Adrenal responses to corticotrophin-releasing factor in conscious hypophysectomized calves

Influence of Stress and Nutrition on Cattle Immunity

Today, the scientific community readily embraces the fact that stress and nutrition impact every physiologic process in the body. At last, the specific mechanisms by which stress and nutrition affect the immune function are being elucidated. The debate among animal scientists concerning the definition and quantification of stress as it relates to animal productivity and well-being is ongoing. However, an increased appreciation and understanding of the effects of stress on livestock production has emerged throughout the scientific community and with livestock producers. The intent of this article is to provide an overview of the general concepts of stress and immunology, and to review the effects of stress and nutrition on the immune system of cattle.

Maternal stress and T-cell differentiation of the developing immune system: possible implications for the development of asthma and atopy

The constant increase in asthma and atopy prevalences--despite improved treatment and knowledge of many aspects of the diseases--has raised growing concern. Accumulating evidence suggests that these increases in atopic diseases are largely attributable to environmental and lifestyle factors, and the lack of systemic childhood infections has in many studies emerged as a major factor. In addition to current high standards of hygiene and the lack or scarcity of such infections, another factor characteristic of our present-day lives could be involved. This review briefly outlines the possibility that prolonged maternal stress associated with sustained excessive cortisol secretion could affect the developing immune system--especially T(H)1/T(H)2 cell differentiation--and further increase the susceptibility to asthma and atopy in genetically predisposed individuals. This hypothesis is critically evaluated in the light of current knowledge.

Corticotropin-releasing hormone stimulates steroidogenesis in cultured human adrenal cells

The effects of corticotropin releasing hormone (CRH) on steroid production by cultures of human fetal adrenal cells was investigated. We found that CRH, at concentrations that have been reported to exist in human fetal serum, stimulated dehydroepiandrosterone sulfate (DS) and cortisol production by cultured fetal zone and neocortical zone cells. A dose-dependent increase in secretion of both steroids was noted, with the cortisol pathway being preferentially enhanced by CRH at high concentrations. Pretreatment of adrenal cells for 3 days made them more responsive to ACTH stimulation and such effects were dose-dependent also. Inclusion of the antagonist, alpha-helical CRH (9-41) blocked CRH-induced stimulation of DS and cortisol over a broad dose range and also interfered with the augmentation of cortisol secretion noted after ACTH in CRH treated cells. CRH had no effects on adrenal cell proliferation or total cell protein. These studies are suggestive that CRH, either of systemic origin or else produced within the adrenal itself, has the potential to be a modulator of adrenal steroid production in the human.

Chemoreflex contribution to adrenocortical function during acute hypoxemia in the llama fetus at 0.6 to 0.7 of gestation

This study tested the hypothesis that the fetal llama, a species adapted to the chronic hypoxia of life at high altitude, demonstrates a potent carotid chemoreflex influence on adrenocortical responses during acute hypoxemia. Plasma ACTH and cortisol concentrations, and mesencephalic and adrenal blood flows were measured during a 1-h period of acute hypoxemia in six intact and four carotid sinus-denervated llama fetuses at 0.6-0.7 of gestation. Fetal PaO2 was reduced from approximately 23 to about 14 mm Hg in both intact and carotid-denervated groups during acute hypoxemia. During hypoxemia, fetal plasma ACTH, adrenal blood flow, and, therefore, delivery of ACTH to the adrenals increased to similar extents in both intact and carotid-denervated fetal llamas. Despite this, the increase in plasma cortisol in hypoxemia in intact fetuses was absent in carotid-denervated fetuses. In addition, the increase in delivery of cortisol to the mesencephalon calculated in intact fetuses during hypoxemia did not occur in the carotid-denervated group. These data suggest that the integrity of the carotid chemoreceptors is indispensable to cortisol release during acute hypoxemia in the llama fetus, even at 0.6-0.7 of gestation.

Source of corticotropin-releasing hormone-like innervation of the adrenal glands of fetal and postnatal sheep

In pre- and postnatal sheep the irregularly shaped adrenal corticomedullary interface is innervated by corticotropin-releasing hormone (CRH)-like beaded fibers. Since CRH-like staining is also seen in the splanchnic nerves, we examined the intermediolateral sympathetic columns (ISC) of the thoracic (T) spinal cords (segments 5-13) of 12 pre- and postnatal sheep for CRH-like immunocytochemical reactivity and in a subset of fetuses, fluorogold immunoreactivity after fluorogold injection to the left adrenal medulla. All animals had CRH-like immunopositive neurons in the ISC from all thoracic segments. The number of CRH-like positive neurons per thoracic segment did not change either from anterior to posterior within groups nor were there differences between groups. Fluorogold immunopositive neurons occurred in the ISC of all injected animals from T5 to T13. Upon double immunostaining, some ISC CRH-like immunopositive neurons also demonstrated fluorogold immunopositivity. Taken together, these studies indicate that there are CRH-like immunopositive neurons in the ISC of pre- and postnatal sheep and that some of these CRH-like immunopositive neurons innervate the adrenal medulla.

