Neural pathways involved in ACTH regulation

Evidence for the Integration of Stress-Related Signals by the Rostral Posterior Hypothalamic Nucleus in the Regulation of Acute and Repeated Stress-Evoked Hypothalamo-Pituitary-Adrenal Response in Rat

A likely adaptive process mitigating the effects of chronic stress is the phenomenon of stress habituation, which frequently reduces multiple stress-evoked responses to the same (homotypic) stressor experienced repeatedly. The current studies investigated putative brain circuits that may coordinate the reduction of stress-related responses associated with stress habituation, a process that is inadequately understood. Initially, two rat premotor regions that respectively regulate neuroendocrine (medial parvicellular region of the paraventricular hypothalamic nucleus [PaMP]) and autonomic (rostral medullary raphe pallidus [RPa]) responses were targeted with distinguishable retrograde tracers. Two to 3 weeks later, injected animals underwent loud noise stress, and their brains were processed for fluorescent immunohistochemical detection of the tracers and the immediate early gene Fos. A rostral region of the posterior hypothalamic nucleus (rPH), and to a lesser extent, the median preoptic nucleus, exhibited the highest numbers of retrogradely labeled cells from both the RPa and PaMP that were colocalized with loud noise-induced Fos expression. Injections of an anterograde tracer in the rPH confirmed these connections and suggested that this region may contribute to the coordination of multiple stress-related responses. This hypothesis was partially tested by posterior hypothalamic injections of small volumes of muscimol, which disrupts normal synaptic functions, before acute and repeated loud noise or restraint exposures. In addition to significantly reduced corticosterone release in response to these two distinct stressors, rPH muscimol disrupted habituation to each stressor modality, suggesting a novel and important contribution of the rostral posterior hypothalamic nucleus in this category of adaptive processes. Significance statement: Habituation to stress is a process that possibly diminishes the detrimental health consequences of chronic stress by reducing the amplitude of many responses when the same challenging conditions are experienced repeatedly. Stress elicits a highly coordinated set of neuroendocrine, autonomic, and behavioral responses that are independently and relatively well defined; however, how the brain achieves coordination of these responses and their habituation-related declines is not well understood. The current studies provide some of the first anatomical and functional results suggesting that a specific region of the hypothalamus, the rostral posterior hypothalamic nucleus, targets multiple premotor regions and contributes to the regulation of acute neuroendocrine responses and their habituation to repeated stress.

Effects of corticotrophin-releasing factor and arginine-vasopressin on proopiomelanocortin (POMC) mRNA levels, release and storage of adrenocorticotrophin from mouse anterior pituitary cells

1. The present studies were undertaken to determine the effects of arginine vasopressin (AVP), corticotrophin-releasing factor (CRF) and AVP in combination with CRF on proopiomelanocortin (POMC) gene expression, and in the release and biosynthesis of POMC products (i.e. ACTH). 2. After a 3 hr treatment, AVP (10(-7) M), CRF (10(-7) M) and AVP (10(-7) M) in combination with CRF (10(-7) M) stimulated ACTH release by 291 +/- 19.2%, 377.4 +/- 25.6% and 462.1 +/- 38.4% (P < 0.01), respectively, with respect to basal secretion; while ACTH content diminished by 84.0 +/- 4.6%, 81.3 +/- 2.1% and 71.0 +/- 1.5% (P < 0.01), respectively, with respect to basal. Total POMC mRNA levels were not affected after a 3 hr treatment. 3. When cells were treated with a wide range of AVP concentrations (10(-11)-10(-7) M) to which we added different concentrations of CRF, modulation of AVP-induced ACTH release was most effective at CRF concentrations of < 10(-10) M. 4. Prolonged (3-6 hr) exposure to as low as 10(-10) M CRF or 10(-9) M AVP resulted in homologous desensitization of ACTH secretion. However, these pretreated cells were able to respond to a further challenge of CRF and AVP. This paper provides new information on the dose ranges over which desensitization and cross-sensitization between CRF and AVP secretory effects take place in mouse anterior pituitary cells.

