Pars intermedia electrical potentials: changes in spike frequency induced by regulatory factors of melanocyte stimulating hormone (MSH) secretion.

Spontaneous action potentials recorded from pars intermedia (PI) cells of frog pituitaries are correlated with spontaneous melanophore stimulating hormone (MSH) secretion from that gland. Those agents known to induce hormone release (e.g., serotonin, isoproterenol) increase the rate of electrical activity, whereas those which inhibit secretion (e.g., norepinephrine, dopamine, GABA) reduce the frequency of spikes. MSH secretion appears to involve cellular depolarization while inhibition of release is associated with cell membrane polarization.