Victor Viau

The role of the medial prefrontal cortex (cingulate gyrus) in the regulation of hypothalamic-pituitary-adrenal responses to stress

In the studies reported here we have examined the role of the medial prefrontal cortex (MpFC) in regulating hypothalamic-pituitary-adrenal (HPA) activity under basal and stressful conditions. In preliminary studies we characterized corticosteroid receptor binding in the rat MpFC. The results revealed high-affinity (Kd approximately 1 nM) binding with a moderate capacity (42.9 +/- 3 fmol/mg) for 3H- aldosterone (with a 50-fold excess of cold RU28362; mineralocorticoid receptor) and high-affinity (Kd approximately 0.5–1.0 nM) binding with higher capacity (183.2 +/- 22 fmol/mg) for 3H-RU 28362 (glucocorticoid receptor). Lesions of the MpFC (cingulate gyrus) significantly increased plasma levels of both adrenocorticotropin (ACTH) and corticosterone (CORT) in response to a 20 min restraint stress. The same lesions had no effect on hormone levels following a 2.5 min exposure to ether. Implants of crystalline CORT into the same region of the MpFC produced a significant decrease in plasma levels of both ACTH and CORT with restraint stress, but again, there was no effect with ether stress. Neither MpFC lesions nor CORT implants had any consistent effect on A.M. or P.M. levels of plasma ACTH or CORT. Manipulations of MpFC function were not associated with changes in the clearance rate for CORT or in corticosteroid receptor densities in the pituitary, hypothalamus, hippocampus, or amygdala. Taken together, these findings suggest that MpFC is a target site for the negative-feedback effects of glucocorticoids on stress-induced HPA activity, and that this effect is dependent upon the nature of the stress.

Functional Cross-Talk Between the Hypothalamic-Pituitary-Gonadal and -Adrenal Axes

Under normal conditions, the adrenal glucocorticoids, the endproduct of the hypothalamic‐pituitary‐adrenal (HPA) axis, provide a frontline of defence against threats to homeostasis (i.e. stress). On the other hand, chronic HPA drive and glucocorticoid hypersecretion have been implicated in the pathogenesis of several forms of systemic, neurodegenerative and affective disorders. The HPA axis is subject to gonadal influence, indicated by sex differences in basal and stress HPA function and neuropathologies associated with HPA dysfunction. Functional cross‐talk between the gonadal and adrenal axes is due in large part to the interactive effects of sex steroids and glucocorticoids, explaining perhaps why several disease states linked to stress are sex‐dependent. Realizing the interactive nature by which the hypothalamic‐pituitary‐gonadal and HPA systems operate, however, has made it difficult to model how these hormones act in the brain. Manipulation of one endocrine system is not without effects on the other. Simultaneous manipulation and assessment of both endocrine systems can overcome this problem. This dual approach in the male rat reveals that testosterone can act and interact on different aspects of basal and stress HPA function. Basal adrenocorticotropic hormone (ACTH) release is regulated by testosterone‐dependent effects on arginine vasopressin synthesis, and corticosterone‐dependent effects on corticotropin‐releasing hormone (CRH) synthesis in the paraventricular nucleus (PVN) of the hypothalamus. In contrast, testosterone and corticosterone interact on stress‐induced ACTH release and drive to the PVN motor neurones. Candidate structures mediating this interaction include several testosterone‐sensitive afferents to the HPA axis, including the medial preoptic area, central and medial amygdala and bed nuclei of the stria terminalis. All of these relay homeostatic information and integrate reproductive and social behaviour. Because these modalities are affected by stress in humans, a dual systems approach holds great promise in establishing further links between the neuroendocrinology of stress and the central bases of sex‐dependent disorders, including psychiatric, cardiovascular and metabolic disease.

