Descending spinal pathways mediating the responses of adrenal tyrosine hydroxylase and catecholamines to insulin and 2-deoxyglucose

We investigated the bilateral organization of the descending spinal sympathetic projections that subserve adrenomedullary stimulation caused by insulin hypoglycemia or 2-deoxyglucose (2DG) cellular glucopenia in adult rats with right-sided hemisection of the cord at C6-C7 (i.e. above the site of origin of the adrenal innervation). Cord hemisection alone caused a minor rise of adrenal tyrosine hydroxylase activity (ATHA) on the intact side as compared to sham-operated animals, but there was no change on the operated side. Catecholamine levels were marginally affected. In rats with hemisection the drugs elevated ATHA significantly in both glands; the response on the intact side was similar to that in sham-operated rats, but the effect on the sectioned side was reduced by one-half. 2DG affected the adrenaline content differently in the two glands: on the intact side it provoked a decrease comparable with that in non-lesioned rats, but had no significant effect on the lesioned side. 2DG acts through higher CNS structures, for complete transection of the cord at T2-T3 prevents the effects of 2DG on ATHA and adrenal adrenaline. We conclude that adrenal tyrosine hydroxylase induction by insulin and 2DG in hemisectioned rats, just as in the case of immobilization stress, is mediated by ipsilateral and, to a lesser degree, contralateral pathways generating net excitatory effects on ATHA; the latter fibers decussate below the low cervical level. The same organization may prevail for control of the concentration of adrenal adrenaline.     

Comparison of HPA axis hormonal responses to naloxone vs psychologically-induced stress

Although the naloxone challenge has been used to draw inferences about the dynamics of the stress response, this procedure has never actually been compared "head to head" with a psychological stressor. In the present study, we asked 14 healthy volunteers to complete the naloxone challenge and the Trier Social Stress Test in an outpatient GCRC laboratory setting so that the degree of correspondence between the two procedures could be examined. Findings indicated that subjects who had greater ACTH responses to naloxone also had greater ACTH responses to the psychologically-induced stressor. This was true for both ACTH peak (r=0.57; p<0.04) and ACTH AUC response (r=0.64; p<0.02) measures; none of the cortisol summary score measures correlated significantly across the two challenges. Associations were also found between subjects' baseline personality characteristics and their ACTH responses to each of the challenges. Furthermore, the kinds of characteristics that predicted greater ACTH response to the pharmacological challenge were similar to the kinds of characteristics that predicted ACTH response to the psychological challenge. In general, higher scores on the NEO dimensions of Extraversion and Openness predicted greater ACTH responses. These findings give preliminary evidence that novelty-seeking behavior may be associated with HPA axis lability. The commonalities in personality predictors between the two challenges further support the notion that a common biological substrate may be, at least partially, responsible for the similarities in responses. However, caution should be used in assuming that responses to naloxone directly parallel responses to physiological stress.     

Serotonin 1A receptor messenger RNA regulation in the hippocampus after acute stress.

BACKGROUND: When rats are subjected to chronic stress for 2 weeks, a significant decrease in hippocampal serotonin (5-HT)1A messenger RNA (mRNA) is observed. We wanted to investigate whether stress, administered for shorter periods of time, would result in decreases in 5-HT1A gene expression in hippocampus. METHODS: In one experiment, rats were either stressed daily for 1 week or implanted with two corticosterone pellets to produce elevated corticosterone levels. In another experiment, rats were subjected to a severe acute stressor and sacrificed 1 day or 1 week after the stressor. RESULTS: We found that 24 hours after the acute stress, rats showed a significant decrease in 5-HT1A mRNA levels in CA1 and the dentate gyrus compared to controls. No significant changes in 5-HT1A mRNA levels were detected in any of the other groups. CONCLUSIONS: Although 1 week of chronic stress is not sufficient to cause significant decreases in hippocampal 5-HT1A mRNA levels, a severe and prolonged acute stress is capable of down-regulating, at least transiently, 5-HT1A mRNA gene expression in hippocampus.     

Role of sex and sex steroids in mediating pituitary-adrenal responses to acute buspirone treatment in sheep

