Osmoresponsive units in sheep median preoptic nucleus

Abstract

Single-unit neural activity in the lamina terminalis, a region implicated in osmoregulation, was studied in alpha-chloralose-anesthetized sheep during mild hyperosmotic stimulation (intracarotid infusions of 1.65 M NaCl, 3 M sorbitol in 0.15 M NaCl, or 3 M urea in 0.15 M NaCl, at 1 ml/min). Twelve of 121 units (9.9%) were activated significantly (by 82 +/- 52%) by 2- to 3-min infusions of 1.65 M NaCl. Eleven of these and one untested unit were excited by hypertonic sorbitol (91 +/- 40% increase). Of five units further tested with urea, two were excited (by 19 and 58%). Isotonic or hypotonic NaCl infusions were without effect (eight osmoresponsive units tested). All responsive units were in the median preoptic nucleus (MnPO; nucleus medianus). MnPO units were compared with neurohypophysial fibers (multiunit recordings). Osmotic response profiles were similar; both MnPO units and neurohypophysial fibers responded equally to hypertonic NaCl and sorbitol but less to equiosmolal urea. Both MnPO units and neurohypophysial fibers responded slowly, taking 50 and 30 s of NaCl infusion, respectively, to show significant increases and approximately 2 min to reach peak activity. Their hemodynamic responses differed, however; neurohypophysial fibers were strongly excited by nitroprusside-induced hypotension (three of three animals) but MnPO osmoresponsive units were not (zero of five units). Osmoresponsive MnPO units may contribute osmotic, but not hemodynamic, inputs to control vasopressin secretion and/or osmoregulatory responses.