MODULATION OF FORCE DURING LOCOMOTION - DIFFERENTIAL ACTION OF CRUSTACEAN CARDIOACTIVE PEPTIDE ON POWER-STROKE AND RETURN-STROKE MOTOR-NEURONS

Crustacean cardioactive peptide (CCAP) elicited expression of the moto r pattern that drives coordinated swimmeret beating in crayfish and mo dulated this pattern in a dose-dependent manner. In each ganglion that innervates swimmerets, neurons with CCAP-like immunoreactivity sent p rocesses to the lateral neuropils, which contain branches of swimmeret motor neurons and the local pattern-generating circuits. CCAP affecte d each of the four functional groups of motor neurons, power-stroke ex citers (PSE), return-stroke exciters (RSE), power-stroke inhibitors (P SI), and return-stroke inhibitors (RSI), that innervate each swimmeret . When CCAP was superfused, the membrane potentials of these neurons b egan to oscillate periodically about their mean potentials. The mean p otentials of PSE and RSI neurons depolarized, and some of these neuron s began to fire during each depolarization. Both intensity and duratio ns of PSE bursts increased significantly. The mean potentials of RSE a nd PSI neurons hyperpolarized, and these neurons were less likely to f ire during each depolarization. When CCAP was superfused in a low Ca2 saline that blocked chemical transmission, these changes in mean pote ntial persisted, but the periodic oscillations disappeared, These resu lts are evidence that CCAP acts at two levels: activation of local pre motor circuits and direct modulation of swimmeret motor neurons. The a ction on motor neurons is differential; PSEs and RSIs are excited, but RSEs and PSIs are inhibited. The consequences of this selectivity are to increase intensity of bursts of impulses that excite power-stroke muscles.