The involvement of nitric oxide and prostaglandins in the cholinergic stimulation of hypothalamie-pituitary-adrenal response during crowding stress.

The aim of the present study was to determine the effect of social crowding stress and significance of nitric oxide (NO) and prostaglandins (PG) generated by constitutive and inducible nitric oxide synthase (NOS) and cyclooxygenase (COX) in the stimulation of hypothalamic-pituitary-adrenal (HPA) axis by cholinergic muscarinic receptor agonist carbachol. Inhibitors of neuronal NOS (nNOS) L-NNA, general NOS L-NAME and inducible NOS (iNOS) aminoguanidine, as well as inhibitors of COX-1, piroxicam, and COX-2, compound NS-398 were administered 15 min prior to carbachol to control or crowded rats (24 rats in cage for 7, during 3 and 7 days). In stressed rats L-NAME, L-NNA and aminoguanidine significantly intensified the carbachol-induced ACTH and corticosterone secretion, like in control rats. Piroxicam, markedly decreased the carbachol-induced ACTH and corticosterone response under either basal or stress conditions. Compound NS-398 did not markedly alter the carbachol-induced HPA response in control and stressed rats. Crowding stress (3 days) significantly impaired the i.c.v. prostaglandin E(2)-induced ACTH response. Corticotropin releasing hormone (CRH) receptor antagonists, alpha-helical CRH [9-14], given i.c.v. did not alter the PGE(2)-evoked corticosterone response in either control or stressed rats, indicating that hypothalamic CRH is not involved in the PGE(2)-induced central stimulation of HPA axis. In control rats L-NAME considerably enhanced, while L-arginine, a physiological NOS substrate, abolished the PGE(2)-induced ACTH and corticosterone response. In stressed rats this NOS blocker significantly increased and L-Arg reduced the stimulatory effect of PGE(2) on ACTH and corticosterone secretion. The carbachol-induced corticosterone response was significantly increased by pretreatment with nNOS inhibitor L-NNA and was considerably reduced by indomethacin, a general COX inhibitor. Pretreatment with both antagonists left the carbachol-induced corticosterone level unchanged, suggesting an independent and reciprocal effect of NO and PG in the cholinergic stimulation of pituitary-adrenocortical response. These results indicate that in the stimulatory action of muscarinic agonist, carbachol, NO is an inhibitory transmitter under basal and crowding stress conditions. This psychosocial stress does not functionally affect the NOS/NO systems. Prostaglandins are involved in the cholinergic muscarinic-induced stimulation of HPA response to a significant extent in non-stressed rats. PGE(2) may be involved in the carbachol-elicited HPA response under basal and stress conditions. Prostaglandins released in response to muscarinic stimulation did not evoke the hypothalamic CRH mediation. NO significantly impairs and PG stimulates the carbachol-induced HPA response in rats under basal and social stress conditions.     

CRF-dependent and CRF-independent mechanisms involved in hypophysial-adrenal system activation by bacterial endotoxin.

The immune system and the hypothalamic-pituitary-adrenal (HPA) axis play important role in the overall inflammatory response. The mechanism through which lipopolysaccharide (LPS, endotoxin) stimulates the HPA axis is not well understood. In order to clarify the role of hypophysiotropic peptides of paraventricular origin in the effect of LPS on ACTH and corticosterone secretion, the effect of LPS was studied on rats with lesions of hypothalamic paraventricular nucleus (PVN). It was shown that 90 min after 2 mg/kg LPS i.p. the ACTH, but not the corticosterone response was effectively blunted in PVN-lesioned rats, as compared to sham operated animals. However, in PVN-lesioned rats 240 min after treatment with LPS a significantly higher plasma ACTH and corticosterone level was monitored. It is, therefore, suggested that in response to LPS activation of HPA both CRF(s)-dependent and CRF(s)-independent mechanisms are involved, even a direct effect of the adrenal cortex should be taken into account.     

Nitric oxide mediates the interleukin-1beta- and nicotine-induced hypothalamic-pituitary-adrenocortical response during social stress.

