Transcriptomic signature, bioactivity and safety of a non-hepatotoxic analgesic generating AM404 in the midbrain PAG region.

Safe and effective pain management is a critical healthcare and societal need. The potential for acute liver injury from paracetamol (ApAP) overdose; nephrotoxicity and gastrointestinal damage from chronic non-steroidal anti-inflammatory drug (NSAID) use; and opioids' addiction are unresolved challenges. We developed SRP-001, a non-opioid and non-hepatotoxic small molecule that, unlike ApAP, does not produce the hepatotoxic metabolite N-acetyl-p-benzoquinone-imine (NAPQI) and preserves hepatic tight junction integrity at high doses. CD-1 mice exposed to SRP-001 showed no mortality, unlike a 70% mortality observed with increasing equimolar doses of ApAP within 72 h. SRP-001 and ApAP have comparable antinociceptive effects, including the complete Freund's adjuvant-induced inflammatory von Frey model. Both induce analgesia via N-arachidonoylphenolamine (AM404) formation in the midbrain periaqueductal grey (PAG) nociception region, with SRP-001 generating higher amounts of AM404 than ApAP. Single-cell transcriptomics of PAG uncovered that SRP-001 and ApAP also share modulation of pain-related gene expression and cell signaling pathways/networks, including endocannabinoid signaling, genes pertaining to mechanical nociception, and fatty acid amide hydrolase (FAAH). Both regulate the expression of key genes encoding FAAH, 2-arachidonoylglycerol (2-AG), cannabinoid receptor 1 (CNR1), CNR2, transient receptor potential vanilloid type 4 (TRPV4), and voltage-gated Ca2+ channel. Phase 1 trial (NCT05484414) (02/08/2022) demonstrates SRP-001's safety, tolerability, and favorable pharmacokinetics, including a half-life from 4.9 to 9.8 h. Given its non-hepatotoxicity and clinically validated analgesic mechanisms, SRP-001 offers a promising alternative to ApAP, NSAIDs, and opioids for safer pain treatment.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation.

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Dopamine injections to the midbrain periaqueductal gray inhibit vocal-motor production in a teleost fish.

Across vertebrates, the midbrain periaqueductal gray (PAG) plays a critical role in social and vocal behavior. Dopaminergic neurotransmission also modulates these behaviors, and dopaminergic innervation of the PAG has been well documented. Nonetheless, the potential role of dopamine in shaping vocal production at the level of the PAG is not well understood. Here, we tested the hypothesis that dopamine modulates vocal production in the PAG, using a well-characterized vertebrate model system for the study of vocal communication, the plainfin midshipman fish, Porichthys notatus. We found that focal dopamine injections to the midshipman PAG rapidly and reversibly inhibited vocal production triggered by stimulation of known vocal-motor structures in the preoptic area / anterior hypothalamus. While dopamine inhibited vocal-motor output, it did not alter behaviorally-relevant parameters of this output, such as vocalization duration and frequency. Dopamine-induced inhibition of vocal production was prevented by the combined blockade of D1- and D2-like receptors but was unaffected by isolated blockade of either D1-receptors or D2-receptors. Our results suggest dopamine neuromodulation in the midshipman PAG may inhibit natural vocal behavior, in courtship and/or agonistic social contexts.

Fluorocitrate and neurotropin confer analgesic effects on neuropathic pain in diabetic rats via inhibition of astrocyte activation in the periaqueductal gray.

