We evaluated the effect of plasma hyperosmolality on behavioral thermoregulation in mice, using a new experimental system. The system consisted of Plexiglas box (dimensions: 50 × 12 × 19. cm) with five computer-controlled Peltier boards (dimensions: 10 × 10. cm) at the bottom. Experiments were conducted in two different settings of the system. An operant behavior setting: each board was first set to 39 °C, and the right-end board was changed to 20 °C for 1. min when a mouse moved to a specific position. A temperature mosaic setting: each board was randomly set to 15 °C, 22 °C, 28 °C, 35 °C, or 39 °C with a 6-min interval, but each board temperature was different from the others at a given time point. Mice were injected subcutaneous (s.c.) isotonic or hypertonic saline (154. mM (IS group) or 2,500. mM (HS group), 10. ml/kg body wt), and exposed to either setting for 90. min. In the operant setting, the HS group showed fewer operant behavior counts than the IS group (11 ± 5 and 25 ± 4 counts, respectively; P < 0.05) with greater increase in body temperature (1.6 ± 0.4 °C vs. 0.0 ± 0.2 °C, respectively; P < 0.05). In the mosaic setting, the HS group selected the board temperature of 35 °C more frequently than the other temperatures (P < 0.05) with the same increase in body temperature. These results may suggest that plasma hyperosmolality modulates behavioral thermoregulatory response to heat and induce regulated hyperthermia.
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