EXPRESSION OF NGF AND NT3 MESSENGER-RNAS IN HIPPOCAMPAL INTERNEURONS INNERVATED BY THE GABAERGIC SEPTOHIPPOCAMPAL PATHWAY

We used in situ hybridization for the detection of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 ( NT3) mRNAs combined with immunocytochemistry against the calcium-bindi ng proteins parvalbumin (PARV), calbindin 28k (CALB), and calretinin ( CALR) to determine the expression of neurotrophins in functionally dis tinct subsets of hippocampal interneurons. Most PARV-immunoreactive ne urons in the hippocampus were NGF mRNA-positive (82%), which correspon ds to 71% of NGF-positive neurons in the hippocampus proper and in the dentate gyrus (excluding granule cells). In contrast, only a subset o f CALB- and CALR-immunoreactive interneurons (24% and 23%, respectivel y) displayed hybridization signals for NGF. Small subsets of PARV- and CALR-positive cells expressed NT3 mRNA, but we did not find hippocamp al interneurons expressing BDNF mRNA. These results show that NGF and NT3 genes are differentially regulated in distinct subsets of GABAergi c cells, and these interneurons are a major source of NGF production i n the hippocampus. We also addressed whether hippocampal interneurons expressing neurotrophins were targets of the GABAergic septo-hippocamp al pathway. We developed a triple-labeling method that combines antero grade tracing of this pathway by means of Phaseolus vulgaris leucoaggl utinin injections, with in situ hybridization for the detection of neu rotrophins, and immunocytochemistry for calcium-binding proteins. Virt ually every PARV-positive neuron innervated by GABAergic septohippocam pal baskets expressed NGF mRNA (86%), whereas 39-59% of CALR- and CALB -positive interneurons that were contacted by GABAergic septohippocamp al axons showed NGF gene expression. A small subset of NT3 mRNA-expres sing interneurons was also innervated by septohippocampal baskets. The se findings show that the GABAergic septohippocampal pathway preferent ially terminates on interneurons expressing NGF mRNA, suggesting that this neurotrophic factor might be involved in the specification of thi s connection and in its maintenance and normal function in the adult b rain.