We investigated the role of nitric oxide (NO) in the interleukin 1beta (IL-1beta) and nicotine induced hypothalamic-pituitary-adrenal axis (HPA) responses, and a possible significance of CRH and vasopressin in these responses under basal and social stress conditions. Male Wistar rats were crowded in cages for 7 days prior to treatment. All compounds were injected i.p., nitric oxide synthase (NOS) inhibitors, alpha-helical CRH antagonist and vasopressin receptor antagonist 15 min before IL-1beta or nicotine. Identical treatment received control non-stressed rats. Plasma ACTH and serum corticosterone levels were measured 1 h after IL-1beta or nicotine injection. L-NAME (2 mg/kg), a general nitric oxide synthase (NOS) inhibitor, considerably reduced the ACTH and corticosterone response to IL-1beta (0.5 microg/rat) the same extent in control and crowded rats. CRH antagonist almost abolished the nicotine-induced hormone responses and vasopressin antagonist reduced ACTH secretion. Constitutive endothelial eNOS and neuronal nNOS inhibitors substantially enhanced the nicotine-elicited ACTH and corticosterone response and inducible iNOS inhibitor, aminoguanidine, did not affect these responses in non-stressed rats. Social stress significantly attenuated the nicotine-induced ACTH and corticosterone response. In crowded rats L-NAME significantly deepened the stress-induced decrease in the nicotine-evoked ACTH and corticosterone response. In stressed rats neuronal NOS antagonist did not alter the nicotine-evoked hormone responses and inducible NOS inhibitor partly reversed the stress-induced decrease in ACTH response to nicotine. These results indicate that NO plays crucial role in the IL-1beta-induced HPA axis stimulation under basal and social stress conditions. CRH and vasopressin of the hypothalamic paraventricular nucleus may be involved in the nicotine induced alterations of HPA axis activity. NO generated by eNOS, but not nNOS, is involved in the stress-induced alterations of HPA axis activity by nicotine.     

Effect of constitutive- and inducible-cyclooxygenase in the carbachol-induced pituitary-adrenocortical response during social stress.

Acetylcholine potently stimulates the hypothalamic-pituitary-adrenal (HPA) axis. Cholinergic receptor agonist carbachol, given intraperitoneally (i.p.) or into the lateral cerebral ventricle (i.c.v.) to non-anesthetized rats acts via multiple pathways to stimulate the HPA axis. The present study sought to determine 1) the functional selectivity of carbachol for cholinergic muscarinic and/or nicotinic receptors involved in the stimulation of HPA axis; 2) the involvement of prostaglandins (PGs) generated by constitutive and inducible cyclooxygenase (COX-1 and COX-2) in the carbachol-induced ACTH and corticosterone secretion in non-stressed rats and animals exposed to social crowding stress for 7 days (24 per a cage for 6). Carbachol was given i.c.v. or i.p. and cholinergic receptor antagonists or cyclooxygenase isoenzyme antagonists were given by the same routes 15 min earlier. One hour after the last injection trunk blood was taken for ACTH and corticosterone determinations. Atropine (0.1 microg i.c.v.), a cholinergic receptor antagonist, totally abolished the carbachol (2 microg i.c.v.)-induced ACTH and corticosterone secretion and mecamylamine (20 microg i.c.v.), a selective nicotinic receptor antagonist, did not affect this secretion. This finding indicates that carbachol functions as a selective central cholinergic muscarinic receptor agonist for the HPA axis stimulation. Crowding stress significantly diminished the carbachol (0.2 mg/kg i.p.)-induced plasma ACTH and corticosterone levels measured 1 hr after administration. Pretreatment with indomethacin (2 mg/kg i.p.), a non-selective cyclooxygenase inhibitor, significantly diminished the ACTH and corticosterone responses to carbachol (0.2 mg/kg i.p.) in control rats and moderately decreased these responses in stressed rats. Piroxicam (0.2 and 2.0 mg/kg i.p.), a COX-1 inhibitor, considerably impaired the carbachol-induced ACTH and corticosterone responses in control rats and markedly diminished these responses in stressed rats. A selective COX-2 blocker, compound NS-398 (0.2 and 2.0 mg/kg i.p.), substantially decreased the carbachol-induced hormones secretion in control rats but did not markedly alter this secretion in stressed rats. These results indicate that in the carbachol-induced HPA axis activation PGs generated by COX-1 are considerably and to a much greater extent involved than PGs generated by COX-2. Social stress markedly diminishes the mediation of PGs generated by COX-1 but PGs synthesized by COX-2 do not substantially participate in the carbachol-induced HPA response.     

Role of nitric oxide in the nicotine-induced pituitary-adrenocortical response.

