Parallel processing by distinct classes of principal neurons in the olfactory cortex

Understanding how distinct neuron types in a neural circuit process and propagate information is essential for understanding what the circuit does and how it does it. The olfactory (piriform, PCx) cortex contains two main types of principal neurons, semilunar (SL) and superficial pyramidal (PYR) cells. SLs and PYRs have distinct morphologies, local connectivity, biophysical properties, and downstream projection targets. Odor processing in PCx is thought to occur in two sequential stages. First, SLs receive and integrate olfactory bulb input and then PYRs receive, transform, and transmit SL input. To test this model, we recorded from populations of optogenetically identified SLs and PYRs in awake, head-fixed mice. Notably, silencing SLs did not alter PYR odor responses, and SLs and PYRs exhibited differences in odor tuning properties and response discriminability that were consistent with their distinct embeddings within a sensory-associative cortex. Our results therefore suggest that SLs and PYRs form parallel channels for differentially processing odor information in and through PCx.

Non-synaptic interactions between olfactory receptor neurons, a possible key feature of odor processing in flies

When flies explore their environment, they encounter odors in complex, highly intermittent plumes. To navigate a plume and, for example, find food, they must solve several challenges, including reliably identifying mixtures of odorants and their intensities, and discriminating odorant mixtures emanating from a single source from odorants emitted from separate sources and just mixing in the air. Lateral inhibition in the antennal lobe is commonly understood to help solving these challenges. With a computational model of the Drosophila olfactory system, we analyze the utility of an alternative mechanism for solving them: Non-synaptic (“ephaptic”) interactions (NSIs) between olfactory receptor neurons that are stereotypically co-housed in the same sensilla. We find that NSIs improve mixture ratio detection and plume structure sensing and do so more efficiently than the traditionally considered mechanism of lateral inhibition in the antennal lobe. The best performance is achieved when both mechanisms work in synergy. However, we also found that NSIs decrease the dynamic range of co-housed ORNs, especially when they have similar sensitivity to an odorant. These results shed light, from a functional perspective, on the role of NSIs, which are normally avoided between neurons, for instance by myelination.

Postnatal development of centrifugal inputs to the olfactory bulb

Processing in primary sensory areas is influenced by centrifugal inputs from higher brain areas, providing information about behavioral state, attention, or context. Activity in the olfactory bulb, the first central processing stage of olfactory information, is dynamically modulated by direct projections from a variety of areas in adult mice. Despite the early onset of olfactory sensation compared to other senses, the development of centrifugal inputs to the olfactory bulb remains largely unknown. Using retrograde tracing across development, we show that centrifugal projections to the olfactory bulb are established during the postnatal period in an area-specific manner. While feedback projections from the piriform cortex are already present shortly after birth, they strongly increase in number during postnatal development with an anterior-posterior gradient. Contralateral projections from the anterior olfactory nucleus are present at birth but only appeared postnatally for the nucleus of the lateral olfactory tract. Numbers of olfactory bulb projecting neurons from the lateral entorhinal cortex, ventral hippocampus, and cortical amygdala show a sudden increase at the beginning of the second postnatal week and a delayed development compared to more anterior areas. These anatomical data suggest that limited top-down influence on odor processing in the olfactory bulb may be present at birth, but strongly increases during postnatal development and is only fully established later in life.

Distinct interhemispheric connectivity at the level of the olfactory bulb emerges during Xenopus laevis metamorphosis

During metamorphosis, the olfactory system of anuran tadpoles undergoes substantial restructuring. The main olfactory epithelium in the principal nasal cavity of Xenopus laevis tadpoles is associated with aquatic olfaction and transformed into the adult air-nose, while a new adult water-nose emerges in the middle cavity. Impacts of this metamorphic remodeling on odor processing, behavior, and network structure are still unexplored. Here, we used neuronal tracings, calcium imaging, and behavioral experiments to examine the functional connectivity between the epithelium and the main olfactory bulb during metamorphosis. In tadpoles, olfactory receptor neurons in the principal cavity project axons to glomeruli in the ventral main olfactory bulb. These projections are gradually replaced by receptor neuron axons from the newly forming middle cavity epithelium. Despite this reorganization in the ventral bulb, two spatially segregated odor processing streams remain undisrupted and behavioral responses to waterborne odorants are unchanged. Contemporaneously, new receptor neurons in the remodeling principal cavity innervate the emerging dorsal part of the bulb, which displays distinct wiring features. Glomeruli around its midline are innervated from the left and right nasal epithelia. Additionally, postsynaptic projection neurons in the dorsal bulb predominantly connect to multiple glomeruli, while half of projection neurons in the ventral bulb are uni-glomerular. Our results show that the “water system” remains functional despite metamorphic reconstruction. The network differences between the dorsal and ventral olfactory bulb imply a higher degree of odor integration in the dorsal main olfactory bulb. This is possibly connected with the processing of different odorants, airborne vs. waterborne.

