Effect of the Photoperiod and Administration of Melatonin on the Pars Tuberalis of Viscacha (Lagostomus maximus maximus): An Ultrastructural Study

Abstract

The pituitary pars tuberalis (PT) is a glandular zone exhibiting welldefined structural characteristics. Morphologically, it is formed by specific secretory cells, folliculostellate cells, and migratory cells coming from the pars distalis. The purpose of this work was to investigate differences in specific cellular characteristics in the PT of viscachas captured in summer (long photoperiod) and winter (short photoperiod), as well as the effects of chronic melatonin administration in viscachas captured in summer and kept under long photoperiod. In summer, the PT-specific cells exhibited celllike characteristics with an important secretory activity and a moderate amount of glycogen. In winter, the PT-specific granulated cells showed ultrastructural variations with signs of a reduced synthesis activity. Also, PT showed a high amount of glycogen and a great number of cells in degeneration. After melatonin administration, the ultrastructural characteristics were similar to those observed in winter, but the amount of glycogen was higher. These results suggest possible functional implications as a result of morphological differences between long and short photoperiods, and are in agreement with the variations of the pituitary-gonadal axis, probably in response to the natural photoperiod changes through the pineal melatonin. The ultrastructural differences observed in PT, after melatonin administration, were similar to those observed in the short photoperiod, thus supporting the hypothesis that these cytological changes are induced by melatonin. Anat Rec,

Influence of photoperiod on hormones, behavior, and immune function

“…When pinealectomized hamsters or sheep were infused with melatonin on a daily basis, the types of responses elicited depended on the duration, but not on the phase, of the daily infusion [20, 38, 56]. Data obtained from similar studies in several other mammalian species support the conclusions that the duration of melatonin is the critical physiological parameter providing photoperiod information [13, 266, 272]. …”

Histological and histochemical study of the duodenum of the plains viscacha (Lagostomus maximus) at different stages of its ontogenetic development

“…) and some endocrine glands and sense organs (Romera et al . , ; Filippa and Mohamed ). However, researches on the digestive system morphology of the plains viscacha are scarce and they refer to the weight and size of the digestive tract and accessory glands (Llanos and Crespo ).…”

Encoding and Decoding Photoperiod in the Mammalian Pars Tuberalis

“…Ultra structural analyses of the PT have demonstrated a seasonal-dependent change in morphology and secretory activity [32,33,34,35]. Similarly, melatonin administration induces SP-like modifications in the PT [36,37]. These anatomical modifications associated with changes in photoperiod and melatonin signal duration are paralleled with changes in immuno-reactivity and/or mRNA expression of βTSH and αGSU in PT specific cells (in Siberian hamsters [38,39,40,41], in European hamsters [42,43], in sheep [29,44], and in mice [45]).…”