Cannabinoid action in the olfactory epithelium

Structural Rearrangement of the Olfactory Epithelium in Male Baikal Yellowfin Sculpins Across the Reproductive Period

The morphological peculiarities of receptor neurons and support cells in the olfactory epithelium of male yellowfin sculpin (Cottocomephorus grewingkii; Dybowski, 1874) were studied during the pre-spawning, spawning (when males do not feed and have a higher sensitivity to female pheromones), and guarding (the fertilized eggs) periods. This study was performed using electron transmission and laser confocal microscopy. Structural changes in the fish olfactory epithelium are associated with the shift in olfactory signals from alimentary to pheromonal. These results expand our knowledge of the odorant-dependent plasticity of the periphery of the fish olfactory system.

The physiological roles of anoctamin2/TMEM16B and anoctamin1/TMEM16A in chemical senses

Chemical senses allow animals to detect and discriminate a vast array of molecules. The olfactory system is responsible of the detection of small volatile molecules, while water dissolved molecules are detected by taste buds in the oral cavity. Moreover, many animals respond to signaling molecules such as pheromones and other semiochemicals through the vomeronasal organ. The peripheral organs dedicated to chemical detection convert chemical signals into perceivable information through the employment of diverse receptor types and the activation of multiple ion channels. Two ion channels, TMEM16B, also known as anoctamin2 (ANO2) and TMEM16A, or anoctamin1 (ANO1), encoding for Ca2+-activated Cl¯ channels, have been recently described playing critical roles in various cell types. This review aims to discuss the main properties of TMEM16A and TMEM16B-mediated currents and their physiological roles in chemical senses. In olfactory sensory neurons, TMEM16B contributes to amplify the odorant response, to modulate firing, response kinetics and adaptation. TMEM16A and TMEM16B shape the pattern of action potentials in vomeronasal sensory neurons increasing the interspike interval. In type I taste bud cells, TMEM16A is activated during paracrine signaling mediated by ATP. This review aims to shed light on the regulation of diverse signaling mechanisms and neuronal excitability mediated by Ca-activated Cl¯ channels, hinting at potential new roles for TMEM16A and TMEM16B in the chemical senses.

ACKR3 in olfactory glia cells shapes the immune defense of the olfactory mucosa

Barrier-forming olfactory glia cells, termed sustentacular cells, play important roles for immune defense of the olfactory mucosa, for example as entry sites for SARS-CoV-2 and subsequent development of inflammation-induced smell loss. Here we demonstrate that sustentacular cells express ACKR3, a chemokine receptor that functions both as a scavenger of the chemokine CXCL12 and as an activator of alternative signaling pathways. Differential gene expression analysis of bulk RNA sequencing data obtained from WT and ACKR3 conditional knockout mice revealed upregulation of genes involved in immune defense. To map the regulated genes to the different cell types of the olfactory mucosa, we employed biocomputational methods utilizing a single-cell reference atlas. Transcriptome analysis, PCR and immunofluorescence identified up-regulation of NF-κB-related genes, known to amplify inflammatory signaling and to facilitate leukocyte transmigration, in the gliogenic lineage. Accordingly, we found a marked increase in leukocyte-expressed genes and confirmed leukocyte infiltration into the olfactory mucosa. In addition, lack of ACKR3 led to enhanced expression and secretion of early mediators of immune defense by Bowman's glands. As a result, the number of apoptotic cells in the epithelium was decreased. In conclusion, our research underlines the importance of sustentacular cells in immune defense of the olfactory mucosa. Moreover, it identifies ACKR3, a druggable G protein-coupled receptor, as a promising target for modulation of inflammation-associated anosmia.

Cannabinoid receptor type 1 regulates sequential stages of migration and morphogenesis of neural crest cells and derivatives in chicken and frog embryos

The main cannabinoid receptor CB1R first shows expression during early neurula stage in chicken (Gallus gallus) embryos, and at early tailbud stage in the frog (Xenopus laevis) embryos. This raises the question of whether CB1R regulates similar or distinct processes during the embryonic development of these two species. Here, we examined whether CB1R influences the migration and morphogenesis of neural crest cells and derivatives in both chicken and frog embryos. Early neurula stage chicken embryos were exposed to arachidonyl-2'-chloroethylamide (ACEA; a CB1R agonist), N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251; a CB1R inverse agonist) or Blebbistatin (nonmuscle Myosin II inhibitor) in ovo and examined during migration of neural crest cells and at condensing cranial ganglia stage. Early tailbud stage frog embryos were bathed in ACEA, AM251 or Blebbistatin, and analyzed at late tailbud stage for changes in craniofacial and eye morphogenesis, and in patterning and morphology of melanophores (neural crest-derived pigment cells). In chicken embryos exposed to ACEA and Myosin II inhibitor, cranial neural crest cells migrated erratically from the neural tube, and the right, but not the left, ophthalmic nerve of the trigeminal ganglia was affected in ACEA- and AM251-treated embryos. In frog embryos with inactivation or activation of CB1R, or inhibition of Myosin II, the craniofacial and eye regions were smaller and/or less developed, and the melanophores overlying the posterior midbrain were more dense, and stellate in morphology, than the same tissues and cells in control embryos. This data suggests that despite differences in the time of onset of expression, normal activity of CB1R is required for sequential steps in migration and morphogenesis of neural crest cells and derivatives in both chicken and frog embryos. In addition, CB1R may signal through Myosin II to regulate migration and morphogenesis of neural crest cells and derivatives in chicken and frog embryos.

SARS-CoV-2 infection of sustentacular cells disrupts olfactory signaling pathways

Loss of olfactory function has been commonly reported in SARS-CoV-2 infections. Recovery from anosmia is not well understood. Previous studies showed that sustentacular cells, and occasionally olfactory sensory neurons (OSNs) in the olfactory epithelium (OE), are infected in SARS-CoV-2-infected patients and experimental animals. Here, we show that SARS-CoV-2 infection of sustentacular cells induces inflammation characterized by infiltration of myeloid cells to the olfactory epithelium and variably increased expression of proinflammatory cytokines. We observed widespread damage to, and loss of cilia on, OSNs, accompanied by downregulation of olfactory receptors and signal transduction molecules involved in olfaction. A consequence of OSN dysfunction was a reduction in the number of neurons in the olfactory bulb expressing tyrosine hydroxylase, consistent with reduced synaptic input. Resolution of the infection, inflammation, and olfactory dysfunction occurred over 3-4 weeks following infection in most but not all animals. We also observed similar patterns of OE infection and anosmia/hyposmia in mice infected with other human coronaviruses such as SARS-CoV and MERS-CoV. Together, these results define the downstream effects of sustentacular cell infection and provide insight into olfactory dysfunction in COVID-19-associated anosmia.