Aspects of autonomic and neuroendocrine function

This article provides a brief review of aspects of autonomic and neuroendocrine function studied initially in collaboration with the late Marian Silver. The importance of the sympathetic innervation to the liver in the control of glycogenolysis was established in anaesthetised animals of various species. Otherwise the work has been carried out mainly in conscious animals under strictly physiological conditions and below behavioural threshold. Investigations of the role of the autonomic innervation to the endocrine pancreas in controlling the release of pancreatic hormones, led to the realisation that the parasympathetic innervation mediates responses to glycaemic stimuli while the sympathetic innervation mediates responses to any form of stress. Studies of adrenal medullary function have confirmed that its threshold for many forms of stress is much higher than that of other components of the sympathetic system and revealed the importance of the pattern of electrical stimulation in determining the rates of release of catecholamines, enkephalins, corticotrophin-releasing factor (CRF) and adrendocorticotrophin (ACTH). The splanchnic sympathetic innervation to the adrenal cortex also plays an important role in determining glucocorticoid output by sensitising the cells to ACTH, probably mainly by the release of vasoactive intestinal peptide (VIP) from cortical nerve terminals. Finally studies of feeding in milk-fed calves have shown that suckling is associated with a remarkable hypertension and tachycardia. These cardiovascular effects are due to a selective sympathetic discharge, which does not involve the adrenal medullae, or the release of neuropeptide Y (NPY) and, at least in the calf, can be attributed to activation of adrenoceptors.

Vascularization of the adrenal cortex: its possible involvement in the regulation of steroid hormone release

This review examines the morphology of the adrenal gland with particular reference to the adrenal vasculature. It examines the possibility that variability in adrenal gland responsiveness may be attributable to neural or hormonal modulation of adrenal blood flow. Changes in the rate of blood flow through the adrenal cortex would be expected to play an important role in the regulation of steroid hormone release. It would affect both the delivery of the major stimulant (ACTH) and the removal of the end product from the steroidogenic cells (the glucocorticoids). In the past, interest in this area has concentrated on the regulation of arterial blood flow, rather than the regulation of venous drainage. The current review examines the concept of vascular damming, and attempts to link the morphological features of the gland with experimental data associated with glucocorticoid release. It is postulated that regulation of venous drainage, via the vascular dam, plays an important role in the storage of the secretory product during the animals' inactive phase, and in the initial rapid rise in plasma levels of the glucocorticoids seen in response to stress or injection of ACTH.

Mifepristone modulation of ACTH and CRH regulation of bovine adrenocorticosteroidogenesis in vitro

Mifepristone (RU486), bovine corticotropin-releasing hormone (CRH), arginine vasopressin (VP), adrenocorticotropin (ACTH1-24), and protein kinase activators (forskolin, [FSK]; phorbol 12-myristate 13-acetate [PMA]) were used in vitro to investigate their direct effect on adrenocorticosteroidogenesis. Bovine adrenocortical fasciculata/reticularis cells (2 x 10(5) viable cells/well) were cultured for 3 d in medium supplemented with 10% fetal calf serum. After incubation for an additional 24 hr in serum-free medium, cells were treated with serum-free medium alone (Control) or various concentrations of ACTH, CRH, VP, FSK, PMA, RU486, and/or various concentrations for 1, 2, 4, or 24 hr. Medium content of cortisol and progesterone were determined by radioimmunoassays. ACTH, CRH, FSK, and PMA each stimulated (P < 0.05) secretion of cortisol in time- and dose-related manners. Although these agents stimulated (P < 0.05) secretion of progesterone in a dose-related manner, medium content of progesterone declined (P < 0.05) over time. The minimal effective doses of ACTH and CRH required to stimulate (P < 0.05) secretion of cortisol relative to the Control over a 4-hr culture period were 0.01 nM and 3 nM, respectively. Relative to observations at 1 hr posttreatment, 24-hr treatment with ACTH or CRH increased the medium content of cortisol by an additional 19.8- and 48-fold, respectively (whereas content of progesterone declined over that time period). VP-stimulated secretion of cortisol was time- (P < 0.05) but not dose-related. Specifically, by 24-hr posttreatment, the medium content of cortisol was increased (P < 0.05) 4.6-fold relative to the quantity of cortisol secreted by 1-hr postaddition of VP (0.01 to 1 microM). Co-treatment with RU486 (1 microM) decreased (p < 0.05) FSK-, ACTH- and CRH-stimulated secretion of cortisol by 77, 27, and 56%, respectively. Similarly, the stimulatory effects of ACTH and CRH on progesterone secretion were reduced (P < 0.05) by 40 and 22%, respectively, by co-addition of RU486. The inhibitory action of RU486 on production of cortisol was no longer apparent by 24 hr after treatment. These observations indicate that RU486 can act as a steroid agonist and as well as an antagonist. These data characterize time- and dose-related direct actions of ACTH, CRH, and RU486 on adrenocorticosteroidogenesis. This information will assist efforts to clarify complex intra-adrenal interactions of neurohormones, growth factors, and endogenous steroids.

