Olfactory regulation by dopamine and DRD2 receptor in the nose

Olfactory behavior is important for animal survival, and olfactory dysfunction is a common feature of several diseases. Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. In analyzing the single-cell RNA sequencing data from mouse and human olfactory mucosa (OM), we found that the mature olfactory sensory neurons (OSNs) express high levels of dopamine D2 receptor (Drd2) rather than other dopamine receptor subtypes. The DRD2 receptor is expressed in the cilia and somata of mature OSNs, while nasal dopamine is mainly released from the sympathetic nerve terminals, which innervate the mouse OM. Intriguingly, genetic ablation of Drd2 in mature OSNs or intranasal application with DRD2 antagonist significantly increased the OSN response to odorants and enhanced the olfactory sensitivity in mice. Mechanistic studies indicated that dopamine, acting through DRD2 receptor, could inhibit odor-induced cAMP signaling of olfactory receptors. Interestingly, the local dopamine synthesis in mouse OM is down-regulated during starvation, which leads to hunger-induced olfactory enhancement. Moreover, pharmacological inhibition of local dopamine synthesis in mouse OM is sufficient to enhance olfactory abilities. Altogether, these results reveal nasal dopamine and DRD2 receptor as the potential peripheral targets for olfactory modulation.

Olfactory behavior is important for food seeking and animal survival. On the other hand, olfactory dysfunction is a common feature of several diseases such as psychiatric disorders, neurodegeneration, and COVID-19 (1–3). Interestingly, the olfactory ability can be regulated by feeding status and external environments (4, 5). Recent studies have made progress in identifying the neural circuits and circulating hormones in olfactory regulation (6–11). However, the peripheral targets modulating olfactory ability remain relatively unexplored (12).Dopamine (DA) is a monoamine neurotransmitter (13, 14), which plays important roles in a variety of brain functions. DA is released by dopaminergic neurons in the central nervous system. In addition, DA can be released by sympathetic nerves in the peripheral tissues including the olfactory mucosa (OM) (15–18). The sympathetic innervation of rodent OM originates predominantly from the superior cervical ganglion (SCG) (17). Tyrosine hydroxylase (TH) is the rate-limiting enzyme for DA synthesis (19). Intriguingly, the Th mRNA is locally translated in the sympathetic nerve axons, which facilitates local DA synthesis (20, 21).There are two types of DA receptors based on sequence homology and function: The excitatory D1-like receptors (DRD1 and DRD5) and inhibitory D2-like receptors (DRD2–DRD4) (22). Activation of DRD2, a Gα_i/o-coupled receptor, can reduce the intracellular levels of cyclic adenosine monophosphate (cAMP). Drd2 is associated with several neuropsychiatric diseases and is the target of some antipsychotic drugs (23–28). In the central nervous system including the olfactory bulb (OB), DA-DRD2 signaling plays important roles in regulating synaptic transmission and plasticity (29–33). However, the function and regulation of DA-DRD2 signaling in the peripheral tissues are relatively less understood.Here we show that DRD2 is expressed in the cilia and somata of mature olfactory sensory neurons (OSNs) in mice. We provide evidence that DA-DRD2 signaling has a tonic inhibition on OSN activity and olfactory function in mice. Intriguingly, hunger greatly reduces the N4-acetylcytidine (ac⁴C) modification of Th mRNA and local DA synthesis in mouse OM, which causes the olfactory enhancement during starvation. We further show that inhibition of local DA synthesis or DRD2 receptor in mouse OM recapitulates enhanced olfactory abilities during starvation. Collectively, these results reveal nasal DA and DRD2 receptor as the potential peripheral targets for olfactory regulation.     

