POTENTIATION OF NERVE GROWTH FACTOR-INDUCED ALTERATIONS IN CHOLINERGIC FIBER LENGTH AND PRESYNAPTIC TERMINAL SIZE IN CORTEX OF LESIONED RATS BY THE MONOSIALOGANGLIOSIDE GM1

The effect of monosialoganglioside GM1 and/or nerve growth factor trea tment on the cholinergic innervation of the rat cortex was studied usi ng both light- and electron-microscopic techniques assisted by image a nalysis. Adult male Wistar rats were unilaterally decorticated and rec eived continuous infusions, via minipump, of vehicle, GM1 (1.5 mg/day) and/or nerve growth factor (12 mu g/day) into the cerebroventricular space. Treatments were initiated immediately post-lesion and ended aft er seven days. Thirty days post-lesion (i.e. 23 days after the end of drug administration) brains were processed for choline acetyltransfera se immunocytochemistry for either light- or electron-microscopic analy sis. At this time-point choline acetyltransferase-immunoreactive neuro ns in the ipsilateral nucleus basalis magnocellularis were significant ly reduced in size especially in the mid portion of this nucleus, in l esion vehicle-treated rats. Moreover, decreases in choline acetyltrans ferase immunoreactive fibre length (ranging from 31 to 50%) and varico sity number (ranging from 26 to 39%) occurred in all cortical layers w ithin a portion of the remaining cortex of these animals. Monosialogan glioside GM1 or nerve growth factor treatment equally attenuated defic its in nucleus basalis magnocellularis cell size and cortical choline acetyltransferase immunoreactive fibre length. However, nerve growth f actor, but not monosialoganglioside GM1 treatment also increased choli ne acetyltransferase-immunoreactive varicosity number above control le vels. In lesioned rats which received both nerve growth factor and the monosialoganglioside GM1, the mean cross-sectional area of nucleus ba salis magnocellularis cholinergic neurons did not differ significantly from control values. By contrast, cortical choline acetyltransferase- immunoreactive fibre length and varicosity number were significantly i ncreased above control values and that induced by nerve growth factor treatment alone. Quantitative electron-microscopic analysis showed tha t cholinergic boutons in cortical layer V were considerably shrunken i n lesioned vehicle-treated rats and that GM1 treatment failed to signi ficantly attenuate this deficit. However, exogenous nerve growth facto r provoked a significant increase (35% above control values) in cortic al cholinergic presynaptic terminal size which was even further augmen ted by concurrent GM1 treatment (69% above control values). This troph ic factor-induced increase in bouton size was confirmed using serial e lectron microscopy and computer-assisted three-dimensional reconstruct ion of the cholinergic varicosities. The number of synaptic contacts i n cortical layer V was also found to be significantly reduced (45% of control values) in lesioned vehicle-treated rats but was maintained at control levels by exogenous GM1 treatment. In addition, a significant increase (95% above control levels) in the number of choline acetyltr ansferase-immunoreactive boutons with synaptic differentiations was no ted in lesioned nerve growth factor-treated rats. Concurrent GM1 and n erve growth factor treatment did not cause a further increase in synap tic number. Cortical cholinergic innervation and the size of choline a cetyltransferase-immunoreactive nucleus basalis magnocellularis neuron s were not altered in unlesioned rats by GM1. nerve growth factor or n erve growth factor and GM1 treatment. These results demonstrate that G M1 can attenuate deficits in cortical cholinergic innervation followin g injury and can further augment nerve growth factor induced synaptic remodelling in the injured adult rat brain.