Synaptic and intrinsic properties of neurons of origin of the perforant path in layer II of the rat entorhinal cortex in vitro

Layer II of the entorhinal cortex (EC) provides the first step in the hippocampal trisynaptic loop via the perforant path projection to the dentate gyrus. While a great deal is known about this projection and the properties of the dentate granule cells, much less information is available concerning the properties of and synaptic inputs to the cells of origin of the pathway in layer II. The present experiments have employed a slice preparation of the rat EC to study the intrinsic membrane properties and synaptic organization of layer II neurons. Two types of neurons could be identified electrophysiologically. The majority were designated type I and displayed a pronounced time-dependent inward rectification in the hyperpolarizing direction. Type II displayed little evidence of this characteristic. However, morphological examination suggested that both types were spiny stellate neurons projecting via the perforant path. Synaptic responses of both types displayed evidence of excitatory inputs mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors. In general, however, at low frequencies the responses were dominated by inhibitory inputs mediated by both GABAA and GABAB receptors. At higher frequencies the bias was shifted much more toward excitation. The contribution of synaptic and intrinsic properties of layer II neurons to the processing capabilities of the EC is discussed.

Adrenal responses to the peptide PACAP in conscious functionally hypophysectomized calves

Intra-aortic infusions of pituitary adenylate cyclase-activating peptide-(1-38) (PACAP) produced a dose-related fall in aortic blood pressure over the range of 4-40 pmol.min-1.kg-1 in the presence of exogenous adrenocorticotropic hormone-(1-24) (ACTH, 2 ng.min-1.kg-1 i.v.; P < 0.01). At the higher dose there was a significant fall in adrenal vascular resistance in the absence, but not in the presence, of ACTH. PACAP also produced a dose-related increase in right adrenal cortisol output over the same range, which was significantly greater in the absence of exogenous ACTH (P < 0.01). At the higher dose, PACAP produced small but significant increases in adrenal epinephrine and norepinephrine output (P < 0.01) both in the presence and the absence of ACTH. There was also a small rise in Met5-enkephalin output, and corticotrophin-releasing factor (CRF) was released in the presence, but not in the absence, of ACTH. It is concluded that PACAP is capable of exerting potent steroidogenic and vasodilator effects in the adrenal gland in the normal conscious calf and of releasing significant amounts of catecholamines, enkephalins, and CRF from the adrenal medulla. These findings identify PACAP as a candidate neuromodulator in the adrenal gland in this species.

The subthalamo-nigral pathway regulates movement and concomitant acetylcholinesterase release from the substantia nigra

Within the substantia nigra acetylcholinesterase is released independently of cholinergic transmission: this release could be related to some aspects of motor control. To investigate this possibility, acetylcholinesterase release was continuously monitored in relation to specific movements evoked by central electrical stimulation. Increased intensities of stimulation of the subthalamic nucleus in awake guinea-pigs produced a behavioural response, ranging from a decrease in spontaneous movement, to chewing, to both chewing and circling movements. Enhancement of acetylcholinesterase release occurred only when large scale movements (circling as well as chewing) were evoked by subthalamic stimulation: however, a similar protocol of stimulation during ketamine-induced anaesthesia did not produce any comparable movements nor any concomitant change in the release of acetylcholinesterase. Perfusion of the glutamate agonist N-methyl-D-aspartate (NMDA) into the substantia nigra also induced an increase in release of acetylcholinesterase from the substantia nigra of conscious animals, whereas (S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) did not significantly enhance acetylcholinesterase levels. It is concluded that AChE release in the substantia nigra can occur as a result of activation of glutamatergic subthalamic afferents, and that this activation may also be associated with changes in movement.

