Noradrenergic induction of odor-specific neural habituation and olfactory memories

Juxtacellular recordings from identified neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the primary source of noradrenergic transmission in the mammalian central nervous system. This small pontine nucleus consists of a densely packed nuclear core-which contains the highest density of noradrenergic neurons-embedded within a heterogeneous surround of non-noradrenergic cells. This local heterogeneity, together with the small size of the LC, has made it particularly difficult to infer noradrenergic cell identity based on extracellular sampling of in vivo spiking activity. Moreover, the relatively high cell density, background activity and synchronicity of LC neurons have made spike identification and unit isolation notoriously challenging. In this study, we aimed at bridging these gaps by performing juxtacellular recordings from single identified neurons within the mouse LC complex. We found that noradrenergic neurons (identified by tyrosine hydroxylase, TH, expression; TH-positive) and intermingled putatively non-noradrenergic (TH-negative) cells displayed similar morphologies and responded to foot shock stimuli with excitatory responses; however, on average, TH-positive neurons exhibited more prominent foot shock responses and post-activation firing suppression. The two cell classes also displayed different spontaneous firing rates, spike waveforms and temporal spiking properties. A logistic regression classifier trained on spontaneous electrophysiological features could separate the two cell classes with 76% accuracy. Altogether, our results reveal in vivo electrophysiological correlates of TH-positive neurons, which can be useful for refining current approaches for the classification of LC unit activity.

The Bruce effect: Representational stability and memory formation in the accessory olfactory bulb of the female mouse

In the Bruce effect, a mated female mouse becomes resistant to the pregnancy-blocking effect of the stud. Various lines of evidence suggest that this form of behavioral imprinting results from reduced sensitivity of the female's accessory olfactory bulb (AOB) to the stud's chemosignals. However, the AOB's combinatorial code implies that diminishing responses to one individual will distort representations of other stimuli. Here, we record extracellular responses of AOB neurons in mated and unmated female mice while presenting urine stimuli from the stud and from other sources. We find that, while initial sensory responses in the AOB (within a timescale required to guide social interactions) remain stable, responses to extended stimulation (as required for eliciting the pregnancy block) display selective attenuation of stud-responsive neurons. Such temporal disassociation could allow attenuation of slow-acting endocrine processes in a stimulus-specific manner without compromising ongoing representations that guide behavior.

Noradrenergic Modulation of the Piriform Cortex: A Possible Avenue for Understanding Pre-Clinical Alzheimer’s Disease Pathogenesis

Olfactory dysfunction is one of the biomarkers for Alzheimer’s disease (AD) diagnosis and progression. Deficits with odor identification and discrimination are common symptoms of pre-clinical AD, preceding severe memory disorder observed in advanced stages. As a result, understanding mechanisms of olfactory impairment is a major focus in both human studies and animal models of AD. Pretangle tau, a precursor to tau tangles, is first observed in the locus coeruleus (LC). In a recent animal model, LC pretangle tau leads to LC fiber degeneration in the piriform cortex (PC), a cortical area associated with olfactory dysfunction in both human AD and rodent models. Here, we review the role of LC-sourced NE in modulation of PC activity and suggest mechanisms by which pretangle tau-mediated LC dysfunction may impact olfactory processing in preclinical stage of AD. Understanding mechanisms of early olfactory impairment in AD may provide a critical window for detection and intervention of disease progression.

Neural and Hormonal Basis of Opposite-Sex Preference by Chemosensory Signals

In mammalian reproduction, sexually active males seek female conspecifics, while estrous females try to approach males. This sex-specific response tendency is called sexual preference. In small rodents, sexual preference cues are mainly chemosensory signals, including pheromones. In this article, we review the physiological mechanisms involved in sexual preference for opposite-sex chemosensory signals in well-studied laboratory rodents, mice, rats, and hamsters of both sexes, especially an overview of peripheral sensory receptors, and hormonal and central regulation. In the hormonal regulation section, we discuss potential rodent brain bisexuality, as it includes neural substrates controlling both masculine and feminine sexual preferences, i.e., masculine preference for female odors and the opposite. In the central regulation section, we show the substantial circuit regulating sexual preference and also the influence of sexual experience that innate attractants activate in the brain reward system to establish the learned attractant. Finally, we review the regulation of sexual preference by neuropeptides, oxytocin, vasopressin, and kisspeptin. Through this review, we clarified the contradictions and deficiencies in our current knowledge on the neuroendocrine regulation of sexual preference and sought to present problems requiring further study.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Noradrenergic Activity in the Olfactory Bulb Is a Key Element for the Stability of Olfactory Memory

Memory stability is essential for animal survival when environment and behavioral state change over short or long time spans. The stability of a memory can be expressed by its duration, its perseverance when conditions change as well as its specificity to the learned stimulus. Using optogenetic and pharmacological manipulations in male mice, we show that the presence of noradrenaline in the olfactory bulb during acquisition renders olfactory memories more stable. We show that while inhibition of noradrenaline transmission during an odor-reward acquisition has no acute effects, it alters perseverance, duration, and specificity of the memory. We use a computational approach to propose a proof of concept model showing that a single, simple network effect of noradrenaline on olfactory bulb dynamics can underlie these seemingly different behavioral effects. Our results show that acute changes in network dynamics can have long-term effects that extend beyond the network that was manipulated.SIGNIFICANCE STATEMENT Olfaction guides the behavior of animals. For successful survival, animals have to remember previously learned information and at the same time be able to acquire new memories. We show here that noradrenaline in the olfactory bulb, the first cortical relay of the olfactory information, is important for creating stable and specific olfactory memories. Memory stability, as expressed in perseverance, duration and specificity of the memory, is enhanced when noradrenergic inputs to the olfactory bulb are unaltered. We show that, computationally, our diverse behavioral results can be ascribed to noradrenaline-driven changes in neural dynamics. These results shed light on how very temporary changes in neuromodulation can have a variety of long-lasting effects on neural processing and behavior.

