Figure 1: Neuro- endocrine adaptive mechanism in farm animals during heat stress conditions
Neuro-endocrine Mechanism of Adaptation in Livestock
Stress responses promote the maintenance of homeostasis and adaptation to the various physiological and psychological challenges pertaining to the changes in the environment. This complex process involves coordinated activation of behavioral, physiological and particularly neuroendocrine regulation in the animal. Functional properties of the stress response systems show large interindividual variation, depending upon a variety of factors including genetic predisposition . Moreover, the relative effect was mainly dependent upon the duration and intensity of the various stressors associated with livestock production.
Neuro-endocrine regulation is one of the principal adaptive responses shown by the animal in extreme stress condition . Hypothalamo-pituitary-adrenal (HPA) axis plays a significant role in the release of several neurotransmitters and hormones which regulates the thermoregulatory mechanisms in animals. The HPA axis gets activated when an animal receives stress through various sense organs and also the response to the stimulus is coordinated by brain center. Further, it might activate the adaptive responses pertaining to the neuro-endocrine system of the animals. The activation of the HPA axis may be related directly or indirectly to factors such as heat stress, drought and nutritional stress and also due to disease occurrence. The corticotrophin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and glucocorticoids are the primary products of HPA axis which ultimately controls the stress response pathways in animals . Moreover, ACTH is the important regulator which helps in the production and secretion of cortisol . Also, several research findings showed that the hormones produced by the adrenal and thyroid glands recognized to have a significant role in thermoregulation and metabolic response in livestock animals [15, 11]. Further, the activation of HPA axis may result in enhanced production glucocorticoids like cortisol, which is indicated as the major stress-relieving hormone and also identified as a reliable biomarker for assessing the severity of stress cutting across species . Also increased level of cortisol would favor hepatic gluconeogenesis which helps in the production of glucose from non-carbohydrate sources and maintains the energy metabolism to support life-sustaining activities. Several authors denoted that the secretion of glucocorticoids is the principal endocrine response to heat stress condition [11,26]. Glucocorticoids have been widely used in experimental research in farm animals and also used as a useful variable for analyzing the animal welfare. Moreover, their stimulation may be influenced by a wide variety of factors with respect to the individual animal and sex, age, and physiological stage . Interestingly, parturition in some species was regulated by the activation of the HPA axis . Application of non-invasive techniques for monitoring glucocorticoid metabolites in fecal samples is a useful tool for assessing the animal welfare of various farm animals. Another important endocrine product secreted from adrenal gland is the aldosterone which involves in the regulation of electrolyte mechanism in the animal. Further, the aldosterone level was reported to be higher in the heat-stressed animal rather than the controlled group animals after providing them with adlibitum feed and water . Moreover, the animals during heat stress conditions are subjected to severe dehydration which might result in the activation of the renin-angiotensin-aldosterone pathway to restore the fluid and electrolyte balance . Likewise, the pineal gland is a neuro endocrine transducer, which is responsible for the melatonin production and influences the seasonal changes pertaining to the reproductive capability in different animals . There are scientific reports available to illustrate the significant increase of melatonin level during heat stress and it clearly indicates the anti stressogenic effect in the animals . Finally, the significant effect of glucocorticoid on melatonin level during heat stress establishes adrenal-pineal gland relationship which was evident in the importance of menacing the animal productivity mainly in tropical countries. Understanding the characteristics of melatonin would assist in implementing a new methodology for the photoperiod-dependent breeding programme in animals by inducing changes in the perception of photoperiod and the annual pattern of reproduction . Similarly, the thyroid gland produces two types of hormones triiodothyronine (T3) and thyroxine (T4) which aid the regulation of metabolic activity in the animal. The decline in the level of metabolic hormones is to reduce the metabolic heat production during heat stress condition in goats . Also, the author denoted that the functions of thyroid glands are mainly dependent on the environmental condition the animals are exposed to. Aleena et al.  reported that the changes in the ambient temperature suppresses the activity of thyroid hormone in blood level and also identified these hormones to be the stress indicators for assessing the heat tolerance in the farm animals. Generally, lower activity of the thyroid gland s helps to reduce the metabolic energy expenditure of animals coping them to extreme environmental conditions. Similar results of inhibited thyroid hormone concentration level were also reported in various livestock species like cattle, sheep and goats were exposed to thermal stress [4,15]. This could be an adaptive response shown by the animals to regulate the internal metabolic heat production without compromising the vital functions of the body . Further, the lower levels of the thyroid hormones could be partially attributed to the compromised nutritional status of the heat-stressed animal, which could be attributed to the lower feed intake associated with behavioral adaptive responses . There are research findings showing the reproductive capability of the animals also are compromised during extreme stress condition and alter the concentration and mechanism of different types of hormones associated with reproductive functions . Further, the plasma estradiol level decreased drastically while the progesterone concentration increased during heat stress condition .
