Brain Research, 224 (1981) 129-139 129 Elsevier/North-Holland Biomedical Press INCREASED GLUTAMATE DECARBOXYLASE ACTIVITY IN THE RED NUCLEUS OF THE ADULT CAT AFTER CEREBELLAR LESIONS ANDRE NIEOULLON and NICOLE DUSTICIER Ddpartement de Neurophysiologie Gdndrale, C.N.R.S., LN.P., B.P. 7l, 13277 Marseille Cedex 9 (France) (Accepted March 12th, 1981) Key words: red nucleus - - glutamate decarboxylase - - neuronal plasticity SUMMARY Glutamate decarboxylase (GAD)activity, a marker for GABAergic structures, was studied in the cat red nucleus. GAD is more concentrated in the rostral than in the caudal third of the structure. GAD levels were measured after chronic unilateral lesions of the cerebellum. Destruction of the dentate area and of the nucleus interpositus induced increases of GAD in the contralateral but not in the ipsilateral red nucleus. Similar changes also occurred in the denervated nucleus ventralis lateralis (VL) and nucleus ventralis anterior (VA) of the thalamus. Results show that loss of the excitatory cerebellar input could lead to changes in inhibitory GABAergic nerve terminals. This increase may be induced transsynapti- cally within existing neurons or, more likely, additional GAD-containing nerve terminals may be formed by axonal sprouting. INTRODUCTION It has been demonstrated many years ago, that synaptic reorganization occurs in the central nervous system of adult mammals. Residual primary afferent fibers in the partially deafferented spinal cord were, by anatomical methods, seen to exhibit collateral sprouts la. Transneuronal events also occur as shown in the denervated area of the rat dentate gyrus of the hippocampus 17. Similar events take place in the cat red nucleus after lesion of its main excitatory afferent originating in the cerebellum. This has been shown by both electrophysio- logical 15,16,3° and anatomical methods 19. These studies provided evidence for sprouting of cortico-rubral nerve terminals, the other main afferent pathway to red nucleus, after lesions of cerebello-rubral afferent fibers. 0006-8993/81/0000-0000/$02.50 © Elsevier/North-Holland Biomedical Press