Function of galanin receptor subtypes in the hippocampus

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DESCRIPTION (provided by applicant): Galanin, a broadly expressed neuropeptide, when applied intracerebroventricularly impairs cognitive performance, dampens seizure activity, and affects sexual activity and feeding behaviors. These diverse pharmacological effects are exerted by at least three known G protein coupled receptors: GALR1, GALR2, and GALR3. The in vivo effects of galanin have been determined by using the peptide galanin and some high affinity chimeric galanin antagonists, M15, M35 and M40, which we introduced. These ligands lack receptor subtype selectivity, and thus the contribution of the specific galanin receptor subtypes to the multiple in vivo effects of galanin is not known. The second messenger systems coupled to activation of the different galanin receptor subtypes have been studied in transfected non-neuronal cells, leaving in doubt whether the same signaling occurs in hippocampal neurons expressing these receptors with their own set of G-proteins. In this study we wish to determine the contribution of GALR1, expressed in pyramidal cells of CA1, and of GALR2, expressed in the granule cells in the dentate gyrus, to signaling in these neurons. We will examine CREB and MAPK pathways and hippocampal excitability, including LTP in the dentate gyrus and CA1 regions, which are markedly depressed by galanin. In the absence of receptor subtype-specific ligands, to determine the roles of specific galanin receptor subtypes we will utilize the GALR1 (-/-) transgenic mice and in vivo down regulation of GALR1 and GALR2 expression by cell penetrating peptide nucleic acid (PNA) type antisense oligonucleotides applied icv. Finally, the subtype(s) specificity of presynaptic galanin receptors that regulate noradrenaline (NE), glutamate and acetylcholine (ACh) release in the hippocampus will be addressed in microdialysis experiments. We hypothesize that galanin acting at GALR1 and GALR2 receptors suppresses hippocampal excitability with consequences for seizure threshold and cognitive function. It is expected that the results will form the basis for understanding the role of different galanin receptor subtypes in cognition, and define these galanin receptor subtypes as putative drug targets for cognitive performance-enhancing drugs. Furthermore, these galanin receptor subtypes participate in the regulation of the firing of serotonergic and noradrenergic cells in the brain, and information regarding their mechanism of action in the hippocampus may contribute to understanding their effects in these monoaminergic systems, and in depression. Also, because galanin has been shown to have anti-epileptic actions, understanding how this peptide regulates seizures could have clinical applications in the treatment of epilepsy.