Inspiratory area excitability and the hypothalamus

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Abstract
Tidal volume of respiration was recorded in 19 cats. Electrodes and hypodermic tubing were implanted in the hypothalamus and in the medullary inspiratory area. Inspiratory responses were evoked with either threshold strength electric current or 20 µl of 1.3% NaHCO3 before and after depression of the hypothalamus with 10 µl of 4% thiopental or high frequency coagulation. Depression of the hypothalamus resulted in a decrease in the electrically evoked inspiratory apnea and a decrease in the bicarbonate-evoked response. Injection of similar amounts of thiopental into the thalamus and dorsal hypothalamus or intravenously did not alter evoked responses. The decrease in evoked respiratory responses was not associated with consistent changes in blood pressure or electromyogram activity. The data support the view that tonic discharges from the hypothalamus increase the excitability of respiratory reflex pathways and responsiveness to bicarbonate.