Evidence for histaminergic nerves in the pyramidal cat

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Abstract
The tail of the pyramidal cat was perfused to allow a study ofthe effect of drugs on the changes in resistance to perfusion resulting from direct stimulation of the sympathetic trunk or during reflex or direct stimulation of the carotid sinus nerve. Evidence of histaminergic activity during the various procedures was based on results showing that isoniazid, a histaminase inhibitor, produced an intensification of the dilator responses, and a histamine liberator, 48/80, reduced the evoked dilator responses and finally, blockade of the direct or reflexly induced vasodilatation by antihistaminics. Reflex activation of the carotid sinus or other baroreceptors by the injection of norepinephrine resulted in a depressor response in the perfused tail which was potentiated by anticholinesterases and blocked by antihistaminics. The results suggest that histaminergic nerves can be activated by direct stimulation of the sympathetic trunk and that one physiological stimulus for the activation may be the baroreceptor depressor reflex.