Dense encoding of natural odorants by ensembles of sparsely activated neurons in the olfactory bulb

Recalibrating Olfactory Neuroscience to the Range of Naturally Occurring Odor Concentrations

Sensory systems enable organisms to detect and respond to environmental signals relevant for their survival and reproduction. A crucial aspect of any sensory signal is its intensity; understanding how sensory signals guide behavior requires probing sensory system function across the range of stimulus intensities naturally experienced by an organism. In olfaction, defining the range of natural odorant concentrations is difficult. Odors are complex mixtures of airborne chemicals emitting from a source in an irregular pattern that varies across time and space, necessitating specialized methods to obtain an accurate measurement of concentration. Perhaps as a result, experimentalists often choose stimulus concentrations based on empirical considerations rather than with respect to ecological or behavioral context. Here, we attempt to determine naturally relevant concentration ranges for olfactory stimuli by reviewing and integrating data from diverse disciplines. We compare odorant concentrations used in experimental studies in rodents and insects with those reported in different settings including ambient natural environments, the headspace of natural sources, and within the sources themselves. We also compare these values to psychophysical measurements of odorant detection threshold in rodents, where thresholds have been extensively measured. Odorant concentrations in natural regimes rarely exceed a few parts per billion, while most experimental studies investigating olfactory coding and behavior exceed these concentrations by several orders of magnitude. We discuss the implications of this mismatch and the importance of testing odorants in their natural concentration range for understanding neural mechanisms underlying olfactory sensation and odor-guided behaviors.

Adult-neurogenesis allows for representational stability and flexibility in early olfactory system

In the early olfactory system, adult-neurogenesis, a process of neuronal replacement results in the continuous reorganization of synaptic connections and network architecture throughout the animal's life. This poses a critical challenge: How does the olfactory system maintain stable representations of odors and therefore allow for stable sensory perceptions amidst this ongoing circuit instability? Utilizing a detailed spiking network model of early olfactory circuits, we uncovered dual roles for adult-neurogenesis: one that both supports representational stability to faithfully encode odor information and also one that facilitates plasticity to allow for learning and adaptation. In the main olfactory bulb, adult-neurogenesis affects neural codes in individual mitral and tufted cells but preserves odor representations at the neuronal population level. By contrast, in the olfactory piriform cortex, both individual cell responses and overall population dynamics undergo progressive changes due to adult-neurogenesis. This leads to representational drift, a gradual alteration in sensory perception. Both processes are dynamic and depend on experience such that repeated exposure to specific odors reduces the drift due to adult-neurogenesis; thus, when the odor environment is stable over the course of adult-neurogenesis, it is neurogenesis that actually allows the representations to remain stable in piriform cortex; when those olfactory environments change, adult-neurogenesis allows the cortical representations to track environmental change. Whereas perceptual stability and plasticity due to learning are often thought of as two distinct, often contradictory processing in neuronal coding, we find that adult-neurogenesis serves as a shared mechanism for both. In this regard, the quixotic presence of adult-neurogenesis in the mammalian olfactory bulb that has been the focus of considerable debate in chemosensory neuroscience may be the mechanistic underpinning behind an array of complex computations.

Cholecystokinin-expressing superficial tufted cells modulate odour representation in the olfactory bulb and olfactory behaviours

In mammals, odour information within the olfactory bulb (OB) is processed by complex neural circuits before being ultimately represented in the action potential activity of mitral/tufted cells (M/Ts). Cholecystokinin-expressing (CCK+) superficial tufted cells (sTCs) are a subset of tufted cells that potentially contribute to olfactory processing in the OB by orchestrating M/T activity. However, the exact role of CCK+ sTCs in modulating odour processing and olfactory function in vivo is largely unknown. Here, we demonstrate that manipulating CCK+ sTCs can generate perception and induce place avoidance. Optogenetic activation/inactivation of CCK+ sTCs exerted strong but differing effects on spontaneous and odour-evoked M/T firing. Furthermore, inactivation of CCK+ sTCs disrupted M/T odour encoding and impaired olfactory detection and odour discrimination. These results establish the role of CCK+ sTCs in odour representation and olfactory behaviours. KEY POINTS: Mice could perceive the activity of CCK+ sTCs and show place avoidance to CCK+ sTC inactivation. Optical activation of CCK+ sTCs increased the percentage of cells with odour response but reduced the odour-evoked response in M/Ts in awake mice. Optical inactivation of CCK+ sTCs greatly decreased spontaneous firing and odour-evoked response in M/Ts. Inactivation of CCK+ sTCs impairs the odour decoding performance of M/Ts and disrupts odour detection and discrimination behaviours in mice. These results indicate that CCK+ sTCs participate in modulating the odour representation and maintaining normal olfactory-related behaviours.

