Effects of dopamine on the Aplysia burster R15

The inhibitory response to dopamine (DA) of the Aplysia burster R15 has been attributed previously either to a K+ conductance increase or to a decrease of the slow inward current (SIC) which is responsible for the bursting activity of R15. The present study reexamines the main points which are at the origin of this controversy. It is shown that the response to short applications of DA is actually sensitive to the extracellular K+ concentration and that this response may always be inverted by hyperpolarization of the cell below EK (at least in a high-K+ solution). The results suggest that short applications of DA induce an increase in K+ conductance rather than a blockade of the SIC. This conclusion is strengthened by the effects of intracellular Cs+ injections and by Ca2+-removal experiments. Prolonged DA perfusions may occasionally induce a second slow response in addition to the initial K+ conductance increase. This slow component of the DA response, which may imply modifications of the synaptic activity, does not seem to result either from a K+ conductance increase or from a blockade of the SIC.