Nucleus reuniens thalami innervates gamma aminobutyric acid positive cells in hippocampal field CA1 of the rat.

The aim of the present study was to investigate whether the nucleus reuniens thalami (RE) innervates inhibitory cells in hippocampal field CA1. Therefore, we examined the RE-CA1 input at the ultrastructural level. RE axons were anterogradely labeled with biotin-conjugated dextran amine (BDA), in combination with pre-embedding gamma aminobutyric acid (GABA)-immunolabeling of neurons in CA1. We observed that part of the BDA-labeled axons formed asymmetrical (i.e. excitatory) synapses on GABA-positive dendrites. Based on these data, which are in line with our previously published electrophysiological observations (Dolleman-Van der Weel, M.J., Lopes da Silva, F.H. and Witter, M.P., Nucleus reuniens thalami modulates activity in hippocampal field CA1 through excitatory and inhibitory mechanisms. J. Neurosci., 17 (1997) 5640-5650), we propose that RE-CA1 input partially influences hippocampal transmission through activation of local inhibitory interneurons.