Unexpected expression of Olfactory Receptors in mesencephalic dopaminergic neurons.

Parkinson’s disease is the second most common neurodegenerative disease after Alzheimer’s disease. One of the most evident pathological hallmarks in PD is the selective loss of mesencephalic dopaminergic (mDA) neurons with the consequent decrease of dopamine in the brain. mDA neurons present two main groups of projecting cells: the A9 neurons of the Substantia Nigra (SN) and the A10 cells of the Ventral Tegmental Area (VTA). A9 neurons controls the nigrostriatal pathway and are involved in regulation of voluntary movements and postural reflexes. Their selective degeneration leads to Parkinson’s disease (PD) and the loss of DA synapses in the striatum is believed to be primary cause for the disruption of the ability to control movements. Until now, the causes of the degeneration of A9 neurons in PD are still unknown and a lot of efforts have been done to determine the molecular differences between the dopaminergic cell subpopulations that could explain the selective susceptibility of A9 neurons. Thanks to new technologies developed in the last decade, like nanoCAGE, it was possible to investigate in depth the gene expression profiling of dopaminergic neurons. NanoCAGE and Affimetrix Exon Array technologies applied on A9 and A10 cells collected with Laser Capture Microdissection, revealed the presence of a subset of olfactory receptors in mDA cells. Expression of mDA-specific ORs was validated by PCR on RNA extracted from mouse midbrain and from LCM-purified A9 and A10 mDA neurons. Furthermore, In situ hybridization confirmed the selected expression of ORs in subpopulations of DA cells. Mesencephalic ORs were classified in a genetic tree for their sequence/structure similarities and their potential ligands were identified by homology modelling. Full length ORs were cloned from the midbrain and ectopically expressed on the cell surface of HEK cells to test their responses to odorants. We observed that a subset of chemical odors stimulated mDA-ORs expressed in heterologous cells. Moreover, performing Ca++ imaging experiments on dopaminergic cells isolated from mouse ventral midbrain, I demonstrated that endogenous ORs are expressed in these neurons and importantly, they are functional and respond to odor stimulation. Altogether these results indicate that odorant receptors might contribute to the normal physiology of dopaminergic cells and open new interesting questions about the role of these receptors in the pathology of mDA neurons in Parkinson’s disease. According to this, odour molecules could be used as agonists to trigger ORs activation in DA neurons that might be new targets of therapeutic intervention.