Intracellular Injection of Guanyl Nucleotides Alters the Serotonin- induced Increase in Potassium Conductance in Aplysia Neuron

The effects of the adenylate cyclase inhibitor GDPßS on the response of Aplysia neuron R15 to serotonin (51-IT) were investigated. Previous studies have demonstrated that 5HT causes an increase in K+ conductance in R15 and that the response is mediated by cAMP. At concentrations in the micromolar range, GDPßS inhibits the stimulation ofadenylate cyclase by 5HT in particulate fractions from Aplysia ganglia. When micromolar concentrations of GDPßS are injected into neuron R15, there is no effect on the resting membrane conductance, but the increase in K+ conductance normally elicited by 5HT is completely inhibited. Furthermore, the decrease in inward current normally elicited by dopamine (DA), which does not appear to involve CAMP, is not affected by micromolar concentrations of GDPßS. In addition, application of 8-benzylthio CAMP to R15 can evoke an increase in K+ conductance even after the injection of GDPßS, which indicates that events subsequent to the activation of adenylate cyclase are not inhibited by the GDP analogue. In contrast, when millimolar concentrations of GDPßS are injected into R15, direct effects on membrane conductance are observed and the response of R15 to 5HT is enhanced . Although these effects of high concentrations of GDPßS are only poorly understood, the results with micromolar concentrations are consistent with the hypothesis that stimulation of adenylate cyclase is necessary for the 5HT-induced increase in K' conductance in neuron R15.