Calcium channels in Xenopus spinal neurons differ in somas and presynapticterminals

Calcium channels play dual roles in cell signaling by promoting membrane de polarization and allowing entry of calcium ions. Patch-clamp recordings of calcium and calcium-dependent currents from the soma of Xenopus spinal neur ons indicate key functional differences from those of presynaptic terminals . Both terminals and somas exhibit prominent high-voltage-activated (HVA) c alcium current, but only the soma expresses additional low-voltage-activate d (LVA) T-type current. Further differences are reflected in the HVA curren t; N- and R-type channels are predominant in the soma while the terminal ca lcium current is composed principally of N type with smaller contribution b y L- and R-type channels. Potential physiological significance for these di fferent distributions of channel types may lie in the differential channel kinetics. Activation of somatic HVA calcium current occurs more slowly than HVA currents in terminals. Additionally, somatic LVA calcium current activ ates and deactivates much more slowly than any HVA calcium current. Fast-ac tivating and -deactivating calcium current may be critical to processing th e rapid exocytotic response in terminals, whereas slow LVA and HVA calcium currents may play a central role in shaping the somatic firing pattern. In support of different kinetic behavior between these two compartments, we fi nd that somatic calcium current activates a prominent slow chloride current not observed in terminal recordings. This current activates in response to calcium entering through either LVA or HVA channels and likely functions a s a modulator of excitability or synaptic input. The restriction of this ch annel type to the soma lends further support to the idea that differential expression of fast and slow channel types in these neurons is dictated by d ifferences in signaling requirements for somatic and terminal compartments.