Nitric oxide (NO) is a major signaling molecule and biological mediator of the hypothalamic-pituitary-adrenal (HPA) axis. We investigated the role of NO formed by endothelial (e), neuronal (n) and inducible (i) nitric oxide synthase (NOS) in the stimulatory effect of nicotine on the HPA axis in rats under basal conditions. Also possible interaction of NOS systems with endogenous prostaglandins (PG) in that stimulation was assessed. NOS and cyclooxygenase inhibitors were administered i.p. 15 min prior to nicotine (2, 5 mg/kg i.p.). Plasma ACTH and serum corticosterone levels were measured 1 h after nicotine injection. NOS blockers given alone did not markedly affect the resting ACTH and corticosterone levels. L-NAME (2-10 mg/kg), a broad spectrum NOS inhibitor considerably and dose dependently enhanced the nicotine-induced ACTH and corticosterone secretion. L-NNA (2 mg/kg) and 7-nitroindazole (7-NI 20 mg/kg), neuronal NOS inhibitors in vivo also significantly augmented the nicotine-induced ACTH and corticosterone levels. L-arginine greatly impaired the nicotine-induced hormone responses and reversed the L-NNA elicited enhancement of the nicotine-evoked ACTH and corticosterone response. In contrast to the constitutive eNOS and nNOS antagonists, an inducible NOS antagonist guanethidine (50-100 mg/kg i.p.) did not substantially affect the nicotine-elicited pituitary-adrenocortical responses. Indomethacin (2 mg/kg i.p.), a non-selective cyclooxygenase blocker abolished the L-NAME and L-NNA-induced enhancement of the nicotine-evoked ACTH and corticosterone response. These results indicate that NO is an inhibitory mediator in the HPA axis activity. Inhibition of its generation by eNOS and nNOS significantly enhances the nicotine-induced HPA response. Under basal conditions iNOS is not involved in the nicotine-induced ACTH and corticosterone secretion. Prostaglandins play an obligatory role in the response of HPA axis to systemic nicotine administration.     

Cocaine-induced stimulation of the rat hypothalamic-pituitary-adrenal axis is progressively attenuated following hourly-interval regimens of the drug.

The role of multiple (iv) injections of cocaine on the rat hypothalamic-pituitary-adrenal (HPA) axis was examined using four different temporal regimens of drug exposure. In intact rats, cocaine (5 mg/kg) consistently stimulated the secretion of adrenocorticotropin hormone (ACTH) and corticosterone over a 6 hr interval regimen. In all experimental groups, administration of the vehicle alone failed to measurably alter the secretion of the aforementioned hormones. When rats where exposed to the drug over a 4 hr interval regimen, a modest attenuation of ACTH, but not corticosterone, secretion was observed following the third and last cocaine injection. To test whether the attenuation of ACTH secretion to cocaine administration was caused by corticosterone-mediated negative feedback, the response of intact and adrenalectomized (ADX) rats over 2 hr and 1 hr interval regimens was compared. In intact rats, both drug interval regimens resulted in a significant attenuation of ACTH secretion following, the second and third injections of the drug. ADX rats, on the other hand, exhibited significant increases in ACTH levels following either interval regimens, though we observed a modest blunting of pituitary responsiveness to the 1 hr regimen. From these results we conclude that in intact rats the activity of the HPA axis is significantly attenuated in response to multiple, acute cocaine injections, and that this decreased response may be at least in part caused by a negative corticoid feedback mechanism.     

Effect of subdiaphragmatic vagotomy and cholinergic agents in the hypothalamic-pituitary-adrenal axis activity.

In the present study, we examined whether the vagus nerve is involved in mediating the stimulation of hypothalamic-pituitary-adrenal (HPA) axis by cholinergic muscarinic and nicotinic agonists, carbachol and nicotine. The site of HPA axis muscarinic stimulation was determined using peripheral (i.p.) and intracerebroventricular (i.c.v.) administration of carbachol, atropine sulphate (AtrS) and atropine hydrobromide (AtrBr). The i.p. carbachol-(0.5 mg/kg)-induced corticosterone response was significantly reduced by i.p. pretreatment with AtrBr (0.1 mg/kg), but was not diminished by i.c.v. AtrS (0.1 mug). The increase in corticosterone secretion induced by i.c.v. carbachol (2 microg) was totally abolished by i.c.v. pretreatment with AtrS (0.1 microg) but was not altered by i.p. AtrBr. Subdiaphragmatic vagotomy performed 2 weeks earlier substantially decreased the i.p. carbachol (0.2 mg/kg)-induced ACTH response and markedly augmented ACTH and corticosterone response to a higher dose of carbachol (0.5 mg/kg) in comparison with the responses in sham operated rats. Vagotomy abolished the stimulatory effect of i.p. nicotine in a low dose (1 mg/kg) on ACTH and corticosterone secretion; the ACTH response to higher dose (2.5 mg/kg) was considerably reduced, while corticosterone response remained unaffected. These results suggest that carbachol given i.c.v. evokes considerable corticosterone response by stimulation of central cholinergic muscarinic receptors. A major part of the i.p. carbachol-induced corticosterone secretion results from peripheral cholinergic muscarinic receptor stimulation. Subdiaphragmatic vagotomy moderately intensified the carbachol-induced ACTH and corticosterone secretion. Vagotomy significantly reduced the nicotine-induced ACTH secretion, possibly by the involvement of vagal afferents. The nicotine-induced corticosterone secretion is not exclusively regulated by circulating ACTH but by various intra-adrenal regulatory components.     

