Abstract
The electrical membrane properties (EMP) of adult mouse dorsal root ganglion (DRG) neurons were characterized by an extensive electrophysiological investigation of 450 cells. The neurons were divided into two types: an Mโtype having an action potential with monophasic falling phase and a Bโtype with a more complex biphasic or triphasic falling phase. Compared to Mโtype, Bโtype were โslowโ neurons with a higher specific membrane resistance (R~m~), and a longer time constant (ฯ), duration of action potential (ฮ__t__), and absolute refractory period (ARP). Bโtype also had a larger amplitude action potential, afterhyperpolarization and positive overshoot. The action potential of the Mโtype neuron had only a Na^+^ component while that of the Bโtype had both a Na^+^ and a Ca^2+^ component. After two days in culture, Mโtype neurons exhibited phase bright cytoplasmic granules, which were seldom observed for Bโtype neurons. Although neuron survival remained constant during the first six days in culture (DIV), the relative frequency of occurrence of the Mโtype decreased from 82 to 50%. Thereafter, it decreased more gradually to a final value of approximately 20% after 40 DIV. It was concluded that at least during the first 6 DIV and possibly through to 40 DIV, Mโtype neurons transformed into Bโtype. Both Mโ and Bโtype neurons showed significant and similar changes in their EMP with increasing DIV (up to 40 DIV). For Mโ and Bโtypes combined, R~m~ increased approximately 142%, ฯ by 204%, and no significant change in specific membrane capacitance was observed. Rheobasic threshold depolarization decreased 58%, while the resting membrane potential decreased by only 19%. These changes in the EMP of adult neurons are strikingly similar to changes in EMP observed in adult denervated muscle and in cultures of either embryonic nerve or muscle. This similarity suggested that the adult DRG neurons in cell culture undergo progressive dedifferentiation because of isolation from their usual trophic interactions. Determination of neuronal membrane electrical characteristics provides a new method for evaluating the effects of various possible trophic agents, e.g., hormones and tissue extracts, on the state of differentiation of neurons in cell culture.