Presence of low voltage activated calcium channels distinguishes touch from pressure sensory neurons in the lamprey spinal cord.

Touch (T) and pressure (P) primary sensory neurons (dorsal cells) in the lamprey spinal cord differ not only with regard to their mechanoreceptive properties but also in their membrane properties. Touch cells have a significantly shorter time delay to the spike onset on a rebound of a hyperpolarizing current pulse than the pressure cells. Different time constants and input resistances partially explain these findings but cannot fully account for the observed differences between T- and P-cells. A more detailed study of membrane properties was therefore required. 3-D reconstructions of dorsal cells reveal a round shape with few processes and thus that they are suitable for voltage clamp analysis. Voltage activated calcium channels with a low threshold were found in a subpopulation of dorsal cells after administration of tetrodotoxin and K+ channel antagonists. These channels were blocked by addition of Co2+. In the short latency T-cells Co2+ increased the latency under current clamp conditions, and inhibited the facilitatory effect on spike activation upon increased hyperpolarization. This effect of Co2+ was not observed in the long latency P-cells. It is likely that the presence of low voltage calcium channels in T-cells are responsible for the differences observed between T- and P-cells. Voltage activated calcium channels with a higher threshold were observed in dorsal cells of both types. These channels were blocked by Co2+ or cadmium. Late outward 'tail' currents were shown to include calcium dependent potassium channels.