Autonomic control of adrenal function

Recent studies of adrenal function in conscious calves are reviewed. These have involved collecting the whole of the adrenal effluent blood from the right adrenal gland at intervals and, where necessary, prior functional hypophysectomy by destruction of the pituitary stalk under general halothane anaesthesia 3 d previously. The adrenal medulla was found to release numerous neuropeptides, in addition to catecholamines, in response to stimulation of the peripheral end of the right splanchnic nerve, which was carried out below behavioural threshold. Many of these responses were enhanced by stimulating intermittently at a relatively high frequency. Intra-aortic infusions of a relatively low dose of acetylcholine (4.5 nmol min-1 kg-1) elicited similar responses. In the adrenal cortex, agonists which either potentiated the steroidogenic response to ACTH or exerted a direct steroidogenic action included VIP, CGRP, CRF and ACh acting via muscarinic receptors. Stimulation of the peripheral end of the right splanchnic nerve strongly potentiated the steroidogenic response to ACTH and there is compelling evidence that the innervation normally plays an important part in cortisol secretion.

Evidence that an extrahypothalamic pituitary corticotropin-releasing hormone (CRH)/adrenocorticotropin (ACTH) system controls adrenal growth and secretion in rats

Within two weeks, hypophysectomy induced in rats a striking decrease in the level of circulating ACTH (the concentration of which was at the limit of sensitivity of our assay system), coupled with a net reduction in the plasma corticosterone concentration and an evident adrenal atrophy. Zona fasciculata, the main producer of glucocorticoids, was decreased in volume, due to a lowering in both the number and average volume of its parenchymal cells. Subcutaneous ACTH infusion (0.1 pmol.min-1), administered during the last week following hypophysectomy, restored the normal blood level of ACTH and completely reversed all effects of hypophysectomy on the adrenals. Subcutaneous infusion for one week with alpha-helical-CRH or corticotropin-inhibiting peptide (1 nmol.min-1), which are competitive inhibitors of CRH and ACTH, evoked a further significant lowering of plasma corticosterone concentration and markedly enhanced adrenal atrophy in hypophysectomized rats. These findings strongly suggest that an extrahypothalamic pituitary CRH/ACTH system may be involved in the maintenance of the growth and steroidogenic secretory activity of the rat adrenal cortex.

Relaxing actions of corticotropin-releasing factor on rat resistance arteries

1. Although it well established that corticotropin-releasing factor (CRF) injected i.v. can cause hypotension and vasodilatation, there is no in vitro evidence that CRF acts as a vasodilator. We have therefore tested the hypothesis that the hypotensive effect of i.v. CRF is due to a direct vasodilator action by carrying out experiments in vitro on rat resistance arteries (i.d. 150-300 microns). 2. Initial in vivo experiments confirmed that CRF (1.5 nmol.kg-1) injected i.v. caused hypotension in rats, this being partially antagonized by the CRF analogue CRF9-41. 3. For the in vitro experiments, vessels were taken from the mesenteric, cerebral and femoral vascular beds, and mounted as ring preparations in an isometric myograph. The vessels were pre-contracted with one of 3 agonists (prostaglandin F2 alpha, arginine vasopressin or noradrenaline) or with a high-potassium solution (K+). 4. With maximal concentrations of the agonists, CRF caused relaxation of mesenteric and cerebral vessels with 10 nM, and near complete relaxation with 100 nM. Femoral vessels pre-constricted with agonists and all vessels pre-constricted with K+ were less affected by CRF. In the mesenteric vessels, with sub-maximal levels of pre-constriction, CRF caused substantial relaxation at 1 nM and could cause complete relaxation at 10 nM. 5. The relaxant effect of CRF on contractions of mesenteric vessels was antagonized by 100 nM CRF9-41. Neither tetraethyl ammonium (30 mM) nor glibenclamide (3 microM) antagonized the relaxant effect of CRF. 6. The relaxant effect of CRF on mesenteric small arteries was found to be unaffected by removal of the endothelium. 7. The results indicate that CRF causes an endothelial-independent vasodilatation of rat resistance arteries under in vitro conditions at concentrations which are consistent with this being an important cause of the hypotension observed with i.v. injection of CRF.

