A cholinergic mechanism involved in the neuronal excitation by H+ in the respiratory chemosensitive structures of the ventral medulla oblongata of rats in vitro.

The mechanism of neuronal excitation by H+ in the medullary chemosensitive structures was analyzed in brains slices of the rat in vitro. Responses of neurons to H+ in the ventral surface layer were compared with responses to various transmitter substances. Neurons excited by H+ were always also excited by acetylcholine (ACH). ACh increased the activity of 70% of superficial ventral medullary neurons. Effects of noradrenaline and serotonin on the activity of neurons were largely opposite to that of H+. Cholinergic blocking agents like atropine, hexamethonium and mecamylamine depressed the H+-elicited excitation of neurons. The cholinesterase inhibitor, eserine, increased the neuronal activity. In the presence of eserine, a solution of low pH caused further increase in discharge of most neurons. The low pH solution prolonged and augmented the excitatory action of ACh on the ventral medullary neurons. It is concluded that the H+-elicited excitation of neurons in the "chemosensitive" structures is dependent upon intact cholinergic transmission in the surface layer. This may be interpreted as resulting from facilitation and/or prolongation of such a chemical transmission by H+.