Neonatal Handling Alters Adrenocortical Negative Feedback Sensitivity and Hippocampal Type II Glucocorticoid Receptor Binding in the Rat

Adult rats handled (H) daily for the first 3 weeks of life show a dramatically altered adrenocortical response to stress. We found that H animals secreted less ACTH and corticosterone (B) during and following the termination of stress than did nonhandled (NH) controls. In contrast, H and NH animals did not differ in basal B secretion at any point in the diurnal cycle, nor in adrenocortical responses to exogenously administered oCRF or ACTH. Moreover, the clearance rate for B was similar in H and NH animals. H animals were more sensitive than NH animals to the inhibitory effects of either B or dexamethasone on stress-induced adrenocortical activity. In a dose-response study, both glucocorticoids administered 3 h prior to testing suppressed the adrenocortical response to a 20-min restraint stress to a greater extent in the H animals. Handling increased type II, glucocorticoid receptor binding capacity in the hippocampus of adult animals (approximately 50% increase in capacity, with no change in affinity). There were no handling-induced changes in type II receptor binding capacity in the hypothalamus or pituitary, nor in type I receptor binding capacity in the hippocampus. Following chronic (5 mg/kg/day) treatment with B, hippocampal type II receptor binding capacity was significantly reduced in the B-treated H animals, compared with saline-treated H animals, and indistinguishable from saline-treated NH animals. Down-regulated H animals, like NH animals, hypersecreted B following the termination of stress in comparison to the saline-treated H animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Starvation: Early Signals, Sensors, and Sequelae*

To identify the sequences of changes in putative signals, reception of these and responses to starvation, we sampled fed and starved rats at 2- to 6-h intervals after removal of food 2 h before dark. Metabolites, hormones, hypothalamic neuropeptide expression, fat depots, and leptin expression were measured. At 2 h, insulin decreased, and FFA and corticosterone (B) increased; by 4 h, leptin and glucose levels decreased. Neuropeptide Y messenger RNA (mRNA) increased 6 h after food removal and thereafter. Adrenal and plasma B did not follow ACTH and were elevated throughout, with a nadir at the dark-light transition. Leptin correlated inversely with adrenal B. Fat stores decreased during the last 12 h. Leptin mRNA in perirenal and sc fat peaked during the dark period, resembling plasma leptin in fed rats. We conclude that 1) within the first 4 h, hormonal and metabolic signals relay starvation-induced information to the hypothalamus; 2) hypothalamic neuropeptide synthesis responds rapidly to the altered metabolic signals; 3) catabolic activity quickly predominates, reinforced by elevated B, not driven by ACTH, but possibly to a minor extent by leptin, and more by adrenal neural activity; and 4) leptin secretion decreases before leptin mRNA or fat depot weight, showing synthesis-independent regulation. (Endocrinology 140: 4015‐ 4023, 1999)

Endogenous cannabinoid signaling is essential for stress adaptation

Secretion of glucocorticoid hormones during stress produces an array of physiological changes that are adaptive and beneficial in the short term. In the face of repeated stress exposure, however, habituation of the glucocorticoid response is essential as prolonged glucocorticoid secretion can produce deleterious effects on metabolic, immune, cardiovascular, and neurobiological function. Endocannabinoid signaling responds to and regulates the activity of the hypothalamic–pituitary–adrenal (HPA) axis that governs the secretion of glucocorticoids; however, the role this system plays in adaptation of the neuroendocrine response to repeated stress is not well characterized. Herein, we demonstrate a divergent regulation of the two endocannabinoid ligands, N-arachidonylethanolamine (anandamide; AEA) and 2-arachidonoylglycerol (2-AG), following repeated stress such that AEA content is persistently decreased throughout the corticolimbic stress circuit, whereas 2-AG is exclusively elevated within the amygdala in a stress-dependent manner. Pharmacological studies demonstrate that this divergent regulation of AEA and 2-AG contribute to distinct forms of HPA axis habituation. Inhibition of AEA hydrolysis prevented the development of basal hypersecretion of corticosterone following repeated stress. In contrast, systemic or intra-amygdalar administration of a CB1 receptor antagonist before the final stress exposure prevented the repeated stress-induced decline in corticosterone responses. The present findings demonstrate an important role for endocannabinoid signaling in the process of stress HPA habituation, and suggest that AEA and 2-AG modulate different components of the adrenocortical response to repeated stressor exposure.

Basal ACTH, corticosterone and corticosterone-binding globulin levels over the diurnal cycle, and age-related changes in hippocampal type I and type II corticosteroid receptor binding capacity in young and aged, handled and nonhandled rats.