Continuous neurogenesis in the adult forebrain is required for innate olfactory responses

Although the functional significance of adult neurogenesis in hippocampal-dependent learning and memory has been well documented, the role of such neurogenesis in olfactory activity is rather obscure. To understand the significance of adult neurogenesis in olfactory functions, we genetically ablated newly born neurons by using tamoxifen-treated Nestin-CreER(T2);neuron-specific enolase-diphtheria toxin fragment A (NSE-DTA) mice. In these mice, tamoxifen-inducible Cre recombinase allows the NSE (Eno2) gene to drive DTA expression in differentiating neurons, leading to the efficient ablation of newly born neurons in the forebrain. These mutant mice were capable of discriminating odors as competently as control mice. Strikingly, although control and mutant mice frequently showed freezing behaviors to a fox scent, a predator odor, mutant mice approached this odor when they were conditioned to associate the odor with a reward, whereas control mice did not approach the odor. Furthermore, although mutant males and females showed normal social recognition behaviors to other mice of a different sex, mutant males displayed deficits in male-male aggression and male sexual behaviors toward females, whereas mutant females displayed deficits in fertility and nurturing, indicating that sex-specific activities, which are known to depend on olfaction, are impaired. These results suggest that continuous neurogenesis is required for predator avoidance and sex-specific responses that are olfaction dependent and innately programmed.     

Management of long‐lasting phantosmia: a systematic review

Background

Interest in the pathophysiology and management of phantom smells has increased rapidly over the last decade. A PubMed search for the term “phantosmia” demonstrated a near‐doubling of articles published on phantosmia within the past 7 years. We aimed to systematically review the literature on the management of phantosmia.

Methods

The PubMed, EMBASE, and Cochrane databases were searched for articles published since January 1990, using terms combined with pertinent Boolean search operators. We included articles evaluating management of phantosmia written in the English language, with original data and a minimum of 6 months of follow‐up, on at least 2 patients and with well‐defined and measurable outcomes.

Results

A total of 2151 unique titles were returned upon the initial search. Of these, 146 abstracts were examined, yielding 7 articles meeting the inclusion criteria. All articles were predominantly level 4 evidence. One prospective level 3 study was included. The studies included a total of 96 patients, with follow‐up ranging from 6 months to 11 years. Endpoints were primarily based on subjective patient responses. Management options included observation and medical and surgical therapy. Olfactory mucosa excision was the only surgical intervention studied, with short‐term symptomatic improvement in 10 of 11 patients. Forty‐one patients were treated medically, which included antipsychotic, antimigraine, and antiseizure medications, transcranial stimulation, and topical cocaine application.

Conclusion

Despite increasing interest in the treatment of phantosmia and reports of successful therapies, there remains a paucity of data and lack of consensus regarding optimal management of this difficult condition.

     

Insect olfaction from model systems to disease control

Great progress has been made in the field of insect olfaction in recent years. Receptors, neurons, and circuits have been defined in considerable detail, and the mechanisms by which they detect, encode, and process sensory stimuli are being unraveled. We provide a guide to recent progress in the field, with special attention to advances made in the genetic model organism Drosophila. We highlight key questions that merit additional investigation. We then present our view of how recent advances may be applied to the control of disease-carrying insects such as mosquitoes, which transmit disease to hundreds of millions of people each year. We suggest how progress in defining the basic mechanisms of insect olfaction may lead to means of disrupting host-seeking and other olfactory behaviors, thereby reducing the transmission of deadly diseases.     

Odorant receptors regulate the final glomerular coalescence of olfactory sensory neuron axons