Cholinergic mechanisms in depression

Evidence supporting a cholinergic hypothesis of depression is presented. First, cholinergic overdrive produces behavioral, neuroendocrine, and polysomnographic features of melancholia, and melancholics exhibit state-independent supersensitivity to cholinergic overdrive. Drugs inducing up-regulation and supersensitivity of cholinergic systems produce behavioral, polysomnographic, and neuroendocrine effects of melancholia when withdrawn. These observations also implicate cholinergic system supersensitivity as a factor in the pathophysiology of certain affective disorders. Cholinergic and monoaminergic mechanisms reciprocally regulate drive-reduction, and substances of abuse either activate monoaminergic networks or antagonize cholinergic systems. These points are consistent with the hypothesis that dynamic interaction between cholinergic and monoaminergic systems is involved in the regulation of mood and affect. Finally, antimuscarinic agents have antidepressant effects. Thus, the hypothesis that supersensitivity of cholinergic systems is involved in the pathophysiology of affective disorders is supported by several lines of evidence. This evidence is reviewed; directions for future research and promising methods of investigation are discussed.

Hypophysiotropic and neuromodulatory regulation of adrenocorticotropin in the human fetal pituitary gland

Synthetic human corticotropin-releasing factor (hCRF) stimulated ACTH secretion by human fetal pituitaries in superfusion and dispersed human fetal pituitary cells cultured on an extracellular matrix in static incubation from 14 to 23 wk gestational age. The action of hCRF in vitro was potentiated by arginine vasopressin (AVP) at all ages studied. 8-Br-cAMP induced a response similar to hCRF. The AVP effect on ACTH was synergistic with both CRF and 8-Br-cAMP. hCRF-mediated secretion of ACTH was noncompetitively inhibited by 24-h pretreatment, or by 3-h concomitant treatment, with dexamethasone. Neither oxytocin, catecholamines, prostaglandins, nor indomethacin exerted significant effects on ACTH secretion, either alone or in combination with hCRF or AVP during the gestational ages studied. These results support a physiologic role for CRF in the regulation of secretion by corticotropic cells as early as 14 wk gestation, by which time corticotropes and ability to secrete ACTH have been demonstrated.

Immunoreactive dynorphin and alpha-neo-endorphin in rat hypothalamo-neurohypophyseal system

Concentrations of dynorphin and alpha-neo-endorphin were measured in rat supraoptic and paraventricular nuclei, median eminence and posterior pituitary. Paraventricular lesions did not alter the level of either peptide in the median eminence or posterior pituitary. Knife cuts in the lateral retrochiasmatic area that severed the supraoptico-hypophyseal tract reduced dynorphin and alpha-neo-endorphin levels by 64-80% in the posterior pituitary and by somewhat less in the median eminence. Knife cuts in the vicinity of the supraoptic nucleus that did not interrupt the supraoptico-hypophyseal tract did not affect peptide concentrations in the posterior pituitary. Our data suggest that dynorphin and alpha-neo-endorphin in the posterior pituitary are in processes of supraoptic nucleus neurons. These neurons also project to the median eminence which may receive fibers from other dynorphin and alpha-neo-endorphin containing cells too.

Modulation of stress-induced ACTH release by corticotropin-releasing factor, catecholamines and vasopressin

The stress-induced release of ACTH is believed to involve the activation of several humoral and neural pathways, including corticotropin-releasing factor (CRF), catecholamines and vasopressin. The essential role of CRF was supported by our observation that immunoneutralization of this releasing factor significantly lowers plasma ACTH levels of ether-stressed rats. However, the presence of a small but measurable residual ACTH secretion suggested the possible involvement of factors other than CRF in the stress response. We report here that pretreatment with a vasopressin antagonist decreases the plasma ACTH levels of ether-stressed rats in later (10-20 min), but not earlier (0-10 min), phases of ether stress. The ganglionic blocker chlorisondamine, inhibits ACTH release during both phases of the response to ether by 40-60% when used alone, and by 100% when administered with anti-CRF antibody. These results support a role of CRF, catecholamines and vasopressin in mediating ACTH release by ether stress.

Oxytocin as well as vasopressin potentiate ovine CRF in vitro

We have investigated the effects of synthetic oxytocin and vasopressin on corticotropin release induced by the 41-residue ovine corticotropin-releasing factor (oCRF) in vitro. Segments of the anterior pituitary glands obtained from male and female Wistar or from female hetero- and homozygous Brattleboro rats were used. Ovine CRF (0.1-2.5 nmol/l) stimulated corticotropin release by pituitaries of Wistar rats and this effect was augmented two- to threefold in the presence of arginine vasopressin (0.09-0.9 mIU/ml) or oxytocin (0.9-90 mIU/ml). A similar phenomenon was demonstrated in Brattleboro rats. These data favor the hypothesis that oxytocin might have a physiological role in the regulation of pituitary-adrenocortical function in homozygous Brattleboro rats which lack vasopressin.

