Reticular Groove and Reticulum Are Innervated by Myenteric Neurons with Different Neurochemical Codes

The reticulum and the reticular groove are functional distinct compartments within the ovine forestomach. While the reticulum takes part in various motor functions, such as mixing, retaining, and rejecting the forestomach ingesta, the reticular groove serves mainly as a bypass between the esophagus and the abomasum. To accomplish these different tasks, the compartments develop specific motility patterns that are controlled by intrinsic neural circuits. In this study the intrinsic innervation by myenteric neurons was analyzed by quadruple immunohistochemistry against cholineacetyl transferase (ChAT), nitric oxide synthase (NOS), substance P (SP), and vasoactive intestinal peptide (VIP). Four neurochemically different subpopulations of myenteric neurons were found in the reticulum and the floor of the reticular groove: ChAT/-, ChAT/SP, NOS/-, and NOS/VIP. The neuronal proportions were calculated relative to all myenteric neurons. Neurons of the reticulum were mostly immunoreactive for ChAT (89% ± 3%), whereas neurons adjacent to the reticular groove predominantly expressed a nitrergic phenotype (62% ± 4%). ChAT-positive neurons were also immunoreactive for SP (ChAT/SP: 64% ± 3% reticulum; 25% ± 1% reticular groove) or were purely cholinergic (ChAT/-: 25% ± 4% reticulum; 13% ± 3% reticular groove). NOS-positive neurons colocalized VIP (NOS/VIP: 10% ± 3% reticulum; 46% ± 1% reticular groove) or none of the other neurotransmitters (NOS/-: 1% ± 1% reticulum; 17% ± 3% reticular groove). Analysis of the soma sizes revealed that in both compartments the nitrergic neurons were significantly larger than the cholinergic neurons. It is suggested that the specific neurochemical code in combination with a specific morphology leads to a precise regulation of the specialized tasks of the reticulum and reticular groove by subpopulations of myenteric neurons.