Odorant receptors (OR) are strongly implicated in coalescence of olfactory sensory neuron (OSN) axons and the formation of olfactory bulb (OB) glomeruli. However, when ORs are first expressed relative to basal cell division and OSN axon extension is unknown. We developed an in vivo fate-mapping strategy that enabled us to follow OSN maturation and axon extension beginning at basal cell division. In parallel, we mapped the molecular development of OSNs beginning at basal cell division, including the onset of OR expression. Our data show that ORs are first expressed around 4 d following basal cell division, 24 h after OSN axons have reached the OB. Over the next 6+ days the OSN axons navigate the OB nerve layer and ultimately coalesce in glomeruli. These data provide a previously unidentified perspective on the role of ORs in homophilic OSN axon adhesion and lead us to propose a new model dividing axon extension into two phases. Phase I is OR-independent and accounts for up to 50% of the time during which axons approach the OB and begin navigating the olfactory nerve layer. Phase II is OR-dependent and concludes as OSN axons coalesce in glomeruli.In the mouse olfactory system, olfactory sensory neurons (OSNs) extend their axons from the olfactory epithelium (OE) to the olfactory bulb (OB), where they converge to form glomeruli. Each OSN expresses only 1 of ∼2,400 candidate odorant receptor (OR) alleles. OSNs expressing the same OR can be widely dispersed in the OE, yet their axons converge in only two to three molecularly specific glomeruli of a possible 3,700 (1). It was first recognized almost 20 y ago that substituting an OR-coding region with that of a different OR resulted in the glomerular convergence of axons at an ectopic location relative to that of the native ORs (2). This led to the suggestion that ORs have an instructive role in the extension and glomerular coalescence of OSN axons, most likely mediated by homophilic fasciculation (3–5).Postnatal OSNs are derived from self-renewing precursors located proximal to the deep basal lamina of the OE (6). Following the division of globose basal stem cells, OSN neuroblasts transiently express Achaete-scute homolog 1 (Ascl1) followed by two phases of differentiation (6–8). Initially, they express growth-associated protein–43 (GAP-43), a marker of immature cells. Subsequently, the OSNs down-regulate GAP-43 and express olfactory marker protein (OMP), a universal marker of mature OSNs.Although there is a consensus on the involvement of ORs in OSN axon glomerular convergence, when ORs exert that influence following basal cell division or axon extension is not known. Moreover, the developmental progression of GAP-43 to OMP, as a measure of OSN differentiation and maturation, or of adenylate cyclase 3 (AC3), a downstream signaling molecule also implicated in axon extension, has not been considered in the context of OR expression or OSN dynamics. Here, we determined when ORs exert their influence on OSN axons. We assessed two fundamental questions: (i) When do OSNs express ORs relative to progenitor cell division and the expression of GAP-43, OMP and AC3? (ii) How does the extension of OSN axons correlate with OR onset and the molecular differentiation of OSNs?     

Human herpesvirus-6 entry into the central nervous system through the olfactory pathway

Viruses have been implicated in the development of neurodegenerative diseases, such as Alzheimer's, Parkinson's, and multiple sclerosis. Human herpesvirus-6 (HHV-6) is a neurotropic virus that has been associated with a wide variety of neurologic disorders, including encephalitis, mesial temporal lobe epilepsy, and multiple sclerosis. Currently, the route of HHV-6 entry into the CNS is unknown. Using autopsy specimens, we found that the frequency of HHV-6 DNA in the olfactory bulb/tract region was among the highest in the brain regions examined. Given this finding, we investigated whether HHV-6 may infect the CNS via the olfactory pathway. HHV-6 DNA was detected in a total of 52 of 126 (41.3%) nasal mucous samples, showing the nasal cavity is a reservoir for HHV-6. Furthermore, specialized olfactory-ensheathing glial cells located in the nasal cavity were demonstrated to support HHV-6 replication in vitro. Collectively, these results support HHV-6 utilization of the olfactory pathway as a route of entry into the CNS.     

Olfactory loss in chronic rhinosinusitis is associated with neuronal activation of c‐Jun N‐terminal kinase

Background

Olfactory inflammation in chronic rhinosinusitis (CRS) is associated with cytokines that may result in the death of olfactory sensory neurons. The principal signaling molecules involved in the apoptotic pathway are c‐Jun N‐terminal kinases (JNK). Although the JNK pathway has emerged as a key player in programmed cell death in neuroinflammation, its specific role in CRS‐associated olfactory loss has not been thoroughly investigated.

Methods

JNK activation was studied in human tissue samples from 9 control and 11 CRS patients by immunohistochemical staining for phosphorylated c‐Jun. A mouse model of inducible olfactory cytokine expression was used to experimentally control inflammation and assess JNK activation over time.

Results

In patients with CRS, activation of c‐Jun is significantly increased relative to non‐CRS control subjects, and there is an associated loss of sensory neurons. In the olfactory inflammation mouse model, prolonged induction of inflammation results in elevation of c‐Jun expression and neuronal apoptosis.

