Neurophysiological response properties of medullary pain-control neurons following chronic treatment with morphine or oxycodone: modulation by acute ketamine.

Descending facilitatory circuitry that involves the rostroventromedial medulla (RVM) exerts a significant role in the development of antinociceptive tolerance and hyperalgesia following chronic morphine treatment. The role of the RVM in the development of antinociceptive tolerance to oxycodone, another clinically used strong opioid, is not yet known. Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, attenuates opioid antinociceptive tolerance, but its effect on RVM cell discharge in opioid-tolerant animals is not known. Here, we compared chronic effects of morphine and oxycodone on the discharge properties of RVM cells and attempted to attenuate chronic treatment-induced changes with ketamine. Parallel recordings of RVM cell discharge and limb withdrawal response were performed under light pentobarbital anesthesia in male rats following sustained systemic treatment with morphine or oxycodone at equianalgesic doses. Ongoing activity and the response to noxious heat and pinch were determined in pronociceptive RVM ON-cells and antinociceptive OFF-cells on the sixth treatment day. Proportions of RVM cell types were not changed. Chronic oxycodone induced antinociceptive tolerance both in limb withdrawal and RVM cell activity. Chronic morphine induced antinociceptive tolerance in limb withdrawal that was accompanied by pronociceptive heat response changes in RVM ON- and OFF-cells. A behaviorally subantinociceptive dose of acute ketamine reversed antinociceptive tolerance both to morphine and oxycodone in limb withdrawal and reversed the chronic morphine-induced pronociceptive discharge changes in RVM cells. The results indicate that an NMDA receptor-dependent descending pronociceptive circuitry involving the RVM has an important role in behavioral antinociceptive tolerance to morphine but not oxycodone.NEW & NOTEWORTHY Morphine and oxycodone are two clinically used strong opioids. Chronic treatment with oxycodone as well as morphine can lead to analgesic tolerance and paradoxical hyperalgesia. Here we show that an N-methyl-d-aspartate receptor-dependent pronociceptive change in discharge properties of rostroventromedial medullary neurons controlling spinal nociception has an important role in antinociceptive tolerance to morphine but not oxycodone. Interestingly, chronic oxycodone did not induce pronociceptive changes in the rostroventromedial medulla.

Descending antinociception induced by secondary somatosensory cortex stimulation in experimental neuropathy: role of the medullospinal serotonergic pathway.

Stimulation of the secondary somatosensory cortex (S2) has attenuated pain in humans and inflammatory nociception in animals. Here we studied S2 stimulation-induced antinociception and its underlying mechanisms in an experimental animal model of neuropathy induced by spinal nerve ligation (SNL). Effect of S2 stimulation on heat-evoked limb withdrawal latency was assessed in lightly anesthetized rats that were divided into three groups based on prior surgery and monofilament testing before induction of anesthesia: 1) sham-operated group and 2) hypersensitive and 3) nonhypersensitive (mechanically) SNL groups. In a group of hypersensitive SNL animals, a 5-HT1A receptor agonist was microinjected into the rostroventromedial medulla (RVM) to assess whether autoinhibition of serotonergic cell bodies blocks antinociception. Additionally, effect of S2 stimulation on pronociceptive ON-cells and antinociceptive OFF-cells in the RVM or nociceptive spinal wide dynamic range (WDR) neurons were assessed in anesthetized hypersensitive SNL animals. S2 stimulation induced antinociception in hypersensitive but not in nonhypersensitive SNL or sham-operated animals. Antinociception was prevented by a 5-HT1A receptor agonist in the RVM. Antinociception was associated with decreased duration of heat-evoked response in RVM ON-cells. In spinal WDR neurons, heat-evoked discharge was delayed by S2 stimulation, and this antinociceptive effect was prevented by blocking spinal 5-HT1A receptors. The results indicate that S2 stimulation suppresses nociception in SNL animals if SNL is associated with tactile allodynia-like hypersensitivity. In hypersensitive SNL animals, S2 stimulation induces antinociception mediated by medullospinal serotonergic pathways acting on the spinal 5-HT1A receptor, and partly through reduction of the RVM ON-cell discharge.NEW & NOTEWORTHY Stimulation of S2 cortex, but not that of an adjacent cortical area, induced descending heat antinociception in rats with the spinal nerve ligation-induced model of neuropathy. Antinociception was bilateral, and it involved suppression of pronociceptive medullary cells and activation of serotonergic pathways that act on the spinal 5-HT1A receptor. S2 stimulation failed to induce descending antinociceptive effect in sham-operated controls or in nerve-ligated animals that had not developed mechanical hypersensitivity.

