Noradrenergic activation in the paraventricular nucleus during acute and chronic immobilization stress in rats: an in vivo microdialysis study

Sensitization to the effects of tumor necrosis factor-alpha: neuroendocrine, central monoamine, and behavioral variations

Consistent with the proposition that cytokines act as immunotransmitters between the immune system and the brain, systemic administration of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha; 1.0-4.0 microg) induced mild illness in CD-1 mice, increased plasma corticosterone concentrations, and altered central norepinephrine, dopamine, and serotonin turnover. The actions of TNF-alpha were subject to a time-dependent sensitization effect. After reexposure to a subeffective dose of the cytokine (1.0 microgram) 14-28 d after initial treatment, marked illness was evident (reduced consumption of a palatable substance and diminished activity and social exploration), coupled with an elevation of plasma corticosterone levels. In contrast, cytokine reexposure 1-7 d after initial treatment did not elicit illness, and at the 1 d interval the corticosterone response to the cytokine was reduced. The increase of norepinephrine release within the paraventricular nucleus of the hypothalamus, as reflected by elevated accumulation of 3-methoxy-4-hydroxyphenylglycol, was augmented at the longer reexposure intervals. In contrast, within the central amygdala and the prefrontal cortex TNF-alpha reexposure at the 1 d interval was associated with a pronounced sensitization-like effect, which was not apparent at longer intervals. Evidently, systemic TNF-alpha proactively influences the response to subsequent treatment; however, the nature of the effects (i.e., the behavioral, neuroendocrine, and central transmitter alterations) vary over time after initial cytokine treatment. It is suggested that the sensitization may have important repercussions with respect to cognitive effects of TNF-alpha and may also be relevant to analyses of the neuroprotective or neurodestructive actions of cytokines.

The role of ascending neuronal pathways in stress-induced release of noradrenaline in the hypothalamic paraventricular nucleus of rats

Central catecholaminergic pathways carrying pain-related signals to the hypothalamic paraventricular nucleus (PVN) were investigated in laboratory rats. Four per cent formalin injected subcutaneously was employed as a stressful stimulus. Neuronal activity in brainstem catecholaminergic and paraventricular neurones was assessed by Fos immunohistochemistry. Stress-induced noradrenaline (NE) release from nerve terminals in the PVN was measured in extracellular fluid by in-vivo microdialysis. Within 30 min, formalin elicited a four- to sixfold increase in plasma ACTH and corticosterone concentrations and intense Fos-like activity was seen in the superficial zones of the lumbar spinal cord ipsilateral to the side of the formalin injection. In brainstem catecholaminergic neurones, the PVN, and midline thalamic nuclei, formalin-induced Fos-immunopositivity was equally present in the ipsi- and contralateral sides of the injection. An immediate elevation (4-5 times higher than baseline levels) of NE levels was measured in both the right and left PVN after a formalin injection into the right paw. Unilateral surgical transections at the medulla-spinal cord junction failed to affect formalin-induced elevations in NE levels in the PVN independently of the side of the formalin injection or the knife cut. Thus, this observation clearly shows that fibres carrying pain-evoked signals ascend bilaterally from the spinal cord to the brainstem and forebrain. Hemisections of the medulla oblongata between the level of A1-A2 NE cell groups and the locus coeruleus reduced but did not eliminate formalin-induced NE release from the PVN ipsilateral to the knife cut. This effect was independent of the side of the formalin injection. In the contralateral PVN, high and similar NE levels were measured in response to a formalin injection into the right or the left leg. The present study indicates that formalin-induced pain signals are carried by sensory fibres to the ipsilateral spinal cord. From there, axons of different dorsal horn neurones reach noradrenergic cells on both sides of the medulla oblongata. The majority of noradrenergic fibers ascend on the same side and innervate the ipsilateral PVN. Since formalin administration resulted in a moderate elevation of NE levels in the PVN on the operated side, the role of other ascending noradrenergic (from the locus coeruleus) or noncatecholaminergic fibres that could modulate NE release from the PVN should be considered.

Alpha2-adrenoceptor-mediated restraint of norepinephrine synthesis, release, and turnover during immobilization in rats

Stress-related release of norepinephrine (NE) in the brain and periphery probably underlies several neuroendocrine and neurocirculatory responses. NE might influence its own synthesis, release, and turnover, by negative feedback regulation via alpha2-adrenoceptors. We examined central and peripheral noradrenergic function by measuring concentrations of NE, dihydroxyphenylglycol (DHPG), and dihydroxyphenylacetic acid (DOPAC) in hypothalamic paraventricular nucleus (PVN) microdialysate and arterial plasma simultaneously during immobilization (IMMO) in conscious rats. The alpha2-adrenoceptor antagonist yohimbine (YOH) was injected i.p. or perfused locally into the PVN via the microdialysis probe. The i.p. YOH increased plasma NE, epinephrine (EPI), DHPG, dihydroxyphenylalanine, and DOPAC levels by 4.3, 7.3, 2.5, 0.6 and 1.8-fold and PVN microdialysate NE, DHPG, and DOPAC by 1. 2, 0.6 and 0.5-fold. The i.p. YOH also enhanced effects of IMMO on plasma and microdialysate NE, DHPG, and DOPAC. YOH delivered via the PVN microdialysis probe did not affect microdialysate or plasma levels of the analytes at baseline and only slightly augmented microdialysate NE responses to IMMO. The results indicate that alpha2-adrenoceptors tonically restrain NE synthesis, release, and turnover in sympathetic nerves and limit IMMO-induced peripheral noradrenergic activation. In the PVN, alpha2-adrenoceptors do not appear to contribute to these processes tonically and exert relatively little restraint on IMMO-induced local noradrenergic activation.

