Novel hippocampal interneuronal subtypes identified using transgenic mice that express green fluorescent protein in GABAergic interneurons

The chief inhibitory neurons of the mammalian brain, GABAergic neurons, are comprised of a myriad of diverse neuronal subtypes. To facilitate the stud y of these neurons, transgenic mice were generated that express enhanced gr een fluorescent protein (EGFP) in subpopulations of GABAergic neurons. In o ne of the resulting transgenic lines, called GIN (GFP-expressing Inhibitory Neurons), EGFP was found to be expressed in a subpopulation of somatostati n-containing GABAergic interneurons in the hippocampus and neocortex. In bo th live and fixed brain preparations from these mice, detailed microanatomi cal features of EGFP-expressing interneurons were readily observed. In stra tum oriens of the hippocampus, EGFP-expressing interneurons were comprised almost exclusively of oriens/alveus interneurons with lacunosum-moleculare axon arborization (O-LM cells). In the neocortex, the somata of EGFP-expres sing interneurons were largely restricted to layers II-IV and upper layer V . In hippocampal area CA1, two previously uncharacterized subtypes of interne urons were identified using the GIN mice: stratum pyramidale interneurons w ith lacunosum-moleculare axon arborization (P-LM cells) and stratum radiatu m interneurons with lacunosum-moleculare axon arborization (R-LM cells). Th ese newly identified interneuronal subtypes appeared to be closely related to O-LM cell, as they selectively innervate stratum lacunosum-moleculare. W hole-cell patch-clamp recordings revealed that these cells were fast-spikin g and showed virtually no spike frequency accommodation. The microanatomica l features of these cells suggest that they function primarily as "input-bi asing" neurons, in that synaptic volleys in stratum radiatum would lead to their activation, which in turn would result in selective suppression of ex citatory input from the entorhinal cortex onto CA1 pyramidal cells.