Paraventricular lesions: hormonal and cardiovascular responses to hemorrhage

Locus Coeruleus Noradrenergic Modulation of Diurnal Corticosterone, Stress Reactivity, and Cardiovascular Homeostasis in Male Rats

INTRODUCTION

Activation of the locus coeruleus-noradrenergic (LC-NA) system during awakening is associated with an increase in plasma corticosterone and cardiovascular tone. These studies evaluate the role of the LC in this corticosterone and cardiovascular response.

METHODS

Male rats, on day 0, were treated intraperitoneally with either DSP4 (50 mg/kg body weight) (DSP), an LC-NA specific neurotoxin, or normal saline (SAL). On day 10, animals were surgically prepared with jugular vein (hypothalamic-pituitary-adrenal [HPA] axis) or carotid artery (hemodynamics) catheters and experiments performed on day 14. HPA axis activity, diurnally (circadian) and after stress (transient hemorrhage [14 mL/kg body weight] or air puff-startle), and basal and post-hemorrhage hemodynamics were evaluated. On day 16, brain regions from a subset of rats were dissected for norepinephrine and corticotropin-releasing factor (CRF) assay.

RESULTS

In DSP rats compared to SAL rats, (1) regional brain norepinephrine was decreased, but there was no change in median eminence or olfactory bulb CRF content; (2) during HPA axis acrophase, the plasma corticosterone response was blunted; (3) after hemorrhage and air puff-startle, the plasma adrenocorticotropic hormone response was attenuated, whereas the corticosterone response was dependent on stressor category; (4) under basal conditions, hemodynamic measures exhibited altered blood flow dynamics and systemic vasodilation; and (5) after hemorrhage, hemodynamics exhibited asynchronous responses.

CONCLUSION

LC-NA modulation of diurnal and stress-induced HPA axis reactivity occurs via distinct neurocircuits. The integrity of the LC-NA system is important to maintain blood flow dynamics. The importance of increases in plasma corticosterone at acrophase to maintain short- and long-term cardiovascular homeostasis is discussed.

NMDA and non-NMDA glutamate receptors in the paraventricular nucleus of the hypothalamus modulate different stages of hemorrhage-evoked cardiovascular responses in rats

Here we report the involvement of N-Methyl-d-Aspartate (NMDA) and non-NMDA glutamate receptors from the paraventricular nucleus of the hypothalamus (PVN) in the mediation of cardiovascular changes observed during hemorrhage and post-bleeding periods. In addition, the present study provides further evidence of the involvement of circulating vasopressin and cardiac sympathetic activity in cardiovascular responses to hemorrhage. Systemic treatment with the V1-vasopressin receptor antagonist dTyr(CH2)5(Me)AVP (50 μg/kg, i.v.) increased the latency to the onset of hypotension during hemorrhage and slowed post-bleeding recovery of blood pressure. Systemic treatment with the β1-adrenergic receptor antagonist atenolol (1 mg/kg, i.v.) also increased the latency to the onset of hypotension during hemorrhage. Moreover, atenolol reversed the hemorrhage-induced tachycardia into bradycardia. Bilateral microinjection of the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) into the PVN blocked the hypotensive response to hemorrhage and reduced the tachycardia during the post-hemorrhage period. Systemic treatment with dTyr(CH2)5(Me)AVP inhibited the effect of LY235959 on hemorrhage-induced hypotension, without affecting the post-bleeding tachycardia. PVN treatment with the selective non-NMDA receptor antagonist NBQX (2 nmol/100 nL) reduced the recovery of blood pressure to normal levels in the post-bleeding phase and reduced hemorrhage-induced tachycardia. Combined blockade of both NMDA and non-NMDA glutamate receptors in the PVN completely abolished the hypotensive response in the hemorrhage period and reduced the tachycardiac response in the post-hemorrhage period. These results indicate that local PVN glutamate neurotransmission is involved in the neural pathway mediating cardiovascular responses to hemorrhage, via an integrated control involving autonomic nervous system activity and vasopressin release into the circulation.

