Opioids Induce Bidirectional Synaptic Plasticity in a Brainstem Pain Center in the Rat

Opioids are powerful analgesics commonly used in pain management. However, opioids can induce complex neuroadaptations, including synaptic plasticity, that ultimately drive severe side effects, such as pain hypersensitivity and strong aversion during prolonged administration or upon drug withdrawal, even following a single, brief administration. The lateral parabrachial nucleus (LPBN) in the brainstem plays a key role in pain and emotional processing; yet, the effects of opioids on synaptic plasticity in this area remain unexplored. Using patch-clamp recordings in acute brainstem slices from male and female Sprague Dawley rats, we demonstrate a concentration-dependent, bimodal effect of opioids on excitatory synaptic transmission in the LPBN. While a lower concentration of DAMGO (0.5 µM) induced a long-term depression of synaptic strength (low-DAMGO LTD), abrupt termination of a higher concentration (10 µM) induced a long-term potentiation (high-DAMGO LTP) in a subpopulation of cells. LTD involved a metabotropic glutamate receptor (mGluR)-dependent mechanism; in contrast, LTP required astrocytes and N-methyl-D-aspartate receptor (NMDAR) activation. Selective optogenetic activation of spinal and periaqueductal gray matter (PAG) inputs to the LPBN revealed that, while LTD was expressed at all parabrachial synapses tested, LTP was restricted to spino-parabrachial synapses. Thus, we uncovered previously unknown forms of opioid-induced long-term plasticity in the parabrachial nucleus that potentially modulate some adverse effects of opioids. PERSPECTIVE: We found a previously unrecognized site of opioid-induced plasticity in the lateral parabrachial nucleus, a key region for pain and emotional processing. Unraveling opioid-induced adaptations in parabrachial function might facilitate the identification of new therapeutic measures for addressing adverse effects of opioid discontinuation such as hyperalgesia and aversion.

Oxoglutarate dehydrogenase complex controls glutamate-mediated neuronal death

Brain injury is accompanied by neuroinflammation, accumulation of extracellular glutamate and mitochondrial dysfunction, all of which cause neuronal death. The aim of this study was to investigate the impact of these mechanisms on neuronal death. Patients from the neurosurgical intensive care unit suffering aneurysmal subarachnoid hemorrhage (SAH) were recruited retrospectively from a respective database. In vitro experiments were performed in rat cortex homogenate, primary dissociated neuronal cultures, B35 and NG108-15 cell lines. We employed methods including high resolution respirometry, electron spin resonance, fluorescent microscopy, kinetic determination of enzymatic activities and immunocytochemistry. We found that elevated levels of extracellular glutamate and nitric oxide (NO) metabolites correlated with poor clinical outcome in patients with SAH. In experiments using neuronal cultures we showed that the 2-oxoglutarate dehydrogenase complex (OGDHC), a key enzyme of the glutamate-dependent segment of the tricarboxylic acid (TCA) cycle, is more susceptible to the inhibition by NO than mitochondrial respiration. Inhibition of OGDHC by NO or by succinyl phosphonate (SP), a highly specific OGDHC inhibitor, caused accumulation of extracellular glutamate and neuronal death. Extracellular nitrite did not substantially contribute to this NO action. Reactivation of OGDHC by its cofactor thiamine (TH) reduced extracellular glutamate levels, Ca2+ influx into neurons and cell death rate. Salutary effect of TH against glutamate toxicity was confirmed in three different cell lines. Our data suggest that the loss of control over extracellular glutamate, as described here, rather than commonly assumed impaired energy metabolism, is the critical pathological manifestation of insufficient OGDHC activity, leading to neuronal death.

Differential activation of spinal and parabrachial glial cells in a neuropathic pain model

The clinical burden faced by chronic pain patients is compounded by affective comorbidities, such as depression and anxiety disorders. Emerging evidence suggests that reactive glial cells in the spinal cord dorsal horn play a key role in the chronification of pain, while supraspinal glia are important for psychological aspects of chronic pain. The lateral parabrachial nucleus (LPBN) in the brainstem is a key node in the ascending pain system, and is crucial for the emotional dimension of pain. Yet, whether astrocytes and microglia in the LPBN are activated during chronic pain is unknown. Here, we evaluated the occurrence of glial activation in the LPBN of male Sprague-Dawley rats 1, 4, and 7 weeks after inducing a chronic constriction injury (CCI) of the sciatic nerve, a prevalent neuropathic pain model. CCI animals developed mechanical and thermal hypersensitivity that persisted for at least 4 weeks, and was mostly reversed after 7 weeks. Using immunohistochemical staining and confocal imaging, we found that CCI caused a strong increase in the expression of the astrocytic marker GFAP and the microglial marker Iba1 in the ipsilateral spinal dorsal horn, with peak expression observed 1 week post-injury. Moreover, morphology analysis revealed changes in microglial phenotype, indicative of microglia activation. In contrast, CCI did not induce any detectable changes in either astrocytes or microglia in the LPBN, at any time point. Thus, our results indicate that while neuropathic pain induces a robust glial reaction in the spinal dorsal horn, it fails to activate glial cells in the LPBN.

