A single corticosterone pretreatment inhibits the hypothalamic-pituitary-adrenal responses to adrenergic and cholinergic stimulation.

The purpose of the present study was to determine whether an increased plasma corticosterone or dexamethasone levels induced by a single corticosterone or dexamethasone injection to conscious rats affects the hypothalamic-pituitary-adrenocortical (HPA) activity induced by adrenergic and cholinergic agonists. Male Wistar rats were pretreated subcutaneously (s.c.) with a single dose of dexamethasone (5 mg/kg) or corticosterone (25 mg/kg) 24 or 48 h before intraperitoneal (i.p.) administration of adrenergic agonists: phenylephrine, an alpha1-adrenergic receptor agonist, clenbuterol, a beta2-adrenergic agonist and noradrenaline acting predominantly on alpha1-adrenoreceptors, and cholinergic agonists: carbachol, a predominant muscarinic receptor agonist and nicotine, a nicotinic receptor agonist. Dexamethasone profoundly decreased the resting ACTH levels in control rats and given 24 h before each of the stimulatory agonist abolished the adrenergic- and cholinergic agonists-induced ACTH and corticosterone responses. Pretreatment with corticosterone of control rats did not substantially alter the resting plasma ACTH and serum corticosterone levels measured 24 and 48 h later. A single pretreatment with corticosterone abolished or powerfully inhibited, perhaps by a feedback mechanism, the ACTH and corticosterone responses induced 24 and 48 h later by all adrenergic and cholinergic agonists used in this study. These results indicate that prolonged administration of corticosterone is not necessary to induce almost complete suppression of the HPA responsiveness to adrenergic or cholinergic stimulation. Chronic treatment with corticosteroids to achieve glucocorticoid receptors desensitization does not seem to be required.     

Blockade of nitric oxide formation impairs adrenergic-induced ACTH and corticosterone secretion.

It has been suggested that adrenergic agents might modulate the L-arginine-NO pathway. Sympathomimetic agonists enhance the basal release of NO, and noradrenaline increases the synthesis of nitric oxide synthase (NOS) in the medial basal hypothalamus in vitro. In the present study possible involvement of NO in central stimulation of the hypothalamic-pituitary-adrenal (HPA) axis by adrenergic agents was investigated in conscious rats. The nitric oxide synthase blocker N(omega)-nitro-L-arginine methyl ester (L-NAME 2 and 10 microg) was administered intracerebroventricularly (i.c.v.) 15 min before the adrenergic agonist given by the same route; 1 h later the rats were decapitated. Plasma levels of ACTH and corticosterone were measured. L-NAME significantly diminished the ACTH and corticosterone response to phenylephrine (30 microg), an alpha1-adrenergic receptor agonist. These hormone responses to clonidine (10 microg), an alpha2-receptor agonist, were dose-dependently suppressed or totally abolished by L-NAME. A significant rise in the ACTH and corticosterone secretion induced by isoprenaline (10 microg), a beta-adrenergic receptor agonist, was only moderately diminished by pretreatment with L-NAME. These results indicate that NOS is considerably involved in central stimulation of the HPA axis by alpha1- and alpha2-adrenergic receptor agonists, and that NO mediates the stimulatory action of these agonists on ACTH and corticosterone secretion. The stimulation induced by beta-adrenergic receptors is only moderately affected by endogenous NO.     

The influence of indomethacin on the acth secretion induced by central stimulation of adrenergic receptors.