Effect of intracerebroventricular benzamil on cardiovascular and central autonomic responses to DOCA-salt treatment

DOCA-salt treatment increases mean arterial pressure (MAP), while central infusion of benzamil attenuates this effect. The present study used c-Fos immunoreactivity to assess the role of benzamil-sensitive proteins in the brain on neural activity following chronic DOCA-salt treatment. Uninephrectomized rats were instrumented with telemetry transmitters for measurement of MAP and with an intracerebroventricular (ICV) cannula for benzamil administration. Groups included rats receiving DOCA-salt treatment alone, rats receiving DOCA-salt treatment with ICV benzamil, and appropriate controls. At study completion, MAP in vehicle-treated DOCA-salt rats reached 142 ± 4 mmHg. In contrast DOCA-salt rats receiving ICV benzamil had lower MAP (124 ± 3 mmHg). MAP in normotensive controls was 102 ± 3 mmHg. c-Fos immunoreactivity was quantified in the supraoptic nucleus (SON) and across subnuclei of the hypothalamic paraventricular nucleus (PVN), as well as other cardiovascular regulatory sites. Compared with vehicle-treated normotensive controls, c-Fos expression was increased in the SON and all subnuclei of the PVN, but not in other key autonomic nuclei, such as the rostroventrolateral medulla. Moreover, benzamil treatment decreased c-Fos immunoreactivity in the SON and in medial parvocellular and posterior magnocellular neurons of the PVN in DOCA-salt rats but not areas associated with regulation of sympathetic activity. Our results do not support the hypothesis that DOCA-salt increases neuronal activity (as indicated by c-Fos immunoreactivity) of other key regions that regulate sympathetic activity. These results suggest that ICV benzamil attenuates DOCA-salt hypertension by modulation of neuroendocrine-related PVN nuclei rather than inhibition of PVN sympathetic premotor neurons in the PVN and rostroventrolateral medulla.

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

Neural sites involved in the sustained increase in muscle sympathetic nerve activity induced by inspiratory capacity apnea: a fMRI study

A maximal inspiratory breath hold (inspiratory capacity apnea) against a closed glottis evokes a large and sustained increase in muscle sympathetic nerve activity (MSNA). Because of its dependence on a high intrathoracic pressure, it has been suggested that this maneuver causes unloading of the low-pressure baroreceptors, known to increase MSNA. To determine the central origins of this sympathoexcitation, we used functional magnetic resonance imaging to define the loci and time course of activation of different brain areas. We hypothesized that, as previously shown for the Valsalvsa maneuver, discrete but widespread regions of the brain would be involved. In 15 healthy human subjects, a series of 90 gradient echo echo-planar image sets was collected during three consecutive 40-s inspiratory capacity apneas using a 3-T scanner. Global signal intensity changes were calculated and subsequently removed by using a detrending technique, which eliminates the global signal component from each voxel's signal intensity change. Whole brain correlations between changes in signal intensity and the known pattern of MSNA during the maneuver were performed on a voxel-by-voxel basis, and significant changes were determined by using a random-effects analysis procedure ( P < 0.01, uncorrected). Significant signal increases emerged in multiple areas, including the rostral lateral medulla, cerebellar nuclei, anterior insula, dorsomedial hypothalamus, anterior cingulate, and lateral prefrontal cortexes. Decreases in signal intensity occurred in the dorsomedial and caudal lateral medulla, cerebellar cortex, hippocampus, and posterior cingulate cortex. Given that many of these sites have roles in cardiovascular control, the sustained increase in MSNA during an inspiratory capacity apnea is likely to originate from a distributed set of discrete areas.

