Differential roles of signal transduction mechanisms in long-term potentiation of excitatory synaptic transmission induced by activation of group I mGluRs in the spinal trigeminal subnucleus oralis

The Trigeminal Sensory System and Orofacial Pain

The trigeminal sensory system consists of the trigeminal nerve, the trigeminal ganglion, and the trigeminal sensory nuclei (the mesencephalic nucleus, the principal nucleus, the spinal trigeminal nucleus, and several smaller nuclei). Various sensory signals carried by the trigeminal nerve from the orofacial area travel into the trigeminal sensory system, where they are processed into integrated sensory information that is relayed to higher sensory brain areas. Thus, knowledge of the trigeminal sensory system is essential for comprehending orofacial pain. This review elucidates the individual nuclei that comprise the trigeminal sensory system and their synaptic transmission. Additionally, it discusses four types of orofacial pain and their relationship to the system. Consequently, this review aims to enhance the understanding of the mechanisms underlying orofacial pain.

ERK1/2-dependent BDNF synthesis and signaling is required for the antidepressant effect of microglia stimulation

Depressed mice have lower numbers of microglia in the dentate gyrus (DG). Reversal of this decline by a single low dose of lipopolysaccharide (LPS) may have antidepressant effects, but there is little information on the molecular mechanisms underlying this effect. It is known that impairment of brain-derived neurotrophic factor (BDNF) signaling is involved in the development of depression. Here, we used a combination of neutralizing antibodies, mutant mice, and pharmacological approaches to test the role of BDNF-tyrosine kinase receptor B (TrkB) signaling in the DG in the effect of microglial stimulation. Our results suggest that inhibition of BDNF signaling by infusion of an anti-BDNF antibody, the BDNF receptor antagonist K252a, or knock-in of the mutant BDNF Val68Met allele abolished the antidepressant effect of LPS in chronically stressed mice. Increased BDNF synthesis in DG, mediated by extracellular signal-regulated kinase1/2 (ERK1/2) signaling but not protein kinase B (Akt)-mammalian target of rapamycin (mTOR) signaling, was essential for the antidepressant effect of microglial stimulation. These results suggest that increased BDNF synthesis through activation of ERK1/2 caused by a single LPS injection and subsequent TrkB signaling are required for the antidepressant effect of hippocampal microglial stimulation.

The Up-regulation of TNF-α Maintains Trigeminal Neuralgia by Modulating MAPKs Phosphorylation and BKCa Channels in Trigeminal Nucleus Caudalis

Trigeminal neuralgia (TN) is a severe chronic neuropathic pain. Despite numerous available medical interventions, the therapeutic effects are not ideal. To control the pain attacks, the need for more contemporary drugs continues to be a real challenge. Our previous study reported that Ca²⁺-activated K⁺ channels (BK_Ca) channels modulated by mitogen-activated protein kinases (MAPKs) in the trigeminal ganglia (TG) neurons play crucial roles in regulating TN, and some research studies demonstrated that inflammatory cytokine tumor necrosis factor alpha (TNF-α) could promote neuropathic pain. Meanwhile, the trigeminal nucleus caudalis (TNC), the first central site of the trigeminal nociceptive pathway, is responsible for processing sensory and pain signals from the peripheral orofacial area. Thus, this study is aimed to further investigate whether TNF-α and MAPKs phosphorylation in the TNC could mediate the pathogenesis of TN by modulating BK_Ca channels. The results showed that TNF-α of the TNC region is upregulated significantly in the chronic constriction injury of infraorbital nerve (ION-CCI) rats model, which displayed persistent facial mechanical allodynia. The normal rats with target injection of exogenous TNF-α to the fourth brain ventricle behaved just like the ION-CCI model rats, the orofacial mechanical pain threshold decreased clearly. Meanwhile, the exogenous TNF-α increased the action potential frequency and reduced the BK_Ca currents of TNC neurons significantly, which could be reversed by U0126 and SB203580, the inhibitors of MAPK. In addition, U0126, SB203580, and another MAPK inhibitor SP600125 could relieve the facial mechanical allodynia by being injected into the fourth brain ventricle of ION-CCI model rats, respectively. Taken together, our work suggests that the upregulation of TNF-α in the TNC region would cause the increase of MAPKs phosphorylation and then the negative regulation of BK_Ca channels, resulting in the TN.

STIM Proteins and Glutamate Receptors in Neurons: Role in Neuronal Physiology and Neurodegenerative Diseases

Neuronal calcium (Ca²⁺) influx has long been ascribed mainly to voltage-gated Ca²⁺ channels and glutamate receptor channels. Recent research has shown that it is also complemented by stromal interaction molecule (STIM) protein-mediated store-operated Ca²⁺ entry (SOCE). SOCE is described as Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. The present review summarizes recent studies that indicate a relationship between neuronal SOCE that is mediated by STIM1 and STIM2 proteins and glutamate receptors under both physiological and pathological conditions, such as neurodegenerative disorders. We present evidence that the dysregulation of neuronal SOCE and glutamate receptor activity are hallmarks of acute neurodegenerative diseases (e.g., traumatic brain injury and cerebral ischemia) and chronic neurodegenerative diseases (e.g., Alzheimer's disease and Huntington's disease). Emerging evidence indicates a role for STIM proteins and glutamate receptors in neuronal physiology and pathology, making them potential therapeutic targets.

