Somatostatin-immunoreactive interneurons contribute to lateral inhibitory circuits in the dentate gyrus of control and epileptic rats

Lateral inhibition, a feature of neuronal circuitry that enhances signaling specificity, has been demonstrated in the rat dentate gyrus. However, neit her the underlying neuronal circuits, nor the ways in which these circuits are altered in temporal lobe epilepsy, are completely understood. This stud y examines the potential contribution of one class of inhibitory interneuro ns to lateral inhibitory circuits in the dentate gyrus of both control and epileptic rats. The retrograde tracer wheat germ agglutinin-apo-horse radis h peroxidase-gold (WGA-apo-HRP-gold) was injected into the septa[ dentate g yrus. Neurons double-labeled for glutamic acid decarboxylase (GAD) and the retrograde tracer are concentrated in the hilus and may contribute to later al inhibition. Neurons double-labeled for somatostatin and the retrograde t racer account for at least 28% of GAD-positive neurons with axon projection s appropriate for generating lateral inhibition in control rats. Despite an overall loss of somatostatin-expressing cells in epileptic animals, the nu mber of somatostatin-positive interneurons with axon projections appropriat e for generating lateral inhibition is similar to that seen in controls. Th ese findings suggest that somatostatinergic interneurons participate in lat eral inhibitory circuits in the dentate gyrus of both control and epileptic rats, and that surviving somatostatinergic interneurons might sprout new a xon collaterals in epileptic animals. Hippocampus 2001;11:418-422. (C) 2001 Wiley-Liss, Inc.