The interleukin-1, interleukin-6, and tumor necrosis factor are associated with the stimulation of stress axis. Cytokines are also stimulating the secretion of the hormone leptin from adipocytes. Significant reduction in the leptin concentration during heat stress condition in several farm animals has also been reported. This was attributed to the drastic reduction in the feed intake and the conversion of all non-carbon sources into glucose as a part of both behavioral and Neuro-endocrine responses . Leptin is now recognized as an inhibitor of stress axis activity . Therefore, both leptin and glucocorticoids complete the negative feedback circuit to suppress stress axis activity to maintain the homeostasis. Future studies are required though to establish the relationship between hormonal mediators, immune cytokines, brain neurochemicals, and stress axis activation for identifying other important traits associated with neuro-endocrine regulation of heat stress in farm animals. The importance of the hypothalamic–pituitary–adrenal (HPA) axis and glucocorticoids in modifying the inflammatory and cytokine response is highlighted in various studies in adrenalectomized animals. Likewise, most of the comparative studies shown the significant variation of endocrine responses in farm animals and these findings strongly established breed differences in the behavior of HPA axis during extreme environmental condition . These efforts might help in developing new heat-tolerant breeds, which can survive and produce optimally in harsh environmental conditions.
The HPA axis
Hypophysiotropic neurons localized in the paraventricular nucleus (PVN) of hypothalamus synthesize and secrete corticotropin-releasing factor (CRF). The CRF was recognized as the principal regulator of the HPA axis. In response to stress, CRF is released into hypophysial portal vessels that influence the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH) . The binding of CRF to its receptor on pituitary corticotropes stimulate the release of ACTH into the systemic circulation . Also, the principal target for circulating ACTH is the adrenal cortex, where it stimulates glucocorticoid synthesis and secretion from the zona fasciculata. Glucocorticoids are the downstream effectors of the HPA axis and regulate the physiological process by inducing changes in the intracellular receptors . The biological effects of glucocorticoids help the animal in coping with extreme stress conditions. However, the extent of synthesis and stimulation of glucocorticoids influence the functioning of HPA axis and might affect the health of the animal .
The physiological actions of the CRF family of peptides are mediated through two different receptor subtypes belongs to the class B family of G-protein coupled receptors . The CRFR1 is expressed at high levels in the brain and pituitary and low levels in other peripheral tissues . The highest levels of CRFR1 expression are found in the anterior pituitary, olfactory bulb, cerebral cortex, hippocampus, and cerebellum . The CRF1 is activated through the binding of CRF-agonist and the ligand binding and subsequent receptor conformational change depend on three different sites in the second and third extracellular areas of CRF1. Considering the major tissues, CRF1 is coupled to a stimulatory G-protein further stimulate the adenylyl pathway, and ligand-binding triggers an overshoot in the cyclic adenosine monophosphate level which acts as a secondary messenger for numerous biological process . However, the signal can be transmitted along multiple signal transduction pathways, depending on the structure of the receptor and the region of its expression. Alternate signaling pathways are stimulated by CRF1 which includes the activation of protein kinase c and mitogenactivated protein kinase . CRF1 was also identified as a potent mediator of endocrine, autonomic, behavioral and immune responses to chronic stress conditions .
Vasopressin (AVP) is a non-peptide that is highly expressed in the PVN, supraoptic (SON), and suprachiasmatic nuclei of the hypothalamus. Further, magnocellular neurons of the PVN and SON act together to stimulate the posterior lobe of the pituitary and release AVP directly into the systemic circulation for retaining the osmotic homeostasis . In addition to magnocellular neurons, parvocellular neurons of the PVN synthesize and release AVP into the portal circulation, where this peptide potentiates the effects of CRF on ACTH release from the anterior pituitary.
The synergistic effects of AVP on ACTH release are mediated through the vasopressin V3 receptor on pituitary corticotropes. Further, binding of AVP to the V3 receptor activates phospholipase C by coupling to Gq proteins. Further, activation of the phospholipase C stimulates protein kinase C, resulting in the potentiation of ACTH release . Several investigators have reported that the expression of AVP in parvocellular neurons of the PVN and V receptor density in pituitary corticotropes is significantly increased in response to chronic stress . Furthermore, it is already identified that AVP plays an important role in the stress response by maintaining the level of ACTH during chronic heat stress condition.
