NEUROPEPTIDE GALANIN INHIBITS OMEGA-CONOTOXIN GVIA-SENSITIVE CALCIUM CHANNELS IN PARASYMPATHETIC NEURONS

1. We determined the effect of the neuropeptide galanin on barium currents (I-Ba) flowing through voltage-gated calcium channels. We voltage clamped parasympathetic neurons dissociated from mudpuppy cardiac ganglia using both the standard whole cell and the perforated-patch variations of the patch-clamp technique. 2. Galanin produced a concentration-dependent inhibition of I-Ba. The maximal inhibition was 50-60% and the concentration that produced half-maximal inhibition (IC50) was 0.42 nM. In mud-puppy parasympathetic neurons, omega-conotoxin-GVIA (CTX)-sensitive channels are the predominant type of calcium channels, and only a small portion of I-Ba is contributed by dihydropyridine-sensitive channels. Galanin preferentially inhibited a portion of the CTX-sensitive current. 3. In currents recorded with the standard whole cell technique, activation of I-Ba was slowed in the presence of galanin. In contrast, in the majority of neurons studied with the perforated-patch technique, galanin decreased I-Ba without altering the kinetics of current activation. With both recording methods, the decrease in I-Ba was greatest with voltage steps to 0 mV and persisted with steps to +50 mV. For control currents, large depolarizing voltage steps (+70 to +120 mV) did not markedly facilitate I-Ba when either recording technique was used. However, the degree of facilitation in galanin was significantly greater with the standard whole cell recording technique. 4. I-Ba exhibited inactivation under the conditions of these experiments. Inactivation of I-Ba recorded during a 900-ms depolarizing voltage step was fitted to a double exponential. Galanin decreased the amplitude of I-Ba but did not alter the time constants of inactivation. The voltage dependence of steady-state inactivation was not significantly affected by galanin. 5. Galanin decreased the amplitude of tail currents measured at -40 mV after positive voltage steps (recorded with the standard whole cell technique). The percent decrease by galanin was greater with steps to +10 mV (45 +/- 6%, mean +/- SE), but was still substantial after steps to the more positive potentials (36 +/- 5%). 6. We conclude that in mudpuppy neurons, galanin primarily inhibited N-type calcium channels and the inhibition involved both voltage-dependent and voltage-independent mechanisms. We suggest that the characteristics of neurotransmitter-induced inhibition of calcium channels may be affected by the recording technique used.