Identification and immunohistochemistry of cholinergic and non-cholinergic circular muscle motor neurons in the guinea-pig small intestine

Sites of action of morphine on the ascending excitatory reflex in the guinea-pig small intestine

The effect of morphine on the ascending excitatory reflex of the circular muscle elicited by radial distension of the gut wall was studied in the isolated guinea-pig small intestine. A three compartment bath, in which an intermediate compartment divided the site of intraluminal stimulation (caudal compartment) from the site of reflex contraction recording (oral compartment), was used. Morphine (0.01-10 microM) applied independently to each compartment, caused a concentration-dependent depression (up to 90%) of the amplitude of distension-evoked reflex contractions. Concentration-response curves to morphine were shifted to the right by naloxone (30 nM) with an apparent pA2 value of about 8.5, which suggests an interaction with opioid mu-receptor subtypes. Our results indicate that morphine not only depressed transmission from excitatory motor neurons to the circular muscle but also neuro-neuronal transmission along the ascending excitatory reflex pathway.

Identification of motor neurons to the longitudinal muscle of the guinea pig ileum

Motor neurons that innervate the longitudinal muscle of the guinea pig ileum were identified by retrograde transport from the longitudinal muscle plexus in organotypic culture. Motor neurons had short projections, less than 3.5 mm long, and never had Dogiel type II morphology; most labeled neurons had morphological characteristics of Dogiel type I neurons. Immunoreactivity for choline acetyltransferase was present in 97% of retrogradely labeled nerve cell bodies, reflecting the dominant cholinergic input to the longitudinal muscle layer. Substance P immunoreactivity was present in 48% of motor neurons, indicating that it or a similar tachykinin that mediates noncholinergic excitatory transmission is likely to be released by a subset of cholinergic motor neurons. This strongly suggests that the difference in frequency dependence of substance P and acetylcholine release is attributable to different release mechanisms rather than to activation of separate populations of motor neurons. Immunoreactivity for the calcium-binding protein calretinin was present in 87% of longitudinal muscle motor neurons. The neurochemical coding of longitudinal muscle motor neurons indicated that they constitute about one quarter of all myenteric neurons and are distinct from circular muscle motor neurons.