Cannabinoids Regulate Sensory Processing in Early Olfactory and Visual Neural Circuits

Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry

Circadian rhythms are biological rhythms that originate from the "master circadian clock," called the suprachiasmatic nucleus (SCN). SCN orchestrates the circadian rhythms using light as a chief zeitgeber, enabling humans to synchronize their daily physio-behavioral activities with the Earth's light-dark cycle. However, chronic/ irregular photic disturbances from the retina via the retinohypothalamic tract (RHT) can disrupt the amplitude and the expression of clock genes, such as the period circadian clock 2, causing circadian rhythm disruption (CRd) and associated neuropathologies. The present review discusses neuromodulation across the RHT originating from retinal photic inputs and modulation offered by endocannabinoids as a function of mitigation of the CRd and associated neuro-dysfunction. Literature indicates that cannabinoid agonists alleviate the SCN's ability to get entrained to light by modulating the activity of its chief neurotransmitter, i.e., γ-aminobutyric acid, thus preventing light-induced disruption of activity rhythms in laboratory animals. In the retina, endocannabinoid signaling modulates the overall gain of the retinal ganglion cells by regulating the membrane currents (Ca2+, K+, and Cl- channels) and glutamatergic neurotransmission of photoreceptors and bipolar cells. Additionally, endocannabinoids signalling also regulate the high-voltage-activated Ca2+ channels to mitigate the retinal ganglion cells and intrinsically photosensitive retinal ganglion cells-mediated glutamate release in the SCN, thus regulating the RHT-mediated light stimulation of SCN neurons to prevent excitotoxicity. As per the literature, cannabinoid receptors 1 and 2 are becoming newer targets in drug discovery paradigms, and the involvement of endocannabinoids in light-induced CRd through the RHT may possibly mitigate severe neuropathologies.

Non-canonical type 1 cannabinoid receptor signaling regulates night visual processing in the inner rat retina

Type 1 cannabinoid receptors (CB1Rs) are expressed in major retinal neurons within the rod-pathway suggesting a role in regulating night visual processing, but the underlying mechanisms remain poorly understood. Using acute rat retinal slices, we show that CB1R activation reduces glutamate release from rod bipolar cell (RBC) axon terminals onto AII and A17 amacrine cells through a pathway that requires exchange proteins directly activated by cAMP (EPAC1/2) signaling. Consequently, CB1R activation abrogates reciprocal GABAergic feedback inhibition from A17 amacrine cells. Moreover, the activation of CB1Rs in vivo enhances and prolongs the time course of the dim-light rod-driven visual responses, an effect that was eliminated when both GABAA and GABAC receptors were blocked. Altogether, our findings underscore a non-canonical mechanism by which cannabinoid signaling regulates RBC dyad synapses in the inner retina to regulate dim-light visual responses to fine-tune night vision.

Food preference-based screening method for identification of effectors of substance use disorders using Caenorhabditis elegans

Substance use disorder (SUD) affects over 48 million Americans aged 12 and over. Thus, identifying novel chemicals contributing to SUD will be critical for developing efficient prevention and mitigation strategies. Considering the complexity of the actions and effects of these substances on human behavior, a high-throughput platform using a living organism is ideal. We developed a quick and easy screening assay using Caenorhabditis elegans. C. elegans prefers high-quality food (Escherichia coli HB101) over low-quality food (Bacillus megaterium), with a food preference index of approximately 0.2, defined as the difference in the number of worms at E. coli HB101 and B. megaterium over the total worm number. The food preference index was significantly increased by loperamide, a μ-opioid receptor (MOPR) agonist, and decreased by naloxone, a MOPR antagonist. These changes depended on npr-17, a C. elegans homolog of opioid receptors. In addition, the food preference index was significantly increased by arachidonyl-2'-chloroethylamide, a cannabinoid 1 receptor (CB1R) agonist, and decreased by rimonabant, a CB1R inverse agonist. These changes depended on npr-19, a homolog of CB1R. These results suggest that the conserved opioid and endocannabinoid systems modulate the food preference behaviors of C. elegans. Finally, the humanoid C. elegans strains where npr-17 was replaced with human MOPR and where npr-19 was replaced with human CB1R phenocopied the changes in food preference by the drug treatment. Together, the current results show that this method can be used to rapidly screen the potential effectors of MOPR and CB1R to yield results highly translatable to humans.

Endogenous cannabinoids in the piriform cortex tune olfactory perception

Sensory perception depends on interactions between external inputs transduced by peripheral sensory organs and internal network dynamics generated by central neuronal circuits. In the sensory cortex, desynchronized network states associate with high signal-to-noise ratio stimulus-evoked responses and heightened perception. Cannabinoid-type-1-receptors (CB1Rs) - which influence network coordination in the hippocampus - are present in anterior piriform cortex (aPC), a sensory paleocortex supporting olfactory perception. Yet, how CB1Rs shape aPC network activity and affect odor perception is unknown. Using pharmacological manipulations coupled with multi-electrode recordings or fiber photometry in the aPC of freely moving male mice, we show that systemic CB1R blockade as well as local drug infusion increases the amplitude of gamma oscillations in aPC, while simultaneously reducing the occurrence of synchronized population events involving aPC excitatory neurons. In animals exposed to odor sources, blockade of CB1Rs reduces correlation among aPC excitatory units and lowers behavioral olfactory detection thresholds. These results suggest that endogenous endocannabinoid signaling promotes synchronized population events and dampen gamma oscillations in the aPC which results in a reduced sensitivity to external sensory inputs.