Effect of cyclooxygenase inhibitors on the vasopressin induced ACTH and corticosterone response during crowding stress.

The aim of the present study was to compare the effect of social stress on the corticotropin releasing hormone (CRH) and arginine vasopressin (AVP)-induced pituitary-adrenocortical activity. Also the significance of prostaglandins (PG) generated by constitutive and inducible cyclooxygenase (COX-1 and COX-2) in the stimulation of hypothalamic-pituitary-adrenal (HPA) axis by AVP under basal and crowding stress conditions was investigated. The control rats were housed 7 in a standard cage and stressed rats were crowded 24 in a cage of the same size during 7 days. The activity of HPA axis was determined by measuring plasma ACTH and serum corticosterone levels 1 h after i.p. AVP administration. Indomethacin (2.0 mg/kg i.p.), a non-selective COX inhibitor, piroxicam (0.2, 2.0, and 5.0 mg/kg), a more potent COX-1 than COX-2 inhibitor, and compound NS-398 (0.2 and 2.0 mg/kg) a selective COX-2 inhibitor, were administered i.p. 15 min prior to AVP (5.0 microg/kg i.p.) to control or crowded rats. The obtained results indicate that social stress for 7 days considerably inhibits the stimulatory action of AVP on ACTH secretion, while it intensifies the CRH-induced ACTH secretion. Indomethacin, piroxicam and NS-398 significantly diminished the AVP-elicited ACTH and corticosterone secretion in non-stressed rats. None of these COX antagonist induced any significant inhibition of the AVP-induced ACTH and corticosterone secretion in stressed rats. Therefore, PG generated by COX-1 or COX-2 do not participate to a significant extent in the HPA stimulation by AVP during crowding stress. These results suggest that social crowding stress desensitizes the PG stimulatory mechanism which considerably mediates the AVP-induced HPA stimulation under basal conditions. The results contrast with a lack of any involvement of PG in the CRH-induced stimulation of HPA response under basal or crowding stress conditions.     

Adrenal splanchnic innervation contributes to the diurnal rhythm of plasma corticosterone in rats by modulating adrenal sensitivity to ACTH

Activity of the hypothalamic-pituitary-adrenal axis is characterized by a diurnal rhythm with an AM nadir and PM peak. Splanchnic nerve transection disrupts the diurnal rhythm in plasma corticosterone; however, there is a controversy as to whether the nerve-mediated effect is 1) via inhibition in the AM vs. excitation in the PM, or 2) involves changes in adrenal sensitivity to ACTH. The present studies were designed to address these issues. Adult male rats were anesthetized and underwent bilateral transection of the thoracic splanchnic nerve or sham transection. One week after surgery, rats were killed in the AM or PM with collection of nonstress plasma for measurement of corticosterone and ACTH. Plasma corticosterone was increased in the PM relative to the AM; however, plasma corticosterone in the PM was attenuated by splanchnic nerve transection, without affecting plasma ACTH. This decrease in PM plasma corticosterone after nerve-transection was 1) associated with decreased adrenal responsivity to ACTH, 2) associated with decreased adrenal cAMP content, 3) prevented by adrenal demedullation, and 4) not affected by removal of adrenal capsaicin-sensitive afferent fibers. Repeated serial blood sampling from individual rats confirmed the excitatory effect of splanchnic innervation in the PM. These results support the hypothesis that the adrenal splanchnic innervation modulates the diurnal rhythm in plasma corticosterone by increasing adrenal responsivity to ACTH and augmenting steroidogenesis in the PM and suggest that alterations in adrenal corticosterone secretion obscured by pulsatile secretion are more clearly revealed with repeated serial blood sampling.     

