Neuromodulation in Chemosensory Pathways

Abstract

Interactions with the environment depend not only on sensory perception of external stimuli but also on processes of neuromodulation regulated by the internal state of an organism. These processes allow regulation of stimulus detection to match the demands of an organism influenced by its general brain state (satiety, wakefulness/sleep state, attentiveness, arousal, learning etc.). The sense of smell is initiated by sensory neurons located in the nasal cavity that recognize environmental odorants and project axons into the olfactory bulb (OB), where they form synapses with several types of neurons. Modulations of early synaptic circuits are particularly important since these can affect all subsequent processing steps. While the precise mechanisms have not been fully elucidated, work from many labs has demonstrated that the activity of neurons in the OB and cortex can be modulated by different factors inducing specific changes to olfactory information processing. The symposium "Neuromodulation in Chemosensory Pathways" at the International Symposium on Olfaction and Taste (ISOT 2016) highlighted some of the most recent advances in state-dependent network modulations of the mouse olfactory system including modulation mediated by specific neurotransmitters and neuroendocrine molecules, involving pharmacological, electrophysiological, learning, and behavioral approaches.

Keywords: hormonal status; learning; mouse olfactory system; network modulations; state-dependent; surprise and expectation; top–down inputs.

Figure 1.

Diagram of bottom-up and top-down inputs into the olfactory bulb. Olfactory sensory neurons (OSNs) located in the olfactory epithelium project axons into the olfactory bulb (OB), transmitting sensory information to this brain region. OSN axons synapse with both projection neurons (mitral cells, MC) and interneurons (periglomerular, PG and external tufted, ET). Mitral cells also make connections with granule cells (GCs) and PGs. Several regions of the brain, including the Raphe, anterior olfactory nucleus (AON) and horizontal limb of the diagonal band of Broca (HDB) also project back to the OB (top–down) to modulate the perception of odors. Courtesy of A.C. Puche, modified from (Aungst et al. 2003) and “The Rat Nervous System” 2^nd Edition.

Figure 2.

Model of neuromodulatory signaling in the olfactory bulb relying upon metabolic cues. Glucose, insulin, and glucagon-like peptide (GLP-1) levels are sensed in mitral cells of the OB through phosphorylation and post-translational changes of a potassium ion channel, Kv1.3. dSAC, deep short axon cells; GLUT4, glucose transporter 4; GLP-1R, glucagon-like peptide receptor 1; GCL, granule cell layer; GLM, glomerular cell layer; MCL, mitral cell layer; PKA, protein kinase A; InsR, insulin receptor kinase.