Role of hypothalamic inputs in maintaining pituitary-adrenal responsiveness in repeated restraint

The role of hypothalamic structures in the regulation of chronic stress responses was studied by lesioning the mediobasal hypothalamus or the paraventricular nucleus of hypothalamus (PVH). Rats were acutely (60 min) and/or repeatedly (for 7 days) restrained. In controls, a single restraint elevated the plasma adrenocorticotropin (ACTH), corticosterone, and prolactin levels. Repeated restraint produced all signs of chronic stress, including decreased body and thymus weights, increased adrenal weight, basal corticosterone levels, and proopiomelanocortin (POMC) mRNA expression in the anterior pituitary. Some adaptation to repeated restraint of the ACTH response, but not of other hormonal responses, was seen. Lesioning of the mediobasal hypothalamus abolished the hormonal response and POMC mRNA activation to acute and/or repeated restraint, suggesting that the hypothalamo-pituitary-adrenal axis activation during repeated restraint is centrally driven. PVH lesion inhibited the ACTH and corticosterone rise to the first restraint by approximately 50%. In repeatedly restrained rats with PVH lesion, the ACTH response to the last restraint was reduced almost to basal control levels, and the elevation of POMC mRNA level was prevented. PVH seems to be important for the repeated restraint-induced ACTH and POMC mRNA stimulation, but it appears to partially mediate other restraint-induced hormonal changes.

Transneuronal tracing from sympathectomized lumbar epaxial muscle in female rats

Pseudorabies virus (PRV) has been used as a transneuronal tracer to study central neural networks, including the central control of the lordosis-producing, lumbar epaxial muscles. Within muscles, however, the sympathetic innervation of blood vessels poses a confounding source of tracer labeling in the CNS. The present study destroyed sympathetic nerves before injection of PRV, thereby allowing for a more selective uptake by somatic motoneurons. Specifically, a focal sympathectomy was created by the injection of dopamine-beta-hydroxylase immunotoxin (DHIT). When PRV was injected into control rats, both somatic motoneurons within the ventral horn of the spinal cord and sympathetic preganglionic neurons within the intermediolateral column (IML) of the spinal cord became labeled. Additionally, labeled neurons were observed in many brain regions, including those previously implicated in the control of the lordosis reflex (e.g., the medullary reticular formation; MRF) and those previously implicated in the control of vasomotor tone (e.g., the rostral ventrolateral medulla; RVLM). When injected into DHIT-pretreated animals, PRV labeling in ventral horn neurons persisted in many animals; however, labeling in IML was eliminated in almost every case. In these animals, PRV labeling was absent in brain areas traditionally associated with vasomotor tone, such as RVLM, whereas labeling persisted in brain areas previously implicated in the control of the lordosis response, such as MRF. The results support the connectivity of spinal and medullary structures with the somatic control of the lordosis-producing muscles and provide a more detailed description of these portions of the putative lordosis-relevant neurocircuitry.

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.

Corticosterone modulation of ACTH secretogogue gene expression in the paraventricular nucleus

This review will describe effects of corticosterone on the temporal dynamics of components within the hypothalamo-pituitary-adrenal (HPA) axis in response to sustained hypovolemia. The characterization of the synthetic and secretory profiles of HPA elements in these rat models reveals the complexities of steroid-mediated regulation of neuroendocrine and corticotrope function during a sustained stress event. Collectively, our data suggest activation of gene transcription and secretion are independently controlled, and that corticosterone affects adrenocorticotropin hormone (ACTH) gene expression in the parvicellular neuroendocrine part of the hypothalamic paraventricular nucleus using two mechanisms: first, an inhibition which contributes to classic negative feedback, and second, a facilitation, which is seen at low plasma concentrations.

Injection of muscimol in dorsomedial hypothalamus and stress-induced Fos expression in paraventricular nucleus

Prior microinjection of the GABA(A)-receptor agonist muscimol into the dorsomedial hypothalamus (DMH) in conscious rats attenuates the increases in heart rate, blood pressure, and circulating adrenocorticotrophic hormone seen in air stress. Here, we examined the effect of similar treatment on air stress- or hemorrhage-induced Fos expression in the paraventricular nucleus (PVN). Muscimol (80 pmol/100 nl per side) or saline (100 nl per side) was microinjected bilaterally into the DMH in conscious rats before either air stress, an emotional or neurogenic stressor, or graded hemorrhage, a physiological stressor. Each stressor evoked a characteristic pattern of Fos expression in the parvocellular and magnocellular PVN after saline. Injection of muscimol into the DMH suppressed Fos expression in the PVN associated with air stress but not with hemorrhage. Injection of muscimol at sites anterior to the DMH and closer to the PVN had no effect on Fos expression in the PVN after air stress. Thus activation of neurons in the DMH is necessary for excitation of neurons in the PVN during air stress but not 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.

