Double labeling of the paravertebral sympathetic C system in the bullfrog with antisera to LHRH and NPY.

The specificity of synaptic contacts between pre- and postganglionic cells in the sympathetic C system has been examined by immunocytochemical localization of two neuropeptides. Sections of bullfrog paravertebral sympathetic ganglia were stained with antibodies to luteinizing hormone releasing hormone (LHRH) and neuropeptide Y (NPY). Preganglionic synaptic boutons containing LHRH immunoreactivity were found to make contact with a subpopulation of postganglionic cell bodies and with some clusters of small intensely fluorescent (SIF) cells. In ganglia 9 and 10, 95.8% of the neurons contacted by LHRH-containing boutons were also positive for NPY-like immunoreactivity and conversely, 99.3% of the neurons that contained NPY-like immunoreactivity were contacted by LHRH-containing boutons. Qualitatively similar results were found in most other paravertebral ganglia. These observations support the conclusions that preganglionic C axons selectively innervate C-type ganglion cells and that virtually all C-type ganglion cells and some SIF cells receive a direct LHRH input. Moreover, they suggest that a pattern of specific connections between two sets of peptidergic neurons is expressed throughout most of the paravertebral sympathetic chain of the bullfrog.