Influence of vasopressin and oxytocin on neoteny in the adult mexican axolotl Ambystoma mexicanum

Abstract

Lysine-vasopressin and oxytocin were tested on the normally neotenous adult Mexican axolotl, Ambystoma mexicanum, to determine their influence on metamorphosis. The salamanders were divided into four groups, and daily injections of saline, lysine-vasopressin, oxytocin, and TSH were made over a 28-day test period. Tail height, gill length, and body weight were measured as indicators of metamorphic change. Additionally, thyroid uptake of '"I was determined.The results showed that the vasopressin and oxytocin groups had a significant (P<O.O1) loss in tail height and body weight compared to controls, but showed no gill rachis resorption or 13'1 uptake. These results suggest that the effects of vasopressin and oxytocin were not mediated by TSH and thyroxine, but rather may be caused by an antagonism of prolactin.In amphibians, neoteny is characterized by the attainment of sexual maturity during the larval stage and usually assumes the permanence of that larval stage. It is generally agreed that thyroxine is responsible for inducing metamorphosis in all nonneotenous species and has been shown to induce precocious metamorphosis even in neotenes. Control of thyroxine secretion is mediated by thyroid-stimulating hormone (TSH) released from the pituitary.The physiological competence of the pituitary and thyroid glands has been demonstra€ed by many researchers (Tassava, '69; Tassava et al., '68; Connelly, "73; Taurog. '74; and Taurog et al., '74). In mammals, the hypothalamus secretes thyrotrophin-releasing hormone (TRH) that stimulates the release of TSH from the pituitary. In amphibians TRH, although present in the pituitary, apparently has no effect in the stimulation of TSH release (Taurog, '74; Gona and Gona, '76 ('76) found that when prolactin secretion was blocked with the drug ergocornine, thyroxine-induced metamorphosis in A. tigrinum was accelerated. However, ergocornine alone was unable to stimulate an increase in radioiodide uptake by the thyroid or to induce meta morphosis in this species, but it did result in a reduction of tail fin height.Jorgensen and Larsen ('60) found that they were able to induce skin molting in toads with the hypothalamically derived neurohypophyseal hormones lysine-vasopressin and oxytocin. They also determined that these hormones did not act directly on the tissues, but rather their action was mediated through the stimula tion of the pituitary gland. Recently, Platt and LiCause ('80) have shown that oxytocin accelerates thyroxine-induced metamorphosis and inhibits the antimetamorphic action of prolactin in larval tiger salamanders. Since lysinevasopressin and oxytocin have a stimulatory effect on the pituitary and since oxytocin has been shown to have an antiprolactin response, it may follow that these neurohypophyseal hormones have an effect on neoteny. Therefore, it should be of interest to observe Address reprint requests to Dan E. Wells, Department of Riology. Indidna llnivrrsity. Rloomington. IN 47105.