No abstract

Previous work has indicated that acute and repeated stress can alter thyroid hormone secretion. Corticosterone, the end product of hypothalamic-pituitary-adrenal (HPA) axis activation and strongly regulated by stress, has been suggested to play a role in hypothalamic-pituitary-thyroid (HPT) axis regulation. In the current study, we sought to further characterize HPT axis activity after repeated exposure to inescapable foot-shock stress (FS), and to examine changes in proposed regulators of the HPT axis, including plasma corticosterone and hypothalamic arcuate nucleus agouti-related protein (AGRP) mRNA levels. Adult male Sprague-Dawley rats were subjected to one daily session of inescapable FS for 14 days. Plasma corticosterone levels were determined during and after the stress on days 1 and 14. Animals were killed on day 15, and trunk blood and brains were collected for measurement of hormone and mRNA levels. Repeated exposure to FS led to a significant decrease in serum levels of 3,5,3′-triiodothyronine (T₃) and 3,5,3′,5′-tetraiodothyronine (T₄). Stress-induced plasma corticosterone levels were not altered by repeated exposure to the stress. Despite the decrease in peripheral hormone levels, thyrotropin-releasing hormone (TRH) mRNA levels within the paraventricular nucleus of the hypothalamus were not altered by the stress paradigm. Arcuate nucleus AGRP mRNA levels were significantly increased in the animals exposed to repeated FS. Additionally, we noted significant correlations between stress-induced plasma corticosterone levels and components of the HPT axis, including TRH mRNA levels and free T₄ levels. Additionally, there was a significant correlation between AGRP mRNA levels and total T₃ levels. Changes in body weight were also correlated with peripheral corticosterone and TRH mRNA levels. These results suggest that repeated exposure to mild-electric foot-shock causes a decrease in peripheral thyroid hormone levels, and that components of the HPA axis and hypothalamic AGRP may be involved in stress regulation of the HPT.

The paraventricular nucleus of the hypothalamus (PVN) serves as the origin of the final common pathway in the secretion of glucocorticoid hormones in response to stress. Various stress-related inputs converge upon the cells of the medial parvocellular division of the PVN. These neurons, which synthesize and release corticotropin-releasing hormone, arginine vasopressin, and other secretagogues, are responsible for a cascade of events which culminates in the adrenocorticotropin-induced release of corticosteroids from the adrenal cortex. Previous data have suggested complex afferent regulation of PVN neurons, although the neuronal pathways by which the effects of stress are mediated remain to be fully disclosed. The present experiment sought to identify forebrain areas potentially involved in afferent regulation of the PVN in response to an acute stressor. Discrete injections of the retrograde tracer Fluoro-gold were delivered to the PVN, and rats were subsequently subjected to an acute swim stress. Brains were processed immunocytochemically for the simultaneous detection of the tracer and Fos, the protein product of the immediate early gene c-fos, utilized as a marker for neuronal activation. The majority of Fluoro-gold/Fos labeled neurons were detected in the parastrial nucleus, the medial preoptic area, the anterior hypothalamic area, the dorsomedial hypothalamic nucleus and adjacent posterior hypothalamic area, and, to a lesser extent, the supramammillary nucleus. These findings are discussed in relation to neural pathways mediating activation and inhibition of the hypothalamic-pituitary-adrenocortical axis.

All stress-related inputs are conveyed to the hypothalamus via several brain areas and integrated in the parvocellular division of the paraventricular nucleus (PVN) where corticotropin-releasing hormone (CRH) is synthesized. Arginine vasopressin (AVP) is present in both magnocellular and parvocellular divisions of the PVN, and the latter population of AVP is colocalized with CRH. CRH and AVP are co-secreted in the face of certain stressful stimuli, and synthesis of both peptides is suppressed by glucocorticoid. CRH and AVP stimulate corticotropin (ACTH) secretion synergistically, but the physiological relevance of the dual corticotroph regulation is not understood. Norepinephrine (NE) is a well known neurotransmitter that regulates CRH neurons in the PVN. We explored the mode of action of NE on CRH and AVP gene transcription in the PVN to examine the effect of the neurotransmitter on multiple genes that are responsible for a common physiological function. After NE injection into the PVN of conscious rats, CRH heteronuclear (hn) RNA increased rapidly and markedly in the parvocellular division of the PVN. AVP hnRNA did not change significantly in either the parvocellular or magnocellular division of the PVN after NE injection. The present results show that the transcription of CRH and AVP genes is differentially regulated by NE, indicating the complexity of neurotransmitter regulation of multiple releasing hormone genes in a discrete hypothalamic neuronal population.