Illicit Drug Use and Smell and Taste Dysfunction: A National Health and Nutrition Examination Survey 2013–2014

Taste and smell dysfunction are suspected to be associated with substance use. However, representative epidemiological studies remain insufficient. This cross-sectional study explored the relationship between drug use (including cannabis or hashish, cocaine, heroin, and methamphetamine) and olfactory/gustatory dysfunction using data from the 2013–2014 National Health and Nutrition Examination Survey. In this study, participants who completed the smell examination with mean age of 59 were classified into four groups: cannabis users (n = 845), participants without cannabis use (n = 794), illicit drug users (n = 450), and participants without illicit drug use (n = 2000). Participants who completed the taste examination with mean age of 58 were also categorised into four groups: cannabis users (n = 810), participants without cannabis use (n = 714), illicit drug users (n = 428), and participants without illicit drug use (n = 1815). Logistic regression models investigated the association between cannabis or illicit drug use and smell or taste dysfunctions among study participants. Odds ratios and 95% confidence intervals were calculated. Finally, we did not find correlations between illicit drug use and dysfunction of taste or smell senses; our findings were consistent in many subgroup analyses. We recommend that further studies explore the mechanism and dose of illicit drug use that could have chemosensory impacts.

Possible role of endocannabinoids in olfactory and taste dysfunctions in Alzheimer's and Parkinson's patients and volumetric changes in the brain

The purpose of this study is to determine the volumes of primary brain regions associated with smell and taste in Alzheimer's and Parkinson's patients and healthy controls using MR imaging and examine volumetric changes in comparison to smell/taste questionnaire and test results and endocannabinoid (EC) levels. The study included 15 AD patients with mild cognitive dysfunction scored as 18 ≤ MMSE ≤ 23, 15 PD patients with scores of 18 < MoCA < 26 and 18 ≤ MMSE ≤ 23, and 15 healthy controls. A taste and smell questionnaire was given to the participants, and their taste and smell statuses were examined using the Sniffin' Sticks smell identification test and Burghart Taste Strips. EC levels were analyzed in the blood serum samples of the participants using the ELISA method. The volumes of the left olfactory bulb (p = 0.001), left amygdala (p = 0.004), left hippocampus (p = 0.008), and bilateral insula (left p = 0.000, right p = 0.000) were significantly smaller in the Alzheimer's patients than the healthy controls. The volumes of the left olfactory bulb (p = 0.001) and left hippocampus (p = 0.009) were significantly smaller in the Parkinson's patients than the healthy controls. A significant correlation was determined between volume reduction in the left Rolandic operculum cortical region and taste dysfunction. EC levels were significantly higher in both AD (p = 0.000) and PD (p = 0.006) in comparison to the controls. Our results showed that volumetric changes occur in the brain regions associated with smell and taste in Alzheimer's and Parkinson's patients. It was observed that ECs played a role in these volumetric changes and the olfactory and taste dysfunctions of the patients.

Possible Role of Endocannabinoids in Olfactory and Taste Dysfunctions in COVID-19 Patients and Volumetric Changes in the Brain

Introduction

COVID-19 infection develops neurologic symptoms such as smell and taste loss. We aimed to determine the volumetric changes in the brain and correlation of possible related biochemical parameters and endocannabinoid levels after COVID-19 recovery.

Methods

Brain magnetic resonance images of recovered COVID-19 patients and healthy volunteers, whose olfactory and gustatory scores were obtained through a questionnaire, were taken, and the volumes of the brain regions associated with taste and smell were measured by automatic and semiautomatic methods. Endocannabinoids (EC), which are critical in the olfactory system, and vitamin B12, zinc, iron, ferritin, thyroid-stimulating hormone (TSH), and thyroxine (T4) levels, which are reported to have possible roles in olfactory disorders, were measured in peripheral blood.

Results

Taste and smell disorder scores and EC levels were found to be higher in recovered COVID-19 patients compared to controls. EC levels were negatively correlated with bilateral entorhinal cortex (ENT) volumes in the COVID-19 group. Subgenual anterior cingulate cortex volumes showed correlations with gustatory complaints and ferritin in recovered COVID-19 patients.

Conclusions

The critical finding of our study is the high EC levels and negative correlation between EC levels and left ENT volumes in recovered COVID-19 patients.

Implications

It is possible that ECs are potential neuromodulators in many conditions leading to olfactory disorders, including COVID-19.

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.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

The Cellular basis of loss of smell in 2019-nCoV-infected individuals

A prominent clinical symptom of 2019-novel coronavirus (nCoV) infection is hyposmia/anosmia (decrease or loss of sense of smell), along with general symptoms such as fatigue, shortness of breath, fever and cough. The identity of the cell lineages that underpin the infection-associated loss of olfaction could be critical for the clinical management of 2019-nCoV-infected individuals. Recent research has confirmed the role of angiotensin-converting enzyme 2 (ACE2) and transmembrane protease serine 2 (TMPRSS2) as key host-specific cellular moieties responsible for the cellular entry of the virus. Accordingly, the ongoing medical examinations and the autopsy reports of the deceased individuals indicate that organs/tissues with high expression levels of ACE2, TMPRSS2 and other putative viral entry-associated genes are most vulnerable to the infection. We studied if anosmia in 2019-nCoV-infected individuals can be explained by the expression patterns associated with these host-specific moieties across the known olfactory epithelial cell types, identified from a recently published single-cell expression study. Our findings underscore selective expression of these viral entry-associated genes in a subset of sustentacular cells (SUSs), Bowman's gland cells (BGCs) and stem cells of the olfactory epithelium. Co-expression analysis of ACE2 and TMPRSS2 and protein-protein interaction among the host and viral proteins elected regulatory cytoskeleton protein-enriched SUSs as the most vulnerable cell type of the olfactory epithelium. Furthermore, expression, structural and docking analyses of ACE2 revealed the potential risk of olfactory dysfunction in four additional mammalian species, revealing an evolutionarily conserved infection susceptibility. In summary, our findings provide a plausible cellular basis for the loss of smell in 2019-nCoV-infected patients.

The cyclic AMP signaling pathway in the rodent main olfactory system

Odor perception begins with the detection of odorant molecules by the main olfactory epithelium located in the nasal cavity. Odorant molecules bind to and activate a large family of G-protein-coupled odorant receptors and trigger a cAMP-mediated transduction cascade that converts the chemical stimulus into an electrical signal transmitted to the brain. Morever, odorant receptors and cAMP signaling plays a relevant role in olfactory sensory neuron development and axonal targeting to the olfactory bulb. This review will first explore the physiological response of olfactory sensory neurons to odorants and then analyze the different components of cAMP signaling and their different roles in odorant detection and olfactory sensory neuron development.

Sustentacular Cell Enwrapment of Olfactory Receptor Neuronal Dendrites: An Update

The pseudostratified olfactory epithelium (OE) may histologically appear relatively simple, but the cytological relations among its cell types, especially those between olfactory receptor neurons (ORNs) and olfactory sustentacular cells (OSCs), prove more complex and variable than previously believed. Adding to the complexity is the short lifespan, persistent neurogenesis, and continuous rewiring of the ORNs. Contrary to the common belief that ORN dendrites are mostly positioned between OSCs, recent findings indicate a sustentacular cell enwrapped configuration for a majority of mature ORN dendrites at the superficial layer of the OE. After vertically sprouting out from the borderlines between OSCs, most of the immature ORN dendrites undergo a process of sideways migration and terminal maturation to become completely invaginated into and enwrapped by OSCs. Trailing the course of the dendritic sideways migration is the mesodendrite (mesentery of the enwrapped dendrite) made of closely apposed, cell junction connected plasma membrane layers of neighboring folds of the host sustentacular cell. Only a minority of the mature ORN dendrites at the OE apical surface are found at the borderlines between OSCs (unwrapped). Below I give a brief update on the cytoarchitectonic relations between the ORNs and OSCs of the OE. Emphasis is placed on the enwrapment of ORN dendrites by OSCs, on the sideways migration of immature ORN dendrites after emerging from the OE surface, and on the terminal maturation of the ORNs. Functional implications of ORN dendrite enwrapment and a comparison with myelination or Remak’s bundling of axons or axodendrites in the central and peripheral nervous system are also discussed.