Behavioral and neurochemical consequences of lipopolysaccharide in mice: anxiogenic-like effects

Systemic administration of lipopolysaccharide (LPS) induces sickness behaviors, as well as alterations of hypothalamic-pituitary-adrenal functioning commonly associated with stressors. In the present investigation, it was demonstrated that systemic LPS treatment induced a sickness-like behavioral profile (reduced active behaviors, soporific effects, piloerection, ptosis), which appeared to be dependent upon the novelty of the environmental context in which animals were tested. As well, LPS induced anxiogenic-like responses, including decreased time spent in the illuminated portion of a light-dark box, reduced open-arm entries in a plus-maze test, and decreased contact with a novel stimulus object in an open-field situation. The behavioral changes were accompanied by increased plasma ACTH and corticosterone levels. As well, LPS induced increased turnover of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the paraventricular nucleus (PVN), median eminence plus arcuate nucleus, hippocampus, as well as NE turnover within the locus coeruleus and DA turnover within the nucleus accumbens. Although these neurochemical variations were reminiscent of those elicited by stressors, LPS was not particularly effective in modifying DA activity within the prefrontal cortex or NE within the amygdala, variations readily induced by stressors. Whether the LPS-induced anxiogenic-like responses were secondary to the illness engendered by the endotoxin remains to be determined. Nevertheless, it ought to be considered that bacterial endotoxin challenge, and the ensuing cytokine changes, may contribute to emotionality and perhaps even anxiety-related behavioral disturbances.

The long term acute phase-like responses that follow acute stressor exposure are blocked by alpha-melanocyte stimulating hormone

Both intracerebroventricular (i.c.v.) IL-1beta and exposure to inescapable tail shock (IS) activate acute phase responses (APRs) that include increases in core body temperature (CBT), increases in hypothalamic-pituitary-adrenal activity, decreases in carrier proteins such as corticosterone binding globulin (CBG), aphagia and adipsia. A variety of data suggested that stressors produce APRs by inducing brain IL-1beta. The current series of studies further explored this possibility by determining whether the functional IL-1beta antagonist, alpha-melanocyte-stimulating hormone (alpha-MSH(1-13)), would block IS-induced APRs. Immediately following i.c.v. alpha-MSH(1-13) administration, rats were exposed to a single session of 100, 5 s, 1.6 mA ISs, or control treatment (home cage control). alpha-MSH(1-13) blocked IS-induced increased CBT, increased plasma corticosterone (CORT), decreased CBG, aphagia and adipsia 24 h after IS. The inhibitory effects of alpha-MSH(1-13) were shown not to be a consequence of alpha-MSH(1-13) producing its actions 24 h after its administration because alpha-MSH(1-13) given 24 h before IS did not block IS-induced increased CBT and CORT during IS. Additionally, alpha-MSH(1-13), given 24 h before IS, had no effect on increased CBT, increased CORT, decreased CBG, adipsia, or aphagia 24 h after IS. These data provide support for a specific mode of action for i.c.v. alpha-MSH(1-13), namely blockade of APRs with no impact on acute hyperthermia or increased levels of CORT produced during IS.

Pituitary adenylate cyclase-activating polypeptide-nerve terminals densely innervate corticotropin-releasing hormone-neurons in the hypothalamic paraventricular nucleus of the rat

Pituitary adenylate cyclase-activating polypeptide (PACAP) is widely distributed in many regions of the hypothalamus including the paraventricular nucleus (PVN). In this study, using well-characterized antibodies against PACAP and corticotropin-releasing hormone (CRH), we identified numerous nerve fibers with PACAP-immunoreactivity (ir) closely apposed to CRH neurons in the medial parvocellular subdivision of the rat PVN. Electron microscopy revealed the presence of synapses between PACAP-ir containing terminals and CRH-perikarya and -dendrites. These morphological observations suggest that PACAP may modulate the activation of the hypothalamic-pituitary-adrenal axis.

Effect of reserpine on 3-methoxy-4-hydroxyphenylethyleneglycol and 3,4-dihydroxyphenylacetic acid in the hippocampus of depression-model rats: an in vivo microdialysis study

Using in vivo microdialysis, we examined the effect of intraperitoneal injection of reserpine (2 mg/kg) on hippocampal 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) and 3,4-dihydroxy-phenylacetic acid (DOPAC), two major metabolites of catecholamine. Responses were examined serially for 12 h in the hippocampus of walking-stress-induced depression-model rats, recovery rats and control rats. Control rats showed a rapid rise followed by a gradual fall of free and total MHPG and a delayed increase of DOPAC in response to reserpine. Depression-model rats showed a significantly blunted biphasic response of free and total MHPG as well as blunted monophasic response of DOPAC compared with control rats. Recovery rats also exhibited a blunted fall response of MHPG. Our findings suggest that the vesicle membrane in the central noradrenaline (NA) neurons could be hyposensitive to reserpine in the depression-model rats.

Response of the hypothalamo-pituitary-adrenal axis to nicotine

Nicotine has been shown to be a potent stimulus for the secretion of the stress-responsive hormones, adrenocorticotropin (ACTH) and prolactin. This paper reviews the findings by our laboratory and others that demonstrate the polysynaptic pathways involved in the neuroendocrine responses to systemic nicotine. It will focus primarily on the hypothalamo-pituitary-adrenal (HPA) axis and the effect of nicotine on ACTH secretion, with supplementary information on prolactin secretion, where relevant. Data are presented demonstrating that nicotine acts via a central mechanism to stimulate indirectly the release of ACTH from the anterior pituitary corticotropes. Nicotine does not appear to act directly at the hypothalamic paraventricular nucleus (PVN), the site of the corticotropin-releasing hormone (CRH) neurons crucial to the regulation of ACTH. However, brainstem catecholaminergic regions projecting to the PVN showed a regionally selective and dose-dependent sensitivity to nicotine, particularly the noradrenergic/adrenergic nucleus tractus solitarius (NTS). A reduction in the modulatory effect of these catecholamines (by neurotoxic lesion, synthetic enzyme inhibitors or adrenergic receptor antagonists) resulted in an inhibition of nicotine-stimulated ACTH secretion. In addition, blockade of nicotinic cholinergic receptors (NAchRs) in the brainstem by the antagonist, mecamylamine, resulted in a dose-dependent reduction in norepinephrine (NE) release from terminals in the PVN, and a concomitant reduction in plasma ACTH. The differential sensitivity of these receptors to the nicotinic agonists, cytisine and nicotine, reflects the heterogeneity of the NAchR subtypes involved. The desensitization characteristics of the neuroendocrine responses to both acute and chronic nicotine exposure are indicative of an alteration in these NAchRs.