When the animal is subjected to heat stress, multiple alterations takes place in the endocrine system such as the HPA axis. Among them, the main feature of the stress reaction is the activation of the HPA axis leading to the increase of ACTH . ACTH is one of the important pituitary hormones which helps in supporting the growth and development of adrenal cortex and stimulating the synthesis and secretion of glucocorticoids. Therefore, the secretion of glucocorticoids depends on the integrity of the HPA axis and further the anterior pituitary secretes ACTH and the adrenal cortex synthesis Cortisol for regulating the behavior and neuroendocrine activities associated with the heat stress [15,46]. Therefore, ACTH and cortisol are often considered as the most important indicator for assessing the quantum of stress.
Proopiomelanocortin (POMC) is a prohormone that is highly expressed in the pituitary and the hypothalamus. POMC is processed into a number of bioactive peptides including ACTH, β-endorphin, β-lipotropic hormone and the melanocortins . In response to CRF, ACTH is released from pituitary corticotropes into the systemic circulation where it binds to its specific receptor in the adrenal cortex. ACTH binds to the melanocortin type 2 receptor (MC2-R) in parenchymal cells of the adrenocortical zona fasciculata. Activation of the MC2-R induces stimulation of cAMP pathway events that induce steroidogenesis and the secretion of glucocorticoids, mineralocorticoids and androgenic steroids . Specifically, ACTH promotes the conversion of cholesterol into 5-5 pregnenolone during the initial step of glucocorticoid biosynthesis.
Glucocorticoids are transported through the circulatory system with the help of carrier proteins. The carrier proteins influence the glucocorticoids to be available rapidly after initiation of the stress responses . Among the various glucocorticoids produced, cortisol is the principal hormone which controls the stress response in large ruminants in particular. Albumin is the major carrier protein for cortisol, although cortisol could be transported by cortisol binding globulin called transcortin. Approximately 1–10% of cortisol circulates as a “free” steroid through the circulatory system. In addition to that, the different types of tissue have the ability to regulate the available cortisol in a proper manner with the help of 11β hydroxysteroid dehydrogenase enzymes which converts cortisone to cortisol during stress.
Glucocorticoids have regulatory actions and mainly help to moderate the mobilization of energy expenditure throughout the body with the objective to retain the homeostasis . Furthermore, glucocorticoids can affect the concentration of non-esterified fatty acids, lactate, plasma insulin and although having a variable effect on glucose metabolism [10,33,37]. In addition to that, cortisol converts all non-carbohydrate sources into glucose through the process of hepatic gluconeogenesis for supporting the energy requirements of vital organs. Glucocorticoid secretion was found to be highly variable in many species including cattle and sheep during stress . Also, glucocorticoids are synthesized in a diurnal pattern that is governed by many factors including genetics .
Negative feedback Mechanisms associated with Glucocorticoids
Glucocorticoids modulate the HPA axis through the negative feedback mechanisms. Glucocorticoids have the capability to inhibit the activation of HPA axis through a delayed feedback mechanism and this control was determined by the circulating glucocorticoid concentration . The delayed feedback system acts through transcriptional alterations and is regulated by GR localized in the brain region. Following binding of glucocorticoids, GRs modulate transcription of HPA components by binding to GREs or through interactions with transcription factors . Glucocorticoids have a low nanomolar affinity for the GR and extensively occupy GRs during periods of elevated glucocorticoid secretion that occur the following stress. Mineralocorticoid receptors (MRs) have a sub nanomolar affinity for glucocorticoids, a restricted expression pattern in the brain and bind glucocorticoids during periods of basal secretion . Also, the distinctive function of these two receptors suggests that MRs regulate the basal HPA tone while GRs mediate the glucocorticoid negative feedback mechanisms .
GRs are widely expressed in the brain, and thus the precise anatomical locus of glucocorticoid negative feedback remains poorly defined. However, two regions of the brain appear to be key sites for glucocorticoid feedback inhibition of the HPA axis. High levels of GR are expressed in hypophysiotropic neurons of the PVN, and local administration of glucocorticoids reduce PVN neuronal activity and attenuate adrenalectomy-induced ACTH hypersecretion. Several research findings suggest that the PVN is an important site for glucocorticoid feedback inhibition of the HPA axis . The hippocampus has been implicated as a secondary site for glucocorticoid negative feedback regulation of the HPA axis . The hippocampus contains a high concentration of both GR and MR and infusion of glucocorticoids into this structure reduces basal and stress-induced glucocorticoid release [51,54].