Multi-functional feedforward inhibitory circuits for cortical odor processing

Feedforward inhibitory circuits are key contributors to the complex interplay between excitation and inhibition in the brain. Little is known about the function of feedforward inhibition in the primary olfactory (piriform) cortex. Using in vivo two-photon targeted patch clamping and calcium imaging in mice, we find that odors evoke strong excitation in two classes of interneurons – neurogliaform (NG) cells and horizontal (HZ) cells – that provide feedforward inhibition in layer 1 of the piriform cortex. NG cells fire much earlier than HZ cells following odor onset, a difference that can be attributed to the faster odor-driven excitatory synaptic drive that NG cells receive from the olfactory bulb. As a consequence, NG cells strongly but transiently inhibit odor-evoked excitation in layer 2 principal cells, whereas HZ cells provide more diffuse and prolonged feedforward inhibition. Our findings reveal unexpected complexity in the operation of inhibition in the piriform cortex.

Parallel pathways for rapid odor processing in lateral entorhinal cortex: Rate and temporal coding by layer 2 subcircuits

SummaryOlfactory information is encoded in lateral entorhinal cortex (LEC) by two classes of layer 2 (L2) principal neurons: fan and pyramidal cells. However, the functional properties of L2 neurons are unclear. Here, we show in awake mice that L2 cells respond rapidly to odors during single sniffs and that LEC is essential for discrimination of odor identity and intensity. Population analyses of L2 ensembles reveals that while rate coding distinguishes odor identity, firing rates are weakly concentration-dependent and changes in spike timing represent odor intensity. L2 principal cells differ in afferent olfactory input and connectivity with local inhibitory circuits and the relative timing of pyramidal and fan cell spikes underlies odor intensity coding. Downstream, intensity is encoded purely by spike timing in hippocampal CA1. Together, these results reveal the unique processing of odor information by parallel LEC subcircuits and highlight the importance of temporal coding in higher olfactory areas.

A non-invasive olfactory bulb measure dissociates Parkinson’s patients from healthy controls and discloses disease duration

Olfactory dysfunction is a prevalent non-motor symptom of Parkinson’s disease (PD). This dysfunction is a result of neurodegeneration within the olfactory bulb (OB), the first processing area of the central olfactory system, and commonly precedes the characteristic motor symptoms in PD by several years. Functional measurements of the OB could therefore potentially be used as an early biomarker for PD. Here, we used a non-invasive method, so-called electrobulbogram (EBG), to measure OB function in PD and age-matched healthy controls to assess whether EBG measures can dissociate PDs from controls. We estimated the spectrogram of the EBG signal during exposure to odor in PD (n = 20) and age-matched controls (n = 18) as well as identified differentiating patterns of odor-related synchronization in the gamma, beta, and theta frequency bands. Moreover, we assessed if these PD-EBG components could dissociate PD from control as well as their relationship with PD characteristics. We identified six EBG components during the initial and later stages of odor processing which dissociated PD from controls with 90% sensitivity and 100% specificity with links to PD characteristics. These PD-EBG components were related to medication, disease duration, and severity, as well as clinical odor identification performance. These findings support using EBG as a tool to experimentally assess PD interventions, potentially aid diagnosis, and the potential development of EBG into an early biomarker for PD.