Coronaviruses: a challenge of today and a call for extended human postmortem brain analyses

While there is abounding literature on virus-induced pathology in general and coronavirus in particular, recent evidence accumulates showing distinct and deleterious brain affection. As the respiratory tract connects to the brain without protection of the blood-brain barrier, SARS-CoV-2 might in the early invasive phase attack the cardiorespiratory centres located in the medulla/pons areas, giving rise to disturbances of respiration and cardiac problems. Furthermore, brainstem regions are at risk to lose their functional integrity. Therefore, long-term neurological as well as psychiatric symptomatology and eventual respective disorders cannot be excluded as evidenced from influenza-A triggered post-encephalitic Parkinsonism and HIV-1 triggered AIDS-dementia complex. From the available evidences for coronavirus-induced brain pathology, this review concludes a number of unmet needs for further research strategies like human postmortem brain analyses. SARS-CoV-2 mirroring experimental animal brain studies, characterization of time-dependent and region-dependent spreading behaviours of coronaviruses, enlightening of pathological mechanisms after coronavirus infection using long-term animal models and clinical observations of patients having had COVID-19 infection are calling to develop both protective strategies and drug discoveries to avoid early and late coronavirus-induced functional brain disturbances, symptoms and eventually disorders. To fight SARS-CoV-2, it is an urgent need to enforce clinical, molecular biological, neurochemical and genetic research including brain-related studies on a worldwide harmonized basis.

In vivo odorant input induces spike timing-dependent plasticity of glutamatergic synapses in developing zebrafish olfactory bulb

Early odorant experience and neural activity are essential for refining developing neural connections. Although neural activity-induced synaptic plasticity is one of the most important cellular mechanisms underlying the refinement of neural circuits, whether and how natural odorant experience induces long-term plasticity in the olfactory bulb remains unknown. In vivo perforated whole-cell recording from mitral cells (MCs) in larval zebrafish showed that odorant experience induced persistent modification of developing olfactory bulb circuits via spike timing-dependent plasticity (STDP). Repetitive odorant stimuli paired with postsynaptic spiking in a critical time window (pre-post, positive timing) resulted in persistent enhancement of glutamatergic inputs from olfactory sensory neurons, but long-term depression within the opposite time window (post-pre, negative timing). Furthermore, spike-timing-dependent potentiation (tLTP) in STDP induced by repetitive odorant stimulation had similar cellular processes to those of electrical stimulation-induced tLTP. Finally, odorant input induced STDP required the activation of postsynaptic N-methyl-d-aspartate receptors (NMDARs). Thus, the NMDAR is likely to be a postsynaptic coincidence detector responsible for the sensory experience-dependent refinement of developing connections.

Complex neural representation of odour information in the olfactory bulb

The most important task of the olfactory system is to generate a precise representation of odour information under different brain and behavioural states. As the first processing stage in the olfactory system and a crucial hub, the olfactory bulb plays a key role in the neural representation of odours, encoding odour identity, intensity and timing. Although the neural circuits and coding strategies used by the olfactory bulb for odour representation were initially identified in anaesthetized animals, a large number of recent studies focused on neural representation of odorants in the olfactory bulb in awake behaving animals. In this review, we discuss these recent findings, covering (a) the neural circuits for odour representation both within the olfactory bulb and the functional connections between the olfactory bulb and the higher order processing centres; (b) how related factors such as sniffing affect and shape the representation; (c) how the representation changes under different states; and (d) recent progress on the processing of temporal aspects of odour presentation in awake, behaving rodents. We highlight discussion of the current views and emerging proposals on the neural representation of odorants in the olfactory bulb.

Locus coeruleus-norepinephrine modulation of sensory processing and perception: A focused review

The locus coeruleus-norepinephrine (LC-NE) system is involved in many brain functions and neurological disorders. In this review we discuss how LC-NE signaling affects the activity of cortical and subcortical sensory neurons, and how it influences perception-driven behaviors associated with mammalian somatosensory, visual, auditory, and olfactory systems. We summarize the consistent as well as seemingly inconsistent findings across brain areas and sensory modalities and propose a framework to understand these phenomena from the perspective of adrenergic receptor expression, dose-dependent physiology and excitation-inhibition balance. We also discuss potential future research directions in this field.

The Pre-synaptic Landscape of Mitral/Tufted Cells of the Main Olfactory Bulb

In olfaction, all volatile odor information is tunneled through the main olfactory bulb (OB). Odor information is then processed before it is transferred to higher brain centers. Odor processing in the OB is carried out by numerous local inhibitory circuits and modulated by top-down input. Top-down modulation of OB function has been shown to act via interneurons but evidence also exists for its direct impact onto the principle mitral and tufted cells (M/Ts). Here, we used monosynaptic rabies trans-synaptic tracing from the OB to map and quantify the local and top-down pre-synaptic landscape of M/Ts and local inhibitory interneurons. We found that M/Ts receive a significant amount of top-down inputs from various brain regions that match qualitatively but not quantitatively those that synapse onto local inhibitory inter-neurons. These results show that M/Ts are direct targets of top-down inputs.

Microdialysis Unveils the Role of the α2-Adrenergic System in the Basolateral Amygdala during Acquisition of Conditioned Odor Aversion in the Rat

Previous work has shown that β-adrenergic and GABAergic systems in the basolateral amygdala (BLA) are involved in the acquisition of conditioned odor aversion (COA) learning. The involvement of α₂-adrenoreceptors, however, is poorly documented. In a first experiment, male Long-Evans rats received infusions of 0.1 μg of the selective α₂-antagonist dexefaroxan (Dex) in the BLA before being exposed to COA learning. In a second experiment, levels of norepinephrine (NE) were analyzed following Dex retrodialysis into the BLA. While microdialysis data showed a significant enhancement of NE release in the BLA with Dex, behavioral results showed that pre-CS infusion of Dex impaired, rather than facilitated, the acquisition of COA. Our results show that the NE system in the BLA is involved in the acquisition of COA, including a strong α₂-receptor modulation until now unsuspected. Supported by the recent literature, the present data suggest moreover that the processes underlying this learning are probably mediated by the balanced effects of NE excitatory/inhibitory signaling in the BLA, in which interneurons are highly involved.

Pheromone-Induced Odor Associative Fear Learning in Rats

Alarm pheromones alert conspecifics to the presence of danger. Can pheromone communication aid in learning specific cues? Such facilitation has an evident evolutionary advantage. We use two associative learning paradigms to test this hypothesis. The first is stressed cage mate-induced conditioning. One pair-housed adult rat received 4 pairings of terpinene + shock over 30 min. Ten minutes after return to the home cage, its companion rat was removed and exposed to terpinene. Single-housed controls were exposed to either terpinene or shock only. Companion rats showed terpinene-specific freezing, which was prevented by β-adrenoceptor blockade. Using Arc to index neuronal activation in response to terpinene re-exposure, stressed cage-mate induced associative learning was measured. Companion rats showed increased neuronal activity in the accessory olfactory bulb, while terpinene + shock-conditioned rats showed increased activity in the main olfactory bulb. Both groups had enhanced activity in the anterior basolateral amygdala and central amygdala. To test involvement of pheromone mediation, in the 2nd paradigm, we paired terpinene with soiled bedding from odor + shock rats or a rat alarm pheromone. Both conditioning increased rats’ freezing to terpinene. Blocking NMDA receptors in the basolateral amygdala prevented odor-specific learning suggesting shock and pheromone-paired pathways converge in the amygdala. An alarm pheromone thus enables cue-specific learning as well as signalling danger.

