Vladimir K. Patchev

The neurosteroid tetrahydroprogesterone counteracts corticotropin-releasing hormone-induced anxiety and alters the release and gene expression of corticotropin-releasing hormone in the rat hypothalamus

The ring-A-reduced progesterone derivative 5 alpha-pregnan-3 alpha-ol-20-one (tetrahydroprogesterone) is synthesized under normal physiological conditions in the brain and is a potent modulator of the GABA receptor. This neurosteroid has significant sedative and anxiolytic properties. Corticotropin-releasing hormone plays a major role in stress-induced activation of the hypothalamo-pituitary-adrenal axis, and sustained hyperactivity of hypothalamic corticotropin-releasing hormone-producing neurons may be causally related to both, increased pituitary-adrenal secretion and behavioural symptoms observed in anxiety and affective disorders. We investigated the effect of tetrahydroprogesterone on corticotropin-releasing hormone-induced anxiety, the basal and methoxamine-stimulated release of corticotropin-releasing hormone from hypothalamic organ explants in vitro, and adrenalectomy-induced up-regulation of the gene expression of corticotropin-releasing hormone in the hypothalamic paraventricular nucleus in rats. At doses of 5 and 10 micrograms i.c.v., tetrahydroprogesterone counteracted the anxiogenic action of 0.5 microgram of corticotropin-releasing hormone. Tetrahydroprogesterone did not alter the basal release of corticotropin-releasing hormone in vitro, but suppressed the stimulatory effect of the alpha 1-adrenergic agonist methoxamine on this parameter. Measurements of the steady-state levels of mRNA coding for corticotropin-releasing hormone by quantitative in situ-hybridization histochemistry revealed that tetrahydroprogesterone was equipotent with corticosterone in preventing adrenalectomy-induced up-regulation of peptide gene expression. Systemic administration of tetrahydroprogesterone also restrained adrenalectomy-induced thymus enlargement. These results demonstrate that tetrahydroprogesterone has anxiolytic effects that are mediated through interactions with hypothalamic corticotropin-releasing hormone in both, genomic and non-genomic fashions.

The neurosteroid tetrahydroprogesterone attenuates the endocrine response to stress and exerts glucocorticoid-like effects on vasopressin gene transcription in the rat hypothalamus

The neurosteroid tetrahydroprogesterone (5α-pregnan-3α-ol-20-one, allopregnanolone, THP), has been previously shown to counteract the anxiogenic effects of corticotropin-releasing hormone (CRH) and to interfere with noradrenergic and corticosteroid-mediated regulation of CRH release and gene transcription. Those observations indicated that, besides its sedative and analgesic activity, THP may also affect the neuroendocrine response to stress in a mode resembling that of corticosteroids. To examine this possibility, we compared the ability of THP, its precursor progesterone (P4), and the glucocorticoids dexamethasone (DEX) and corticosterone (CORT) to influence the pituitary-adrenal response to acute emotional stress and the adrenalectomy-induced increase in the gene transcription of the stress-related peptide arginine vasopressin (AVP) and of corticosteroid receptors (MR and GR) in the brain. Pretreatment of rats with a single dose of THP or P4 (50 μg/kg) significantly attenuated the elevation of plasma adrenocorticotropin (ACTH) and serum corticosterone after emotional stress; both steroids were, however, less potent than a similar dose of DEX. Administration of 1 mg of THP, CORT, or P4 to adrenalectomized (ADX) rats attenuated the increase in AVP mRNA levels in the ventromedial subdivision of the hypothalamic paraventricular nucleus (PVN), as compared with vehicle-treated ADX rats. However, whereas CORT and P4 influenced the ADX-induced increase in the transcription of both types of corticosteroid receptors in the hippocampus, these were unaffected by THP. In contrast to the glucocorticoids, THP and P4 failed to decrease plasma ACTH levels in rats deprived of endogenous steroids. These results demonstrate that the neurosteroid THP and its precursor P4 resemble glucocorticoids in their suppression of the pituitary-adrenal response to emotional stress; however, THP influences the transcription of glucocorticoid-responsive genes in brain structures involved in the regulation of the hypothalamo-pituitary-adrenal system in a fashion that is quite distinct from that obtained with glucocorticoids.

