alpha 1/alpha 2 Selectivity ratio in a series of agonists and their relation to pre/postsynaptic activity ratios

Intrathecal ketorolac enhances antinociception from clonidine

Although both alpha2-adrenergic agonists and cyclooxygenase inhibitors produce analgesia, their exact sites of action and interaction remain unclear. A previous report demonstrated a surprising inhibition of antinociception in rats from intrathecal clonidine by co-administered ketorolac. There are no other reports of interaction between these two classes of analgesics. We therefore reexamined this interaction, determining the effect of intrathecal clonidine and ketorolac alone and in combination in normal rats. Clonidine, but not ketorolac, produced antinociception to noxious hind paw thermal stimulation. The addition of ketorolac significantly enhanced the effect of clonidine, indicating a synergistic interaction for analgesia. Although the reasons for the discrepancy between this and the previous report are unclear, these results are consistent with previous studies that indicate an antinociceptive action of intrathecal alpha2-adrenergic agonists in the normal condition, a lack of such effect for cyclooxygenase inhibitors, and positive reinforcing effects of these two systems when co-stimulated.

IMPLICATIONS

Spinal injection of the alpha2-adrenergic agonist clonidine and the cyclooxygenase inhibitor ketorolac results in a synergistic interaction for antinociception in normal animals, suggesting that the combination of these drugs will enhance rather than detract from the analgesia of either alone.

Pharmacological analysis of vasodilator responses to alpha 2-adrenoceptor agonists in isolated rat common carotid arteries

Using the cannula inserting method, vasodilator responses to alpha 2-adrenoceptor agonists (clonidine, guanabenz, DJ7141 and xylazine) were investigated in isolated and perfused rat common carotid arteries. Alpha 2-adrenoceptor agonists dose-dependently induced a vasodilation in preparations preconstricted by noradrenaline. The potencies were in the order of clonidine greater than guanabenz greater than DJ-7141 greater than or equal to xylazine. Removal of the endothelium inhibited ACh-induced vasodilation, but not the alpha 2-agonist-induced dilation. Atropine treatment inhibited ACh-induced vasodilation, but not the alpha 2-agonist-induced dilation. Alpha 2-agonist-induced dilations were not modified by beta-blockade, which significantly suppressed isoprenaline-induced vasodilations. The potent alpha 2-adrenoceptor antagonist DG5128 did not influence the alpha 2-agonist-induced vasodilation. In preparations preconstricted by PGF2 alpha, clonidine and xylazine never induced a vasodilation, and clonidine frequently induced vasoconstrictions that were completely blocked by bunazosin. It is concluded that alpha 2-adrenoceptor agonist-induced vasodilation is independent from the existence of the endothelium, and that it is not related to vascular beta- and alpha 2-adrenoceptors and muscarinic receptors, suggesting that the alpha 2-adrenoceptor agonist-induced vasodilation is due to an antagonistic activity towards the vascular alpha 1-adrenoceptors.

Differential alpha-1 and alpha-2 adrenergic effects on hypothalamic corticotropin-releasing factor and plasma adrenocorticotropin

There is presently no consensus as to the nature of the catecholaminergic influence on the regulation of corticotropin-releasing factor. The potential role that the alpha-adrenergic system plays was investigated by measuring hypothalamic corticotropin-releasing factor-like immunoreactivity and plasma adrenocorticotropin, following manipulation of alpha-1 and alpha-2 adrenergic receptor activation. Administration of the alpha-1 agonist methoxamine did not significantly alter either plasma adrenocorticotropin or hypothalamic corticotropin-releasing factor. Administration of the alpha-2 agonist clonidine resulted in a 24-fold increase in plasma adrenocorticotropin and a significant decrease in median eminence corticotropin-releasing factor, consistent with its release. Corticotropin-releasing factor in the remainder of the hypothalamus was not altered. Concurrent administration of clonidine with the selective alpha-2 antagonist yohimbine prevented the clonidine-induced changes in plasma adrenocorticotropin and hypothalamic corticotropin-releasing factor, consistent with the clonidine effect being mediated through alpha-2 receptors. Concurrent administration of clonidine with methoxamine did not prevent these effects, suggesting that the effect of clonidine was not mediated through presynaptic inhibition of noradrenergic adrenergic neurotransmission. Inhibition of protein synthesis by anisomycin induced changes in corticotropin-releasing factor and adrenocorticotropin which were not altered by combined treatment with methoxamine or clonidine. These data suggest differential roles for alpha-1 and alpha-2 systems in the regulation of corticotropin-releasing factor. Results from alpha-2 adrenergic activation were consistent with stimulation of corticotropin-releasing factor release, an effect mediated by a postsynaptic alpha-2 mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic treatment with antidepressant drugs and ECT differentially modifies the hypothermic action of clonidine and guanfacine