We had previously demonstrated that indomethacin affected the corticosterone secretion induced by central stimulation of alpha-but not beta-adrenergic receptors in conscious rats. In the present study we investigated whether hypothalamic and/or pituitary prostaglandins (PGs) were involved in the central adrenergic stimulation of ACTH secretion. Indomethacin, 2 mg/kg ip or 10 microg intracerebroventricularly (icv), was administered 15 min before phenylephrine (30 microg icv), an alpha-adrenergic agonist, clonidine (10 microg), an alpha2-adrenergic agonist, and isoprenaline (20 microg) or clenbuterol (10 microg), a beta1- or beta2-adrenergic agonist. One hour after the last injection the rats were decapitated and plasma levels of ACTH were measured. The present results show that the ACTH responses induced by icv administration of phenylephrine and clonidine were considerably impaired by icv or ip pretreatment with indomethacin, an inhibitor of prostaglandin synthesis. Indomethacin given by either route only slightly diminished the isoprenaline-induced ACTH response and did not substantially alter the clenbuterol-induced response. The adrenergic-induced ACTH responses were more potently inhibited by ip than by icv pretreatment with indomethacin, which may result from a stronger inhibition of PGs synthesis in the median eminence and anterior pituitary by ip pretreatment with indomethacin than in hypothalamic structures by its icv administration. These results indicate a significant involvement of PGs in central stimulation of the hypothalamic-pituitary adrenal (HPA) axis by alpha1- and alpha2- but not beta-adrenergic receptors.     

Pharmacological identification of the major subtypes of adrenoceptors involved in the canine external carotid vasoconstrictor effects of adrenaline and noradrenaline.

This study investigated the potential effects of adrenaline and noradrenaline on the external carotid blood flow of vagosympathectomised dogs and the receptor mechanisms involved. One minute (1 min) intracarotid infusions of adrenaline and noradrenaline produced dose-dependent decreases in external carotid blood flow without changes in blood pressure or heart rate. These responses, which remained unaffected after saline, were: (i) mimicked by the adrenoceptor agonists, phenylephrine (alpha1) and BHT933 (6-Ethyl-5,6,7,8-tetrahydro-4H-oxazolo [4,5-d] azepin-2-amine dihydrochloride; alpha2); (ii) abolished after phentolamine (2000 microg/kg) unmasking a vasodilator component (subsequently blocked by propranolol; 1000 microg/kg); and (iii) partly blocked by rauwolscine (30 and 100 microg/kg), and subsequently abolished by prazosin (100 microg/kg). Accordingly, rauwolscine (100 and 300 microg/kg) markedly blocked the responses to BHT933 without affecting those to phenylephrine; likewise, prazosin (100 microg/kg) markedly blocked the responses to phenylephrine without affecting those to BHT933. These results show that both alpha1- and alpha2-adrenoceptors mediate vasoconstriction within the canine external carotid circulation. Moreover, after blockade of alpha1/alpha2-adrenoceptors, both adrenaline and noradrenaline exhibit a beta-adrenoceptor-mediated vasodilator component.     

Effect of L-NAME, a specific nitric oxide synthase inhibitor, on corticotropin-releasing hormone-elicited ACTH and corticosterone secretion.

This study was designed to determine the role of endogenous nitric oxide (NO) in the corticotropin-releasing hormone (CRH)-induced ACTH and corticosterone secretion, as well as possible involvement of hypothalamic dopamine and noradrenaline in that secretion in conscious rats. CRH given i.p. stimulated dose-dependently the pituitary-adrenocortical activity measured 1 h later. Dexamethasone (0.2 mg/kg i.p.) injected 1 h before CRH (1 microg/kg i.p.) totally abolished the CRH-elicited ACTH and corticosterone secretion, indicating a predominantly pituitary site of CRH-evoked stimulation. L-arginine (120 mg/kg i.p.) and N(omega)-nitro-L-arginine methyl ester (L-NAME 5-10 mg/kg i.p.) did not markedly affect the basal plasma ACTH and corticosterone levels. L-NAME given 15 min before CRH markedly, but not significantly, augmented the CRH-induced ACTH response, and enhanced more potently and significantly the corticosterone response. Pretreatment with L-arginine, a substrate for NOS, slightly diminished the CRH-induced ACTH response and considerably reduced the corticosterone response. L-arginine also significantly reversed the L-NAME-evoked increase in the CRH-induced ACTH and corticosterone secretion. L-NAME did not markedly alter the CRH-induced hypothalamic dopamine and noradrenaline levels, while L-arginine significantly increased noradrenaline level. However, those alterations were not directly correlated with the observed changes in ACTH and corticosterone secretion. These results indicate that in conscious rats NO plays a marked inhibitory role in the CRH-induced ACTH secretion and inhibits more potently corticosterone secretion. Hypothalamic dopamine and noradrenaline do not seem to be directly involved in the observed alterations in ACTH and corticosterone secretion.     