Forced swim test-induced neurochemical endocrine, and immune changes in the rat

The forced swim test (FST) is a behavioral paradigm that is widely used as a screening test for antidepressant activity in rodents. The objectives of the present study were to characterize the corticosterone and immune responses and in addition to examine neurotransmitter levels, in five brain regions at intervals (15, 30, 60, 90, and 120 min) following the second exposure to the FST. There was a significant but transient reduction in noradrenaline and 5-HT concentrations, in the hypothalamus 15 min post-FST exposure. 5-HT turnover in the frontal cortex and amygdala was significantly increased between 20-120 min post-FST exposure. The FST elicited a robust corticosterone response that peaked significantly at 30 min and had almost returned to baseline 120 min after exposure. There was a significant reduction in total white blood cell count 120 min after the FST, which was accompanied by a significantly reduced percentage of lymphocytes 90 and 120 min post-FST exposure. In addition, there was a significant but transient suppression of both PHA and Con A-induced lymphocyte proliferation 15 min following FST exposure. This study demonstrates that there are neurochemical changes that are coincident with the endocrine and immune changes associated with FST exposure in rats. Furthermore, this model could be used to examine the effects of manipulation of this stress response by antidepressant drugs. Such an investigation could add to our understanding of the interactions between antidepressants, stress and the neuroendocrine and immune systems.

Stress-induced expression of immediate early genes in the brain and peripheral organs of the rat

Stress causes rapid and transient expression of immediate early genes (IEGs) in the brain, and the monitoring of IEGs has enabled the visualization of the neurocircuitry of stress. Previous studies have postulated that stressors can be divided into two categories; processive and systemic. The neural circuits of brain activation differ between the two kinds of stressors. For example, processive stressors, such as immobilization (IMO), induce c-fos mRNA first in the cortical and limbic areas and then in the paraventricular hypothalamic nucleus (PVH), while c-fos expression in the PVH precedes that in other areas in animals subjected to systemic stressors. We further show that prior exposure to IMO stress for 6 days, or implantation of corticosterone pellets suppresses the induction of c-fos, fos B, jun B and NGFI-B, but not that of NGFI-A in the rat PVH. Plasma glucocorticoid may be an important factor regulating stress-induced IEG expression. It is well known that AP-1 and glucocorticoid receptors (GR) interact and suppress each other. Thus, decreased AP-1 levels in chronically stressed animals may help enhance the negative feedback effects of GR and prevent hypersecretion of glucocorticoid, which is implicated in the pathogenesis of stress-related diseases. IMO stress induces rapid expression of c-fos, c-jun and NGFI-A mRNAs in the heart and stomach. These were observed in the ventricular myocardium and coronary arteries, and in the epithelium, smooth muscles and arteries of the stomach after 30 min of IMO. IEG expression in the peripheral organs may provide a molecular basis for stress-induced psychosomatic disorders.

The role of paraventricular nucleus of hypothalamus in stress-ulcer formation in rats

The rat stress model of restraint and cold water immersion was used to investigate the effect of stimulating the paraventricular nucleus (PVN) of hypothalamus on the development of stress-induced gastric ulceration. The results were (1) electric stimulation of the PVN increased the stress ulceration, while electrolytic lesion of the PVN decreased it; (2) intracerebroventricular injection (i.c.v.) of acetylcholine (Ach) enhanced the effect of PVN stimulation on stress ulcers, and the M-receptor was involved; (3) i.c.v. norepinephrine (NE) attenuated the effect of PVN stimulation on stress ulcers in a dose-dependent manner, and the beta-receptor was involved; (4) i.c.v. 5-hydroxytryptamine (5-HT) enhanced the effect of PVN stimulation on stress ulcers; (5) electrolytic lesions of dorsal raphe nucleus (DR) attenuated the effect of PVN stimulation on stress ulcers, while electrolytic lesions of the locus ceruleus (LC) aggravated the effect; (6) thyroidectomy, adrenalectomy, ovariectomy, vagotomy and sympathectomy all attenuated the effect of PVN stimulation on stress ulcers; (7) electric stimulation of the PVN produced no effect on gastric juice volume, acidity, total acid output, pepsin activity or the gastric barrier mucus; but greatly reduced gastric mucosal blood flow. These results indicate that the PVN is an important brain site regulating the development of stress-induced gastric ulcers, that the classical neurotransmitters Ach, NE and 5-HT are involved, and that in the periphery, both the parasympathetic and sympathetic nervous systems and the three endocrine glands (thyroid, adrenal and gonad) take part in the effect.

Chemical sensory deafferentation abolishes hypothalamic pituitary activation induced by noxious stimulation or electroacupuncture but only decreases that caused by immobilization stress. A c-fos study

We have shown in previous c-fos studies that noxious stimulation or electroacupuncture in deeply anaesthetized rats activate the hypothalamic pituitary corticotrope axis in a specific way. C-fos expression was more pronounced in the arcuate than the paraventricular hypothalamic nuclei, and none occurred in the pituitary intermediate lobe. The absence of the usual autonomic responses to psychological stress, such as tachycardia or blood pressure elevation, suggested a specific action of the somatosensory input on the hypothalamic pituitary axis. To prove this hypothesis, c-fos expression was examined in the paraventricular, arcuate and other hypothalamic nuclei, the pituitary gland, and the A1 and A2 medullary catecholaminergic cell groups of animals deprived of nociceptive primary afferent input by neonatal capsaicin. After noxious stimulation or electroacupuncture, no c-fos enhancement occurred in any of those sites in capsaicin-treated animals, and there was no increased plasma release of adrenocorticotropic hormone. In contrast, the hypothalamic pituitary c-fos activation provoked by immobilization stress though markedly decreased, was not abolished by capsaicin, whereas plasma release of adrenocorticotropic hormone remained undiminished. These findings suggest that noxious stimulation or electroacupuncture act on the hypothalamic pituitary corticotrope axis through an exclusively physical effect depending on the noxious signal elicited in the somatosensory pathway. They also demonstrate the occurrence of a minor somatosensory physical component after forced immobilization, acting on the hypothalamic pituitary axis probably together with the prevalent component of emotional arousal elicited by this form of stress.

