Regional Distribution of Thyroid Stimulating Hormone Activity in the Pituitary Gland of the Atlantic Stingray, Dasyatis Sabina

The possibility that the elasmobranch pituitary contains thyroid stimulating hormone (TSH) activity was investigated by measuring the increase in the release of thyroxine from thyroid glands of the Atlantic stingray, Dasyatiq sabina, incubated with homogenates of various pituitary regions. The ventral lobe of the pars distalis contained most of the TSH activity, with lesser amounts in the neurointermediate lobe. Histological techniques were not sensitive enough to detect changes in the thyroid associated with the increase in thyroxine release. It is concluded that the elasmobranch pituitary contains TSH activity but its functional significance remains to be determined. Few studies have been conducted to examine the functional relationship between the pituitary and the thyroid gland of elasmobranchs. Dodd and Goddard (unpublished but cited by Dent and Dodd, 1961) hypophysectomized adult dogfish, Scyliorhinus caniculus, but found no histological changes in the thyroid after 2 years, Whereas Vivien (1964) found that after decapitation of Scylim'hinus embryos the thyroid failed to complete its differentiation. The latter result is, of course, open to several interpretations since decapitation removes more than the pituitary. Injection of homoplastic pituitary homogenates into Scyliorhinus resulted in histological signs of stimulation of the thyroid gland (Vivien, 1941; Olivereau, 1954). Unfortunately, histological methods of assessing thyroid activity are frequently both insensitive (Sage and Robins, 1970) and unreliable (Swift, 1960). Ferguson, Dodd, Hunter, and Dodd (unpublished data summarized by Dodd et al. (1963» Using the McKenzie mouse assay found thyroid stimUlating hormone (TSH) activity in all parts of the S. caniculus pituitary, most of it being ---P ' The University of Texas, Marine Science Institute, ort Aransas, TX 78373. ---Manuscript accepted May 1972. FISHERY BULLETIN: VOL. 71, NO. I. 1973. in the ventral lobe. However, the highest activity found was much less than that found in the posterior lobe of the mouse pituitary, which presumably does not contain TSH. Their results could be interpreted as suggesting that the small amount of TSH activity found in the dogfish pituitary was of no significance. The interpretation of assays of lower vertebrate TSH on mammalian assay systems is further complicated by the probability of phylogenetic specificity of hormone action. It is known that teleost TSH is relatively inactive on the mammalian thyroid (Fontaine, 1969); similarly it is possible that if there is an elasmobranch TSH it may have low activity on mammalian tissues. In a recent review Gorbman (1969) states that "definite proof of a TSH-like principle in elasmobranch pituitaries remains to be provided." In an attempt to elucidate this problem we investigated the stimulatory effects of homogenates of the different regions of the pituitary of Dasyatis sabina on thyroxine release from the animal's own thyroid gland in vitro. This technique eliminates the problem of phylogenetic specificity, and, by measuring thyroxine release, avoids the problems of interpretation associated with histological assessment of thyroid activity.