Osmoreceptors in the lamina terminalis inhibit thermoregulatory punting in sheep.

In response to increased core temperature, mammals initiate evaporative cooling responses (e.g. sweating, punting). These responses result in dehydration and plasma hypertonicity if the fluid losses are not replaced. The aim of the present study was to determine whether osmoreceptors in the lamina terminalis (LT) may exert an inhibitory influence on evaporative thermoregulatory cooling responses. We studied the effect of infusions of either hypertonic 1.65M NaCl or 3M sorbitol in 0.15M NaCl at 1.6 ml/min for 10 min into a carotid artery, on the rate of punting of 4 conscious sheep exposed to an ambient temperature of 39°C for 1,-2 hours. Both intracarotid (IC) 1.65M NaCl or 3M sorbitol caused rapid and large reduction in punting (127 ± 18 to 65 ± 15 and 110 ± 15 to 76 ± 24 breaths/rain respectively) that reversed immediately infusions ended. Control IC infusion of isotonic saline did not inhibit panting during heat exposure. In 3 sheep in which the osmoregulatory region of the lamina term±nails had been ablated, IC infusion of 1.65M NaCl or 3M sorbitol did not reduce punting. Drinking in response to IC hypertonic saline was also abolished in these 3 sheep with LT lesions. We propose that osmoreceptors in the region of the lamina terminal±s, probably located in the organum vaseulosum laminae term±nails and/or subfornical organ, may exert inhibitory osmoregulatory influences on evaporative cooling mechanisms. [ABSTRACT FROM AUTHOR]