Fed, but not fasted, adrenalectomized rats survive the stress of hemorrhage and hypovolemia

Integration of energy homeostasis and stress by parvocellular neurons in rat hypothalamic paraventricular nucleus

KEY POINTS

Central regulation of energy homeostasis and stress are believed to be reciprocally regulated, i.e. excessive food intake suppresses, while prolonged hunger exacerbates, stress responses in vivo. This relationship may be mediated by neuroendocrine parvocellular corticotropin-releasing hormone (CRH) neurons in the hypothalamic paraventricular nucleus that receive both stress- and feeding-related input. We find that hunger strongly and selectively potentiates, while re-feeding suppresses, a cellular analogue of a stress response induced by acute glucopenia in CRH neurons in rat hypothalamic slices. Neuronal activation in response to glucopenia was mediated synaptically, via the relative enhancement of glutamate over GABA input. These results illustrate how acute stress responses may be initiated in vivo and show that it is reciprocally integrated with energy balance via local hypothalamic mechanisms acting at the level of CRH neurons and their afferent terminals.

ABSTRACT

Increased food intake is a common response to help cope with stress, implying the existence of a previously postulated but imperfectly understood, inverse relationship between the regulation of feeding and stress. We have identified components of the neural circuitry that can integrate these homeostatic responses. Prior fasting (∼24 h) potentiates, and re-feeding suppresses, excitatory responses to acute glucopenia in about half of the corticotropin releasing hormone (CRH)-expressing, putatively neurosecretory, stress-related neurons in the paraventricular nucleus of the hypothalamus studied. Glucoprivation stress ex vivo resulted from a preferential relative increase in excitatory (glutamatergic) over inhibitory (GABAergic) inputs. Putative preautonomic cells were less sensitive to fasting, and showed a predominant inhibition to acute glucopenia. We conclude that hunger may sensitize hypothalamic stress responses by acting via local mechanisms, at the level of CRH neurons and their presynaptic inputs. Those mechanisms involve neither presynaptic ATP-sensitive potassium channels nor postsynaptic ATP levels.

Evaluating the stress response as a bioindicator of sub-lethal effects of crude oil exposure in wild house sparrows (Passer domesticus)

Petroleum can disrupt endocrine function in humans and wildlife, and interacts in particularly complex ways with the hypothalamus-pituitary-adrenal (HPA) axis, responsible for the release of the stress hormones corticosterone and cortisol (hereafter CORT). Ingested petroleum can act in an additive fashion with other stressors to cause increased mortality, but it is not clear exactly why--does petroleum disrupt feedback mechanisms, stress hormone production, or both? This laboratory study aimed to quantify the effects of ingested Gulf of Mexico crude oil on the physiological stress response of house sparrows (Passer domesticus). We examined baseline and stress-induced CORT, negative feedback, and adrenal sensitivity in house sparrows given a 1% oil or control diet (n = 12 in each group). We found that four weeks on a 1% oil diet did not alter baseline CORT titers or efficacy of negative feedback, but significantly reduced sparrows' ability to secrete CORT in response to a standardized stressor and adrenocorticotropin hormone injection, suggesting that oil damages the steroid-synthesizing cells of the adrenal. In another group of animals on the same 1% oil (n = 9) or control diets (n = 8), we examined concentrations of eight different blood chemistry parameters, and CORT in feathers grown before and during the feeding experiments as other potential biomarkers of oil exposure. None of the blood chemistry parameters differed between birds on the oil and control diets after two or four weeks of feeding, nor did feather CORT differ between the two groups. Overall, this study suggests that the response of CORT to stressors, but not baseline HPA function, may be a particularly sensitive bioindicator of sub-lethal chronic effects of crude oil exposure.

Chronic exposure to a low dose of ingested petroleum disrupts corticosterone receptor signalling in a tissue-specific manner in the house sparrow (Passer domesticus)

Stress-induced concentrations of glucocorticoid hormones (including corticosterone, CORT) can be suppressed by chronic exposure to a low dose of ingested petroleum. However, endocrine-disrupting chemicals could interfere with CORT signalling beyond the disruption of hormone titres, including effects on receptors in different target tissues. In this study, we examined the effects of 6 weeks of exposure to a petroleum-laced diet (1% oil weight:food weight) on tissue mass and intracellular CORT receptors in liver, fat, muscle and kidney (metabolic tissues), spleen (an immune tissue) and testes (a reproductive tissue). In the laboratory, male house sparrows were fed either a 1% weathered crude oil (n = 12) or a control diet (n = 12); glucocorticoid receptors and mineralocorticoid receptors were quantified using radioligand binding assays. In oil-exposed birds, glucocorticoid receptors were lower in one metabolic tissue (liver), higher in another metabolic tissue (fat) and unchanged in four other tissues (kidney, muscle, spleen and testes) compared with control birds. We saw no differences in mineralocorticoid receptors between groups. We also saw a trend towards reduced mass of the testes in oil-exposed birds compared with controls, but no differences in fat, kidney, liver, muscle or spleen mass between the two groups. This is the first study to examine the effects of petroleum on CORT receptor density in more than one or two target tissues. Given that a chronic low dose of ingested petroleum can affect stress-induced CORT titres as well as receptor density, this demonstrates that oil can act at multiple levels to disrupt an animal's response to environmental stressors. This also highlights the potential usefulness of the stress response as a bioindicator of chronic crude oil exposure.