Cannabinoid Control of Olfactory Processes: The Where Matters

Olfaction has a direct influence on behavior and cognitive processes. There are different neuromodulatory systems in olfactory circuits that control the sensory information flowing through the rest of the brain. The presence of the cannabinoid type-1 (CB1) receptor, (the main cannabinoid receptor in the brain), has been shown for more than 20 years in different brain olfactory areas. However, only over the last decade have we started to know the specific cellular mechanisms that link cannabinoid signaling to olfactory processing and the control of behavior. In this review, we aim to summarize and discuss our current knowledge about the presence of CB1 receptors, and the function of the endocannabinoid system in the regulation of different olfactory brain circuits and related behaviors.

Cannabinoids, Chemical Senses, and Regulation of Feeding Behavior

The herb Cannabis sativa has been traditionally used in many cultures and all over the world for thousands of years as medicine and recreation. However, because it was brought to the Western world in the late 19th century, its use has been a source of controversy with respect to its physiological effects as well as the generation of specific behaviors. In this regard, the CB1 receptor represents the most relevant target molecule of cannabinoid components on nervous system and whole-body energy homeostasis. Thus, the promotion of CB1 signaling can increase appetite and stimulate feeding, whereas blockade of CB1 suppresses hunger and induces hypophagia. Taste and flavor are sensory experiences involving the oral perception of food-derived chemicals and drive a primal sense of acceptable or unacceptable for what is sampled. Therefore, research within the last decades focused on deciphering the effect of cannabinoids on the chemical senses involved in food perception and consequently in the pattern of feeding. In this review, we summarize the data on the effect of cannabinoids on chemical senses and their influences on food intake control and feeding behavior.

TMEM16A calcium-activated chloride currents in supporting cells of the mouse olfactory epithelium

Glial-like supporting (or sustentacular) cells are important constituents of the olfactory epithelium that are involved in several physiological processes such as production of endocannabinoids, insulin, and ATP and regulation of the ionic composition of the mucus layer that covers the apical surface of the olfactory epithelium. Supporting cells express metabotropic P2Y purinergic receptors that generate ATP-induced Ca²⁺ signaling through the activation of a PLC-mediated cascade. Recently, we reported that a subpopulation of supporting cells expresses also the Ca²⁺-activated Cl^- channel TMEM16A. Here, we sought to extend our understanding of a possible physiological role of this channel in the olfactory system by asking whether Ca²⁺ can activate Cl^- currents mediated by TMEM16A. We use whole-cell patch-clamp analysis in slices of the olfactory epithelium to measure dose-response relations in the presence of various intracellular Ca²⁺ concentrations, ion selectivity, and blockage. We find that knockout of TMEM16A abolishes Ca²⁺-activated Cl^- currents, demonstrating that TMEM16A is essential for these currents in supporting cells. Also, by using extracellular ATP as physiological stimuli, we found that the stimulation of purinergic receptors activates a large TMEM16A-dependent Cl^- current, indicating a possible role of TMEM16A in ATP-mediated signaling. Altogether, our results establish that TMEM16A-mediated currents are functional in olfactory supporting cells and provide a foundation for future work investigating the precise physiological role of TMEM16A in the olfactory system.

Protein calorie malnutrition, nutritional intervention and personalized cancer care

Cancer patients often experience weight loss caused by protein calorie malnutrition (PCM) during the course of the disease or treatment. PCM is expressed as severe if the patient has two or more of the following characteristics: obvious significant muscle wasting, loss of subcutaneous fat; nutritional intake of <50% of recommended intake for 2 weeks or more; bedridden or otherwise significantly reduced functional capacity; weight loss of >2% in 1 week, 5% in 1 month, or 7.5% in 3 months. Cancer anorexia-cachexia syndrome (CACS) is a multifactorial condition of advanced PCM associated with underlying illness (in this case cancer) and is characterized by loss of muscle with or without loss of fat mass. Cachexia is defined as weight loss of more than 5% of body weight in 12 months or less in the presence of chronic disease. Hence with a chronic illness on board even a small amount of weight loss can open the door to cachexia. These nutritional challenges can lead to severe morbidity and mortality in cancer patients. In the clinic, the application of personalized medicine and the ability to withstand the toxic effects of anti-cancer therapies can be optimized when the patient is in nutritional homeostasis and is free of anorexia and cachexia. Routine assessment of nutritional status and appropriate intervention are essential components of the effort to alleviate effects of malnutrition on quality of life and survival of patients.

Effects of oral Δ9-tetrahydrocannabinol on the cerebral processing of olfactory input in healthy non-addicted subjects

BACKGROUND

Considering the increasing acknowledgment of the human sense of smell as a significant component of the quality of life, olfactory drug effects gain potential clinical importance. A recent observation in a human experimental context indicated that Δ⁹-tetrahydrocannabinol (THC) impaired the subject's performance in olfactory tests. To further analyze the role of THC in human olfaction, the present report addresses its effects on the central processing of olfactory stimuli.

METHODS

Employing a placebo-controlled randomized crossover design, an oral dose of 20 mg THC was administered in 15 healthy volunteers. The central processing of olfactory input, consisting of short pulses of gaseous vanillin or hydrogen sulfide, and for comparison, of non-odorous but painful carbon dioxide, were investigated before and after administration of THC or placebo in a pharmacological functional magnet resonance imaging study.

RESULTS

Following THC administration, the vanillin stimuli lost their pleasantness and became hedonically inert. This observation had its functional correlate in reduced stimulus-associated brain activations located in the left amygdala, the hippocampus and superior temporal pole (peak MNI coordinates x = - 27, y = - 1, z = - 26 mm p = 0.039). Differences in amygdala activations were significantly correlated with the corresponding differences in vanillin pleasantness (p = 0.025). By contrast, no effects were observed on the perception of processing of H₂S stimuli.

CONCLUSIONS

The results support that THC induced a modulation of the central processing of olfactory input. The THC-induced reduction in the pleasantness of a pleasurable odor was accompanied by reduced activations in the limbic system. Results agree with previous observation of negative effects of cannabinoids on the human sense of smell and strengthen the evidence that THC-based medications will be among drugs with olfactory side effects.