Potentiated response of corticotropin (ACTH) to repeated moderate hemorrhage requires amygdalar neuronal processing

We examined the role of the amygdala in the potentiation of the corticotropin (ACTH) response to a 10 mg/kg hemorrhage by a 1-hour episode of equivalent hypovolemia done 24 h earlier. Unanesthetized rats were studied on the fourth (D1) and fifth (D2) day after chronic implantation of arterial and venous catheters. Immunocytochemistry for Fos protein indicated that neurons in the central and medial nuclei of the caudal amygdala were activated by hemorrhage. We then tested the effect of excitotoxic destruction of the neurons in these areas by bilateral injections of ibotenic acid 10 days prior to catheter placement. In rats that were hemorrhaged on both D1 and D2, the responses of ACTH and corticosterone increased significantly from the first (H1) to the second hemorrhage (H2) in a control group injected with saline (p < 0.05) and in lesioned groups without bilateral damage of the Fos-responsive areas (p < 0.01). In the group with bilateral damage to these sites, the responses to H1 and H2 did not differ. Additional rats had H1 on D2 to control for the long-term effects of the chronic cannulation. The responses of ACTH to H1 on either D1 or D2 did not differ between the saline-injected controls and any of the lesioned groups. In contrast, the response of ACTH to H2 on D2 in rats with bilateral damage of the caudal amygdala was not significant and was less than the response of ACTH to H2 in both rats with unilateral damage of this area (p < 0.05) and those injected with saline (p < 0.05). We conclude that bilateral neuronal processing within the caudal amygdala is required for the potentiation of the response of ACTH to H2 by H1.

Differential recruitment of hypothalamic neuroendocrine and ventrolateral medulla catecholamine cells by non-hypotensive and hypotensive hemorrhages

We performed c-fos expression experiments in conscious rats to quantify the threshold and extent of activation of hypothalamic neuroendocrine cells in response to non-hypotensive and hypotensive hemorrhages allowing us to assess whether their pattern of recruitment corresponded to known oxytocin, vasopressin and ACTH release patterns. Also, because previous studies have implicated ventrolateral medulla catecholamine cells in the generation of certain hypothalamic neuroendocrine cell responses, we examined the response of ventrolateral medulla catecholamine cells to non-hypotensive and hypotensive hemorrhages and directly tested their role in regulating neuroendocrine cell responses to hypotensive hemorrhage. Animals were subjected to hemorrhages of 0, 4, 8, 12 or 16 ml/kg BW, the latter two levels being hypotensive. We found that only supraoptic nucleus vasopressin cells were significantly activated by the smallest non-hypotensive hemorrhage (4 ml/kg), which corresponds to reports that only vasopressin is released into the plasma after a small hemorrhage. Hypotensive hemorrhages resulted in significant recruitment of paraventricular and supraoptic oxytocin and vasopressin cells and parvocellular cells of the medial division of the paraventricular nucleus. Vasopressin cells were recruited in much greater numbers than oxytocin cells, which is in agreement with previous findings that there is a greater release of vasopressin than oxytocin into the plasma after hypotensive hemorrhage. In addition, medial parvocellular cells of the paraventricular nucleus, most likely to be tuberoinfundibular-projecting corticotropin-releasing factor cells, were activated by hypotensive hemorrhage only when arterial pressure dropped below 60 mmHg which also corresponds well with the plasma release response of ACTH. Ventrolateral medulla catecholamine cells were only recruited by hypotensive hemorrhages. While caution must be exercised in interpreting an absence of response, this certainly suggests that catecholamine cells are unlikely to have a role in the activation of supraoptic neurosecretory cells in response to non-hypotensive hemorrhages. Unilateral lesions of the ventrolateral medulla catecholamine cell column, corresponding primarily to the location of A1 noradrenergic cells, significantly reduced the hypotensive hemorrhage-induced activation of hypothalamic vasopressin, oxytocin and medial parvocellular paraventricular nucleus cells. This suggests that A1 noradrenergic cells contribute to the activation of these neuroendocrine cell populations, including oxytocin cells, which is an unexpected finding. More significantly, however, because the reduction in responsiveness after A1 lesions was similar for all cell categories, it seems likely that other factors must determine the differential recruitment of hypothalamic neuroendocrine cells in response to a hypotensive hemorrhage.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Corticosterone can facilitate as well as inhibit corticotropin-releasing hormone gene expression in the rat hypothalamic paraventricular nucleus