The conserved endocannabinoid anandamide modulates olfactory sensitivity to induce hedonic feeding in C. elegans

The ability of cannabis to increase food consumption has been known for centuries. In addition to producing hyperphagia, cannabinoids can amplify existing preferences for calorically dense, palatable food sources, a phenomenon called hedonic amplification of feeding. These effects result from the action of plant-derived cannabinoids that mimic endogenous ligands called endocannabinoids. The high degree of conservation of cannabinoid signaling at the molecular level across the animal kingdom suggests hedonic feeding may also be widely conserved. Here, we show that exposure of Caenorhabditis elegans to anandamide, an endocannabinoid common to nematodes and mammals, shifts both appetitive and consummatory responses toward nutritionally superior food, an effect analogous to hedonic feeding. We find that anandamide's effect on feeding requires the C. elegans cannabinoid receptor NPR-19 but can also be mediated by the human CB1 cannabinoid receptor, indicating functional conservation between the nematode and mammalian endocannabinoid systems for the regulation of food preferences. Furthermore, anandamide has reciprocal effects on appetitive and consummatory responses to food, increasing and decreasing responses to inferior and superior foods, respectively. Anandamide's behavioral effects require the AWC chemosensory neurons, and anandamide renders these neurons more sensitive to superior foods and less sensitive to inferior foods, mirroring the reciprocal effects seen at the behavioral level. Our findings reveal a surprising degree of functional conservation in the effects of endocannabinoids on hedonic feeding across species and establish a new system to investigate the cellular and molecular basis of endocannabinoid system function in the regulation of food choice.

Chemical Constituents of Essential Oils Used in Olfactory Training: Focus on COVID-19 Induced Olfactory Dysfunction

The recent increase in the number of patients with post-viral olfactory dysfunction (PVOD) following the outbreak of COVID-19 has raised the general interest in and concern about olfactory dysfunction. At present, no clear method of treatment for PVOD has been established. Currently the most well-known method to improve the symptoms of olfactory dysfunction is "olfactory training" using essential oils. The essential oils used in olfactory training typically include rose, lemon, clove, and eucalyptus, which were selected based on the odor prism hypothesis proposed by Hans Henning in 1916. He classified odors based on six primary categories or dimensions and suggested that any olfactory stimulus fits into his smell prism, a three-dimensional space. The term "olfactory training" has been used based on the concept of training olfactory sensory neurons to relearn and distinguish olfactory stimuli. However, other mechanisms might contribute to how olfactory training can improve the recovery of the olfactory sense. Possibly, the essential oils contain chemical constituents with bioactive properties that facilitate the recovery of the olfactory sense by suppressing inflammation and enhancing regeneration. In this review, we summarize the chemical constituents of the essential oils of rose, lemon, clove, and eucalyptus and raise the possibility that the chemical constituents with bioactive properties are involved in improving the symptoms of olfactory dysfunction. We also propose that other essential oils that contain chemical constituents with anti-inflammatory effects and have binding affinity with SARS-CoV-2 can be new candidates to test their efficiencies in facilitating the recovery.

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.

Orchestration of the circadian clock and its association with Alzheimer's disease: Role of endocannabinoid signaling

Circadian rhythms are 24-hour natural rhythms regulated by the suprachiasmatic nucleus, also known as the "master clock". The retino-hypothalamic tract entrains suprachiasmatic nucleus with photic information to synchronise endogenous circadian rhythms with the Earth's light-dark cycle. However, despite the robustness of circadian rhythms, an unhealthy lifestyle and chronic photic disturbances cause circadian rhythm disruption in the suprachiasmatic nucleus's TTFL loops via affecting glutamate and γ-aminobutyric acid-mediated neurotransmission in the suprachiasmatic nucleus. Recently, considerable evidence has been shown correlating CRd with the incidence of Alzheimer's disease. The present review aims to identify the existence and signalling of endocannabinoids in CRd induced Alzheimer's disease through retino-hypothalamic tract- suprachiasmatic nucleus-cortex. Immunohistochemistry has confirmed the expression of cannabinoid receptor 1 in the suprachiasmatic nucleus to modulate the circadian phases of the master clock. Literature also suggests that cannabinoids may alter activity of suprachiasmatic nucleus by influencing the activity of their major neurotransmitter γ-aminobutyric acid or by interacting indirectly with the suprachiasmatic nucleus's two other major inputs i.e., the geniculo-hypothalamic tract-mediated release of neuropeptide Y and serotonergic inputs from the dorsal raphe nuclei. Besides, the expression of cannabinoid receptor 2 ameliorates cognitive deficits via reduction of tauopathy and microglial activation. In conclusion, endocannabinoids may be identified as a putative target for correcting CRd and decelerating Alzheimer's disease.