Implications of estrogen-dependent brain organization for gender differences in hypothalamo-pituitary-adrenal regulation.

Estrogens, derived from the aromatization of testosterone in the brain, account for sex‐specific organization of neural circuits controlling gonadotropin release and sexual behavior. This study examines the possible organizing role of perinatal gonadal steroids in the manifestation of known, albeit unexplained, male‐female differences in basal and stress‐related adrenocortical secretion. We document here the existence of gender‐specific differences in the gene expression of hypothalamic corticotropin‐releasing hormone (CRH), and hippocampal and hypothalamic glucocorticoid receptors (GR), diurnal corticosterone secretion, as well as in the responsiveness of CRH and GR mRNA levels to exogenous estradiol. In addition, we report that neonatal estrogenization of female rats profoundly affects several regulatory substrates of the hypothalamo‐pituitary‐adrenal (HPA) axis, namely, the gene expression of CRH, arginine‐vasopressin (AVP) and GR in the brain, and the responsiveness of these parameters to estrogen. The neonatal treatment appeared to “defeminize” a number of neuroendocrine mechanisms related to HPA function; these changes were reminiscent of those observed in earlier studies on sexual differentiation of reproductive behavior and hormonal secretion. The results indicate a pivotal role for estrogens during early development for the determination of gender‐specific differences in HPA function in the mature animal and demonstrate for the first time that the brain‐organizing actions of gonadal steroids may extend to nonreproductive neuroendocrine axes.—Patchev, V. K., Hayashi, S., Orikasa, C., Almeida, O. F. X. Implications of estrogen‐dependent brain organization for gender differences in hypothalamo‐pituitary‐adrenal regulation. FASEB J. 9, 419–423 (1995)

Chronic melatonin treatment and the hypothalamo-pituitary-adrenal axis in the rat: attenuation of the secretory response to stress and effects on hypothalamic neuropeptide content and release.

The pituitary-adrenal secretory response to acute and chronic stress, suppressibility of adrenocortical secretions by exogenous glucocorticoids, and hypothalamic content and in vitro release of the two major peptidergic activators of the hypothalamo-pituitary-adrenal (HPA) axis, corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP), were examined in rats receiving daily melatonin (MEL) injections coincident with the circadian increment of endogenous pineal and adrenocortical secretory activity. After 7 days of MEL administration, the rats displayed a significant attenuation of the adrenocortical secretory response to acute and chronic stress. Chronic MEL treatment also prevented the decline in adrenocorticotropic hormone (ACTH) release resulting from chronic stress exposure. Hypothalamic CRH content was significantly lower in rats receiving MEL treatment, while AVP remained largely unaltered; however, MEL administration counteracted the chronic stress-induced decrease in hypothalamic AVP content and in vitro release. When exposed to dexamethasone in vitro, hypothalamic explants from MEL-treated rats responded with a stronger suppression of CRH and AVP release than those originating from vehicle-injected animals. These observations indicate that MEL attenuates the adrenocortical response to stress and influences the biosynthesis, release and glucocorticoid responsiveness of hypothalamic ACTH secretagogues.

Gender specificity in the neural regulation of the response to stress

Pronounced gender-related differences are observable in the regulation of the limbic-hypothalamic-pituitary-adrenal (LHPA) activity under basal and stress-related conditions, and by circulating glucocorticoid levels. This article reviews recent studies that have unequivocally demonstrated that these differences emerge from the organizational effects of gonadal steroids during early brain development. Although largely masked by the dominating role of glucocorticoids in maintaining feedback thresholds, gonadal steroids continue to exert gender-specific activational effects on the LHPA axis through adulthood. The importance of these modulatory effects of gonadal steroids may be reflected in gender differences in the incidence of psychopathologies that are accompanied by symptoms of LHPA dysregulation. One goal of this review is to highlight the need for further investigations into the (still elusive) cellular and molecular mechanisms underlying the activational effects of sex steroids, which may provide leads for neuroprotective hormone replacement strategies.

