Euthanize EIIa-GCaMP mice (offspring from the pairing of B6.FVB-Tg(EIIa-cre)C5379Lmgd/J [RRID:IMSR_JAX:003724; cat. 003724; Jax Labs] and B6J.Cg-Gt(ROSA)26Sortm96(CAG-GCaMP6s)Hze/MwarJ [RRID:IMSR_JAX:028866; cat. 028865; Jax Labs]), by inhalation of isoflurane and thoracotomy. Remove the entire colon from mouse and place into a Sylgard-lined Petri dish (Dow Corning). Circulate oxygenated artificial cerebral spinal fluid at room-temp (ACSF, containing in mM: 117.9 NaCl [cat. S9888, Sigma], 4.7 KCL [cat. P3911; Sigma], 25 NaHCO3[catS6014; Sigma], 1.3 NaH2PO4, [cat. S8282; Sigma] 1.2 MgSO47H2O [cat. 230391; Sigma], 2.5 CaCl2 [cat. C1016; Sigma], 11.1 D-glucose [cat. G5767; Sigma], 2 sodium butyrate [cat. B5887; Sigma], and 20 sodium acetate [cat. S2889; Sigma]) with 4μM nifedipine (cat. N7634; Sigma) and 3μM indomethacin (cat. I7378; Sigma) added. Cut the colon open longitudinally and pin flat using minutien pins (Fine Science Tools; cat. 26002-20) with mucosal side facing down. A small amount of stretch should be applied when pinning the colon tissue for optimal imaging conditions. Transfer to the stage of an upright fluorescence microscope (DM6 FS, Leica) equipped with camera (Prime 95B, Photometrics, Roper Scientific) and software (Metamorph, Molecular Devices) for calcium imaging and slowly heat up circulating fluid to 35-37 C (or desired temperature) using a heated water bath.