Endocannabinoid Modulation of Stimulus-Specific Adaptation in Inferior Colliculus Neurons of the Rat

Cannabinoid receptors (CBRs) are widely distributed in the brain, including the inferior colliculus (IC). Here, we aim to study whether endocannabinoids influence a specific type of neuronal adaptation, namely, stimulus-specific adaptation (SSA) found in some IC neurons. SSA is important because it has been found as early as the level of the midbrain and therefore it may be a neuronal correlate of early indices of deviance detection. Furthermore, recent studies have demonstrated a direct link between SSA and MMN, that is widely used as an outcome measure in a variety of human neurodegenerative disorders. SSA is considered a form of short-term plasticity, and CBRs have been shown to play a role in short-term neural plasticity. Therefore, it is reasonable to hypothesize that endocannabinoids may play a role in the generation or modulation of SSA. We recorded single units in the IC under an oddball paradigm stimulation. The results demonstrate that cannabinoid agonists lead to a reduction in the neuronal adaptation. This change is due to a differential increase of the neuronal firing rate to the standard tone alone. Furthermore, we show that the effect is mediated by the cannabinoid receptor 1 (CBR1). Thus, cannabinoid agonists down-modulate SSA in IC neurons.

Endocannabinoid signaling enhances visual responses through modulation of intracellular chloride levels in retinal ganglion cells

Type 1 cannabinoid receptors (CB1Rs) are widely expressed in the vertebrate retina, but the role of endocannabinoids in vision is not fully understood. Here, we identified a novel mechanism underlying a CB1R-mediated increase in retinal ganglion cell (RGC) intrinsic excitability acting through AMPK-dependent inhibition of NKCC1 activity. Clomeleon imaging and patch clamp recordings revealed that inhibition of NKCC1 downstream of CB1R activation reduces intracellular Cl⁻ levels in RGCs, hyperpolarizing the resting membrane potential. We confirmed that such hyperpolarization enhances RGC action potential firing in response to subsequent depolarization, consistent with the increased intrinsic excitability of RGCs observed with CB1R activation. Using a dot avoidance assay in freely swimming Xenopus tadpoles, we demonstrate that CB1R activation markedly improves visual contrast sensitivity under low-light conditions. These results highlight a role for endocannabinoids in vision and present a novel mechanism for cannabinoid modulation of neuronal activity through Cl⁻ regulation.

DOI:http://dx.doi.org/10.7554/eLife.15932.001

Cannabinoid receptor signaling induces proliferation but not neurogenesis in the mouse olfactory epithelium

The olfactory epithelium actively generates neurons through adulthood, and this neurogenesis is tightly regulated by multiple factors that are not fully defined. Here, we examined the role of cannabinoids in the regulation of neurogenesis in the mouse olfactory epithelium. In vivo proliferation and cell lineage studies were performed in mice (C57BL/6 and cannabinoid type 1 and 2 receptor deficient strains) treated with cannabinoids directly (WIN 55,212–2 or 2-arachidonylglycerol ether) or indirectly via inhibition of cannabinoid hydrolytic enzymes. Cannabinoids increased proliferation in neonatal and adult mice, and had no effect on proliferation in cannabinoid type 1 and 2 receptor deficient adult mice. Pretreatment with the cannabinoid type1 receptor antagonist AM251 decreased cannabinoid-induced proliferation in adult mice. Despite a cannabinoid-induced increase in proliferation, there was no change in newly generated neurons or non-neuronal cells 16 d post-treatment. However, cannabinoid administration increased apoptotic cell death at 72 hours post-treatment and by 16 d the level of apoptosis dropped to control levels. Thus, cannabinoids induce proliferation, but do not induce neurogenesis nor non-neuronal cell generation. Cannabinoid receptor signaling may regulate the balance of progenitor cell survival and proliferation in adult mouse olfactory epithelium.

Preferential epithelial expression of type-1 cannabinoid receptor (CB1R) in the developing canine embryo

The use of cannabinoid receptor agonists is gaining a strong interest both in human and veterinary medicine. The potential use of cannabimimetic compounds in companion animals was reviewed in 2007 for their role in tissue inflammation and pain. A better knowledge of type-1 cannabinoid receptor (CB1R) expression on the target population may help in risk management in order to prevent unwanted side effects. We used 30-days old canine embryos to describe the distribution of CB1R by means of immunohistochemistry with a commercially available antibody.CB1R immunoreactivity was mainly epithelial and included most structures of central and peripheral nervous system, inner ear, olfactory epithelium and related structures, eye and thyroid. Further investigative research on the role of the endocannabinoid system in the developmental biology field is needed, however, we show that in the canine species we must consider pregnancy as risk condition for developmental abnormalities that may arise upon the use of CB1R receptor agonists.

Distribution of the Endocannabinoid System in the Central Nervous System

The endocannabinoid system consists of endogenous cannabinoids (endocannabinoids), the enzymes that synthesize and degrade endocannabinoids, and the receptors that transduce the effects of endocannabinoids. Much of what we know about the function of endocannabinoids comes from studies that combine localization of endocannabinoid system components with physiological or behavioral approaches. This review will focus on the localization of the best-known components of the endocannabinoid system for which the strongest anatomical evidence exists.

Metamorphic remodeling of the olfactory organ of the African clawed frog, Xenopus laevis

The amphibian olfactory system undergoes massive remodeling during metamorphosis. The transition from aquatic olfaction in larvae to semiaquatic or airborne olfaction in adults requires anatomical, cellular, and molecular modifications. These changes are particularly pronounced in Pipidae, whose adults have secondarily adapted to an aquatic life style. In the fully aquatic larvae of Xenopus laevis, the main olfactory epithelium specialized for sensing water-borne odorous substances lines the principal olfactory cavity (PC), whereas a separate olfactory epithelium lies in the vomeronasal organ (VNO). During metamorphosis, the epithelium of the PC is rearranged into the adult "air nose," whereas a new olfactory epithelium, the adult "water nose," forms in the emerging middle cavity (MC). Here we performed a stage-by-stage investigation of the anatomical changes of the Xenopus olfactory organ during metamorphosis. We quantified cell death in all olfactory epithelia and found massive cell death in the PC and the VNO, suggesting that the majority of larval sensory neurons is replaced during metamorphosis in both sensory epithelia. The moderate cell death in the MC shows that during the formation of this epithelium some cells are sorted out. Our results show that during MC formation some supporting cells, but not sensory neurons, are relocated from the PC to the MC and that they are eventually eliminated during metamorphosis. Together our findings illustrate the structural and cellular changes of the Xenopus olfactory organ during metamorphosis.

Effects of 20 mg oral Δ(9) -tetrahydrocannabinol on the olfactory function of healthy volunteers

AIMS

Olfactory loss impairs the patient's quality of life. In individualized therapies, olfactory drug effects gain clinical importance. Molecular evidence suggests that among drugs with potential olfactory effects is Δ(9) -tetrahydrocannabinol (THC), which is approved for several indications, including neuropathic pain or analgesia in cancer patients. The present study aimed at assessing the olfactory effects of THC to be expected during analgesic treatment.

METHODS

The effects of 20 mg oral THC on olfaction were assessed in a placebo-controlled, randomized cross-over study in healthy volunteers. Using an established olfactory test (Sniffin' Sticks), olfactory thresholds, odour discrimination and odour identification were assessed in 15 subjects at baseline and 2 h after THC administration.

