The distribution of nerve fibers containing
enkephalin (ENK)-like immunoreactivity was examined in the rectum of aganglionosis rats (AGRs) which completely lack the intramural
ganglion cells in the large intestine, and was compared with that of their normal littermates. Furthermore, Met5-enkephalin-Arg6-Gly7-Leu8 (
MEAGL)-like immunoreactive neurons projecting to the rectum were examined using retrograde tracing combined with immunohistochemistry in the major pelvic
ganglion of normal male rats. In the intermuscular space of the aganglionic rectum of AGRs, unlike the pattern of the normal intermuscular plexus, moderate numbers of ENK-like-immunoreactive fibers were arranged in an irregular, coarse network; greatly diminished numbers of immunoreactive fibers were found in the submucosa. No ENK-like-immunoreactive fibers were seen in the circular muscle layer and mucosa. In the normal rat rectum, ENK-like-immunoreactive fibers were seen throughout all layers, and immunoreactive nerve cells were found predominantly in the myenteric plexus of
colchicine-treated animals.
Fluoro-Gold injected into the upper rectum labelled numerous principal
ganglion neurons in the major pelvic and inferior mesenteric ganglia. Less than 10% of tracer-labelled neurons were positive for
fluorescein immunolabelling of
MEAGL in the major pelvic
ganglion; no immunoreactive neurons were found in the inferior mesenteric
ganglion. In the major pelvic
ganglion of the
colchicine-treated normal rats, about 5% of principal
ganglion neurons were immunoreactive for
MEAGL. Comparison of serial
paraffin sections of the major pelvic
ganglion stained for
tyrosine hydroxylase (TH),
MEAGL and
vasoactive intestinal polypeptide (VIP), respectively, revealed that more than half of
MEAGL-like immunoreactive neurons were also positive for TH; there was no case showing co-existence of
MEAGL with VIP in the principal neurons. These results indicate that a small number of
enkephalin-containing neurons in the major pelvic
ganglion project to the rectum, and that more than half of these neurons are postganglionic sympathetic. They may terminate mainly in the myenteric ganglia in the rectum.