Endocannabinoids inhibit neurogenic inflammation in murine joints by a non-canonical cannabinoid receptor mechanism

Cannabinoid receptor 2 evolutionary gene loss makes parrots more susceptible to neuroinflammation

In vertebrates, cannabinoids modulate neuroimmune interactions through two cannabinoid receptors (CNRs) conservatively expressed in the brain (CNR1, syn. CB1) and in the periphery (CNR2, syn. CB2). Our comparative genomic analysis indicates several evolutionary losses in the CNR2 gene that is involved in immune regulation. Notably, we show that the CNR2 gene pseudogenized in all parrots (Psittaciformes). This CNR2 gene loss occurred because of chromosomal rearrangements. Our positive selection analysis suggests the absence of any specific molecular adaptations in parrot CNR1 that would compensate for the CNR2 loss in the modulation of the neuroimmune interactions. Using transcriptomic data from the brains of birds with experimentally induced sterile inflammation we highlight possible functional effects of such a CNR2 gene loss. We compare the expression patterns of CNR and neuroinflammatory markers in CNR2-deficient parrots (represented by the budgerigar, Melopsittacus undulatus and five other parrot species) with CNR2-intact passerines (represented by the zebra finch, Taeniopygia guttata). Unlike in passerines, stimulation with lipopolysaccharide resulted in neuroinflammation in the parrots linked with a significant upregulation of expression in proinflammatory cytokines (including interleukin 1 beta (IL1B) and 6 (IL6)) in the brain. Our results indicate the functional importance of the CNR2 gene loss for increased sensitivity to brain inflammation.

The impact of perceived stress on the hair follicle: Towards solving a psychoneuroendocrine and neuroimmunological puzzle

While popular belief harbors little doubt that perceived stress can cause hair loss and premature graying, the scientific evidence for this is arguably much thinner. Here, we investigate whether these phenomena are real, and show that the cyclic growth and pigmentation of the hair follicle (HF) provides a tractable model system for dissecting how perceived stress modulates aspects of human physiology. Local production of stress-associated neurohormones and neurotrophins coalesces with neurotransmitters and neuropeptides released from HF-associated sensory and autonomic nerve endings, forming a complex local stress-response system that regulates perifollicular neurogenic inflammation, interacts with the HF microbiome and controls mitochondrial function. This local system integrates into the central stress response systems, allowing the study of systemic stress responses affecting organ function by quantifying stress mediator content of hair. Focusing on selected mediators in this "brain-HF axis" under stress conditions, we distill general principles of HF dysfunction induced by perceived stress.

Cannabinoid control of neurogenic inflammation

A significant number of cannabinoids are known to have analgesic and anti-inflammatory properties in various diseases. Due to their presynaptic/terminal location, cannabinoid receptors can inhibit synaptic transmission and have the potential to regulate neurogenic inflammation. Neurogenic inflammation occurs when a noxious signal is detected in the periphery initiating an antidromic axon reflex in the same sensory neurone leading to depolarization of the afferent terminal. Neuropeptides are subsequently released and contribute to vasodilation, plasma extravasation and modulation of immune cells. Endocannabinoids, synthetic cannabinoids and phytocannabinoids can reduce neuroinflammation by inhibiting afferent firing and inflammatory neuropeptide release. Thus, in addition to a direct effect on vascular smooth muscle and inflammatory cells, cannabinoids can reduce inflammation by silencing small diameter neurones. This review examines the neuropharmacological processes involved in regulating antidromic depolarization of afferent nerve terminals by cannabinoids and the control of neurogenic inflammation in different diseases.

Neuromodulation of metabolic functions: from pharmaceuticals to bioelectronics to biocircuits

Neuromodulation of central and peripheral neural circuitry brings together neurobiologists and neural engineers to develop advanced neural interfaces to decode and recapitulate the information encoded in the nervous system. Dysfunctional neuronal networks contribute not only to the pathophysiology of neurological diseases, but also to numerous metabolic disorders. Many regions of the central nervous system (CNS), especially within the hypothalamus, regulate metabolism. Recent evidence has linked obesity and diabetes to hyperactive or dysregulated autonomic nervous system (ANS) activity. Neural regulation of metabolic functions provides access to control pathology through neuromodulation. Metabolism is defined as cellular events that involve catabolic and/or anabolic processes, including control of systemic metabolic functions, as well as cellular signaling pathways, such as cytokine release by immune cells. Therefore, neuromodulation to control metabolic functions can be used to target metabolic diseases, such as diabetes and chronic inflammatory diseases. Better understanding of neurometabolic circuitry will allow for targeted stimulation to modulate metabolic functions. Within the broad category of metabolic functions, cellular signaling, including the production and release of cytokines and other immunological processes, is regulated by both the CNS and ANS. Neural innervations of metabolic (e.g. pancreas) and immunologic (e.g. spleen) organs have been understood for over a century, however, it is only now becoming possible to decode the neuronal information to enable exogenous controls of these systems. Future interventions taking advantage of this progress will enable scientists, engineering and medical doctors to more effectively treat metabolic diseases.