Selenium deficiency impairs corticosterone and leptin responses to adrenocorticotropin in the rat

Selenium deficiency causes oxidative stress and impairs steroidogenesis in vitro. Leptin is closely related to the hypothalamo-pituitary-adrenal (HPA) axis. Leptin inhibits the HPA axis at the central level while corticosteroids have been shown to stimulate leptin secretion in most studies. We hypothesized that oxidative stress impairs adrenal steroidogenesis and decreases leptin production in vivo. The goal of this study was to investigate in rats the effects of selenium deficiency and oxidative stress on adrenal function and on leptin concentrations. Weanling rats were fed a selenium-deficient (Se-) or selenium-sufficient (Se+) diet for 4-10 weeks. Selenium deficiency caused a marked decrease in liver (> or = 99%) and adrenal (> or = 81%) glutathione peroxidase (GPx) activities. Selenium deficiency did not affect basal and short-term adrenocorticotropin (ACTH) stimulated corticosterone or leptin concentrations. In contrast, after long-term ACTH stimulation, selenium deficiency caused a doubling in adrenal isoprostane content and blunted the increase in corticosterone and leptin concentrations observed in Se+ animals. Plasma leptin concentrations were 50% lower in Se- compared to Se+ animals following long-term ACTH. Our results suggest that oxidative stress causes a decrease in circulating corticosterone in response to ACTH, and, as a consequence, a decrease in plasma leptin concentrations.     

Prenatal stress alters circadian activity of hypothalamo–pituitary–adrenal axis and hippocampal corticosteroid receptors in adult rats of both gender

Prenatal stress impairs activity of the hypothalamo–pituitary–adrenal (HPA) axis in response to stress in adult offspring. So far, very few data are available on the effects of prenatal stress on circadian functioning of the HPA axis. Here, we studied the effects of prenatal stress on the circadian rhythm of corticosterone secretion in male and female adult rats. To evaluate the effects of prenatal stress on various regulatory components of corticosterone secretion, we also assessed the diurnal fluctuation of adrenocorticotropin, total and free corticosterone levels, and hippocampal corticosteroid receptors. Finally, in the search of possible maternal factors, we studied the effects of repeated restraint stress on the pattern of corticosterone secretion in pregnant female rats. Results demonstrate that prenatal stress induced higher levels of total and free corticosterone secretion at the end of the light period in both males and females, and hypercorticism over the entire diurnal cycle in females. No diurnal fluctuation of adrenocorticotropin was observed in any group studied. The effects of prenatal stress on corticosterone secretion could be mediated, at least in part, by a reduction in corticosteroid receptors at specific times of day. Results also show that prepartal stress alters the pattern of corticosterone secretion in pregnant females. Those data indicate that prenatally stressed rats exhibit an altered temporal functioning of the HPA axis, which, taken together with their abnormal response to stress, reinforces the idea of a general homeostatic dysfunction in those animals. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 302–315, 1999     

Effect of adrenergic antagonists and cyclooxygenase inhibitors on the nicotine-induced hypothalamic-pituitary-adrenocortical activity.

Nicotine is a potent stimulus for the hypothalamic-pituitary-adrenal (HPA) axis. Systemic nicotine acts via central mechanisms to stimulate by multiple pathways the release of ACTH from the anterior pituitary corticotrops and corticosterone from the adrenal cortex. Nicotine may stimulate indirectly the hypothalamic paraventricular nucleus, the site of the corticotropin-releasing hormone (CRH) neurons which activates ACTH release. In the present studies an involvement of adrenergic system and prostaglandins synthesized by constitutive cyclooxygenase (COX-1) and inducible cyclooxygenase (COX-2) in the nicotine-induced HPA response in rats was investigated. Nicotine (2.5-5 mg/kg i.p.) significantly increased plasma ACTH and corticosterone levels measured 1 hr after administration. Adrenergic receptor antagonists or COX inhibitors were injected i.p. 15 min prior to nicotine and the rats were decapitated 1 hr after the last injection. Prazosin (0.01-0.1 mg/kg), an alpha1-adrenergic antagonist, significantly decreased the nicotine-evoked ACTH and corticosterone secretion. Yohimbine (0.1-1.0 mg/kg), an alpha2-adrenergic antagonist, moderately diminished ACTH response, and propranolol (0.1-10 mg/kg), a beta-adrenergic antagonist, did not significantly alter the nicotine-induced hormones secretion. Pretreatment with piroxicam (0.2-2.0 mg/kg), a COX-1 inhibitor, considerably impaired the nicotine-induced ACTH and corticosterone secretion. Compound NS-398 (0.2-5.0 mg/kg), a selective COX-2 blocker did not markedly alter these hormones secretion, and indomethacin (2 mg/kg), a non-selective COX inhibitor significantly diminished ACTH response. These results indicate that systemic nicotine stimulates the HPA axis indirectly, and both adrenergic system and prostaglandins are significantly involved in this stimulation. Noradrenaline, stimulating postsynaptic alpha1-adrenergic receptors, and prostaglandins, synthesized by COX-1 isoenzyme, are of crucial significance in the nicotine-induced ACTH and corticosterone secretion.     

