A further study of the cardiovascular responses to central administration of acetylcholine in rats.

General characteristics and the nature of cardiovascular responses to acetylcholine (ACh) injected into the lateral cerebral ventricle (LCV), the 4th cerebral ventricle (4th CV) and the cisterna magna (CM) were studied. Acetylcholine, depending on the route of injection, the dose and the peripheral vascular tone, produced both increases and decreases in blood pressure and heart rate. After injection of central anticholinesterase agents (physostigmine, neostigmine, armin), the pressor response to ACh was reversed to a depressor response, with or without a secondary pressor response, accompanied by a bradycardia. The vascular effects were selectively abolished by central administration of atropine, indicating that they are mediated via muscarinic cholinoceptors. It is concluded that cardiovascular responses to ACh can be initiated at a number of cardiovascular loci situated at various levels of the rat brain. It seems that both muscarinic cholinoceptors mediating pressor responses and those mediating depressor responses exist in the cardiovascular loci of all these brain levels, but not in an equal proportion. The hypothesis is proposed that the muscarinic cholinoceptors responsible for pressor effects are more sensitive to ACh and functionally probably more competent than muscarinic cholinoceptors responsible for depressor effects, the latter being in a markedly greater number. The present study provides additional support for the idea that ACh participates in the physiological control of the rat cardiovascular system, as both a central cholinergic excitatory and inhibitory regulator.