Developmental expression profiles and distinct regional estrogen responsiveness suggest a novel role for the steroid receptor coactivator SRC-1 as discriminative amplifier of estrogen signaling in the rat brain

The regional distribution, developmental profiles, and gonadectomy‐ and estrogen‐induced changes in the density of transcripts encoding the steroid receptor coactivator‐1 (SRC‐1) were examined in the female rat brain by semiquantitative in situ hybridization. The results demonstrate striking differences in the abundance of SRC‐1 mRNA in discrete brain regions throughout ontogeny. Whereas transcript densities gradually decreased with age in the cerebral cortex, they peaked prominently during the peripubertal period in the hypothalamic medial preoptic area (MPOA) and ventromedial nucleus (VMN). Gonadectomy and estrogen substitution influenced SRC‐1 mRNA levels in sexually mature animals in a region‐specific fashion. Ovariectomy resulted in a down‐regulation of SRC‐1 mRNA levels in the VMN, a brain region richly endowed with estrogen receptors and playing a major role in neuroendocrine control of reproductive functions. In contrast, SRC‐1 transcript levels were significantly up‐regulated after estradiol treatment. Interestingly, SRC‐1 expression in the cortex was refractory to alterations of the estrogen milieu. The obtained SRC‐1 mRNA expression profiles during development clearly demonstrate brain region specificity and regulation by estrogen, thus it is proposed that SRC‐1 amplifies estrogen receptor‐dependent transcription in a temporally and spatially coordinated manner and therefore contributes to the functional specialization of brain areas involved in the regulation of reproduction.

Chronic Melatonin Treatment Counteracts Glucocorticoid-Induced Dysregulation of the Hypothalamic-Pituitary-Adrenal Axis in the Rat

Transient exposure of rats to high doses of dexamethasone (DEX; 500 µg/day for 5 days) produced a host of symptoms that are indicative of hypothalamic-pituitary-adrenal (HPA) axis dysregulation, such as increased adrenocortical secretion over 24 h, blunted and prolonged secretory response to emotional stress, refractoriness of adrenocorticotropin in vitro release to stimulation with the secretagogues corticotropin-releasing hormone (CRH) and vasopressin, decreased levels of mRNA encoding type II corticosteroid receptors in the hippocampus and increased numbers of transcripts encoding CRH in the paraventricular nucleus. Daily administration of melatonin (MEL; 80 µg/kg) concomitantly with, and for 5 days after discontinuation of, glucocorticoid treatment ‘normalized’ most of the symptoms of impaired HPA regulation caused by the exposure to DEX. While none of the treatments used caused major shifts in circadian patterns of corticosterone secretion, MEL administration was associated with diminished overall corticosterone secretion and increased sensitivity to glucocorticoid feedback. Taken together, these findings indicate that chronic MEL treatment may protect several regulatory components of the HPA axis from glucocorticoid-induced deterioration.