Group I metabotropic glutamate receptor activation induces TRPC6-dependent calcium influx and RhoA activation in cultured human kidney podocytes

Researches have shown that mice lacking the metabotropic glutamate receptor 1 (mGluR) showed albuminuria, remodeling of F-actin, with loss of stress fibers. Selective group I mGluRs agonist (S)-3,5-dihydroxyphenylglycine (DHPG) attenuated albuminuria in several rodent models of nephrotic syndrome. However, the molecular mechanism is obscure. Using a human podocyte cell line, we here investigated the molecular mechanisms of group I mGluRs-induced calcium influx and the formation of stress fibers. Our data showed that group I mGluRs activation by DHPG induced a significant calcium influx, and promoted cytoskeletal stress fiber formation and focal adhesions in podocytes. Pre-incubating podocytes with non-selective inhibitor of transient receptor potential channels (TRPC), or the knockdown of TRPC6 attenuated the calcium influx and the stress fiber formation induced by DHPG. Further, DHPG resulted in an increase of active RhoA expression. However, the knockdown of RhoA by siRNA abolished the DHPG-induced increase in stress fibers. Additionally, nonselective inhibitors of TRPC or TRPC6 knockdown clearly inhibited RhoA activation induced by DHPG, as assessed by Glutathione-S-transferase pull-down assay followed by Western blotting. Taken together, our findings suggest TRPC6 regulates actin stress fiber formation and focal adhesions via the RhoA pathway in response to group I mGluRs activation. Our data can potentially explain the mechanism of protective action of group I mGluRs in glomerular podocyte injury.

Trigeminal long-term potentiation as a cellular substrate for migraine

Most previous studies suggest that the subnucleus caudalis (Vc) of spinal trigeminal nucleus (Vsp) plays a key role in the generation and maintenance of migraine, a type of primary headache, by participating in the trigeminovascular system. Furthermore, the excitability of the Vc with the stimulation of the peripheral nociceptive fibers innervating the intracranial vessels or dura matter is regarded as a main cellular substrate for migraine. Here, a revised hypothesis is introduced, reinforcing the previous hypothesis and complementing it. This hypothesis suggests that, besides the Vc, much broader areas of the trigeminal sensory nuclei (Vsn), i.e., the principal sensory nucleus (Vp), the oralis nucleus (Vo), and the interpolaris nucleus (Vi), contribute to process and integrate pain signals generated in the head. In addition, the plasticity of synaptic transmission between nuclei or subnuclei in the Vsn, in particular, the Vsp, can be a cellular model for migraine, in the same way as the hippocampal synaptic plasticity is a model for learning and memory. This hypothesis will contribute to the discovery of new therapeutic tools for patients with migraine.

The Emerging Role of Metabotropic Glutamate Receptors in the Pathophysiology of Chronic Stress-Related Disorders

Chronic stress-related psychiatric conditions such as anxiety, depression, and alcohol abuse are an enormous public health concern. The etiology of these pathologies is complex, with psychosocial stressors being among the most frequently discussed risk factors. The brain glutamatergic neurotransmitter system has often been found involved in behaviors and pathophysiologies resulting from acute stress and fear. Despite this, relatively little is known about the role of glutamatergic system components in chronic psychosocial stress, neither in rodents nor in humans. Recently, drug discovery efforts at the metabotropic receptor subtypes of the glutamatergic system (mGlu1-8 receptors) led to the identification of pharmacological tools with emerging potential in psychiatric conditions. But again, the contribution of individual mGlu subtypes to the manifestation of physiological, molecular, and behavioral consequences of chronic psychosocial stress remains still largely unaddressed. The current review will describe animal models typically used to analyze acute and particularly chronic stress conditions, including models of psychosocial stress, and there we will discuss the emerging roles for mGlu receptor subtypes. Indeed, accumulating evidence indicates relevance and potential therapeutic usefulness of mGlu2/3 ligands and mGlu5 receptor antagonists in chronic stress-related disorders. In addition, a role for further mechanisms, e.g. mGlu7-selective compounds, is beginning to emerge. These mechanisms are important to be analyzed in chronic psychosocial stress paradigms, e.g. in the chronic subordinate colony housing (CSC) model. We summarize the early results and discuss necessary future investigations, especially for mGlu5 and mGlu7 receptor blockers, which might serve to suggest improved therapeutic strategies to treat stress-related disorders.