Probing pain aversion in rats with the "Heat Escape Threshold" paradigm

The aversive aspect of pain constitutes a major burden faced by pain patients. This has been recognized by the pain research community, leading to the development of novel methods focusing on affective-motivational behaviour in pain model animals. The most common tests used to assess pain aversion in animals require cognitive processes, such as associative learning, complicating the interpretation of results. To overcome this issue, studies in recent years have utilized unconditioned escape as a measure of aversion. However, the vast majority of these studies quantify jumping - a common escape behaviour in mice, but not in adult rats, thus limiting its use. Here, we present the "Heat Escape Threshold" (HET) paradigm for assessing heat aversion in rats. We demonstrate that this method can robustly and reproducibly detect the localized effects of an inflammatory pain model (intraplantar carrageenan) in male and female Sprague-Dawley rats. In males, a temperature that evoked unconditioned escape following carrageenan treatment also induced real-time place avoidance (RTPA). Systemic morphine more potently alleviated carrageenan-induced heat aversion (as measured by the HET and RTPA methods), as compared to reflexive responses to heat (as measured by the Hargreaves test), supporting previous findings. Next, we examined how blocking of excitatory transmission to the lateral parabrachial nucleus (LPBN), a key node in the ascending pain system, affects pain behaviour. Using the HET and Hargreaves tests, we show that intra-LPBN application of glutamate antagonists reverses the effects of carrageenan on both affective and reflexive pain behaviour, respectively. Finally, we employed the HET paradigm in a generalized opioid-withdrawal pain model. Withdrawal from a brief systemic administration of remifentanil resulted in a long-lasting and robust increase in heat aversion, but no change in reflexive responses to heat. Taken together, these data demonstrate the utility of the HET paradigm as a novel tool in preclinical pain research.

GABAergic CaMKIIα+ Amygdala Output Attenuates Pain and Modulates Emotional-Motivational Behavior via Parabrachial Inhibition

Pain and emotion are strongly regulated by neurons in the central nucleus of the amygdala (CeA), a major output of the limbic system; yet, the neuronal signaling pathways underlying this modulation are incompletely understood. Here, we characterized a subpopulation of CeA neurons that express the CaMKIIα gene (CeACAM neurons) and project to the lateral parabrachial nucleus (LPBN), a brainstem region known for its critical role in distributing nociceptive and other aversive signals throughout the brain. In male Sprague Dawley rats, we show that CeACAM-LPBN neurons are GABAergic and mostly express somatostatin. In anaesthetized rats, optogenetic stimulation of CeACAM-LPBN projections inhibited responses of LPBN neurons evoked by electrical activation of Aδ- and C-fiber primary afferents; this inhibition could be blocked by intra-LPBN application of the GABAA receptor antagonist bicuculline. CeACAM-LPBN stimulation also dampened LPBN responses to noxious mechanical, thermal, and chemical stimuli. In behaving rats, optogenetic stimulation of CeACAM-LPBN projections attenuated nocifensive responses to mechanical pressure and radiant heat, disrupted the ability of a noxious shock to drive aversive learning, reduced the defensive behaviors of thigmotaxis and freezing, induced place preference, and promoted food consumption in sated rats. Thus, we suggest that CeACAM-LPBN projections mediate a form of analgesia that is accompanied by a shift toward the positive-appetitive pole of the emotional-motivational continuum. Since the affective state of pain patients strongly influences their prognosis, we envision that recruitment of this pathway in a clinical setting could potentially promote pain resilience and recovery.SIGNIFICANCE STATEMENT Pain and emotion interact on multiple levels of the nervous system. Both positive and negative emotion may have analgesic effects. However, while the neuronal mechanisms underlying "stress-induced analgesia" have been the focus of many studies, the neuronal substrates underlying analgesia accompanied by appetitive emotional-motivational states have received far less attention. The current study focuses on a subpopulation of amygdala neurons that form inhibitory synapses within the brainstem lateral parabrachial nucleus (LPBN). We show that activation of these amygdalo-parabrachial projections inhibits pain processing, while also reducing behaviors related to negative affect and enhancing behaviors related to positive affect. We propose that recruitment of this pathway would benefit pain patients, many of whom suffer from psychological comorbidities such as anxiety and depression.