Parallel processing by distinct classes of principal neurons in the olfactory cortex

Understanding the specific roles that different neuron types play within a neural circuit is essential for understanding what that circuit does and how it does it. The primary olfactory (piriform, PCx) cortex contains two main types of principal neurons: semilunar (SL) and pyramidal (PYR). SLs and PYRs have different morphologies, connectivity, biophysical properties, and downstream projections, predicting distinct roles in cortical odor processing. The prevailing model is that odor processing in PCx occurs in two stages, where SLs are the primary recipients of olfactory bulb (OB) input, and PYRs receive and transform information from SLs. Here we recorded from optogenetically-identified SLs and PYRs in awake, head-fixed mice. We found differences in SL and PYR odor-evoked activity that reflect their different connectivity profiles. But SL responses did not precede PYR responses and suppressing SL activity had little effect on PYR odor responses. These results suggest that SLs and PYRs form parallel odor processing channels.

Imbalanced post- and extrasynaptic SHANK2A functions during development affect social behavior in SHANK2-mediated neuropsychiatric disorders

Mutations in SHANK genes play an undisputed role in neuropsychiatric disorders. Until now, research has focused on the postsynaptic function of SHANKs, and prominent postsynaptic alterations in glutamatergic signal transmission have been reported in Shank KO mouse models. Recent studies have also suggested a possible presynaptic function of SHANK proteins, but these remain poorly defined. In this study, we examined how SHANK2 can mediate electrophysiological, molecular, and behavioral effects by conditionally overexpressing either wild-type SHANK2A or the extrasynaptic SHANK2A(R462X) variant. SHANK2A overexpression affected pre- and postsynaptic targets and revealed a reversible, development-dependent autism spectrum disorder-like behavior. SHANK2A also mediated redistribution of Ca2+-permeable AMPA receptors between apical and basal hippocampal CA1 dendrites, leading to impaired synaptic plasticity in the basal dendrites. Moreover, SHANK2A overexpression reduced social interaction and increased the excitatory noise in the olfactory cortex during odor processing. In contrast, overexpression of the extrasynaptic SHANK2A(R462X) variant did not impair hippocampal synaptic plasticity, but still altered the expression of presynaptic/axonal signaling proteins. We also observed an attention-deficit/hyperactivity-like behavior and improved social interaction along with enhanced signal-to-noise ratio in cortical odor processing. Our results suggest that the disruption of pre- and postsynaptic SHANK2 functions caused by SHANK2 mutations has a strong impact on social behavior. These findings indicate that pre- and postsynaptic SHANK2 actions cooperate for normal neuronal function, and that an imbalance between these functions may lead to different neuropsychiatric disorders.

Distinct interhemispheric connectivity at the level of the olfactory bulb emerges during Xenopus laevis metamorphosis

The olfactory system of anuran tadpoles requires substantial restructuring to adapt to the lifestyle of the adult frogs. Xenopus laevis tadpoles have a single main olfactory epithelium in the principal nasal cavity associated with aquatic olfaction. After metamorphosis, this epithelial surface is transformed into the adult air-nose and a new epithelium, the adult water-nose, is present in the middle cavity. Impacts of this massive remodeling on odor processing, behavior and network structure are still unexplored. In the present study, we used neuronal tracings, calcium imaging and a behavioral assay to examine the functional connectivity between the epithelium and the main olfactory bulb during metamorphosis. In tadpoles, olfactory receptor neurons in the principal cavity epithelium project axons to glomeruli in the ventral main olfactory bulb. During metamorphosis, these projections are gradually replaced by receptor neuron axons emerging from the newly forming middle cavity epithelium. Despite this metamorphotic reorganization in the ventral bulb, two spatially and functionally segregated odor processing streams remain undisrupted. In line with this, metamorphotic rewiring does not alter behavioral responses to waterborne odorants. Contemporaneously, newly formed receptor neurons in the remodeling principal cavity epithelium project their axons to the dorsal part of the bulb. The emerging neuronal networks of the dorsal and ventral main olfactory bulb show substantial differences. Glomeruli around the midline of the dorsal bulb are innervated from the left and right nasal epithelia. In addition, postsynaptic projection neurons in the dorsal bulb predominantly have smaller tufts and connect to multiple glomeruli, while more than half of projection neurons in the ventral bulb have a single, bigger tuft. Our results show that during the metamorphotic reconstruction of the olfactory network the water system remains functional. Differences of the neuronal network of the dorsal and ventral olfactory bulb imply that a higher degree of odor integration takes place in the dorsal main olfactory bulb. This is likely connected with the processing of different odorants, airborne vs. waterborne, in these two parts of the olfactory bulb.