DISTRIBUTION PATTERN AND ULTRASTRUCTURAL-LOCALIZATION OF RXT1, AN ORPHAN NA+ CL--DEPENDENT TRANSPORTER, IN THE CENTRAL-NERVOUS-SYSTEM OF RATS AND MICE/

The cellular and subcellular localization of Rxt1 protein, an orphan N a+/Cl--dependent transporter, was investigated in the central nervous system of rats and mice, with rabbit polyclonal antibodies specificall y directed against its C-terminal region. At the light microscope leve l, the distribution of Rxt1, visualized by the immunoperoxidase method , was found to be similar in rats and mice. Labelled elements were pre sent in numerous gray matter regions of the central nervous system, fr om the olfactory bulb to the spinal cord. In all labelled regions, imm unoreactivity was confined to the neuropil where both a diffuse labell ing of low intensity and an intense punctate staining were noted. To f urther identify the nature of the cellular elements bearing the puncta te staining, possible changes in this labelling pattern were investiga ted: (i) in deep cerebellar nuclei and lateral vestibular nucleus of t he Lurcher mutant mouse, in which all cerebellar Purkinje cells are mi ssing and (ii) in the rat cervical spinal cord, 10 days after multiple resections of dorsal roots. The vast majority of the punctate structu res, delineating the neuronal perikaryal and stem dendritic contours, had disappeared in the mutant mouse, providing evidence that they belo ng to Purkinje cell axon terminals. In rhizotomized rats, the intense labelling in laminae I and III had disappeared, demonstrating that it occurred in subclasses of axonal projections of primary afferent fibre s. These results strongly suggest that Rxt1 is present in presynaptic axon terminals. The electron microscopic study was carried out in the hippocampus, cerebellum and lateral vestibular nucleus of control mice , where Rxt1-labelled punctate structures were found to be abundant. I mmunostaining was confined to axon terminals, particularly in hippocam pal and cerebellar messy fibres and in Purkinje cell axonal terminatio ns of the cerebellar deep nuclei and lateral vestibular nucleus. In th e cerebellar cortex, axon terminals belonging to inhibitory local circ uit neurons (basket and Golgi cells), were free of labelling. The obse rvations reported in this study have shown that: (1) The Rxt1 transpor ter is neuron-specific, and is expressed by only some classes or even subclasses of neuronal systems. (2) This transporter can be encountere d in excitatory axons using glutamate as neurotransmitter (hippocampal and cerebellar messy fibres; primary afferent fibres), as well as in inhibitory axons known by their GABAergic nature (Purkinje cell axon t erminals) where it might be involved in the re-uptake process of one o r several molecules released from corresponding terminals. (C) 1997 IB RO.