Association between dietary zinc intake and olfactory dysfunction: a study based on the NHANES database

OBJECTIVE

The primary objective of this study was to find the association between dietary zinc intake and the prevalence of olfactory disorders using data from the National Health and Nutrition Examination Survey (NHANES).

METHODS

A cross-sectional study was conducted using the 2013-2014 NHANES data. A linear regression model was constructed with dietary zinc intake as the independent variable and olfactory dysfunction as the dependent variable. Initially, in the unadjusted model, weighted logistic regression analysis was carried out for continuous variables, and stratified analysis was conducted for categorical variables. Subsequently, three models were created to perform subgroup analysis by adjusting for different confounding factors, further investigating the relationship between dietary zinc intake and olfactory dysfunction. Finally, restricted cubic spline (RCS) models adjusting for all confounding factors were utilized to study the nonlinear associations of age and dietary zinc intake with olfactory dysfunction and their relevant thresholds.

RESULTS

A total of 2958 samples were analyzed in this study. Weighted logistic regression analysis displayed a negative relationship between dietary zinc intake and the prevalence of olfactory dysfunction in the population of non-Hispanic whites and other Hispanics, as well as in individuals with body mass index (BMI) ≥ 25 kg/m2 (OR < 1, P < 0.05). The P values for the multiplicative interaction terms adjusting for all confounding factors were not significant (P for interaction > 0.05). In the three regression models adjusting for different confounding factors, dietary zinc intake was significantly negatively related to olfactory dysfunction in all populations (Crude: OR 0.63, 95% CI 0.44-0.91; Model I: OR 0.58, 95% CI 0.38-0.90; Model II: OR 0.59, 95% CI 0.35-1.00). Subgroup analysis based on BMI showed a remarkable negative relationship between dietary zinc intake and olfactory dysfunction in the group with BMI of 25-30 kg/m2 (Crude: OR 0.50, 95% CI 0.28-0.90, P = 0.012; Model I: OR 0.49, 95% CI 0.24-1.00, P = 0.021) and the group with BMI ≥ 30 kg/m2 (Crude: OR 0.55, 95% CI 0.33-0.92, P = 0.013; Model I: OR 0.51, 95% CI 0.29-0.88, P = 0.005; Model II: OR 0.51, 95% CI 0.29-0.91, P = 0.004). RCS analysis revealed a remarkable nonlinear association of age and dietary zinc intake with olfactory dysfunction (P-non-linear < 0.05). The prevalence of olfactory dysfunction was considerably higher in individuals aged 60 and above compared to those under 60 years old. Daily dietary zinc intake within the range of 9.60-17.45 mg was a protective factor for olfactory dysfunction, while intake outside this range increased the prevalence of olfactory dysfunction.

CONCLUSION

Daily dietary zinc intake within the range of 9.60-17.45 mg has a protective effect against olfactory dysfunction. Intake outside this range increases the prevalence of olfactory dysfunction. The prevalence of olfactory dysfunction is significantly higher in individuals aged 60 and above compared to those under 60 years old. For individuals with a BMI of 25-30 kg/m2 and a BMI ≥ 30 kg/m2, dietary zinc intake is negatively correlated with olfactory dysfunction. Therefore, it is recommended that these populations increase their dietary zinc intake to develop healthier lifestyles and maintain olfactory health.

Stability and flexibility of odor representations in the mouse olfactory bulb

Dynamic changes in sensory representations have been basic tenants of studies in neural coding and plasticity. In olfaction, relatively little is known about the dynamic range of changes in odor representations under different brain states and over time. Here, we used time-lapse in vivo two-photon calcium imaging to describe changes in odor representation by mitral cells, the output neurons of the mouse olfactory bulb. Using anesthetics as a gross manipulation to switch between different brain states (wakefulness and under anesthesia), we found that odor representations by mitral cells undergo significant re-shaping across states but not over time within state. Odor representations were well balanced across the population in the awake state yet highly diverse under anesthesia. To evaluate differences in odor representation across states, we used linear classifiers to decode odor identity in one state based on training data from the other state. Decoding across states resulted in nearly chance-level accuracy. In contrast, repeating the same procedure for data recorded within the same state but in different time points, showed that time had a rather minor impact on odor representations. Relative to the differences across states, odor representations remained stable over months. Thus, single mitral cells can change dynamically across states but maintain robust representations across months. These findings have implications for sensory coding and plasticity in the mammalian brain.