Hypoxic Adaptation of Mitochondrial Metabolism in Rat Cerebellum Decreases in Pregnancy

Function of brain amino acids as neurotransmitters or their precursors implies changes in the amino acid levels and/or metabolism in response to physiological and environmental challenges. Modelling such challenges by pregnancy and/or hypoxia, we characterize the amino acid pool in the rat cerebellum, quantifying the levels and correlations of 15 amino acids and activity of 2-oxoglutarate dehydrogenase complex (OGDHC). The parameters are systemic indicators of metabolism because OGDHC limits the flux through mitochondrial TCA cycle, where amino acids are degraded and their precursors synthesized. Compared to non-pregnant state, pregnancy increases the cerebellar content of glutamate and tryptophan, decreasing interdependence between the quantified components of amino acid metabolism. In response to hypoxia, the dependence of cerebellar amino acid pool on OGDHC and the average levels of arginine, glutamate, lysine, methionine, serine, phenylalanine, and tryptophan increase in non-pregnant rats only. This is accompanied by a higher hypoxic resistance of the non-pregnant vs. pregnant rats, pointing to adaptive significance of the hypoxia-induced changes in the cerebellar amino acid metabolism. These adaptive mechanisms are not effective in the pregnancy-changed metabolic network. Thus, the cerebellar amino acid levels and OGDHC activity provide sensitive markers of the physiology-dependent organization of metabolic network and its stress adaptations.

Up-regulation of 2-oxoglutarate dehydrogenase as a stress response

2-Oxoglutarate dehydrogenase multienzyme complex (OGDHC) operates at a metabolic cross-road, mediating Ca(2+)- and ADP-dependent signals in mitochondria. Here, we test our hypothesis that OGDHC plays a major role in the neurotransmitter metabolism and associated stress response. This possibility was assessed using succinyl phosphonate (SP), a highly specific and efficient in vivo inhibitor of OGDHC. Animals exposed to toxicants (SP, ethanol or MnCl(2)), trauma or acute hypoxia showed intrinsic up-regulation of OGDHC in brain and heart. The known mechanism of the SP action as OGDHC inhibitor pointed to the up-regulation triggered by the enzyme impairment. The animal behavior and skeletal muscle or heart performance were tested to correlate physiology with the OGDHC regulation and associated changes in the glutamate and cellular energy status. The SP-treated animals exhibited interdependent changes in the brain OGDHC activity, glutamate level and cardiac autonomic balance, suggesting the neurotransmitter role of glutamate to be involved in the changed heart performance. Energy insufficiency after OGDHC inhibition was detectable neither in animals up to 25 mg/kg SP, nor in cell culture during 24 h incubation with 0.1 mM SP. However, in animals subjected to acute ethanol intoxication SP did evoke energy deficit, decreasing muscular strength and locomotion and increasing the narcotic sleep duration. This correlated with the SP-induced decrease in NAD(P)H levels of the ethanol-exposed neurons. Thus, we show the existence of natural mechanisms to up-regulate mammalian OGDHC in response to stress, with both the glutamate neurotransmission and energy production potentially involved in the OGDHC impact on physiological performance. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Behavioral impact of the regulation of the brain 2-oxoglutarate dehydrogenase complex by synthetic phosphonate analog of 2-oxoglutarate: implications into the role of the complex in neurodegenerative diseases

Decreased activity of the mitochondrial 2-oxoglutarate dehydrogenase complex (OGDHC) in brain accompanies neurodegenerative diseases. To reveal molecular mechanisms of this association, we treated rats with a specific inhibitor of OGDHC, succinyl phosphonate, or exposed them to hypoxic stress. In males treated with succinyl phosphonate and in pregnancy-sensitized females experiencing acute hypobaric hypoxia, we revealed upregulation of brain OGDHC (within 24 hours), with the activity increase presumably representing the compensatory response of brain to the OGDHC inhibition. This up-regulation of brain OGDHC was accompanied by an increase in exploratory activity and a decrease in anxiety of the experimental animals. Remarkably, the hypoxia-induced elevation of brain OGDHC and most of the associated behavioral changes were abrogated by succinyl phosphonate. The antagonistic action of hypoxia and succinyl phosphonate demonstrates potential therapeutic significance of the OGDHC regulation by the phosphonate analogs of 2-oxoglutarate.