Systematic characterisation of sex differences in the serotonergic modulation of the hypothalamic-pituitary-adrenal (HPA) axis may assist with our understanding of why stress-related disorders are disproportionately represented in women. In this study, we examined the acute effects of buspirone, a serotonergic 1A receptor subtype agonist, on the endocrine endpoints of adrenocorticotrophin (ACTH) and cortisol secretion in gonadectomised male and female sheep. Each sheep was treated with an acute i.v. injection containing vehicle or buspirone (0.03, 0.1 and 0.3 mg/kg) in the presence and absence of sex steroid replacement (SSR). In males, SSR treatment consisted of testosterone (2 x 200 mg s.c. pellets) and, in females, the mid-luteal phase of the oestrus cycle was simulated by treatment with oestradiol (1 cm s.c. implant) and an intravaginal controlled internal drug release device containing 0.3 g progesterone. ACTH, cortisol, testosterone and progesterone were measured in jugular blood. Basal ACTH levels were higher in males, whereas basal cortisol levels were higher in females, regardless of sex steroid status. The magnitude of the increase in ACTH and cortisol secretion following buspirone treatment was dose-dependent. There were no differences in the ACTH responses of males and females to buspirone treatment, either in the presence or absence of sex steroid replacement. However, although the cortisol response to buspirone was greater in females, there was no discernable effect of sex steroid status in addition to this sex difference on either basal or buspirone-stimulated cortisol release. We conclude that the larger basal and buspirone-stimulated cortisol response measured in females may reflect a sex difference, either in the sensitivity of the adrenal gland to ACTH or in the catecholaminergic innervation of the adrenal gland. The lack of effect of sex and sex steroids in the ACTH secretory response to buspirone may indicate that the sex differences in serotonergic modulation of the HPA axis, as reported previously by our group, were mediated via serotonergic receptor subtypes other than the 1A receptor.     

Social stress in laboratory rats: hormonal responses and immune cell distribution

Previous experiments with chronically coexisting groups of Long-Evans rats indicated differences in many aspects of blood cellular immunity between winner and loser rats. The present study investigated the specific hormonal response patterns of winners and losers in relation to changes in numbers of blood immune cells. At the beginning of a 7 day period of chronic confrontation, a partition wall was removed between two neighboring rat groups, each containing a male-female pair. Fights for dominance between the males resulted in fast establishment of stable dominance relationships. At day 7 of the confrontation, winner males showed stable concentrations of CBG (corticosteroid-binding globulin) and even reduced titers of total CORT (corticosterone). In contrast, a marked decrease in CBG and unaffected total CORT concentrations were determined in loser males. Increased norepinephrine (NE) and epinephrine (E) titers were evident only in losers. In addition, reduced testosterone titers were observed in the bitten loser male subgroup. All male subgroups lost body mass with most pronounced reductions in loser males. Confrontation caused a marked granulocytosis, especially in loser males. NE concentrations in loser males correlated with the percentage of granulocytes. Numbers of CD4 T-cells were lowered in all loser males and in non-biting winners. In not-bitten losers also a reduced number of CD8 T-cells was determined. Interestingly, higher pre-confrontational NE titers were detected in future bitten loser and future biting winner males relatively to not-bitten losers and non-biting winners. The present report indicates that differential hormonal response patterns may play an important role in some of the immunological differences observed between winner and loser males under stressful social conditions.     

Neural circuits mediating stress.

Stress has been linked to the pathophysiology and pathogenesis of mood and anxiety disorders. Over the past few years, our understanding of the brain and neuroendocrine circuits that are linked to the stress response have increased dramatically. This article reviews a series of animal and human studies aimed at understanding what are the pathways by which stress is perceived, processed, and transduced into a neuroendocrine response. We focus on the classic stress circuit: the limbic-hypothalamic-pituitary-adrenal (LHPA) axis. These studies indicate that the LHPA stress circuit is a complex system with multiple control mechanisms and that these mechanisms are altered in pathological states, such as chronic stress and depression. These studies also suggest that the interactions between the LHPA and other neurotransmitters, such as serotonin, may provide the neurobiological substrate by which stress may affect mood.     

Role of the solitary tract nucleus in mediating nociceptive evoked cardiorespiratory responses

We compared the cardiorespiratory reflex responses evoked by noxious stimulation of the forelimb and cornea. Due to the depressant effects of anaesthesia on visceral reflexes we compared data from an unanaesthetised decerebrate rat model--the working heart-brainstem preparation (WHBP), with the anaesthetised rat. In both experimental models stimulation of the forelimb (mechanical pinch) evoked a tachycardia (WHBP: 19 +/- 2 bpm) and a decrease in respiratory cycle length (WHBP: from 4.1 +/- 0.2 to 2.3 +/- 0.1 s). The magnitude of response in anaesthetised animals depended on anaesthetic depth. Mechanical stimulation of the cornea evoked a bradycardia (-49.2 +/- 4.8 bpm) and an increase in respiratory cycle length from 4 +/- 0.36 to 5.88 +/- 0.2 s which was only present in the WHBP. In the WHBP activation of forelimb and corneal nociceptors both elicited significant pressor effects; in anaesthetised rats there were inconsistent changes in arterial pressure. To determine a role for the nucleus of the solitary tract (NTS) in mediating nociceptive evoked responses in the WHBP, synaptic transmission was blocked reversibly following bilateral microinjections of cobalt chloride. The heart rate responses evoked from either forelimb or corneal nociceptors were attenuated by approximately 50% (P < 0.05). A similar effect was observed using isoguvacine, a GABAA receptor agonist, to hyperpolarise NTS neurones. In conclusion, activation of forelimb and corneal nociceptors evoked contrasting patterns of cardiorespiratory response in the WHBP while in the anaesthetised rat the magnitude of the cardiorespiratory response to forelimb stimulation was quantitatively dependent on anaesthetic dose. In the WHBP, NTS neurones appear important for mediating the cardiac component of the reflex response following stimulation of nociceptive reflex pathways.     

