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An animal model of voluntary oral morphine consumption would allow for a pre-clinical evaluation of new treatments aimed at reducing opioid intake in humans. However, the main limitation of oral morphine consumption in rodents is its bitter taste, which is strongly aversive. Taste aversion is often overcome by the use of adulterants, such as sweeteners, to conceal morphine taste or bitterants in the alternative bottle to equalize aversion. However, the adulterants' presence is the cause for consumption choice and, upon removal, the preference for morphine is not preserved. Thus, current animal models are not suitable to study treatments aimed at reducing consumption elicited by morphine itself. Since taste preference is a learned behavior, just-weaned rats were trained to accept a bitter taste, adding the bitterant quinine to their drinking water for one week. The latter was followed by allowing the choice of quinine or morphine (0.15 mg/mL) solutions for two weeks. Then, quinine was removed, and the preference for morphine against water was evaluated. Using this paradigm, we show that rats highly preferred the consumption of morphine over water, reaching a voluntary morphine intake of 15 mg/kg/day. Morphine consumption led to significant analgesia and hyperlocomotion, and to a marked deprivation syndrome following the administration of the opioid antagonist naloxone. Voluntary morphine consumption was also shown to generate brain oxidative stress and neuroinflammation, signs associated with opioid dependence development. We present a robust two-bottle choice animal model of oral morphine self-administration for the evaluation of therapeutic interventions for the treatment of morphine dependence.
Learn More >Tropisetron is an alpha 7 nicotinic acetylcholine receptor (α7nAChR) agonist and is a commonly used antiemetic clinically. α7nAChRs activation modulating nociception transmissions and cholinergic anti-inflammation may decrease neuropathic pain. This study was set to investigate the effects of tropisetron on neuropathic pain and neuroinflammation as well as the underlying mechanisms in rats.
Learn More >Migraine is a complex neurovascular disorder that is one of the leading causes of disability and a reduced quality of life. Even with such a high societal impact, our understanding of the cellular and molecular mechanisms that contribute to migraine headaches are poorly understood. To address this complex disorder, several groups have performed genome-wide associated studies (GWAS) to elucidate migraine susceptibility genes, with many identifying TRPM8, a cold-sensitive cation channel expressed in peripheral afferents innervating the trigeminovascular system, and the principal mediator of cold and cold pain associated with injury and disease. Interestingly, these migraine-associated single nucleotide polymorphisms (SNPs) reside in noncoding regions of TRPM8, with those correlated with reduced migraine risk exhibiting lower TRPM8 expression and decreased cold sensitivity. Nonetheless, as a role for TRPM8 in migraine has yet to be defined, we sought to address this gap in our knowledge using mouse genetics and TRPM8 antagonism to determine if TRPM8 channels or neurons are required for migraine-like pain (mechanical allodynia and facial grimace) in inducible migraine models. Our results show that both evoked and spontaneous pain behaviors are dependent on both TRPM8 channels and neurons, as well as required in both acute and chronic migraine models. Moreover, inhibition of TRPM8 channels prevented acute but not established chronic migraine-like pain. However, chronic pain could not be prevented with TRPM8 inhibition. These results are consistent with its association with migraine in genetic analyses and establish that TRPM8 channels are a component of the underlying mechanisms of migraine.
Learn More >Visceral hypersensitivity is a major clinic symptom in patients with irritable bowel syndrome (IBS). Anterior cingulate cortex (ACC) is involved in processing the information of pain. Both G protein-coupled receptor kinase 6 (GRK6) and P2Y purinoceptor 6 (P2Y6) are associated with neuroinflammation and pathological pain. The aim of this study was to investigate the interaction between GRK6 and P2Y6 in ACC in the development of visceral hypersensitivity of adult offspring rats with prenatal maternal stress (PMS).
