GABAergic transmission and enhanced modulation by opioids and endocannabinoids in adult rat rostral ventromedial medulla

Mild Traumatic Brain Injury-Induced Augmented Postsurgical Pain Is Driven by Central Serotonergic Pain-Facilitatory Signaling

BACKGROUND

Individuals recovering from mild traumatic brain injury (mTBI) have increased rates of acute and chronic pain. However, the mechanism through which mTBI triggers heightened pain responses and the link between mTBI and postsurgical pain remain elusive. Recent data suggest that dysregulated serotonergic pain-modulating circuits could be involved. We hypothesized that mTBI triggers dysfunction in descending serotonergic pain modulation, which exacerbates acute pain and delays pain-related recovery after surgery.

METHODS

Using mouse models of mTBI and hindpaw incision for postsurgical pain in C57BL/6J mice, mechanical withdrawal thresholds were assessed throughout the postsurgical period. To determine whether mTBI leads to persistent alteration of endogenous opioid tone, mu-opioid receptors (MORs) were blocked with naloxone. Finally, the role of descending serotonergic signaling on postsurgical allodynia in animals with mTBI was examined using ondansetron (5-HT 3 receptor antagonist) or a serotonin-specific neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), to ablate descending serotonergic fibers. The treatment effects on withdrawal thresholds were normalized to baseline (percentage of maximum possible effect, MPE%), and analyzed using paired t -test or 2-way repeated-measures ANOVA with post hoc multiple comparisons.

RESULTS

Post-mTBI mice demonstrated transient allodynia in hindpaws contralateral to mTBI, while no nociceptive changes were observed in sham-mTBI animals (mean difference, MD, MPE%, post-mTBI day 3: -60.9; 95% CI, -88.7 to -35.0; P < .001). After hindpaw incision, animals without mTBI exhibited transient allodynia, while mice with prior mTBI demonstrated prolonged postsurgical allodynia (MD-MPE% postsurgical day 14: -65.0; 95% CI, -125.4 to -4.5; P = .04). Blockade of MORs using naloxone transiently reinstated allodynia in mTBI animals but not in sham-mTBI mice (MD-MPE% post-naloxone: -69.9; 95% CI, -94.8 to -45.1; P < .001). Intrathecal administration of ondansetron reversed the allodynia observed post-mTBI and postincision in mTBI mice (compared to vehicle-treated mTBI mice, MD-MPE% post-mTBI day 3: 82.7; 95% CI, 58.5-106.9; P < .001; postsurgical day 17: 62.5; 95% CI, 38.3-86.7; P < .001). Both the acute allodynia after TBI and the period of prolonged allodynia after incision in mTBI mice were blocked by pretreatment with 5,7-DHT (compared to sham-mTBI mice, MD-MPE% post-mTBI day 3: 0.5; 95% CI, -18.5 to 19.5; P = .99; postsurgical day 14: -14.6; 95% CI, -16.7 to 45.9; P = .48). Similar behavioral patterns were observed in hindpaw ipsilateral to mTBI.

CONCLUSIONS

Collectively, our results show that descending serotoninergic pain-facilitating signaling is responsible for nociceptive sensitization after mTBI and that central endogenous opioid tone opposes serotonin's effects. Understanding brain injury-related changes in endogenous pain modulation may lead to improved pain control for those with TBI undergoing surgery.

Bulbospinal nociceptive ON and OFF cells related neural circuits and transmitters

The rostral ventromedial medulla (RVM) is a bulbospinal nuclei in the descending pain modulation system, and directly affects spinal nociceptive transmission through pronociceptive ON cells and antinociceptive OFF cells in this area. The functional status of ON and OFF neurons play a pivotal role in pain chronification. As distinct pain modulative information converges in the RVM and affects ON and OFF cell excitability, neural circuits and transmitters correlated to RVM need to be defined for an in-depth understanding of central-mediated pain sensitivity. In this review, neural circuits including the role of the periaqueductal gray, locus coeruleus, parabrachial complex, hypothalamus, amygdala input to the RVM, and RVM output to the spinal dorsal horn are discussed. Meanwhile, the role of neurotransmitters is concluded, including serotonin, opioids, amino acids, cannabinoids, TRPV1, substance P and cholecystokinin, and their dynamic impact on both ON and OFF cell activities in modulating pain transmission. Via clarifying potential specific receptors of ON and OFF cells, more targeted therapies can be raised to generate pain relief for patients who suffer from chronic pain.