Influence of nitric oxide synthase inhibitors on the vasopressin-induced pituitary-adrenocortical activity and hypothalamic catecholamine levels.

In the present study the role of endogenous nitric oxide (NO) in the vasopressin-induced ACTH and corticosterone secretion was investigated in conscious rats. Vasopressin (AVP 5 microg/kg i.p.) considerably augmented ACTH and corticosterone secretion. L-arginine (120 and 300 mg/kg i.p.) did not significantly alter the AVP-induced secretion of those hormones. Nitric oxide synthase (NOS) blockers N(omega)-nitro-L-arginine (L-NNA) and its methyl ester (L-NAME) given i.p. 15 min before AVP markedly increased the AVP-induced ACTH secretion. L-NNA (2 mg/kg) more potently and significantly increased the AVP-induced ACTH secretion, whereas L-NAME elicited a weaker and not significant effect. Both those NOS antagonists intensified significantly and to a similar extent the AVP-induced corticosterone secretion. L-arginine (120 mg/kg i.p.) reversed the L-NNA-induced rise in the AVP-stimulated ACTH secretion and substantially diminished the accompanying corticosterone secretion. Neither vasopressin alone nor in combination with L-arginine and L-NAME evoked any significant alterations in the hypothalamic noradrenaline and dopamine levels. L-NNA (2 and 10 mg/kg i.p.) elicited a dose dependent and significant decrease in the hypothalamic noradrenaline level. The hypothalamic dopamine level was not significantly altered by any treatment. These results indicate that in conscious rats endogenous NO has an inhibitory influence on the AVP-induced increase in ACTH and corticosterone secretion. L-NNA is significantly more potent than L-NAME in increasing the AVP-induced ACTH secretion. This may be connected with a considerable increase by L-NNA of hypothalamic noradrenergic system activation which stimulates the pituitary-adrenal axis in addition to specific inhibition of NOS.     

Central catecholaminergic control of ACTH secretion

Plasma adrenocorticotropic hormone (ACTH) has been measured after an intra-third ventricular administration of noradrenaline, an adrenergic agonist or an adrenergic antagonist. Centrally administered noradrenaline caused a significant increase in ACTH secretion. The alpha-agonist phenylephrine also increased the ACTH level. However, neither the alpha-antagonist phentolamine nor beta-agonist isoproterenol affected the ACTH level. The beta-antagonist propranolol evoked a significant elevation in ACTH. Passive immunoneutralization was examined with anti-rat corticotropin-releasing factor (CRF) rabbit serum, anti-arginine vasopressin (AVP) rabbit serum and normal rabbit serum (NRS) on the intra-third ventricular noradrenaline-induced ACTH secretion to study the involvement of endogenous CRF. An intra-third ventricular administration of noradrenaline caused a significant increase of ACTH levels in NRS-injected rats and anti-AVP-injected rats, whereas an i.v. anti-rat CRF injection significantly reduced the intra-third ventricular noradrenaline-induced ACTH secretion. These results suggest that central catecholamine stimulated ACTH secretion via the alpha-adrenergic mechanism and that endogenous CRF is at least partly involved in the noradrenaline-induced ACTH secretion.     

Role of calcium ion in acth-induced steroidogenesis in humans

In order to examine the role of calcium ion in ACTH-induced steroidogenesis in humans, we carried out infusion of a pharmacological dose of ACTH (4.2 micrograms/kg) and a physiological dose of ACTH (0.0084 microgram/kg) for 120 min, and infusion of dibutyryl cyclic AMP (DBcAMP) [0.33 mg/kg/min] for 20 min, in 22 normal subjects with or without verapamil treatment (360 mg/day, orally) for 5 days. The subjects were pretreated with 1.0 mg of dexamethasone and 5.0 mg of enalapril daily for 2 days before each infusion test to inhibit endogenous ACTH and angiotensin II. Following infusion of 0.0084 microgram/kg of ACTH, plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01) were significantly increased; with chronic verapamil treatment plasma levels of corticosterone (P less than 0.05) and cortisol (P less than 0.02) were significantly lower than those without verapamil. On the other hand, 4.2 micrograms/kg of ACTH for 120 min significantly increased the plasma levels of several steroid hormones, although there were no differences between the infusion with and without verapamil. Infusion of DBcAMP for 20 min significantly increased plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01), but verapamil did not affect the steroidogenic response to the DBcAMP infusion. The present results suggest that steroidogenesis induced by a physiological dose of ACTH differs from that after a pharmacological dose of ACTH or DBcAMP, and is mediated mainly by calcium ion as an intracellular messenger in man.     