Seasonal glucocorticoid responses to capture in wild free-living mammals

We determined baseline and capture-induced glucocorticoid concentrations during two different seasons in three species of wild free-living rodents: brown lemmings ( Lemmus trimucronatus), golden-mantled ground squirrels ( Spermophilus saturatus), and yellow-pine chipmunks ( Tamias amoenus). Initial blood samples were obtained within 3 min of capture, so that initial glucocorticoid levels reflect baseline titers of undisturbed animals. Animals were held for an additional 30 min, when a second blood sample was taken to measure stress-induced glucocorticoid titers. The primary glucocorticoid differed in each species. Lemmings secreted extremely large amounts of corticosterone (as high as 8,000 ng/ml). These high concentrations were accompanied by high corticosterone-binding globulin capacity and resistance to negative feedback. Squirrels and chipmunks secreted a mixture of cortisol and corticosterone (10–400 ng/ml). In males of all three species and female squirrels and chipmunks, glucocorticoid levels were significantly elevated 30 min after capture. Baseline and 30-min glucocorticoid levels differed seasonally in each species. Levels were higher during summer (with no snow cover) than in spring (with ∼60% snow cover) in female lemmings, higher during breeding than before hibernation in squirrels, and higher postreproductively than during breeding in chipmunks. Together, these data indicate that glucocorticoid responses to stress in these free-living species are similar to those in laboratory species, but the magnitude of the response appears to depend on life-history features specific to each species.

Glucocorticoid regulation of the inflammatory response to injury

During the first half of the 20th century, physiologists were interested in the adrenal glands primarily because adrenalectomized animals failed to survive even mild degrees of systemic stress. It eventually became clear that hormones secreted by the adrenal cortex were critical for survival and, in this context, adrenal cortical hormones were widely considered to support or stimulate important responses to stress or injury. With the purification and manufacture of adrenal cortical hormones in the 1930s and 1940s, clinicians suddenly discovered the potent anti-inflammatory actions of glucocorticoids (GCs). This dramatic, and unexpected, discovery has dominated clinical and laboratory research into GC actions throughout the second half of the 20th century. More recent research is again reporting GC-induced stimulatory effects on a variety of inflammatory response components. These effects are usually observed at low GC concentrations, close to concentrations that are observed in vivo during basal, unstimulated states. For example, GC-mediated stimulation has been reported for the hepatic acute-phase response, for cytokine secretion, expression of cytokine/chemokine receptors, and for the pro-inflammatory mediator, macrophage migration inhibition factor. It seems clear that the long-held clinical view that GCs act solely as anti-inflammatory agents needs to be re-assessed. Varying doses of GCs do not lead simply to varying degrees of inflammation suppression, but rather GCs can exert a full range of effects from permissive to stimulatory to suppressive.

Glucocorticoid regulation of diverse cognitive functions in normal and pathological emotional states

The glucocorticoid hormone cortisol is essential for many forms of regulatory physiology and for cognitive appraisal. Cortisol, while associated with fear and stress response, is also the hormone of energy metabolism and it coordinates behavioral adaptation to the environmental and internal conditions through the regulation of many neurotransmitters and neural circuits. Cortisol has diverse effects on many neuropeptide and neurotransmitter systems thus affecting functional brain systems. As a result, cortisol affects numerous cognitive domains including attention, perception, memory, and emotional processing. When certain pathological emotional states are present, cortisol may have a role in differential activation of brain regions, particularly suppression of hippocampal activation, enhancement of amygdala activity, and dendritic reshaping in these regions as well as in the ventral prefrontal cortex. The coordinated actions of glucocorticoid regulation on various brain systems such as those implicated in emotional processing can lead to perceptual and cognitive adaptations and distortions of events that may be relevant for understanding mood disorders.

