High voltage-activated Ca2+ currents in rat supraoptic neurones: biophysical properties and expression of the various channel alpha1 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 alpha1 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 Cav1.2, Cav1.3 (forming L-type channels), Cav2.1 (P/Q-type), Cav2.2 (N-type) and Cav2.3 subunits (R-type). Each subunit exhibited further specificity in its distribution throughout the nucleus, and we particularly observed strong immunostaining of Cav1.3 and Cav2.3 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