Osmosensitive neurons in the rat's dorsal midbrain.

In experiment 1, multiple unit recordings were taken simultaneously from lateral preoptic and dorsal midbrain areas during a series of intracarotid hypertonic and isotonic NaCl injections. Subjects were 15 hooded rats (11 males and 4 ovariectomized females) under urethane anesthesia. Results showed that the neuronal reactions to a series of hypertonic NaCl injections (0.30 M, 0.45 M, 0.60 M and 0.75 M) were at least as strong in the dorsal midbrain as in the lateral preoptic area. Strength of neuronal reaction correlated with osmolarity of the NaCl solution injected. Control isotonic NaCl injections were ineffective, and the (monitored) force of injection was found not to affect the results. In experiment 2 with 15 hooded rats (9 males and 6 ovariectomized females), and two male Wistar rats under urethane anesthesia, recording from dorsal midbrain units were made during intracarotid injections of hypertonic and isotonic NaCl solutions. In addition, other sensory stimulations, including tail pinches, were presented. Of the 52 units studied, 39 cells (75%) reacted to injections of hypertonic NaCl, but not the isotonic (control) solution (Normosol-R). Again, strength of neuronal reaction correlated with osmolarity of the NaCl solution injected, and force of injections was found not to influence results. Eleven cells reacted to hypertonic NaCl injections but not to tail pinch. This and other evidence indicated that certain dorsal midbrain cells were specifically osmosensitive, and not merely showing general 'arousal' reactions to the injections. These results indicate that, for the rat, the osmosensitive zone extends into the midbrain. The functional significance of these findings is discussed.