Modulatory effects of myomodulin on the excitability and membrane currentsin retzius cells of the leech

Modulatory effects of myomodulin on the excitability and membrane currents in Retzius cells of the leech. J. Neurophysiol. 82: 216-225, 1999. Ion chan nel modulation by the peptide myomodulin (MM) has been demonstrated in a wi de variety of organisms including Aplysia, Lymnaea, and Pleurobranchaea. Th is neural and muscular modulation has been shown to be important for shapin g and modifying behavior. In this paper, we report that MM modulates severa l distinct ionic channels in another species, the medicinal leech Hirudo me dicinalis. Experiments have focused on the Retzius cell (R) because the R c ell is a multifunction neuron that has been implicated in a number of behav iors including feeding, swimming, secretion, thermal sensing, and the touch elicited shortening reflex and its plasticity. Previous work had identifie d a MM-like peptide in the leech and demonstrated that this peptide modulat ed the excitability of the R cell. Using combined current- and voltage-clam p techniques to examine the effects of MM on the R cell, we found that in r esponse to a step pulse, MM increased the excitability of the R cell such t hat the cell fires more action potentials with a shorter latency to the fir st action potential. We found that this effect was mediated by the activati on of a Na+-mediated inward current near the cell resting membrane potentia l. Second, we found that MM differentially modulated the potassium currents I, and I,. No effect of MM was found on I,, whereas MM significantly reduc ed both the peak and steady-state amplitudes of I, by 49 +/- 2.9% and 43 +/ - 7.2%, respectively (means +/- SE). Finally we found that MM reduced the a mplitude of the Ca2+ current by similar to 20%. The ionic currents modulate d by MM are consistent with the overall effect of MM on the cellular activi ty of the R cell. An understandings of the cellular mechanisms by which MM modulates the activity of the R cell should help us to better understand th e roles of both MM and the R cell in a variety of behaviors in the leech.