RESULTS

Δ(9) -Tetrahydrocannabinol impaired the performance of subjects (n = 15) in the olfactory test. Specifically, olfactory thresholds were increased and odour discrimination performance was reduced. This resulted in a significant drop in composite threshold, discrimination, identification (TDI) olfactory score by 5.5 points (from 37.7 ± 4.2 to 32.2 ± 5.6, 95% confidence interval for differences THC vs. placebo, -7.8 to -2.0, P = 0.003), which is known to be a subjectively perceptible impairment of olfactory function.

CONCLUSIONS

Considering the resurgence of THC in medical use for several pathological conditions, the present results indicate that THC-based analgesics may be accompanied by subjectively noticeable reductions in olfactory acuity. In particular, for patients relying on their sense of smell, this might be relevant information for personalized therapy strategies.

An endocannabinoid system is present in the mouse olfactory epithelium but does not modulate olfaction

Endocannabinoids modulate a diverse array of functions including progenitor cell proliferation in the central nervous system, and odorant detection and food intake in the mammalian central olfactory system and larval Xenopus laevis peripheral olfactory system. However, the presence and role of endocannabinoids in the peripheral olfactory epithelium have not been examined in mammals. We found the presence of cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2) receptor protein and mRNA in the olfactory epithelium. Using either immunohistochemistry or calcium imaging we localized CB1 receptors on neurons, glia-like sustentacular cells, microvillous cells and progenitor-like basal cells. To examine the role of endocannabinoids, CB1- and CB2- receptor-deficient (CB1(-/-)/CB2(-/-)) mice were used. The endocannabinoid 2-arachidonylglycerol (2-AG) was present at high levels in both C57BL/6 wildtype and CB1(-/-)/CB2(-/-) mice. 2-AG synthetic and degradative enzymes are expressed in wildtype mice. A small but significant decrease in basal cell and olfactory sensory neuron numbers was observed in CB1(-/-)/CB2(-/-) mice compared to wildtype mice. The decrease in olfactory sensory neurons did not translate to impairment in olfactory-mediated behaviors assessed by the buried food test and habituation/dishabituation test. Collectively, these data indicate the presence of an endocannabinoid system in the mouse olfactory epithelium. However, unlike in tadpoles, endocannabinoids do not modulate olfaction. Further investigation on the role of endocannabinoids in progenitor cell function in the olfactory epithelium is warranted.

Truffles contain endocannabinoid metabolic enzymes and anandamide

Truffles are the fruiting body of fungi, members of the Ascomycota phylum endowed with major gastronomic and commercial value. The development and maturation of their reproductive structure are dependent on melanin synthesis. Since anandamide, a prominent member of the endocannabinoid system (ECS), is responsible for melanin synthesis in normal human epidermal melanocytes, we thought that ECS might be present also in truffles. Here, we show the expression, at the transcriptional and translational levels, of most ECS components in the black truffle Tuber melanosporum Vittad. at maturation stage VI. Indeed, by means of molecular biology and immunochemical techniques, we found that truffles contain the major metabolic enzymes of the ECS, while they do not express the most relevant endocannabinoid-binding receptors. In addition, we measured anandamide content in truffles, at different maturation stages (from III to VI), through liquid chromatography-mass spectrometric analysis, whereas the other relevant endocannabinoid 2-arachidonoylglycerol was below the detection limit. Overall, our unprecedented results suggest that anandamide and ECS metabolic enzymes have evolved earlier than endocannabinoid-binding receptors, and that anandamide might be an ancient attractant to truffle eaters, that are well-equipped with endocannabinoid-binding receptors.

Exotic models may offer unique opportunities to decipher specific scientific question: the case of Xenopus olfactory system

The fact that olfactory systems are highly conserved in all animal species from insects to mammals allow the generalization of findings from one species to another. Most of our knowledge about the anatomy and physiology of the olfactory system comes from data obtained in a very limited number of biological models such as rodents, Zebrafish, Drosophila, and a worm, Caenorhabditis elegans. These models have proved useful to answer most questions in the field of olfaction, and thus concentrating on these few models appear to be a pragmatic strategy. However, the diversity of the organization and physiology of the olfactory system amongst phyla appear to be greater than generally assumed and the four models alone may not be sufficient to address all the questions arising from the study of olfaction. In this article, we will illustrate the idea that we should take advantage of biological diversity to address specific scientific questions and will show that the Xenopus olfactory system is a very good model to investigate: first, olfaction in aerial versus aquatic conditions and second, mechanisms underlying postnatal reorganization of the olfactory system especially those controlled by tyroxine hormone.

Peripheral modulation of smell: fact or fiction?

Despite studies dating back 30 or more years showing modulation of odorant responses at the level of the olfactory epithelium, most descriptions of the olfactory system infer that odorant signals make their way from detection by cilia on olfactory sensory neurons to the olfactory bulb unaltered. Recent identification of multiple subtypes of microvillar cells and identification of neuropeptide and neurotransmitter expression in the olfactory mucosa add to the growing body of literature for peripheral modulation in the sense of smell. Complex mechanisms including perireceptor events, modulation of sniff rates, and changes in the properties of sensory neurons match the sensitivity of olfactory sensory neurons to the external odorant environment, internal nutritional status, reproductive status, and levels of arousal or stress. By furthering our understanding of the players mediating peripheral olfaction, we may open the door to novel approaches for modulating the sense of smell in both health and disease.

Neuropeptide Y enhances olfactory mucosa responses to odorant in hungry rats

Neuropeptide Y (NPY) plays an important role in regulating appetite and hunger in vertebrates. In the hypothalamus, NPY stimulates food intake under the control of the nutritional status. Previous studies have shown the presence of NPY and receptors in rodent olfactory system, and suggested a neuroproliferative role. Interestingly, NPY was also shown to directly modulate olfactory responses evoked by a food-related odorant in hungry axolotls. We have recently demonstrated that another nutritional cue, insulin, modulates the odorant responses of the rat olfactory mucosa (OM). Therefore, the aim of the present study was to investigate the potential effect of NPY on rat OM responses to odorants, in relation to the animal's nutritional state. We measured the potential NPY modulation of OM responses to odorant, using electro-olfactogram (EOG) recordings, in fed and fasted adult rats. NPY application significantly and transiently increased EOG amplitudes in fasted but not in fed rats. The effects of specific NPY-receptor agonists were similarly quantified, showing that NPY operated mainly through Y1 receptors. These receptors appeared as heterogeneously expressed by olfactory neurons in the OM, and western blot analysis showed that they were overexpressed in fasted rats. These data provide the first evidence that NPY modulates the initial events of odorant detection in the rat OM. Because this modulation depends on the nutritional status of the animal, and is ascribed to NPY, the most potent orexigenic peptide in the central nervous system, it evidences a strong supplementary physiological link between olfaction and nutritional processes.

