Acta Pharmacologica Sinica (2009) 30: 1276–1282; doi: 10.1038/aps.2009.121; published online 24 August 2009

Aim: Since the distal part of the intestine is targeted by a wide range of pathogens, the motility of the recto-anal region has been the object of many experimental and clinical observations.  In this study, we investigated descending motor responses in the anal canal as a measure of the activation of autonomic reflex pathways underlying evacuatory recto-anal activity.

Methods: The partitioned organ bath method was used to register motor responses of the anal canal as induced by balloon distension of the rectum in isolated rat recto-anal preparations.

Results: Distension-induced descending responses of the anal canal comprised contractions (with distension at a distance of 15 mm ), initial contractions and secondary relaxations (at 10 mm ) and short contractions followed by deep relaxations (at 3− 5 mm ).  Decreasing the distance between the distension stimulus and the anal canal resulted in a decreased contraction response and increased relaxation.  Tetrodotoxin (0.1 μmol/L) inhibited these responses.  Atropine (0.3 μmol/L) decreased contraction and did not change the relaxation response.  N^G-nitro-L-arginine (0.5 mmol/L) enhanced contraction in both the absence and presence of atropine.  L-arginine (0.5 mmol/L) inhibited contraction and extended relaxation in atropine-pretreated preparations.  The actions of N^G-nitro-L-arginine and L-arginine were more pronounced in the aboral direction.  ChAT-positive nerve fibers were observed in myenteric ganglia of the rectum and the anal canal.  The density of NADPH-diaphorase-positive neurons was higher in the anal canal region.

Conclusion: Our results suggest that locality-dependent activation of the descending reflex neuromuscular communications underlie evacuatory activity in the recto-anal region.  This activation response involves long excitatory cholinergic and non-cholinergic pathways along the rectum and short inhibitory nitrergic pathways located predominantly in the anal canal region.

Keywords: rectum; anal canal; motor reflexes; NADPH-diaphorase