FIBROBLAST GROWTH FACTOR-II PROTECTS ENTORHINAL LAYER-II GLUTAMATERGIC NEURONS FROM AXOTOMY-INDUCED DEATH

The entorhinal cortex is a major relay between the hippocampus and oth er cortical and subcortical regions. Glutamatergic axons from layer II neurons form the entorhinal cortical projection to the hippocampus vi a the perforant pathway. We have demonstrated previously that lesion o f the perforant pathway causes the death of similar to 30% of entorhin al layer II (ECL2) neurons. To elucidate mechanisms contributing to ne uronal death and to investigate strategies preventing it, we identifie d the phenotype of the vulnerable neuronal population. Sections were i mmunolabeled with antibodies to the neuronal markers NeuN, glutamate, and calbindin-D-28k, and to receptors for fibroblast growth factor-2 ( FGFR1) and NMDA (NMDAR1) and were examined using confocal microscopy. Calbindin immunoreactivity was strikingly lamina-specific to ECL2, whe re one-third of all ECL2 neurons were calbindin-positive. Localization of glutamate revealed that half of the glutamatergic ECL2 neurons coe xpressed calbindin. Quantification using unbiased stereology at 9 week s after lesion of the perforant pathway revealed that the only ECL2 ne uronal population that experienced a significant (70%) loss (20% of th e total) was the population of glutamatergic ECL2 neurons that did not coexpress calbindin. All ECL2 neurons expressed FGFR1; therefore, we tested the role of FGF-2 in the survival of glutamatergic ECL2 neurons , We grafted fibroblasts genetically engineered to express nerve growt h factor or FGF-2 and found that only FGF-2 grafts prevented loss of t he vulnerable glutamatergic/calbindin-negative neurons. We present a h ypothesis for the selective vulnerability of these glutamatergic/calbi ndin-negative ECL2 neurons and address the role of FGF-2 in neuronal r escue.