We have used in situ hybridization to investigate how basal levels of circulating corticosterone modulate CRH gene transcription in the neuroendocrine parvicellular part of the hypothalamic paraventricular nucleus (PVHmpd) during sustained hypovolemia. In the absence of the stressor, the accumulation rate of the CRH primary transcript exhibited a dose dependency on low maintained levels of plasma corticosterone similar to that previously reported for the mature messenger RNA (mRNA); levels declined as plasma corticosterone increased. In response to hypovolemia, the absence of corticosterone compromised CRH gene transcription mechanisms to mount the activated response seen in intact animals. However, adrenalectomized rats with low doses of corticosterone (insufficient to normalize thymus weights) showed an augmented mRNA response compared with that in intact animals. When replaced corticosterone normalized thymus weights, the magnitude of the mRNA response was reduced to that seen in intact animals. In contrast to CRH gene regulation, PVHmpd proenkephalin mRNA levels were unaffected by corticosterone concentrations. These results suggest that corticosterone affects CRH gene transcription in the PVHmpd using two mechanisms: first, inhibition, which probably uses type II glucocorticoid receptor-dependent mechanisms and contributes to classic negative feedback; and second, facilitation, which is seen at low plasma concentrations and maintains gene transcription in the presence of sustained stress, possibly using type I mechanisms. This suggests that one reason why adrenal insufficiency severely compromises survival of sustained stress is that CRH gene transcription cannot be maintained without previous exposure to low levels of plasma corticosterone.

Role of paraventricular nucleus parvicellular neurons in the compensatory responses to graded hemorrhage

The goal of this study was to determine the role of the parvicellular component of the paraventricular hypothalamic nucleus (PVH) in the compensatory responses to blood loss. Male Sprague-Dawley rats were prepared with bilateral ibotenate lesions of the parvicellular PVH (PVHx; n = 5) or with sham lesions (Sham; n = 8). After >10 days recovery, hemorrhage was performed by gradual withdrawal of 16 ml/kg blood over 34 min via an indwelling femoral arterial catheter while the rats were conscious and unrestrained. Basal serum corticosterone levels, plasma renin concentration (PRC), mean arterial pressure, and heart rate did not differ between PVHx and Sham, whereas basal hematocrit was lower in PVHx than Sham (40 +/- 1 vs. 44 +/- 1; P < 0.05). After hemorrhage, corticosterone increased fourfold in Sham (P < 0.001) but did not increase significantly in PVHx. However, the blood pressure, heart rate, PRC, and hemodilution responses to hemorrhage were the same in Sham and PVHx during both the normotensive (7-13 ml/kg blood loss) and hypotensive (16 ml/kg blood loss) phases. In conclusion, the parvicellular PVH is essential for the corticosterone response, but not for the cardiovascular or renin responses to blood loss.

