A COMBINED RETROGRADE TRACER AND GABA-IMMUNOCYTOCHEMICAL STUDY OF THEPROJECTION FROM NUCLEUS INTERPOSITUS POSTERIOR TO THE POSTERIOR LOBE C(2) ZONE OF THE CAT CEREBELLUM

The extent to which the cells of origin of the cerebellar nucleocortic al pathway are immunopositive for gamma-aminobutyric acid (GABA) was i nvestigated in four cats using retrograde labelling of nucleocortical neurons in combination with immunocytochemistry. Neurons were retrogra dely labelled by injection of fluorescent (coumarin)-tagged latex micr ospheres into the c(2) zone in the rostral part of the paramedian lobu le. The zone was identified electrophysiologically by the characterist ics of the climbing fibre responses evoked on the cerebellar surface b y percutaneous stimulation applied to the left and right forepaws in p entobarbitone-anaesthetized animals. Sections of the cerebellum contai ning the retrogradely labelled neurons were processed for GABA immunoc ytochemistry using a fluorescent (rhodamine)-tagged immunoglobulin, Wh en viewed with epifluorescence microscopy and appropriate filter block s the retrogradely labelled nucleocortical neurons could be visualized in the same sections as the GABA-immunopositive neurons. Almost all o f a total of 254 labelled nucleocortical neurons were located in nucle us interpositus posterior, where a total of 711 GABAergic neurons were also found. None of these cells contained coumarin-tagged beads and d isplayed immunoreactivity for GABA (i.e. none was double-labelled). Wh en compared by area of their cell body, the nucleocortical and GABA-im munopositive neurons appeared to form two partially overlapping popula tions. The mean cell area of the nucleocortical neurons was 620 +/- 23 3 mu m(2) (SD), whereas the GABA-immunopositive neurons were much smal ler, with a mean cell area of 220 +/- 115 mu m(2). The results suggest that GABA does not play a major role in the nucleocortical pathway to the c(2) zone of the rostral paramedian lobule of the cat cerebellum.