In vivo measurement of the dynamics of norepinephrine in an olfactory bulb following ischemia-induced olfactory dysfunction and its responses to dexamethasone treatment

Information on the dynamics of molecules following olfactory dysfunction remains essential for understanding the molecular events involved in the pathological process of olfactory dysfunction. This study for the first time demonstrates a method based on the combination of in vivo microdialysis with high performance liquid chromatography (HPLC) and electrochemical detection (ECD) for the measurement of the dynamics of norepinephrine (NE) in the olfactory bulbs of Sprague-Dawley rats following olfactory dysfunction induced by brain ischemia and its responses toward dexamethasone treatment. The method possesses a high spatial resolution and benefits from in vivo microdialysis and high selectivity and is thus capable of measuring NE in the olfactory bulb of rats. With this method, the basal level of NE in the olfactory bulb was evaluated to be ca. 235 ± 25 nM (n = 6). This level was found to increase by 260 ± 90% at a time point of 240 min after brain ischemia with bilateral ligation of both common carotid arteries. The increase was found to be suppressed upon the treatment of the animals with 0.2% dexamethasone in the olfactory bulb. These results suggest that NE is involved in the pathological process of ischemia-induced olfactory dysfunction and this information is useful to further understand the molecular events involved in olfactory dysfunction.

Olfactory bulb acetylcholine release dishabituates odor responses and reinstates odor investigation

Habituation and dishabituation modulate the neural resources and behavioral significance allocated to incoming stimuli across the sensory systems. We characterize these processes in the mouse olfactory bulb (OB) and uncover a role for OB acetylcholine (ACh) in physiological and behavioral olfactory dishabituation. We use calcium imaging in both awake and anesthetized mice to determine the time course and magnitude of OB glomerular habituation during a prolonged odor presentation. In addition, we develop a novel behavioral investigation paradigm to determine how prolonged odor input affects odor salience. We find that manipulating OB ACh release during prolonged odor presentations using electrical or optogenetic stimulation rapidly modulates habituated glomerular odor responses and odor salience, causing mice to suddenly investigate a previously ignored odor. To demonstrate the ethological validity of this effect, we show that changing the visual context can lead to dishabituation of odor investigation behavior, which is blocked by cholinergic antagonists in the OB.

Noradrenergic effects on olfactory perception and learning

We here review modulation of olfactory guided behavioral tasks by noradrenaline. In this review we focus on modulation of the main olfactory system in adult rodents. We detail behavioral paradigms commonly used and discuss how sensory perception and learning can be measured using these paradigms. We then describe neuromodulatory effects on several aspects of olfactory processing, including detection and encoding. We describe how memory duration, specificity and duration are affected by noradrenergic modulation.

α2-Adrenergic receptor activation promotes long-term potentiation at excitatory synapses in the mouse accessory olfactory bulb

The formation of mate recognition memory in mice is associated with neural changes at the reciprocal dendrodendritic synapses between glutamatergic mitral cell (MC) projection neurons and GABAergic granule cell (GC) interneurons in the accessory olfactory bulb (AOB). Although noradrenaline (NA) plays a critical role in the formation of the memory, the mechanism by which it exerts this effect remains unclear. Here we used extracellular field potential and whole-cell patch-clamp recordings to assess the actions of bath-applied NA (10 µM) on the glutamatergic transmission and its plasticity at the MC-to-GC synapse in the AOB. Stimulation (400 stimuli) of MC axons at 10 Hz but not at 100 Hz effectively induced N-methyl-d-aspartate (NMDA) receptor-dependent long-term potentiation (LTP), which exhibited reversibility. NA paired with subthreshold 10-Hz stimulation (200 stimuli) facilitated the induction of NMDA receptor-dependent LTP via the activation of α₂-adrenergic receptors (ARs). We next examined how NA, acting at α₂-ARs, facilitates LTP induction. In terms of acute actions, NA suppressed GC excitatory postsynaptic current (EPSC) responses to single pulse stimulation of MC axons by reducing glutamate release from MCs via G-protein coupled inhibition of calcium channels. Consequently, NA reduced recurrent inhibition of MCs, resulting in the enhancement of evoked EPSCs and spike fidelity in GCs during the 10-Hz stimulation used to induce LTP. These results suggest that NA, acting at α₂-ARs, facilitates the induction of NMDA receptor-dependent LTP at the MC-to-GC synapse by shifting its threshold through disinhibition of MCs.

History-Dependent Odor Processing in the Mouse Olfactory Bulb

In nature, animals normally perceive sensory information on top of backgrounds. Thus, the neural substrate to perceive under background conditions is inherent in all sensory systems. Where and how sensory systems process backgrounds is not fully understood. In olfaction, just a few studies have addressed the issue of odor coding on top of continuous odorous backgrounds. Here, we tested how background odors are encoded by mitral cells (MCs) in the olfactory bulb (OB) of male mice. Using in vivo two-photon calcium imaging, we studied how MCs responded to odors in isolation versus their responses to the same odors on top of continuous backgrounds. We show that MCs adapt to continuous odor presentation and that mixture responses are different when preceded by background. In a subset of odor combinations, this history-dependent processing was useful in helping to identify target odors over background. Other odorous backgrounds were highly dominant such that target odors were completely masked by their presence. Our data are consistent in both low and high odor concentrations and in anesthetized and awake mice. Thus, odor processing in the OB is strongly influenced by the recent history of activity, which could have a powerful impact on how odors are perceived.

SIGNIFICANCE STATEMENT We examined a basic feature of sensory processing in the olfactory bulb. Specifically, we measured how mitral cells adapt to continuous background odors and how target odors are encoded on top of such background. Our results show clear differences in odor coding based on the immediate history of the stimulus. Our results support the argument that odor coding in the olfactory bulb depends on the recent history of the sensory environment.