Modulation by octopamine of olfactory responses to nonpheromone odorants in the cockroach, Periplaneta americana L

Olfactory receptor cells in insects are modulated by neurohormones. Recordings from cockroach olfactory sensilla showed that a subset of sensory neurons increase their responses to selected nonpheromone odorants after octopamine application. With octopamine application, recordings demonstrated increased firing rates by the short but not the long alcohol-sensitive sensilla to the nonpheromone volatile, hexan-1-ol. Within the same sensillum, individual receptor cells are shown to be modulated independently from each other, indicating that the octopamine receptors reside in the receptor not in the accessory cells. A uniform decrease in the amplitude of electroantennogram, which is odorant independent, is suggested to reflect the rise in octopamine concentration in the antennal hemolymph. Perception of general odorants measured as behavioral responses changed qualitatively under octopamine treatment: namely, repulsive hexan-1-ol became neutral, whereas neutral eucalyptol became attractive. Octopamine induced a change in male behavioral responses to general odors that were essentially the same as in the state of sexual arousal. Our findings suggest that sensitivity to odors having different biological significances is modulated selectively at the peripheral as well as other levels of olfactory processing.

Acute Immobilization Stress Modulate GABA Release from Rat Olfactory Bulb: Involvement of Endocannabinoids-Cannabinoids and Acute Stress Modulate GABA Release

We studied the effects of cannabinoids and acute immobilization stress on the regulation of GABA release in the olfactory bulb. Glutamate-stimulated 3H-GABA release was measured in superfused slices. We report that cannabinoids as WIN55, 212-2, methanandamide, and 2-arachidonoylglycerol were able to inhibit glutamate- and KCl-stimulated 3H-GABA release. This effect was blocked by the CB1 antagonist AM281. On the other hand, acute stress was able per se to increase endocannabinoid activity. This effect was evident since the inhibition of stimulated GABA release by acute stress was reversed with AM281 and tetrahydrolipstatin. Inhibition of the endocannabinoid transport or its catabolism showed reduction of GABA release, antagonized by AM281 in control and stressed animals. These results point to endocannabinoids as inhibitory modulators of GABA release in the olfactory bulb acting through an autocrine mechanism. Apparently, stress increases the endocannabinoid system, modulating GABAergic synaptic function in a primary sensory organ.

The localization and physiological effects of cannabinoid receptor 1 in the brain stem auditory system of the chick

Fast, temporally-precise, and consistent synaptic transmission is required to encode features of acoustic stimuli. Neurons of nucleus magnocellularis (NM) in the auditory brain stem of the chick possess numerous adaptations to optimize the coding of temporal information. One potential problem for the system is the depression of synaptic transmission during a prolonged stimulus. The present study tested the hypothesis that cannabinoid receptor 1 (CB1) signaling may limit synaptic depression at the auditory nerve-NM synapse. In situ hybridization was used to confirm that CB1 mRNA is expressed in the cochlear ganglion; immunohistochemistry was used to confirm the presence of CB1 protein in NM. These findings are consistent with the common presynaptic locus of CB1 in the brain. Rate-dependent synaptic depression was then examined in a brain slice preparation before and after administration of WIN 55,212-2 (WIN), a potent CB1 agonist. WIN decreased the amplitude of excitatory postsynaptic currents (EPSCs) and also reduced depression across a train of stimuli. The effect was most obvious late in the pulse train and during high rates of stimulation. This CB1-mediated influence could allow for lower, but more consistent activation of NM neurons, which could be of importance for optimizing the coding of prolonged, temporally-locked acoustic stimuli.

The styryl dye FM1-43 suppresses odorant responses in a subset of olfactory neurons by blocking cyclic nucleotide-gated (CNG) channels

Many olfactory receptor neurons use a cAMP-dependent transduction mechanism to transduce odorants into depolarizations. This signaling cascade is characterized by a sequence of two currents: a cation current through cyclic nucleotide-gated channels followed by a chloride current through calcium-activated chloride channels. To date, it is not possible to interfere with these generator channels under physiological conditions with potent and specific blockers. In this study we identified the styryl dye FM1-43 as a potent blocker of native olfactory cyclic nucleotide-gated channels. Furthermore, we characterized this substance to stain olfactory receptor neurons that are endowed with cAMP-dependent transduction. This allows optical differentiation and pharmacological interference with olfactory receptor neurons at the level of the signal transduction.

Delta-9-tetrahydrocannabinol may palliate altered chemosensory perception in cancer patients: results of a randomized, double-blind, placebo-controlled pilot trial

BACKGROUND

A pilot study (NCT00316563) to determine if delta-9-tetrahydrocannabinol (THC) can improve taste and smell (chemosensory) perception as well as appetite, caloric intake, and quality of life (QOL) for cancer patients with chemosensory alterations.

PATIENTS AND METHODS

Adult advanced cancer patients, with poor appetite and chemosensory alterations, were recruited from two sites and randomized in a double-blinded manner to receive either THC (2.5 mg, Marinol(®); Solvay Pharma Inc., n = 24) or placebo oral capsules (n = 22) twice daily for 18 days. Twenty-one patients completed the trial. At baseline and posttreatment, patients completed a panel of patient-reported outcomes: Taste and Smell Survey, 3-day food record, appetite and macronutrient preference assessments, QOL questionnaire, and an interview.

RESULTS

THC and placebo groups were comparable at baseline. Compared with placebo, THC-treated patients reported improved (P = 0.026) and enhanced (P < 0.001) chemosensory perception and food 'tasted better' (P = 0.04). Premeal appetite (P = 0.05) and proportion of calories consumed as protein increased compared with placebo (P = 0.008). THC-treated patients reported increased quality of sleep (P = 0.025) and relaxation (P = 0.045). QOL scores and total caloric intake were improved in both THC and placebo groups.

CONCLUSIONS

THC may be useful in the palliation of chemosensory alterations and to improve food enjoyment for cancer patients.

Endocannabinoid modulation in the olfactory epithelium

Appetite, food intake, and energy balance are closely linked to the endocannabinoid system in the central nervous system. Now, endocannabinoid modulation has been discovered in the peripheral olfactory system of larval Xenopus laevis. The endocannabinoid 2-AG has been shown to influence odorant-detection thresholds according to the hunger state of the animal. Hungry animals have increased 2-AG levels due to enhanced synthesis of 2-AG in sustentacular supporting cells. This renders olfactory receptor neurons, exhibiting CB1 receptors, more sensitive at detecting lower odorant concentrations, which probably helps the animal to locate food. Since taste and vision are also influenced by endocannabinoids, this kind of modulation might boost sensory inputs of food in hungry animals.

Autonomic modulation of olfactory signaling

The olfactory epithelium is extensively innervated by sympathetic nerve endings, which release norepinephrine, and parasympathetic nerve endings, which release acetylcholine. Because olfactory sensory neurons have adrenergic and muscarinic receptors in addition to odorant receptors, autonomic stimulation can modulate the responses of olfactory sensory neurons to odorants. Recent studies have shed light on the molecular mechanisms that underlie crosstalk between muscarinic and odorant receptor signaling. The emerging view is that the stimulation of odorant receptor signaling by odorants, which is the earliest step in olfaction, can be substantially regulated by the autonomic nervous system.