The role of corticotrophin releasing factor in relation to the neural control of adrenal function in conscious calves

1. Adrenal responses to intra-aortic infusions of pure synthetic ovine corticotrophin releasing factor (CRF), and to electrical stimulation of the preganglionic sympathetic innervation, have been investigated in functionally hypophysectomized conscious calves, in the presence and absence of a specific CRF antagonist. 2. CRF exerted a substantial steroidogenic effect on the adrenal gland of functionally hypophysectomized calves when infused intra-aortically at a dose (1.3 ng min-1 kg-1) below that which caused any fall in the arterial blood pressure. This response was significantly reduced, but not abolished by a concomitant infusion of CRF-antagonist into the aorta. 3. The steroidogenic effect of CRF was significantly reduced in the presence of exogenous adrenocorticotrophic hormone (ACTH) (2 ng min-1 kg-1, I.V.) and the surviving response was completely abolished by CRF-antagonist. 4. Stimulation of the peripheral end of the splanchnic nerve at 4 Hz in functionally hypophysectomized calves given exogenous ACTH produced a rise in mean adrenal output of the same order of magnitude as did exogenous CRF under the same conditions. The response to splanchnic nerve stimulation was apparently unaffected by CRF-antagonist although release of endogenous CRF from the gland was significantly increased thereby. 5. These results indicate that release of CRF from the adrenal gland during splanchnic nerve stimulation in the calf does not contribute significantly to the steroidogenic response thereto.

Comparison of ACTH and corticotropin-releasing hormone effects on rat adrenal steroidogenesis in vitro

The effects of equimolar concentrations (10(-9) M) of ACTH and corticotropin-releasing hormone (CRH) on the secretory activity of zona glomerulosa (ZG) and zonae fasciculata and reticularis (ZF/ZR) of rat adrenals were investigated in vitro by high-pressure liquid chromatography. ACTH enhanced the output of all the post-progesterone steroids (11-deoxycorticosterone, 18-hydroxy-11-deoxycorticosterone, corticosterone, 18-hydroxycorticosterone, and aldosterone) both by isolated ZG or ZF/ZR cells and by adrenal slices. CRH raised the secretory activity exclusively of adrenal slices, and its effect was less than half that of ACTH. However, the extents to which the various post-progesterone hormones contributed to the ACTH- or CRH-induced rises in the overall adrenal secretory activity were similar. The hypothesis is discussed that CRH acts on the rat adrenal gland by eliciting a local production of ACTH.

Endocrine responses to intra-aortic infusions of acetylcholine in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine (4.5 nmol min-1 kg-1 for 10 min) have been investigated in conscious, functionally hypophysectomized, 3- to 6-week-old calves, in the presence and absence of exogenous ACTH (2 ng min-1 kg-1, I.V.). 2. Acetylcholine produced a substantial fall in adrenal vascular resistance, which was significantly reduced in the presence of exogenous ACTH, while producing minimal changes in aortic blood pressure and heart rate. 3. There was also a significant rise in right adrenal cortisol output which was sufficient to produce a measurable rise in plasma cortisol concentration. The effect could be accounted for by the increase in adrenal ACTH presentation. It was abolished by pre-treatment with atropine (0.2 mg kg-1). A small but significant rise in aldosterone output during acetylcholine infusions was also abolished in the presence of ACTH. 4. Both adrenaline and noradrenaline were released during intra-aortic acetylcholine infusions and these responses were substantially reduced, but not abolished, by pre-treatment with atropine. 5. Acetylcholine also stimulated the release of corticotrophin-releasing factor (CRF) and [Met5]enkephalins from the gland. The output of CRF was enhanced and that of free [Met5]enkephalin was significantly reduced in the presence of exogenous ACTH. All these responses were largely, but not completely, suppressed by atropine. 6. Acetylcholine also promoted the release of the pancreatic hormones glucagon, insulin and pancreatic polypeptide (PP). The amounts of pancreatic glucagon and insulin that were released were highly dependent on the concentration of glucose in the circulating plasma and all these responses were abolished by atropine. 7. It is concluded that acetylcholine is capable of stimulating the release of a wide variety of agonists from the adrenal gland when infused intra-aortically at a dose of 4.5 nmol min-1 kg-1. The increase in cortisol output appears to be secondary to an increase in blood flow whereas the adrenal medullary responses are not, and appear to be due largely, but not entirely, to activation of muscarinic receptors.