Projections of excitatory and inhibitory motor neurones to the circular and longitudinal muscle of the guinea pig colon

The aim of this study was to identify myenteric pathways to the circular an d longitudinal muscle of the guinea pig proximal colon. To identify excitat ory and inhibitory muscle motoneurones, we applied the neuronal retrograde tracer DiI onto the circular or longitudinal muscle layer and performed add itional immunohistochemistry for nitric oxide synthase (NOS) and choline ac etyltransferase (ChAT). On average 166 +/- 81 circular muscle motoneurones (CMMN) and 100 +/- 74 longitudinal muscle motoneurones (LMMN) were labelled by DiI tracing. Myenteric pathways innervating the muscle were either asce nding (DiI-labelled neurones with oral projections) or descending (DiI-labe lled neurones with anal projections). The circular muscle was preferentiall y innervated by ascending pathways (66.0 +/-9.1%). Most ascending CMMN were ChAT-positive (87.2 +/-8.5%), whereas descending CMMN were mainly NOS-posi tive (82.3 +/- 14.6%). Most ascending (62.2 +/- 11.1%) and descending (82.0 +/- 12.5%) CMMN had circumferential projection preferences (circumferentia l projections were longer than projections along the longitudinal gut axis) . In contrast to the polarised projections to the circular muscle, the long itudinal muscle was equally innervated by ascending (46.2 +/- 15.1%) and de scending (53.9 +/- 15.1%) neurones. Ascending and descending pathways to th e longitudinal muscle consisted predominantly of ChAT-positive neurones (98 .1 +/-1.9% and 68.0 +/-8.5%, respectively), and both pathways had prominent longitudinal projection preferences. Only 25.5% of the descending LMMN wer e NOS-positive. In conclusion, the circular muscle in the proximal colon is innervated by descending inhibitory (NOS-positive neurones) and ascending excitatory (ChAT-positive neurones) pathways. In contrast, the longitudinal muscle is primarily innervated by ascending and descending excitatory moto neurones, and only a small proportion of the descending pathway consisted o f inhibitory motoneurones.