The endocannabinoid 2-arachidonoyl-glycerol controls odor sensitivity in larvae of Xenopus laevis

Cannabinoids modulate the activity of many neuronal cells, among them sensory neurons in the olfactory epithelium. Here we show that the endocannabinoid 2-arachidonoyl-glycerol (2-AG) is synthesized in both olfactory receptor neurons and glia-like sustentacular cells in larval Xenopus laevis. Its production in the latter depends on the hunger state of the animal. The essential effect of 2-AG in olfactory receptor neurons is the control of odorant detection thresholds via cannabinoid CB(1) receptor activation. Hunger renders olfactory neurons more sensitive. Endocannabinoid modulation in the nose may therefore substantially influence food-seeking behavior.

The endocannabinoid 2-arachidonoyl-glycerol controls odor sensitivity in larvae of Xenopus laevis

Cannabinoids modulate the activity of many neuronal cells, among them sensory neurons in the olfactory epithelium. Here we show that the endocannabinoid 2-arachidonoyl-glycerol (2-AG) is synthesized in both olfactory receptor neurons and glia-like sustentacular cells in larval Xenopus laevis. Its production in the latter depends on the hunger state of the animal. The essential effect of 2-AG in olfactory receptor neurons is the control of odorant detection thresholds via cannabinoid CB(1) receptor activation. Hunger renders olfactory neurons more sensitive. Endocannabinoid modulation in the nose may therefore substantially influence food-seeking behavior.

Endothelin evokes distinct calcium transients in neuronal and non-neuronal cells of rat olfactory mucosa primary cultures

The olfactory system is regulated by several nervous and hormonal factors, and there is a growing body of evidence that some of these modulations already take place in the olfactory mucosa (OM). We recently suggested that, among others, vasoactive peptides might play multifaceted roles in different OM cells. Here we studied the effect of the vasoconstrictive peptide endothelin (ET) in the rat OM. We identified different components of the ET system both in the olfactory mucosa and in long-term primary culture of OM cells, composed of olfactory sensory neurons (OSNs) lying on a blend of non-neuronal OM cells (nNCs). We demonstrated that ET receptors are differentially expressed on OM cells, and that ET might be locally matured by the endothelin-converting enzyme ECE-1 located in OSNs. Using calcium imaging, we showed that ET triggers robust dose-dependent Ca(2+) responses in most OM cells, which consist of a transient phase, followed, in nNCs, by a sustained plateau phase. All transient responses depended on intracellular calcium release, while the sustained plateau phase also depended on subsequent external calcium entry. Using both pharmacology and spotting lethal (sl/sl) mutant rats, lacking functional ET(B) receptors, we finally demonstrated that these effects of ET are mediated through ET(B) receptors in OSNs and ET(A) receptors in nNCs.The present study therefore identifies endothelin as a potent endogenous modulator of the olfactory mucosa; specific endothelin-mediated Ca(2+) signals may serve distinct signaling functions, and thereby suggest differential functional roles of endothelin in both neuronal and non-neuronal OM cells.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

A new challenge-development of test systems for the infochemical effect

BACKGROUND, AIM, AND SCOPE

Many-if not all-organisms depend on so-called infochemicals, chemical substances in their surroundings which inform the receivers about their biotic and abiotic environment and which allow them to react adequately to these signals. Anthropogenic substances can interfere with this complex chemical communication system. This finding is called infochemical effect. So far, it is not known to what extent anthropogenic discharges act as infochemicals and influence life and reproduction of organisms in the environment because adequate testing methods to identify chemicals which show the infochemical effect and to quantify their effects have not been developed yet. The purpose of this article is to help and find suitable test designs.

MAIN FEATURES

Test systems used in basic research to elucidate the olfactory cascade and the communication of environmental organisms by infochemicals are plentiful. Some of them might be the basis for a quantified ecotoxicological analysis of the infochemical effect. In principle, test systems for the infochemical effect could be developed at each step of the chemosensory signal transduction and processing cascade.

RESULTS

Experimental set-ups were compiled systematically under the aspect whether they might be usable for testing the infochemical effect of single chemicals in standardized quantifying laboratory experiments. For an appropriate ecotoxicological assessment of the infochemical effect, experimental studies of many disciplines, such as molecular biology, neurobiology, physiology, chemical ecology, and population dynamics, should be evaluated in detail before a decision can be made which test system, respectively which test battery, might be suited best. The test systems presented here are based on the knowledge of the genetic sequences for olfactory receptors, binding studies of odorants, signal transmission, and reactions of the receivers on the level of the organisms or the populations. The following basic approaches are conceivable to identify the role of an infochemical: binding studies to the odorant-binding protein or to the odorant receptor binding protein (e.g., by in situ hybridization and immunohistochemical studies), measurement of electrical signals of the receptor cells in the tissue (e.g., electroolfactograms, electroantennograms), registration of phenotypic changes (e.g., observation under the microscope), behavioral tests (e.g., in situ online biomonitoring, use of T-shaped olfactometers, tests of avoidance responses), measurement of population changes (e.g., cell density or turbidity measurements), and multispecies tests with observation of community structure and community function. The main focus of this study is on aquatic organisms.

DISCUSSION

It is evident that the infochemical effect is a very complex sublethal endpoint, and it needs further studies with standardized quantitative methods to elucidate whether and to what extent the ecosystem is affected. The collection of approaches presented here is far from being complete but should serve as a point of depart for further experimental research.

CONCLUSIONS

This article is the first to compare various approaches for testing the infochemical effect. The development of a suitable test system will not be easy as there are a multitude of relevant chemicals, a multitude of relevant receptors, and a multitude of relevant reactions, and it must be expected that the effective concentrations are very low. The chemical communication is of utmost importance for the ecosystem and justifies great endeavors to find solutions to these technical problems.

RECOMMENDATIONS AND PERSPECTIVES

The infochemical effect is a new chapter in ecotoxicology. Will a new endpoint, the so-called infochemical effect, be required in addition to the actual standard test battery of Annex 5 to Commission Directive 92/69/EEC (EC 1992)? Finding the answer to this question is a big challenge that could be met by a comprehensive research project.

Highly specific responses to amine odorants of individual olfactory receptor neurons in situ

The main olfactory system of larval Xenopus laevis is made up of at least two subsystems consisting of subsets of olfactory receptor neurons (ORNs) with different transduction mechanisms. One ORN subset lacks the canonical cAMP transduction pathway and responds to amino acid odorants. The second subset has the cAMP transduction pathway but as yet suitable odorants are unknown. Here we report the identification of amines as proper olfactory stimuli for larval X. laevis using functional Ca(2+) imaging and slice preparations of the olfactory system. The response profiles of individual ORNs to a number of amines were extremely complex and mostly highly specific. The great majority of amine-sensitive ORNs responded also to forskolin, an activator of the olfactory cAMP transduction pathway. Most amine-induced responses could be attenuated by the cyclic nucleotide-gated channel inhibitor LY83583. This confirms that most amine-responsive olfactory receptors (ORs) are coupled to the cAMP-dependent transduction pathway. Furthermore, we show that trace amine-associated receptors (TAARs), which have been shown to act as specific ORs for amines in mammals, are expressed in the olfactory organ of X. laevis. The TAARs expressed in Xenopus cannot, however, explain the complex responses of individual ORNs to amines because there are too few of them. This indicates that, in addition to TAARs, there must be other receptor families involved in the detection of amines.

