Cholinergic Modulation Of Neural Circuit In The Mouse Olfactory Bulb

Objective: Acetylcholine is an important neurotransmitter in the central nervous system. Cholinergic neurons in the basal forebrain regulate attention as well as learning and memory by releasing acetylcholine. The main olfactory bulb(MOB) receives dense centrifugal inputs from both cholinergic neurons and GABAergic neurons in the horizontal limb of the diagonal band of Broca(HDB).The cholinergic projection is believed to play important roles in the olfactory information processing and olfactory memory. Behavioral experiments suggest that cholinergic projection affects odors discrimination and olfactory memory formation in animals. However, it remains untested how signal processing in the olfactory system is modulated by rapid and selective activation of HDB cholinergic neurons. Recent studies found that patients with neurodegenerative diseases such as Alzheimer’s disease are more likely to be accompanied by a loss of cholinergic neurons and a defect in olfaction. Hense,understanding the roles of cholinergic system in olfactory information processing may be have therapeutic significance for the pathogenesis and early diagnosis of neurodegenerative diseases. The research focuses on how the neuralcircuit in the main olfactory bulb is regulated by acetylcholine. Method: The recombinant adenovirus was injected into HDB of the mouse brain by stereotactic system, and then the distribution of cholinergic fibers in the olfactory bulb was observed. 300?m horizontal olfactory bulb acute slices were prepared for patch clamp recordings. Periglomerular cells(PGC) in the glomerular layer and mitral cells(MC) in the mitral cell layer were recorded by t recording electrodes with neurobiotin. Acetylcholine was applied by a puffing electrode near PG cells. After the recording, the brain slices were fixed with 4%paraformaldehyde and stained with Cy3 second antibody. The images were scanned by laser scanning confocal microscope, and the cell morphology was determined. All electrophysiological data were analyzed and processed by Clampex 10.2 software(Axon). Results : Cholinergic fibers projected to extensive regions in MOB, mainly concentrated in the glomerular layer.Acetylcholine enhanced spontaneous and evoked firing rates remarkably in PGCs expressing vesicular GABA transporter(VGAT). In addition,acetylcholine induced inward currents in PGCs in voltage clamp mode. The increase of firing rate and inward currents in PGCs were blocked by mAChR antagonist atropine, but not the synaptic blockers. Nicotine had no such effect on PGCs. Acetylcholine by activating mAChR increased excitatory postsynaptic inputs in PGCs by activating mAChR. MCs, the major projection neurons in MOB were inhibited by mAChR activation. This inhibition was blocked by GABAA receptor antagonist. The discharges of dopaminergic PGCs in MOB was inhibited directly by acetylcholine. Conclusion: Acetylcholine increases the excitability of GABAergic PGCs through mAChR activation, which isindependent of synaptic transmission. Acetylcholine inhibits mitral cells indirectly by activating GABAergic periglomerular cells and regulate the outputs of MOB. Acetylcholine inhibits dopaminergic PGCs through mAChR activation. Acetylcholine plays an important role in regulating neural circuit in MOB, which may be closely related to olfactory discrimination and olfactory memory.