Effect of naloxone on central adrenergic stimulation of corticosterone secretion

The interaction of central opioid with adrenergic system in stimulation of the hypothalamic-pituitary-adrenal (HPA) axis was investigated in conscious rats. All the tested drugs were administered intracerebroventricularly (i.c.v.), naloxone 15 min before the adrenergic agonists. Phenylephrine (30 micrograms), clonidine (10 micrograms) and isoprenaline (10 micrograms), the alpha 1-, alpha 2- and beta-adrenergic agonists as well as noradrenaline (10 micrograms) and adrenaline (10 micrograms) significantly increased the serum corticosterone levels, measured 1 h after drug administration. Naloxone (0.01-1 microgram) did not markedly influence the corticosterone response to phenylephrine and isoprenaline and diminished by a half the response to clonidine. Naloxone also significantly decreased the corticosterone response to noradrenaline but did not substantially alter the response to adrenaline. Noradrenaline potently stimulates the HPA axis via alpha 2-adrenoceptors, whereas adrenaline involves mainly alpha 1- and beta-adrenergic receptors. These results indicate that central opioid system is significantly involved in mediating the pituitary-adrenocortical response to clonidine and noradrenaline but not phenylephrine, isoprenaline and adrenaline. This suggests an interaction of opioids at central alpha 2- but not alpha 1-receptor sites.

Effect of brain mast cells degranulation on the corticosterone response to stimulation of central opioid receptors in rats

The influence of brain mast cells degranulation, exerted by compound 48/80 given intracerebroventricularly, on the opioid-induced stimulation of the pituitary-adrenal axis, measured indirectly through corticosterone secretion, was investigated in conscious rats. The mu- and delta-opioid receptor agonists Leu-enkephalinamide, morphine and beta-endorphin, given intracerebroventricularly, dose-dependently increased the serum corticosterone levels. The effect of Leu-enkephalinamide was not changed by pretreatment with the histamine H1- and H2-receptor antagonists mepyramine and cimetidine. When Leu-enkephalinamide, morphine and beta-endorphin were given 3 hr after compound 48/80, the serum corticosterone levels were higher than after either one of these drugs given separately, which suggests an independent mechanism of action. In rats pretreated 24 hr earlier with compound 48/80, i.e., when brain mast cells were completely degranulated, the stimulating effect of morphine was almost abolished and the effects of Leu-enkephalinamide and beta-endorphin were considerably reduced. Since the histamine levels in the whole brain and thalamus were elevated 3 and 24 hr after administration of compound 48/80, these results suggest that factors other than histamine depletion from brain mast cells by compound 48/80 may be responsible for the dramatic impairment of the stimulating effect of morphine, Leu-enkephalinamide and beta-endorphin on the pituitary-adrenal axis.

The intracerebroventricularly administered mast cells degranulator compound 48/80 increases the pituitary-adrenocortical activity in rats

The effect of brain mast cells degranulation by compound 48/80 on the pituitary-adrenocortical activity, measured indirectly through corticosterone secretion, and the involvement of a histaminergic mechanism in that stimulation was investigated in conscious rats. All the drugs were given intracerebroventricularly (icv), histamine antagonists 15 min prior to compound 48/80. Compound 48/80 induced a significant dose- and time-related increase in the serum corticosterone levels. That increase, measured 1 h after administration of compound 48/80, was moderately diminished by icv pretreatment of rats with mepyramine and cimetidine, histamine H1- and H2-receptor antagonists. Three hours after administration of compound 48/80 mast cells of the thalamus and the hypothalamus were completely degranulated. At the same time the thalamus and the whole brain histamine levels were substantially higher than in the saline-treated control rats. The above results suggest that histamine liberated from the brain mast cells and central histamine receptors play a moderate role in increasing the pituitary-adrenocortical activity by compound 48/80.

