Fasting reduces the number of TRH immunoreactive neurons in the hypothalamic paraventricular nucleus of male rats, but not in mice

[Display omitted] •Leptin induces pSTAT3 in PVHTRH neurons of rats, but not in mice.•Fasting reduces the number of PVHTRH immunoreactive neurons of rats, but not in mice.•Leptin deficient mice display reduced T4 levels, but normal number of PVHTRH neurons. During fasting or weight loss, the fall in leptin levels leads to suppression of thyrotropin-releasing hormone (TRH) expression in the paraventricular nucleus of the hypothalamus (PVH) and, consequently, inhibition of the hypothalamic-pituitary-thyroid (HPT) axis. However, differently than rats, just few PVHTRH neurons express the leptin receptor in mice. In the present study, male adult rats and mice were submitted to 48 -h fasting to evaluate the consequences on proTRH peptide expression at the PVH level. Additionally, the proTRH peptide expression was also assessed in the brains of leptin-deficient (Lepob/ob) mice. We observed that approximately 50 % of PVHTRH neurons of leptin-injected rats exhibited phosphorylation of the signal transducer and activator of transcription 3 (pSTAT3), a marker of leptin receptor activation. In contrast, very few PVHTRH neurons of leptin-injected mice exhibited pSTAT3. Rats submitted to 48 -h fasting showed a significant reduction in the number of PVHTRH immunoreactive neurons, as compared to fed rats. On the other hand, no changes in the number of PVHTRH immunoreactive neurons were observed between fasted and fed mice. Next, the number of TRH immunoreactive cells was determined in the PVH, dorsomedial nucleus of the hypothalamus and nucleus raphe pallidus of Lepob/ob and wild-type mice and no significant differences were observed, despite reduced plasma T4 levels in Lepob/ob mice. Taken together, these findings provide additional evidence of the important species-specific differences in the mechanisms used by fasting and/or leptin to regulate the HPT axis.