ULTRASTRUCTURAL-STUDY OF THE GABAERGIC AND CEREBELLAR INPUT TO THE NUCLEUS-RETICULARIS TEGMENTI PONTIS

The nucleus reticularis tegmenti pontis is an intermediate of the cere brocerebellar pathway and serves as a relay centre for sensorimotor an d visual information. The central nuclei of the cerebellum provide a d ense projection to the nucleus reticularis tegmenti pontis, but it is not known to what extent this projection is excitatory or inhibitory, and whether the terminals of this projection contact the neurons in th e nucleus reticularis tegmenti pontis that give rise to the messy fibr e collaterals innervating the cerebellar nuclei. In the present study the nucleus reticularis tegmenti pontis of the cat was investigated at the ultrastructural level following anterograde and retrograde transp ort of wheat germ agglutinin coupled to horseradish peroxidase (WGA-HR P) from the cerebellar nuclei combined with postembedding GABA immunoc ytochemistry. The neuropil of this nucleus was found to contain many W GA-HRP labeled terminals, cell bodies and dendrites, but none of these pre-or postsynaptic structures was double labeled with GABA. The vast majority of the WGA-HRP labeled terminals contained clear spherical v esicles, showed asymmetric synapses, and contacted intermediate or dis tal dendrites. Many of the postsynaptic elements of the cerebellar aff erents in the nucleus reticularis tegmenti pontis were retrogradely la beled with WGA-HRP, while relatively few were GABAergic. We conclude t hat all cerebellar terminals in the nucleus reticularis tegmenti ponti s of the cat are nonGABAergic and excitatory, and that they contact pr edominantly neurons that project back to the cerebellum. Thus, the rec iprocal circuit between the cerebellar nuclei and the nucleus reticula ris tegmenti pontis appears to be well designed to function as an exci tatory reverberating loop. (C) 1997 Elsevier Science B.V.