Interactions between three slow potassium responses controlled by three distinct receptors in Aplysia neurones

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Abstract
A voltage clamp study was made of the K+ permeability increases produced in certain identifiable neurons of the cerebral ganglion of Aplysia by any 1 of 3 distinct agonists (carbachol, histamine and dopamine). The 3 K+ responses involve 3 distinct receptors, as shown by the selective effects of reversible antagonists and by the differential, irreversible effects of trimethyloxonium (TMO) ions. Prolonged exposure of the neurons to one of the agonists reduces the response to the same agonist (desensitization) and, over the low concentration range, doubling the concentration of an agonist leads to supra-linear summation (potentiation). Prolonged exposure of the neurons to one of the agonists also reduces the response to the other agonists (cross-desensitization) and combined application of 2 agonists reveals cross-potentiation. The time course of desensitization (onset and decay) was the same for the histamine and carbachol responses and, except at very high concentration, was indistinguishable from that of cross-desensitization. Potentiation was similar in the 2 agonist systems and did not differ significantly from cross-potentiation. The results can be interpreted by assuming that the responses to the 3 agonists involve specific steps followed by common reaction steps, and that some of the common reaction steps control potentiation and desensitization. The responses to carbachol and histamine differ in their voltage sensitivity. This suggests that 1 or more of the specific steps are voltage-sensitive. Although an increase of the intracellular Ca2+ concentration can itself open K+ channels, and also inhibit the responses to the 3 agonists, an increase of internal Ca2+ does not appear to play an important role either in the development of the response or in the desensitization process.