ELECTROPHYSIOLOGY OF RETROHIPPOCAMPAL NEURONS

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The hippocampal formation can be divided into two circuits of neurons on the basis of anatomical connections and the behavioral correlates of firing of cells. The "Ammon's horn circuit" is an essentially unidirectional loop of excitatory neurons from the entorhinal cortex (EC) through the dentate gyrus, Ammon's horn (the so-called tri-synaptic pathway), and subiculum. Entorhinal, Ammon's horn and subicular neurons are known to fire action potentials when the animal is in a particular location in the recording environment (place cells). In those parts of the "presubicular circuit" that have been examined (presubiculum, postsubiculum, some anterior thalamic nuclei and retrosplenial cortex), neurons fire action potentials when the animal faces a particular direction within the recording environment (head direction cells). This project will examine the electrophysiology of three retrohippocampal regions: a) the pre and postsubiculum (members of the head direction cell circuit); b) the subiculum (member of the place cell circuit); and c) the entorhinal Cortex (the structure that controls information flow between the two circuits). The electrophysiological properties of single neurons, their local connectivity and their contributions to the pre-/postsubiculum to entorhinal to subiculum pathway will be studied in slices and in anesthetized animals. The purpose will be to: a) characterize the cells of the electrophysiologically undefined subicular complex; b) identify the specific neuronal elements in the disynaptic pathway that serves to transfer information between two distinct limbic system circuits; and c) to provide electrophysiological criteria for interpreting recordings from other preparations such as extracellular unit data from freely behaving animals. This work is important in the understanding of navigation, a function that is frequently lost in dementias. Also, because all output from the hippocampal formation goes through the subiculum or the entorhinal cortex, this work will be important in the understanding of the spread of epileptic activity to other parts of brain.