Regulation of hypothalamic neuropeptidemRNA levels during negative energy balance challenges

This dissertation examined the regulation of specific hypothalamic peptide mRNA levels in response to negative energy balance challenges by in situ hybridization. First, a role for the orexins as physiological important regulators of energy balance was evaluated in rats. Intact and ovariectomized female rats (with or without estradiol treatment) were food deprived for 48 h. This study revealed that: (1) food deprivation had no effect on orexin mRNA levels in female rats irrespective of ovarian steroid background. Further, experimental diabetes was induced in male rats by streptozotocin administration and normal and diabetic rats were food deprived for 48 h. This study revealed that: (2) conditions of extreme negative energy balance had no effect on orexin mRNA levels in male rats, suggesting that changes in orexin mRNA levels are not a consistent correlate of negative energy balance challenges. Secondly, the time course of change in hypothalamic neuropeptide Y (NPY), agouti-related protein (AGRP) and proopiomelanocortin (POMC) mRNA levels were determined during food deprivation, food restriction, and refeeding in mice by in situ hybridization. The work demonstrated that: (3) NPY and AGRP mRNA levels are significantly increased within 6 h of food deprivation, while POMC mRNA decreased only after 24 h deprivation. (4) Moderate food restriction (60% of ad libitum intake) for 3 d, 7 d, or 14 d induced graded increases in NPY and AGRP mRNA levels, while after 3 d food restriction there were no further decrease in POMC mRNA levels. (5) The magnitude of the effects of 24 h deprivation was comparable to 7 d restriction (NPY and AGRP). (6) Refeeding for 6 h, but not 2 h, reduced the food deprivation-induced changes in NPY mRNA, did not affect AGRP mRNA, and restored POMC mRNA levels to ad libitum control levels. (7) These results suggest potential differential regulation of POMC and AGRP (the endogenous melanocortin antagonist) mRNA levels in response to energy balance challenges. The contribution of leptin in reversing the deprivation-induced changes in these peptide mRNA levels was examined. (8) Leptin administration after deprivation did not affect the food deprivation-induced changes in NPY, AGRP or POMC mRNA levels.