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.     

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.     

Role of prostaglandins and nitric oxide in the lipopolysaccharide-induced ACTH and corticosterone response.

This study was designed to determine the role of endogenous prostaglandins (PG) and nitric oxide (NO) in the lipopolysaccharide (LPS)-induced ACTH and corticosterone secretion in conscious rats. LPS (0.5 and 1 mg/kg) given i.p. stimulated the hypothalamic-pituitary-adrenocortical (HPA) activity measured 2 h later. A non-selective cyclooxygenase inhibitor indomethacin (10 mg/kg i.p.), piroxicam (2 mg/kg i.p.), a more potent antagonist of constitutive cyclooxygenase (COX-1) and compound NS-398 (2 mg/kg i.p.), a selective inhibitor of inducible cyclooxygenase (COX-2) given 30 min before LPS (1 mg/kg i.p.) significantly diminished both the LPS-induced ACTH and corticosterone secretion. COX-2 blocker was the most potent inhibitor of ACTH secretion (72.3%). Nomega-nitro-L-arginine methyl ester (L-NAME 2 and 10 mg/kg i.p.), a non-selective nitric oxide synthase (NOS) blocker given 15 min before LPS did not substantially alter plasma ACTH and corticosterone levels 2 h later. Aminoguanidine (AG 100 mg/kg i.p.), a selective inducible nitric oxide synthase (iNOS) inhibitor, considerably enhanced ACTH and corticosterone secretion induced by a lower dose (0.5 mg/kg) of LPS and did not significantly alter this secretion after a larger dose (1 mg/kg) of LPS. L-NAME did not markedly affect the indomethacin-induced inhibition of ACTH and corticosterone response. By contrast, aminoguanidine abolished the indomethacin-induced reduction of ACTH and corticosterone secretion after LPS. These results indicate an opposite action of PG generated by cyclooxygenase and NO synthesized by iNOS in the LPS-induced HPA-response.     

Role of brain adrenoceptors in the corticortopin-releasing factor-induced central activation of sympatho-adrenomedullary outflow in rats

We investigated the role played by catecholamine-dependent pathways in modulating the ability of centrally administered corticotropin releasing factor (CRF) to activate sympatho-adrenomedullay outflow, using urethane-anesthetized rats. The CRF (1.5 nmol/animal, i.c.v.)-induced elevations of both plasma noradrenaline and adrenaline were attenuated by phentolamine (a non-selective alpha adrenoceptor antagonist) [125 and 250 microg (0.33 and 0.66 micromol)/animal], Heat (a selective alpha(1) adrenoceptor antagonist) [10 and 30 microg (30 and 90 nmol)/animal, i.c.v.] and clonidine (a selective alpha(2) adrenoceptor agonist) [100 microg (0.375 micromol)/animal, i.c.v.]. On the other hand, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of both catecholamines was not influenced by RS 79948 (a selective alpha(2) adrenoceptor antagonist) [10 and 30 microg (7.2 and 72 nmol)/animal, i.c.v.]. Furthermore, the CRF (1.5 nmol/animal, i.c.v.)-induced elevation of noradrenaline was attenuated by sotalol (a non-selective beta adrenoceptor antagonist) [125 and 250 microg (0.4 and 0.8 micromol)/animal, i.c.v.], while that of adrenaline was not influenced by sotalol. These results suggest that centrally administered CRF-induced elevation of plasma noradrenaline is mediated by an activation of alpha(1) and beta adrenoceptors in the brain, and that of plasma adrenaline is mediated by an activation of alpha(1) adrenoceptors in the brain. Furthermore, central alpha(2) adrenoceptors are involved in modulating the CRF-induced elevation of both plasma catecholamines.     

