Heme oxygenase immunoreactive neurons in the rat intestine and their relationship to nitrergic neurons

Carbon monoxide (CO), like nitric oxide (NO), is a putative gaseous neurotransmitter. CO is produced by the enzyme heme oxygenase (HO) acting on a family of heme-containing compounds. Two isomers of HO have been characterized (HO-1, HO-2). In the CNS and in peripheral ganglia HO-2 occurs in a majority of neurons. NO and CO function as transmitters of enteric neurons but the relative distribution of enteric neurons utilizing these gaseous transmitters is unknown in rodent. We have studied the distribution of HO-2 immunoreactivity and NO synthase (NOS) activity within the rat ileum.

Tissue sections and primary neuronal cell cultures were incubated with a HO-2 specific antibody, and then assessed or reprocessed for NOS activity using NADPH-dependent diaphorase staining.

HO-2 immunoreactivity was expressed in subpopulations of myenteric and submucosal neurons. Approximately 45% of the ganglion cells in tissue section were HO-2 positive. This was similar in proportion to those found to stain for NOS activity, and 10% of HO-2 positive neurons also contained NOS. HO-2 immunoreactivity was also found in epithelial cells within the villi, and in interstitial cells around the myenteric plexus and within the smooth muscle. In culture, the distribution and colocalisation of HO-2 and NOS positive neurons was similar to that in tissue sections. We identified labelled neurons as either Dogiel Type I or II; only Type II cells colocalized NOS and HO-2.

Neurons, endocrine-like cells and interstitial cells with the capacity for CO production are distributed throughout the ileum and some neurons have the capacity to synthesize both NO and CO as gaseous messengers.