RESPIRATORY RESPONSES TO COMBINED HYPOXIA AND HYPOTHERMIA IN RATS AFTER POSTERIOR HYPOTHALAMIC-LESIONS

Urethane-anaesthetised rats were exposed to hypoxia (7% O-2 in N-2) fo r 5 min periods while body core temperature (T-bc) was maintained with in the normal range (37-38 degrees C) using an abdominal heat exchange r. Animals were exposed to hypoxia and after placement of electrolytic lesions in either the anterior (n = 6) or posterior hypothalamus (It = 6). Neither lesion altered respiration while rats breathed air at ei ther T-bc. At normal T-bc, rats responded to hypoxia with increased ve ntilation throughout the exposure period. This response was unchanged by lesions in either location. At reduced T-bc rats responded to hypox ia with an initial increase in ventilation followed by depression to b elow air-breathing levels. This depressive response was unchanged afte r anterior hypothalamic lesions but eliminated after posterior hypotha lamic lesions. It is concluded that neurons either originating in the posterior hypothalamus, or passing through it, play a role in the inte raction between cold and hypoxia which leads to inhibition of respirat ion.