The effect of acute, chronic and chronic intermittent stress on the central noradrenergic system

The objective of this investigation was to examine the immediate and long term effects of acute, chronic and chronic intermittent stress on the central noradrenergic system of rats. Male Sprague-Dawley rats were subjected to one hour of physical immobilization stress either as a single exposure, or as 14 exposures applied either on consecutive days, or randomly over 60 days. Animals were sacrificed immediately, 6 h and 24 h following the last stressor. Levels of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylethylene-glycol sulfate (MHPG-sulfate) were measured in the hypothalamus, hippocampus, cerebral cortex and locus coeruleus region and beta-adrenergic receptor (BAR) density was determined in the cortex. Immediately after acute stress, a significant reduction in hypothalamic NE levels and marked increases in MHPG-sulfate levels in all four brain regions were observed. In contrast immediately after the last stressor of a chronic or chronic intermittent stress regimen, no change in NE concentration was observed while levels of MHPG-sulfate in the four brain regions showed a smaller increase than that observed after an acute stressor. Acute stress induced changes normalized within 6 h while chronic and chronic intermittently stressed animals had altered NE or MHPG-sulfate levels in certain brain regions for up to 6-24 h. Cortical BAR binding parameters remained unchanged after all stress paradigms.

alpha1B adrenoceptors in rat paraventricular nucleus overlap with, but do not mediate, the induction of c-Fos expression by osmotic or restraint stress

A role has been suggested for hypothalamic alpha1 adrenoceptors in the acute stress-induced activation of the hypothalamic-pituitary-adrenal axis. Using a polyclonal antiserum against the rat alpha1B adrenergic receptor protein, we have demonstrated alpha1B receptor immunoreactivity in neurons and especially in punctate cell processes in the rat paraventricular nucleus. The distribution of alpha1B receptor immunoreactivity overlapped in part with the distributions of c-Fos immunoreactivity induced in the paraventricular nucleus by either restraint stress or hypertonic saline administration. However, intraperitoneal pretreatment with the alpha1 receptor antagonist prazosin (0.5 or 5.0 mg/kg) failed to attenuate stress-induced c-Fos expression in the paraventricular nucleus. Prazosin also failed to attenuate the secretion of corticosterone following restraint stress. Thus, we conclude that neither acute secretory activity nor activation of gene transcriptional responses mediated by c-Fos in the hypothalamic pituitary adrenal axis following these stressors are dependent upon hypothalamic alpha1 adrenergic receptors.

Effect of fasting and immobilization stress on estrogen receptor immunoreactivity in the brain in ovariectomized female rats

The present study examined the effect of 48-h fasting and 1-h immobilization on estrogen receptor immunoreactivity in selected hypothalamic areas and the nucleus of the solitary tract (NTS) in ovariectomized rats. Fasting induced an increase in ER-immunoreactive cells in the paraventricular nucleus (PVN), periventricular nucleus (PeVN) and NTS compared with the unfasted control group. Similarly, immobilization caused an increase in ER-positive cells in the same areas, PVN, PeVN and NTS, versus the non-immobilized group. There was no significant increase in the number of ER-immunoreactive cells in the preoptic area (POA), arcuate nucleus (ARC) or ventromedial hypothalamic nucleus (VMH) following fasting and immobilization. Our previous work in ovariectomized rats with estrogen microimplants in the brain revealed that the PVN and A2 region of the NTS are the feedback sites of estrogen in activating the neural pathway to suppress pulsatile LH secretion during 48-h fasting. The result in the food-deprived rats suggests that estrogen modulation of the suppression of LH secretion during fasting is partly due to the increase in estrogen receptors in the PVN and A2 region. The physiological significance of the increase in neural ER following immobilization remains to be elucidated.

Activation of anterior lobe corticotrophs by electroacupuncture or noxious stimulation in the anaesthetized rat, as shown by colocalization of Fos protein with ACTH and beta-endorphin and increased hormone release

A marked expression of the c-fos proto-oncogene has been recently reported in cells of the anterior lobe of the pituitary gland in rats subject to electroacupuncture or noxious thermal stimulation under pentobarbital anaesthesia. The present study was undertaken to identify the activated pituitary cells. Following both kinds of stimulation, most Fos-immunoreactive anterior lobe cells showed colocalization with adrenocorticotropic hormone or beta-endorphin immunoreactivity. No c-fos expression occurred in pituitary cells immunoreactive for growth hormone, prolactin, luteinizing hormone, or thyrotropin-stimulating hormone. A marked rise of adrenocorticotropic hormone and beta-endorphin concentrations occurred in plasma. In the hypothalamus, c-fos expression was increased in the mediobasal nuclei-namely, the arcuate nucleus-and in the paraventricular nucleus, but more in the former. It is suggested that somatosensory noxious input, or the partly noxious input evoked by electroacupuncture, activate the hypothalamo-pituitary-adrenocortical axis as in common forms of stress, but with a specific activation of the mediobasal hypothalamic nuclei and no stimulation of intermediate lobe cells. Opiate release from the pituitary gland may contribute to acupuncture analgesia or the intrinsic antinociceptive reactions triggered by noxious stimulation.

