APPOSITIONS MADE BY AXONS OF DESCENDING INTERNEURONS IN THE GUINEA-PIG SMALL-INTESTINE, INVESTIGATED BY CONFOCAL MICROSCOPY

There are four major classes of descending interneurons in the myenter ic plexus of the guinea-pig small intestine. In this study, the connec tions made by two of these classes of descending interneurons with oth er interneurons and with inhibitory motor neurons have been investigat ed using confocal, conventional fluorescence and electron microscopy. The terminals of descending interneurons known to contain both bombesi n (BN) and nitric oxide synthase (NOS) were identified by BN immunorea ctivity (IR). Cholinergic interneurons known to contain somatostatin ( SOM) were identified by SOM-IR. The connections of these two groups of interneurons with the following three classes of nerve cell bodies we re examined those with NOS-LR that also contain gamma-aminobutyric aci d (GABA) (inhibitory motor neurons), those with only NOS-IR (descendin g interneurons and inhibitory motor neurons) and those with only GABA- IR (motor neurons). The BN-IR and SOM-IR interneurons were found to fo rm connections with each other, and both types of interneurons provide d inputs to motor neurons. Most previous analyses of interconnections in the enteric plexuses have been by conventional fluorescence microsc opy and electron microscopy. In the present work these are compared wi th confocal microscopy. BN-IR varicosities formed pericellular baskets around each class of nerve cell that were easily identifiable with al l techniques. Using confocal microscopy, BN-IR varicosities that were in contact with NOS-IR and GABA-IR nerve cells were quantified. Confoc al microscopy demonstrated over twice as many contacts as were shown b y a previous electron microscopic study. In contrast, conventional flu orescence microscopy showed little indication that SOM-IR varicosities formed inputs to NOS-IR or GABA-IR nerve cells, despite the fact that confocal microscopy revealed direct appositions and electron microsco py revealed synapses. This study has shown that confocal analysis is a valuable adjunct to conventional fluorescence microscopy for determin ing neuronal circuitry. Moreover, it allows a more rapid collection of data than does electron microscopy. It is concluded that chains of BN -IR and SOM-IR interneurons form descending pathways in the small inte stine and that both types of interneuron connect with muscle motor neu rons. (C) 1997 Elsevier Science B.V.