The prototypic antidepressant drug, imipramine, but not Hypericum perforatum (St. John's Wort), reduces HPA-axis function in the rat.

Dysregulation in corticotropin-releasing hormone (CRH) secretion in the hypothalamus-pituitary-adrenal (HPA) axis may be involved in the etiology of major depressive disorder (MDD). Chronic therapy with standard antidepressant drugs, such as imipramine, can downregulate HPA axis function, indicating that the HPA axis may be an important target for antidepressant action. We tested several doses of a standardized commercial preparation of Hypericum perforatum plant extract (popularly known as St. John's Wort), a medicinal herb used for treating mild depressive symptoms, to determine whether it also modulated HPA axis function. Chronic imipramine treatment (daily injections for 8 weeks) of male Sprague-Dawley rats significantly downregulated circulating plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone compared to animals treated with saline. However, chronic St. John's Wort treatment (daily gavage for 8 weeks) had no effect on plasma ACTH or corticosterone, even at the highest doses tested. Our results confirm previous findings that imipramine may have significant peripheral HPA axis-mediated effects. However, our data does not support any role for H. perforatum in modulation of HPA axis function, suggesting that alternative pathways may be involved in mediating its antidepressant effects.     

Prenatal stress alters circadian activity of hypothalamo-pituitary-adrenal axis and hippocampal corticosteroid receptors in adult rats of both gender.

Prenatal stress impairs activity of the hypothalamo-pituitary-adrenal (HPA) axis in response to stress in adult offspring. So far, very few data are available on the effects of prenatal stress on circadian functioning of the HPA axis. Here, we studied the effects of prenatal stress on the circadian rhythm of corticosterone secretion in male and female adult rats. To evaluate the effects of prenatal stress on various regulatory components of corticosterone secretion, we also assessed the diurnal fluctuation of adrenocorticotropin, total and free corticosterone levels, and hippocampal corticosteroid receptors. Finally, in the search of possible maternal factors, we studied the effects of repeated restraint stress on the pattern of corticosterone secretion in pregnant female rats. Results demonstrate that prenatal stress induced higher levels of total and free corticosterone secretion at the end of the light period in both males and females, and hypercorticism over the entire diurnal cycle in females. No diurnal fluctuation of adrenocorticotropin was observed in any group studied. The effects of prenatal stress on corticosterone secretion could be mediated, at least in part, by a reduction in corticosteroid receptors at specific times of day. Results also show that prepartal stress alters the pattern of corticosterone secretion in pregnant females. Those data indicate that prenatally stressed rats exhibit an altered temporal functioning of the HPA axis, which, taken together with their abnormal response to stress, reinforces the idea of a general homeostatic dysfunction in those animals.     

Influence of cyclooxygenase inhibitors on the central histaminergic stimulations of hypothalamic - pituitary - adrenal axis.