Aging is associated in the 344/N Fischer rat with decreased stress responsivity of central and peripheral catecholaminergic systems and impairment of the hypothalamic-pituitary-adrenal axis

The effects of acute stress on various indices of sympatho-adrenal, sympathoneural functions and hypothalamic-pituitary-adrenal (HPA) axis were examined both at central and peripheral sites in healthy, intact male Fischer 344/N rats of increasing age. Extracellular fluid (ECF) levels of norepinephrine (NE), its metabolites dihydroxyphenylglycol (DHPG), and methoxyhydroxyphenylglycol (MHPG), and of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC), were measured 24 h after implantation of a microdialysis probe in the paraventricular nucleus (PVN) of the hypothalamus, and samples collected at 30-min. intervals during immobilization (IMMO). ECF levels of NE, DHPG, MHPG, and DOPAC were at baseline similar in both age groups, and all increased significantly in response to IMMO. The IMMO-induced increases in ECF levels of NE and MHPG were, however, significantly smaller in old than in young rats. Plasma levels of the dihydroxyphenylalanine (DOPA), -NE, epinephrine (EPI), DHPG, MHPG, dopamine (DA), DOPAC and HVA, were determined in different groups of young and old rats, cannulated in the tail artery, at baseline, and after 5, 30, 60, and 120 min of IMMO. Basal levels of DOPA, DHPG, MHPG, DA, DOPAC, HVA, NE and EPI were significantly higher in old than in young rats, and increased in plasma during IMMO. However, the magnitude of the increase in the majority of these compounds was significantly smaller in old than in young rats. Basal plasma levels of ACTH were similar among age groups, and basal plasma levels of corticosterone showed a significant aging-associated decline. Two i.v. doses (2 and 20 micrograms/kg BW) of rat CRF elicited significantly greater and delayed ACTH, and greater corticosterone responses in older rats, consistent with the pattern encountered in hypothalamic CRF deficiency. An i.v. injection of ACTH evoked lower corticosterone responses in the older (18 and 24 month old) than in the younger (2 and 8 month old) groups of rats, consistent with secondary adrenocortical atrophy in older animals. Steady-state mRNA levels of mineralocorticoid and glucocorticoid receptors were significantly decreased in the hippocampus of the 8-, 18-, and 24-month-old rats, compatible with maturational rather than senescent changes. CRF mRNA levels in the paraventricular nucleus of the hypothalamus, and levels of POMC mRNA in the anterior pituitary were significantly reduced with age. In conclusion, in this strain of rats, aging is associated with diminished responsiveness of central, and peripheral catecholaminergic systems to acute stress, and progressive hypothalamic CRH deficiency.

Effects of various stressors on in vivo norepinephrine release in the hypothalamic paraventricular nucleus and on the pituitary-adrenocortical axis

The hypothalamic-pituitary adrenocortical (HPA) system and sympathoneural and adrenomedullary systems are major effector systems that serve to maintain homeostasis during stress. Corticotropin-releasing hormone (CRH) in the paraventricular nucleus (PVN) of the hypothalamus, a determinant of both HPA and autonomic responses to stress, is under the control of many neurotransmitters and neuropeptides. Norepinephrine (NE) potently stimulates CRH neurons in the PVN; however, the physiologic role of NE in stress-induced activation of the HPA is unknown. In the present study we exposed animals to various stressors (immobilization (IMMO), cold (COLD), hemorrhage (HEM), hypoglycemia elicited by insulin administration (INS), pain and tissue damage caused by formalin injection (FORM) and sc injection of physiological saline (SAL), all of which are known to activate the HPA axis. Injection of physiological saline iv was used as a control. In vivo microdialysis was used to assess stressor- and intensity-specific activation of the PVN noradrenergic system, based on measurements of NE, its intraneuronal metabolite dihydroxyphenylglycol (DHPG), and the dopamine metabolite, dihydroxyphenylacetic acid (DOPAC). Simultaneously with microdialysate collections, blood samples were obtained via catheters in the femoral artery to measure plasma ACTH and corticosterone (CORT) levels as dependent measures, to assess stress-induced activation of the HPA axis. At their highest intensities, all the stressors significantly increased levels of PVN microdialysate NE, DHPG, and DOPAC, and plasma ACTH and CORT. PVN NE levels varied across stressors, with IMMO and FORM more potent than INS, COLD, or HEM. INS and HEM evoked proportionately larger plasma ACTH responses than did IMMO, FORM, and COLD. Plasma CORT responses were largest during IMMO, FORM, and HEM. Except for COLD and HEM, there was a strong correlation of plasma ACTH levels with levels of NE, DHPG, and DOPAC in PVN microdialysate. The data suggest that, except for COLD or HEM, there is a strong positive correlation of PVN noradrenergic activation and activity of the HPA axis. With stressors such as IMMO and FORM, NE synthesis, reflected by DOPAC changes, is strongly positively correlated with activity of the HPA axis. Furthermore, the results indicate substantial stressor specificity of PVN catecholaminergic and of HPA responses to different stressors and are inconsistent with a founding tenet of Selye's stress theory, the doctrine of nonspecificity, which defines stress as the nonspecific response of the body to any demand.