Catecholaminergic regulation of the hypothalamic-pituitary-adrenocortical activity

The significance and site of adrenergic receptors involved in the control of the hypothalamic-pituitary-adrenal axis (HPA) activity was assessed indirectly by estimation of serum corticosterone levels 1 h after drug administration to conscious rats. Adrenergic drugs were given intracerebroventricularly (icv) and intraperitoneally (ip), the antagonists 15 min prior to the agonists. Noradrenaline, adrenalin and isoproterenol given by either route increased dose-dependently the serum corticosterone levels. The corticosterone response to icv noradrenaline was almost abolished by icv pretreatment with propranolol, a beta-adrenergic antagonist, and yohimbine, and alpha 2-receptor blocker, and was also considerably reduced by prazosin, an alpha 1-adrenergic antagonist. When given ip, these antagonists did not significantly influence the noradrenaline induced corticosterone response, which suggests a suprapituitary site of action of noradrenaline in stimulation of the HPA. The corticosterone response to icv adrenalin was suppressed by prazosin given by either route. The corticosterone response to ip adrenalin was almost abolished by pretreatment with yohimbine, and also significantly diminished by propranolol given by the same route. The increase in corticosterone secretion, induced by isoproterenol given by either route, was abolished by ip injection of propranolol. These results indicate that noradrenaline stimulates the HPA via alpha- and beta-adrenergic receptors, mainly at the suprapituitary level. Adrenalin increases that activity both via central and pituitary alpha- and beta-adrenoceptors. Isoproterenol activates the HPA by stimulation of pituitary beta-receptors.

The influence of alpha-fluoromethylhistidine and histamine receptor antagonists on the beta-endorphin-induced corticosterone response

Involvement of a central histaminergic mechanism in the stimulating effect of beta-endorphin (beta-End) on the pituitary-adrenocortical activity, measured indirectly through corticosterone secretion, was investigated in conscious rats. The rise in serum corticosterone levels, induced by beta-End injected intraventricularly (icv) was considerably impaired by pretreatment with naltrexone, an opioid receptor antagonist. The stimulating effect of beta-End was almost totally suppressed by a prior icv administration of mepyramine, a histamine H1-receptor antagonist, and also considerably reduced by pretreatment with cimetidine, an H2-receptor antagonist. The strongest suppression, by 83%; of the beta-End-induced corticosterone response was evoked by a prior administration of alpha-fluoromethylhistidine, an inhibitor of neuronal histamine synthesis in the brain. These results indicate that both the brain neuronal histamine and central histamine H1- and H2-receptors are considerably involved in the beta-endorphin-induced stimulation of the pituitary-adrenocortical activity.

Effects of systemic and intracerebroventricular phenylephrine and clonidine on corticosterone secretion in rats

A site of action of adrenergic drugs and type of receptors involved in stimulating the pituitary-adrenocortical activity, assessed indirectly through corticosterone (CS) secretion, was investigated in conscious rats. Adrenergic drugs were administered intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.), the antagonists always 15 min prior to the agonist and 1 h later the trunk blood was collected for CS determination. Phenylephrine (PHE) (alpha 1-agonist) given by either route increased dose-dependently the serum CS levels. Such effect of PHE given i.p. was abolished by i.p. pretreatment with prazosin (PRA) (alpha 1-antagonist), and similar effect of i.c.v. administered PHE was considerably suppressed by i.c.v. pretreatment with PRA. I.c.v. pretreatment with yohimbine (YO) (alpha 2-antagonist), did not influence the effect of PHE. Clonidine (CLON) given i.c.v. significantly increased the serum CS level and such effect was halved by i.c.v. pretreatment with either YO and PRA. I.p. administered PRA did not decrease the CLON-induced CS response. YO given i.p. considerably reduced the CLON-induced CS response in both low and high doses thus indicating the interaction with pre- and post-synaptic alpha-adrenoceptors. These results suggest that PHE activated the pituitary-adrenocortical axis by a selective stimulation of alpha 1-adrenoceptors located in the hypothalamus and possibly on anterior pituitary corticotrophs. CLON seems to stimulate the secretion of CS by activating both alpha 2- and alpha 1-adrenoceptors in the hypothalamus.