title: Local circuit interactions between oriens/alveus interneurons and CA1 pyramidal cells in hippocampal slices: electrophysiology and morphology.
abstract: Electrophysiological and anatomical techniques were used to determine the role, in the hippocampal circuitry, of local circuit neurons located at the oriens/alveus border (O/A interneurons). Intracellular recording from these cells showed that their response characteristics were clearly nonpyramidal: high input resistance, short membrane time constant, short-duration action potential, pronounced, brief afterhyperpolarizations (AHP), and nondecremental firing during intrasomatic depolarizing current pulses. Intracellular Lucifer yellow (LY) injection and subsequent fluorescence microscopy confirmed their nonpyramidal nature. O/A interneuron somata were bipolar or multipolar; their dendrites projected mostly parallel to the alveus, except for 1 or 2 processes that turned perpendicularly, and ascended through stratum oriens and pyramidale and into radiatum. Their axons were seen to branch profusely in stratum oriens and pyramidale. Simultaneous intracellular recordings from O/A interneurons and CA 1 pyramidal cells showed that pyramidal cells directly excite these interneurons. Major hippocampal afferents also directly excited the O/A interneurons. In a small number of interneuron-pyramidal pairs, stimulation of the O/A interneuron directly inhibited pyramidal cells. In one case, reciprocal connections were observed: The pyramidal cell excited the interneuron, and the interneuron inhibited the pyramidal cell. In 1 interneuron-to-interneuron pair, an inhibitory connection from O/A interneuron to stratum pyramidale interneuron was also observed. With intracellular HRP injections into O/A interneurons and subsequent electron microscopy, we observed that O/A interneuron axons made contacts with pyramidal and nonpyramidal cells. HRP-filled symmetric synaptic contacts were found on pyramidal cell dendrites and somata. HRP-filled axons also made contacts with pyramidal cell initial segments. HRP-filled O/A interneuron axon contacts were also found on nonpyramidal cell dendrites in stratum oriens. These electrophysiological and anatomical results suggest that O/A interneurons make synaptic contact with pyramidal cells and may mediate feedforward and feedback inhibition onto CA 1 pyramidal cells.
mesh_terms: D000818:Animals; D003712:Dendrites / Q000648:ultrastructure; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D006624:Hippocampus / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D006735:Horseradish Peroxidase; D066298:In Vitro Techniques; D007395:Interneurons / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D007546:Isoquinolines; D008564:Membrane Potentials; D008854:Microscopy, Electron; D009474:Neurons / Q000145:classification / Q000502:physiology; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D005456:Fluorescent Dyes; D007546:Isoquinolines; C017475:lucifer yellow; D006735:Horseradish Peroxidase