The chemical coding and projections of neurons containing neuropeptide Y (NPY) have been investigated in the myenteric plexus of the guinea-pig small intestine. Chemical coding was determined by investigating the colocalization of NPY immunoreactivity with the immunoreactivities for bombesin (BN), 5-HT, nitric oxide synthase (NOS), and somatostatin. Projections were determined by studying the consequences of nerve lesions created by myectomy and myotomy operations. NPY immunoreactivity was localized in four classes of myenteric neuron, anally projecting interneurons, neurons that projected anally and to the circular muscle, neurons projecting to the longitudinal muscle and in a small population of secretomotor neurons that projected to the mucosa. The interneurons and muscle motor neurons both had Dogiel type I morphology, whereas the secretomotor neurons had fine branching processes. Of the NPY-immunoreactive Dogiel type I neurons, 98% were also immunoreactive for NOS; conversely, 82% of NOS-immunoreactive neurons were immunoreactive for NPY. BN was also colocalized with NPY and NOS; 30% of the NPY-immunoreactive neurons were BN/NOS/NPY-immunoreactive. No nerve cells had BN and NPY immunoreactivity without NOS immunoreactivity.
The presence of NPY immunoreactivity was investigated in three classes of descending interneurons that are distinguished by their reactivities for somatostatin, 5-HT and NOS. NPY immunoreactivity was never colocalized with 5-HT or somatostatin, but most NPY-immunoreactive descending interneurons whose terminals formed pericellular baskets were also reactive for BN and NOS. The average projection lengths of the NPY interneurons was 2-3mm, in the anal direction.
Evaluation of immunoreactivity for BN, NOS and NPY revealed three major populations of anally directed circular muscle motor neurons, with BN/NOS/NPY, BN/NOS and NOS/NPY immunoreactivities. Examination of simultaneous labeling after nerve lesions showed that NOS/NPY neurons had short anal projections, averaging about 2-3mm, and neurons with BN immunoreactivity were longer, having average projections of about 5-8mm.