Brain histamine participates in central regulation of the hypothalamic-pituitary-adrenal (HPA) axis. Endogenous prostaglandins modulate signal transduction of different neurotransmitters involved in activation of HPA axis. In the present experiment we investigated whether endogenous prostaglandins are involved in the stimulation of ACTH and corticosterone secretion by histaminergic systems in the rat brain. Histamine (50 microg), histamine-trifluoromethyl-toluidine derivative (HTMT, 75microg) a selective and potent H(1)-receptor agonist, and amthamine (50 microg) a H(2)-receptor agonist given intracerebroventricularly (i.c.v.) to non-anesthetized rats considerably increased ACTH and corticosterone secretion 1h after administration. A non-selective cyclooxygenase inhibitor indomethacin (2 mg/kg i.p. or 10 microg i.c.v.), piroxicam (0.02 and 0.2 microg i.c.v.) a more potent antagonist of constitutive cyclooxygenase (COX-1) and compound NS-398 (0.1 and 1.0 microg i.c.v.), a selective inhibitor of inducible cyclooxygenase (COX-2) were given 15 min before histamine and histamine receptor agonists. One hour after the last injection trunk blood from decapitated rats was collected for hormones determination. The histamine-induced ACTH and corticosterone secretion was significantly diminished by piroxicam and was not markedly altered by indomethacin and compound NS-398. The HTMT-elicited increase in ACTH and corticosterone secretion was significantly prevented by indomethacin and was not affected by piroxicam or compound NS-398. The amthamine-evoked increase in ACTH and corticosterone secretion was not markedly influenced by any cyclooxygenase blocker applied in the present experiment. These results indicate that the histamine H(1)-receptor transmitted central stimulation of the HPA axis is considerably mediated by prostaglandins generated by consititutive cyclooxygenase, whereas stimulation transmitted via H(2)-receptor does not significantly depend on endogenous prostaglandins mediation.     

The influence of sex and gonadectomy on the growth hormone and corticosterone response to hexarelin in the rat

The present study is designed to investigate the role of sex and gonadal status in the growth hormone (GH) and corticosterone response to hexarelin (HEXA), a GH-releasing peptide, which also causes a stimulatory action on the hypothalamic-pituitary-adrenal (HPA) axis. HEXA (80 microg/Kg) was administered intracarotid to anesthetized intact or gonadectomized male (ORC) and female (OVX) middle-aged rats. The GH stimulatory response to HEXA was gender-related since the GH increase was significantly (p < 0.001) higher in intact males (area under the curve, AUC= 12560 +/- 1784 ng/ml.45 min) than in females (AUC= 4628 +/- 257 ng/ml.45 min). This sex difference does not depend on circulating gonadal steroids since it persists in ORC (AUC = 11980 +/- 1136 ng/ml.45 min) and OVX (AUC = 5539 +/- 614 ng/ml.45 min) rats. The different effects of HEXA on corticosterone secretion detected in male and female rats are probably dependent on the prevailing activity of the HPA axis. In fact, in male rats that have low basal corticosterone levels, HEXA caused an increase in corticosterone secretion, which was significantly (p< 0.05) higher in ORC than in intact rats. The increase in corticosterone secretion by HEXA both in intact and OVX females was delayed, probably due to the elevated initial corticosterone levels, which could have activated the glucocorticoid negative feedback. We suggest that gender-specific patterns in the regulation of the hypothalamus-pituitary function could be responsible for the GH and corticosterone sexually differentiated responses to HEXA.     

Different activation of ACTH and corticosterone release in response to various stressors in rats

The aim of this study was to investigate the reaction of the hypothalamo-pituitary-adrenocortical (HPA) system to various stressors (fasting, crowding, cold and heat) by measuring blood ACTH and corticosterone (CORT) concentration as well as the cholesterol (CHOL) content in the adrenals. To examine the effects of stress termination, the rats were returned and kept under control conditions for the same period as that of stress duration (supposed recovery period). According to our results HPA system was activated by all the stressors applied. Heat seems to be the strongest stressor since the exposure of animals to a high ambient temperature resulted in the greatest rise of plasma ACTH concentration as well as CORT synthesis and secretion. These values remained elevated after the stress termination i.e. after the rats had been returned to room temperature. Fasting seems to be the weakest stressor given because it causes the smallest increase in blood ACTH and CORT concentrations. Moreover, in refed rats the HPA function was fully recovered. In conclusion, the various stressors applied seem to induce a different response of the HPA system as judged by quantitative changes in ACTH and CORT release.     

Effect of cyclooxygenase inhibitors on the CRH-induced pituitary-adrenocortical activity during crowding stress.

