RELATIONSHIPS BETWEEN STRIATIN-CONTAINING NEURONS AND CORTICAL OR THALAMIC AFFERENT-FIBERS IN THE RAT STRIATUM - AN ULTRASTRUCTURAL-STUDY BY DUAL LABELING

Striatin, a recently isolated rat brain calmodulin-binding protein bel onging to the WD-repeat protein family, is thought to be part of a cal cium signal transduction pathway presumably specific to excitatory syn apses, at least in the striatum. This study was aimed to specify the c ellular and subcellular localization of striatin. and to determine the possible synaptic relationships between the two main excitatory affer ent pathways, arising from the cerebral cortex and the thalamus, and t he striatin-containing elements, in the rat striatum. Anterograde trac t-tracing by means of biotinylated dextran amine injection in the fron toparietal cerebral cortex or the parafascicular nucleus of the thalam us was combined with immunogold detection of striatin. Striatin-immuno reactivity was confined to the neuronal somatodendritic compartment, i ncluding spines. Whereas 90-95% of the striatal neurons were striatin- positive, only about 50% of the sections of dendritic spines engaged i n asymmetrical synaptic contacts exhibited striatin labelling. Among t he sections of striatin-immunopositive dendritic spines, the number of immunogold particles ranged from one to more than seven, indicating a n heterogeneity of the spine labelling. Moreover, within each class of spines presenting at least two silver-gold particles, the distributio n of the particles varied from a clear-cut alignment under the postsyn aptic densities (24-33% of spines) to a location distant from the syna ptic area. In the cell bodies and dendrites, striatin labelling was us ually not associated with the cytoplasmic membrane nor with the postsy naptic densities. In the striatum ipsilateral to the tracer injections , only 34.8% of the synaptic contacts formed by corticostriatal affere nts involved striatin-positive elements (slightly labelled dendritic s pines), whereas 56.7% of the synaptic contacts formed by thalamostriat al boutons were made on striatin-positive targets (mostly dendrites). In both cases, striatin labelling was usually not associated with the postsynaptic density. Most of the immunoreactive dendritic spines were in contact with unidentified afferents. These data reveal that striat in is expressed in the vast majority of the cell bodies of striatal sp iny neurons, but is heterogeneously distributed among the dendritic sp ines of those neurons. Data also indicate a preferential relationship between striatin-containing structures and afferents from the parafasc icular thalamic nucleus with respect to the frontoparietal cerebral co rtex. But, at the dendritic spine level, striatin may be involved in s ignal transduction mechanisms involving as yet unidentified excitatory afferents to striatal neurons. (C) 1998 IBRO. Published by Elsevier S cience Ltd.