Top-down feedback enables flexible coding strategies in the olfactory cortex

In chemical sensation, multiple models have been proposed to explain how odors are represented in the olfactory cortex. One hypothesis is that the combinatorial identity of active neurons within sniff-related time windows is critical, whereas another model proposes that it is the temporal structure of neural activity that is essential for encoding odor information. We find that top-down feedback to the main olfactory bulb dictates the information transmitted to the piriform cortex and switches between these coding strategies. Using a detailed network model, we demonstrate that feedback control of inhibition influences the excitation-inhibition balance in mitral cells, restructuring the dynamics of piriform cortical cells. This results in performance improvement in odor discrimination tasks. These findings present a framework for early olfactory computation, where top-down feedback to the bulb flexibly shapes the temporal structure of neural activity in the piriform cortex, allowing the early olfactory system to dynamically switch between two distinct coding models.

Relationship between Olfactory Function and BMI in Normal Weight Healthy Subjects and Patients with Overweight or Obesity

Smell plays a critical role in food choice and intake by influencing energy balance and body weight. Malnutrition problems or modified eating behaviors have been associated with olfactory impairment or loss. The obesity epidemic is a serious health problem associated with an increased risk of mortality and major physical comorbidities. The etiopathogenesis of obesity is complex and multifactorial, and one of the main factors contributing to the rapid increase in its incidence is the environment in which we live, which encourages the overconsumption of foods rich in energy, such as saturated fats and sugars. By means of the “Sniffin’ Sticks” test, we measured the olfactory threshold, discrimination and identification score (TDI score) in patients of the Obesity Center of the University Hospital (OC; n = 70) and we compared them with that of healthy normal weight controls (HC; n = 65). OC patients demonstrated a significantly lower olfactory function than HC subjects both general and specific for the ability to discriminate and identify odors, even when they were considered separately as females and males. For OC patients, a negative correlation was found between body mass index (BMI) and olfactory scores obtained by each subject, both when they were divided according to gender and when they were considered all together. Besides, normosmic OC patients showed a significantly lower BMI than hyposmic ones. A reduced sense of smell may contribute to obesity involving the responses of the cephalic phase, with a delay in the achievement of satiety and an excessive intake of high-energy foods and drinks.

CHIME: CMOS-Hosted in vivo Microelectrodes for Massively Scalable Neuronal Recordings

Mammalian brains consist of 10s of millions to 100s of billions of neurons operating at millisecond time scales, of which current recording techniques only capture a tiny fraction. Recording techniques capable of sampling neural activity at high spatiotemporal resolution have been difficult to scale. The most intensively studied mammalian neuronal networks, such as the neocortex, show a layered architecture, where the optimal recording technology samples densely over large areas. However, the need for application-specific designs as well as the mismatch between the three-dimensional architecture of the brain and largely two-dimensional microfabrication techniques profoundly limits both neurophysiological research and neural prosthetics. Here, we discuss a novel strategy for scalable neuronal recording by combining bundles of glass-ensheathed microwires with large-scale amplifier arrays derived from high-density CMOS in vitro MEA systems or high-speed infrared cameras. High signal-to-noise ratio (<25 μV RMS noise floor, SNR up to 25) is achieved due to the high conductivity of core metals in glass-ensheathed microwires allowing for ultrathin metal cores (down to <1 μm) and negligible stray capacitance. Multi-step electrochemical modification of the tip enables ultra-low access impedance with minimal geometric area, which is largely independent of the core diameter. We show that the microwire size can be reduced to virtually eliminate damage to the blood-brain-barrier upon insertion and we demonstrate that microwire arrays can stably record single-unit activity. Combining microwire bundles and CMOS arrays allows for a highly scalable neuronal recording approach, linking the progress in electrical neuronal recordings to the rapid progress in silicon microfabrication. The modular design of the system allows for custom arrangement of recording sites. Our approach of employing bundles of minimally invasive, highly insulated and functionalized microwires to extend a two-dimensional CMOS architecture into the 3rd dimension can be translated to other CMOS arrays, such as electrical stimulation devices.