Stimulation of the Locus Ceruleus Modulates Signal-to-Noise Ratio in the Olfactory Bulb

Norepinephrine (NE) has been shown to influence sensory, and specifically olfactory processing at the behavioral and physiological levels, potentially by regulating signal-to-noise ratio (S/N). The present study is the first to look at NE modulation of olfactory bulb (OB) in regards to S/N in vivo We show, in male rats, that locus ceruleus stimulation and pharmacological infusions of NE into the OB modulate both spontaneous and odor-evoked neural responses. NE in the OB generated a non-monotonic dose-response relationship, suppressing mitral cell activity at high and low, but not intermediate, NE levels. We propose that NE enhances odor responses not through direct potentiation of the afferent signal per se, but rather by reducing the intrinsic noise of the system. This has important implications for the ways in which an animal interacts with its olfactory environment, particularly as the animal shifts from a relaxed to an alert behavioral state.SIGNIFICANCE STATEMENT Sensory perception can be modulated by behavioral states such as hunger, fear, stress, or a change in environmental context. Behavioral state often affects neural processing via the release of circulating neurochemicals such as hormones or neuromodulators. We here show that the neuromodulator norepinephrine modulates olfactory bulb spontaneous activity and odor responses so as to generate an increased signal-to-noise ratio at the output of the olfactory bulb. Our results help interpret and improve existing ideas for neural network mechanisms underlying behaviorally observed improvements in near-threshold odor detection and discrimination.

The neurobiology of safety and threat learning in infancy

What an animal needs to learn to survive is altered dramatically as they change from dependence on the parent for protection to independence and reliance on self-defense. This transition occurs in most altricial animals, but our understanding of the behavioral neurobiology has mostly relied on the infant rat. The transformation from dependence to independence occurs over three weeks in pups and is accompanied by complex changes in responses to both natural and learned threats and the supporting neural circuitry. Overall, in early life, the threat system is quiescent and learning is biased towards acquiring attachment related behaviors to support attachment to the caregiver and proximity seeking. Caregiver-associated cues learned in infancy have the ability to provide a sense of safety throughout lifetime. This attachment/safety system is activated by learning involving presumably pleasurable stimuli (food, warmth) but also painful stimuli (tailpinch, moderate shock). At about the midway point to independence, pups begin to have access to the adult-like amygdala-dependent threat system and amygdala-dependent responses to natural dangers such as predator odors. However, pups have the ability to switch between the infant and adult-like system, which is controlled by maternal presence and modification of stress hormones. Specifically, if the pup is alone, it will learn fear but if with the mother it will learn attachment (10-15days of age). As pups begin to approach weaning, pups lose access to the attachment system and rely only on the amygdala-dependent threat system. However, pups learning system is complex and exhibits flexibility that enables the mother to override the control of the attachment circuit, since newborn pups may acquire threat responses from the mother expressing fear in their presence. Together, these data suggest that the development of pups' threat learning system is not only dependent upon maturation of the amygdala, but it is also exquisitely controlled by the environment. Most notably the mother can switch pup learning between attachment to threat learning in a moment's notice. This enables the mother to navigate pup's learning about the world and what is threatening and what is safe.

Exciting experiences make neurons less excitable

Neurons in the brain of a female mouse that respond to the scent of a given male become suppressed after mating.

GABAB Receptors Tune Cortical Feedback to the Olfactory Bulb

Sensory perception emerges from the confluence of sensory inputs that encode the composition of external environment and top-down feedback that conveys information from higher brain centers. In olfaction, sensory input activity is initially processed in the olfactory bulb (OB), serving as the first central relay before being transferred to the olfactory cortex. In addition, the OB receives dense connectivity from feedback projections, so the OB has the capacity to implement a wide array of sensory neuronal computation. However, little is known about the impact and the regulation of this cortical feedback. Here, we describe a novel mechanism to gate glutamatergic feedback selectively from the anterior olfactory cortex (AOC) to the OB. Combining in vitro and in vivo electrophysiological recordings, optogenetics, and fiber-photometry-based calcium imaging applied to wild-type and conditional transgenic mice, we explore the functional consequences of circuit-specific GABA type-B receptor (GABABR) manipulation. We found that activation of presynaptic GABABRs specifically depresses synaptic transmission from the AOC to OB inhibitory interneurons, but spares direct excitation to principal neurons. As a consequence, feedforward inhibition of spontaneous and odor-evoked activity of principal neurons is diminished. We also show that tunable cortico-bulbar feedback is critical for generating beta, but not gamma, OB oscillations. Together, these results show that GABABRs on cortico-bulbar afferents gate excitatory transmission in a target-specific manner and thus shape how the OB integrates sensory inputs and top-down information.

SIGNIFICANCE STATEMENT

The olfactory bulb (OB) receives top-down inputs from the olfactory cortex that produce direct excitation and feedforward inhibition onto mitral and tufted cells, the principal neurons. The functional role of this feedback and the mechanisms regulating the balance of feedback excitation and inhibition remain unknown. We found that GABAB receptors are expressed in cortico-bulbar axons that synapse on granule cells and receptor activation reduces the feedforward inhibition of spontaneous and odor-driven mitral and tufted cells' firing activity. In contrast, direct excitatory inputs to these principal neurons remain unchanged. This study demonstrates that activation of GABAB receptors biases the excitation/inhibition balance provided by cortical inputs to the OB, leading to profound effects on early stages of sensory information processing.

Cell-Type-Specific Modulation of Sensory Responses in Olfactory Bulb Circuits by Serotonergic Projections from the Raphe Nuclei

Serotonergic neurons in the brainstem raphe nuclei densely innervate the olfactory bulb (OB), where they can modulate the initial representation and processing of olfactory information. Serotonergic modulation of sensory responses among defined OB cell types is poorly characterized in vivo Here, we used cell-type-specific expression of optical reporters to visualize how raphe stimulation alters sensory responses in two classes of GABAergic neurons of the mouse OB glomerular layer, periglomerular (PG) and short axon (SA) cells, as well as mitral/tufted (MT) cells carrying OB output to piriform cortex. In PG and SA cells, brief (1-4 s) raphe stimulation elicited a large increase in the magnitude of responses linked to inhalation of ambient air, as well as modest increases in the magnitude of odorant-evoked responses. Near-identical effects were observed when the optical reporter of glutamatergic transmission iGluSnFR was expressed in PG and SA cells, suggesting enhanced excitatory input to these neurons. In contrast, in MT cells imaged from the dorsal OB, raphe stimulation elicited a strong increase in resting GCaMP fluorescence with only a slight enhancement of inhalation-linked responses to odorant. Finally, optogenetically stimulating raphe serotonergic afferents in the OB had heterogeneous effects on presumptive MT cells recorded extracellularly, with an overall modest increase in resting and odorant-evoked responses during serotonergic afferent stimulation. These results suggest that serotonergic afferents from raphe dynamically modulate olfactory processing through distinct effects on multiple OB targets, and may alter the degree to which OB output is shaped by inhibition during behavior.

