Somatostatin Contributes to In Vivo Gamma Oscillation Modulation and Odor Discrimination in the Olfactory Bulb

Neuropeptides are systematically encountered in local interneurons, but their functional contribution in neural networks is poorly documented. In the mouse main olfactory bulb (MOB), somatostatin is mainly concentrated in local GABAergic interneurons restricted to the external plexiform layer (EPL). Immunohistochemical experiments revealed that the sst2 receptor, the major somatostatin receptor subtype in the telencephalon, is expressed by mitral cells, the MOB principal cells. As odor-activated mitral cells synchronize and generate gamma oscillations of the local field potentials, we investigated whether pharmacological manipulations of sst2 receptors could influence these oscillations in freely behaving mice. In wild-type, but not in sst2 knock-out mice, gamma oscillation power decreased lastingly after intrabulbar injection of an sst2-selective antagonist (BIM-23627), while sst2-selective agonists (octreotide and L-779976) durably increased it. Sst2-mediated oscillation changes were correlated with modifications of the dendrodendritic synaptic transmission between mitral and granule cells. Finally, bilateral injections of BIM-23627 and octreotide respectively decreased and increased odor discrimination performances. Together, these results suggest that endogenous somatostatin, presumably released from EPL interneurons, affects gamma oscillations through the dendrodendritic reciprocal synapse and contributes to olfactory processing. This provides the first direct correlation between synaptic, oscillatory, and perceptual effects induced by an intrinsic neuromodulator.