Daily limited access to sweetened drink attenuates hypothalamic-pituitary-adrenocortical axis stress responses.

Stress can promote palatable food intake, and consumption of palatable foods may dampen psychological and physiological responses to stress. Here we develop a rat model of daily limited sweetened drink intake to further examine the linkage between consumption of preferred foods and hypothalamic-pituitary-adrenocortical axis responses to acute and chronic stress. Adult male rats with free access to water were given additional twice-daily access to 4 ml sucrose (30%), saccharin (0.1%; a noncaloric sweetener), or water. After 14 d of training, rats readily learned to drink sucrose and saccharin solutions. Half the rats were then given chronic variable stress (CVS) for 14 d immediately after each drink exposure; the remaining rats (nonhandled controls) consumed their appropriate drinking solution at the same time. On the morning after CVS, responses to a novel restraint stress were assessed in all rats. Multiple indices of chronic stress adaptation were effectively altered by CVS. Sucrose consumption decreased the plasma corticosterone response to restraint stress in CVS rats and nonhandled controls; these reductions were less pronounced in rats drinking saccharin. Sucrose or saccharin consumption decreased CRH mRNA expression in the paraventricular nucleus of the hypothalamus. Moreover, sucrose attenuated restraint-induced c-fos mRNA expression in the basolateral amygdala, infralimbic cortex, and claustrum. These data suggest that limited consumption of sweetened drink attenuates hypothalamic-pituitary-adrenocortical axis stress responses, and calories contribute but are not necessary for this effect. Collectively the results support the hypothesis that the intake of palatable substances represents an endogenous mechanism to dampen physiological stress responses.

Estrogen potentiates adrenocortical responses to stress in female rats.

It is well established that estrogens markedly enhance the glucocorticoid response to acute stress in females. However, the precise mechanism responsible for this regulation is poorly understood. Here, we tested whether estrogens enhance the activation of the paraventricular nucleus (PVN) of the hypothalamus by measuring stress-induced c-fos mRNA expression in the PVN of restraint-stressed ovariectomized (OVX) rats treated with physiologically relevant doses of estradiol (E(2)), the major female estrogen. As expected, E(2) enhanced plasma corticosterone responses to restraint in OVX females. However, E(2) markedly attenuated the stress-induced c-fos gene expression in the PVN and inhibited plasma ACTH responses in these animals. Furthermore, E(2)-inhibitory effects were mimicked by progesterone (P) alone or in combination with E(2). Interestingly, the suppressive central effects of both E(2) and P were apparently independent of basal paraventricular corticotropin-releasing hormone (CRH) transcription, since these ovarian steroids did not significantly affect PVN CRH mRNA expression in unstressed rats. These unexpected findings suggested that E(2) promotes glucocorticoid hypersecretion in females by additional peripheral (i.e., adrenal) mechanisms. Indeed, E(2) markedly enhanced plasma corticosterone responses and adrenal corticosterone content in dexamethasone-blocked OVX rats challenged with varying doses of exogenous ACTH. These results suggest that enhanced adrenal sensitive to ACTH is an important physiological mechanism mediating E(2)-related glucocorticoid hypersecretion in stressed females.

Chronic stress induces adrenal hyperplasia and hypertrophy in a subregion-specific manner.

The adrenal gland is an essential stress-responsive organ that is part of both the hypothalamic-pituitary-adrenal axis and the sympatho-adrenomedullary system. Chronic stress exposure commonly increases adrenal weight, but it is not known to what extent this growth is due to cellular hyperplasia or hypertrophy and whether it is subregion specific. Moreover, it is not clear whether increased production of adrenal glucocorticoid after chronic stress is due to increased sensitivity to adrenocorticotropic hormone (ACTH) vs. increased maximal output. The present studies use a 14-day chronic variable stress (CVS) paradigm in adult male rats to assess the effects of chronic stress on adrenal growth and corticosterone steroidogenesis. Exogenous ACTH administration (0-895 ng/100 g body wt) to dexamethasone-blocked rats demonstrated that CVS increased maximal plasma and adrenal corticosterone responses to ACTH without affecting sensitivity. This enhanced function was associated with increased adrenal weight, DNA and RNA content, and RNA/DNA ratio after CVS, suggesting that both cellular hyperplasia and hypertrophy occurred. Unbiased stereological counting of cells labeled for Ki67 (cell division marker) or 4,6-diamidino-2-phenylindole (nuclear marker), combined with zone specific markers, showed that CVS induced hyperplasia in the outer zona fasciculata, hypertrophy in the inner zona fasciculata and medulla, and reduced cell size in the zona glomerulosa. Collectively, these results demonstrate that increased adrenal weight after CVS is due to hyperplasia and hypertrophy that occur in specific adrenal subregions and is associated with increased maximal corticosterone responses to ACTH. These chronic stress-induced changes in adrenal growth and function may have implications for patients with stress-related disorders.

The medial prefrontal cortex differentially regulates stress-induced c-fos expression in the forebrain depending on type of stressor.