SIGNIFICANCE STATEMENT

Modulation of the circuits that process sensory information can profoundly impact how information about the external world is represented and perceived. This study investigates how the serotonergic system modulates the initial processing of olfactory information by the olfactory bulb, an obligatory relay between sensory neurons and cortex. We find that serotonergic projections from the raphe nuclei to the olfactory bulb dramatically enhance the responses of two classes of inhibitory interneurons to sensory input, that this effect is mediated by increased glutamatergic drive onto these neurons, and that serotonergic afferent activation alters the responses of olfactory bulb output neurons in vivo These results elucidate pathways by which neuromodulatory systems can dynamically regulate brain circuits during behavior.

Heterogeneous effects of norepinephrine on spontaneous and stimulus-driven activity in the male accessory olfactory bulb

Norepinephrine (NE) release has been linked to experience-dependent plasticity in many model systems and brain regions. Among these is the rodent accessory olfactory system (AOS), which is crucial for detecting and processing socially relevant environmental cues. The accessory olfactory bulb (AOB), the first site of chemosensory information processing in the AOS, receives dense centrifugal innervation by noradrenergic fibers originating in the locus coeruleus. Although NE release has been linked to behavioral plasticity through its actions in the AOB, the impacts of noradrenergic modulation on AOB information processing have not been thoroughly studied. We made extracellular single-unit recordings of AOB principal neurons in ex vivo preparations of the early AOS taken from adult male mice. We analyzed the impacts of bath-applied NE (10 μM) on spontaneous and stimulus-driven activity. In the presence of NE, we observed overall suppression of stimulus-driven neuronal activity with limited impact on spontaneous activity. NE-associated response suppression in the AOB came in two forms: one that was strong and immediate (21%) and one other that involved gradual, stimulus-dependent monotonic response suppression (47%). NE-associated changes in spontaneous activity were more modest, with an overall increase in spontaneous spike frequency observed in 25% of neurons. Neurons with increased spontaneous activity demonstrated a net decrease in chemosensory discriminability. These results reveal that noradrenergic signaling in the AOB causes cell-specific changes in chemosensory tuning, even among similar projection neurons.NEW & NOTEWORTHY Norepinephrine (NE) is released throughout the brain in many behavioral contexts, but its impacts on information processing are not well understood. We studied the impact of NE on chemosensory tuning in the mouse accessory olfactory bulb (AOB). Electrophysiological recordings from AOB neurons in ex vivo preparations revealed that NE, on balance, inhibited mitral cell responses to chemosensory cues. However, NE's effects were heterogeneous, indicating that NE signaling reshapes AOB output in a cell- and stimulus-specific manner.

Physical Interaction Is Required in Social Buffering Induced by a Familiar Conspecific

In social animals, signals released from fearless conspecifics attenuate fear responses, namely social buffering. The presence of conspecific odor can suppress the expression of freezing response of conditioned mice. The present study investigated if physical social experience is required for this social buffering effect. The mice were exposed to donors, donor bedding (collected from cages of donors), or fresh bedding as control, respectively, for 10 days (1 hour daily) in prior to fear conditioning test. The fear expression test was examined in presence of donor bedding. The results showed that only the donor group mice showed reduced freezing time than the other two groups in the fear memory test. This phenomenon indicated that physical interaction might be required for the social buffering effect.

Activation of raphe nuclei triggers rapid and distinct effects on parallel olfactory bulb output channels

The serotonergic raphe nuclei are involved in regulating brain states over timescales of minutes and hours. We examined more rapid effects of raphe activation on two classes of principal neurons in the mouse olfactory bulb, mitral and tufted cells, which send olfactory information to distinct targets. Brief stimulation of the raphe nuclei led to excitation of tufted cells at rest and potentiation of their odor responses. While mitral cells at rest were also excited by raphe activation, their odor responses were bidirectionally modulated, leading to improved pattern separation of odors. In vitro whole-cell recordings revealed that specific optogenetic activation of raphe axons affected bulbar neurons through dual release of serotonin and glutamate. Therefore, the raphe nuclei, in addition to their role in neuromodulation of brain states, are also involved in fast, sub-second top-down modulation similar to cortical feedback. This modulation can selectively and differentially sensitize or decorrelate distinct output channels.

fMRI study of olfaction in the olfactory bulb and high olfactory structures of rats: Insight into their roles in habituation

Cerebral blood volume (CBV) fMRI with ultrasmall superparamagnetic iron oxide particles (USPIO) as a contrast agent was used to investigate olfactory processing in rats. fMRI data were acquired in sixteen 0.75-mm coronal slices covering the olfactory bulb (OB) and higher olfactory regions (HOR), including the anterior olfactory nucleus and piriform cortex. For each animal, multiple consecutive fMRI measurements were made during a 3-h experiment session, with each measurement consisting of a baseline period, an odorant stimulation period, and a recovery period. Two different stimulation paradigms with a stimulation period of 40s or 80s, respectively, were used to study olfactory processing. Odorant-induced CBV increases were robustly observed in the OB and HOR of each individual animal. Olfactory adaptation, which is characterized by an attenuation of responses to continuous exposure or repeated stimulations, has different characteristics in the OB and HOR. For adaptation to repeated stimuli, while it was observed in both the OB and HOR, CBV responses in the HOR were attenuated more significantly than responses in the OB. In contrast, within each continuous 40-s or 80-s odor exposure, CBV responses in the OB were stable and did not show adaptation, but the CBV responses in the HOR were state dependent, with no adaptation during initial exposures, but significant adaptation during following exposures. These results support previous reports that HOR plays a more significant role than OB in olfactory habituation. The technical approach presented in this study should enable more extensive fMRI studies of olfactory processing in rats.

