Postnatal development of the inferior olivary complex in the rat: IV. Synaptogenesis of GABAergic afferents, analyzed by glutamic acid decarboxylase immunocytochemistry

Abstract

The postnatal maturation of the GABAergic innervation of the rat inferior olive was studied with an antiserum to glutamic acid decarboxylase (GAD), the GABA‐synthesizing enzyme. GAD‐positive axons were present at a very low density in the periolivary and interlamellar regions of newborn rats, as well as in certain precise areas of the lamellae, at the mediodorsal limit. The immature distribution indicates that the GABAergic projections reach the inferior olive shortly before birth and that the greater part of synaptogenesis and the establishment of the adult organization occurs postnatally. Light and electron microscopic analyses disclosed that the maturation of this system of olivary afferents passes through three well‐defined stages: (1) During the first, or immature stage (from P0 to P5), GAD immunoreactivity is not confined to axon terminals, as in adult rats. The labeled fibers penetrate progressively into the periphery of the lamellae and reach their centers in an irregular manner by the end of the immature stage. This staggered invasion of the lamellae accentuates intraregional olivary differences and begins to take the adult configuration. As fiber penetration advances, the density of labeled axons establishing synaptic contacts increases, while the number of completely immunostained fibers decreases. This distribution prevails until the end of the immature stage and suggests that the GABAergic afferent projections remain in a “waiting compartment” from their prenatal arrival until the moment they invade the olivary parenchyma. (2). The second stage is designated as an intermediate stage of maturation and lasts from P7 to P10. During this period, GAD axoplasmic compartmentation occurs, and henceforth only axon terminals exhibit GAD immunoreactivity. Concomitantly, intraregional differences in the pattern of innervation become more marked, because of the continuing irregular distribution of the growing labeled axons. This intermediate maturational stage is also characterized by a rapid increase in labeled axon terminals bearing synaptic complexes and by the formation of complex synaptic arrangements, the protoglomeruli. From the beginning of protoglomeruli formation, GAD‐positive axon terminals are one of their constituents, and they are systematically localized at the periphery of the incipient dendritic protrusions. (3) The final stage of maturation takes place from P10 to P15. During this stge, the adultlike pattern of GABAergic innervation of the inferior olive is attained. Toward P15, intraregional differences in GAD immunoreactivity are similar to those of the adult rat. As expeceted, the synaptology of this afferent system reaches in adult complexity, especially at the glomeruli, where olivary neurons (which are electrotonically coupled through dendrodendritic gap junctions by P15) receive GAD‐positive axon terminals and establish synaptic contacts on the dendritic appendages linked by gap junctions. The spatiotemporal sequence of the establishment of GAD immunoreactivity indicates that GABAergic input, except in olivary regions projecting to the vestibulocerebellum, develops after the formation of most olivocerebellar projections. Therefore, the olivocerebellar system, despite its precocity, acquires n adult functional network only toward the end of the second postnatal week, when GAD‐positive axon terminals synpase on the dendrites linked by gap junctions. In fact, this complex synaptic arrangement, implied in the modulation of the ratio of electrotonic couupling between olivary neurons, is essential for the spatiofunctional organization of the olivocerebellar system.