Basal corticosterone (B) levels increase with age in the rat, a result of decreased negative-feedback inhibition of hypothalamic-pituitary-adrenal (HPA) activity. Postnatal handling increases CNS negative-feedback sensitivity and appears to attenuate some of the changes occurring in the HPA axis in later life. In the experiments described here, we have examined basal HPA function in young (6-8 months) and old (22 months), handled (H) and nonhandled (NH) rats in relation to changes in corticosteroid receptor binding. Among young animals, there were no group differences in basal adrenocorticotropin (ACTH) or B levels at any point in the diurnal cycle. In contrast, plasma ACTH and B levels during the PM phase were significantly higher in old NH animals in comparison to old H animals and to both groups of young animals. The H and NH groups did not differ in in vivo adrenal responsiveness to exogenous ACTH. As expected, ACTH sensitivity was greater in all groups during the PM phase and in general, old animals showed a greater response to ACTH regardless of the treatment group. There were no differences across the groups in AM plasma corticosterone-binding globulin (CBG) levels. However, during the PM phase of the cycle, CBG levels were significantly lower and the percentage of B in the free form was significantly higher in the old NH animals. As expected, levels of free B during the PM phase of the cycle were significantly higher in the old NH animals. Thus, there is a significant increase in the PM corticoid signal in the old NH animals that occurs as a function of elevated B and decreased CBG levels; these age-related changes in basal HPA activity were not seen in the old H animals. Type I (mineralocorticoid-like) receptor binding in the hippocampus did not differ as a function of handling and was significantly reduced with age in both H and NH animals. Type II (glucocorticoid) receptor binding decreased as a function of age in both H and NH animals, but was consistently higher in the H animals. There were no differences in type II receptor binding in the hypothalamus or pituitary as a function of age or handling. These data suggest that the increase in basal HPA activity occurring in aged rats is largely restricted to the dark phase of the cycle and is attenuated by postnatal handling, a treatment that increases hippocampal type II corticosteroid receptor binding.

Recruitment of Prefrontal Cortical Endocannabinoid Signaling by Glucocorticoids Contributes to Termination of the Stress Response

The mechanisms subserving the ability of glucocorticoid signaling within the medial prefrontal cortex (mPFC) to terminate stress-induced activation of the hypothalamic–pituitary–adrenal (HPA) axis are not well understood. We report that antagonism of the cannabinoid CB1 receptor locally within the mPFC prolonged corticosterone secretion following cessation of stress in rats. Mice lacking the CB1 receptor exhibited a similar prolonged response to stress. Exposure of rats to stress produced an elevation in the endocannabinoid 2-arachidonoylglycerol within the mPFC that was reversed by pretreatment with the glucocorticoid receptor antagonist RU-486 (20 mg/kg). Electron microscopic and electrophysiological data demonstrated the presence of CB1 receptors in inhibitory-type terminals impinging upon principal neurons within layer V of the prelimbic region of the mPFC. Bath application of corticosterone (100 nm) to prefrontal cortical slices suppressed GABA release onto principal neurons in layer V of the prelimbic region, when examined 1 h later, which was prevented by application of a CB1 receptor antagonist. Collectively, these data demonstrate that the ability of stress-induced glucocorticoid signaling within mPFC to terminate HPA axis activity is mediated by a local recruitment of endocannabinoid signaling. Endocannabinoid activation of CB1 receptors decreases GABA release within the mPFC, likely increasing the outflow of the principal neurons of the prelimbic region to contribute to termination of the stress response. These data support a model in which endocannabinoid signaling links glucocorticoid receptor engagement to activation of corticolimbic relays that inhibit corticosterone secretion.