Role of HPA axis in Neuro-endocrine Regulation
The biological functioning and regulation of HPA axis is the primary requirement of adaptive mechanisms in the farm animals. The HPA axis is activated in response to the different environmental stressor, particularly heat stress in order to help the animal to cope with extreme environmental conditions . Cortisol is recognized to be the stress relieving hormone and synthesized as the main product of the activation of the HPA axis and evokes a response from specific target receptors, eventually retaining homeostasis back to normal stage . However, prolonged stimulation of the HPA axis results in suppression of immune function and reproductive efficiency. Cortisol also was identified as a biological marker for assessing the severity of stress in farm animals [57,3].
The energy stored in animals for production purpose is deviated from the neuroendocrine response to help the animal cope with the extremely stressful condition . Neuro endocrine mechanisms are the paramount process of adaptation to various stressors . The SAM axis and the HPA axis act together to stimulate and integrate all the adaptive mechanisms.
Activation of the HPA axis is a complex mechanism that involves regulation of both neuronal and endocrine regulators. Glucocorticoids play a prominent role in regulating the magnitude and duration of HPA axis activation [60,73]. When the animals are subjected to extreme heat stress condition, elevated levels of circulating glucocorticoids inhibit HPA activity at the level of the hypothalamus and pituitary. Further, the HPA axis is also subject to glucocorticoid independent regulation and the neuro-endocrine effects are also modulated by CRF binding proteins which found to be the higher concentration in the systemic circulation and in the pituitary gland . Two principal neuro-endocrine adaptive mechanisms are elicited, when an animal encounters the environmental stress which includes sympatheticadrenal- medullary (SAM) and the hypothalamic–pituitary–adrenal (HPA) axes. These axis act together which ultimately result in various stress responses encompassing the interplay of adaptive responses of various organs and receptors to overcome extreme stress condition [4,6]. Activation of the SAM axis occurs rapidly on exposure to stressful condition to stimulate the autonomic nervous system culminating in the secretion of the catecholamines adrenaline and noradrenaline .
The activation of the SAM and HPA axes are considered effective mechanisms to assist the animals in adapting to changes in its environment they reflect the alteration in metabolic rate, peripheral circulation, respiration, and energy availability which help the animal to survive in extreme environmental conditions .
The hypothalamic-pituitary-thyroid axis (HPT) axis plays a critical role in the regulation of energy expenditure by affecting basal metabolic rate through the actions of thyroid hormones. The HPT axis is under the control of neurons located in the medial region of the PVN nucleus of the hypothalamus that synthesizes and release thyrotropin-releasing hormone (TRH) into the pituitary gland . The TRH stimulates the release of thyrotropin (TSH) from the anterior pituitary, which in turn stimulate the synthesis and release of thyroid hormones in the target thyroid gland. Mainly two types of thyroid hormones are produced such as Triiodothyronine (T3) and Thyroxine (T4). The T3 is the main biologically active hormone recognized by its greater affinity for thyroid hormone receptors while T4 is the storage hormone which is converted to T3 by the activity of deiodinase enzymes located within most target tissues aided by the central nervous system . Thyroid hormones are recognized as the key regulators of metabolic activity in domestic animals . Both acute and chronic stress causes transient activation of the HPT axis. This was facilitated by the increases in TSH concentration as a result of the direct stimulatory effect of glucocorticoids on the pituitary thyrotrope . However, prolonged stress is invariably associated with decreased HPT activity in farm animals in an effort to reduce the metabolic heat production during heat stress. Similarly, reduced HPT activity is mediated by glucocorticoids on TRH production in the hypothalamus . Likewise, increased somatostatin secretion as a result of enhanced intrahypothalamic CRH release might also influence the reduced TSH secretion during heat stress . In addition to the inhibition in TSH production, the extended activity of the HPA axis also reduces the conversion of T4 to T3 in peripheral tissues . Similarly, the adipocyte hormone leptin regulates this axis by increasing TRH levels in the fed state. Leptin directly stimulates the TRH in the hypothalamic paraventricular nucleus and indirectly by regulating proopiomelanocortin neurons in the hypothalamic arcuate nucleus . However, the indirect pathway is fully functional in lean animals, it is inactive during diet-induced obesity due to leptin resistance. Moreover, the primary target site for leptin that mediates its effect on the HPT axis is the arcuate nucleus, further removal of this nucleus abolishes both nutritional stress and leptin-induced regulation of the HPT axis. However, more investigations are needed in this pathway for understanding the underlying mechanisms pertaining to HPT axis in the future.
