The etiology of denervation hypersensitivity was studied using a rat model. Degeneration of the myenteric plexus was produced by direct application of 0.1% benzalkonium chloride to the serosal surface of the distal colon. Thirty days later, the treated group was compared with a control group undergoing a sham operation. The treated group showed that the decreased number of ganglion cells of the myenteric plexus on routine stain and acetylcholinesterase staining demonstrated the myenteric plexus in the treated group diminished acetylcholinesterase activity. Methacholine (1%), a muscarinic agonist, increased intraluminal pressure in treated but not control rats. The dose-response curve of colonic muscle strips to oxotremorine showed a shift to the left, indicating greater sensitivity, in the treated bowel, with a 50% effective dose (ED50) of 2.5 x 10(-8) in treated muscle and 2.2 x 10(-7) in controls. Binding studies using [3H]quinuclidinyl benzilate ([3H]QNB) showed that the specific maximal binding (Bmax) for [3H]QNB was greater in treated than in untreated animals (228 +/- 26.1 and 135 +/- 42.9 fmol/mg protein, respectively; P < 0.01), even though the dissociation constants (Kd) were the same (0.398 +/- 0.083 and 0.406 +/- 0.065). These findings show that acquired denervation hypersensitivity in this model is due to an increase in the number of muscarinic acetylcholine receptors.