Summary: A new study reveals how dangerous inflammations in the brain are caused during the aging process. Researchers say failure with the CB1 to control activity of immune cells may result in chronic neuroinflammation. They say it may be possible to break this vicious cycle in the future with drugs that contain THC, which is a powerful CB1 receptor activator.
Source: University of Bonn.
The so-called CB1 receptor is responsible for the intoxicating effect of cannabis. However, it appears to act also as a kind of “sensor” with which neurons measure and control the activity of certain immune cells in the brain. A recent study by the University of Bonn at least points in this direction. If the sensor fails, chronic inflammation may result – probably the beginning of a dangerous vicious circle. The publication appears in the journal Frontiers in Molecular Neuroscience.
The activity of the so-called microglial cells plays an important role in brain aging. These cells are part of the brain’s immune defense: For example, they detect and digest bacteria, but also eliminate diseased or defective nerve cells. They also use messenger substances to alert other defense cells and thus initiate a concerted campaign to protect the brain: an inflammation.
This protective mechanism has undesirable side effects; it can also cause damage to healthy brain tissue. Inflammations are therefore usually strictly controlled. “We know that so-called endocannabinoids play an important role in this”, explains Dr. Andras Bilkei-Gorzo from the Institute of Molecular Psychiatry at the University of Bonn. “These are messenger substances produced by the body that act as a kind of brake signal: They prevent the inflammatory activity of the glial cells.”
Endocannabinoids develop their effect by binding to special receptors. There are two different types, called CB1 and CB2. “However, microglial cells have virtually no CB1 and very low level of CB2 receptors,” emphasizes Bilkei-Gorzo. “They are therefore deaf on the CB1 ear. And yet they react to the corresponding brake signals – why this is the case, has been puzzling so far.”
Neurons as “middlemen”
The scientists at the University of Bonn have now been able to shed light on this puzzle. Their findings indicate that the brake signals do not communicate directly with the glial cells, but via middlemen – a certain group of neurons, because this group has a large number of CB1 receptors. “We have studied laboratory mice in which the receptor in these neurons was switched off,” explains Bilkei-Gorzo. “The inflammatory activity of the microglial cells was permanently increased in these animals.”
In contrast, in control mice with functional CB1 receptors, the brain’s own defense forces were normally inactive. This only changed in the present of inflammatory stimulus. “Based on our results, we assume that CB1 receptors on neurons control the activity of microglial cells,” said Bilkei-Gorzo. “However, we cannot yet say whether this is also the case in humans.”
This is how it might work in mice: As soon as microglial cells detect a bacterial attack or neuronal damage, they switch to inflammation mode. They produce endocannabinoids, which activate the CB1 receptor of the neurons in their vicinity. This way, they inform the nerve cells about their presence and activity. The neurons may then be able to limit the immune response. The scientists were able to show that neurons similarly regulatory the other major glial cell type, the astroglial cells.
During ageing the production of cannabinoids declines reaching a low level in old individuals. This could lead to a kind of vicious circle, Bilkei-Gorzo suspects: “Since the neuronal CB1 receptors are no longer sufficiently activated, the glial cells are almost constantly in inflammatory mode. More regulatory neurons die as a result, so the immune response is less regulated and may become free-running.”
It may be possible to break this vicious circle with drugs in the future. It is for instance hoped that cannabis will help slow the progression of dementia. Its ingredient, tetrahydrocannabinol (THC), is a powerful CB1 receptor activator – even in low doses free from intoxicating effect. Last year, the researchers from Bonn and colleagues from Israel were able to demonstrate that cannabis can reverse the aging processes in the brains of mice. This result now suggest that an anti-inflammatory effect of THC may play a role in its positive effect on the ageing brain.
Source: Andras Bilkei-Gorzo – University of Bonn
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Original Research: Open access research for “Cannabinoid 1 Receptor Signaling on Hippocampal GABAergic Neurons Influences Microglial Activity” by Frank Ativie, Joanna A. Komorowska, Eva Beins, Önder Albayram, Till Zimmer, Andreas Zimmer, Dario Tejera, Michael Heneka and Andras Bilkei-Gorzo in Frontiers in Molecular Neuroscience. Published August 28 2018.
Cannabinoid 1 Receptor Signaling on Hippocampal GABAergic Neurons Influences Microglial Activity
Microglia, the resident immune cells of the brain, play important roles in defending the brain against pathogens and supporting neuronal circuit plasticity. Chronic or excessive pro-inflammatory responses of microglia damage neurons, therefore their activity is tightly regulated. Pharmacological and genetic studies revealed that cannabinoid type 1 (CB1) receptor activity influences microglial activity, although microglial CB1 receptor expression is very low and activity-dependent. The CB1 receptor is mainly expressed on neurons in the central nervous system (CNS)—with an especially high level on GABAergic interneurons. Here, we determined whether CB1 signaling on this neuronal cell type plays a role in regulating microglial activity. We compared microglia density, morphology and cytokine expression in wild-type (WT) and GABAergic neuron-specific CB1 knockout mice (GABA/CB1−/−) under control conditions (saline-treatment) and after 3 h, 24 h or repeated lipopolysaccharide (LPS)-treatment. Our results revealed that hippocampal microglia from saline-treated GABA/CB1−/− mice resembled those of LPS-treated WT mice: enhanced density and larger cell bodies, while the size and complexity of their processes was reduced. No further reduction in the size or complexity of microglia branching was detected after LPS-treatment in GABA/CB1−/− mice, suggesting that microglia in naïve GABA/CB1−/− mice were already in an activated state. This result was further supported by correlating the level of microglial tumor necrosis factor α (TNFα) with their size. Acute LPS-treatment elicited in both genotypes similar changes in the expression of pro-inflammatory cytokines (TNFα, interleukin-6 (IL-6) and interleukin 1β (IL-1β)). However, TNFα expression was still significantly elevated after repeated LPS-treatment in WT, but not in GABA/CB1−/− mice, indicating a faster development of tolerance to LPS. We also tested the possibility that the altered microglia activity in GABA/CB1−/− mice was due to an altered expression of neuron-glia interaction proteins. Indeed, the level of fractalkine (CX3CL1), a neuronal protein involved in the regulation of microglia, was reduced in hippocampal GABAergic neurons in GABA/CB1−/− mice, suggesting a disturbed neuronal control of microglial activity. Our result suggests that CB1 receptor agonists can modulate microglial activity indirectly, through CB1 receptors on GABAergic neurons. Altogether, we demonstrated that GABAergic neurons, despite their relatively low density in the hippocampus, have a specific role in the regulation of microglial activity and cannabinoid signaling plays an important role in this arrangement.