Intrinsic osmosensitivity of subfornical organ neurons

The constancy of plasma osmolality demands that salt and water concentratio n within the extracellular fluid be constantly monitored and regulated with in a few percentage points. The circumventricular organs in general, and th e subfornical organ in particular, have long been proposed to be the site o f the osmosensitivity. Isolated subfornical organ neurons of male rats were studied using the whole-cell patch-clamp technique and both action potenti al frequency and whole cell currents were measured as bath osmolality was c hanged, from 240 to 330 mOsm, by altering the amount of mannitol and mainta ining the concentrations of electrolytes constant. Out of 64 cells, 66% res ponded to changes in bath osmolality in a predictable manner, exhibiting a hyperpolarization and decrease in spike frequency in hypo-osmotic solutions and a depolarization and increase in action potential frequency during hyp erosmotic exposure. Cells (34%) defined as non-responders exhibited no sign ificant modulation during identical changes in extracellular osmolality. Th e responses to changing extracellular osmolality were dose dependent; the a ctivity of subfornical organ neurons was significantly modulated by changes in extracellular osmolality of less than 10 mOsm. By regression analysis, this osmosensitivity was approximately 0.1 Hz/mOsm change throughout a +/- 10 mOsm range and was maintained throughout the range of osmolalities studi ed (270-330 mOsm). The mechanism underlying this osmosensitivity remains un clear, although the non-selective cation conductance and the volume-activat ed chloride conductance do not seem to be involved. This intrinsic osmosensitivity of subfornical organ within the normal physi ological range supports the view that this circumventricular structure play s a role in normal osmoregulation. (C) 2000 IBRO. Published by Elsevier Sci ence Ltd. All rights reserved.