Changes in Olfactory Sensory Neuron Physiology and Olfactory Perceptual Learning After Odorant Exposure in Adult Mice

The adult olfactory system undergoes experience-dependent plasticity to adapt to the olfactory environment. This plasticity may be accompanied by perceptual changes, including improved olfactory discrimination. Here, we assessed experience-dependent changes in the perception of a homologous aldehyde pair by testing mice in a cross-habituation/dishabituation behavioral paradigm before and after a week-long ester-odorant exposure protocol. In a parallel experiment, we used optical neurophysiology to observe neurotransmitter release from olfactory sensory neuron (OSN) terminals in vivo, and thus compared primary sensory representations of the aldehydes before and after the week-long ester-odorant exposure in individual animals. Mice could not discriminate between the aldehydes during pre-exposure testing, but ester-exposed subjects spontaneously discriminated between the homologous pair after exposure, whereas home cage control mice cross-habituated. Ester exposure did not alter the spatial pattern, peak magnitude, or odorant-selectivity of aldehyde-evoked OSN input to olfactory bulb glomeruli, but did alter the temporal dynamics of that input to make the time course of OSN input more dissimilar between odorants. Together, these findings demonstrate that odor exposure can induce both physiological and perceptual changes in odor processing, and suggest that changes in the temporal patterns of OSN input to olfactory bulb glomeruli could induce differences in odor quality.

Computational modeling suggests distinct, location-specific function of norepinephrine in olfactory bulb and piriform cortex

Noradrenergic modulation from the locus coerulus is often associated with the regulation of sensory signal-to-noise ratio. In the olfactory system, noradrenergic modulation affects both bulbar and cortical processing, and has been shown to modulate the detection of low concentration stimuli. We here implemented a computational model of the olfactory bulb and piriform cortex, based on known experimental results, to explore how noradrenergic modulation in the olfactory bulb and piriform cortex interact to regulate odor processing. We show that as predicted by behavioral experiments in our lab, norepinephrine can play a critical role in modulating the detection and associative learning of very low odor concentrations. Our simulations show that bulbar norepinephrine serves to pre-process odor representations to facilitate cortical learning, but not recall. We observe the typical non-uniform dose—response functions described for norepinephrine modulation and show that these are imposed mainly by bulbar, but not cortical processing.

The "ram effect": new insights into neural modulation of the gonadotropic axis by male odors and socio-sexual interactions

Reproduction in mammals is controlled by the hypothalamo-pituitary-gonadal (HPG) axis under the influence of external and internal factors such as photoperiod, stress, nutrition, and social interactions. Sheep are seasonal breeders and stop mating when day length is increasing (anestrus). However, interactions with a sexually active ram during this period can override the steroid negative feedback responsible for the anoestrus state, stimulate luteinizing hormone (LH) secretion and eventually reinstate cyclicity. This is known as the “ram effect” and research into the mechanisms underlying it is shedding new light on HPG axis regulation. The first step in the ram effect is increased LH pulsatile secretion in anestrus ewes exposed to a sexually active male or only to its fleece, the latter finding indicating a “pheromone-like” effect. Estradiol secretion increases in all ewes and this eventually induces a LH surge and ovulation, just as during the breeding season. An exception is a minority of ewes that exhibit a precocious LH surge (within 4 h) with no prior increase in estradiol. The main olfactory system and the cortical nucleus of the amygdala are critical brain structures in mediating the ram effect since it is blocked by their inactivation. Sexual experience is also important since activation (increased c-fos expression) in these and other regions is greatly reduced in sexually naïve ewes. In adult ewes kisspeptin neurons in both arcuate and preoptic regions and some preoptic GnRH neurons are activated 2 h after exposure to a ram. Exposure to rams also activates noradrenergic neurons in the locus coeruleus and A1 nucleus and increased noradrenalin release occurs in the posterior preoptic area. Pharmacological modulation of this system modifies LH secretion in response to the male or his odor. Together these results show that the ram effect can be a fruitful model to promote both a better understanding of the neural and hormonal regulation of the HPG axis in general and also the specific mechanisms by which male cues can overcome negative steroid feedback and trigger LH release and ovulatory cycles.

Noradrenergic plasticity of olfactory sensory neuron inputs to the main olfactory bulb

Sensory responses are modulated by internal factors including attention, experience, and brain state. This is partly due to fluctuations in neuromodulatory input from regions such as the noradrenergic locus ceruleus (LC) in the brainstem. LC activity changes with arousal and modulates sensory processing, cognition, and memory. The main olfactory bulb (MOB) is richly targeted by LC fibers and noradrenaline profoundly influences MOB circuitry and odor-guided behavior. Noradrenaline-dependent plasticity affects the output of the MOB; however. it is unclear whether noradrenergic plasticity also affects the input to the MOB from olfactory sensory neurons (OSNs) in the glomerular layer. Noradrenergic terminals are found in the glomerular layer, but noradrenaline receptors do not seem to acutely modulate OSN terminals in vitro. We investigated whether noradrenaline induces plasticity at the glomerulus. We used wide-field optical imaging to measure changes in odor responses following electrical stimulation of LC in anesthetized mice. Surprisingly, odor-evoked intrinsic optical signals at the glomerulus were persistently weakened after LC activation. Calcium imaging selectively from OSNs confirmed that this effect was due to suppression of presynaptic input and was prevented by noradrenergic antagonists. Finally, suppression of responses to an odor did not require precise coincidence of the odor with LC activation. However, suppression was intensified by LC activation in the absence of odors. We conclude that noradrenaline release from LC has persistent effects on odor processing already at the first synapse of the main olfactory system. This mechanism could contribute to arousal-dependent memories.

Properties and mechanisms of olfactory learning and memory

Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.

Functional imaging of cortical feedback projections to the olfactory bulb

Processing of sensory information is substantially shaped by centrifugal, or feedback, projections from higher cortical areas, yet the functional properties of these projections are poorly characterized. Here, we used genetically-encoded calcium sensors (GCaMPs) to functionally image activation of centrifugal projections targeting the olfactory bulb (OB). The OB receives massive centrifugal input from cortical areas but there has been as yet no characterization of their activity in vivo. We focused on projections to the OB from the anterior olfactory nucleus (AON), a major source of cortical feedback to the OB. We expressed GCaMP selectively in AON projection neurons using a mouse line expressing Cre recombinase (Cre) in these neurons and Cre-dependent viral vectors injected into AON, allowing us to image GCaMP fluorescence signals from their axon terminals in the OB. Electrical stimulation of AON evoked large fluorescence signals that could be imaged from the dorsal OB surface in vivo. Surprisingly, odorants also evoked large signals that were transient and coupled to odorant inhalation both in the anesthetized and awake mouse, suggesting that feedback from AON to the OB is rapid and robust across different brain states. The strength of AON feedback signals increased during wakefulness, suggesting a state-dependent modulation of cortical feedback to the OB. Two-photon GCaMP imaging revealed that different odorants activated different subsets of centrifugal AON axons and could elicit both excitation and suppression in different axons, indicating a surprising richness in the representation of odor information by cortical feedback to the OB. Finally, we found that activating neuromodulatory centers such as basal forebrain drove AON inputs to the OB independent of odorant stimulation. Our results point to the AON as a multifunctional cortical area that provides ongoing feedback to the OB and also serves as a descending relay for other neuromodulatory systems.

