Sex Dimorphic Responses of the Hypothalamus-Pituitary-Thyroid Axis to Maternal Separation and Palatable Diet.

Neonatal stress contributes to the development of obesity and has long-lasting effects on elements of the hypothalamus-pituitary-thyroid (HPT) axis. Given the importance of thyroid hormones in metabolic regulation, we studied the effects of maternal separation and a high-fat/high-carbohydrate diet (HFC), offered from puberty or adulthood, on HPT axis activity of adult male and female Wistar rats. Pups were non-handled (NH) or maternally separated (MS) 3 h/day at postnatal days (Pd) 2-21. In a first experiment, at Pd60, rats had access to chow or an HFC diet (cookies, peanuts, chow) for 1 month. Male and female NH and MS rats that consumed the HFC diet increased their caloric intake, body weight, and serum insulin levels; fat weight increased in all groups except in MS males, and serum leptin concentration increased only in females. Mediobasal hypothalamus (MBH) Pomc expression increased in NH-HFC females and Npy decreased in NH-HFC males. MS males showed insulinemia and hypercortisolemia that was attenuated by the HFC diet. The HPT axis activity response to an HFC diet was sex-specific; expression of MBH thyrotropin-releasing hormone-degrading ectoenzyme (Trhde) increased in NH and MS males; serum TSH concentration decreased in NH males, and T4 increased in NH females. In a second experiment, rats were fed chow or an HFC diet from Pd30 or 60 until Pd160 and exposed to 1 h restraint before sacrifice. Regardless of neonatal stress, age of diet exposition, or sex, the HFC diet increased body and fat weight and serum leptin concentration; it induced insulinemia in males, but in females only in Pd30 rats. The HFC diet's capacity to curtail the hypothalamus-pituitary-adrenal axis response to restraint was impaired in MS males. In restrained rats, expression of Trh in the paraventricular nucleus of the hypothalamus, Dio2 and Trhde in MBH, and serum thyroid hormone concentration were altered differently depending on sex, age of diet exposition, and neonatal stress. In conclusion, metabolic alterations associated to an HFC-diet-induced obesity are affected by sex or time of exposition, while various parameters of the HPT axis activity are additionally altered by MS, pointing to the complex interplay that these developmental influences exert on HPT axis activity in adult rats.

Neonatal Maternal Separation Alters, in a Sex-Specific Manner, the Expression of TRH, of TRH-Degrading Ectoenzyme in the Rat Hypothalamus, and the Response of the Thyroid Axis to Starvation.

Hypothalamic-pituitary-thyroid (HPT) axis activity is important for energy homeostasis, and is modified by stress. Maternal separation (MS) alters the stress response and predisposes to metabolic disturbances in the adult. We therefore studied the effect of MS on adult HPT axis activity. Wistar male and female pups were separated from their mothers 3 h/d during postnatal day (PND)2-PND21 (MS), or left nonhandled (NH). Open field and elevated plus maze tests revealed increased locomotion in MS males and anxiety-like behavior in MS females. At PND90, MS females had increased body weight gain, Trh expression in the hypothalamic paraventricular nucleus, and white adipose tissue mass. MS males had increased expression of TRH-degrading enzyme in tanycytes, reduced TSH and T3, and enhanced corticosterone serum concentrations. MS stimulated brown adipose tissue deiodinase 2 activity in either sex. Forty-eight hours of fasting (PND60) augmented serum corticosterone levels similarly in MS or NH females but more in MS than in NH male rats. MS reduced the fasting-induced drop in hypothalamic paraventricular nucleus-Trh expression of males but not of females and abolished the fasting-induced increase in Trh expression in both sexes. Fasting reduced serum concentrations of TSH, T4, and T3, less in MS than in NH males, whereas in females, TSH decreased in MS but not in NH rats, but T4 and T3 decreased similarly in NH and MS rats. In conclusion, MS produced long-term changes in the activity of the HPT axis that were sex specific; response to fasting was partially blunted in males, which could affect their adaptive response to negative energy balance.

Voluntary exercise adapts the hypothalamus-pituitary-thyroid axis in male rats.

