The molecular mechanism for reversed dopamine D2 receptor cellular output and defecation control, enabled by constitutive ghrelin receptor activity

Dopamine is likely to be the relevant neurotransmitter responsible for coordinating voluntary defecation in the lumbro-sacral defecation center. In these neurons dopamine acts via the dopamine D2 receptor (DRD2) and atypically elicits excitatory responses, leading to increased colonic motility. These neurons also express the ghrelin receptor (GHSR), whose agonist, ghrelin, appears to be absent from the central nervous system, yet stimulating this receptor also increases colonic motility. In intact animals, we have shown a functional interaction between DRD2 and GHSR in controlling colonic motility. We have also shown that these receptors are present in the same neurons.We sought to understand the mechanism for reversed DRD2 activity and the role for GHSR in this reversed activity.Experiments were conducted in accordance with the National Health and Medical Research Council guidelines for the care and use of animals and with approval from the Florey Institute of Neuroscience and Mental Health Animal Ethics Committee (FINMH-20-024).In spinal defecation neurons, those excited by dopamine were also excited by GHSR agonists. These excitatory responses required intracellular calcium stores. This GHSR-dependent switch in DRD2 coupling to calcium mobilization also occurred in recombinant cells but was not the result of a switch in the G protein preference of DRD2. Moreover, allostery between these receptors using ligand binding was undetectable, nor were they close enough to invoke dimerization as assessed by super-resolution microscopy. Instead, we observe a GHSR-dependent priming of phospholipase C beta (PLC-β), dependent on GHSR’s constitutive activity. In cells co-expressing a GHSR polymorph lacking constitutive activity, DRD2 coupling to calcium was restored by priming with a low concentration ghrelin. This requirement for PLC-β is seen in spinal defecation neurons, where PLC-β inhibition reverses the dopamine response of these cells from excitatory to inhibitory. Together our data indicate that dopamine mediated excitation is dependent on GHSR constitutive activity via a dominant second-messenger switch. This work has broad implications for switching of metabotropic neurotransmitter responses via alternative G protein-coupled receptor modulation and more directly for treatment of Parkinson’s-associated chronic constipation. 2021 NHMRC Ideas Grant; 2021/GNT2012657 (SGBF, JBF) SGBF is an ARC Future Fellow (FT180100543) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.