Learn More >Trigeminal neuralgia (TN) is a severe form of neuropathic pain frequently associated with anxiety. The chronic constriction injury of the infraorbital nerve (CCI-ION) of rodents is a well-established model to study sensory alterations related to TN. However, few studies have addressed the emotional component of pain, which is fundamental to increase its translational capability. Emission of ultrasonic vocalization (USV) is considered a reliable measure of the emotional state of rats. Rats emit 50-kHz USVs in social and appetitive situations, whereas 22-kHz USVs may index a negative state. Studies suggest that persistent pain causes reduction in 50-kHz calls, but this may also indicate anxiety-like behavior. Thus, we hypothesize that CCI-ION would decrease 50-kHz calls and that pharmacological pain relief would restore USVs, without interfering with anxiety-like behavior. On day 15 after surgery, male rats were treated with local lidocaine, midazolam or carbamazepine to determine their effect on facial mechanical hyperalgesia, USV and anxiety-like behavior. The results showed that CCI-ION induced hyperalgesia, which was attenuated by lidocaine or carbamazepine, developed anxiety-like behavior, which was reduced only by midazolam, and displayed a reduced number of 50-kHz calls, compared to sham. Lidocaine and carbamazepine increased 50-kHz calls emitted by CCI-ION rats, but midazolam failed to change them. These data add information on the translational aspects of CCI-ION model and carbamazepine treatment for trigeminal neuropathic pain. Furthermore, they suggest that the reduction of USV in persistent pain conditions is related to spontaneous pain and reinforce the idea that it reflects the emotional component of pain.
Learn More >Neuropathic pain is a chronic and sometimes intractable condition caused by lesions or diseases of the somatosensory nervous system. Many drugs are available but unfortunately do not provide satisfactory effects in patients, producing limited analgesia and undesirable side effects. Thus, there is an urgent need to develop new pharmaceutical agents to treat neuropathic pain. To date, highly specific agents that modulate a single target, such as receptors or ion channels, never progress to the clinic, which may reflect the diverse etiologies of neuropathic pain seen in the human patient population. Therefore, the development of multifunctional compounds exhibiting two or more pharmacological activities is an attractive strategy for addressing unmet medical needs for the treatment of neuropathic pain. To develop novel multifunctional compounds, key pharmacophores of currently used clinical pain drugs, including pregabalin, fluoxetine and serotonin analogs, were hybridized to the side chain of tianeptine, which has been used as an antidepressant. The biological activities of the hybrid analogs were evaluated at the human transporters of neurotransmitters, including serotonin (hSERT), norepinephrine (hNET) and dopamine (hDAT), as well as mu (μ) and kappa (κ) opioid receptors. The most advanced hybrid of these multifunctional compounds, , exhibited multiple transporter inhibitory activities for the uptake of neurotransmitters with IC values of 70 nM, 154 nM and 2.01 μM at hSERT, hNET and hDAT, respectively. Additionally, compound showed partial agonism (EC = 384 nM) at the μ-opioid receptor with no influence at the κ-opioid receptor. In in vivo pain animal experiments, the multifunctional compound showed significantly reduced allodynia in a spinal nerve ligation (SNL) model by intrathecal administration, indicating that multitargeted strategies in single therapy could considerably benefit patients with multifactorial diseases, such as pain.
Learn More >Inflammation is a natural response of the organism to an infection, trauma, or cellular stress. Pain is the first symptom of acute and chronic inflammation. The standard class of medication to treat inflammatory pain is the nonsteroidal anti-inflammatory drug (NSAID). These drugs are associated with severe side effects such as gastric ulcers, gastritis, or internal bleeding. One of NSAIDs, Dipyrone® (metamizole) is largely used in many European and South American countries despite its dubious effectivity and its withdrawal from the market of several countries. Here, aiming to identify a new anti-inflammatory drug prototype based on Dipyrone® structure, a set of 27 molecules were virtually screened, and 4 compounds containing the active metabolite 4-aminoantipyrine and 1,4-dioxo-2-butenyl fragment were selected for docking, synthesis, and biological evaluation. The selection was based on the number of H-bonds and π- π stacking interactions between the inhibitor and the amino acids within the binding site of the enzyme. Carrageenan-induced paw edema, acetic acid-induced writhing, and formalin assays were used to evaluate inflammation and pain response. The selected compounds 1-4 inhibited the involvement of biogenic amines in the formation of paw edema. Compounds 1-4 also reduced pain in the inflammatory response phase. It has to be noted that 4-AA may cause agranulocytosis, which should be borne in mind when developing drug candidates of similar structure. Our new drug prototypes based on 4-aminoantipyrine and 1,4-dioxo-2-butenyl moieties open a gate for developing a prototype of nonsteroidal anti-inflammatory drugs.