Cellular registration without behavioral recall of olfactory sensory input under general anesthesia

BACKGROUND

Previous studies suggest that sensory information is "received" but not "perceived" under general anesthesia. Whether and to what extent the brain continues to process sensory inputs in a drug-induced unconscious state remain unclear.

METHODS

One hundred seven rats were randomly assigned to 12 different anesthesia and odor exposure paradigms. The immunoreactivities of the immediate early gene products c-Fos and Egr1 as neural activity markers were combined with behavioral tests to assess the integrity and relationship of cellular and behavioral responsiveness to olfactory stimuli under a surgical plane of ketamine-xylazine general anesthesia.

RESULTS

The olfactory sensory processing centers could distinguish the presence or absence of experimental odorants even when animals were fully anesthetized. In the anesthetized state, the c-Fos immunoreactivity in the higher olfactory cortices revealed a difference between novel and familiar odorants similar to that seen in the awake state, suggesting that the anesthetized brain functions beyond simply receiving external stimulation. Reexposing animals to odorants previously experienced only under anesthesia resulted in c-Fos immunoreactivity, which was similar to that elicited by familiar odorants, indicating that previous registration had occurred in the anesthetized brain. Despite the "cellular memory," however, odor discrimination and forced-choice odor-recognition tests showed absence of behavioral recall of the registered sensations, except for a longer latency in odor recognition tests.

CONCLUSIONS

Histologically distinguishable registration of sensory processing continues to occur at the cellular level under ketamine-xylazine general anesthesia despite the absence of behavioral recognition, consistent with the notion that general anesthesia causes disintegration of information processing without completely blocking cellular communications.

Broadly tuned and respiration-independent inhibition in the olfactory bulb of awake mice

Olfactory representations are shaped by brain state and respiration. The interaction and circuit substrates of these influences are unclear. Granule cells (GCs) in the main olfactory bulb (MOB) are presumed to sculpt activity reaching the cortex via inhibition of mitral/tufted cells (MTs). GCs potentially make ensemble activity more sparse by facilitating lateral inhibition among MTs and/or enforce temporally precise activity locked to breathing. Yet the selectivity and temporal structure of wakeful GC activity are unknown. We recorded GCs in the MOB of anesthetized and awake mice and identified state-dependent features of odor coding and temporal patterning. Under anesthesia, GCs were sparsely active and strongly and synchronously coupled to respiration. Upon waking, GCs desynchronized, broadened their tuning and largely fired independently from respiration. Thus, during wakefulness, GCs exhibited stronger odor responses with less temporal structure. We propose that during wakefulness GCs may shape MT odor responses through broadened lateral interactions rather than respiratory synchronization.

Stressors impair odor recognition memory via an olfactory bulb-dependent noradrenergic mechanism

Non-associative habituation and odor recognition tasks have been widely used to probe questions of social recognition, odor memory duration, and odor memory specificity. Among others, these paradigms have provided valuable insight into how neuromodulation, and specifically norepinephrine/noradrenaline (NE) influences odor memory. In general, NE levels are modulated by arousal, stress, and behavioral state, but there is sparse evidence of a direct relationship between NE and odor memory in adult rodents. The present study uses simple mild psychological stressors (bright light and sound) to modulate NE levels physiologically in order to probe stressors NE-dependent effect on odor recognition memory. In rats with bilateral bulbar cannulations, we show that these stressors modulate olfactory memory and that this effect is at least partially mediated by the olfactory bulb. Specifically, we show that the presence of stressors during the acquisition of odor memory suppresses memory for an odor when tested 30 min after familiarization to that odor. This suppression is blocked by infusing NE antagonists into the olfactory bulb prior to odor acquisition. Additionally, we find that infusion of bulbar NE is sufficient to suppress odor memory in a manner mimicking that of our stressors. These effects are unlikely to be solely mediated by locomotor/exploratory changes produced by stressors, although these stressors influence certain behaviors not directly related to odor investigation. This study provides important information about how behaviorally relevant changes in NE can influence top-down sensory processing and odor memory.

Odor representation in the olfactory bulb under different brain states revealed by intrinsic optical signals imaging

The olfactory system responds to the same stimulus with great variability according to the current state of the brain. At the levels of multi-unit activity and local field potentials, the response of the olfactory bulb (OB) to a given olfactory stimulus during a state of lower background activity is stronger than the response that occurs during higher background activity, but the distribution pattern of activity remains similar. However, these results have only been established at the individual neuron and neuron cluster scales in previous studies. It remains unclear whether these results are consistent at a larger scale (e.g., OB regions); therefore, intrinsic optical signals imaging was employed in the present study to clarify this issue. The basal brain states of rats were manipulated by using different levels of anesthesia. Under a state of low basal brain activity, the intensity of the activity pattern elicited in the dorsal OB by a given odorant was significantly higher than that under high basal brain activity, but the topography was highly similar across different brain states. These results were consistent across the levels of individual neurons, neuron clusters, glomeruli, and the OB regions, which suggest that the OB contains as yet unknown neural mechanisms that ensure the high-fidelity representation of the same olfactory stimulation under different brain states.

Duration and specificity of olfactory nonassociative memory

Olfactory habituation is a simple form of nonassociative memory in which responsiveness to stable but behaviorally nonsignificant stimuli is decreased. Olfactory habituation has recently become a paradigm widely used to probe the neural substrate underlying olfactory perception and memory. This simple behavioral paradigm has been used successfully used to probe many aspects of olfactory processing, and it has recently become clear that the neural processes underlying olfactory habituation can depend on the task parameters used. We here further investigate memory specificity and duration using 2 variations in task parameters: the number of habituation trials and the time delay between habituation and cross-habituation testing. We find that memory specificity increases with the number of habituation trials but decreases with time after the last habituation trial.

