ANSTO health researchers have contributed to an international study published in Nature Neuroscience that sheds light on the mechanism by which anti-anxiety drugs act on the brain which could lead to cognitive impairment in vulnerable individuals.
The research critically depended on a unique laboratory model developed at ANSTO known as the “Guwiyang Wurra -TSPO knockout” (a healthy mouse that lacks an evolutionary, ancient protein normally present in mitochondria, the organelle that provides a cell with energy. Because of the importance of the protein for energy generation, its name in Dharawal language is Guwiyang Wurra “fire mouse”).
The study suggested that the anti-anxiety drug was not acting on nerve cells directly but on microglial cells (cells of the brain’s own intrinsic immune system that can gather around nerve cells and their connections, the synapses) and that the movement of microglial cells was interfering with dendritic spines (small protrusions from the neurons at the tip of which the synaptic connections to other nerve cells are located).
“This observation is important because long-term use of anti-anxiety medication is thought to contribute to an acceleration of dementia and how that might occur was not known,” said co-author ANSTO Prof. Richard Banati.
“The knowledge gained in this work by a large international team helps in the development of anti-anxiety drugs without such detrimental cognitive effects. The specific experiment looked closely into how the long-term use of anti-anxiety drugs, such as diazepam, can alter the complex wiring of the brain.
“We have neurons and each neuron connects to another neuron by what is called a synapse. Here, the research team recognized the importance of other neighboring cells, microglial cells.
“These are small and highly mobile cells that are part of the non-neuronal matrix in which nerve cells are embedded. This matrix makes up a substantial part of the brain and is actually directly influencing the functioning of neural networks. The compound that was studied, diazepam, didn’t go directly to the long spines and synaptic connections between the nerve cell itself, but to the microglia.
“By doing so, the drug changed the normal activity of microglial cells and indirectly the maintenance function that microglia have around synaptic nerve cell connections. It is intriguing to see how the brain’s local immune system, of which microglial cells are part, directly participates in the overall functional integrity of the brain.
There are a number of serious illness conditions, such as dementia but notably also those characterized by often extreme or prolonged fatigue, such as we see now in ‘long COVID’ or after accidental or therapeutic radiation exposure, where we know that the immune system responds very strongly.
“If the connections between neurons are severed by the activity of the microglial cells, then it’s almost like unplugging neural connections, and that would explain, how very subtle changes could drive a further progression of dementia, or—more speculatively—cause severe fatigue.
“The conceptual significance of the work for me is that it shows us that we might want to view the brain not only as a telephone switchboard with point-to-point connections but as a switchboard in an unusual environment.”
You can think of the collective motion of the microglial cells as being similar to what occurs in lava lamps. The microglial cells create an amorphous but still locally confined dynamic, like bubbles that go up and then down when driven by heat.
And this ever-shifting, localized activity can interfere with the more static wire connections, in extreme cases, perhaps comparable to small, local cable melts that affect the whole system which otherwise looks fine.
The overlapping of the immune system (glial cells) and the nervous system (neurons) is important in understanding the underlying cellular mechanism.
Both systems mediate between the internal world of the organism and the input from the environment. This self/non-self interaction shows itself in a dynamic equilibrium in which connections are formed by the nervous system and modulated or even severed by cells of the immune system.
“The use of the powerful TSPO knockout mouse model provided evidence that the mitochondrial protein TSPO was involved in the remodeling of dendritic connections by microglial cells. Anti-anxiety drugs, such as diazepam, bind with TSPO.
“In a genetically modified animal like a TSPO knockout mouse, the side effects that are described for diazepam simply do not occur. Diazepam that was administered to laboratory models, showed a reduction in dendrite spines, while these defects did not occur in the TSPO knockout model,” said Prof. Banati.
Based on the findings, the authors concluded, that as a consequence of anti-anxiety drug use (benzodiazepines), the TSPO-mediated loss of dendritic spines accelerated cognitive decline.
It was also possible that chronic use of drugs such as the benzodiazepines altered the function of microglial cells, which could promote disease-specific pathological changes in the brain.