The aim of the present study was to determine the effect of social stress and significance of prostaglandins (PG) generated by constitutive and inducible cyclooxygenase (COX-1 and COX-2) in the stimulation of hypothalamic-pituitary-adrenal (HPA) axis by corticotropin releasing hormone (CRH) under basal and social crowding stress conditions. The stressed rats were crowded in groups of 24 to a cage for 3 or 7 days, whereas the control animals were haused in groups of 7 to a cage of the same size. The activity of HPA axis was determined by measuring plasma ACTH and serum corticosterone levels 1 h after i.p. CRH administration. Inhibitors of COX-1, piroxicam (0.2, 2.0, and 5.0 mg/kg), and COX-2, compound NS-398 (0.2 and 2.0 mg/kg), were administered i.p. 15 min prior to CRH (0.1 microg/kg i.p.) to control or crowded rats. The obtained results indicate that social stress for 3 and 7 days markedly intensifies the stimulatory action of CRH on ACTH secretion. Neither piroxicam nor NS-398 induce any significant effect on the CRH-elicited ACTH and corticosterone secretion in non-stressed or crowded rats. Therefore, PG generated by COX-1 or COX-2 do not participate to a significant extent in the stimulation of HPA axis by CRH under either basal conditions or during crowding stress. These results also indicate that the stimulatory action of CRH on ACTH secretion is not only completely resistant to desensitization but is sensitized during social crowding stress. The results contrast with a significant involvement of PG in the vasopressin-induced stimulation of HPA response during crowding stress.     

Corticosterone secretion induced by chronic isolation in neonatal rats is sexually dimorphic and accompanied by elevated ACTH

Rat pups repeatedly subjected to brief periods of isolation during the stress hyporesponsive period (SHRP) exhibit varied neuroendocrine and behavioral changes as neonates and as adults. For example, neonatal rats exhibit increased circulating corticosterone after 1-h isolation on postnatal day 9 (P9) only if they were isolated daily from P2 to P8 [McCormick, C.M., Kehoe, P., Kovacs, S., 1998. Corticosterone release in response to repeated, short episodes of neonatal isolation: evidence of sensitization. Int. J. Dev. Neurosci. 16, 175-185]. It is not known if the increase in adrenocortical response on P9 following repeated isolation is mediated by increased pituitary ACTH secretion. The present study examined the responsivity of the hypothalamic-pituitary-adrenal (HPA) axis during the SHRP following brief, repeated isolation or acute pharmacological manipulation. Removal from the nest for 1 h daily on P4-8 increased circulating corticosterone after 1-h isolation on P9 by approximately twofold. Neither unhandled nor handled controls showed a corticosterone response to 1-h isolation on P9. The increased corticosterone was sexually dimorphic, with only females showing the sensitization response. Other findings suggest that the hormonal response is centrally mediated; chronically isolated pups of both sexes exhibit increased plasma ACTH following 1-h isolation on P9. While we could not detect an increase in Fos immunoreactivity (IR) on P9 in the hypothalamic paraventricular nucleus (PVN) of chronically isolated pups, acute pharmacological activation of serotonin 2A/2C receptors produced robust activation of ACTH and corticosterone secretion as well as expression of Fos in the PVN on P9. We conclude that chronic isolation stress limited to the SHRP stimulates the neonatal HPA axis, and that the adrenal response is sexually dimorphic. In addition, PVN neurons can express Fos IR on P9 in response to a very potent activation of the HPA axis.     

Periodic maternal deprivation and lesion of anterodorsal thalami nuclei induce alteration on hypophyso adrenal system activity in adult rats

The hypothalamic-pituitary-adrenal (HPA) axis is normally regulated by extrahypothalamic limbic structures, among these, the anterodorsal thalami nuclei (ADTN), which exert an inhibitory influence on HPA, in basal and acute stress conditions in rats. In the present work we have investigated whether neonatal maternal deprivation (MD) produces long-term changes in the ADTN regulation of HPA activity. Maternal deprivation, in female rats, for 4.5 hs daily, during the first 3 weeks of life, produced at 3 months old, a significant decrease in plasma ACTH concentration (p<0.001) and an increase in plasma corticosterone (C) (p<0.001), compared to control non-deprived rats (NMD). Also MD showed higher plasma epinephrine (E) and norepinephrine (NE) levels than NMD rats. The increase of NE (66.6% p<0.001) was higher than that observed in E (19%). After 30 days of ADTN lesion, plasma ACTH values were higher than in sham lesioned rats, in both NMD and MD animals. ACTH response was greater in MD rats. Plasma C, in NMD, was higher, whereas in MD lesioned animals, it was significantly lower than in sham lesioned. In MD rats, lesion produced a significant increase in plasma E and NE (p<0.001), and again, NE increase was higher than E increase. The more accentuated increase of NE than E, suggests sympathetic nervous system hyperactivity. In summary, neonatal maternal deprivation induces long-term alterations on HPA axis sensitivity and medullo adrenal secretion; enhanced sympathetic nervous system activity and, therefore affected the ADTN inhibitory influence on ACTH and adrenal glands secretion.