Enkephalin reduces calcium action potentials in Rohon-Beard neurons in vivo

Rohon-Beard neurons of the Xenopus spinal cord develop somatic Ca++-dependent action potentials at early embryonic stages. Because many Ca++-dependent events are blocked or reduced by enkephalin in other neuronal systems, we tested the effect of met-enkephalin on the Ca++ spikes of Rohon-Beard cells. Our results indicate that met-enkephalin, which has no effect on the resting membrane potential or conductance in these cells, reversibly and specifically shortens their Ca++ action potentials, probably by decreasing the Ca++ conductance. Enkephalin has a similar effect on Xenopus dorsal root ganglion cells in vivo. Examination of Rohon-Beard cells at various stages of development shows that enkephalin shortens Ca++ spikes from the earliest times that the spikes appear (approximately 21 hr after fertilization) to the latest times they are detectable (7 to 9 days). This very early response suggests either that enkephalin binds directly to the Ca++ channels or that an "enkephalin receptor" is among the earliest neuronal phenotypes to appear in these cells.