Afferent and efferent synaptic connections of somatostatin-immunoreactive neurons in the rat fascia dentata.

The aim of this study was to determine whether somatostatin (SS)-immunoreactive neurons of the rat fascia dentata are involved in specific excitatory circuitries that may result in their selective damage in models of epilepsy. Synaptic connections of SS-immunoreactive neurons were determined at the electron microscopic level by using normal and colchicine pretreated rats. Vibratome sections prepared from both fascia dentata of control animals and from rats that had received an ipsilateral lesion of the entorhinal cortex 30-36 hours before sacrifice were immunostained for SS by using a monoclonal antibody (SS8). Correlated light and electron microscopic analysis demonstrated that many SS-immunoreactive neurons in the hilus send dendritic processes into the outer molecular layer of the fascia dentata, and dendrites of the same neurons occupy broad areas in the dentate hilar area. The majority of SS-immunoreactive axon terminals form symmetric synapses with the granule cell dendrites in the outer molecular layer and also innervate deep hilar neurons. Via their dendrites in the outer molecular layer, the SS-immunoreactive neurons receive synaptic inputs from perforant pathway axons which were identified by their anterograde degeneration following entorhinal lesions. The axons from the entorhinal cortex are the first segment of the main hippocampal excitatory loop. The hilar dendrites of the same SS-immunoreactive cells establish synapses with the mossy axon collaterals which represent the second member in this excitatory neuronal chain. These observations suggest that SS-immunoreactive neurons in the dentate hilar area may be driven directly by their perforant path synapses and via the granule cells which are known to receive a dense innervation from the entorhinal cortex. These observations demonstrate that SS-immunoreactive neurons in the hilar region are integrated in the main excitatory impulse flow of the hippocampal formation.