The hypothalamic-pituitary thyroid (HPT) axis modulates energy homeostasis. Its activity decreases in conditions of negative energy balance but the effects of chronic exercise on the axis are controversial and unknown at hypothalamic level. Wistar male rats were exposed for up to 14 days to voluntary wheel running (WR), or pair-feeding (PF; 18% food restriction), or to repeated restraint (RR), a mild stressor. WR and RR diminished food intake; body weight gain decreased in the 3 experimental groups, but WAT mass and serum leptin more intensely in the WR group. WR, but not RR, produced a delayed inhibition of central markers of HPT axis activity. At day 14, in WR rats paraventricular nucleus-pro-TRH mRNA and serum TSH levels decreased, anterior pituitary TRH-receptor 1 mRNA levels increased, but serum thyroid hormone levels were unaltered, which is consistent with decreased secretion of TRH and clearance of thyroid hormones. A similar pattern was observed if WR animals were euthanized during their activity phase. In contrast, in PF animals the profound drop of HPT axis activity included decreased serum T3 levels and hepatic deiodinase 1 activity; these changes were correlated with an intense increase in serum corticosterone levels. WR effects on HPT axis were not associated with changes in the activity of the hypothalamic-pituitary adrenal axis, but correlated positively with serum leptin levels. These data demonstrate that voluntary WR adapts the status of the HPT axis, through pathways that are distinct from those observed during food restriction or repeated stress.

Tanycyte pyroglutamyl peptidase II contributes to regulation of the hypothalamic-pituitary-thyroid axis through glial-axonal associations in the median eminence.

Pyroglutamyl peptidase II (PPII), a highly specific membrane-bound metallopeptidase that inactivates TRH in the extracellular space, is tightly regulated by thyroid hormone in cells of the anterior pituitary. Whether PPII has any role in the region where axons containing hypophysiotropic TRH terminate, the median eminence, is unknown. For this purpose, we analyzed the cellular localization and regulation of PPII mRNA in the mediobasal hypothalamus in adult, male rats. PPII mRNA was localized in cells lining the floor and infralateral walls of the third ventricle and coexpressed with vimentin, establishing these cells as tanycytes. PPII mRNA extended in a linear fashion from the tanycyte cell bodies in the base of the third ventricle to its cytoplasmic and end-feet processes in the external zone of the median eminence in close apposition to pro-TRH-containing axon terminals. Compared with vehicle-treated, euthyroid controls, animals made thyrotoxic by the i.p. administration of 10 microg L-T(4) daily for 1-3 d, showed dramatically increased accumulation of silver grains in the mediobasal hypothalamus and an approximately 80% increase in enzymatic activity. PPII inhibition in mediobasal hypothalamic explants increased TRH secretion, whereas i.p. injection of a specific PPII inhibitor increased cold stress- and TRH-induced TSH levels in plasma. We propose that an increase in circulating thyroid hormone up-regulates PPII activity in tanycytes and enhances degradation of extracellular TRH in the median eminence through glial-axonal associations, contributing to the feedback regulation of thyroid hormone on anterior pituitary TSH secretion.

Thyrotropin-releasing hormone regulates the diurnal variation of pyroglutamyl aminopeptidase II activity in the male rat adenohypophysis.

OBJECTIVE: Thyrotropin-releasing hormone (TRH) is inactivated in the extracellular compartment by pyroglutamyl aminopeptidase II (PPII), a narrow specificity ectopeptidase present in the brain and in the lactotrophs of the adenohypophysis. TRH and various hypothalamic/paracrine agents regulate the activity of PPII on the surface of adenohypophyseal cells in primary culture. The activity of the hypothalamic-pituitary-thyroid axis presents circadian variations including an increase of serum thyrotropin levels in the early hours of the day. The purpose of this study was to determine whether adenohypophyseal PPII activity fluctuates during the daytime in the male rat and the role of TRH in these regulatory events in vivo. RESULTS: Adenohypophyseal PPII specific activity and mRNA levels presented diurnal variations. A decrease in specific activity occurred with a minimum between 0930 and 1130 h, associated with increased serum thyrotropin levels. PPII mRNA levels were lowest at 0800 h. Intraperitoneal injection at 0800 or 1000 h of [3-Me-His(2)]-TRH, a potent agonist of the TRH receptor, reduced PPII specific activity at 30 min post-injection which was followed by a return to basal levels at 2 h. A second phase of decrease occurred between 4 and 8 h post-injection. Intravenous injection of a TRH-immune serum induced, at 2 h post-injection, an increase in adenohypophyseal PPII specific activity, which lasted up to 6 h. CONCLUSIONS: Adenohypophyseal PPII activity and mRNA levels fluctuate during the day; TRH down-regulates PPII activity in vivo, contributing to some of these variations. These new findings, and previous data, suggest that adenohypophyseal PPII activity varies in distinct physiological events, in response to endocrine and hypothalamic/paracrine factors, potentially modulating responses to TRH.