Currently, effective treatments for diabetic neuropathic pain (DNP) are still unmet clinical needs. Activation of astrocytes in the ventrolateral region of periaqueductal gray (vlPAG) has a regulating effect on pain responses. The present study was designed to confirm that repeated intra-vlPAG injection of fluorocitrate (FC), a selective inhibitor of astrocyte activation or intraperitoneal (IP) injection of neurotropin, a widely prescribed analgesic drug for chronic pain, inhibited the activation of astrocytes in vlPAG and thus produced an analgesic effect on DNP. An in vivo model was developed to study DNP in rats. The changes in mechanical withdrawal threshold (MWT) and activation levels of astrocytes in the vlPAG were evaluated in all experimental rats. Compared with normal rats, vlPAG-based glial fibrillary acid protein (GFAP) was clearly upregulated, whereas the MWTs of DNP rats were markedly diminished. The intra-vlPAG injections of FC or IP injections of neurotropin attenuated the alterations both in MWTs and expression levels of GFAP in vlPAG in DNP rats. Collectively, these findings suggest the antinociceptive effects of FC and neurotropin in DNP rats, which were associated with suppressing the activation of astrocytes in vlPAG.

Enhanced responsiveness to hypoxic panicogenic challenge in female rats in late diestrus is suppressed by short-term, low-dose fluoxetine: Involvement of the dorsal raphe nucleus and the dorsal periaqueductal gray.

BACKGROUND: Acute hypoxia, which is panicogenic in humans, also evokes panic-like behavior in male rats. Panic disorder is more common in women and susceptibility increases during the premenstrual phase of the cycle. AIMS: We here investigated for the first time the impact of hypoxia on the expression of panic-like escape behavior by female rats and its relationship with the estrous cycle. We also evaluated functional activation of the midbrain panic circuitry in response to this panicogenic stimulus and whether short-term, low-dose fluoxetine treatment inhibits the hyper-responsiveness of females in late diestrus. METHODS: Male and female Sprague Dawley rats were exposed to 7% O2. Females in late diestrus were also tested after short-term treatment with fluoxetine (1.75 or 10 mg/kg, i.p.). Brains were harvested and processed for c-Fos and tryptophan hydroxylase immunoreactivity in the periaqueductal gray matter (PAG) and dorsal raphe nucleus (DR). RESULTS: Acute hypoxia evoked escape in both sexes. Overall, females were more responsive than males and this is clearer in late diestrus phase. In both sexes, hypoxia induced functional activation (c-Fos expression) in non-serotonergic cells in the lateral wings of the DR and dorsomedial PAG, which was greater in late diestrus than proestrus (lowest behavioral response to hypoxia). Increased responding in late diestrus (behavioral and cellular levels) was prevented by 1.75, but not 10 mg/kg fluoxetine. DISCUSSION: The response of female rats to acute hypoxia models panic behavior in women. Low-dose fluoxetine administered in the premenstrual phase deserves further attention for management of panic disorders in women.

Tumor necrosis factor-α modulates GABAergic and dopaminergic neurons in the ventrolateral periaqueductal gray of female mice.

Neuroimmune signaling is increasingly identified as a critical component of various illnesses, including chronic pain, substance use disorder, and depression. However, the underlying neural mechanisms remain unclear. Proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), may play a role by modulating synaptic function and long-term plasticity. The midbrain structure periaqueductal gray (PAG) plays a well-established role in pain processing, and although TNF-α inhibitors have emerged as a therapeutic strategy for pain-related disorders, the impact of TNF-α on PAG neuronal activity has not been thoroughly characterized. Recent studies have identified subpopulations of ventrolateral PAG (vlPAG) with opposing effects on nociception, with dopamine (DA) neurons driving pain relief in contrast to GABA neurons. Therefore, we used slice physiology to examine the impact of TNF-α on neuronal activity of both these subpopulations. We focused on female mice since the PAG is a sexually dimorphic region and most studies use male subjects, limiting our understanding of mechanistic variations in females. We selectively targeted GABA and DA neurons using transgenic reporter lines. Following exposure to TNF-α, there was an increase in excitability of GABA neurons along with a reduction in glutamatergic synaptic transmission. In DA neurons, TNF-α exposure resulted in a robust decrease in excitability along with a modest reduction in glutamatergic synaptic transmission. Interestingly, TNF-α had no effect on inhibitory transmission onto DA neurons. Collectively, these data suggest that TNF-α differentially affects the function of GABA and DA neurons in female mice and enhances our understanding of how TNF-α-mediated signaling modulates vlPAG function.NEW & NOTEWORTHY This study describes the effects of TNF-α on two distinct subpopulations of neurons in the vlPAG. We show that TNF-α alters both neuronal excitability and glutamatergic synaptic transmission on GABA and dopamine neurons within the vlPAG of female mice. This provides critical new information on the role of TNF-α in the potential modulation of pain, since activation of vlPAG GABA neurons drives nociception, whereas activation of dopamine neurons drives analgesia.

