Thyrotrophic Hormone Secretion in Rats After Partial or Total Interruption of Neural Aiferents to the Medial Basal Hypothalamus

Abstract

The neural connections to the medial basal hypothalamus (MBH) were permanently interrupted by means of a bayonet-shaped small knife fixed on the electrode carrier of a sterotaxic instrument. The effects of the following "deafferentations" upon thyroid function were studied: complete deafferentation of the MBH leaving it in contact only with the pituitary; incomplete deafferentation in which only the lateral, dorsal and caudal connections of the MBH were severed; anterior frontal cuts of half-dome shape with a diameter of 1.5 mm at the base (these cuts were placed at the level of the optic chiasm and extended from the base of the hypothalamus to the paraventricular nuclei); posterior frontal cuts similar to the former method but placed 1.0 mm caudal to the optic chiasm. Half the animals were kept for 4 weeks on a normal diet. The others received the normal diet for 2 weeks and then were given a diet containing 0.15% propylthiouracil (PTU) for the 14 days prior to autopsy. Thyroid weight and cell height, serum PBI [protein-bound iodine], thyroidal 24-hr. radioiodine uptake, biological half-life of thyroidal radioiodine and the thyroid: serum radioiodine ratio (T:S) were measured as criteria of thyroid function. Complete deafferentiation was found to cause a moderate decrease in pituitary thyrotropin secretion. Incomplete deafferentation and anterior frontal cuts had no appreciable effect on thyroid function. Posterior frontal cuts, which excluded the anterior hypothalamus from the region able to act on the pituitary, caused a marked decrease in thyroid function. A thyrotrophic response to PTU was observed in all animals, but the response was only moderate after complete deafferentation and very slight after posterior frontal cuts. Data supports the view that the hypothalamic region between the suprachias-matic nuclei and the anterior border of the ventromedial nuclei ("thyrotrophic area") plays an essential role in the hypothalamic control of pituitary thyrotropin secretion.