No evidence for central histamine-receptor involvement with the hypotensive effect of clonidine in cats.

In conscious hypertensive cats, intraventricular (i.c.v.) administration of clonidine (25 mug), induced hypotension and bradycardia. Pretreatment with metiamide (2 mg i.c.v.) did not significantly antagonise either the hypotension or bradycardia induced by clonidine (25 mug), but induced marked behavioural changes. Central pretreatment with mepyramine (200 mug, i.c.v.) or procaine (600 mug i.c.v.), reduced the hypotension evoked by clonidine (25 mug), but no antagonism of the clonidine-induced bradycardia was apparent. Central phentolamine (200 mug, i.c.v.) or tolazoline (200 mug, i.c.v.) antagonised the hypotension and bradycardia evoked by i.c.v. clonidine.