• olfactory bulb
• somatostatin
• somatostatin receptor 4 (sst4)

The olfactory bulb (OB) is the first central relay of the olfactory sense. Through its laminar organization with well-defined cytoarchitecture and physiology, it mediates perception and integration of the olfactory signals. Sensory neuron axons synapse directly onto the apical dendrites of its principal neurons, the mitral and tufted cells, which the relay the olfactory information directly toward the olfactory cortex and other brain regions. As in most sensory system, synaptic transmission is finely tuned by GABAergic inhibitory interneurons which process the incoming information at different levels of the neural network.
The main bulbar interneuron populations, known to process the olfactory inputs, are the periglomerular and granule cells. But based on their morphology, localization and neurochemical properties, various interneurons sub-populations coexist whose specific roles in the synaptic and cellular plasticity of the bulbar network are extensively studied. Among them, some interneurons express neuropeptides. More than just neurochemical markers, neuropeptides are known to act as important regulators of neurochemical activity, able to regulate neuronal excitability of neural networks.
Somatostatin (SRIF) is one of the most abundant neuropeptide in the adult central nervous system (CNS). Six G-protein-coupled receptors, sst1, sst2A, sst2B, sst3, sst4, and sst5 mediate their cellular actions. SRIF is found in the mammalian OB, with immunoreactive cells and fibers found in the inner granule cell and external plexiform layers and to a lesser extent in the pe-ripheral glomerular layer. Binding and immunohistochemical data evidenced a differential distribution of sst2, sst3 and sst4 receptors through the bulbar layers, suggesting that the peptide modulates distinct levels of olfactory processing from information processing to integration or memorization of the olfactory signals (see Martel et al, 2014 in appendix).
The aim of this work was to decipher the functional role of the sst4 receptor in the physiology of the OB.
Based on preliminary 125-SRIF binding results showing a prominent sst4-type labeling in the glomerular layer, I focused on glomerular neurons. Immuno-histochemical studies performed on coronal sections of mouse OB shown that sst4 receptors are restricted to a calretinin-positive subpopulation of TH-negative GABAergic interneurons in the GL .
On the other side a behavioral study was performed to investigate the functional consequences of sst4 invalidation on olfactory performances. Wild-type and sst4 KO male mice were compared in a battery of behavioral protocols aimed at detecting olfactory acuity, discrimination or memory defects, using spontaneous odor exploration and discrimination tasks and olfactory operant conditioning in an automated olfactometer. However, no major olfactory defect was observed in sst4-invalidated mice during the behavioral screening.