Seasonal changes in plasma glucocorticoid concentrations in free-living vertebrates

The vertebrate stress response helps animals respond to environmental dangers such as predators or storms. An important component of the stress response is glucocorticoid (GC) release, resulting from activation of the hypothalamic-pituitary-adrenal axis. After release, GCs induce a variety of behavioral and physiological changes that presumably help the animal respond appropriately to the situation. Consequently, GC secretion is often considered an obligatory response to stressful situations. Evidence now indicates, however, that free-living species from many taxa can seasonally modulate GC release. In other words, the magnitudes of both unstressed and stressed GC concentrations change depending upon the time of year. This review examines the growing evidence that GC concentrations in free-living reptiles, amphibians, and birds, but not mammals, are commonly elevated during the breeding season. This evidence is then used to test three hypotheses with different focuses on GC's energetic or behavioral effects, as well as on GC's role in preparing the animal for subsequent stressors. These hypotheses attempt to place annual GC rhythms into a physiological or behavioral context. Integrating seasonal differences in GC concentrations with either different physiological states or different life history stages provides clues to a new understanding of how GCs actually help in survival during stress. Consequently, understanding seasonal modulation of GC release has far-reaching importance for both the physiology of the stress response and the short-term survival of individual animals.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Stress and inflammatory disease: widening roles for serotonin and substance P

Serotonin has been implicated in mediating the hypothalamo-pituitary-adrenal (HPA) axis response to stress and is an important therapeutic target for a number of psychiatric disorders including depression. The neurokinin substance P has been shown to inhibit stress-induced HPA axis activity and we have demonstrated that endogenous substance P is able to reduce the duration of the HPA axis response to stress suggesting an important role in the termination of the stress response. This may be important in controlling the transition from acute to chronic stress and substance P has recently attracted attention as a potential antidepressant.In addition to these central effects, serotonin and substance P are considered to be pro-inflammatory agents. Despite being implicated in mediating inflammation there have been few studies investigating the effects of manipulations of serotonergic or substance P systems on chronic inflammatory disease. Treatment of rats with adjuvant-induced arthritis(AA), a model of chronic inflammatory stress, with a substance P antagonist specific for the NK1 receptor subtype resulted in a reduction in hind paw inflammation suggesting substance P may influence inflammation. We have noted that depletion of whole body serotonin and selective central depletion of serotonin results in a decrease in the severity of inflammation in rats with adjuvant arthritis. Furthermore, treatment with a selective serotonin reuptake inhibitor results in an earlier onset and increased severity of inflammation in adjuvant arthritis, confirming a pro-inflammatory role for serotonin. Serotonin is also present in the immune tissues and concentrations in the spleen fall following the development of inflammation in adjuvant arthritis. Concentrations of serotonin are significantly higher in normal female spleen than in males, and this may underlie the greater predisposition of females to certain autoimmune diseases.There is increasing evidence of a role for transmitters such as serotonin and substance P,both centrally and peripherally, in mediating a wide variety of inflammatory and psychiatric disorders. A better understanding of the mechanisms of action of these transmitters and the development of suitable drugs targeting specific receptor subtypes has great potential to impact on clinical practice in the near future. The purpose of this review is to consider the separate roles of serotonin and substance P in relation to HPA axis stress responses, in the context of a model of chronic inflammatory disease, highlighting novel directions of current research for each of these transmitters.

Testosterone receptor blockade after trauma and hemorrhage attenuates depressed adrenal function

Although the testosterone receptor antagonist flutamide restores the depressed immune function in males after trauma and hemorrhage, it remains unknown whether this agent has any salutary effects on adrenal function. To study this, male rats underwent laparotomy and were bled to and maintained at a blood pressure of 40 mmHg until 40% of the shed blood volume was returned in the form of Ringer lactate. Animals were then resuscitated and flutamide (25 mg/kg body wt) was administered subcutaneously. Plasma adrenocorticotropic hormone (ACTH) and corticosterone, as well as adrenal corticosterone and cAMP were measured 20 h after resuscitation. In additional animals, ACTH was administered and ACTH-induced corticosterone release and adrenal cAMP were determined. The results indicate that adrenal contents of corticosterone and cAMP were significantly decreased and morphology was altered after hemorrhage. Administration of flutamide improved corticosterone content, restored cAMP content, and attenuated adrenal morphological alterations. Flutamide also improved the diminished ACTH-induced corticosterone release and adrenal cAMP response at 20 h after hemorrhage and resuscitation. Furthermore, the diminished corticosterone response to ACTH stimulation in the isolated adrenal preparation was improved with flutamide. These results suggest that flutamide is a useful adjunct for improving adrenal function in males following trauma and hemorrhage.

