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Dual effects of proctolin on the rhythmic burst activity of the cardiac ganglion.

The neuropeptide proctolin has distinguishable excitatory effects upon premotor cells and motorneurons of Homarus cardiac ganglion. Proctolin's excitation of the small, premotor, posterior cells is rapid in onset (5-10 s) and readily reversible (less than 3 min). Prolonged bursts in small cells often produce a "doublet" ganglionic burst mode via interactions with large motorneuron burst-generating driver potentials. In contrast to small cell response, proctolin's direct excitatory effects upon motorneuron are slow in onset (60-90 s to peak) and long-lasting (10-20 min). The latter include: a concentration-dependent (10(-9)-10(-7)M) depolarization of the somatic membrane potential; increases in burst frequency and enhancement of the rate of depolarization of the interburst pacemaker potential. Experiments on isolated large cells indicate: the slow depolarization is produced by a decrease in the resting GK and proctolin can produce or enhance motorneuron autorhythmicity . A two- tiered non-hierarchical network model is proposed. The differential pharmacodynamics exhibited by the two cell types accounts for the sequential modes of ganglionic burst activity produced by proctolin.