminish the K conductance (cholinergic or depolarization induced) that remained intact under these different experimental conditions. The depolarization-induced influx of Ca was shown to block the cholinergically controlled K conductance, but Ca was excluded as the possible mediator of the diminution in K conductance caused by the three blocking neurones. An intracellular injection of Ca ions into the medial cells was shown to activate a variety of changes in membrane conductance; in particular, two K-conductance increases: an early, TEA-sensitive one, and a slowly developing, TEA-insensitive one. Both the permeant cyclic AMP analogue p-chlorophenylthioadenosine 3',5'-monophosphate (CPT-cyclic AMP) and the phosphodiesterase inhibitors amino-phylline and isobutyl-1-methylxanthine (IBMX) were shown to block the depolarization-induced K conductance, and to reduce, though not eliminate, the slowly developing K conductance activated by an intracellular injection of Ca.(ABSTRACT TRUNCATED AT 400 WORDS)