The medial prefrontal cortex (mPFC) plays an important inhibitory role in the hypothalamic-pituitary-adrenal (HPA) axis response. The involvement of the mPFC appears to depend on the type of stressor, preferentially affecting 'psychogenic' stimuli. In this study, we mapped expression of c-fos mRNA to assess the neural circuitry underlying stressor-specific actions of the mPFC on HPA reactivity. Thus, groups of mPFC-lesioned and sham-operated rats were restrained for 20 min or exposed to ether fumes for 2 min. In both cases, the animals were killed at 40 min from the onset of stress. Interestingly, bilateral lesions of the mPFC significantly enhanced c-fos mRNA expression in the hypothalamic paraventricular nucleus of restrained animals, an effect that was paralleled by potentiation of circulating ACTH concentrations in these animals. On the other hand, lesions of the mPFC did not affect neither PVN c-fos mRNA expression nor plasma ACTH concentrations in animals exposed to ether. Lesions of the mPFC also enhanced c-fos activation in the medial amygdala following restraint, but not following ether exposure. Additional regions whose activity was affected by mPFC lesions or stressor differences included the ventrolateral division of the bed nucleus of the stria terminalis, CA3 hippocampus, piriform cortex, and dorsal endopiriform nucleus. Expression of c-fos mRNA was nearly absent in the central amygdala of all stressed animals, regardless of lesion. Furthermore, prefrontal cortex lesions did not change stress-induction levels of c-fos in the CA1 hippocampus, dentate gyrus, anteromedial division of the bed nucleus of the stria terminalis, lateral septum, and claustrum. Taken together, this study indicates that the medial prefrontal cortex differentially regulates cellular activation of specific stress-related brain regions, thus exerting stressor-dependent inhibition of the HPA axis.

Stress integration after acute and chronic predator stress: differential activation of central stress circuitry and sensitization of the hypothalamo-pituitary-adrenocortical axis.

Predator exposure is a naturalistic stressor of high ethological relevance. In the current study, our group examined central and peripheral integration of stress responses in rats after acute or repeated exposure to a natural predator (cat). Acute cat exposure rapidly induced hypothalamo-pituitary-adrenocortical (HPA) axis activation and paraventricular nucleus (PVN) CRH mRNA production. Repeated daily cat exposure (7 and 14 d) also up-regulated PVN mRNA CRH expression, but did not result in frank adrenocortical hyperactivity. Unlike other chronic homotypic stress regimens, repeated cat exposure facilitated corticosterone secretion after the 6th or 13th day of exposure. Notably, ACTH secretion and central amygdaloid nucleus CRH mRNA expression were enhanced in animals that were preexposed to the holding chamber relative to chamber-naive rats, suggesting that contextual cues can sensitize subsequent responses to a fearful stimulus. Analysis of c-fos activation was then used to identify brain circuits activated by acute predator stress. Cat exposure elicited a pattern of central c-fos activation that differed substantially from that after either restraint or hypoxia. Predator-specific c-fos mRNA induction was observed in several brain regions comprising the hypothetical brain defense circuit (bed nucleus of the stria terminalis, medial region of the ventromedial nucleus, and dorsal premammillary nucleus). Surprisingly, acute cat exposure did not induce c-fos expression in the PVN. In summary, the data indicate that 1) predation stress invokes a unique stress circuitry that promotes homotypic sensitization of the HPA axis, and 2) familiarization of animals to testing environments can prime central stress pathways to respond robustly to novel threats.

CNS glucagon-like peptide-1 receptors mediate endocrine and anxiety responses to interoceptive and psychogenic stressors.

Responses to stressors serve to adjust physiology and behavior to increase short-term survival at the potential expense of increasing susceptibility to disease over the long term. We show that glucagon-like peptide-1 (7-36) amide (GLP-1) increases levels of the stress-activated hormones ACTH and corticosterone when administered directly into the rat brain and increases levels of anxiety as measured by the elevated plus maze. The endocrine response is preferentially activated by GLP-1 administration in the paraventricular nucleus of the hypothalamus, whereas the anxiety response is preferentially activated by administration in the central nucleus of the amygdala. Furthermore, GLP-1 antagonists block increases in stress hormones associated with the toxin LiCl and both the endocrine and anxiety responses to vertical heights. Although diverse neural circuits must necessarily process disparate stressors, the current data implicate a role for the GLP-1 system as a critical mediator of multiple stress responses.

Stress activation of cortex and hippocampus is modulated by sex and stage of estrus.

Sex plays a major role in stress integration and stress-related affective disease states. Notably, neurocircuits regulating organismic responses to stress are prime targets for central gonadal steroid action. To assess the roles of sex and estrous cycle in central stress integration, we analyzed c-fos mRNA expression in hypothalamic-pituitary-adrenocortical-related regions of stressed male and cycling female (proestrous, estrous, and diestrous) rats. At 60 min after the onset of acute restraint stress, all animal groups showed induction of c-fos mRNA in the frontal cortex, cingulate cortex, piriform cortex, hippocampus, hypothalamic paraventricular nucleus (PVN), medial amygdala, and lateral septum. However, the magnitude of c-fos induction in cortical and hippocampal regions was substantially lower in proestrous and estrous females compared with males and diestrous females. Sex- and estrus cycle-related changes are region specific, as no difference in c-fos induction occurred in the hypothalamic PVN, medial amygdala, or ventrolateral septum in any group. Furthermore, induction of c-fos mRNA in limbic cortexes (but not hippocampus) was positively correlated with progesterone and negatively correlated with ACTH levels. Taken together, this study indicates that cortical structures are differentially stress activated in females depending on the phase of the estrous cycle, perhaps in a progesterone-dependent fashion.