Are the short‐photoperiod‐induced decreases in serum prolactin responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters?

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Abstract
Serum prolactin (PRL) decreases in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters following short‐photoperiod exposure. Both species also exhibit short‐photoperiod‐induced changes in body and lipid mass, but in opposite directions; Syrian hamsters increase and Siberian hamsters decrease their body weight, changes reflected nearly exclusively in their carcass lipid content. The purpose of these experiments was to determine whether the photoperiod‐induced changes in PRL were responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters by using the strategy of experimentally producing serum PRL levels opposite to those normally associated with the photoperiod in which the animals were housed. In long photoperiods serum PRL was reduced to short‐day levels by subcutaneous (s.c.) CB‐154 (bromoergocryptine, a dopamine agonist) injections. In short photoperiods, serum PRL was elevated to long‐day levels in Syrian hamsters by ectopic pituitary explants, and in Siberian hamsters, serum PRL was elevated by chronic s.c. infusions of ovine PRL (oPRL). In both species, manipulations of serum PRL did not affect food intake, carcass composition, or the wet weight of various white and brown adipose tissue pads (WAT and BAT, respectively). Body weight increased in CB‐154‐treated Syrian hamsters and decreased in Siberian hamsters, an effect partially reversed by coadministration of oPRL in Syrian, but not Siberian, hamsters. Thus, lowering serum PRL to short‐day levels in long‐day‐housed hamsters of both species mimicked the directional change in body weight appropriate for each species when they are exposed to short days. This effect of CB‐154 on body weight may be a result of some as yet unidentified effect of dopaminergic stimulation on overall growth since 1) these changes in body weight were not reflected as changes in lipid mass, as occurs naturally following short‐day exposure for each species, and 2) neither species exhibited a reciprocal change in body weight when serum PRL was experimentally elevated in short days. Alternatively, it may be that once the energetic responses to short‐day exposure have been fully expressed, the ability of PRL to stimulate the target sites of action for PRL for these responses may be decreased. BAT protein content, cytochrome oxidase activity (measures of metabolic growth of this tissue), and retroperitoneal total and specific lipoprotein lipase (LPL) activities were increased by CB‐154 treatment in Syrian hamsters. These measures, as well as total and specific BAT LPL activities, and resting, minimal, and norepinephrine (NE)‐stimulated oxygen consumption were not affected by the PRL manipulations per se. However, CB‐154‐treated Siberian hamsters had increased NE‐stimulated oxygen consumption. Chronic s.c. infusions of oPRL in short‐day‐housed Siberian hamsters did not alter in vivo lipogensis in WAT or BAT pads or in liver or muscle. The results of these experiments suggest that the changes in energy balance associated with changes in the photoperiod (melatonin) in these hamster species cannot simply be attributed to changes in serum PRL.

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