Role of the hypothalamic paraventricular nucleus in cardiovascular regulation

1. The paraventricular hypothalamic nucleus (PVH) is a complex structure with both neuroendocrine and autonomic functions. It is a major source of vasopressin and the primary source of corticotropin-releasing factor. In addition, parvicellular PVH neurons have reciprocal connections with brainstem autonomic centres and directly innervate sympathetic preganglionic neurons. Evidence is reviewed which indicates that in conscious rats PVH activation increases blood pressure, heart rate, renal nerve activity and plasma renin activity. 2. In conscious rats, a non-hypotensive haemorrhage (13 mL/kg blood loss over 24 min) results in increased numbers of Fos-immunoreactive cell nuclei within both magnocellular and parvicellular PVH neurons, including the ventral medial parvicellular regions known to contain neuronal projections to brainstem autonomic centres and spinal cord sympathetic preganglionic neurons. 3. Cell-selective ibotenate lesions of the parvicellular PVH significantly blunt the corticosterone response but do not alter blood pressure, heart rate or plasma renin concentration response to non-hypotensive or hypotensive haemorrhage. This and earlier studies indicate that, while the PVH is necessary for the corticosterone response and contributes to increased vasopressin release during blood loss, it does not play an important role in the sympathetic nervous system and renin-angiotensin responses to hypovolaemia and hypotension. 4. There is evidence to indicate that the parvicellular PVH serves as a necessary relay for cardiovascular and renin responses to certain behavioural stressors. We propose that cardiovascular information relayed to parvicellular PVH autonomic regions may be used to modulate behavioural, rather than homeostatic, effects on haemodynamics and renin release.

Involvement of the PVN and BST in 1K1C hypertension in the rat

The paraventricular nucleus of the hypothalamus (PVN) and the bed nucleus of the stria terminalis (BST) are two sources of central nervous system (CNS)-derived arginine vasopressin (AVP), a nonapeptide that has been implicated in central autonomic regulation and in particular in cardiovascular regulation, through its actions within the CNS. These experiments were designed to determine if either the PVN or the BST were involved in the development of Goldblatt one-kidney one-clip (1K1C) hypertension in the rat. In order to test this hypothesis, ibotenic acid lesions of the PVN or electrolytic lesions of the BST were undertaken in both normotensive (sham-operated) rats and in 1K1C rats. In both cases the development of 1K1C hypertension was inhibited over the 18-21 days following surgery. Lesions of the PVN did not alter normal blood pressure regulation in the sham-operated animals, whereas lesions to the BST did affect normal blood pressure regulation, resulting in a dramatic increase in blood pressure during the initial days following surgery. These studies suggest that the PVN and BST are involved in the development of 1K1C hypertension in the rat, moreover the BST may also play a role in central cardiovascular control.

Effects of footshock stress upon spleen and peripheral blood lymphocyte mitogenic responses in rats with lesions of the paraventricular nuclei

To assess the role of the hypothalamic paraventricular nucleus (PVN) in mediating stressor-induced immune alterations, male Lewis rats were subjected to a 1-h session of intermittent footshock stress or home cage conditions 6 days after receiving bilateral or sham PVN lesions. Splenic and peripheral blood lymphocyte proliferative responses to the non-specific mitogens, concanavalin A (ConA) and phytohemagglutinin (PHA), were subsequently measured as were plasma corticosterone levels. In sham-operated rats, footshock markedly elevated plasma corticosterone levels and concurrently suppressed the proliferative responses of peripheral blood and splenic lymphocytes. In PVN-lesioned rats, however, the shock-induced suppression of lymphocyte proliferation in the peripheral blood and the elevation of plasma corticosterone were significantly attenuated, while lymphocyte proliferation in the spleen was suppressed below the level of the sham-treated animals. Thus, by utilizing ablation studies, we have determined that the PVN may play a direct role in the alteration of lymphocyte function during stress, and an intact PVN buffers the effect of stress on the responsiveness of spleen lymphocytes to non-specific mitogens.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Effect of diazepam on endocrine and cardiovascular responses to head-up tilt in humans

Effects of the GABAergic drug diazepam (0.15 mg kg-1, i.v.) on cardiovascular and endocrine responses to 50 degrees head-up tilt were evaluated in seven men. During the initial phase of tilt (normotensive phase), increases in heart rate (HR) and total peripheral resistance (TPR) and decreases in cardiac output were unaffected by diazepam. Also the associated increase in plasma noradrenaline did not change, while response in plasma ACTH was diminished and in plasma cortisol abolished by diazepam (F(1,10) = 6.45; P < 0.03). After 42 +/- 4 min of sustained tilt with saline (control) and 47 +/- 6 min (n.s.) after diazepam, presyncopal symptoms appeared (hypotensive phase) associated with decreases in HR, MAP, and TPR (P < 0.01). This episode induced a 2-3-fold increase in plasma ACTH, beta-endorphin, prolactin, cortisol (< 0.01), and a moderate increase in plasma adrenaline (P < 0.05). Diazepam did not significantly change cardiovascular and endocrine responses to the hypotensive phase of tilt. Results indicate that diazepam attenuates the cortisol part of pituitary-adrenal responses to moderate, but not to severe, central hypovolaemia in humans with no effect on cardiovascular tolerance.