Expression of insulin system in the olfactory epithelium: first approaches to its role and regulation

Food odours are major determinants for food choice; their detection is influenced by nutritional status. Among different metabolic signals, insulin plays a major role in food intake regulation. The aim of the present study was to investigate a potential role of insulin in the olfactory mucosa (OM), using ex vivo tissues and in vitro primary cultures. We first established the expression of insulin receptor (IR) in rat olfactory mucosa. Transcripts of IR-A and IR-B isoforms, as well as IRS-1 and IRS-2, were detected in OM extracts. Using immunocytochemistry, IR protein was located in olfactory receptor neurones, sustentacular and basal cells and in endothelium of the lamina propria vessels. Moreover, the insulin binding capacity of OM was quite high compared to that of olfactory bulb or liver. Besides the main pancreatic insulin source, we demonstrated insulin synthesis at a low level in the OM. Interestingly 48 h of fasting, leading to a decreased plasmatic insulin, increased the number of IR in the OM. Local insulin concentration was also enhanced. These data suggest a control of OM insulin system by nutritional status. Finally, an application of insulin on OM, aiming to mimic postprandial insulin increase, reversibly decreased the amplitude of electro-olfactogramme responses to odorants by approximately 30%. These data provide the first evidence that insulin modulates the most peripheral step of odour detection at the olfactory mucosa level.

Expression of adiponectin receptor 1 in olfactory mucosa of mice

AdipoR1 and AdipoR2 are receptors for the adipocyte-derived hormone adiponectin, which is an important regulator of glucose and lipid metabolism, and which has also been implicated in the control of food intake and energy homeostasis. In the present study, we have demonstrated that AdipoR1 is expressed in mature sensory neurons of the olfactory mucosa of mice, in a pattern reminiscent of the olfactory marker protein. AdipoR1 expression levels in the olfactory mucosa have been observed to increase gradually during late embryogenesis until adulthood. No local expression of adiponectin has been detected in nasal tissues, indicating that serum adiponectin is the ligand for AdipoR1 in olfactory sensory neurons. As the serum adiponectin concentration is regulated depending on adipose tissue mass, with a reduction of adiponectin levels being seen in obesity, AdipoR1 function in the olfactory epithelium seems to be directly linked to the nutritional status of the body, suggesting a potential modulatory role for AdipoR1 in the adjustment of the olfactory system to energy balance requirements.

The infochemical effect-a new chapter in ecotoxicology

BACKGROUND, AIM, AND SCOPE

Organisms use chemical cues in their surrounding, so-called infochemicals, as important source of information about their biotic and abiotic environment. The scope of this work is to transfer the knowledge on infochemicals obtained in chemical ecology into ecotoxicology, compare the observations with ecotoxicological standard tests, with other sublethal effects, and deduce consequences for the legal situation of environmental chemicals.

MAIN FEATURES

General principles were elaborated from the compiled information from literature on the structures and roles of natural infochemicals. The experiences gained in chemical ecology and in ecotoxicology led to the discovery of the infochemical effect: Anthropogenic substances can influence the chemical communication of environmental organisms. This finding is supported by a close look at fragrances and other common anthropogenic substances in the environment.

RESULTS

Increasing scientific knowledge shows how complex the chemical communication of environmental organisms is. Infochemicals are released by senders and detected by receivers. The relevant concentrations of infochemicals are very low, usually in the nano- to micromolar range and they do not seem to have common structural features. Knowledge about natural infochemicals is still poor and not consistent. The chemical cues fluctuate specifically in time and space resulting in dynamic response patterns in the ecosystem. Organisms can react to infochemicals in very specific ways by behavioral, morphological, or physiological responses; activities that are relevant for their survival as vital reactions such as flight, food uptake, or mating are affected. Anthropogenic substances at minor concentrations can interfere in the complex chemical communication web of infochemicals, possibly leading to increased vulnerability of populations.

DISCUSSION

The findings show clearly that the actual description of the interplay of organisms in the ecosystem is still very simplified and we are far from understanding the interactions completely. Anthropogenic discharges may play a role on the chemical communication and, hence, on the behavior and interactions of organisms in the ecosystem. The description of the infochemical effect opens a new chapter in ecotoxicology. It is a challenge to develop a suitable test system for the infochemical effect with the knowledge of the multitude of possible reactions and of the high specificity of infochemicals. Problems during the performance and evaluation of standard tests might be related to reactions due to infochemicals in the test systems which have not been considered so far.

CONCLUSIONS

The roles of anthropogenic infochemicals in the environment and the role of natural infochemicals in laboratory tests have been underestimated up to now.

RECOMMENDATIONS AND PERSPECTIVES

The discrepancy between the biological relevance and the lack of data about infochemicals in the environment reveals the necessity of further research. According to the actual findings, infochemicals are so decisive for the interactions in the ecosystem that they should not be neglected in ecotoxicology. The discovery of the infochemical effect is comparable to the detection that environmental substances can act as hormones. Sublethal effects with impacts on the ecosystem, such as the infochemical effect, will receive higher appraisal in the ecotoxicology of the future. It needs to be clarified to which extent anthropogenic discharges disturb the natural chemical communication web. A systematic analysis of this very complex field will be needed to know whether a new ecotoxicological endpoint, the infochemical effect, will have to be taken up in the standard repertoire. The knowledge on infochemicals might require some adjustments of the legal framework on environmental chemicals in future. Looking closer at the infochemical effect will lead to a new understanding of the complexity of environmental communities.

Presynaptic protein distribution and odour mapping in glomeruli of the olfactory bulb of Xenopus laevis tadpoles

The sensory input layer in the olfactory bulb (OB) is typically organized into spheroidal aggregates of dense neuropil called glomeruli. This characteristic compartmentalization of the synaptic neuropil is a typical feature of primary olfactory centres in vertebrates and most advanced invertebrates. In the present work we mapped the location of presynaptic sites in glomeruli across the OB using antibodies to presynaptic vesicle proteins and presynaptic membrane proteins in combination with confocal microscopy. In addition the responses of glomeruli upon mucosal application of amino acid-odorants and forskolin were monitored using functional calcium imaging. We first describe the spatial distribution of glomeruli across the main olfactory bulb (MOB) in premetamorphic Xenopus laevis. Second, we show that the heterogeneous organization of glomeruli along the dorsoventral and mediolateral axes of the MOB is associated with a differential distribution of synaptic vesicle proteins. While antibodies to synaptophysin, syntaxin and SNAP-25 uniformly labelled glomeruli in the whole MOB, intense synaptotagmin staining was present only in glomeruli in the lateral, and to a lesser extent in the intermediate, part of the OB. Interestingly, amino acid-responsive glomeruli were always located in the lateral part of the OB, and glomeruli activated by mucosal forskolin application were exclusively located in the medial part of the OB. This correlation between odour mapping and presynaptic protein distribution is an additional hint on the existence of different subsystems within the main olfactory system in larval Xenopus laevis.