Activity disruption causes degeneration of entorhinal neurons in a mouse model of Alzheimer's circuit dysfunction

Neurodegenerative diseases are characterized by selective vulnerability of distinct cell populations; however, the cause for this specificity remains elusive. Here, we show that entorhinal cortex layer 2 (EC2) neurons are unusually vulnerable to prolonged neuronal inactivity compared with neighboring regions of the temporal lobe, and that reelin + stellate cells connecting EC with the hippocampus are preferentially susceptible within the EC2 population. We demonstrate that neuronal death after silencing can be elicited through multiple independent means of activity inhibition, and that preventing synaptic release, either alone or in combination with electrical shunting, is sufficient to elicit silencing-induced degeneration. Finally, we discovered that degeneration following synaptic silencing is governed by competition between active and inactive cells, which is a circuit refinement process traditionally thought to end early in postnatal life. Our data suggests that the developmental window for wholesale circuit plasticity may extend into adulthood for specific brain regions. We speculate that this sustained potential for remodeling by entorhinal neurons may support lifelong memory but renders them vulnerable to prolonged activity changes in disease.

The cellular mechanism by which the rostral ventromedial medulla acts on the spinal cord during chronic pain

Clinical therapies for chronic pain are limited. While targeted drugs are promising therapies for chronic pain, they exhibit insufficient efficacy and poor targeting. The occurrence of chronic pain partly results from central changes caused by alterations in neurons in the rostral ventromedial medulla (RVM) in the brainstem regulatory pathway. The RVM, which plays a key role in the descending pain control pathway, greatly contributes to the development and maintenance of pain. However, the exact roles of the RVM in chronic pain remain unclear, making it difficult to develop new drugs targeting the RVM and related pathways. Here, we first discuss the roles of the RVM and related circuits in chronic pain. Then, we analyze synaptic transmission between RVM neurons and spinal cord neurons, specifically focusing on the release of neurotransmitters, to explore the cellular mechanisms by which the RVM regulates chronic pain. Finally, we propose some ideas for the development of drugs targeting the RVM.

Cannabinoids in the descending pain modulatory circuit: Role in inflammation

The legalization of cannabis in some states has intensified interest in the potential for cannabis and its constituents to lead to novel therapeutics for pain. Our understanding of the cellular mechanisms underlying cannabinoid actions in the brain have lagged behind opioids; however, the current opioid epidemic has also increased attention on the use of cannabinoids as alternatives to opioids for pain, especially chronic pain that requires long-term use. Endogenous cannabinoids are lipid signaling molecules that have complex roles in modulating neuronal function throughout the brain. In this review, we discuss cannabinoid functions in the descending pain modulatory pathway, a brain circuit that integrates cognitive and emotional processing of pain to modulate incoming sensory inputs. In addition, we highlight areas where further studies are necessary to understand cannabinoid regulation of descending pain modulation.

Compensatory Activation of Cannabinoid CB2 Receptor Inhibition of GABA Release in the Rostral Ventromedial Medulla in Inflammatory Pain

The rostral ventromedial medulla (RVM) is a relay in the descending pain modulatory system and an important site of endocannabinoid modulation of pain. Endocannabinoids inhibit GABA release in the RVM, but it is not known whether this effect persists in chronic pain states. In the present studies, persistent inflammation induced by complete Freund's adjuvant (CFA) increased GABAergic miniature IPSCs (mIPSCs). Endocannabinoid activation of cannabinoid (CB1) receptors known to inhibit presynaptic GABA release was significantly reduced in the RVM of CFA-treated rats compared with naive rats. The reduction in CFA-treated rats correlated with decreased CB1 receptor protein expression and function in the RVM. Paradoxically, the nonselective CB1/CB2 receptor agonist WIN55212 inhibited GABAergic mIPSCs in both naive and CFA-treated rats. However, WIN55212 inhibition was reversed by the CB1 receptor antagonist rimonabant in naive rats but not in CFA-treated rats. WIN55212-mediated inhibition in CFA-treated rats was blocked by the CB2 receptor-selective antagonist SR144528, indicating that CB2 receptor function in the RVM is increased during persistent inflammation. Consistent with these results, CB2 receptor agonists AM1241 and GW405833 inhibited GABAergic mIPSC frequency only in CFA-treated rats, and the inhibition was reversed with SR144528. When administered alone, SR144528 and another CB2 receptor-selective antagonist AM630 increased mIPSC frequency in the RVM of CFA-treated rats, indicating that CB2 receptors are tonically activated by endocannabinoids. Our data provide evidence that CB2 receptor function emerges in the RVM in persistent inflammation and that selective CB2 receptor agonists may be useful for treatment of persistent inflammatory pain.

SIGNIFICANCE STATEMENT

These studies demonstrate that endocannabinoid signaling to CB1 and CB2 receptors in adult rostral ventromedial medulla is altered in persistent inflammation. The emergence of CB2 receptor function in the rostral ventromedial medulla provides additional rationale for the development of CB2 receptor-selective agonists as useful therapeutics for chronic inflammatory pain.

