Multiple peptides converge to activate the same voltage-dependent current in a central pattern-generating circuit

The stomatogastric ganglion of the crab, Cancer borealis, is modulated by >20 different substances, including numerous neuropeptides. One of these peptides, proctolin, activates an inward current that shows strong outward rectification (Golowasch and Marder, 1992). Decreasing the extracellular Ca(2+) concentration linearizes the current-voltage curve of the proctolin-induced current. We used voltage clamp to study the currents evoked by proctolin and five additional modulators [C. borealis tachykinin-related peptide Ia (CabTRP Ia), crustacean cardioactive peptide, red pigment-concentrating hormone, TNRNFLRFamide, and the muscarinic agonist pilocarpine] in stomatogastric ganglion neurons, both in the intact ganglion and in dissociated cell culture. Subtraction currents yielded proctolin-like current-voltage relationships for all six substances, and the current-voltage curves of all six substances showed linearization in low external Ca(2+). The lateral pyloric neuron responded to all six modulators, but the ventricular dilator neuron only responded to a subset of them. Bath application of saturating concentrations of proctolin occluded the response to CabTRP and vice versa. N-(6-Aminohexyl)-5-chloro-1-napthalensulfonamide, a calmodulin inhibitor, increased the amplitude and altered the voltage dependence of the responses elicited by CabTRP and proctolin. Together, these data indicate that all six substances converge onto the same voltage-dependent current, although they activate different receptors. Therefore, differential network responses evoked by these substances may primarily depend on the receptor distribution on network neurons.