C Y Bowers

Rats were injected with monosodium 1-glutamate (MSG) daily for the 1st 5 days of life and allowed to mature. This is known to cause selective destruction of neurons in the retina and in the arcuate nucleus of the hypothalamus. The adult animals had a significant increase in body fat without an increase in weight, a marked reduction in pituitary, thyroid, adrenal, gonadal and prostate weights. Pituitary, hypothalamic and serum thyrotropin (TSH) were significantly reduced in the males. Serum growth hormone (GH) was markedly reduced in both sexes and the serum prolactin (Prl) was increased significantly in females. FSH did not appear to be abnormal and the LH may have been increased in the males. Serum T4 was significantly reduced in females. The fertility of the females was normal, but treated males mated with normal females showed a marked reduction in fertility and, although the litter sizes of the offspring were normal, the birth weights of the pups of both sexes were significantly reduced. These persistent alterations in neuroendocrine function indicate that lesions produced by neonatal MSG treatment provide a convenient model for studying hypothalamic function.

LH-releasing hormone (LRH2), purified from porcine hypothalamic tissue, was tested in the human being. The LRH was injected iv into the following 8 subjects: 2 untreated men, 2 untreated women, 2 men pretreated with ethinyl estradiol, and 2 women pretreated with an oral contraceptive preparation (Lyndiol). The design was that of a 4-factor partially nested factorial experiment in which serum LH and FSH levels were measured by specific radioimmunoassays and compared with the values obtained after injection of a control solution of lysine vasopressin. The data indicated a highly significant increase (p <0.01) in serum LH at 23.5 min and a significant increase (p <0.05) in serum FSH at 24.5 min after administration of LRH. There was a 3.7-fold mean increase in serum LH levels with a range of 1.9–7.5, and a 2.2-fold mean increase in serum FSH levels (1.2–4.8). The response to LRH was not significantly influenced by either the sex of the subject or pretreatment with a steroid suppressing LH release. This study demonstrates that administration of LRH of porcine origin can induce release of LH in both men and women and thus is not species specific.

Thyrotropin-releasing hormone (TRH, pyroglutamic acid-histidine-proline-amide) binds to a seven-transmembrane-spanning, G protein-coupled receptor. We tested the hypothesis that Tyr106 of the third transmembrane helix of the TRH receptor (TRH-R) binds pyroglutamyl of TRH by mutating Tyr106 to Phe and replacing the ring carbonyl of the TRH pyroglutamyl moiety with a methylene group ([Pro1]TRH). Compared to the affinity of wild-type TRH-R for TRH, the affinities of [Phe106]TRH-R for TRH and of wild-type TRH-R for [Pro1]TRH were 100,000- and 110,000-fold lower, respectively. The affinity of [Phe106]TRH-R for [Pro1]TRH was only 16-fold lower than that for TRH, demonstrating a lack of additivity of the effects of these changes in the receptor and ligand. These data provide compelling evidence that the hydroxyl group of Tyr106 of the TRH-R binds the TRH pyroglutamyl carbonyl group. To our knowledge, this represents the highest affinity, non-covalent bond yet observed between single functional groups of a GPCR and ligand and is the first delineation of a direct binding interaction between a residue in the transmembrane core of a GPCR and a specific moiety of a peptide agonist.