Antagonism of the antidiarrhoeal effect of clonidine and the lethal effect of noradrenaline in rats: a reliable procedure to evaluate the in-vivo alpha 1- and alpha 2-blocking activity of drugs?

Stimulation of faecal excretion in rats by alpha 2-adrenergic antagonists

The effects of several alpha-adrenoceptor antagonists on faecal output and water content in rats were investigated. Fed rats were treated either subcutaneously (s.c.) or orally with phentolamine, idazoxan, yohimbine, 1-(2-pyrimidinyl) piperazine (PmP) or prazosin. Drug potencies were compared on the basis of the dose inducing excretion of 1 g dry weight of faeces (AD1) by rats that do not normally excrete any faecal pellet during the observation time. The alpha 2-antagonist, idazoxan (AD1 = 0.25 mg kg-1, s.c.) was approximately 2.5, 4 and 8 times more potent than PmP, phentolamine and yohimbine in promoting faecal excretion. Prazosin, an alpha 1-antagonist with putative affinity for the alpha 2B-receptor subtype, was the least effective (AD1 greater than 5 mg kg-1, s.c.). The same compounds also increased the water content of faeces and had similar potencies by the oral route. Both clonidine (0.15 mg kg-1, s.c.) and atropine (0.2 mg kg-1, s.c.) significantly prevented the effects of all antagonists on faecal excretion. The present results are consistent with the view that rat colon is under tonic inhibitory control of prejunctional alpha 2-adrenergic receptors, whose blockage by specific antagonists induces faecal excretion. The alpha 2A-receptor subtype appears to be the most likely candidate for controlling faecal excretion through inhibition of acetylcholine release.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (2). Alpha-adrenoceptor agonists

Eighteen chemically dissimilar alpha-adrenoceptor agonists were assessed in 6 different tests. The antidiarrheal activity of the compounds was highly correlated with the diuretic activity. Both activities were highly correlated with the inhibition of [3H]clonidine or [3H]idazoxan binding; the degree of correlation with inhibition of [3H]WB4101 binding or with antiptotic activity was much less. Antiptotic activity was better correlated, although still poorly, with inhibition of [3H]WB4101 binding than with inhibition of [3H]clonidine or [3H]idazoxan binding. The in vivo antiptotic/diuretic or antiptotic/antidiarrheal potency ratios both reflected the known alpha 2/alpha 1-selectivity of the compounds tested and were significantly correlated with the in vitro potency ratios between inhibition of [3H]WB4101 binding on one hand and of [3H]clonidine or [3H]idazoxan binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor agonistic activity and selectivity is discussed.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (1). Alpha-adrenoceptor antagonists

Twenty one chemically dissimilar alpha-adrenoceptor antagonists were assessed in 7 different tests. The potencies of the compounds to protect rats from norepinephrine lethality were closely correlated with their potencies for inhibition of [3H]WB4101, but not of [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding to rat brain homogenates. The reverse was found for the potencies of the compounds to prevent the antidiarrheal effect of clonidine. The in vivo anti-norepinephrine/anti-clonidine potency ratios were highly correlated with the ratios between the in vitro potency to inhibit [3H]WB4101 binding on one hand and [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor antagonistic activity and selectivity is discussed.

Yohimbine attenuates the delayed lethality induced in mice by amitraz, a formamidine pesticide

We have found that a single dose of amitraz, a formamidine pesticide, produces death in mice 2-5 days after dosing. To further examine this phenomenon, adult albino mice of both sexes were treated with either yohimbine (10 mg/kg, i.p.) or deionized water (6 ml/kg, i.p.), immediately before an injection of amitraz (600 mg/kg, i.p.) and twice daily thereafter for 8 days. Male mice treated with water were more susceptible than water-treated females to the lethal effects of amitraz. In addition, yohimbine treatment significantly decreased the number of deaths in both sexes. These data suggest that an alpha 2-adrenergic mechanism is involved in the delayed lethality produced by amitraz.