High voltage-activated Ca2+ currents in rat supraoptic neurones:: Biophysical properties and expression of the various channel α1 subunits

The diversity of Ca2+ currents was studied in voltage-clamped acutely dissociated neurones from the rat supraoptic nucleus (SON), and the expression of the various corresponding pore-forming al subunits determined by immunohistochemistry. We observed the presence of all high voltage-activated L-, N-, P/Q- and R-type currents. We did not observe low-voltage-activated T-type current. The multimodal current/voltage relationships of L- and R-type currents indicated further heterogeneity within these current types, each exhibiting two components that differed by a high (-20 mV) and a lower (-40 mV) threshold potential of activation, L- and R-type currents were fast activating and showed time-dependent inactivation, conversely to N- and P/Q-type currents, which activated more slowly and did not inactivate. The immunocytochemical staining indicated that the soma and proximal dendrites of SON neurones were immunoreactive for Ca(v)1.2, Ca(v)1.3 (forming L-type channels), Ca(v)2.1 (P/Q-type), Ca(v)2.2 (N-type) and Ca(v)2,3 subunits (R-type). Each subunit exhibited further specificity in its distribution throughout the nucleus, and we particularly observed strong immunostaining of Ca-v.3 and Ca(v)2.3 subunits within the dendritic zone of the SON. These data show a high heterogeneity of Ca2+ channels in SON, neurones, both in their functional properties and cellular distribution. The lower threshold and rapidly activating L- and R-type currents should underlie major Ca2+ entry during action potentials, while the slower and higher threshold N- and P/Q-type currents should be preferentially recruited during burst activity. It will be of key interest to determine their respective role in the numerous Ca2+-dependent events that control the activity and physiology of SON neurones.