Plasticity of hippocampal corticosteroid receptors during aging in the rat

Aging is commonly associated with dysregulation of the hypothalamo‐pituitary‐adrenal axis and cognitive impairment. On the basis of suggestions that these disruptions ensue from changes in the hippocampal complement of corticosteroid (mineralocorticoid and glucocorticoid) receptors (MR and GR), we examined the availability of hippocampal MR and GR by measuring the in vivo uptake of 3H‐aldosterone and 3H‐dexamethasone (selective MR and GR agonists, respectively); MR and GR mRNA levels were also measured. We observed age‐related declines in both the synthesis of MR and GR and the uptake of their respective ligands. Whereas MR mRNA levels and ligand uptake declined in parallel, GR binding declined more steeply than GR mRNA. This latter result, together with our finding that aged rats show impaired corticosteroid receptor mRNA and protein up‐regulation after corticosteroid withdrawal, indicates decreased transcription of MR and GR genes and posttranslational modification of GR mRNA during aging. Given that corticosteroids can influence MR and GR synthesis and binding, and based on the finding that aged subjects show reduced basal secretion of corticosterone, we propose that this relative hypocorticalism may be responsible for the changes observed in MR and GR activity, which then leads to disturbances in neuroendocrine regulation and cognitive function in aged subjects.—Hassan, A. H. S., Patchev, V. K., vonRosenstiel, P., Holsboer, F., Almeida, O. F. X. Plasticity of hippocampal corticosteroid receptors during aging in the rat. FASEB J. 13, 115–122 (1999)

Increased Arginine Vasopressin Secretion May Participate in the Enhanced Susceptibility of Lewis Rats to Inflammatory Disease

Lewis (LEW/N) and Fisher (F344/N) rats are histocompatible inbred strains characterized respectively by susceptibility and resistance to inflammatory disease. LEW/N rats have deficient corticotropin-releasing hormone (CRH), ACTH and corticosterone responses to inflammation, and increased circulating and hypothalamic concentrations of arginine vasopressin (AVP). CRH is produced locally at inflammatory sites, where it acts as a proinflammatory agent, while AVP has been reported to exert immunopotentiating effects in vivo and in vitro. In order to further investigate the mechanism of increased AVP secretion in LEW/N rats, we measured AVP and CRH mRNA in several hypothalamic nuclei in LEW/N and F344/N rats, using in situ hybridization histochemistry. LEW/N rats had increased AVP mRNA concentrations in the supraoptic (SON) and paraventricular (PVN), but not in the suprachiasmatic (SCN) nuclei. CRH mRNA, on the other hand, was decreased in the PVN of LEW/N rats. To examine the potential role of AVP and CRH in the exaggerated inflammatory responses of LEW/N rats, we pretreated young female Lewis and Fischer rats with AVP- and/or CRH-neutralizing rabbit antisera and elicited subsequently an inflammatory response by a nuchal subcutaneous injection of carrageenin. We demonstrated that both antisera decreased significantly the leukocyte concentration in the inflammatory exudate in LEW/N rats, but found no synergistic or addictive effects between them. We conclude that previously observed differences in hypothalamic AVP and CRH contents between LEW/N and F344/N rats correspond to differences in the steady state mRNA levels of the two neuropeptides and that both AVP and CRH participate in the excessive inflammatory response of Lewis rats as locally active proinflammatory agents.

Activational Effects of Gonadal Steroids on Hypothalamo‐Pituitary‐Adrenal Regulation in the Rat Disclosed by Response to Dexamethasone Suppression

Previous studies demonstrated that gonadal steroids secreted during perinatal life permanently ‘organize’ the mechanisms governing hypothalamo‐pituitary‐adrenal (HPA) function, leading to sexually differentiated patterns of pituitary‐adrenal activity under basal and stress conditions. In this paper, we show that gonadal steroids can also exert ‘activational’ effects upon the HPA system. Examination of the ability of different doses of dexamethasone to suppress the nocturnal increase in corticosterone secretion and to attenuate the gene expression of CRH in the hypothalamic paraventricular nucleus of intact and gonadectomized male and female rats revealed that ovarian steroids make an important contribution to the higher sensitivity of the pituitary‐adrenal axis in females to glucocorticoid suppression, whereas testicular steroids may be causal to the male’s moderate responsiveness to glucocorticoid feedback. These findings may be implicated in a number of psychiatric and neurological disease states commonly associated with impaired HPA regulation, but which may be primarily rooted in altered gonadal steroid secretion.