The Canonical Transient Receptor Potential family of ion channel subunits comprise 7 distinct gene products (TRPC1-7), most of which are expressed in the central nervous system. Three subfamily members of this class (TRPC1, TRPC4, and TRPC5) are highly expressed in the hippocampus. These ion channel subunits forms homomeric and heteromeric channels with distinct characteristics. In addition, the currents mediated by these proteins are activated or potentiated by phospholipase C via subtypes of G protein-coupled receptors and tyrosine kinase receptors. In particular, Gq/11-linked receptors such as the type 1 metabotropic glutamate and M1, M3, and M5 muscarinic receptors, activate these currents by altering plasma membrane PIP2 and increasing intracellular Ca2+ concentrations. The function of the TRPC1/4/5 subfamily in hippocampus is not known. We hypothesize that hippocampal function is modulated by receptors that activate or potentiate these excitatory ion channels. We have generated mice lacking the TRPC4, TRPC5, both TRPC4 and TRPC5 genes, obtained the TRPC1 knockout mouse, and are breeding TRPC1/4/5 triple knockout mice. Here we propose to use these mice to understand the function of these ion channels in the hippocampus using protein localization, behavioral studies, acute brain slice recordings of hippocampus, and recordings of isolated hippocampal neurons. The results of these studies should clarify receptor-mediated alteration of hippocampal-mediated spatial and contextual memory, and identify new therapeutic targets for diseases and surgeries that affect the hippocampus.
Long term memory and spatial navigation rely upon the function of the hippocampus, a large region of the brain located beneath the cortical surface. Damage to the hippocampus, from oxygen starvation, encephalitis, medial temporal lobe epilepsy, tumors, and Alzheimer's disease, may result in amnesia, the inability to form or retain new memories. The basic functional unit of the hippocampus (and brain) is the richly modifiable synapse, the point where one neuron passes information to another. Synapses are endowed with a dense array of proteins that enable a panoply of modulatory influences. Foremost among the proteins that mediate synaptic function are ion channels. The function of these proteins is to initiate and control information flow from one synapse to another, and in particular, to initiate changes in intracellular calcium that control neurotransmitter release. The ion channels that are absolutely required for synaptic function in the hippocampus are largely known. Less well known are the functions of modulatory ion channels, known as TRP channels. These ion channels are activated by special classes of receptors and allow calcium to flow directly into neurons. In this proposal, we outline how we will determine the function of the canonical subfamily of TRPC channel subunits comprised of TRPC1, TRPC4, and TRPC5. Currently there are no pharmacological agents that specifically block these channels. We have genetically modified mice that lack each of these genes, as well as mice lacking combinations of all 3 of the genes. We will conduct experiments with these genetically modified mice in order to understand changes in function of the hippocampus, as determined by behavioral studies and detailed electrophysiological studies of the neurons normally expressing these channels. The long term practical benefit of such studies is to better understand how the hippocampus works, as well as define potentially new therapeutic targets that may be used to ameliorate conditions such as learning and memory deficits that occur in development, oxygen starvation during birth, central nervous system surgeries and cancers, and Alzheimer's disease.