The hypothermia inducing action of clonidine and guanfacine was abolished by yohimbine and idazoxan pretreatment which suggests an alpha 2-adrenoceptor involvement in this effect. The effects of acute and chronic treatment with the antidepressant drugs desipramine (DMI), amitriptyline (AMI), maprotiline (MAP), mianserin (MIAN), iprindol (IPR), alaproclate (ALA) and electroconvulsive treatment (ECT) on the hypothermic action of the alpha 2-adrenoceptor agonists clonidine and guanfacine were studied. Acute administration of MIAN potentiated clonidine induced hypothermia whereas acute MIAN, IPR and ALA potentiated guanfacine induced hypothermia. Repetitive DMI, AMI and MAP treatment attenuated clonidine-induced hypothermia whereas guanfacine-induced hypothermia was potentiated by chronic treatment with DMI, AMI, MAP and MIAN, ECT applied without anaesthesia attenuated both clonidine and guanfacine hypothermia, however, under ethyl ether anaesthesia ECT was effective only towards guanfacine hypothermia. This discrepancy is discussed in terms of the relative selectivity of the agonists used, the reliability of agonist studies for indexing receptor function, and possible pharmacokinetic interaction.

Baroreceptor function in man following peripheral alpha 1- and alpha 2-adrenoceptor stimulation

Methoxamine and alpha-methyl-noradrenaline were administered to six healthy male subjects on separate days as rapid bolus injections until blood pressure increased by approximately 30 mmHg; Valsalva's Manoeuvre was carried out on each occasion. Propranolol (80 mg) or placebo was administered (random order, double-blind, weekly intervals) and the observations were repeated after 2 h. Baroreceptor sensitivity (delta R-R interval ms/mmHg systolic BP) was less (p less than 0.05) with alpha-methyl-noradrenaline than methoxamine. Propranolol abolished the differences in baroreceptor-mediated bradycardia following alpha-methyl-noradrenaline and methoxamine, and shifted the baroreceptor sensitivity regression lines (p less than 0.05) to the left. During the release phase of Valsalva's Manoeuvre baroreceptor sensitivity was increased following propranolol. The smaller baroreceptor-mediated bradycardia response observed with alpha-methyl-noradrenaline does not support the hypothesis that pre-synaptic alpha-adrenoreceptors have a physiological role in the modulation of baroreceptor function in man, and may be due to alpha-methyl-noradrenaline having beta 1-agonist activity.

Animal pharmacology of guanfacine

The pharmacologic data obtained from animal experiments with guanfacine, a novel, centrally acting antihypertensive agent, are reviewed. When given orally, guanfacine lowers systemic blood pressure in conscious DOCA-NaCl-hypertensive rats, Grollman rats and spontaneously hypertensive rats in a dose-dependent manner. It is also effective in renal hypertensive cats. Guanfacine reduces blood pressure in cats, rabbits and rats after injection into the lateral cerebral ventricle and in dogs after infusion into the vertebral artery at intravenously ineffective doses. Vagally mediated reflex bradycardia in dogs is enhanced. The preganglionic splanchnic (sympathetic) nerve activity is reduced in cats. In rats, guanfacine reduces the noradrenaline turnover in the brain stem. All these findings indicate a central site of action. Peripheral alpha-adrenoceptor stimulant properties of guanfacine have been demonstrated in various studies. In addition to postsynaptic stimulant effects, presynaptic guanfacine-induced inhibition of sympathetic heart nerve stimulation is antagonized by rauwolscine but not by prazosin, indicating a highly preferential alpha 2-agonistic presynaptic action of the drug. In receptor binding studies using rat cortex membranes and human platelets, guanfacine exhibited a high selectivity for alpha 2 adrenoceptors. Guanfacine has the advantage over other centrally acting antihypertensives of being less sedative and causing no rebound hypertension after discontinuation of treatment. The latter is mainly due to its pharmaco-kinetic properties.

Effects of peripheral alpha 1- and alpha 2-adrenoceptor agonists on baroreceptor responsiveness in conscious dogs

Baroreceptor responsiveness was investigated in conscious dogs following increasing doses (i.v.) of the selective alpha-adrenoceptor agonists methoxamine (alpha 1) and oxymetazoline (alpha 2), in the presence and absence of beta-adrenoceptor blockade. The study was repeated in another group of dogs with background afferent baroreceptor nerve activity reduced by continuous infusion of sodium nitroprusside. Both agonists dose dependently increased mean arterial pressure and reflexly decreased heart rate. In dogs pretreated with a beta-adrenoceptor antagonist a correlation between increase in mean arterial pressure (increase up to 70 mmHg) and increase in heart period (baroreceptor responsiveness) indicated no difference in the regression lines between methoxamine and oxymetazoline for both the normotensive and the sodium nitroprusside groups. However, in the dogs not pretreated with a beta-adrenoceptor antagonist the slope of the regression line for oxymetazoline was steeper than that for methoxamine (P less than 0.01) in the normotensive group. In the sodium nitroprusside group the regression line for oxymetazoline was situated significantly to the left of the methoxamine line (P less than 0.05). It is suggested that this greater bradycardic response to the alpha 2-adrenoceptor agonist oxymetazoline was caused by suppression of the cardiac sympathetic component (presynaptic modulation of noradrenaline release) in addition to the vagal activation and the sympathetic withdrawal component of the reflex. This indicates that drugs with alpha 2-adrenoceptor agonistic activity can influence a reflex physiological situation under conditions of low sympathetic nerve activity.