BIPHASIC RESPONSE OF SPINAL GABAERGIC NEURONS AFTER A LUMBAR RHIZOTOMY IN THE ADULT-RAT

The expression of gamma-aminobutyric acid (GABA) and of the isoforms o f the enzyme involved in its synthesis, glutamic acid decarboxylase (G AD), is modified in several rat brain structures in different injury m odels. The aim of the present work was to determine whether such plast icity of the GABAergic system also occurred in the deafferented adult rat spinal cord, a model where a major reorganization of neural circui ts takes place. GABAergic expression following unilateral dorsal rhizo tomy was studied by means of non-radioactive in situ hybridization to detect GAD(67) mRNA and by immunohistochemistry to detect GAD(67) prot ein and GABA. Three days following rhizotomy the number of GAD(67) mRN A-expressing neurons was decreased in the superficial layers of the de afferented horn, while GABA immunostaining of axonal fibres located in this region was highly increased. Seven days after lesion, on the oth er hand, many GAD(67) mRNA-expressing neurons were bilaterally detecte d in deep dorsal and ventral layers, this expression being correlated with the increased detection of GAD(67) immunostained somata and with the reduction of GABA immunostaining of axons. GABA immunostaining was frequently found to be associated with reactive astrocytes that exhib ited intense immunostaining for glial fibrillary acidic protein (GFAP) but remained GAD(67) negative. These results indicate that degenerati on of afferent terminals induces a biphasic response of GABAergic spin al neurons located in the dorsal horn and show that many spinal neuron s located in deeper regions re-express GAD(67), suggesting a possible participation of the local GABAergic system in the reorganization of d isturbed spinal networks.