Electrophysiology and morphology of vasoactive-intestinal-peptide-immunoreactive neurones of the guinea-pig ileum.

Simultaneous intracellular staining and electrophysiological recording techniques have been applied to neurones of guinea-pig myenteric plexus-longitudinal muscle preparations. With micro-electrodes filled with a solution of the fluorescent dye Lucifer Yellow, neurones were first characterized morphologically and electrophysiologically, and subsequently subjected to an indirect immunohistochemical method for the detection of vasoactive intestinal peptide (VIP)-like immunoreactivity. Cross-correlations of morphology, electrophysiology and VIP immunoreactivity were successfully achieved in a total of 164 neurones. Sixty-three had the slow after-hyperpolarization characteristic of AH neurones; 101 cells displayed fast excitatory post-synaptic potentials (e.p.s.p.s) in response to transmural or focal stimulation and were therefore, by definition, S neurones. Unequivocal VIP immunoreactivity was observed in 25 (25%) S neurones, which, with only one exception, had Dogiel Type I morphology (i.e. many short soma processes and a single long process). In contrast, AH neurones had Dogiel Type II morphology (i.e. smooth soma with several long processes) and none showed VIP immunoreactivity. In addition to cholinergic fast e.p.s.p.s., non-cholinergic slow synaptic inputs were evoked in seventeen of the twenty-two VIP-immunoreactive S neurones tested. Both the fast and slow e.p.s.p.s could be elicited by stimulation of the preparation, oral or aboral to the site of recording. These observations demonstrate that, in the guinea-pig ileum, myenteric plexus neurones showing VIP immunoreactivity are of a single electrophysiological type (S neurones) and belong to essentially one morphological class (Dogiel Type I).