SYNAPTIC ORGANIZATION OF NEUROPEPTIDE-CONTAINING PREGANGLIONIC BOUTONS IN LUMBAR SYMPATHETIC-GANGLIA OF GUINEA-PIGS

Within the lumbar sympathetic ganglia of guinea pigs, the endings of d ifferent populations of neuropeptide-containing preganglionic neurons form well-defined pericellular baskets of boutons around target neuron s in specific functional pathways. We have used multiple-labelling imm unofluorescence, confocal microscopy, and ultrastructural immunocytoch emistry to investigate synaptic organisation within pericellular baske ts labelled for immunoreactivity to calcitonin gene-related peptide (C GRP), substance P (SP), or the pro-enkephalin-derived peptide, met-enk ephalin-arg-gly-leu (MERGL) in relation to their target neurons. Diffe rent functional populations of neurons, identified by their neurochemi cal profile, showed a significant degree of spatial clustering and pre dicted well the distribution of specific classes of pericellular baske ts. Most of the boutons in a basket were completely surrounded by Schw ann cell processes and did not form synapses. The synapses that were p resent were made mostly onto dendrites enclosed by the Schwann cell sh eath surrounding the neuron within the basket. These dendrites probabl y originated from neurochemically similar neighbouring neurons. Nevert heless, some of the boutons in the baskets did form synapses with the cell body or proximal dendrites of the neuron they surrounded. Occasio nally, cell bodies received a relatively high number of synapses and c lose appositions from boutons in a pericellular basket. Synaptic conve rgence of two immunohistochemically distinct types of preganglionic in puts was found in baskets of SP-immunoreactive or MERGL immunoreactive , but not CGRP-immunoreactive, boutons. Taken together, our results sh ow that the appearance of pericellular baskets is primarily due to the packing of the target neurons. The grouping of functionally similar c lasses of neurons with their pathway-specific projections of peptide-c ontaining preganglionic neurons suggests that peptides could exert the ir effects in relatively well-defined zones within the ganglia. J. Com p. Neurol. 398:551-567, 1998. (C) 1998 Wiley-Liss, Inc.