Slow excitatory synaptic transmission in dopaminergic neurons: the role of glutamate receptor delta 1 (GluD1) – SExSy

Glutamate (Glu) is the main excitatory neurotransmitter in the vertebrate central nervous system. Its effect are mediated by the activation of ionotropic and metabotropic receptor (iGlu and mGlu). Among iGlu, the role of the delta family, and especially of GluD1, in synaptic functions is still undefined. The functional significance of GluD1 has been inferred from human genetic studies reporting that the GRID1 gene, coding for GluD1, is a strong candidate gene for several psychiatric disorder such as schizophrenia. Interestingly, GluD1 knockout mice (GluD1-KO) mice exhibit hyperactivity, depression-like behavior, hyperaggressiveness, deficits in social interaction, altered expression of other iGlu. Recently, I showed that GluD1 is widely expressed in the rodent brain with high expression levels in regions such as the cerebral cortex, hippocampus, striatum and midbrain. GluD1 is mainly localized at postsynaptic density of excitatory synapses (Hepp et al, 2014). However little is known to date about the cellular and synaptic functions of GluD1 as well as its role in synaptic circuitry.

Unlike other iGlu, GluDs have been considered until very recently as orphan receptors with no identified ligand triggering their opening. However, I recently contributed in de-orphanizing GluD receptors by showing that they open upon activation of type I mGlu. I have shown that deletion of GluD1 or loss of functional GluD1 channel impairs slow mGlu-mediated synaptic transmission and activity of midbrain DA neurons. In vivo, the firing of these neurons comprise phasic discharges or burst episodes of action potentials, this latter mode of firing being associated with higher release of DA in target brain structure and conveys, amongst other things, motivationally relevant information to the forebrain. The bursting activity in these DA neurons was almost completely abolished in GluD1-KO mice or after expression of a pore mutant GluD1. My results indicate that GluD1 is crucial for the normal activity of DA neurons and in their integrative properties of glutamatergic afferences.

The major goals of this proposal is to elucidate the functional role of GluD1 in excitatory synaptic transmission at single cell and network levels, in vitro and in vivo, and, to develop new molecular and opto-pharmacological tools allowing to assess the specific role of GluD1 in specific pathways. This will consist in assessing cell autonomous effect of GluD1 on the activity of DA neurons, establishing the cellular/molecular origin of dysfunctions observed in DA neurons of GluD1-KO mice, and restoring/altering selectively and locally the function of DA neurons mice using genetic conditional tools and in vitro/in vivo electrophysiology. The role of GluD1 in specific afferent excitatory synaptic transmission onto DA neurons will be probed by optogenetics and in vitro electrophysiology. The presence of GluD1 at several input synapses will be further examined at the ultrastructural level. In order to spatially and temporally manipulate GluD1 channel function, we will develop a opto-chemical genetic strategy by attaching synthetic photosensitive ligand onto a genetically engineered GluD1 to allow inhibition of only that specific protein with light.

The feasibility of this project is supported by my published and unpublished results and my own expertise in gene transfer, electrophysiology, genetic manipulations, and optogenetics combined with the established expertise of the collaboration. Proposed experiments are based on techniques, equipments and biological tools available in the lab. This project, for which I have gathered critical technical collaborations, will generate useful genetic tools for future study of GluD1, and unravel the importance of GluD1 contribution to DA cell excitability and brain connectivity.