The Sympathetic-adrenal-Medullary Axis
The sympathoadrenal system consists of the adrenal glands and associated receptors. The sympathetic adrenal medullary (SAM) axis coordinates all the responses of diverse stressors by mediating the release of epinephrine from the adrenal medulla and norepinephrine from peripheral sympathetic nerves. Also, the interrelation between the central nervous system and pituitary coordinates the SAM axis activation resulting in the, release of β-endorphin which helps in circulating glucocorticoids and catecholamines to interact with a wide variety of cells to alter the metabolic and immune mechanisms .
Catecholamines are released to circulatory system as part of the body’s stress response. It plays a pivotal role in integrating certain adaptive mechanisms, mainly the flight and fright responses . The classical catecholamines includes dopamine, epinephrine and norepinephrine . Additionally, catecholamines plays an enormous role in regulating the cardiopulmonary system, increasing the cardiac output, respiration rate and redistribute blood flow to the pulmonary organs, further those organs necessary for mounting responses to the various stresses . The catecholamines interact with adrenergic receptors present in the cell membranes of the visceral organs and smooth muscles, further result in the activation of signaling pathways and consequent alterations of various endocrine organ functions . Catecholamines are mainly circulated through blood and it could alter the effects on afferent sensory nerves impacting central nervous system function . However, these rapid responses may be necessary for survival, sustained elevation of circulating catecholamines for prolonged periods of time can also produce pathological conditions, such as cardiac hypertrophy, hypertension and posttraumatic stress disorder .
The sympathetic component of the sympathoadrenal system comprises preganglionic neurons, where they synapse with postganglionic neurons that project and innervate the target tissues . Moreover, preganglionic neurons release the neurotransmitter acetylcholine that stimulates the postganglionic neurons to release norepinephrine directly into the target tissue. In the adrenal arm of the sympathoadrenal system, preganglionic neurons extend from the spinal cord to ganglia in the adrenal medulla, where the terminals appose endocrine cells are called chromaffin cells. Acetylcholine stimulates synthesis of catecholamines in chromaffin cells in the adrenal medulla and the secretion of epinephrine and norepinephrine into the peripheral blood circulatory system. In addition, species differences were established for the level of norepinephrine and epinephrine released from the adrenal medulla .
Activation of brain stem noradrenergic neurons and sympathetic medullary system further contribute to the endocrine responses to various stressful stimuli. Similarly to the HPA axis, stress evoked activation of these systems promotes the mobilization of resources to compensate for adverse effects pertaining to severe stress conditions. Also, the locus coeruleus (LC) contains the largest cluster of noradrenergic neurons in the brain and innervates large segments of the neuroaxis. The LC has been implicated in a wide array of physiological and behavioral functions including emotion, vigilance, memory and various adaptive responses. Furthermore, a wide array of stressful stimuli activates LC neurons, alter their electrophysiological activity, and induce norepinephrine release . Stimulation of the LC elicits several stress associated responses including ACTH release, anxiogenic like behaviors, and suppression of immune functions. In addition, there are interactions between CRF and NE neurons in the CNS. Further, central administration of CRF alters the activity of LC neurons and NE catabolism in terminal regions. Finally, dysfunction of catecholaminergic neurons in the LC has been implicated in the stress associated disorders.
Climate change has emerged as the major threat to the livestock production. Extended exposure of increased temperature coupled with high relative humidity compromises the ability of farm animal to control the body heat which ultimately affects their feed intake, milk production, and reproductive efficiency, resulting in severe economic constraints for dairy farmers. Therefore, understanding in depth the various adaptive responses of animals may provide future directions for coping them to the devastating effects of heat stress. Neuroendocrine response is the principal regulator of stress response in animals and it forms the basis for regulating and coordinating all other adaptive response in domestic animals. The detailed discussions on various mechanisms and pathways associated with neuro-endocrine regulation have identified several biological markers such as cortisol, T3, T4, epinephrine, and norepinephrine. Further, in-depth understanding of the hidden intricacies of neural and endocrine regulation of adaptive responses in animals provides the scope for intervening points to protect them against the adverse impact of heat stress. There are, however, breed differences established for the level of neuroendocrine responses in various species.
Conflict of Interests
The authors declare that they have no conflict of interests.