Role of neuropeptide Y (NPY) in cardiovascular responses to stress

Neuropeptide Y (NPY), a putative sympathetic neurotransmitter and neuromodulator, is released during sympathetic nerve stimulation and causes vasoconstriction and cardiodepression. Whether the release of NPY contributes to stress-induced cardiovascular responses was assessed by studying i/ plasma levels of circulating NPY-immunoreactivity (NPY-ir) during various stress paradigms and ii/ mechanisms of action of NPY in the cardiovascular system of the rat. Plasma NPY-ir was increased by all stress situations, such as transfer to a new environment (by 52%), exposure to cold water (4⁰C) (by 117%) and hemorrhage (4 ml/300 g body weight) (by 231%). The cold water, stressinduced, maximal increase in circulating plasma NPY-ir was delayed in relation to the peak pressor response by 10–20 min. Administration of NPY caused dose-dependent pressor responses that were greater in pithed rats - which have all centrally mediated circulatory reflexes removed - than in conscious rats. Infusion of a low pressor (8.5 ± 1.5 mm Hg) dose of norepinephrine into conscious rats potentiated NPY-mediated pressor responses 2-fold and also tended to increase bradycardic effect of a higher dose of NPY (by 19%). Thus hypertensive and bradycardic actions of NPY appear to depend on the level of adrenergic activity and on the interactions at the level of vascular smooth muscle, heart, and baroreceptor reflexes. During a hyperadrenergic state such as stress, cardiovascular effects of NPY may be greatly accentuated. NPY may enhance and prolong the stressinduced hypertensive responses while antagonizing adrenergic cardiostimulation.     

Glucocorticoid inhibition of stress-induced changes in hypothalamic corticotrophin-releasing factor messenger RNA and proenkephalin A messenger RNA

We have examined the effects of localised administration of the synthetic glucocorticoid dexamethasone on basal and stress-induced levels of mRNA for both corticotrophin-releasing factor (CRF) and proenkephalin A, in parvocellular neurons of the paraventricular nucleus (PVN). Unilateral implants of cholesterol directly over the PVN had no effect on either basal or stress-induced increase of CRF and proenkephalin A message. However, unilateral implants of dexamethasone significantly decreased basal CRF mRNA compared to the contralateral, unimplanted side and prevented the increase in CRF and proenkephalin A mRNA 4 hours after ip hypertonic saline stress. These findings confirm the PVN as a site for glucocorticoid feedback. Furthermore the data demonstrates a steroid-sensitive stress mediated increase in proenkephalin A mRNA which provides additional evidence for a role of the endogenous opiates in the hypothalamic neuroendocrine response to stress.     

Role of renal nerve activity, plasma catecholamines and plasma vasopressin in cardiovascular responses to intracisternal neurotoxins in the rabbit

We have examined the acute (0-3 h) effect of intracisternally administered 5,7-dihydroxytryptamine (DHT) and 6-hydroxydopamine (6-OHDA) on blood pressure, heart rate, renal nerve activity, plasma adrenaline, plasma noradrenaline and plasma vasopressin in conscious rabbits. The increase in blood pressure seen following 5,7-DHT treatment was associated with increases in adrenaline and vasopressin levels and renal nerve activity throughout the response. The increase in blood pressure which followed 6-OHDA administration was associated with an increase in renal nerve activity alone. These findings indicate that the rise in blood pressure elicited by these drugs involves an increase in sympathetic nerve activity. The absence of a rise in vasopressin levels during the response to 6-OHDA suggests that the rise in blood pressure seen in these animals is due entirely to a bulbospinal sympathoexcitatory pathway.     

Studies on the cellular localization of biochemical responses to catecholamines in the brain.

