Central cardiovascular responses of histamine and homodimaprit in normal and hypophysectomized rats.

Central administration of histamine in conscious rats produced half the pressor response in hypophysectomized rats compared to normal animals. The pressor response was blocked completely by hexamethonium in hypophysectomized rats but not at all in normal rats. Homodimaprit given centrally, gave pressor effects of equal magnitude in both groups. In normal rats, it caused significantly higher blood blood pressure following ganglionic blockade, whereas in hypophysectomized rats hexamethonium did not block the response to homodimaprit. These results, plus previous findings, show that histamine, given centrally, raises blood pressure in normal conscious animals by releasing vasoactive substances from the brain. In hypophysectomized rats it stimulates sympathetic output. Homodimaprit, on the other hand, produces its pressor action by the release of vasoactive substances from the brain both in normal and hypophysectomized animals. A hypothesis is proposed to suggest that parallel neuronal processing occurs in the brain when histamine is given.