Role of mGluR5 in modulating hippocampal neuronal and synaptic function

The principal excitatory neurotransmitter in the brain, L-glutamate, has its physiological actions via three types of ionotropic receptor (iGluRs), named after the agonists N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate, as well as a family of G-protein coupled metabotropic glutamate receptors (mGluRs). Whereas iGluRs directly mediate excitatory synaptic transmission, mGluRs play a more modulatory role. mGluRs comprise eight receptors that can be divided into three groups based on sequence homology, signalling mechanisms and pharmacology. Group I receptors (mGluR1 and 5) are located postsynaptically whereas group II (mGluR2 and 3) and group III (mGluR4, 6, 7, 8) receptors are mainly expressed presynaptically with some postsynaptic and glial expression. The CA1 region of the rodent hippocampus has been used extensively to investigate the roles of glutamate receptors in synaptic transmission and plasticity. Synaptic plasticity is the process by which the efficiency of synaptic transmission is altered; the two main long-lasting forms are termed long-term potentiation (LTP) and long-term depression (LTD). Most forms of LTP, and many forms of LTD, are triggered by activation of NMDA receptors and subsequently involve alterations in AMPA receptor-mediated transmission. However, activation of mGluRs is also involved in plasticity induction. Synaptic plasticity is involved in a variety of brain functions, such as normal development of the nervous system and in learning and memory formation, and may also be involved in drug addiction and some neurological disorders. The rodent CA3:CA1 synapse lacks group II mGluRs and expresses group III receptors only early in development. In contrast, group I receptors are present throughout development. Activation of group I receptors using agonists such as DHPG elicits a variety of effects on CA1 pyramidal neurones, including changes in membrane resistance, depolarisation, a loss of spike frequency adaptation and intracellular calcium mobilisation. In addition, activation of group I receptors induces LTD of AMPA receptor-mediated transmission in both juvenile and adult rats. Localisation studies suggest that mGluR5 is the major group I subtype expressed in CA1 principal neurones, with little mGluR1 present. A conventional method of identifying subtypes involved in specific effects is to use selective antagonists; e.g. MPEP to block mGluR5 and LY367385 to block mGluR1. However, use of antagonists has produced some conflicting results; although the mGluR5 selective antagonist MPEP may block LTD induced by DHPG, this is not universally the case. Similarly, the block by MPEP of mGluR-dependent LTD induced by paired-pulse low frequency stimulation (PP-LFS) is not consistent. An alternative method of investigating subtype-specific effects is to use positive allosteric modulators (PAMs) that selectively potentiate the actions of endogenous transmitter or exogenous agonists at individual receptor types. The aims of this project will be to utilise novel mGluR5-selective PAMs to investigate the roles of mGluR5 in regulating neuronal and synaptic function. Electrophysiological recordings (extracellular or whole-cell as appropriate) will be performed from hippocampal brain slices. By combining perfusion of mGluR5 PAMs with sub-threshold doses of the group I mGluR agonist DHPG, we will determine the role of mGluR5 in regulating neuronal properties (such as membrane potential and input resistance) and excitatory transmission. In addition, the receptors will be activated synaptically by using stimulus paradigms known to activate mGluRs (e.g. PP-LFS to induce LTD or trains of high frequency stimulation to induce an mGluR-dependent synaptic response and induce LTP). The effect of PAMs on these stimulus trains will be investigated. Calcium imaging will also be used to investigate the role of mGluR5 in controlling intracellular calcium levels.