Quantitative and morphological analysis of dentate granule cells with recurrent basal dendrites from normal and epileptic rats.

Granule cells with recurrent basal dendrites (RBDs) were previously reported in both control and epileptic rats. RBDs are dendrites that arise from the basal half of granule cell bodies and curve toward and extend into the molecular layer. They are increased in frequency in the pilocarpine model of epilepsy. The present study was undertaken to analyze the distribution and morphology of granule cells with RBDs and the synaptic connections of RBDs. Granule cells were labeled by retrograde transport of biocytin. Those with an RBD were found throughout the granule cell layer, but were most numerous at the hilar border. The morphology of these cells varied in the different depths of the granule cell layer; the angle of their cell body's long axis was mainly vertical at the hilar margin, and changed to virtually horizontal close to the molecular layer border. Quantitative data on the distribution of granule cells with RBDs and the angle of the cell body's long axis confirmed these descriptions. At the electron microscopic level, RBDs showed the typical features of dendrites and formed numerous axodendritic and axospinous synapses with labeled and unlabeled axon terminals. These results showed that RBDs of granule cells from epileptic rats are postsynaptic to axon terminals, including mossy fibers, and thus are involved in a similar synaptic circuitry as apical dendrites of granule cells from these animals.