Neuromodulation of olfactory transformations
Introduction
Neuromodulation can be defined as a neurochemical process that serves to modify (modulate) the computations performed by a neuron or network as a function of task demands or behavioral state. This modulation can be mediated by neurochemicals arising from extrinsic sources (including acetylcholine from basal forebrain nuclei, norepinephrine from the locus coeruleus, and other amines and peptides), as well as those released from neurons intrinsic to a local network (including ancillary effects of classical neurotransmitters mediated by metabotropic receptors as well as the release or co-release of aminergic or peptide neuromodulators). While the functional distinction between neurotransmitters and neuromodulators is imperfect, the latter are characterized by effects that are best understood as changes in neuronal or network state that alter responsivity to synaptic inputs.
Olfactory networks (Figure 1) are constructed to extract and recognize pertinent signals from unpredictable, chemically noisy environments, and must be able to adapt flexibly to changes in the sensory environment as well as to internal state variables such as hunger or alarm. These adaptive transformations are often regulated by neuromodulatory inputs, a wide variety of which innervate the olfactory system. Consequently, the olfactory system has served as an important model system for understanding the functional roles and computational mechanisms of neuromodulation, particularly in mammals. One of its advantages in this role is that there has been somewhat less of a tendency to associate individual neuromodulators with broad behavioral states such as attention or arousal. While these relationships may have merit, their presumption also can introduce bias into experimental design and interpretation. In contrast, the construction of multiscale models of neuromodulation can elucidate the relationships between cellular and synaptic effects and the resulting systems-level transformations of (sensory) input, providing rich insights into the complexity and contingencies of neural circuit function. We here review some examples of the neuromodulatory regulation of specific functional computations hypothesized for olfactory circuitry in adult rodents.
Section snippets
Olfactory processing and neuromodulation
Chemical signals are transduced by primary olfactory sensory neurons (OSNs) in the nasal cavities, which project directly to the central nervous system to form discrete neuropilar glomeruli within the olfactory bulb (OB). Within these glomeruli, OSN axonal arbors interact with mitral and tufted cells (principal neurons) along with several local interneuron species that together form glomerular microcircuits (reviewed in [1, 2]). Computational studies of experimental data have shown that these
Synthesis
Neuromodulators regulate olfactory networks and modify their computational transformations at multiple processing stages. Whereas the individual cellular effects of different neuromodulators clearly differ, some of their observed functional effects on network processing appear similar or closely integrated. Nevertheless, the functional effects of different modulators tend to be labeled differently, partially for historical reasons, even when their concrete effects are difficult to distinguish.
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
References (46)
- et al.
Glomerular microcircuits in the olfactory bulb
Neural Netw
(2009) Construction of odor representations by olfactory bulb microcircuits
Prog Brain Res
(2014)Early transformations in odor representation
Trends Neurosci
(2010)Circuit oscillations in odor perception and memory
Prog Brain Res
(2014)Context-driven activation of odor representations in the absence of olfactory stimuli in the olfactory bulb and piriform cortex
Front Behav Neurosci
(2014)- et al.
Nonlinear effects of noradrenergic modulation of olfactory bulb function in adult rodents
J Neurophysiol
(2011) Sequential mechanisms underlying concentration invariance in biological olfaction
Front Neuroeng
(2012)- et al.
A model of cholinergic modulation in olfactory bulb and piriform cortex
J Neurophysiol
(2013) Olfactory cortical neurons read out a relative time code in the olfactory bulb
Nat Neurosci
(2013)- et al.
Central olfactory processing