Forebrain nuclei involved in autonomic control

Several key regions of the forebrain are involved in regulation of autonomic functions. These areas include the several areas within the hypothalamus (viz., paraventricular hypothalamic nucleus, lateral hypothalamic area, posterior periventricular area, and zona incerta), the basal forebrain (viz., central nucleus of the amygdala and bed nucleus of the stria terminalis), and the cerebral cortex (viz., insular and medial prefrontal cortex). All these areas have been implicated on anatomical grounds to be part of a central autonomic network involving multiple interconnecting circuits. Apart from these complex interconnections, most of these areas project to the lower brain stem where they are capable of influencing the cell groups which innervate the vagal and sympathetic preganglionic neurons or in some cases, like the paraventricular hypothalamic nucleus and the lateral hypothalamic area, provide direct projections to these neurons.

Excitotoxic lesions of the paraventricular hypothalamus: metabolic and cardiac effects

The excitotoxin, N-methyl-D-aspartic acid (NMDA), was used to lesion cell bodies, but not fibers-of-passage, in the paraventricular hypothalamus. Bilateral injections of NMDA (12.6 nmol/100 nl) were made into the paraventricular hypothalamus in halothane-anesthetized male Sprague-Dawley rats. Water intake, food intake, urine output and body weight were measured daily for 26 days after lesioning. Lesioned rats exhibited a modest, but significant, reduction in the rate of gain of body weight, which was most closely correlated with decreases in food intake. Water intake and urine output were not significantly different among the groups. Resting blood pressure, heart rate and baroreflex sensitivity (using the infusion of phenylephrine method) were similar in conscious animals of both groups, 4-5 weeks after lesioning. Neuronal loss, primarily of parvocellular elements, was evident in the paraventricular hypothalamus and neuronal loss frequently extended into the ventro-medial thalamus adjacent to the paraventricular hypothalamus in NMDA-lesioned rats. In a second experiment, injections of NMDA were given acutely into the paraventricular hypothalamus of halothane-anesthetized rats. Upon recovery from anesthesia, behavioral excitation and increases in blood pressure and heart rate were evident for 1-2 hr. Histological examination of hearts taken 48 hr after injection of NMDA revealed a largely mononuclear inflammatory infiltration, hyperemia and myocardial hemorrhage and focal myocardial necrosis. Inflammatory and degenerative changes were most prominent in the left ventricular subendocardium. The cardiomyopathy possessed similarities with catecholamine-induced myocardial necrosis. The results indicated that NMDA-induced lesions of parvocellular elements of the paraventricular hypothalamus did not cause hyperphagia or obesity or alter the resting systemic circulatory function. However, an inflammatory cardiomyopathy, termed "excitotoxin-induced myocardial necrosis", was associated with injections of NMDA into the hypothalamus. Excitotoxin-induced myocardial necrosis may complicate any hemodynamic studies performed in rats in which lesions of the CNS have been produced by means of application of excitotoxins.