Learn More >Pain strongly affects public health, both because of the patient suffering and the socioeconomic impact. The available drugs for pain treatment are not fully effective and have many adverse effects. Therefore, there is a need to obtain new analgesic compounds. This study evaluated the antinociceptive effect of N-(3-(phenylselanyl)prop-2-yn-1-yl)benzamide (SePB), an organoselenium compound containing the benzamide moiety, through time (15-120 min) and dose-response (1-50 mg/kg) curves in thermal and chemical mice models of nociception, as well as the involvement of the serotonergic system in this effect. The open-field test (OFT) was carried out to assess locomotor activity. SePB (10 mg/kg) induced an increase in the latency to nociception response in the tail immersion test from 30 min. In the dose-response curves, SePB at different doses reduced latency time to nociceptive response in the tail immersion and hot plate tests, and reduced the licking time in the glutamate test, demonstrating antinociceptive effect, without altering the locomotor activity of mice. WAY100635 (0.5 mg/kg, subcutaneously, a 5-HT receptor antagonist), ketanserin (0.3 mg/kg, intraperitoneally, a 5-HT receptor antagonist), but not ondansetron (0.5 mg/kg, intraperitoneally, a 5-HT receptor antagonist), administered 15 min before SePB, prevented the increased latency to nociceptive response induced by SePB in the tail immersion test, demonstrating that 5-HT and 5-HT receptors are involved in the antinociceptive effect of SePB. Upon more studies evaluating SePB antinociceptive effects in chronic pain models and its toxicity, this compound could be indicated as an interesting molecule to treat pain.
Learn More >Hepatocyte growth factor (HGF) is a secretory protein that is involved in various biological activities such as angiogenesis, neuroprotection, and anti-inflammatory effects. Intramuscular injection of an HGF-encoding plasmid DNA (pCK-HGF-X7) has been shown to produce pain-relieving effects in a rodent model and patients with neuropathic pain.To further investigate the underlying mechanism, we investigated the anti-inflammatory effects of HGF in the context of neuropathic pain. Consistent with previous data, intramuscular injection of pCK-HGF-X7 showed pain relieving effects up to 8 weeks and pharmacological blockade of the c-Met receptor hindered this effect, which suggest that the analgesic effect was c-Met receptor-dependent. At the histological level, macrophage infiltration in the dorsal root ganglion (DRG) was significantly decreased in the pCK-HGF-X7 injected group. Moreover, HGF treatment significantly downregulated the LPS-mediated induction of pro-inflammatory cytokines in primary cultured DRG neurons. Taken together, these data suggest that HGF-encoding plasmid DNA attenuates neuropathic pain via controlling the expression of pro-inflammatory cytokines.
Learn More >Sensitization of trigeminal ganglion neurons contributes to primary headache disorders such as migraine, but the specific neuronal and non-neuronal trigeminal subtypes that are involved remain unclear. We thus developed a cell atlas in which human and mouse trigeminal ganglia are transcriptionally and epigenomically profiled at single-cell resolution. These data describe evolutionarily conserved and human-specific gene expression patterns within each trigeminal ganglion cell type, as well as the transcription factors and gene regulatory elements that contribute to cell-type-specific gene expression. We then leveraged these data to identify trigeminal ganglion cell types that are implicated both by human genetic variation associated with migraine and two mouse models of headache. This trigeminal ganglion cell atlas improves our understanding of the cell types, genes, and epigenomic features involved in headache pathophysiology and establishes a rich resource of cell-type-specific molecular features to guide the development of more selective treatments for headache and facial pain.
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