Identification of motor neurons to the longitudinal muscle of the guinea pig ileum

Motor neurons that innervate the longitudinal muscle of the guinea pig ileum were identified by retrograde transport from the longitudinal muscle plexus in organotypic culture. Motor neurons had short projections, <3.5 mm long, and never had Dogiel type II morphology; most labeled neurons had morphological characteristics of Dogiel type I neurons. Immunoreactivity for choline acetyltransferase was present in 97% of retrogradely labeled nerve cell bodies, reflecting the dominant cholinergic input to the longitudinal muscle layer. Substance P immunoreactivity was present in 48% of motor neurons, indicating that it or a similar tachykinin that mediates noncholinergic excitatory transmission is likely to be released by a subset of cholinergic motor neurons. This strongly suggests that the difference in frequency dependence of substance P and acetylcholine release is attributable to different release mechanisms rather than to activation of separate populations of motor neurons. Immunoreactivity for the calcium-binding protein calretinin was present in 87% of longitudinal muscle motor neurons. The neurochemical coding of longitudinal muscle motor neurons indicated that they constitute about one quarter of all myenteric neurons and are distinct from circular muscle motor neurons.