Benzodiazepines are psychoactive drugs that share similar pharmacological properties, such as sedative, hypnotic (sleep-inducing), anxiolytic, and anticonvulsive action and are widely used adjuncts to anesthesia to induce central muscle relaxation and amnesia (1). In addition, these drugs also affect immunity. The effect of benzodiazepines depends on the activation of binding sites, such as the central and peripheral benzodiazepine receptors.
The central benzodiazepine receptor is mainly present in the central nervous system (CNS) and forms part of the γ-aminobutyric acid (GABA)A receptor complex (2). The peripheral benzodiazepine receptor is a ubiquitously expressed protein of the outer mitochondrial membrane termed translocator protein 18 kDa (TSPO), structurally and functionally different from the GABAA receptor (3).
The TSPO is expressed in platelets, immune cells, endothelium, vascular smooth muscle, bone marrow, endocrine cells and to a lesser extent in the CNS where it is associated with glial cells (4, 5). Upregulation of the TSPO is observed in many CNS diseases, and some TSPO ligands are currently under investigation as therapeutic means for promoting neuroprotection, axonal regeneration, and modulating inflammation (6, 7).
Diazepam (DZ), which is a mixed-type benzodiazepine that can act on both central and peripheral benzodiazepine receptors, has been demonstrated to have an inhibitory effect on T cell function (8–12). However, the action on professional antigen-presenting cells such as macrophages (Mϕs) and dendritic cells (DCs) is not well characterized. In this work, we studied whether DZ impairs two undesired immune responses as septic shock and murine experimental autoimmune encephalomyelitis (EAE).
The invasion of microbial pathogens into the bloodstream is characterized by a systemic pro-inflammatory response, which can lead to severe sepsis and septic shock. On the one hand, the innate immune cells as Mϕs, DCs, and neutrophils detect pathogen molecules by diverse receptors among which are the TLRs (13). Mϕ-derived cytokines, such as IL-6, TNF-α, and IL-1β, have been identified as central mediators in the pathogenesis of septic shock and the resultant multiple organ dysfunction syndrome that can lead to death (13).
Here we described the bias from the classical activation of innate cells induced by LPS towards anti-inflammatory profiles when cultured with DZ, which prevented acute responses dependent on these cell populations. DCs are professional antigen-presenting cells and have a key role in initiating and regulating adaptive immune responses. Additionally, the DCs are also critical in suppressing immune responses and conserving peripheral tolerance through the generation of anergic and/or regulatory T cells and fine-tuning the response by changing the T-helper (Th1)/Th2/Th17 balance (14).
Both immature and semi-mature DCs have been associated with an induction of tolerance through the generation of Treg cells, the induction of apoptosis, or the anergy of autoreactive effector cells (15–17). In this work, we demonstrate the impaired LPS-induced activation when DC were co-treated with DZ and its inability to initiate Th1 and Th17 adaptive inflammatory responses.
On the other hand, EAE is a well-accepted model that mimics many of the clinical and pathological features of multiple sclerosis (MS). This pathology can be induced in C57BL/6 mice through immunization with MOG35–55 in CFA and produce monophasic or a chronic, sustained form of EAE. This model is characterized by a high induction of Th1 and Th17 autoimmune responses, mononuclear inflammatory infiltration and demyelination. Mϕ and CD4+ T cells are the main cell types in the inflammatory infiltrate (18). Here, we show that DZ not only prevented the onset of autoimmune responses, but more importantly, it also improved the clinical signs when it was administered therapeutically in vivo, once the disease was established, maintaining a low clinical score until 35 days post-immunization (dpi).
In summary, this work describes the immunomodulatory effects of Diazepam on the different stages of the immune response. DZ interferes with the activation of innate cells such as dendritic cells and macrophages induced by inflammatory stimuli and impairs the initiation and development of adaptive inflammatory responses (Th1 and Th17). Furthermore, DZ also favors the development of tolerogenic and anti-inflammatory responses such as Tregs or antigen-specific IL-10 producing cells. Our work contributes in a descriptive way to the knowledge of the immunomodulatory properties of this type of psychoactive drugs.
reference link :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8329586/
Original Research: Closed access.
“Long-term diazepam treatment enhances microglial spine engulfment and impairs cognitive performance via the mitochondrial 18 kDa translocator protein (TSPO)” by Yuan Shi et al. Nature Neuroscience