Heme oxygenase-2 distribution in anorectum: colocalization with neuronal nitric oxide synthase

Recent investigations have suggested carbon monoxide (CO) as a putative mes senger molecule. Although several studies have implicated the heme oxygenas e (HO) pathway, responsible for the endogenous production of CO, in the neu romodulatory control of the internal anal sphincter (IAS), its exact role i s not known. Nitric oxide, produced by neuronal nitric oxide synthase (nNOS ) of myenteric neurons, is an important inhibitory neural messenger molecul e mediating nonadrenergic noncholinergic (NANC) relaxation of the IAS. The present studies were undertaken to investigate in detail the presence and c oexistence of heme oxygenase-2 (HO-2) with nNOS in the opossum anorectum. I n perfusion-fixed, frozen-sectioned tissue, HO-2 immunoreactive (IR) and nN OS IR nerves were identified using immunocytochemistry. Ganglia containing HO-2 IR neuronal cell bodies were present in the myenteric and submucosal p lexuses throughout the entire anorectum. Colocalization of HO-2 IR and nNOS IR was nearly 100% in the IAS and decreased proximally from the anal verge . In the rectum, colocalization of HO-2 IR and nNOS IR was similar to 70%. Additional confocal microscopy studies using c-Kit staining demonstrated th e localization of HO-2 IR and nNOS IR in interstitial cells of Cajal (ICC) of the anorectum. From the high rate of colocalization of HO-2 IR and nNOS IR in the IAS as well as the localization of HO-2 IR and nNOS LR in ICC in conjunction with earlier studies of the HO pathway we speculate an interact ion between HO and NOS pathways in the NANC inhibitory neurotransmission of the IAS and rectum.