Electrophysiologic identification of preganglionic neurons in rat dorsal motor nucleus and analysis of vagus afferent projections.

Electrophysiological studies on preganglionic neurons (PGNs) in the dorsal motor nucleus (DMN) of the vagus nerve has been hampered by technical limitations. Conventional electrical stimulation of the vagus nerve with cathodal square-wave pulses activates both preganglionic and afferent fibers. Thus, some PGNs cannot be identified because the anticipated antidromic responses would be blocked due to collision with those orthodromic responses evoked with shorter latencies by activation of fast-conducting afferent fibers. Projections of vagus afferent fibers to PGNs are difficult to analyse with conventional methods because a preceding antidromic response may affect an orthodromic response which has a slightly longer latency. A new stimulation method was designed, consisting of anodal triangular pulse stimulation and spontaneous-spike triggered stimulation. In chlorase-urethane-anesthetized rats, unitary responses of the DMN to electrical stimulation of the ipsilateral vagus nerve were recorded. When only an orthodromic response by a DMN neuron was recorded with conventional stimulation, application of anodal triangular pulse stimulation revealed an antidromic response, so that the cell in question could be identified as a PGN. Some neurons that produced only an antidromic response to conventional stimulation, revealed an orthodromic response on spontaneous-spike triggered stimulation, which blocked the antidromic response due to collision. With these procedures, orthodromic responses due to vagus afferent projections were recorded in 35% of the identified PGNs, mostly due to C and partly, A fiber activations. All these projections were polysynaptic in nature. In conclusion, one-third of the PGNs of the DMN are involved in vagovagal reflexes, which occur through multisynaptic pathways.