Neurochemical characterization of extrinsic innervation of the guinea pig rectum

The presence of markers for parasympathetic, sympathetic, and glutamatergic or peptidergic sensory innervation was investigated by using in vitro tracing with biotinamide, combined with immunohistochemistry, to characterise quantitatively extrinsic axons to myenteric ganglia of the guinea pig rectum. Of biotinamide‐filled varicose axons, 3.6 ± 1.3% were immunoreactive for tyrosine hydroxylase (TH) and 16.0 ± 4.8% for vesicular acetylcholine transporter (VAChT). TH and vesicular monoamine transporter (VMAT1) showed high coexistence (83–100%), indicating that varicosities lacking TH immunoreactivity also lacked VMAT1. VAChT was detectable in 77% of choline acetyltransferase (ChAT)‐immunoreactive varicosities. Calcitonin gene‐related peptide (CGRP) was detected in 5.3 ± 1.6% of biotinamide‐labeled varicosities, the vesicular glutamate transporter (VGluT) 1 in 2.8 ± 0.8%, and VGluT2 in 11.3 ± 4.2% of varicosities of extrinsic origin. Varicosities from the same axon showed consistent immunoreactivity. A novel type of nerve ending was identified, with branching, flattened lamellar endings, similar to the intraganglionic laminar endings (IGLEs) of the proximal gut. Rectal IGLEs were frequently immunoreactive for VGluT1 and VGluT2. Thus most varicose axons of extrinsic origin, which innervate rectal myenteric ganglia, lack detectable levels of immunoreactivity for TH, VMAT1, VAChT, ChAT, VGluT1/2, or CGRP, under conditions in which these markers are readily detectable in other axons. Although some unlabeled varicosities may belong to afferent axons that lack detectable CGRP or VGluT1/2 in the periphery, this suggests that a large proportion of axons do not release any of the major autonomic or sensory transmitters. We speculate that this may vary under particular circumstances, for example, inflammation or obstruction of the gut. J. Comp. Neurol. 470:357–371, 2004. © 2004 Wiley‐Liss, Inc.