M5 muscarinic receptor has been the hardest receptor to study as there is no specific pharmacology targeted against the receptor and no high level of expression has been detected in tissues. However, the effects of single knockout mice were studied and it was observed that in M5 muscarinic receptor knockout mice, there is a moderate decrease in ligand binding in the lung and bladder, two organs expressing M5 muscarinic receptor. In addition, it has been proposed that M5 muscarinic receptor may be involved in interstitial cystitis, a urinary bladder disorder.