Absence of paresthesia during high-rate spinal cord stimulation reveals importance of synchrony for sensations evoked by electrical stimulation.

Electrically activating mechanoreceptive afferents inhibits pain. However, paresthesia evoked by spinal cord stimulation (SCS) at 40-60 Hz becomes uncomfortable at high pulse amplitudes, limiting SCS "dosage." Kilohertz-frequency SCS produces analgesia without paresthesia and is thought, therefore, not to activate afferent axons. We show that paresthesia is absent not because axons do not spike but because they spike asynchronously. In a pain patient, selectively increasing SCS frequency abolished paresthesia and epidurally recorded evoked compound action potentials (ECAPs). Dependence of ECAP amplitude on SCS frequency was reproduced in pigs, rats, and computer simulations and is explained by overdrive desynchronization: spikes desychronize when axons are stimulated faster than their refractory period. Unlike synchronous spikes, asynchronous spikes fail to produce paresthesia because their transmission to somatosensory cortex is blocked by feedforward inhibition. Our results demonstrate how stimulation frequency impacts synchrony based on axon properties and how synchrony impacts sensation based on circuit properties.

TgF344-AD Rat Model of Alzheimer's Disease: Spatial Disorientation and Asymmetry in Hemispheric Neurodegeneration.

Background: The TgF344-AD ratline represents a transgenic animal model of Alzheimer's disease. We previously reported spatial memory impairment in TgF344-AD rats, yet the underlying mechanism remained unknown. We, therefore, set out to determine if spatial memory impairment in TgF344-AD rats is attributed to spatial disorientation. Also, we aimed to investigate whether TgF344-AD rats exhibit signs of asymmetry in hemispheric neurodegeneration, similar to what is reported in spatially disoriented AD patients. Finally, we sought to examine how spatial disorientation correlates with working memory performance. Methods: TgF344-AD rats were divided into two groups balanced by sex and genotype. The first group underwent the delayed match-to-sample (DMS) task for the assessment of spatial orientation and working memory, while the second group underwent positron emission tomography (PET) for the assessment of glucose metabolism and microglial activity as in-vivo markers of neurodegeneration. Rats were 13 months old during DMS training and 14-16 months old during DMS testing and PET. Results: In the DMS task, TgF344-AD rats were more likely than their wild-type littermates to display strong preference for one of the two levers, preventing working memory testing. Rats without lever-preference showed similar working memory, regardless of their genotype. PET revealed hemispherically asymmetric clusters of increased microglial activity and altered glucose metabolism in TgF344-AD rats. Conclusions: TgF344-AD rats display spatial disorientation and hemispherically asymmetrical neurodegeneration, suggesting a potential causal relationship consistent with past clinical research. In rats with preserved spatial orientation, working memory remains intact.

Amygdaloid administration of tetrapentylammonium attenuates development of pain and anxiety-like behavior following peripheral nerve injury.