Morphine-induced kinase activation and localization in the periaqueductal gray of male and female mice.

Morphine is a potent opioid analgesic with high propensity for the development of antinociceptive tolerance. Morphine antinociception and tolerance are partially regulated by the midbrain ventrolateral periaqueductal gray (vlPAG). However, the majority of research evaluating mu-opioid receptor signaling has focused on males. Here, we investigate kinase activation and localization patterns in the vlPAG following acute and chronic morphine treatment in both sexes. Male and female mice developed rapid antinociceptive tolerance to morphine (10 mg/kg i.p.) on the hot plate assay, but tolerance did not develop in males on the tail flick assay. Quantitative fluorescence immunohistochemistry was used to map and evaluate the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2), protein kinase-C (PKC), and protein kinase-A (PKA). We observed significantly greater phosphorylated ERK 1/2 in the vlPAG of chronic morphine-treated animals which co-localized with the endosomal marker, Eea1. We note that pPKC is significantly elevated in the vlPAG of both sexes following chronic morphine treatment. We also observed that although PKA activity is elevated following chronic morphine treatment in both sexes, there is a significant reduction in the nuclear translocation of its phosphorylated substrate. Taken together, this study demonstrates increased activation of ERK 1/2, PKC, and PKA in response to repeated morphine treatment. The study opens avenues to explore the impact of chronic morphine treatment on G-protein signaling and kinase nuclear transport.

Blockade of GPR55 in dorsal periaqueductal gray produces anxiety-like behaviors and evocates defensive aggressive responses in alcohol-pre-exposed rats.

GPR55 is a receptor expressed in several central nervous system areas, including the periaqueductal gray (PAG). Current knowledge of GPR55 physiology in PAG only covers pain integration, but it is involved in other actions such as anxiety, panic, motivated behaviors, and alcohol intake. In the present study, juvenile male Wistar rats were unexposed (alcohol-naïve group; A-naïve) or exposed to alcohol for 5 weeks (alcohol-pre-exposed group; A-pre-exposed). Posteriorly, animals received intra dorsal-PAG (D-PAG) injections of vehicle (10% DMSO), LPI (1 nmol/0.5 µl) and ML-193 (1 nmol/0.5 µl, a selective GPR55 antagonist). Finally, defensive burying behavior (DBB) paradigm and alcohol preference were evaluated. Compared to the A-naïve group, the A-pre-exposed vehicle group had higher (p < 0.05): (i) time of immobility; (ii) latency to and duration of burying; and (iii) alcohol consumption. In both groups (i.e., A-naïve and A-pre-exposed) treatment with LPI: (i) decreased duration of burying (p < 0.05); (ii) suppressed time of immobility; and (iii) increased alcohol intake (p < 0.05). On the other hand, treatment with ML-193: (i) decreased duration of immobility in A-pre-exposed (but not in A-naïve rats); (ii) promoted an aggressive response against the shock-probe in A-pre-exposed rats (p < 0.05); and (iii) increased alcohol intake (p < 0.05). Our results suggest that blockade of GPR55 in D-PAG is associated with anxiety-like behaviors, defensive aggressive behaviors, and higher alcohol intake, whereas LPI in D-PAG produced anxiolytic-like effects (probably GPR55-mediated), but not prevention of alcohol intake.

The primary motor cortex electrical and chemical stimulation attenuates the chronic neuropathic pain by activation of the periaqueductal grey matter: The role of NMDA receptors.