Regulation of plasma osteocalcin by corticosterone and norepinephrine during restraint stress

Osteocalcin (OC), an extracellular calcium-binding protein of bone origin, is synthesized by osteoblasts and binds with high specificity to bone mineral crystals. A small, but relatively consistent portion of newly synthesized OC which is released to circulation has been well correlated with histological indices of osteoblastic activity. Synthesis of OC is regulated by numerous hormones including glucocorticoids. We previously reported that mild mental stressors such as cage change or cold exposure decreased rat plasma OC by up to 40% within 1 h. A similar response was induced in a time- and dose-related manner by injection of physiological levels of corticosterone (CS), the active glucocorticoid in rats. Prone immobilization by foot restraint of conscious rats for up to 2 h (IMMO) is a well-characterized model of classic "fight-or-flight" response. This model induces an immediate and prolonged elevation of CS, as well as the catecholamines epinephrine (E) and norepinephrine (NE). In marked contrast to milder stressors, immobilization induced an immediate increase of plasma OC, greater than 50% within 5-20 min, which returned toward normal after 2 h of restraint. Selective ablation of the hormones by adrenal medulectomy, adrenalectomy, or blockade of sympathetic ganglia did not abolish the initial rapid rise of plasma OC. Even before IMMO, plasma OC was increased by about 50% in the absence of sympathetic neural function or adrenal CS production. The presence of both CS and NE, but not E, was required to return plasma OC concentrations to basal levels. This strongly suggests interaction of CS and NE to regulate plasma OC and its release from bone. As expected, prior cold exposure lowered plasma OC, but did not abolish a subsequent increase in response to IMMO, nor did IMMO repeated daily for 7 days. The stimulus for the initial rapid elevation of OC is unknown, but likely to be of importance in the role OC plays in response to stress. Further investigation of the OC under mental stress should help to understand the function of this abundant and highly conserved bone protein.

Effects of immobilization on in vivo release of norepinephrine in the bed nucleus of the stria terminalis in conscious rats

Release of norepinepriine (NE) and its metabolites in the bed nucleus of the stria terminalis (BNST) was examined using in vivo microdialysis in conscious rats before, during and after 2 h of immobilization. Microdialysate levels of NE and of dihydroxyphenylglycol (DHPG) increased by 170-290% above basal levels during the 1st h of immobilization and decreased gradually thereafter. In contrast, levels of dihydroxyphenylacetic acid (DOPAC) increased gradually over the entire period of immobilization, peaking at 110% above baseline levels. These findings indicate that in rats a single immobilization is attended by increased synthesis, release and reuptake of NE within the BNST. The results are consistent with previous findings relating to stress-induced release of NE in the hypothalamic paraventricular nucleus, central nucleus of the amygdala and cerebral cortex and suggest concurrent noradrenergic activation in several brains centers during acute stress.

Long-term control of neuronal excitability by corticosteroid hormones

Hippocampal CA1 neurons express both mineralocorticoid and glucocorticoid receptors. Due to the difference in affinity of the two receptor types for corticosterone and variations in endogenous steroid levels, occupation of the receptors will range between a situation of predominant mineralocorticoid receptor activation and conditions where both receptor types are occupied. It was observed that local signal transduction is regulated by activation of the corticosteroid receptors. Particularly, transmission mediated by biogenic amines appears to be sensitive to steroid control. The data indicate that cholinergic and serotonergic responses are small with predominant mineralocorticoid receptor activation, while additional glucocorticoid receptor activation results in large responses; the reverse has been found for noradrenalin. The steroid-dependent control over transmission by biogenic amines will influence local excitability and therefore functional processes in which the hippocampal system is involved.

Idazoxan-induced reductions in cortical glucose use are accompanied by an increase in noradrenaline release: complementary [14C]2-deoxyglucose and microdialysis studies

The autoradiographic [14C]2-deoxyglucose procedure was used to map function-related alterations in local cerebral glucose use following acute administration of the alpha 2-adrenoceptor antagonist, idazoxan (0.3-3 mg kg-1 s.c.). The most prominent feature of the results obtained was the significant reduction in glucose use in certain locus coeruleus projection areas. Thus, in various cortical, hippocampal and thalamic regions, as well as structures involved in auditory and visual function, idazoxan administration was associated with a 13-20% decrease in glucose use. In a complementary microdialysis study, the effect of idazoxan on extracellular noradrenaline levels in the frontal cortex of rats, manipulated in the same fashion as during the [14C]2-deoxyglucose procedure (i.e. following the application of surgery and partial restraint), was examined. Both surgery and restraint were associated with a modest but significant increase in basal noradrenaline release (+31% and +26%, respectively). Subsequent administration of idazoxan (3 mg kg-1 s.c.) evoked a further increase in noradrenaline release, the magnitude of which was the same as that observed following its administration to freely-moving rats (+113%). These combined data suggest that idazoxan-induced reductions in cerebral glucose use, at least in the frontal cortex, may occur as a consequence of the increase in noradrenaline release. In addition, it appears that surgery and partial restraint do not alter alpha 2-adrenoceptor tone in the frontal cortex.

Decreased stress responsivity of central and peripheral catecholaminergic systems in aged 344/N Fischer rats

We investigated the effects of stress on central and peripheral sympatho-adrenal and sympatho-neural functions in healthy, intact young (3-4 mo) and aged (24 mo) male Fischer 344/N rats. Extracellular fluid (ECF) levels of the catecholamines norepinephrine (NE), dihydroxyphenylglycol (DHPG), methoxyhydroxyphenylglycol (MHPG), and dihydroxyphenylacetic acid (DOPAC) were obtained by microdialysis in the paraventricular nucleus (PVN) of the hypothalamus at baseline and during immobilization (IMMO). The baseline levels of these substances were similar in both age groups, and their concentrations increased significantly in response to IMMO. The IMMO-induced increases of NE and MHPG, however, were significantly smaller in old than in young rats. Plasma levels of the catecholamines NE, DHPG, MHPG, DOPAC, dihydroxyphenylalanine (DOPA), epinephrine (EPI), dopamine (DA), and HVA were also determined in young and old rats during IMMO. Basal levels of these substances were significantly higher in old than in young rats. The magnitude of the IMMO-induced increases in the majority of these compounds however, was significantly smaller in old than in young rats. We conclude that, at the basal state, aging in the Fischer rat is associated with normal PVN ECF, but high plasma catecholamine levels; at stress state, however, old rats have substantially lesser activation of their central and peripheral catecholaminergic systems than young rats.

