The role of nicotinic acetylcholine receptors of astrocytes in neuron-glial interactions – glial nAChRs

Astrocytes, the main glial cells of the brain, express a large set of receptors for neurotransmitters that allow them to respond to neuronal activity, often by elevations of their intracellular Ca2+ concentration. In turn, activated astrocytes release transmitters onto neurons, modulating neuronal synaptic transmission. But, in spite of the increasing amounts of reports concerning neuron-glia interactions in the last years, the specific role of most of the receptors expressed by astrocytes in the regulation of neuronal activity is still elusive. Moreover, the implications for neuron-glia interactions in pathological conditions remains poorly understood. Glial nicotinic acetylcholine receptors (nAChRs), in particular those with high Ca2+-permeability, are good candidates to participate in the bidirectional communication between astrocytes and neurons. These receptors could also be implicated in the effects of nicotine in the brain as well as in dysfunctions present in patients with deficits in the nicotinic system. The main aim of my research is to evaluate the role of astrocytic nAChRs in neuron-glia interactions and the implications for these glial receptors in the central effects of nicotine. My team will study how the activation of glial nAChRs influence neuronal activity in acute hippocampal slices of wild-type mice and of knockout mice for one or two specific nAChR subunits. Although the presence of functional nAChRs in hippocampal astrocytes has been controversial, our preliminary results indicate that astrocytic nAChRs are functional in hippocampal acute slices. Patch-clamp recordings and single cell RT-PCR for nAChR subunits are currently performed to characterize the functional properties and the subunit composition of native nAChRs expressed by astrocytes. We have already observed nAChR-mediated currents and the expression of different nAChR subunits in astrocytes. In a second step, we will evaluate how the activation of nAChRs on astrocytes influences hippocampal neuronal activity. First, we will establish the sub-cellular localization of these receptors in single astrocytes. Indeed, it is unknown whether they are present at the soma or at the level of the numerous astrocytic processes envelopping the synapses. To rapidly and locally activate these receptors in astrocytes, we will use a UV photolysis (uncaging) system for caged carbachol. Nicotinic responses will be measured by combining Ca2+ imaging and patch-clamp recordings. Then, the uncaging system will be used to focally and selectively activate single astrocytes while a neighbouring neuron is simultaneously recorded with a patch pipette. This will allow us to study the modulation of neuronal activity by glial nAChRs. In a third step, we will determine whether there is a synergic effect of nicotine and type-B monoamine oxidase (MAO-B) inhibitors important psychoactive molecules present in tobacco on nAChR-mediated neuron-glia interactions. The MAO-B inhibitors block the glial MAO-B enzyme responsible for breaking down some biogenic amine neurotransmitters such as dopamine. A reduction on MAO-B activity may therefore increase the levels of dopamine and other neurotransmitters in the brain, contributing to the nicotine action. We will perform in vivo administrations of nicotine and/or MAO-B inhibitors and we will analyze the subunit composition of nAChRs, the nAChR-mediated Ca2+ responses in astrocytes and the glial nAChRs-dependent neuronal activity in slices of treated mice. Our results should clarify the role of nAChR-mediated neuron-glia interactions. The identification of the glial nAChR subtypes influencing neuronal activity as well as the description of synergic effects of MAO-B inhibitors and nicotine at the cellular level could be useful to understand the implications for neuron-glia interactions in the central effects of nicotine, a step forward to create new therapeutic targets for smoking cessation. In the futur, the pertinence of our results wil..