Conclusion

Activation of neuronal JNK is a feature of chronic olfactory inflammation that is associated with neuronal apoptosis. Given that inhibition of JNK activity is neuroprotective in other settings, antagonism of this pathway may have therapeutic potential in the management of inflammatory olfactory loss or other disorders linked to olfactory neuronal apoptosis.

     

Formyl peptide receptors are candidate chemosensory receptors in the vomeronasal organ

The identification of receptors that detect environmental stimuli lays a foundation for exploring the mechanisms and neural circuits underlying sensation. The mouse vomeronasal organ (VNO), which detects pheromones and other semiochemicals, has 2 known families of chemoreceptors, V1Rs and V2Rs. Here, we report a third family of mouse VNO receptors comprising 5 of 7 members of the formyl peptide receptor (FPR) family. Unlike other FPRs, which function in the immune system, these FPRs are selectively expressed in VNO neurons in patterns strikingly similar to those of V1Rs and V2Rs. Each FPR is expressed in a different small subset of neurons that are highly dispersed in the neuroepithelium, consistently coexpress either Gα_i2 or Gα_o, and lack other chemoreceptors examined. Given the presence of formylated peptides in bacteria and mitochondria, possible roles for VNO FPRs include the assessment of conspecifics or other species based on variations in normal bacterial flora or mitochondrial proteins.     

Ectopic vesicular neurotransmitter release along sensory axons mediates neurovascular coupling via glial calcium signaling

Neurotransmitter release generally is considered to occur at active zones of synapses, and ectopic release of neurotransmitters has been demonstrated in a few instances. However, the mechanism of ectopic neurotransmitter release is poorly understood. We took advantage of the intimate morphological and functional proximity of olfactory receptor axons and specialized glial cells, olfactory ensheathing cells (OECs), to study ectopic neurotransmitter release. Axonal stimulation evoked purinergic and glutamatergic Ca²⁺ responses in OECs, indicating ATP and glutamate release. In axons expressing synapto-pHluorin, stimulation evoked an increase in synapto-pHluorin fluorescence, indicative of vesicle fusion. Transmitter release was dependent on Ca²⁺ and could be inhibited by bafilomycin A1 and botulinum toxin A. Ca²⁺ transients in OECs evoked by ATP, axonal stimulation, and laser photolysis of NP-EGTA resulted in constriction of adjacent blood vessels. Our results indicate that ATP and glutamate are released ectopically by vesicles along axons and mediate neurovascular coupling via glial Ca²⁺ signaling.     

Unitary response of mouse olfactory receptor neurons

The sense of smell begins with odorant molecules binding to membrane receptors on the cilia of olfactory receptor neurons (ORNs), thereby activating a G protein, G(olf), and the downstream effector enzyme, an adenylyl cyclase (ACIII). Recently, we have found in amphibian ORNs that an odorant-binding event has a low probability of activating sensory transduction at all; even when successful, the resulting unitary response apparently involves a single active Gα(olf)-ACIII molecular complex. This low amplification is in contrast to rod phototransduction in vision, the best-quantified G-protein signaling pathway, where each photoisomerized rhodopsin molecule is well known to produce substantial amplification by activating many G-protein, and hence effector-enzyme, molecules. We have now carried out similar experiments on mouse ORNs, which offer, additionally, the advantage of genetics. Indeed, we found the same low probability of transduction, based on the unitary olfactory response having a fairly constant amplitude and similar kinetics across different odorants and randomly encountered ORNs. Also, consistent with our picture, the unitary response of Gα(olf)(+/-) ORNs was similar to WT in amplitude, although their Gα(olf)-protein expression was only half of normal. Finally, from the action potential firing, we estimated that ≤19 odorant-binding events successfully triggering transduction in a WT mouse ORN will lead to signaling to the brain.     