Studies were undertaken to determine the cellular localization of the cyclic adenosine monophosphate (cAMP) response of various forebrain regions to beta-adrenoceptor stimulation. Using brain slices, it was found that the gliotoxin, fluorocitrate (FC), which blocks metabolism selectively in glial cells, virtually abolished the cAMP response to beta-receptor stimulation whereas the neurotoxin, kainic acid (KA), was without effect. FC was confirmed by electrophysiological recording to be selective for glial cells in the brain slices. Similar results were found for these agents on in vivo brain cAMP responses to beta-receptor stimulation using a new microdialysis technique to measure in vivo responses. It is concluded that the cAMP response to beta-adrenoceptor stimulation in various regions of the forebrain occurs predominantly in glia. To determine if this could be correlated with a second biochemical response to beta-receptor stimulation, preliminary studies were undertaken on the localization of the immediate early gene, c-fos, produced in the brain after in vivo stimulation of beta-receptors. It was found that unlike the cAMP responses the c-fos response to beta-receptor stimulation occurs predominantly in neurons. The possible relationship of these two responses is discussed.     

The role of corticotropin-releasing factor--norepinephrine systems in mediating the effects of early experience on the development of behavioral and endocrine responses to stress.

Naturally occurring variations in maternal care in early postnatal life are associated with the development of individual differences in behavioral and hypothalamic-pituitary-adrenal responses to stress in the rat. These effects appear to be mediated by the influence of maternal licking and grooming on the development of central corticotropin-releasing factor (CRF) systems, which regulate the expression of behavioral, endocrine, and autonomic responses to stress through activation of forebrain noradrenergic systems. These findings provide a neurobiologic basis for the observed relationship between early life events and health in adulthood. In more recent studies, we explored the behavioral transmission of individual differences in stress reactivity, and thus, vulnerability to stress-induced illness, across generations.     

Paraventricular lesions: hormonal and cardiovascular responses to hemorrhage

The responses of adrenocorticotropin (ACTH), renin, epinephrine and norepinephrine and arterial pressure and heart rate (HR) to hypotensive hemorrhage were examined before and 1 h after lesion of the paraventricular nuclei (PVN) in pentobarbital-anesthetized rats and 1 day before and 4 days after lesion of the PVN in conscious rats. The ACTH response to hemorrhage was abolished 1 h (n = 8) and 4 days (n = 14) after PVN lesion whereas the ACTH response in the sham groups (in both anesthetized and conscious studies, n = 8 and 16 respectively) remained intact. PVN lesion had no effect on basal ACTH levels 4 days after lesion. The responses of renin, epinephrine, norepinephrine and mean arterial blood pressure (MABP) and HR to hemorrhage were not affected 1 h or 4 days after PVN lesion. Resting levels of the above variables did not change 4 days after lesion. The PVN lesion had a small (but significant) effect on the baroreceptor reflex in the conscious study (reflex changes in HR caused by phenylephrine- or nitroglycerin-induced change in MABP) and had no effect on the baroreceptor reflex in the anesthetized study. The group with PVN lesions gained more weight 6 days after lesion than the group with sham lesions. We conclude that the PVN are part of a neural pathway involved in ACTH regulation during perturbations of the cardiovascular system and on weight gain and that PVN lesions have little or no effect on resting or stimulated (hemorrhage) levels of renin, epinephrine, norepinephrine, HR and MABP or on the baroreceptor reflex.     

Role of catecholamines in glucagon-induced thermogenesis

Summary. The present work was undertaken in order to investigate whether the observed thermogenesis following glucagon injection requires the participation of catecholamines. Our experiments aim at studying the effects of intraperitoneal injection of glucagon on metabolic rates, plasma catecholamine and fuel metabolites in guanethidine-treated ducklings reared at thermoneutrality (25°C). The chronic guanethidine treatment induced a marked decrease in catecholamines levels in peripheral tissues (heart, muscle and intestine) but not in adrenals. At thermoneutrality, intraperitoneal injection of glucagon had lower thermogenic effects in guanethidine-treated compared to control ducklings. Glucagon injection elicited a concomitant increase of plasma norepinephrine, metabolic rate and energy metabolites in control ducklings, whereas in guanethidine-treated ducklings, the plasma catecholamines and metabolic rate did not undergo any consistent change. The thermogenic action of glucagon in birds involves at least the mobilization of lipids and catecholaminergic system stimulation.     

Depletion of central catecholamines reduces pressor responses to arginine vasopressin

Previous reports that central administration of arginine vasopressin (AVP) increases turnover of brain catecholamines raise the possibility that the pressor responses which follow central administration of AVP may be mediated, in part, by central catecholamines. To test this hypothesis, rats were given intraventricular injections of vehicle, or of the neurotoxin, 6-hydroxydopamine, which resulted in significant depletions of hypothalamic and medulla oblongata noradrenalin and hypothalamic dopamine, but not of medullary dopamine or of hypothalamic and medullary 5-hydroxytryptamine. Following a one week recovery, these conscious rats, fitted with indwelling arterial catheters, were given intraventricular injections of AVP; the increases in arterial pressure and heart rate were significantly reduced in the catecholamine-depleted animals. These data support the hypothesis that the pressor and tachycardia responses to intraventricular AVP are mediated, in part, by central catecholamine-containing neurons.