BACKGROUND: The central amygdaloid nucleus (CeA) is involved in processing and descending regulation of pain. Amygdaloid mechanisms underlying pain processing and control are poorly known. Here we tested the hypothesis that perioperative CeA administration of tetrapentylammonium (TPA), a non-selective THIK-1 channel blocker and thereby inhibitor of microglia, attenuates development of chronic neuropathic pain and comorbid anxiety-like behavior. METHODS: Rats with a spared nerve injury (SNI) model of neuropathy or sham operation had a chronic cannula for drug microinjections into the CeA or a control injection site. Monofilament test was used to evaluate pain, and light-dark box (LDB) to assess anxiety. RESULTS: Perioperative CeA treatment with TPA (30 µg/day up to the third postoperative day, D3) significantly attenuated the development of pain and anxiety-like behavior. In the late phase (> D14), CeA administration of TPA (3-30 µg) failed to influence pain. Perioperative minocycline (microglia inhibitor; 25 µg), MK-801 (an N-Methyl-D-aspartate receptor antagonist; 0.1 µg), vehicle or TPA in a control injection site failed to attenuate pain development. CONCLUSIONS: Perioperative treatment of the CeA with TPA delayed development of neuropathic pain and comorbid anxiety-like behavior, while TPA treatment failed to influence maintenance of established neuropathic pain. The failures to attenuate pain development with CeA administrations of minocycline or MK-801 do not support the hypothesis that the TPA-induced prophylactic effect was due to inhibition of amygdaloid microglia or N-methyl-D-aspartate receptors. While TPA in the CeA proved to have a prophylactic effect on SNI-induced pain behavior, the underlying mechanism still remains to be studied.

Microglial pannexin-1 channel activation is a spinal determinant of joint pain.

Chronic joint pain such as mechanical allodynia is the most debilitating symptom of arthritis, yet effective therapies are lacking. We identify the pannexin-1 (Panx1) channel as a therapeutic target for alleviating mechanical allodynia, a cardinal sign of arthritis. In rats, joint pain caused by intra-articular injection of monosodium iodoacetate (MIA) was associated with spinal adenosine 5'-triphosphate (ATP) release and a microglia-specific up-regulation of P2X7 receptors (P2X7Rs). Blockade of P2X7R or ablation of spinal microglia prevented and reversed mechanical allodynia. P2X7Rs drive Panx1 channel activation, and in rats with mechanical allodynia, Panx1 function was increased in spinal microglia. Specifically, microglial Panx1-mediated release of the proinflammatory cytokine interleukin-1ß (IL-1ß) induced mechanical allodynia in the MIA-injected hindlimb. Intrathecal administration of the Panx1-blocking peptide 10panx suppressed the aberrant discharge of spinal laminae I-II neurons evoked by innocuous mechanical hindpaw stimulation in arthritic rats. Furthermore, mice with a microglia-specific genetic deletion of Panx1 were protected from developing mechanical allodynia. Treatment with probenecid, a clinically used broad-spectrum Panx1 blocker, resulted in a striking attenuation of MIA-induced mechanical allodynia and normalized responses in the dynamic weight-bearing test, without affecting acute nociception. Probenecid reversal of mechanical allodynia was also observed in rats 13 weeks after anterior cruciate ligament transection, a model of posttraumatic osteoarthritis. Thus, Panx1-targeted therapy is a new mechanistic approach for alleviating joint pain.

The medullary dorsal reticular nucleus as a relay for descending pronociception induced by the mGluR5 in the rat infralimbic cortex.

