Relation between electrophysiological class and neuropeptide content of guinea pig sympathetic prevertebral neurons

1. Sympathetic neurons in superior mesenteric ganglion and inferior mesenteric ganglion (IMG) isolated from guinea pigs were classified as tonic, phasic, or long after hyperpolarizing (LAH) on the basis of their discharge characteristics and the different types of potassium currents recorded from them with the soma under single-microelectrode voltage clamp. 2. Passive electrical properties showed a progressive increase in input resistance across the prevertebral ganglia in the rostrocaudal direction when compared with those previously reported for the same classes of neurons in celiac ganglia (CG). 3. The proportions of tonic, phasic, and LAH neurons changed markedly in a rostrocaudal progression from 37, 14, and 49%, respectively, in the CG to 80, 18, and 2%, respectively, overall in the IMG. 4. Three populations of neurons distinguished immunohistochemically by their content of somatostatin (SOM), neuropeptide Y (NPY), or neither neuropeptide were present in different proportions in each prevertebral ganglion. The proportions of SOM, NPY, and no peptide neurons changed from 27, 34, and 39%, respectively, in the CG to 45, 20, and 35%, respectively, in the IMG. There was no significant difference in these distributions between the sexes. 5. Individual electrophysiologically characterized neurons were filled with biocytin and later examined for SOM immunoreactivity. All SOM-positive neurons (9/9) in the CG but only 7/10 in the IMG were tonic, whereas SOM-negative neurons were classified in all electrophysiological classes. 6. Other than this one group of sympathetic neurons (constituting nearly 30% of neurons in both CG and IMG), the three electrophysiological classes do not correlate directly with the three neurochemical types so far identified. This is consistent with the existence of more than three functional groups of sympathetic neurons in the prevertebral ganglia. The findings also suggest that a major inhibitory effect on mucosal secretion, as well as on motility, is mediated by peripheral reflex pathways along much of the length of the gastrointestinal tract.