INVIVO DEVELOPMENT OF VOLTAGE-DEPENDENT IONIC CURRENTS IN EMBRYONIC XENOPUS SPINAL NEURONS

Initial evidence that electrical excitability is both an early aspect of neuronal differentiation and a developmentally regulated property w as obtained from recordings of action potentials in vivo. Subsequently , the analysis of the underlying voltage-dependent currents during ear ly stages of embryogenesis was facilitated by investigation of dissoci ated neurons and muscle cells differentiating in culture. Calcium and potassium currents play a major role in the differentiation of the act ion potential of Xenopus spinal neurons, and calcium influx triggers s pecific features of neuronal differentiation. However, the extent to w hich differentiation of currents in vitro parallels that in vivo is un certain. We have undertaken a study of in vivo differentiation of thes e macroscopic currents in Xenopus embryos. Spinal cords were isolated from embryos at several early stages of neurogenesis. Neurons in these isolated spinal cords were accessible to patch-clamp electrodes. Neur onal currents were recorded within 1 hr to assure that the characteris tics of the currents resulted from developmental events occurring in v ivo prior to the experiment. Whole-cell voltage-clamp recordings from neurons in these acutely isolated and intact embryonic spinal cords de monstrate that both the delayed-rectifier and inactivating potassium c urrent and a low-voltage-activated calcium current mature in a manner closely parallel to that observed in culture. The results validate tho se from the culture system and indicate that the spinal cord is anothe r region of the CNS accessible to cellular analysis in an intact prepa ration.