Role of interleukin-1 in stress responses. A putative neurotransmitter

Recently, the central roles of interleukin-1 (IL-1) in physical stress responses have been attracting attention. Stress responses have been characterized as central neurohormonal changes, as well as behavioral and physiological changes. Administration of IL-1 has been shown to induce effects comparable to stress-induced changes. IL-1 acts on the brain, especially the hypothalamus, to enhance release of monoamines, such as norepinephrine, dopamine, and serotonin, as well as secretion of corticotropin-releasing hormone (CRH). IL-1-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis in vivo depends on secretion of CRH, an intact pituitary, and the ventral noradrenergic bundle that innervates the CRH-containing neurons in the paraventricular nucleus of the hypothalamus. Recent studies have shown that IL-1 is present within neurons in the brain, suggesting that IL-1 functions in neuronal transmission. We showed that IL-1 in the brain is involved in the stress response, and that stress-induced activation of monoamine release and the HPA axis were inhibited by IL-1 receptor antagonist (IL-1Ra) administration directly into the rat hypothalamus. IL-1Ra has been known to exert a blocking effect on IL-1 by competitively inhibiting the binding of IL-1 to IL-1 receptors. In the latter part of this review, we will attempt to describe the relationship between central nervous system diseases, including psychological disorders, and the functions of IL-1 as a putative neurotransmitter.

Corticotropin-releasing factor administered into the locus coeruleus, but not the parabrachial nucleus, stimulates norepinephrine release in the prefrontal cortex

Previous studies have indicated that intracerebroventricular application of corticotropin-releasing factor (CRF) activates noradrenergic neurons in the brain stem locus coeruleus (LC) and norepinephrine (NE) metabolism in several brain regions. To assess whether CRF has direct effects on LC noradrenergic neurons, CRF was infused into the LC and concentrations of NE and its metabolites were measured in microdialysates collected from the medial prefrontal cortex (PFM). Infusion of 100 ng of CRF into the LC significantly increased dialysate concentrations of NE and of its catabolite MHPG in the ipsilateral PFM, whereas no significant changes were observed following infusion of artificial CSF. No response was observed when the infusions of CRF occurred outside of the LC, including those in the parabrachial nucleus. Although CRF administered into the LC slightly increased dialysate concentrations of NE in the contralateral PFM, this effect was not statistically significant. The effect of CRF injected into the LC on dialysate NE was prevented by combination with a 10-fold excess of the CRF antagonist alpha-helical CRF9-41, indicating some specificity in the response. These results are consistent with anatomical and electrophysiological evidence suggesting that CRF may directly activate noradrenergic neurons in or close to the LC.

Catecholamine release in the rat hypothalamic paraventricular nucleus in response to haemorrhage, desipramine and potassium

In vivo microdialysis and HPLC were used to measure catecholamine release in the rat hypothalamic paraventricular nucleus (PVN) during haemorrhage. The effects of noradrenaline uptake blockade with 1 microM desipramine (DMI) and a depolarising concentration of potassium (100 mM) through the probe were also examined. Dialysis probes implanted in the PVN of urethane anesthetised rats were perfused with modified Ringer solution at 1.1 microliter/min. Thirty minute collections were analysed for DOPA, noradrenaline, DOPAC, HVA and 5-HIAA. Basal concentrations, in the absence of DMI, were: DOPA 203.6 +/- 44.0 pg/ml, noradrenaline 128.0 +/- 20.4 pg/ml; DOPAC 5.6 +/- 0.7, HVA 5.1 +/- 2.2 and 5-HIAA 87.2 +/- 17.8 ng/ml. Basal noradrenaline was doubled in the presence of DMI while basal and stimulated DOPA, DOPAC, HVA and 5-HIAA were not affected by DMI. Haemorrhage resulted in a significant noradrenaline release (48% over resting levels) in the presence of DMI (n = 10, P < 0.05); in the absence of DMI, a smaller and non-significant increase (30% over basal levels) was observed. Potassium-induced depolarisation caused a significant two- and four-fold increase in noradrenaline release (P < 0.001), with decreases in the dopamine metabolites DOPAC (31%, 44%) and HVA (35%, 28%), and the serotonin metabolite, 5-HIAA (41%, 33%), in the presence and absence of DMI, respectively. The catecholamine precursor DOPA did not vary throughout either experiment. The results indicate that haemorrhage induces a 48% increase in noradrenaline release in the rat PVN which provides evidence for a role of noradrenergic projections to the PVN in cardiovascular control.

Histaminergic neurons mediate restraint stress-induced increases in the activity of noradrenergic neurons projecting to the hypothalamus

The role of histamine in mediating restraint stress-induced increases in the activity of noradrenergic neurons projecting to the hypothalamus was evaluated in male rats. Noradrenergic neuronal activity was estimated by measuring concentrations of the norepinephrine metabolite 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) in the paraventricular and medial preoptic nuclei which contain terminals of these neurons. Placement of rats within restraining tubes rapidly increased MHPG but not norepinephrine concentrations in the paraventricular and medial preoptic nuclei. Depletion of neuronal histamine by alpha-fluoromethylhistidine and antagonism of H1 receptors by mepyramine attenuated, whereas blockade of H2 receptors by zolantidine did not prevent the stress-induced increases in MHPG concentrations. Neither mepyramine nor zolantidine affected MHPG concentrations in hypothalamic regions of nonstressed rats. These results indicate that histaminergic neurons contribute to the stress-induced increase the activity of noradrenergic neurons projecting to the hypothalamus via an action at H1 receptors.

