INTERACTION OF CONVERGENT PATHWAYS THAT INHIBIT N-TYPE CALCIUM CURRENTS IN SENSORY NEURONS

Norepinephrine and GABA inhibit omega-conotoxin GVIA-sensitive (N-type ) calcium current in embryonic sensory neurons by separate pathways. W e have investigated the mechanisms that limit the modulation of N curr ent by varying the level of activation for a single pathway or simulta neously activating multiple pathways. Calcium currents were measured w ith tight-seal, whole-cell recording methods. Simultaneous application of the two transmitters at saturating concentrations produced a large r inhibition of the current than either transmitter by itself, but the maximal inhibition was not linearly additive. Maximal, direct activat ion of GTP-binding proteins by intracellular application of guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S) resulted in a similar limit t o the inhibition; furthermore, GTP gamma S did not enhance the maximal inhibition produced by co-application of transmitters. Interventions downstream in the modulatory pathway (e.g. direct activation of protei n kinase C or inhibition of protein phosphatases) were also unable to alter the maximal limit for inhibition. These results suggest that tra nsmitter-mediated inhibition is not limited by receptor number, levels of G-protein or protein kinase C activation, or degree of phosphoryla tion; rather, the extent of inhibition may be limited by the structura l properties of the N channels themselves.