Indications for GABA-immunoreactive axo-axonic contacts on the intraspinal arborization of a Ib fiber in cat: A confocal microscope study

Abstract

Confocal microscopy was used to detect GABA-immunoreactive axo-axonic appositions, indicating possible synaptic contacts, on lb fiber terminals in the lumbosacral spinal cord. A lb fiber from posterior biceps-semitendinosus muscles was labeled by intra-axonal ejection of tetramethylrhodamine dextran (red), and serial sections of S1-L7 spinal cord segments were processed for GABA immunocytochemistry revealed by fluorescein isothiocynate (green). Appositions between GABA-immunoreactive structures and the labeled fiber appeared as yellow spots because of the presence of both fluorochromes in small volumes (0.3 * 0.3 * 0.5 μm3) of tissue. These spots were identified as probable axo-axonic contacts when: (1) they were observed in two to four serial confocal planes, indicating that they did not occur by chance; and (2) their sizes, shapes, and locations were similar to those of axo-axonic contacts found on la terminals, known to bear presynaptic boutons, and resembled the axo-axonic synapses described in electron microscope studies of lb boutons in Clarke's column. A total of 59 presumed axo-axonic contacts was observed on two lb collaterals, representing an estimated 20% of the total complement. In a three-dimensional reconstruction of one collateral, they were mostly located in terminal positions, and some branches bore more contacts than others. Such differential distribution could not results from chance appositions between GABAergic structures and lb arborization and further supported the identification of axo-axonic contacts. Segmental lb collaterals bear axo-axonic synapses thai might ensure differential funneling of information toward different targets.