Neuropeptides: important regulators of joint homeostasis

PURPOSE

This review explores the mechanisms of joint pain with a special focus on the role of neuropeptides in pain transmission and their potential role in the progression of joint degeneration as seen in osteoarthritis.

METHODS

A literature search was performed on papers published between January 1990 and September 2017 using the Web of Science Core Collection, MEDLINE and Scopus databases.

RESULTS

What is seen in the subchondral bone and synovia is mirrored in the central nervous system (CNS). Substance P, calcitonin gene-related peptide, vasoactive intestinal peptide and neuropeptide Y are the major peptides involved both in the generation of pain as well as reducing pain post-joint trauma. The interplay between them and other neuropeptides and cytokines influence how noxious stimuli are transduced, transmitted and modulated for a final pain perception as part of a complex cascade of events. There is a close interaction between the different components in the joint that together cross-talk to adapt to load and catabolic factors during injury and inflammation.

CONCLUSION

The articular joint should be seen as an organ where local joint pain development and maintenance is influenced by interplay between the local transmitters in the joints as well as their dependence on the CNS. A slow-release cocktail of mixed antibodies targeted against neuropeptides and receptor blockers/stimulators involved in the events of early joint pain or any inflammatory joint disease is a future treatment target.

LEVEL OF EVIDENCE

V.

Attenuation of early phase inflammation by cannabidiol prevents pain and nerve damage in rat osteoarthritis

Osteoarthritis (OA) is a multifactorial joint disease, which includes joint degeneration, intermittent inflammation, and peripheral neuropathy. Cannabidiol (CBD) is a noneuphoria producing constituent of cannabis that has the potential to relieve pain. The aim of this study was to determine whether CBD is anti-nociceptive in OA, and whether inhibition of inflammation by CBD could prevent the development of OA pain and joint neuropathy. Osteoarthritis was induced in male Wistar rats (150-175 g) by intra-articular injection of sodium monoiodoacetate (MIA; 3 mg). On day 14 (end-stage OA), joint afferent mechanosensitivity was assessed using in vivo electrophysiology, whereas pain behaviour was measured by von Frey hair algesiometry and dynamic incapacitance. To investigate acute joint inflammation, blood flow and leukocyte trafficking were measured on day 1 after MIA. Joint nerve myelination was calculated by G-ratio analysis. The therapeutic and prophylactic effects of peripheral CBD (100-300 μg) were assessed. In end-stage OA, CBD dose-dependently decreased joint afferent firing rate, and increased withdrawal threshold and weight bearing (P < 0.0001; n = 8). Acute, transient joint inflammation was reduced by local CBD treatment (P < 0.0001; n = 6). Prophylactic administration of CBD prevented the development of MIA-induced joint pain at later time points (P < 0.0001; n = 8), and was also found to be neuroprotective (P < 0.05; n = 6-8). The data presented here indicate that local administration of CBD blocked OA pain. Prophylactic CBD treatment prevented the later development of pain and nerve damage in these OA joints. These findings suggest that CBD may be a safe, useful therapeutic for treating OA joint neuropathic pain.

Early blockade of joint inflammation with a fatty acid amide hydrolase inhibitor decreases end-stage osteoarthritis pain and peripheral neuropathy in mice

Background

The endocannabinoid system has been shown to reduce inflammatory flares and pain in rodent models of arthritis. A limitation of endocannabinoids is that they are rapidly denatured by hydrolysing enzymes such as fatty acid amide hydrolase (FAAH) which renders them physiologically inert. Osteoarthritis (OA) is primarily a degenerative joint disease; however, it can incorporate mild inflammation and peripheral neuropathy. The aim of this study was to determine whether early blockade of FAAH bioactivity could reduce OA-associated inflammation and joint neuropathy. The ability of this treatment to prevent end-stage OA pain development was also tested.

Methods

Physiological saline or sodium monoiodoacetate (MIA; 0.3 mg) was injected into the right knee of male C57Bl/6 mice (20–42 g) and joint inflammation (oedema, blood flow and leukocyte trafficking) was measured over 14 days. Joint inflammation was also measured in a separate cohort of animals treated on day 1 with either saline or the FAAH inhibitor URB597 (0.03–0.3 mg/kg topical onto the knee joint). In other experiments, von Frey hair tactile sensitivity was determined on days 1 and 14 in MIA-injected mice treated prophylactically with URB597 (0.3 mg/kg s.c. over the knee joint on days 0–3). Saphenous nerve myelination was also assessed in these animals on day 14 by G-ratio analysis.

Results

Intra-articular injection of MIA caused an increase in joint oedema (P < 0.0001), blood flow (P < 0.05), leukocyte rolling (P < 0.05) and adherence (P < 0.001) on day 1 after treatment which subsequently resolved over later time points. This acute inflammatory response was ameliorated by local URB597 treatment. Prophylactic local administration of URB597 prevented MIA-induced saphenous nerve demyelination, and chronic joint pain was also attenuated.

Conclusions

These data indicate that local inhibition of FAAH in MIA-injected knees can reduce acute inflammatory changes associated with the model. Prophylactic treatment of OA mice with the endocannabinoid hydrolysis inhibitor URB597 was also shown to be neuroprotective and prevented the development of joint pain at later time points.