Role of Arginine-Vasopressin (AVP) in Stress-Induced Inhibition of Testicular Steroidogenesis in Normal and in AVP-Deficient Rats*

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Abstract
Immobilization stress has induced in rats a state of testicular desensitization to gonadotropins as well as a post-cAMP blockade of testosterone (T) biosynthesis. Since arginine-vasopressin (AVP) has antagonized in rats the in vitro T-releasing effect of human CG [chorionic gonadotropin] whether AVP might be involved in stress-induced inhibition of T biosynthesis was verified. In Sprague-Dawley and Long-Evans adult male rats chronically cannulated in the jugular vein, a small but statistically significant rise of plasma AVP levels was observed after 2 h of immobilization stress. The i.v. infusion of AVP (1 .mu.g/kg per h) to chronically cannulated rats induced a fall of plasma T levels. A dose-dependent inhibition of plasma T values was also observed 3 h after i.p. administration of AVP (1, 5, 25 .mu.g/kg) in animals killed by decapitation. An antagonist of AVP pressor activity [1-(.beta.-mercapto-.beta.1.beta.-cyclopentamethylenepropionic acid), 2-(O-methyl)tyrosine] AVP, antagonized, when injected i.p. at a dose of 30 .mu.g/kg, the T-inhibitory effects of 3 h of immobilization stress. No consistent changes in plasma LH [luteinizing hormone] levels were observed in these experiments. To further evaluate the role of AVP in stress-induced T inhibition, AVP-deficient Brattleboro rats were submitted to 2 or 3 h of immobilization stress concomitantly with rats of the original Long-Evans strain. After 2 h and even more after 3 h of stress, plasma T levels fell in Long-Evans rats together with basal and human CG- or cAMP-stimulated T production by Percoll purified Leydig cells. In Brattleboro rats, 2 h of stress had no effects on plasma T levels nor in vitro basal or stimulated T production, whereas 3 h of immobilization were as effective as in Long-Evans animals. At least part of T inhibitory effects of immobilization, those occurring during the first 2 h of stress, are due to an AVP-induced, post-cAMP blockade of T biosynthesis. Since plasma corticosterone, during 2 h of stress, rose to similar, albeit smaller, levels in Brattleboro rats as compared to those in Long-Evans animals, this glucocorticoid does not seem to be involved in the testicular effects of stress. Since Brattleboro rats had plasma LH and T levels as well as a testicular T production capacity comparable to those of the original Long-Evans strain, then AVP plays no vital role in the control of the male rat reproductive system, although it may be involved in the fine-tuning of T production.

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