To determine whether short periods of fasting can suppress the activity of the reproductive axis in normal healthy men, eight men were studied on a fed day and again after 48 h of fasting. Subjects were between 20-32 yr of age and ranged from 84-119% of normal body weight. Blood samples were collected on day 1 (a fed day) and day 3 (after 48 h of fasting) at 15-min intervals from 0800-1600 h through indwelling venous catheters. Fasting for 48 h resulted in a significant decrease in mean LH, FSH, and testosterone concentrations. The mean LH concentration decreased from 2.94 +/- 0.59 IU/L on the fed day to 1.07 +/- 0.14 IU/L after 48 h of fasting, and there was an accompanying decrease in LH pulse frequency (from 5.13 +/- 0.29 to 2.63 +/- 0.62 pulses/8 h) and mean baseline LH concentration (from 1.83 +/- 0.52 to 0.51 +/- 0.07 IU/L), but no significant decrease in LH pulse amplitude. In a second study, blood samples were collected from five subjects who were allowed to eat normally between days 1 and 3; these individuals showed no difference in LH secretion. To begin to examine the possibility that an activation of the hypothalamic-pituitary-adrenal axis leads to the suppression of reproductive hormone levels that occurred after 48 h of fasting, cortisol levels were measured in all plasma samples. There was no significant difference in mean cortisol concentrations on fed vs. fasted days or when cortisol concentrations were examined as hourly means across the 2 days. These results indicate that activity of the reproductive axis can be suppressed in normal healthy men by 48 h of fasting. It appears unlikely that activation of the adrenal axis is the cause of this suppression of reproductive axis activity.

No abstract

Cross-sectional and short-term prospective studies in humans support the concept that low energy availability, and not other factors associated with exercise, causes the development of exercise-induced reproductive dysfunction. To rigorously test this hypothesis, we performed a longitudinal study, examining the role of low energy availability on both the development and the reversal of exercise-induced amenorrhea, using a monkey model (Macaca fascicularis). Eight adult female monkeys developed amenorrhea (defined as absence of menses for at least 100 d, with low and unchanging concentrations of LH, FSH, E2, and P4) after gradually increasing their daily exercise to 12.3 +/- 0.9 km/d of running over a 7- to 24-month period. Food intake remained constant during exercise training. To test whether amenorrhea is caused by low energy availability, four of the eight amenorrheic monkeys were provided with supplemental calories (138-181% of calorie intake during amenorrhea) while they maintained their daily training. All four monkeys exhibited increased reproductive hormone levels and reestablished ovulatory cycles, with recovery times for circulating gonadotropin levels ranging from 12-57 d from the initiation of supplemental feeding. The rapidity of recovery within the reproductive axis in a given monkey was directly related to the amount of energy that was consumed during the period of supplemental feeding (r = -0.97; P < 0.05). Repeated measurements of plasma T3 concentrations, a marker of cellular energy availability, revealed a tight correlation between the changes in reproductive function and T3 levels, such that T3 significantly decreased (27%) with the induction and significantly increased (18%) with the reversal of amenorrhea (P < 0.05). These data provide strong evidence that low energy availability plays a causal role in the development of exercise-induced amenorrhea.

There has been little research on effects of chronic stressors on neuroendocrine function in adolescence despite increasing evidence of enduring effects of stressors during this period on behaviour in adulthood. We previously reported that social stress (SS: daily 1 h isolation and new cage partner for 16 days) in adolescence altered locomotor responses to psychostimulants in adulthood. Here, we investigated neuroendocrine responses over the duration of the procedure that may underlie the enduring effects of SS. SS rats were compared to rats undergoing daily isolation only (ISO) and controls (CTL) to determine responses to acute and repeated isolation with and without social instability. At 30 days of age (first isolation), higher plasma corticosterone and corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus and in the central nucleus of the amygdala (CeA) were found in males caged with a new partner (SS) after isolation than those returned to their original partner (ISO). On day 45, SS males and females showed less habituation (higher bioactive levels of corticosterone based on plasma corticosterone and corticosteroid binding globulin levels) to the 16th episode of isolation than did ISO. SS and ISO had higher baseline expression of CRH mRNA in the PVN on day 45 than did CTL, and only CTL had increased levels after isolation. CRH mRNA expression in the CeA increased to a first isolation in CTL and to a 16th isolation in SS but not in ISO males. Modest differences in social interactions were observed between SS and ISO when returned to their cages after isolation. The results suggest that mild social stressors in adolescence impede neuroendocrine adaptation to homotypic stressors. The resultant increase in exposure to glucocorticoids over adolescence may alter ongoing brain development and increase vulnerability to psychopathology.