Optogenetic Evidence for a Direct Circuit Linking Nociceptive Transmission through the Parabrachial Complex with Pain-Modulating Neurons of the Rostral Ventromedial Medulla (RVM)

The parabrachial complex (PB) is a functionally and anatomically complex structure involved in a range of homeostatic and sensory functions, including nociceptive transmission. There is also evidence that PB can engage descending pain-modulating systems, the best characterized of which is the rostral ventromedial medulla (RVM). Two distinct classes of RVM neurons, “ON-cells” and “OFF-cells,” exert net pronociceptive and anti-nociceptive effects, respectively. PB was recently shown to be a relay of nociceptive information to RVM ON- and OFF-cells. The present experiments used optogenetic methods in a lightly anesthetized rat and an adult RVM slice to determine whether there are direct, functionally relevant inputs to RVM pain-modulating neurons from PB. Whole-cell patch-clamp recordings demonstrated that PB conveys direct glutamatergic and GABAergic inputs to RVM neurons. Consistent with this, in vivo recording showed that nociceptive-evoked responses of ON- and OFF-cells were suppressed by optogenetic inactivation of archaerhodopsin (ArchT)-expressing PB terminals in RVM, demonstrating that a net inhibitory input to OFF-cells and net excitatory input to ON-cells are engaged by acute noxious stimulation. Further, the majority of ON- and OFF-cells responded to optogenetic activation of channelrhodopsin (ChR2)-expressing terminals in the RVM, confirming a direct PB influence on RVM pain-modulating neurons. These data show that a direct connection from the PB to the RVM conveys nociceptive information to the pain-modulating neurons of RVM under basal conditions. They also reveal additional inputs from PB with the capacity to activate both classes of RVM pain-modulating neurons and the potential to be recruited under different physiological and pathophysiological conditions.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Postnatal maturation of the spinal-bulbo-spinal loop: brainstem control of spinal nociception is independent of sensory input in neonatal rats

The rostroventral medial medulla (RVM) is part of a rapidly acting spino-bulbo-spinal loop that is activated by ascending nociceptive inputs and drives descending feedback modulation of spinal nociception. In the adult rat, the RVM can facilitate or inhibit dorsal horn neuron inputs but in young animals descending facilitation dominates. It is not known whether this early life facilitation is part of a feedback loop. We hypothesized that the newborn RVM functions independently of sensory input, before the maturation of feedback control. We show here that noxious hind paw pinch evokes no fos activation in the RVM or the periaqueductal gray at postnatal day (P) 4 or P8, indicating a lack of nociceptive input at these ages. Significant fos activation was evident at P12, P21, and in adults. Furthermore, direct excitation of RVM neurons with microinjection of DL-homocysteic acid did not alter the net activity of dorsal horn neurons at P10, suggesting an absence of glutamatergic drive, whereas the same injections caused significant facilitation at P21. In contrast, silencing RVM neurons at P8 with microinjection of lidocaine inhibited dorsal horn neuron activity, indicating a tonic descending spinal facilitation from the RVM at this age. The results support the hypothesis that early life descending facilitation of spinal nociception is independent of sensory input. Since it is not altered by RVM glutamatergic receptor activation, it is likely generated by spontaneous brainstem activity. Only later in postnatal life can this descending activity be modulated by ascending nociceptive inputs in a functional spinal-bulbo-spinal loop.

Modulation of Acid-sensing Ion Channel 1a by Intracellular pH and Its Role in Ischemic Stroke

An important contributor to brain ischemia is known to be extracellular acidosis, which activates acid-sensing ion channels (ASICs), a family of proton-gated sodium channels. Lines of evidence suggest that targeting ASICs may lead to novel therapeutic strategies for stroke. Investigations of the role of ASICs in ischemic brain injury have naturally focused on the role of extracellular pH in ASIC activation. By contrast, intracellular pH (pHi) has received little attention. This is a significant gap in our understanding because the ASIC response to extracellular pH is modulated by pHi, and activation of ASICs by extracellular protons is paradoxically enhanced by intracellular alkalosis. Our previous studies show that acidosis-induced cell injury in in vitro models is attenuated by intracellular acidification. However, whether pHi affects ischemic brain injury in vivo is completely unknown. Furthermore, whereas ASICs in native neurons are composed of different subunits characterized by distinct electrophysiological/pharmacological properties, the subunit-dependent modulation of ASIC activity by pHi has not been investigated. Using a combination of in vitro and in vivo ischemic brain injury models, electrophysiological, biochemical, and molecular biological approaches, we show that the intracellular alkalizing agent quinine potentiates, whereas the intracellular acidifying agent propionate inhibits, oxygen-glucose deprivation-induced cell injury in vitro and brain ischemia-induced infarct volume in vivo Moreover, we find that the potentiation of ASICs by quinine depends on the presence of the ASIC1a, ASIC2a subunits, but not ASIC1b, ASIC3 subunits. Furthermore, we have determined the amino acids in ASIC1a that are involved in the modulation of ASICs by pHi.