Effect of Continuous Light Exposure on Pituitary Gonadotrophs of the Loach, Misgurnus anguillicaudatus

Adult and young loaches, M isgurT/,us anguillicaudatus, of both sexes were exposed to continuous light for periods of 30 and 60 days during the postspawning period. The treatment caused a significant promotion of vitellogenesis and spermatogenesis in affected ovaries and testes of adult fish. In these fish, a remarkable activation of pituitary gonadotrophs of the vesicular cell type was observed electron microscopically. The vesicular cells showed a notable increase both in number and in size and, characteristically, a progressive expansion of their cytoplasmic vesicles. Pituitary gonadotrophs of the globular cell type were also observed to be activated under the influence of constant illumination, but the response was rather slight when compared with that of the vesicular cells even after 60 days of the treatment. In addition, exposure of young females 1i9 continuous light did efficiently induce a precocious differentiation of the vesicular cells in their pituitary glands, in parallel with a precocious occurrence of yolk accumulation in ovarian oocytes. The results indicate that the vesicular gonadotropic cells may be implicated primarily in the mechanism of vitellogenesis in females and spermatogenesis in males of the loach. For the purpose of identifying a gonadotropic cell type(s) of the pituitary gland in teleost fishes, various experimental attempts have hitherto been carried out to cause functional alterations of pituitary cells1)2)3). In our previous ultrastructural studies on the pituitary gland of the loach, Misgurnus anguillicaudatus, we confirmed, by some experimental means, the presence of two distinct types of gonadotrophs, termed the globular and the vesicular cell, along with thyrotroph and somatotroph in the proximal pars distalis of pituitaries of the fish4)5)6). Moreover, these studies suggested that each of the two types of gonadotrophs might take different parts in different stages of development and maturation of the gonad of the loach. However, the exact roles played by the two cell types have not been settled yet. Simon and Reinboth7) distinguished two distinct types of gonadotrophs light microscopically in the pituitary gland of Lepomis maMochirus. Treatment of the fish with a long photoperiod during their sexually quiescent period caused a stimulated development of the gonad accompanied with a marked hypertrophy of one of the two gonadotropic cell types7). This led us to examine whether a continuous exposure of immature or sexually regressed loaches to a long photoperiod could promote * This work was supported in part by a Grant-in-Aid for Scientific Research to H.T. from the Ministry of Education, Science and Culture of Japan. ** Laboratory of Fresh-Water Fish-OUlture, FacUlty of Fisheries, Hokkaido University (~t:e~*~*.if[~'ffllr~**M~.~) -107Bull. Fac. Fish. Hokkaido Univ. 32(2). 1981. gonadal development through an increased activity of either of the two types of pituitary gonadotrophs. It was expected that the resulting changes of the cells might give an important clue for discriminating functional differences between the two types of gonadotrophs of the loach. The present study was thus conducted to observe ultrastructural changes of pituitary gonadotrophs of loaches of both sexes subjected to continuous light exposure during their postspawning period. Material and Methods The loach, Misgurnus anguillicaudatus, used in the present study were collected in the suburbs ofHakodate in late August 1979, when they were at the end of the spawning period. Thirty-two adult loaches, 8.5-12.5 cm in body length and 4.0-13.2 g in body weight, and 30 young fish, 6.5-8.5 cm in body length and 1.8-3.5 g in body weight, of both sexes were selected for the present study. Beginning from 6 days before the start of experiment, they were acclimated to 0.6% NaCI solution in laboratory aquaria with continuous aeration and filtration. The fish could survive laboratory conditions of the present study better in the saline solution than in ordinary freshwater. They were fed daily on commercial fish food throughout the course of the present study. Continuous light exposure of the fish was begun on September 1 and ended on October 31, 1979, lasting for 60 days. At the start of experiment, 9 adult and 6 young fish of both sexes were killed and they served as initial controls. The remaining 23 adult and 24 young loaches were equally divided into an experimental and a control group, each comprizing the same number of males and females except for control adults with 5 males and 6 females. Fish of the experimental group were exposed to constant illumination by two 20-W fluorescent lamps in a room where room temperature was maintained between 18 and 22°C. Fish of the control group were kept under natural light and temperature which ranged from 15 to 20°C during the period of experiment. They were killed 30 and 60 days after the start of experiment, and changes of their gonads and pituitary glands were examined. Pituitary glands were excised out immediately after decapitation. They were fixed with glutaraldehyde-paraformaldehyde mixture in 0.2 M cacodylate buffer (pH 7.4) for about 3 hours at room temperature, followed by postfixation in 1 % osmium tetroxide in the same buffer for about 2 hours at 4°C, and embedded in Epon. Ultrathin sections stained double with uranyl acetate and lead citrate were observed with a Hitachi HU-12 electron microscope. Semithin sections of the Epon-embedded specimens stained with methylene blue were also observed for light microscopic comparison. Gonads of the control and experimental fish were fixed in Bouin's fluid and stained with Delafield's hematoxylin and eosin for histological inspection.