Metabotropic glutamate receptor 5 (mGluR5) activation in the infralimbic cortex (IL) induces pronociceptive behavior in healthy and monoarthritic rats. Here we studied whether the medullary dorsal reticular nucleus (DRt) and the spinal TRPV1 are mediating the IL/mGluR5-induced spinal pronociception and whether the facilitation of pain behavior is correlated with changes in spinal dorsal horn neuron activity. For drug administrations, all animals had a cannula in the IL as well as a cannula in the DRt or an intrathecal catheter. Heat-evoked paw withdrawal was used to assess pain behavior in awake animals. Spontaneous and heat-evoked discharge rates of single DRt neurons or spinal dorsal horn wide-dynamic range (WDR) and nociceptive-specific (NS) neurons were evaluated in lightly anesthetized animals. Activation of the IL/mGluR5 facilitated nociceptive behavior in both healthy and monoarthritic animals, and this effect was blocked by lidocaine or GABA receptor agonists in the DRt. IL/mGluR5 activation increased spontaneous and heat-evoked DRt discharge rates in healthy but not monoarthritic rats. In the spinal dorsal horn, IL/mGluR5 activation increased spontaneous activity of WDR neurons in healthy animals only, whereas heat-evoked responses of WDR and NS neurons were increased in both experimental groups. Intrathecally administered TRPV1 antagonist prevented the IL/mGluR5-induced pronociception in both healthy and monoarthritic rats. The results suggest that the DRt is involved in relaying the IL/mGluR5-induced spinal pronociception in healthy control but not monoarthritic animals. Spinally, the IL/mGluR5-induced behavioral heat hyperalgesia is mediated by TRPV1 and associated with facilitated heat-evoked responses of WDR and NS neurons.

Regulation of neuropathic pain behavior by amygdaloid TRPC4/C5 channels.

Pain per se may increase anxiety and conversely, anxiety may increase pain. Therefore, a positive feedback loop between anxiety and pain possibly contributes to pain and suffering in some pathophysiological pain conditions, such as that induced by peripheral nerve injury. Recent results indicate that transient receptor channels 4 and 5 (TRPC4/C5) in the amygdala have anxiogenic effects in rodents, while their role in chronic pain conditions is not known. Here, we studied whether the amygdaloid TRPC4/C5 that are known to have anxiogenic properties contribute to the maintenance of sensory or affective aspects of pain in an experimental model of peripheral neuropathy. Rats with a spared nerve injury (SNI) model of neuropathy in the left hind limb had a chronic cannula for microinjections of drugs into the right amygdala or the internal capsule (a control site). Sensory pain was assessed by determining mechanical hypersensitivity with calibrated monofilaments and affective pain by determining aversive place-conditioning. Amygdaloid treatment with ML-204, a TRPC4/C5 antagonist, produced a dose-related (5-10 µg) antihypersensitivity effect, without obvious side-effects. Additionally, amygdaloid administration of ML-204 reduced affective-like pain behavior. In the internal capsule, ML-204 had no effect on hypersensitivity or affective-like pain in SNI animals. In healthy controls, amygdaloid administration of ML-204 failed to influence pain behavior induced by mechanical stimulation or noxious heat. The results indicate that the amygdaloid TRPC4/C5 contribute to maintenance of pain hypersensitivity and pain affect in neuropathy.

Descending effect on spinal nociception by amygdaloid glutamate varies with the submodality of noxious test stimulation.

Amygdala has an important role in the processing of primary emotions, such as fear. Additionally, amygdala is involved in processing and modulation of pain. While the amygdala, particularly its central nucleus (CeA), has been shown to contribute to pain control, the descending pain regulation by the CeA is still only partly characterized. Here heat and mechanical nociception was tested in both hind limbs of healthy rats with a chronic guide cannula for microinjection of glutamate into the CeA of the left or right hemisphere. The aim was to assess whether the descending pain regulatory effect by glutamate in the amygdala varies with the submodality or the body side of nociceptive testing, brain hemisphere or the amygdaloid glutamate receptor. Motor performance was assessed with the Rotarod test. Amygdaloid glutamate, independent of the treated hemisphere, produced a dose-related heat and mechanical antinociception that varied with the submodality of testing. Heat antinociception was short lasting (minutes), bilateral and not reversed by blocking the amygdaloid NMDA receptor with MK-801. In contrast, mechanical antinociception lasted longer (>20 min), was predominantly contralateral and reversed by blocking the amygdaloid NMDA receptor. At an antinociceptive dose, amygdaloid glutamate failed to influence motor performance. The results indicate that independent of the brain hemisphere, the spatial extent and duration of the descending antinociceptive effect induced by amygdaloid glutamate varies with the amygdaloid glutamate receptor and the submodality of pain.