Effect of ACTH on Single Unit Activity in the Diencephalon of Intact and Hypophysectomized Rats

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Abstract
Single unit recordings were made extracellularly in the diencephalon of intact and hypophysectomized male rats 4–6 days after operation. Hypophysectomy had no effect on the resting discharge pattern in the hypothalamus. The frequency of firing was also studied before and after intravenous administration of purified (40 mU/100 g) or synthetic (0.4 µg/100 g) ACTH. Administration of this low dose of ACTH resulted in a very rapid increase of spike frequency in 19 % of the neurons in intact rats (latency 3.1 min) and in 32.5 % of the neurons in hypophysectomized rats (latency 1.6 min). Most of the neurons studied were located in the anterior hypothalamus, but no spatial organization of neurons showing a consistent change in frequency was observed. The data are compatible with the theory of shortloop-feedback of ACTH and suggest diffuse rather than localized hypothalamic involvement in this mechanism.

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