Respiration Gates Sensory Input Responses in the Mitral Cell Layer of the Olfactory Bulb

Circuit formation and sensory perception in the mouse olfactory system

In the mouse olfactory system, odor information is converted to a topographic map of activated glomeruli in the olfactory bulb (OB). Although the arrangement of glomeruli is genetically determined, the glomerular structure is plastic and can be modified by environmental stimuli. If the pups are exposed to a particular odorant, responding glomeruli become larger recruiting the dendrites of connecting projection neurons and interneurons. This imprinting not only increases the sensitivity to the exposed odor, but also imposes the positive quality on imprinted memory. External odor information represented as an odor map in the OB is transmitted to the olfactory cortex (OC) and amygdala for decision making to elicit emotional and behavioral outputs using two distinct neural pathways, innate and learned. Innate olfactory circuits start to work right after birth, whereas learned circuits become functional later on. In this paper, the recent progress will be summarized in the study of olfactory circuit formation and odor perception in mice. We will also propose new hypotheses on the timing and gating of olfactory circuit activity in relation to the respiration cycle.

Improvement of olfactory fMRI activation and hemodynamic response function curve with respiration correction

BACKGROUND

Odor perception is highly dependent on respiration, however, the asynchronization between inspiration and odor delivery results in a decreased activation and hemodynamic response function (HRF) curve in olfactory functional MRI (fMRI). This study aimed to investigate the effect of a data post-processing method to minimize the asynchronization and consequently improve the olfactory activation and HRF curve and compare it between different TRs (3 s and 1 s).

NEW METHOD

A new data post-processing method of respiration correction based on olfactory perception was investigated.

COMPARISON WITH EXISTING METHODS

The odor stimuli of olfactory fMRI were mostly based on odor deliveries. But for the reason of respiration, the time of olfactory perception might be a little different. The actual onset time and duration of odor stimuli were modified based on the respiration data.

RESULTS

For 3 s TR data, an increased number of activated voxels and higher HRF curves response height were obtained with the respiration correction method in the primary olfactory cortex (POC) (P = 0.034 and 0.009) and insula (P = 0.024 and 0.001), compared with the uncorrected method. But the activated voxels and response height of the anterior cingulate cortex (ACC) did not differ between uncorrected and respiration correction methods (P = 0.102 and 0.200). The time to peak of the HRF curve was shorter with the respiration correction method in all ROIs (all P < 0.05), compared with the uncorrected method. Whereas there were no significant differences between the uncorrected and corrected results for 1 s TR data.

CONCLUSIONS

The post-processing method of respiration correction could effectively minimize the asynchronization between respirations and odor deliveries, and improve the activations and HRF curves for a routine 3 s TR olfactory fMRI data.

The influence of the respiratory cycle on reaction times in sensory-cognitive paradigms

Behavioural and electrophysiological studies point to an apparent influence of the state of respiration, i.e., whether we inhale or exhale, on brain activity and cognitive performance. Still, the prevalence and relevance of such respiratory-behavioural relations in typical sensory-cognitive tasks remain unclear. We here used a battery of six tasks probing sensory detection, discrimination and short-term memory to address the questions of whether and by how much behaviour covaries with the respiratory cycle. Our results show that participants tend to align their respiratory cycle to the experimental paradigm, in that they tend to inhale around stimulus presentation and exhale when submitting their responses. Furthermore, their reaction times, but not so much their response accuracy, consistently and significantly covary with the respiratory cycle, differing between inhalation and exhalation. This effect is strongest when analysed contingent on the respiratory state around participants' responses. The respective effect sizes of these respiration-behaviour relations are comparable to those seen in other typical experimental manipulations in sensory-cognitive tasks, highlighting the relevance of these effects. Overall, our results support a prominent relation between respiration and sensory-cognitive function and show that sensation is intricately linked to rhythmic bodily or interoceptive functions.

Temporal Relations between Cortical Network Oscillations and Breathing Frequency during REM Sleep

Nasal breathing generates a rhythmic signal which entrains cortical network oscillations in widespread brain regions on a cycle-to-cycle time scale. It is unknown, however, how respiration and neuronal network activity interact on a larger time scale: are breathing frequency and typical neuronal oscillation patterns correlated? Is there any directionality or temporal relationship? To address these questions, we recorded field potentials from the posterior parietal cortex of mice together with respiration during REM sleep. In this state, the parietal cortex exhibits prominent θ and γ oscillations while behavioral activity is minimal, reducing confounding signals. We found that the instantaneous breathing frequency strongly correlates with the instantaneous frequency and amplitude of both θ and γ oscillations. Cross-correlograms and Granger causality revealed specific directionalities for different rhythms: changes in θ activity precede and Granger-cause changes in breathing frequency, suggesting control by the functional state of the brain. On the other hand, the instantaneous breathing frequency Granger causes changes in γ frequency, suggesting that γ is influenced by a peripheral reafference signal. These findings show that changes in breathing frequency temporally relate to changes in different patterns of rhythmic brain activity. We hypothesize that such temporal relations are mediated by a common central drive likely to be located in the brainstem.

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.

