STEADY-STATE OSMOTIC MODULATION OF CATIONIC CONDUCTANCE IN NEURONS OFRAT SUPRAOPTIC NUCLEUS

Whole cell patch-clamp recordings were obtained from isolated rat supr aoptic nucleus magnocellular neurosecretory cells (MNCs). Under curren t clamping, hyperosmolality produced by the addition of 10-30 mM manni tol depolarized each of 25 cells tested. In contrast, reducing fluid o smolality from 295 to 265 mosmol/kgH2O had the reverse effect, hyperpo larizing 18 of 21 MNCs. Voltage-clamp recordings in 43 cells revealed that the effects of hypo- and hyperosmolality, respectively, were caus ed by decreases and increases in a nonselective cation conductance rev ersing near -41 mV. Current-voltage analysis in Na+-free solution reve aled that the reversal potentials of currents elicited by increases an d decreases in osmolality both shifted to a value near -90 mV, suggest ing that a single ionic conductance is modulated by these stimuli. The relation between cationic conductance and osmolality was specific, se nsitive (+2.14% . mosmol-1. kgH2O-1), and well-fit by linear regressio n (r = 0.96; n = 22 cells) between 275 and 325 mosmol/kgH2O. These res ults indicate that MNCs express a depolarizing current that is active under steady-state conditions and that the up- or downregulation of th is current contributes to the excitation or inhibition of these cells upon acute exposure to hypo- or hyperosmolar conditions.