Abstract
Gastrointestinal motility is coordinated by enteric neurons. Both inhibitory and excitatory motor neurons innervate the syncytium consisting of smooth muscle cells (SMCs) interstitial cells of Cajal (ICC) and PDGFRα(+) cells (SIP syncytium). Confocal imaging of mouse small intestines from animals expressing GCaMP3 in ICC were used to investigate inhibitory neural regulation of ICC in the deep muscular plexus (ICC-DMP). We hypothesized that Ca(2+) signaling in ICC-DMP can be modulated by inhibitory enteric neural input. ICC-DMP lie in close proximity to the varicosities of motor neurons and generate ongoing Ca(2+) transients that underlie activation of Ca(2+)-dependent Cl(-) channels and regulate the excitability of SMCs in the SIP syncytium. Electrical field stimulation (EFS) caused inhibition of Ca(2+) for the first 2-3 s of stimulation, and then Ca(2+) transients escaped from inhibition. The NO donor (DEA-NONOate) inhibited Ca(2+) transients and Nω-Nitro-L-arginine (L-NNA) or a guanylate cyclase inhibitor (ODQ) blocked inhibition induced by EFS. Purinergic neurotransmission did not affect Ca(2+) transients in ICC-DMP. Purinergic neurotransmission elicits hyperpolarization of the SIP syncytium by activation of K(+) channels in PDGFRα(+) cells. Generalized hyperpolarization of SIP cells by pinacidil (K(ATP) agonist) or MRS2365 (P2Y1 agonist) also had no effect on Ca(2+) transients in ICC-DMP. Peptidergic transmitter receptors (VIP and PACAP) are expressed in ICC and can modulate ICC-DMP Ca(2+) transients. In summary Ca(2+) transients in ICC-DMP are blocked by enteric inhibitory neurotransmission. ICC-DMP lack a voltage-dependent mechanism for regulating Ca(2+) release, and this protects Ca(2+) handling in ICC-DMP from membrane potential changes in other SIP cells.