Multiple neuroendocrine responses to chronic social stress: interaction between individual characteristics and situational factors

After four weeks of individual housing, male Wistar rats (selected for high or low spontaneous aggressiveness by multiple round-robin encounters) were housed three per cage and submitted to four weeks of chronic social stress consisting of changing membership in the social groups by daily rotation of the animals among cages every day according to a random permutation procedure. In addition, half the males in each condition were housed with three females. Each environmental condition triggered different neuroendocrine changes. Cohabitation with females increased the hypothalamo-pituitary-adrenocortical axis activity, including enlargement of adrenals and increased circulating corticosterone levels. On the other hand, daily rotation of the rats between different social groups activated part of the sympathetic nervous system, such as increased phenylethanolamine N-methyl transferase (PNMT) activity in the adrenals. The level of aggressiveness, however, had no direct influence but interacted with environmental factors on such neuroendocrine measures as circulating testosterone or plasma renin activity. These results indicate that during chronic stress, there is no single, unique response by the animal, but a highly complex set of neuroendocrine changes, dependent on the interaction between individual characteristics (the level of aggressiveness is an example) and situational factors.

Stimulation of the paraventricular nucleus with glutamate activates interscapular brown adipose tissue thermogenesis in rats

The paraventricular nucleus (PVN) of the hypothalamus is involved in the control of energy balance in rodents through its influences on feeding, pituitary hormone secretion and the autonomic nervous system. In the present study, selective stimulation of PVN neurons by means of local microinjection of the excitatory amino acid glutamate (100 mM, 500 mM or 100 nl) led to a concentration-dependent increase in interscapular brown adipose tissue (IBAT) temperature in urethane-anaesthetized rats. This effect could be prevented by pretreatment with the sympathetic ganglionic blocker, chlorisondamine chloride, or the beta-adrenergic receptor antagonist, propranolol, but not by hypophysectomy, implicating the involvement of the sympathetic outflow. Thermogenesis in brown adipose tissue (BAT) is an important accompaniment of overfeeding in small mammals, and previous studies have shown that signals generated in response to feeding gain access to the PVN. The present finding that direct activation of PVN neurons stimulates thermogenesis in BAT, taken together with data that the PVN receives dietary signals from the gut, support the view that PVN neurons may function in monitoring the balance between energy intake and its expenditure.

Excitotoxin-induced myocardial necrosis

The excitotoxins, kainic acid and N-methyl-D-aspartic acid (NMDA), were injected bilaterally into the paraventricular hypothalamus of rats. Kainic acid elicited pressor responses, tachycardia and sudden cardiac death in Nembutal-anesthetized rats. Injections of NMDA caused cardiovascular stimulation on cessation of halothane anesthesia. Intramyocardial hemorrhage, hyaline myocardial necrosis and predominantly mononuclear inflammation were evident 48 h following NMDA. Labetalol pretreatment did not protect from nor did i.v. NMDA cause these changes. Intrahypothalamic excitotoxin injections cause deleterious myocardial changes.

CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus

The CNS cell groups that innervate the sympathoadrenal preganglionic neurons of rats were identified by a transneuronal viral cell body labeling technique combined with neurotransmitter immunohistochemistry. Pseudorabies virus was injected into the adrenal gland. This resulted in retrograde viral infections of the ipsilateral sympathetic preganglionic neurons (T4-T13) and caused retrograde transneuronal cell body infections in 5 areas of the brain: the caudal raphe nuclei, ventromedial medulla, rostral ventrolateral medulla, A5 cell group, and paraventricular hypothalamic nucleus (PVH). In the spinal cord, the segmental distribution of virally infected neurons was the same as the retrograde cell body labeling observed following Fluoro-gold injections in the adrenal gland except there was almost a 300% increase in the number of cells labeled and a shift in cell group distribution. These results imply there are local interneurons that regulate the sympathoadrenal preganglionic neurons. In the medulla oblongata, serotonin (5-HT)-, substance P (SP)-, thyrotropin-releasing hormone-, Met-enkephalin-, and somatostatin-immunoreactive neurons of the raphe pallidus and raphe obscurus nuclei and the ventromedial medulla were infected. In the ventromedial and rostral ventrolateral medulla, immunoreactive phenylethanolamine-N-methyltransferase, SP, neuropeptide Y, somatostatin, and enkephalin neurons were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the hypothalamus, tyrosine hydroxylase- and SP-immunoreactive neurons of the dorsal parvocellular PVH were infected. Only a few immunoreactive vasopressin, oxytocin, Met-enkephalin, neurotensin, and somatostatin PVH neurons were labeled.