α(1A)-Adrenergic regulation of inhibition in the olfactory bulb

By regulating inhibition at dendrodendritic synapses between mitral and granule cells (GCs), noradrenergic neurons extending from the brainstem provide an input essential for odour processing in the olfactory bulb (OB). In the accessory OB (AOB), we have recently shown that noradrenaline (NA) increases GABA inhibitory input on to mitral cells (MCs) by exciting GCs. Here, we show that GCs in the main OB (MOB) exhibit a similar response to NA, indicating a common mechanism for noradrenergic regulation of GCMC inhibition throughout the OB. In GCs of the MOB, NA (10 μM) produced a robust excitatory effect that included a slow afterdepolarization that followed a train of action potentials evoked by a current stimulus. The depolarization and slow afterdepolarization in GCs were blocked by the α1A-adrenergic receptor (AR) selective antagonist WB 4101 (30 nm) and mimicked by the α(1A)-AR selective agonist A 61603 (1 μM). In recordings from MCs, A 61603 (30 nm-1 μM) produced a sizeable increase in the frequency of spontaneous and miniature IPSCs, an effect completely abolished by the GABAA receptor antagonist gabazine (5 μM). Likewise, activation of β-ARs increased the frequency of spontaneous IPSCs; however, this effect was smaller and confined to the first postnatal weeks. NA enhanced inhibition in MCs across a broad concentration range (0.1-30 μM) and its effects were completely abolished by a mixture of α1- and β-AR antagonists (1 μM prazosin and 10 μM propranolol). Furthermore, the general α2-AR agonist clonidine (10 μM) failed to affect sIPSC frequency. Thus, the NA-mediated increase in GCMC inhibition in the OB results mostly from activation of the α1A-AR subtype.

Noradrenergic and cholinergic modulation of olfactory bulb sensory processing

Neuromodulation in sensory perception serves important functions such as regulation of signal to noise ratio, attention, and modulation of learning and memory. Neuromodulators in specific sensory areas often have highly similar cellular, but distinct behavioral effects. To address this issue, we here review the function and role of two neuromodulators, acetylcholine (Ach) and noradrenaline (NE) for olfactory sensory processing in the adult main olfactory bulb. We first describe specific bulbar sensory computations, review cellular effects of each modulator and then address their specific roles in bulbar sensory processing. We finally put these data in a behavioral and computational perspective.

Age-dependent adrenergic actions in the main olfactory bulb that could underlie an olfactory-sensitive period

Many sensory systems are endowed with mechanisms of neural plasticity that are restricted to a sensitive period in the young developing animal. In this study, we performed experiments in slices of the main olfactory bulb (OB) from rats to examine possible age-dependent cellular mechanisms of plasticity in the olfactory system. We focused on the neurotransmitter norepinephrine (NE), shown to be important in different forms of olfactory learning, examining whether two specific cellular effects of NE previously observed in rats less than P14 extended to older animals. These included an acute reduction in GABAergic synaptic transmission from granule cells (GCs) onto output mitral cells (MCs) and an enhancement in gamma frequency (30-70 Hz) oscillations that persists long after removal of NE. We found that NE failed to reduce GC-to-MC transmission or enhance gamma oscillations in older rats at P18-23. The loss of NE actions on both phenomena appeared to reflect an age-dependent loss of function of α(2)-adrenergic receptors. In addition, we found that NE induced an age-dependent enhancement of transient excitation in MCs, providing a mechanism to link the acute decrease in GC-to-MC inhibition to the long-term increase in gamma oscillations through increases in intracellular calcium. The age-dependent cellular mechanisms that we describe could underlie an olfactory-sensitive period in newborn rodents.

Action of the noradrenergic system on adult-born cells is required for olfactory learning in mice

We have previously shown that an experience-driven improvement in olfactory discrimination (perceptual learning) requires the addition of newborn neurons in the olfactory bulb (OB). Despite this advance, the mechanisms which govern the selective survival of newborn OB neurons following learning remain largely unknown. We propose that activity of the noradrenergic system is a critical mediator providing a top-down signal to control the selective survival of newly born cells and support perceptual learning. In adult mice, we used pharmacological means to manipulate the noradrenergic system and neurogenesis and to assess their individual and additive effects on behavioral performance on a perceptual learning task. We then looked at the effects of these manipulations on regional survival of adult-born cells in the OB. Finally, using confocal imaging and electrophysiology, we investigated potential mechanisms by which noradrenaline could directly influence the survival of adult-born cells. Consistent with our hypotheses, direct manipulation of noradrenergic transmission significantly effect on adult-born cell survival and perceptual learning. Specifically, learning required both the presence of adult-born cell and noradrenaline. Finally, we provide a mechanistic link between these effects by showing that adult-born neurons receive noradrenergic projections and are responsive to noradrenaline. Based upon these data we argue that noradrenergic transmission is a key mechanism selecting adult-born neurons during learning and demonstrate that top-down neuromodulation acts on adult-born neuron survival to modulate learning performance.

Modeling habituation in rat EEG-evoked responses via a neural mass model with feedback

Habituation is a generic property of the neural response to repeated stimuli. Its strength often increases as inter-stimuli relaxation periods decrease. We propose a simple, broadly applicable control structure that enables a neural mass model of the evoked EEG response to exhibit habituated behavior. A key motivation for this investigation is the ongoing effort to develop model-based reconstruction of multi-modal functional neuroimaging data. The control structure proposed here is illustrated and validated in the context of a biophysical neural mass model, developed by Riera et al. (Hum Brain Mapp 27(11):896-914, 2006; 28(4):335-354, 2007), and of simplifications thereof, using data from rat EEG response to medial nerve stimuli presented at frequencies from 1 to 8 Hz. Performance was tested by predictions of both the response to the next stimulus based on the current one, and also of continued stimuli trains over 4-s time intervals based on the first stimulus in the interval, with similar success statistics. These tests demonstrate the ability of simple generative models to capture key features of the evoked response, including habituation.

All in a sniff: olfaction as a model for active sensing

Sensation is an active process involving the sampling and central processing of external stimuli selectively in space and time. Olfaction in particular depends strongly on active sensing due to the fact that-at least in mammals-inhalation of air into the nasal cavity is required for odor detection. This seemingly simple first step in odor sensation profoundly shapes nearly all aspects of olfactory system function, from the distribution of odorant receptors to the functional organization of central processing to the perception of odors. The dependence of olfaction on inhalation also allows for profound modulation of olfactory processing by changes in odor sampling strategies in coordination with attentional state and sensory demands. This review discusses the role of active sensing in shaping olfactory system function at multiple levels and draws parallels with other sensory modalities to highlight the importance of an active sensing perspective in understanding how sensory systems work in the behaving animal.