Suppression of Amygdalar Endocannabinoid Signaling by Stress Contributes to Activation of the Hypothalamic-Pituitary-Adrenal Axis

Endocannabinoids inhibit hypothalamic–pituitary–adrenal (HPA) axis activity; however, the neural substrates and pathways subserving this effect are not well characterized. The amygdala is a forebrain structure that provides excitatory drive to the HPA axis under conditions of stress. The aim of this study was to determine the contribution of endocannabinoid signaling within distinct amygdalar nuclei to activation of the HPA axis in response to psychological stress. Exposure of rats to 30-min restraint stress increased the hydrolytic activity of fatty acid amide hydrolase (FAAH) and concurrently decreased content of the endocannabinoid/CB1 receptor ligand N-arachidonylethanolamine (anandamide; AEA) throughout the amygdala. In stressed rats, AEA content in the amygdala was inversely correlated with serum corticosterone concentrations. Pharmacological inhibition of FAAH activity within the basolateral amygdala complex (BLA) attenuated stress-induced corticosterone secretion; this effect was blocked by co-administration of the CB1 receptor antagonist AM251, suggesting that stress-induced decreases in CB1 receptor activation by AEA contribute to activation of the neuroendocrine stress response. Local administration into the BLA of a CB1 receptor agonist significantly reduced stress-induced corticosterone secretion, whereas administration of a CB1 receptor antagonist increased corticosterone secretion. Taken together, these findings suggest that the degree to which stressful stimuli reduce amygdalar AEA/CB1 receptor signaling contributes to the magnitude of the HPA response.

Molecular basis for the development of individual differences in the hypothalamic-pituitary-adrenal stress response

Summary1.Several years ago, investigators described the effects of infantile handling on the development of hypothalamic-pituitary-adrenal (HPA) responses to stress in the rat. Rat pups exposed to brief periods of innocuous handling early in life showed reduced HPA responses to a wide variety of stressors, and the effect persists throughout the life of the animal. These effects are robust and provide an excellent model for understanding how early environmental stimuli, which are external to the organism, alter neural differentiation and, thus, neuroendocrine responsivity to stress.2.This paper reviews the endocrine mechanisms affected by early handling and our current understanding of the neural transduction of environmental events and their effects at the level of the target neurons (in the hippocampus and frontal cortex).3.In brief, handling serves to increase glucocorticoid receptor gene transcription, increasing sensitivity to glucocorticoid negative feedback regulation and, thus, altering the activity within hypothalamic corticotropin-releasing factor/vasopressin neurons. Together these changes serve to determine neuroendocrine responsivity to stress.

Both estrogen receptor α and estrogen receptor β agonists enhance cell proliferation in the dentate gyrus of adult female rats

This study investigated the involvement of estrogen receptors α and β in estradiol-induced enhancement of hippocampal neurogenesis in the adult female rat. Subtype selective estrogen receptor agonists, propyl-pyrazole triol (estrogen receptor α agonist) and diarylpropionitrile (estrogen receptor β agonist) were examined for each receptor’s contribution, individual and cooperative, for estradiol-enhanced hippocampal cell proliferation. Estradiol increases hippocampal cell proliferation within 4 h [Ormerod BK, Lee TT, Galea LA (2003) Estradiol initially enhances but subsequently suppresses (via adrenal steroids) granule cell proliferation in the dentate gyrus of adult female rats. J Neurobiol 55:247–260]. Therefore, animals received s.c. injections of estradiol (10 μg), propyl-pyrazole triol and diarylpropionitrile alone (1.25, 2.5, 5.0 mg/0.1 ml dimethylsulfoxide) or in combination (2.5 mg propyl-pyrazole triol+2.5 mg diarylpropionitrile/0.1 ml dimethylsulfoxide) and 4 h later received an i.p. injection of the cell synthesis marker, bromodeoxyuridine (200 mg/kg). Diarylpropionitrile enhanced cell proliferation at all three administered doses (1.25 mg, P<0.008; 2.5 mg, P<0.003; 5 mg, P<0.005), whereas propyl-pyrazole triol significantly increased cell proliferation (P<0.0002) only at the dose of 2.5 mg. Our results demonstrate both estrogen receptor α and estrogen receptor β are individually involved in estradiol-enhanced cell proliferation. Furthermore both estrogen receptor α and estrogen receptor β mRNA was found co-localized with Ki-67 expression in the hippocampus albeit at low levels, indicating a potential direct influence of each receptor subtype on progenitor cells and their progeny. Dual receptor activation resulted in reduced levels of cell proliferation, supporting previous studies suggesting that estrogen receptor α and estrogen receptor β may modulate each other’s activity. Our results also suggest that a component of estrogen receptor–regulated cell proliferation may take place through alternative ligand and/or cell-signaling mechanisms.