Effect of histamine and antihistaminics on central vasomotor loci

Abstract Histamine (50–500 μg) injected into the cerebral ventricles (ICV) produced a pressor response in cats, which was abolished by spinal section and blocked by small doses of mepyramine (100 μg ICV) and tripelennamine (100 μg ICV). Central administration of mepyramine alone produced a depressor response and blocked both reflex as well as direct excitability of the medullary vasomotor centre. On the other hand ICV tripelennamine either had no effect on the blood pressure or elicited a mild pressor response and facilitated the pressor responses to carotid occulsion as well as electrically evoked medullary responses. These results suggest the presence of histaminergic receptors in the central vasomotor loci. However, mepyramine and tripelennamine had divergent effects unrelated to their antihistaminic activity.