Involvement of D1 dopamine receptors in the nicotine-induced noradrenaline release from hypothalamic and preoptic noradrenaline nerve terminal systems

Nicotine (4 × 2 mg/kg, i.p.) was given every 30 min for 2 h to male rats. Some rats were pretreated with the D1 dopamine (DA) receptor antagonist SCH 23390 (1 mg/kg, i.p.) or with the D2 DA receptor antagonist raclopride (1 mg/kg, i.p.), 5 min before nicotine treatment. Hypothalamic and preoptic catecholamine levels were measured by quantitative histofluorimetry in discrete DA and noradrenaline nerve terminal systems.Nicotine treatment produced a depletion of catecholamine stores in noradrenaline and DA nerve terminals of the hypothalamus, the preoptic area and the median eminence, an action which was counteracted by SCH 23390 but not by raclopride.The results indicate that hypothalamic D1 DA receptors may regulate the sensitivity of the nicotinic cholinoceptors and increase their ability to release hypothalamic noradrenaline. A possible role of D1 DA receptor antagonists to reduce the ability of nicotine treatment to produce rapid increases in LH, prolactin and corticosterone secretion and tonic arousal is implicated.     

Inhibitory effect of ouabain on epinephrine-induced stimulation of adrenal corticosterone secretion in rats.

Chronic administration of ouabain (3 mg/Kg body weight, subcutaneously, once daily for consecutive 15 days) definitely inhibited epinephrine-induced increase of adrenal corticosterone secretion. The inhibition rate increased along with frequency of ouabain administration. Increase in adrenal corticosterone synthesis and secretion by ACTH (20-50 mU/rat) administration was partially suppressed by pretreatment with chronic ouabain administration. A slight but significant increase of adrenal corticosterone secretion caused by epinephrine administration in hypophysectomized rats was also inhibited by pretreatment with ouabain administration. Chronic administration of neither phentolamine (1 mg/rat, intraperitoneally, once daily for consecutive 15 days) nor propranolol (3 mg/Kg body weight, subcutaneously, once daily for consecutive 15 days) caused significant changes in adrenal corticosterone secretion in response to ACTH as well as to epinephrine. Chronic administration of ouabain in rats causes not only elevated secretion of ACTH from anterior pituitary but also functional change in adrenals leading to suppression of corticosterone secretion in response to ACTH or epinephrine administration.     

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.     

Amphetamine-induced inhibition of central noradrenergic neurons: a pharmacological analysis

The amphetamine-induced inhibition of brain noradrenaline (NA) containing neurons in the rat locus coeruleus (LC) was pharmacologically analyzed utilizing single unit recording techniques. The presynaptic α-receptor blocking agent yohimbine (10 mg/kg i.p., 30 min before) largely prevented the amphetamine-induced depression of LC units in contrast to prazosin (0.6 mg/kg i.p., 30 min) or phenoxybenzamine (20 mg/kg, 30 min) which both slow preference for postsynaptic α-receptors. The β-receptor blocking agent, propranolol (10 mg/kg, 30 min), as well as the peripherally but not centrally active α-receptor blocking drug phentolamine (10 mg/kg, i.p., 30 min), also did not block the amphetamine effect. The LC inhibition by amphetamine was blocked by pretreatment with reserpine (10 mg/kg, i.p., 5 h), which caused almost total depletion of brain catecholamines. However, unlike the amphetamine-induced inhibition of central dopamine (DA) neurons the NA cell inhibition was not blocked by pretreatment with a tyrosine hydroxylase inhibitor (α-MT, 50 or 250 mg/kg i.p., 30 min). These results suggest that the amphetamine-induced inhibition of NA neurons in the LC is an indirect effect, mediated via activation of central α-receptors of presynaptic character. The lack of antagonism by α-MT indicate that the NA release by amphetamine, unlike its effect on brain DA, is not critically dependent on the rate of tyrosine hydroxylation. Thus the euphoriant action of amphetamine, which is blocked by α-MT, may be associated with release of DA rather than NA in brain.     

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.     

