.Veuros,icme I.etters, 83 11987) _._7 231 227 t'lscvicr Scientific Publishers Ireland l.td. NSI. 05049 The dentato-olivary projection in the rat as a presumptive GABAergic link in the olivo-cerebello-olivary loop. An ultrastructural study Pierre Angaut and Constantino Sotelo l,ahoratoire ~h" .'v'euromorphologie, INSER,~ U-106. ll~pital de la Salp~;tri~"re. t'ari., " Fram'e ' (Received and accepted 23 September 1987) Key words' Rat; Autoradiography, 7-Aminobutyric acid immunocytochemistrv: Dentato-olivary pro ec- tion It is here shown that autoradiographically labelled axon terminals of the dentato-olivary projection t'c~rm a heterogeneous population, t lowever, a majority of them constitute an even class of synapses, char- actcrizcd by their small axonal size, their content in pleiomorphic vesicles, and the establishment of s}m- metric synapses on small dendrites, about 5% of which are linked through a gap junction. The same materi- al, used for immunocytochemistry of GABA with the postembedding technique, discloses that a majority of boutons with cytological features similar to the dentato-olivary terminals are GABA-immunorcactisc. espcciall~ those synapsing on dendrites linked by gap junctions. The cerebello-olivary projection, despite its heterogeneity, thus appears as part of the GABAergic system which governs the synaptic modul,ttion of the electrotonic coupling between olivary neurons. It has been proposed that the olivocerebellar system is involved in the spatial orga- nization of the Purkinje cell synchrony [3, 7]. This organization seems to result from the electrotonic coupling of all the inferior olivary neurons [5, 9], in association with a selcctive functional decoupling of different sets of neurons through the modulatory synaptic action ofa GABAergic system [7]. Two recent findings-- (i) the presumptive GABAergic nature of the cerebello-olivary system [6], and (ii) the constant associa- tion of glutamic acid decarboxylase (GAD)-immunoreactive boutons synapsing on the small dendritic appendages linked by gap junctions in the olivary glomeruli [8] have suggested that the cerebello-olivary system could be the candidate for the synaptic modulation of the olivary electrotonic coupling. The above-mentioned hypothesis remains speculative as long as the precise synap- tology of the cerebello-olivary pathway is unknown. This projection has been amply described anatomically in the rat as both profuse and well organized topographically (2), which could favor powerful and highly localized effects upon olivary cells. So Correspondence: P. Angaut, Laboratoire de Neuromorphologie, INSERM U-106, HF~pital de la Salpfi- tri,Sre, F-75013 Paris, I"rance. [1304-394087 $ 03.50 (c~) 19~47 l'~lsevier Scientific Publishers Ireland I.td.