Commissural afferents innervate glutamate decarboxylase immunoreactive non-pyramidal neurons in the guinea pig hippocampus.

The innervation of GABAergic hippocampal neurons by commissural fibers was investigated in the guinea pig by a combined anterograde degeneration - immunocytochemical technique. Presumed GABAergic neurons were identified by immunocytochemistry for glutamate decarboxylase (GAD) and the commissural fibers by electron-dense degeneration following contralateral transection of the fimbria. Commissural afferents were found to establish asymmetric synaptic contacts with non-pyramidal GAD-immunoreactive neurons located in subpyramidal and suprapyramidal zones of region CA1. The observed connection suggests that inhibition of pyramidal cells may occur in a feed-forward manner as postulated by electrophysiological studies.