BACKGROUND: Motor cortex stimulation (MCS) is proper as a non-pharmacological therapy for patients with chronic and neuropathic pain (NP). AIMS: This work aims to investigate if the MCS in the primary motor cortex (M1) produces analgesia and how the MCS could interfere in the MCS-induced analgesia. Also, to elucidate if the persistent activation of N-methyl-d-aspartic acid receptor (NMDAr) in the periaqueductal grey matter (PAG) can contribute to central sensitisation of the NP. METHODS: Male Wistar rats were submitted to the von Frey test to evaluate the mechanical allodynia after 21 days of chronic constriction injury (CCI) of the sciatic nerve. The MCS was performed with low-frequency (20 µA, 100 Hz) currents during 15 s by a deep brain stimulation (DBS) device. Moreover, the effect of M1-treatment with an NMDAr agonist (at 2, 4, and 8 nmol) was investigated in CCI rats. The PAG dorsomedial column (dmPAG) was pretreated with the NMDAr antagonist LY 235959 (at 8 nmol), followed by MCS. RESULTS: The MCS decreased the mechanical allodynia in rats with chronic NP. The M1-treatment with an NMDA agonist at 2 and 8 nmol reduced the mechanical allodynia in CCI rats. In addition, dmPAG-pretreatment with LY 235959 at 8 nmol attenuated the mechanical allodynia evoked by MCS. CONCLUSION: The M1 cortex glutamatergic system is involved in the modulation of chronic NP. The analgesic effect of MCS may depend on glutamate signaling recruitting NMDAr located on PAG neurons in rodents with chronic NP.

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury.

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Effects of Mild and Moderate Monoclonal Antibody Dose on Inflammation, Bone Loss, and Activation of the Central Nervous System in a Female Collagen Antibody-induced Arthritis Mouse Model.

Induction of severe inflammatory arthritis in the collagen antibody-induced arthritis (CAIA) murine model causes extensive joint damage and pain-like behavior compromising analysis. While mild models are less severe, their reduced, variable penetrance makes assessment of treatment efficacy difficult. This study aimed to compare macroscopic and microscopic changes in the paws, along with central nervous system activation between a mild and moderate CAIA model. Balb/c mice (n=18) were allocated to control, mild, and moderate CAIA groups. Paw inflammation, bone volume (BV), and paw volume (PV) were assessed. Histologically, the front paws were assessed for joint inflammation, cartilage damage, and pre/osteoclast-like cells and the lumbar spinal cord and the periaqueductal gray (PAG) region of the brain for glial reactivity. A moderate CAIA dose induced (1) significantly greater local paw inflammation, inflammatory cell infiltration, and PV; (2) significantly more osteoclast-like cells on the bone surface and within the surrounding soft tissue; and (3) significantly greater glial reactivity within the PAG compared with the mild CAIA model. These findings support the use of a moderate CAIA model (higher dose of monoclonal antibodies with low-dose lipopolysaccharide) to induce more consistent histopathological features, without excessive joint destruction.

Donepezil prevents morphine tolerance by regulating N-methyl-d-aspartate receptor, protein kinase C and CaM-dependent kinase II expression in rats.

Current studies have indicated that donepezil as a cholinesterase inhibitor can attenuate morphine-induced tolerance. The present study aimed to evaluate the possible role of N-methyl-d-aspartate receptors (NMDARs), protein kinase C (PKC) and CaM-dependent kinase II (CaMKII) pathways in this effect. Female Wistar rats received daily morphine (10 mg/kg, i.p.) alone or in combination with donepezil (1.5 or 2 mg/kg, gavaged) for 14 days. The analgesic effect was assessed by Von-frey, hotplate and tail flick test. On the 15th day, the periaqueductal gray (PAG) and lumbar spinal cord of rats were dissected. Then, protein levels of NMDAR-NR1, NR2B, PKCγ and CaMKIIα were tested using Western blot method. The results showed that morphine tolerance was seen after 8-10 days of injection compared with control group, while daily co-administration of donepezil with morphine prolonged the occurrence of analgesic tolerance. Western blot showed that morphine significantly increased NR1, PKCγ and CaMKIIα expressions in PAG, and significantly increased PKCγ and CaMKIIα in spinal cord. In contrast, donepezil downregulated NR1 and PKCγ in PAG, and downregulated PKCγ and CaMKIIα in spinal cord. Moreover, donepezil alone activates NR1 and NR2B in spinal cord, which needs to be further studied. Thus, the present results suggest that the attenuation effects of donepezil on morphine tolerance are possibly mediated by preventing morphine-induced upregulations in NR1, PKCγ and CaMKIIα expressions.

