Presynaptic modulation of synaptic inhibition onto hippocampal pyramidal neurons

PROJECT SUMMARY Throughout the hippocampus, pyramidal cells integrate excitatory and inhibitory inputs from a diverse population of interneurons. These cells provide feedforward and feedback inhibition onto CA1 pyramidal cells, and can be recruited via glutamate spillover, a phenomenon that describes non-synaptic actions of glutamate diffusing away from the synaptic cleft. The prevailing view, at present, is that glutamate spillover leads to activation of presynaptic group III metabotropic (mGluRIII) glutamate receptors, which inhibit synaptic excitation and inhibition only onto interneurons, not CA1 pyramidal cells. Against this interpretation, our preliminary data indicates that spillover activation of mGluRIII receptors also exists at inhibitory synapses formed onto CA1 pyramidal cells, when experiments are performed under experimental conditions that mimic more closely the ionic composition of the cerebrospinal fluid. The main goals in this application are: (i) to determine how mGluRIII activation via glutamate spillover varies between excitatory and inhibitory inputs, and among inhibitory inputs formed by different classes of interneurons; (ii) to identify the intracellular signaling mechanisms that couple mGluRIII activation to neurotransmitter release; (iii) the determine the sub-cellular location of different type of mGluRIII receptors at different types of inhibitory synapses onto CA1 pyramidal cells; (iv) to determine how mGluRIII activation alters encoding of spatial information by hippocampal CA1 place cells in vivo. The proposed experiments capitalize on in vitro and in vivo electrophysiology, and EM approaches. These findings will generate new information on the operating principles of neuronal circuits involved in spatial map representation and will contribute to the future development of effective strategies to treat disease states associated with hippocampal neural circuit dysfunction, like epilepsy and autism spectrum disorder.