Prolactin and Triiodothyronine Modulate Seasonal Variation in Brown Adipose Tissue in Djungarian Hamsters

Seasonal photoperiodic cycles regulate energy rheostasis in many animals. Brown adipose tissue (BAT) and the liver are key metabolic organs that show remarkable plasticity in response to photoperiodic conditions. Prolactin (PRL) and thyroid hormones are endocrine signals that transmit neural-derived photoperiodic time measurement encoding to regulate peripheral tissues. Using Djungarian hamsters (Phodopus sungorus), a well-established seasonal model, we examined the effects of photoperiod, triiodothyronine (T3), and PRL on liver and brown adipose tissue morphology and lipid content. Tissue mass, brown adipose tissue somatic index, and histological lipid content were quantified in three experiments that investigated tissue variation (1) across photoinduced seasonal cycles, and the sufficiency of acute (2) triiodothyronine, and (3) prolactin administration. Long photoperiod (LP) exposure maintained large BAT and higher lipid content, whereas short photoperiod (SP) exposure reduced BAT mass and lipid content. T3 treatment increased the BAT mass but did not impact lipid content. Conversely, PRL treatment increased BAT lipid content but did not impact tissue mass. Together, these findings identify triiodothyronine and prolactin as critical regulators of metabolic tissues.