Methods in Rodent Chemosensory Cognition

Olfactory information processing and learning are highly developed computational abilities of rodents. These attributes can be exploited to ask questions at several levels of complexity, from aspects of odorant binding by olfactory receptors to higher order learning about the predictive consequences of odorant stimulus presentation. Quantitative understanding of rodent odorant sampling patterns, both baseline nasal breathing and odorant-stimulated sniffing, is critical to elucidating mechanisms of olfactory information processing, from primary olfactory receptors to cortical centers that synthesize olfactory percepts from preprocessed multimodal inputs. This chapter outlines an innovative new method for measuring breathing and sniffing rates in unrestrained mice while the mice are performing odor-guided tasks in a computer controlled olfactometer.The method described here involves implantation of a wireless pressure sensor in the mouse that reports on thoracic pressure transients caused by breathing and sniffing. Recordings of pressure sensor outputs are made simultaneously with optically-sensed nose pokes by the mouse into an odor delivery port or a water delivery port. Odorant delivery timing and water reward delivery are also recorded simultaneously. This method allows for breathing and sniffing dependent thoracic pressure transients to be recorded with high temporal precision before, during, and after the mouse approaches an odor delivery port, samples the delivered odor, and obtains a water reward contingent on the identity of the odor that was presented and sampled.

Strong, weak and neuron type dependent lateral inhibition in the olfactory bulb

In many sensory systems, different sensory features are transmitted in parallel by several different types of output neurons. In the mouse olfactory bulb, there are only two output neuron types, the mitral and tufted cells (M/T), which receive similar odor inputs, but they are believed to transmit different odor characteristics. How these two neuron types deliver different odor information is unclear. Here, by combining electrophysiology and optogenetics, it is shown that distinct inhibitory networks modulate M/T cell responses differently. Overall strong lateral inhibition was scarce, with most neurons receiving lateral inhibition from a handful of unorganized surrounding glomeruli (~5% on average). However, there was a considerable variability between different neuron types in the strength and frequency of lateral inhibition. Strong lateral inhibition was mostly found in neurons locked to the first half of the respiration cycle. In contrast, weak inhibition arriving from many surrounding glomeruli was relatively more common in neurons locked to the late phase of the respiration cycle. Proximal neurons could receive different levels of inhibition. These results suggest that there is considerable diversity in the way M/T cells process odors so that even neurons that receive the same odor input transmit different odor information to the cortex.

Sparsened neuronal activity in an optogenetically activated olfactory glomerulus

Glomeruli are the functional units of olfactory information processing but little remains known about their individual unit function. This is due to their widespread activation by odor stimuli. We expressed channelrhodopsin-2 in a single olfactory sensory neuron type, and used laser stimulation and simultaneous in vivo calcium imaging to study the responses of a single glomerulus to optogenetic stimulation. Calcium signals in the neuropil of this glomerulus were representative of the sensory input and nearly identical if evoked by intensity-matched odor and laser stimuli. However, significantly fewer glomerular layer interneurons and olfactory bulb output neurons (mitral cells) responded to optogenetic versus odor stimuli, resulting in a small and spatially compact optogenetic glomerular unit response. Temporal features of laser stimuli were represented with high fidelity in the neuropil of the glomerulus and the mitral cells, but not in interneurons. Increases in laser stimulus intensity were encoded by larger signal amplitudes in all compartments of the glomerulus, and by the recruitment of additional interneurons and mitral cells. No spatial expansion of the glomerular unit response was observed in response to stronger input stimuli. Our data are among the first descriptions of input-output transformations in a selectively activated olfactory glomerulus.

Bicarbonate directly modulates activity of chemosensitive neurons in the retrotrapezoid nucleus

KEY POINTS

Changes in CO₂ result in corresponding changes in both H⁺ and HCO₃^- and despite evidence that HCO₃^- can function as an independent signalling molecule, there is little evidence suggesting HCO₃^- contributes to respiratory chemoreception. We show that HCO₃^- directly activates chemosensitive retrotrapezoid nucleus (RTN) neurons. Identifying all relevant signalling molecules is essential for understanding how chemoreceptors function, and because HCO₃^- and H⁺ are buffered by separate cellular mechanisms, having the ability to sense both modalities adds additional information regarding changes in CO₂ that are not necessarily reflected by pH alone. HCO₃^- may be particularly important for regulating activity of RTN chemoreceptors during sustained intracellular acidifications when TASK-2 channels, which appear to be the sole intracellular pH sensor, are minimally active.

ABSTRACT

Central chemoreception is the mechanism by which the brain regulates breathing in response to changes in tissue CO₂ /H⁺ . The retrotrapezoid nucleus (RTN) is an important site of respiratory chemoreception. Mechanisms underlying RTN chemoreception involve H⁺ -mediated activation of chemosensitive neurons and CO₂ /H⁺ -evoked ATP-purinergic signalling by local astrocytes, which activates chemosensitive neurons directly and indirectly by maintaining vascular tone when CO₂ /H⁺ levels are high. Although changes in CO₂ result in corresponding changes in both H⁺ and HCO₃^- and despite evidence that HCO₃^- can function as an independent signalling molecule, there is little evidence suggesting HCO₃^- contributes to respiratory chemoreception. Therefore, the goal of this study was to determine whether HCO₃^- regulates activity of chemosensitive RTN neurons independent of pH. Cell-attached recordings were used to monitor activity of chemosensitive RTN neurons in brainstem slices (300 μm thick) isolated from rat pups (postnatal days 7-11) during exposure to low or high concentrations of HCO₃^- . In a subset of experiments, we also included 2',7'-bis(2carboxyethyl)-5-(and 6)-carboxyfluorescein (BCECF) in the internal solution to measure pHi under each experimental condition. We found that HCO₃^- activates chemosensitive RTN neurons by mechanisms independent of intracellular or extracellular pH, glutamate, GABA, glycine or purinergic signalling, soluble adenylyl cyclase activity, nitric oxide or KCNQ channels. These results establish HCO₃^- as a novel independent modulator of chemoreceptor activity, and because the levels of HCO₃^- along with H⁺ are buffered by independent cellular mechanisms, these results suggest HCO₃^- chemoreception adds additional information regarding changes in CO₂ that are not necessarily reflected by pH.