Neurons and Astrocytes in Ventrolateral Periaqueductal Gray Contribute to Restraint Water Immersion Stress-Induced Gastric Mucosal Damage via the ERK1/2 Signaling Pathway.

BACKGROUND: The restraint water immersion stress (RWIS) model includes both psychological and physical stimulation, which may lead to gastrointestinal disorders and cause gastric mucosal damage. The ventrolateral periaqueductal gray (VLPAG) contributes to gastrointestinal function, but whether it is involved in RWIS-induced gastric mucosal damage has not yet been reported. METHODS: The expression of glial fibrillary acidic protein, neuronal c-Fos, and phosphorylated extracellular signal regulated kinase 1/2 in the VLPAG after RWIS was assessed using western blotting and immunocytochemical staining methods. Lateral ventricle injection of astrocytic toxin L-a-aminoadipate and treatment with extracellular signal-regulated kinase (ERK)1/2 signaling pathway inhibitor PD98059 were further used to study protein expression and distribution in the VLPAG after RWIS. RESULTS: The expression of c-Fos, glial fibrillary acidic protein, and phosphorylated extracellular signal regulated kinase 1/2 in the VLPAG significantly increased following RWIS and peaked at 1 hour after RWIS. Lateral ventricle injection of the astrocytic toxin L-a-aminoadipate significantly alleviated gastric mucosal injury and decreased the activation of neurons and astrocytes. Treatment with the ERK1/2 signaling pathway inhibitor PD98059 obviously suppressed gastric mucosal damage as well as the RWIS-induced activation of neurons and astrocytes in the VLPAG. CONCLUSIONS: These results suggested that activation of VLPAG neurons and astrocytes induced by RWIS through the ERK1/2 signaling pathway may play a critical role in RWIS-induced gastric mucosa damage.

Minocycline alleviates the symptoms of morphine withdrawal via the CaMKII-Ras-ERK signaling pathway.

OBJECTIVE: To investigate the effect of minocycline on morphine withdrawal symptoms. METHODS: We established a rat model of morphine dependence, then injected the animals with naloxone to induce withdrawal symptoms. Minocycline was injected into the midbrain periaqueductal gray and its effect on withdrawal symptoms and Ca2+-dependent protein kinase (CaMKII), Ras, and phospho-extracellular signal-regulated kinase (p-ERK) expression was observed. RESULTS: Minocycline inhibited withdrawal symptoms such as "wet dog" shakes, teeth chatter, and ptosis, perhaps by inhibiting the activation of microglia and the expression of CaMKII, Ras, and p-ERK. Minocycline had no effect on the behavior of control rats or on CaMKII, Ras, or p-ERK expression. CONCLUSION: Minocycline alleviates morphine withdrawal symptoms by inhibiting the activation of microglia and downregulating the expression of CaMKII, Ras, and p-ERK.

GPR55 in the brain and chronic neuropathic pain.

There is a clear need for novel and improved therapeutic strategies for alleviating chronic neuropathic pain, as well as a need for better understanding of brain mechanisms of neuropathic pain, which are less understood than spinal and peripheral mechanisms. The G protein-coupled receptor 55 (GPR55), is a lysophosphatidylinositol (LPI)-sensitive receptor that has also been involved in cannabinoid signaling. It is expressed throughout the central nervous system, including the periaqueductal gray (PAG), a brainstem area and key element of the descending pain control system. Behaviors, pharmacology, biochemistry tools, and stereotaxic microinjections were used to determine if GPR55 plays a role in pain control in a chronic constriction injury (CCI) neuropathic pain model in rats. It was found that the blockade of GPR55 action in the PAG can restore and drive a descending control system to mitigate neuropathic pain. Our data demonstrate that GPR55 play a role in the descending pain control system, and identify GPR55 at supraspinal level as a neuropathic pain brain mechanism.

