Neurochemical coding and projection patterns of gastrin-releasing peptide-immunoreactive myenteric neurone subpopulations in the guinea-pig gastric fundus

The aim of this study was to characterise the projection and neurochemical coding patterns of gastrin-releasing peptide (GRP)-containing subpopulations of myenteric neurones in the guinea-pig gastric fundus. For this purpose, we used retrograde tracing with the dye DiI and immunohistochemistry against GRP, choline acetyltransferase (ChAT), enkephalin (ENK), substance P (SP) and neuropeptide Y (NPY). Cell counts revealed that 44% of the myenteric neurones were GRP-positive. Of the GRP-positive neurones, 92% were ChAT-positive and, hence, 8% were presumptively nitric oxide synthase positive (NOS). The GRP-positive subpopulations were ChAT/GRP (40% of all GRP neurones), ChAT/NPY/GRP (25%), ChAT/SP/GRP/±ENK (20%), ChAT/ENK/GRP (8%), NOS/NPY/GRP/±ENK (5%) and NOS/GRP (3%). The tracing experiments revealed the relative contributions of the various GRP-positive subpopulations to the innervation of the circular muscle and the mucosa. GRP immunoreactivity was detected in 46 and 38% of the DiI-labelled muscle and mucosa neurones, respectively. GRP was almost exclusively found in ascending ChAT-positive mucosa and muscle neurones. The populations encoded ChAT/SP/GRP/±ENK and ChAT/ENK/GRP projected predominantly to the circular muscle, whereas the ChAT/NPY/GRP and ChAT/GRP populations had primarily projections to the mucosa. GRP was colocalised with ChAT, ENK and/or SP in varicose nerve fibres innervating the circular muscle and the muscularis mucosae, whereas in the mucosal epithelium GRP was mainly present in nerve fibres containing ChAT and NPY. The data suggest that in the guinea-pig gastric fundus, the ChAT/SP/GRP/±ENK and ChAT/ENK/GRP neurones are ascending excitatory muscle motor neurones, whereas the ChAT/NPY/GRP and ChAT/GRP neurones are very likely involved in the regulation of mucosal functions.