Integration of thermal and osmotic afferent signals in the preoptic/anterior hypothalamic neurons

Our in vivo experiments were aimed at studying afferent links of the preoptic/anterior hypothalamus system, which integrates thermal and osmotic homeostasis. Special attention was paid to using stimulation intensities within the normal physiological range. The experiments were carried out on adult cats anesthetized with ketamine (25mg/kg) and inhalation of nitrous oxide (75 vol.%). Short-term shifts in the osmotic pressure within the cerebral vasculature were induced by infusions of 200-500 microl 3.0% or 0.2% NaCl solutions into the homolateral a. carotis (hyper- and hypoosmotic stimulations, respectively). Thermal stimulation was provided by local heating or cooling of the contralateral forelimb pad skin (+/-7.0 degrees C range). Reactions of preoptic/anterior hypothalamus neurons were classified into four types: monophasic activation, monophasic inhibition, and biphasic responses including excitation followed by inhibition or primary inhibition followed by activation. Monophasic activation was a very common occurrence among preoptic/anterior hypothalamus neuronal reactions. The responsiveness of thermosensitive preoptic/anterior hypothalamus neurons to hyperosmotic stimulation turned to be noticeably higher than that to hypoosmotic stimulation. Practically equal proportions of warm- and cold-sensitive neurons demonstrated changes in the firing activity resulting from intracarotid infusions of 3.0% NaCl solution. Infusions of 0.2% NaCl solution induced firing rate modifications in 26% (12/46) of warm-sensitive and in 32% (18/39) of cold-sensitive neurons. Cold-sensitive neurons displayed a higher sensitivity to a short-term osmotic pressure elevation in the cerebral vasculature (63%, 33/52) than warm-sensitive neurons did (43%, 22/52, P<0.05). In our study, a maximum similarity in the response types was observed when hyperosmotic infusion and skin cooling were applied, while a maximum disagreement was found when hyperosmotic stimulations were combined with skin heating. There is no doubt that preoptic/anterior hypothalamus neurons play a crucial role in the maintenance of body temperature. Several studies have also shown that osmoregulation can be affected by shifts in peripheral and hypthalmic temperatures. Information on the neurol mechanisms of interactions between the thermo- and osmoregulatory circuits in the hypothalamus remains limited. We discuss the obtained data considering the "set-point theory" of thermal hameostasis maintenance.