[Effect of intrathecal injection of acetylcholine on phrenic nerve firing activity in rabbits].

Experiments were performed on 50 anesthetized, immobilized and vagotomized rabbits to investigate how acetylcholine (ACh) regulates respiration. The phrenic nerve impulses were led by a bipolar silver electrode and recorded in a X-Y plotter. The results are as follows: 1. The inspiratory firing activity of phrenic nerve following intrathecal injection of ACh was increased by 30.6 +/- 11.6% (P less than 0.01). 2. The excitatory action of ACh on phrenic motoneurons had no correlation with blood pressure but was associated with the direct action of ACh on phrenic motoneurons. 3. The excitatory effect of ACh on phrenic nucleus could be blocked by atropine, but not by hexamethonium, phentolamine or propranolol. 4. The electric activities of phrenic nerve were not altered by intrathecal injection of any of the four receptor-blockers mentioned above. The results suggest that the phrenic nucleus could be excited by intrathecal injection of ACh and this excitatory effect is mediated by M-receptors. It is assumed that the cholinergic system does not show any tonic excitatory action on the phrenic nucleus under normal physiological conditions. Thus it seems that cholinergic transmitter may participate in the integration of signals for respiratory drive at the level of spinal cord.