Response of the parotid gland of red kangaroos,Macropus rufus,to phenylephrine stimulation

Intracarotid infusions of l-phenylephrine at 1.0 or 10 nmol kg(-1) min(-1) were accompanied by increases in salivary amylase activity, protein, potassium, magnesium and chloride relative to cholinergically-stimulated saliva. Intravenous infusions of phenylephrine at the same dose rates had a lesser effect on salivary composition particularly protein. Propranolol administered with phenylephrine via the carotid artery, at an antagonist/agonist ratio of 10:1, was much more effective in blocking the phenylephrine-induced changes in salivary composition than equimolar infusion of phentolamine with phenylephrine. It was concluded that alpha(1)-adrenoreceptors were not present in functionally significant numbers in the gland and that the effect of phenylephrine on the kangaroo parotid was mediated by beta-adrenoreceptors. As the phenylephrine dose rates in the kangaroos were comparable with those used to determine alpha-adrenergic responses of eutherian salivary glands and as phentolamine appeared to have minor beta-sympathomimetic activity, at least one subtype of beta-adrenoreceptors in macropods may not be identical to its eutherian counterpart.     

Delta-opioid receptor agonist reduces severity of postresuscitation myocardial dysfunction

Postresuscitation myocardial dysfunction is recognized as a leading cause of early death after initially successful cardiopulmonary resuscitation (CPR). In the present study, we hypothesized that a delta-opioid receptor agonist would decrease the severity of postresuscitation myocardial dysfunction and improve survival. Fifteen Sprague-Dawley rats, fasted overnight with access to water, were anesthetized by an injection of 45 mg/kg ip pentobarbital sodium. Additional doses of 10 mg/kg were administered at hourly intervals but not within 30 min before induced ventricular fibrillation (VF). Either the delta-opioid receptor agonist pentazocine (300 microg/kg), pentazocine pretreated with the opioid receptor-blocking agent naloxone (1 mg/kg), or saline placebo was injected into the right atrium after 5 min of untreated VF and 3 min before initiation of CPR. After an additional 8 min of CPR administration, defibrillation was attempted. All animals were successfully resuscitated. Left ventricular rate of pressure increase at 40 mmHg and cardiac index values were significantly improved in pentazocine-treated animals, which also had significantly longer survival times (60 +/- 11 vs. 16 +/- 7 h; P < 0.01). Except for ease of defibrillation, the beneficial effects of pentazocine were completely abolished by pretreatment with naloxone. The concept of pharmacological hibernation employing a delta-opioid receptor agonist is a novel and promising intervention for minimizing global ischemic injury during CPR and postresuscitation myocardial dysfunction.     

Tracheal mucosal blood flow responses to autonomic agonists

In lightly anesthetized adult sheep, we determined tracheal mucosal blood flow (Qtr) by measuring the steady-state uptake of dimethyl ether from a tracheal chamber created by an endotracheal tube provided with two cuffs. Qtr normalized for carotid arterial pressure [Qtr(n)] was determined before and after the exposure of the tracheal mucosa to aerosolized phenylephrine (0.25-2.0 mg), isoproterenol (0.05-0.8 mg), and methacholine (2.5-20 mg). The same doses of methacholine were also administered during the intravenous infusion of vasopressin. The measurements were repeated after intravenous pretreatment with the respective antagonists phentolamine, propranolol, and atropine. Mean +/- SE base-line Qtr(n) was 1.2 +/- 0.1 ml.min-1.mmHg-1.10(2). The autonomic antagonists had no effect on mean Qtr(n). Phenylephrine produced a dose-dependent decrease in mean Qtr(n) (-70% at the highest dose), which was blunted by phentolamine, and isoproterenol produced a dose-dependent increase in mean Qtr(n) (40% at the highest dose), which was blocked by propranolol. Methacholine failed to alter mean Qtr(n) even when Qtr was first decreased by vasopressin. We conclude that in lightly anesthetized adult sheep 1) base-line Qtr(n) is not under adrenergic or cholinergic control, 2) a locally administered alpha-adrenergic agonist decreases and beta-adrenergic agonist increases Qtr(n) via specific receptor activation, and 3) a locally administered cholinergic muscarinic agonist has no effect on Qtr(n).