Valence opponency in peripheral olfactory processing

A hallmark of complex sensory systems is the organization of neurons into functionally meaningful maps, which allow for comparison and contrast of parallel inputs via lateral inhibition. However, it is unclear whether such a map exists in olfaction. Here, we address this question by determining the organizing principle underlying the stereotyped pairing of olfactory receptor neurons (ORNs) in Drosophila sensory hairs, wherein compartmentalized neurons inhibit each other via ephaptic coupling. Systematic behavioral assays reveal that most paired ORNs antagonistically regulate the same type of behavior. Such valence opponency is relevant in critical behavioral contexts including place preference, egg laying, and courtship. Odor-mixture experiments show that ephaptic inhibition provides a peripheral means for evaluating and shaping countervailing cues relayed to higher brain centers. Furthermore, computational modeling suggests that this organization likely contributes to processing ratio information in odor mixtures. This olfactory valence map may have evolved to swiftly process ethologically meaningful odor blends without involving costly synaptic computation.

In complex sensory systems, neurons are typically organized into functionally meaningful maps. This arrangement allows specific stimulus attributes, such as color or spatial contrast, to be computed via lateral inhibition (1). In olfaction, however, it is unclear whether such a functional sensory map exists. In both rodents and flies, sensory neurons which project to nearby glomeruli—processing units in the first olfactory relay center—do not necessarily respond to structurally similar odorants (2, 3), suggesting an absence of chemotopic organization at this circuit level.Might a functional olfactory map instead exist in the periphery? Drosophila melanogaster provides a unique opportunity to address this question, as the receptors, ligands, and behavioral outputs have been characterized for many olfactory receptor neurons [ORNs, (3–8)] (SI Appendix, Table S1). Each sensillum typically houses two and up to four ORNs, which are named “A,” “B,” “C,” or “D” in descending order of their stereotypical extracellular spike amplitudes (9). ORN pairing in a sensillum is also stereotyped—whereby a neuron expressing a particular receptor always neighbors an ORN expressing another specific receptor (4, 6, 8)—implying functional importance for such organization. Indeed, lateral inhibition broadly occurs between compartmentalized ORNs across sensillum types (10).Interestingly, the sensillum is the only such place in the olfactory circuit where short-range lateral inhibition is commonly observed between specific input channels (10–12). Previous work has revealed that direct ephaptic interaction is sufficient to mediate lateral inhibition between electrically coupled ORNs (11). Furthermore, systematic morphological analysis of Drosophila antennal sensilla through serial block-face scanning electron microscopy shows that the basic anatomical features that support ephaptic coupling between ORNs—the close apposition of neuronal processes in a confined compartment (13, 14)—are conserved across sensillum types (15). Taken together, these studies consistently support the notion that ephaptic coupling occurs broadly across olfactory sensilla. Each sensillum can thus be considered a processing unit for olfactory computation, and understanding the organizing principle of ORN pairing will elucidate whether a functional olfactory map exists and how it is arranged.     

Budesonide irrigation with olfactory training improves outcomes compared with olfactory training alone in patients with olfactory loss

Background

Olfactory training (OT) helps many patients with olfactory loss, but unfortunately it is ineffective for a significant number of patients. Budesonide irrigations are widely used to help patients with paranasal sinus inflammation, but have never been tested as a treatment for olfactory loss. We sought to examine the effect of adding budesonide irrigation to olfactory training on patients with olfactory loss without any visible sign of sinonasal inflammation.

Methods

In this randomized, controlled trial, 138 patients with olfactory loss and without any visible sign of sinonasal inflammation were randomized to either OT with saline irrigations or OT with budesonide irrigations. The University of Pennsylvania Smell Identification Test (UPSIT) was administered at the beginning of the study and at 6 months.

Results

A total of 133 patients completed the study. Forty‐seven patients (35.3%) had a clinically significant change in UPSIT score. Among those in the budesonide irrigation + olfactory therapy group, 43.9% improved, compared with 26.9% in the saline irrigation + olfactory therapy group (p = 0.039); this corresponds to an odds ratio of 3.93 (95% confidence interval, 1.20‐12.88) in a fully adjusted model (p = 0.024). Younger age and shorter duration of olfactory loss were also significant predictors of improvement.

Conclusion

Adding budesonide irrigation to olfactory training significantly improved olfactory ability compared with olfactory training plus saline irrigation.