LOCAL PROJECTIONS OF GABAERGIC NEURONS IN THE DENTATE GYRUS AND CA1 REGION IN THE RAT HIPPOCAMPAL-FORMATION

Using retrograde transport of wheat germ agglutinin conjugated colloid al gold (WGA-gold) combined with immunoreactivity for glutamate decarb oxylase (GAD), a specific synthesizing enzyme for gamma-aminobutyric a cid (GABA), local projections of GABAergic neurons in the dentate gyru s and CA1 were examined. In the hilus of the dentate gyrus, it was fou nd that GABAergic neurons in the granule cell layer projected to the i psilateral upper leaf of the molecular layer, with a mediolateral exte nsion of more than 1.2 mm and a rostrocaudal extension of over 0.8 mm. Non-GABAergic neurons in nearly the entire hilar area were found to p roject to the ipsilateral upper leaf of the molecular layer. In the do rsal CA1 region, GABAergic neurons in the stratum pyramidale and radia tum converged onto the ipsilateral stratum pyramidal/oriens, with a me diolateral extension of over 1 mm and a rostrocaudal extension of over 0.7 mm. These results provide direct evidence that in both the dentat e gyrus and CA1, GABAergic interneurons from a fairly large field conv erge onto a very small target area. This suggests that the output sign als from GABAergic neurons in the dentate gyrus and CA1, and non-GABAe rgic neurons in the dentate gyrus, may propagate beyond the anatomical limits contained in conventional slice preparations of the hippocampa l formation.