Mineralocorticoid and glucocorticoid receptors in the brain. Implications for ion permeability and transmitter systems

In this review we have argued that corticosteroid hormones represent an endocrine signal that can influence neuronal communication. The steroids bind to intracellular receptors in the brain, resulting in slow effects that involve gene transcription, but they may also evoke rapid effects via membrane receptors. The signal carried by the corticosteroids is therefore divergent with respect to the dimension of space and time. Within the rat brain, at least two intracellular receptor subtypes, i.e. MRs and GRs, bind corticosterone. The affinity, density and localization of the MRs is different from the GRs, although the actual properties may vary somewhat depending on the condition of the animal. In general, due to the difference in affinity, low corticosteroid levels result in a predominant MR occupation, while higher steroid levels additionally occupy GRs. Recent studies indicate that predominant MR occupation is important for the maintenance of ongoing transmission in certain brain regions and for neuroprotection. By contrast, additional GR occupation (for a limited period of time) results in an attenuation of local excitability; yet, prolonged exposure to high steroid levels may become an endangering condition for neurons. Since predominant MR occupation on the one hand and additional GR occupation on the other hand induce different cellular actions, the ratio of MR/GR occupation is an important factor determining the net effect of corticosteroid hormones in the brain. How coordinated MR- and GR-mediated effects control neuronal communication under various physiological and pathological conditions will be a challenge for future research.

Stress, antidepressant drugs, and the locus coeruleus

This review presents a synthesis of a large body of seemingly inconsistent literature on the role of the locus coeruleus-norepinephrine (LC-NE) system and the corticotropin-releasing hormone (CRH)-median eminence system in mediating the CNS effects of stress and the therapeutic effects of antidepressant drugs. The clinical implications of these findings for the etiology and treatment of stress-related psychiatric disorders such as depression will be discussed.

Glycine stimulates striatal dopamine release in conscious rats

1. Glycine is an inhibitory neurotransmitter in the spinal cord and brainstem. The mechanism of this inhibition is via binding of glycine to specific receptors, increasing transmembrane Cl- conductance and hyperpolarizing neurones. Strychnine selectively antagonizes these effects. The role of glycinergic neurones in supraspinal regions is poorly understood. 2. Effects of glycine on release of catecholamines in the striatum were examined by microdialysis in freely-moving rats. Transcription of the genes encoding strychnine-sensitive glycine receptors was assessed in the striatum and substantia nigra, by use of reverse transcription followed by the polymerase chain reaction. 3. Glycine administered via the microdialysis probe dose-dependently increased concentrations of dopamine and its metabolites, dihydroxyphenylacetic acid and homovanillic acid, in the perfusate, indicating increased local release and metabolism of dopamine. Strychnine markedly attenuated these responses. Whereas striatal tissue did not contain mRNA for either the adult or neonatal form of strychnine-sensitive glycine receptor, nigral tissue contained a message for the adult form. 4. The results suggest that dopaminergic cells in the substantia nigra synthesize strychnine-sensitive glycine receptors and transport the receptors to terminals in the striatum. Occupation of the glycine receptors then exerts a net stimulatory effect on striatal dopamine release in vivo.

Aging prolongs the stress-induced release of noradrenaline in rat hypothalamus

A stress-induced increase in noradrenaline (NA) release was measured by intracerebral microdialysis in the hypothalamic paraventricular nucleus of freely moving Wistar-Kyoto rats at three different ages (6, 18 and 24 months). NA levels in 20-min dialysate samples were measured by high-performance liquid chromatography with electrochemical detection. Microdialysis sampling was done at the baseline during a 20-min immobilization stress and for the next 100 min. Basal NA release was not significantly different in the three age groups. The immobilization stress increased NA levels (247, 197 and 234% of the baseline for the 6-, 18- and 24-month animals, respectively) which was not significantly different in the three groups. In the two younger groups NA returned to the baseline in the first sample after the end of the stress (t = 40 min) whereas in the 24-month group it remained significantly higher for longer (until t = 60 min). Stress-induced release of hypothalamic NA thus appears to be prolonged in old rats.

Stress update Adaptation of the hypothalamic-pituitary-adrenal axis to chronic stress

The hypothalamic-pituitary-adrenal (HPA) axis exhibits a circadian rhythm, activation by stress, and inhibition by corticosteroids. Activity in the HPA axis is very sensitive to inhibition by corticosteroids when they are administered exogenously. When stress-induced corticosteroid secretion occurs, however, normal activity in the HPA is not inhibited and may even be augmented. Experiments in rats have shown that stress also induces facilitation of subsequent activity in the HPA axis that appears to balance the inhibitory effects of corticosterone and thus maintains responsiveness to new, acute stresses in chronically stressed rats. Stress-induced facilitation of HPA axis activity may be mediated by a parallel stress-induced (CRH-dependent) increase in the capacity of brain noradrenergic cell groups to respond to acute stress. A continually responsive HPA axis, even under conditions of chronic stress, appears to be important for survival. Stress-induced increases in glucocorticoid secretion to levels sufficient to occupy glucocorticoid receptors enable appropriate thermoregulatory and cardiovascular responses to acute stress. There is, however, an overall metabolic cost to the animal of maintaining continued activity in the HPA axis during chronic stress.

Immobilization stress rapidly decreases hypothalamic corticotropin-releasing hormone secretion in vitro in the male 344/N Fischer rat

Corticotropin-Releasing-Hormone (CRH) is the principal secretagogue for plasma ACTH and corticosterone secretion and plays an important role in coordinating a variety of physiological and behavioral responses to stress. To explore whether there is a rapid change in the secretory response of the hypothalamic CRH neuron during acute stress, we report here a study of the effects of KCl and norepinephrine (NE) on CRH release in vitro from rat hypothalami explanted after 5, 30, 60, and 120 minutes of immobilization. We also measured the plasma levels of ACTH, beta-endorphin, corticosterone, prolactin, GH, and TSH at these intervals. As the duration of immobilization increased, KCl and NE-induced CRH release in vitro progressively fell. After reaching a maximal rise after 30 minutes of immobilization, plasma ACTH, beta-endorphin, and prolactin progressively fell in plasma, whereas corticosterone remained elevated up to 120 minutes; TSH and GH secretion rapidly declined and remained suppressed. Taken together, these data suggest that during immobilization stress, the responsiveness of the hypothalamic CRH neuron rapidly falls, owing either to CRH depletion and/or desensitization to NE, and this is paralleled by a concomitant decrease in pituitary-adrenal responsiveness.