Elevated CSF osmolality inhibits thermoregulatory heat loss responses.

The effect of intracerebroventricular (icv) infusions of hypertonic NaCl or sucrose on thermoregulatory responses to heat was studied in conscious rabbits to test the idea that brain osmoreceptors are involved in the inhibition of evaporative heat loss and elevation of body temperature in dehydrated mammals at high ambient temperature (Ta). In rabbits hydrated ad lib and resting at a Ta of 33 degrees C, icv infusion (3.4 microliter/min) of hypertonic (1,500 mosm) NaCl or sucrose in artificial cerebrospinal fluid (ACSF) produced a significant reduction in respiratory frequency (f) and in ear skin temperature (Te) and a rise in brain temperature (preoptic area, Tpoa). icv infusion of ACSF alone or ACSF + NaCl at 500 and at 750 mosm had no effect on f or on Te or Tpoa. Infusion of NaCl + ACSF at 1,000 mosm reduced f but did not affect Te or Tpoa. In hydrated rabbits, icv infusion of 1,500 mosm NaCl abolished the rise in f and in Te elicited by POA heating at Ta of 25 degrees C. In dehydrated rabbits at 33 degrees C, f was below hydrated levels and icv infusion of water (6.8 microliter/min) produced a reversible elevation in f. These findings suggest that brain osmoreceptors can influence thermoregulation in hot environments.