Massively parallel microwire arrays integrated with CMOS chips for neural recording

Multi-channel electrical recordings of neural activity in the brain is an increasingly powerful method revealing new aspects of neural communication, computation, and prosthetics. However, while planar silicon-based CMOS devices in conventional electronics scale rapidly, neural interface devices have not kept pace. Here, we present a new strategy to interface silicon-based chips with three-dimensional microwire arrays, providing the link between rapidly-developing electronics and high density neural interfaces. The system consists of a bundle of microwires mated to large-scale microelectrode arrays, such as camera chips. This system has excellent recording performance, demonstrated via single unit and local-field potential recordings in isolated retina and in the motor cortex or striatum of awake moving mice. The modular design enables a variety of microwire types and sizes to be integrated with different types of pixel arrays, connecting the rapid progress of commercial multiplexing, digitisation and data acquisition hardware together with a three-dimensional neural interface.

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.

Leptin modulates olfactory discrimination and neural activity in the olfactory bulb

AIM

Leptin is an important peptide hormone that regulates food intake and plays a crucial role in modulating olfactory function. Although a few previous studies have investigated the effect of leptin on odor perception and discrimination in rodents, research on the neural basis underlying the behavioral changes is lacking. Here we study how leptin affects behavioral performance during a go/no-go task and how it modulates neural activity of mitral/tufted cells in the olfactory bulb, which plays an important role in odor information processing and representation.

METHODS

A go/no-go odor discrimination task was used in the behavioral test. For in vivo studies, single unit recordings, local field potential recordings and fiber photometry recordings were used. For in vitro studies, we performed patch clamp recordings in the slice of the olfactory bulb.

RESULTS

Behaviorally, leptin affects performance and reaction time in a difficult odor-discrimination task. Leptin decreases the spontaneous firing of single mitral/tufted cells, decreases the odor-evoked beta and high gamma local field potential response, and has bidirectional effects on the odor-evoked responses of single mitral/tufted cells. Leptin also inhibits the population calcium activity in genetically identified mitral/tufted cells and granule cells. Furthermore, in vitro slice recordings reveal that leptin inhibits mitral cell activity through direct modulation of the voltage-sensitive potassium channel.

CONCLUSIONS

The behavioral reduction in odor discrimination observed after leptin administration is likely due to decreased neural activity in mitral/tufted cells, caused by modulation of potassium channels in these cells.

Partial depletion of dopaminergic neurons in the substantia nigra impairs olfaction and alters neural activity in the olfactory bulb

Olfactory dysfunction is a major non-motor symptom that appears during the early stages of Parkinson’s Disease (PD), a neurodegenerative disorder characterized by loss of dopaminergic neurons in the substantia nigra (SN). Depletion of SN dopaminergic neurons by 6-hydroxydopamine (6-OHDA) is widely used as a model for PD and ultimately results in motor deficits. However, it is largely unknown whether olfactory behavior and, more importantly, neural activity in the olfactory bulb (OB) are impaired prior to the appearance of motor deficits. We partially depleted the SN dopaminergic population in mice by injection of 6-OHDA. Seven days after injection of 6-OHDA, motor ability was unchanged but olfactory-driven behaviors were significantly impaired. Injection of 6-OHDA into the SN significantly increased the power of the ongoing local field potential in the OB for all frequency bands, and decreased odor-evoked excitatory beta responses and inhibitory high-gamma responses. Moreover, 6-OHDA treatment led to increased odor-evoked calcium responses in the mitral cells in the OB of awake mice. These data suggest that the olfactory deficits caused by depletion of the SN dopaminergic population are likely due to abnormal hyperactivity of the mitral cells in the OB.

Restoring wild-type-like CA1 network dynamics and behavior during adulthood in a mouse model of schizophrenia

Schizophrenia is a severely debilitating neurodevelopmental disorder. Establishing a causal link between circuit dysfunction and particular behavioral traits that are relevant to schizophrenia is crucial to shed new light on the mechanisms underlying the pathology. We studied an animal model of the human 22q11 deletion syndrome, the mutation that represents the highest genetic risk of developing schizophrenia. We observed a desynchronization of hippocampal neuronal assemblies that resulted from parvalbumin interneuron hypoexcitability. Rescuing parvalbumin interneuron excitability with pharmacological or chemogenetic approaches was sufficient to restore wild-type-like CA1 network dynamics and hippocampal-dependent behavior during adulthood. In conclusion, our data provide insights into the network dysfunction underlying schizophrenia and highlight the use of reverse engineering to restore physiological and behavioral phenotypes in an animal model of neurodevelopmental disorder.