Analysis of the galanin-induced decrease in membrane excitability in mudpuppy parasympathetic neurons

Previously, we showed that the neuropeptide galanin hyperpolarizes and decreases membrane excitability of mudpuppy parasympathetic neurons [Konopka L. M., McKeon T. W. and Parsons R. L. (1989) J. Physiol. 410, 107-122]. We also demonstrated that membrane excitability remains depressed when the agonist-induced potential change is negated electrotonically. We hypothesized that galanin inhibits the membrane conductances associated with spike generation. However, we cannot rule out the possibility that the decreased excitability is due to a galanin-induced increase in membrane potassium conductance which reduces the effectiveness of subsequent depolarizing stimuli. Therefore, in the present study we tested, with the galanin-induced hyperpolarization negated, whether the galanin-induced increased membrane potassium conductance was responsible for the decreased excitability. The results showed that the galanin-induced decreased excitability was not dependent on the peak amplitude of the galanin-induced hyperpolarization. Furthermore, the decreased excitability occurred in cells in which there was no measurable galanin-induced hyperpolarization. Moreover, in most cells the galanin-induced decrease in input resistance, measured at the peak of the hyperpolarization (3-25 mV), was less than 15% and when the hyperpolarization was negated electronically, the decrease was even less (approximately 2%). These results indicated that when the hyperpolarization was negated, the galanin-induced increase in potassium conductance was not responsible for the decreased excitability. In preparations pretreated with 5 mM tetraethylammonium, galanin decreased excitability which indicated that a galanin-induced decrease in the calcium-dependent potassium current was not necessary for the decreased excitability. Galanin also decreased excitability in preparations exposed to either 1-3 microM tetrodotoxin or 100-200 microM cadmium. Following galanin application, the threshold for initiation of tetrodotoxin-insensitive spikes was shifted to more positive membrane potentials. Galanin also decreased the amplitude and hyperpolarizing afterpotential of barium spikes in the absence of any agonist-induced hyperpolarization. These observations confirmed that galanin decreased the voltage-dependent calcium conductance. In the present study, we showed that when the hyperpolarization was negated, galanin decreased excitability by shifting the threshold for spike generation regardless of whether voltage-dependent sodium or calcium currents were primarily responsible for the depolarizing component of the action potential.