The role of brain acetylcholine in the pressor response to centrally infected neostigmine.

In unanaesthetized rats, neostigmine (2.5 mug) injected into the cerebral ventricles produces a rise in arterial blood pressure. The effect on the rise was examined after lowering the acetylcholine (ACh) level of the brain by hemicholinium (HC-3) similarly injected.The pressor response was not significantly altered 2 h after injection of 50 mug HC-3 when the ACh level was reduced by 54%. It was greatly attenuated 2 h after injection of 75 mug when the ACh level was reduced by 76%. However, a similar attenuation was observed as early as 30 min after this injection when the ACh level was reduced by 18% only.It is concluded that the pressor response is not mediated by undestroyed ACh but is a direct central effect of neostigmine.