Physiological Modulation of Intestinal Motility by Enteric Dopaminergic Neurons and the D2Receptor: Analysis of Dopamine Receptor Expression, Location, Development, and Function in Wild-Type and Knock-Out Mice

Abstract

Dopaminergic neurons are present in both plexuses of the murine bowel and are upregulated after extrinsic denervation but play unknown roles in enteric nervous system (ENS) physiology. Transcripts encoding dopamine (DA) receptors D₁–D₅ were analyzed by reverse transcription-PCR in stomach ≈ duodenum ≈ ileum ≈ proximal ≫ distal colon. Dissected muscle and myenteric plexus contained transcripts encoding D₁–D₃ and D₅, whereas mucosa contained D₁ and D₃–D₅. D₁–D₅ expression began in fetal gut [embryonic day 10 (E10)], before the appearance of neurons (E12), and was sustained without developmental regulation through postnatal day 1. In situ hybridization revealed that subsets of submucosal and myenteric neurons contained mRNA encoding D₂ or D₃. Immunoblots confirmed that D₁, D₂, and D₅ receptor proteins were present from stomach through distal colon. Subsets of submucosal and myenteric neurons were also D₁, D₂, or D₃ immunoreactive. When double labeled by in situ hybridization, these neurons contained mRNA encoding the respective receptors. Total gastrointestinal transit time (TGTT) and colonic transit time (CTT) were measured in mice lacking D₂, D₃, or D₂ plus D₃. Both TGTT and CTT were decreased significantly (motility increased) in D₂ and D₂ plus D₃, but not D₃, knock-out animals. Mice lacking D₂ and D₂ plus D₃ but not D₃ were smaller than wild-type littermates, yet ate significantly more and had greater stool frequency, water content, and mass. Because motility is abnormal when D₂ is absent, the net inhibitory DA effect on motility is physiologically significant. The early expression of DA receptors is also consistent with the possibility that DA affects ENS development.