Measuring plasminogen activator inhibitor activity in plasma by two enzymatic assays

We compared two methods that measure plasminogen activator inhibitor (PAI) activity in plasma based on the ability of PAI to inhibit tissue plasminogen activator (tPA) or urokinase (uPA) in order to determine which method most accurately measures plasma PAI activity after stressors, like hemorrhage. Plasma PAI activity was significantly elevated after hemorrhage in both assays. Using standard curves derived from rhPAI-1, we found that the tPA-PAI assay was more sensitive than the uPA-PAI assay. However, we measured a 10-fold difference in PAI activity as measured between assays, suggesting that some endogenous plasma constituents (tPA, uPA, plasminogen or plasmin) may interfere with the accurate determination of PAI activity. Increasing the amount of plasma in each assay led to a progressive increase in PAI activity. However, removing either tPA or plasminogen from the tPA-PAI assay unmasked the presence of some endogenous tPA and plasminogen. Furthermore, increasing plasma volume in either assay increases measured plasma tPA, but not uPA. Finally, plasma tPA is elevated after hemorrhage, whereas plasma uPA is not. These results suggest that endogenous tPA and plasminogen may interfere with the measurement of plasma PAI activity in the tPA-PAI assay after hemorrhage or other stresses. The uPA-PAI assay does not have this confounding problem because endogenous uPA does not interfere with the assay, nor does it rise during hemorrhage.

Sensitivity to glucocorticoid-mediated fast-feedback regulation of the hypothalamic-pituitary-adrenal axis is dependent upon stressor specific neurocircuitry

Fos-protein immunoreactivity (Fos-IR) was used to identify neurocircuits potentially participating in the regulation of hypothalamic-pituitary-adrenal (HPA) axis sensitivity to glucocorticoid-mediated fast-feedback in rats exposed to the physical stressor, hemorrhage, or the psychological stressor, airpuff startle. Marked regional brain differences in the Fos-IR expression were observed in response to these stressors. Specifically, after hemorrhage, nuclear Fos-IR increased in the nucleus of the solitary tract and other brainstem regions known to regulate hemodynamic processes including the supraoptic nucleus, and the magnocellular division of hypothalamic paraventricular nucleus (PVN). In contrast, after airpuff startle Fos-IR increased in the dorsomedial and lateral hypothalamus as well as in the lateral septum. Thus, activation of brainstem neurocircuits predominated after hemorrhage whereas activation of forebrain neurocircuits predominated after airpuff startle. In other regions, the magnitude of stressor-induced Fos-IR expression varied in a region-specific manner. When stressor exposure was preceded by administration of corticosterone to achieve levels within the physiological range after stressors, HPA axis responses were suppressed in response to the airpuff startle but not to either a small or moderate hemorrhage.

IN CONCLUSION

(1) fast-feedback mediated inhibition of HPA axis activity is critically dependent upon stressor modality; (2) this apparent selectivity is reflected by differences in the nature of the neurocircuitry mediating these stressors. It is suggested that determination of the central actions of glucocorticoids in mediating fast-feedback regulation of the HPA axis requires evaluation of the interactions between activated glucocorticoid receptors and intracellular signaling cascades evoked by convergent neuronal input.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Blood flow, vascular resistance, and blood volume after hemorrhage in conscious adrenalectomized rat

Hemorrhage leads to cardiovascular collapse and death in adrenal-insufficient animals. To determine whether the cardiovascular collapse is due to vasodilation and/or failure to restore blood volume, we used radiolabeled microspheres and 125I-labeled albumin to measure blood flow and blood volume in conscious adrenalectomized (ADX) rats after 15 ml.kg-1.3 min-1 hemorrhage. In ADX rats, hemorrhage led to a greater fall than in sham rats in blood flow in the stomach, small intestines, cecum, colon, spleen, hepatic portal vein, kidney, testis, lung, thymus, bone, fat, forebrain, cerebellum, and brainstem. The greater fall in blood flow was caused by an increase in vascular resistance in these organs except brain and hepatic artery. Sham rats maintained or increased brain and hepatic artery blood flow after hemorrhage whereas flow decreased and remained depressed in ADX rats. ADX rats failed to restore blood volume, whereas sham rats completely restored blood flow by 2 h. We conclude that cardiovascular collapse in ADX rats does not result from vasodilatation but may result from a failure to restore blood volume. The failure to restore blood volume and the low blood flow to organs, especially brain and liver, may contribute to mortality in ADX rats after hemorrhage.

High-performance liquid chromatographic determination of quinine in rat biological fluids

A high-performance liquid chromatographic (HPLC) method with ultraviolet detection for the determination of quinine in rat biological fluids is described. Due to its selectivity and sensitivity, the proposed method can be used in the case of such rat biological fluids as cerebrospinal fluid (CSF) and perilymph for which the accessible volumes are limited to 100 microl and 10 microl, respectively. Consequently, the assay method has been applied to the measurements of quinine concentration in rat plasma, CSF and perilymph samples.