The distribution of corticotropin releasing factor-like immunoreactive neurons in rat brain

Using the indirect immunofluorescent technique, corticotropin releasing factor (CRF)-like immunoreactive nerve fibers and cell bodies were observed to be widely distributed in rat brain. A detailed stereotaxic atlas of CRF-like immunoreactive neurons was prepared. Large numbers of CRF-containing perikarya were observed in the nucleus paraventricularis, with scattered cells in the following nuclei: accumbens, interstitialis stria terminalis, preopticus medialis, supraopticus, periventricularis hypothalami, amygdaloideus centralis, dorsomedialis, substantia grisea centralis, parabrachialis dorsalis and ventralis, tegmenti dorsalis lateralis, vestibularis medialis, tractus solitarius and reticularis lateralis. The most intense staining of CRF-containing fibers was observed in the external lamina of the median eminence. Moderate numbers of CRF-like fibers were observed in the following nuclei: lateralis and medialis septi, tractus diagonalis, interstitialis stria terminalis, preopticus medialis, supraopticus, periventricularis thalami and hypothalami, paraventricularis, anterior ventralis and medialis thalami, rhomboideus, amygdaloideus centralis, habenulae lateralis, dorsomedialis, ventromedialis, substantia grisea centralis, cuneiformis, parabrachialis dorsalis and ventralis, tegmenti dorsalis lateralis, cerebellum, vestibularis medialis, reticularis lateralis, substantia gelatinosa trigemini and lamina I and II of the dorsal horn of the spinal cord. The present findings suggest that a CRF-like peptide may be involved in a neurotransmitter or neuromodulator role, as well as a hypophysiotropic role.

Neuropeptides in the median eminence: their sources and destinations

By radioimmunoassay and immunocytochemical techniques, 14 neuropeptides have been measured and localized in the rat median eminence. Neuropeptides with inhibitory or stimulatory effects on the anterior pituitary hormones as well as posterior pituitary hormones are present in the median eminence in the highest concentrations of the central nervous system. All these peptides (LH-RH, TRH, somatostatin, CRF, vasopressin, oxytocin) are of preoptic or hypothalamic origin and they are transported to the median eminence by loop-like fiber systems through the lateral retrochiasmatic area. Within the median eminence, the pericapillary space constitutes the main common pathway. Three major transport routes--axons, vessels, liquor spaces--are separated from each others by only basement membranes, which allow free communications downwards to the pituitary but also backwards to the central nervous system.

Immunocytochemical localization of CRF in the ovine hypothalamus

A population of neuronal cell bodies and their fiber pathways have been elucidated within the ovine hypothalamus. The immunoreactive neurons were located in the anterior and dorsal hypothalamus interspersed throughout the paraventricular nucleus. These perikarya were only observed when an antiserum that was generated against the C-terminal of CRF was employed. A dense fiber projection traversed the medial-basal hypothalamus and ended within the palisade-contact zone of the median eminence and neural stem. Fibers were revealed by antisera generated against both the N-terminal and the C-terminal of CRF. Antisera pre-absorbed with synthetic CRF failed to yield immunoreactivity.

Intrahypothalamic terminals of stress conducting fibers

Surgical stress did not elevate plasma corticosterone level in rats with bilateral surgical transection of the lateral retrochiasmatic area (RCAL). After RCAL transections light and electron microscopic terminal degeneration was observed in both the external and the internal layers of the median eminence as well as in the arcuate and ventromedial nuclei. Incisions lateral to the medial forebrain bundle did not prevent the stress-induced rise in plasma corticosterone level and were not followed by degeneration in the median eminence. Transection of the medial forebrain bundle rostral or caudal to the RCAL caused degeneration in both the median eminence and the arcuate nucleus.

Identification of harman in the rat arcuate nucleus

This communication describes the presence of 1-methyl-beta-carboline (harman) in the hypophysiotropic area of the hypothalamus which incorporates the arcuate nucleus. Diethyl ether extracts of borate-buffered arcuate homogenates were subjected to silica column chromatography and thin-layer chromatography (TLC). Identification of the beta-carboline was accomplished by use of fluorescent spectrometry, gas chromatography (GC), mass spectrometry (MS) and combined gas chromatography-mass spectrometry (GC-MS).