Neuropeptide W, an endogenous NPBW1 and NPBW2 ligand, produced an analgesic effect via activation of the descending pain modulatory system during a rat formalin test.

Neuropeptide W (NPW) messenger ribonucleic acid (mRNA) and NPBW1 and/or NPBW2 mRNA are expressed in the descending pain inhibitory system. In the present study, we examined whether NPW microinjected into the descending pain inhibitory system, such as the periaqueductal gray (PAG), locus coeruleus (LC), and rostral ventromedial medulla (RVM), produces an analgesic effect using a rat formalin test. Microinjections of NPW into the PAG ipsilateral and contralateral to the formalin-injected side, LC ipsilateral and contralateral to the formalin-injected side, and RVM produced an analgesic effect. In the RVM study, the analgesic effect was antagonized by WAY100135, a 5-HT1A antagonist, and enhanced by prazosin, an α1 antagonist, and SB269970, a 5-HT7 antagonist. Naloxone, an opioid antagonist, also antagonized the effect of NPW in the RVM study. In the ipsilateral LC study, the analgesic effect was antagonized by WAY100135, idazoxan, an α2 antagonist, and naloxone and was enhanced by prazosin and SB269970. In the contralateral LC study, the analgesic effect was antagonized by prazosin, idazoxan, SB269970, and naloxone. The analgesic effect was antagonized by WAY100135, SB269970, idazoxan, and naloxone in the ipsilateral and contralateral PAG studies. These findings strongly suggest that NPBW1/W2 activation by NPW microinjection into the RVM, LC, and PAG affect the descending pain modulatory system and produce anti-nociceptive and pro-nociceptive effects in the rat formalin test.

Role of 5-HT1A and 5-HT2C receptors of the dorsal periaqueductal gray in the anxiety- and panic-modulating effects of antidepressants in rats.

Antidepressant drugs are first-line treatment for panic disorder. Facilitation of 5-HT1A receptor-mediated neurotransmission in the dorsal periaqueductal gray (dPAG), a key panic-associated area, has been implicated in the panicolytic effect of the selective serotonin reuptake inhibitor fluoxetine. However, it is still unknown whether this mechanism accounts for the antipanic effect of other classes of antidepressants drugs (ADs) and whether the 5-HT interaction with 5-HT2C receptors in this midbrain area (which increases anxiety) is implicated in the anxiogenic effect caused by short-term treatment with ADs. The results showed that previous injection of the 5-HT1A receptor antagonist WAY-100635 in the dPAG blocked the panicolytic-like effect caused by chronic systemic administration of the tricyclic AD imipramine in male Wistar rats tested in the elevated T-maze. Neither chronic treatment with imipramine nor fluoxetine changed the expression of 5-HT1A receptors in the dPAG. Treatment with these ADs also failed to significantly change ERK1/2 (extracellular-signal regulated kinase) phosphorylation level in this midbrain area. Blockade of 5-HT2C receptors in the dPAG with the 5-HT2C receptor antagonist SB-242084 did not change the anxiogenic effect caused by a single acute injection of fluoxetine or imipramine in the Vogel conflict test. These results reinforce the view that the facilitation of 5-HT1A receptor-mediated neurotransmission in the dPAG is a common mechanism involved in the panicolytic effect caused by chronic administration of ADs. On the other hand, the anxiogenic effect observed after short-term treatment with these drugs does not depend on 5-HT2C receptors located in the dPAG.

Electrical stimulation of the posterior insula induces mechanical analgesia in a rodent model of neuropathic pain by modulating GABAergic signaling and activity in the pain circuitry.

The insula has emerged as a critical target for electrical stimulation since it influences pathological pain states. We investigated the effects of repetitive electrical stimulation of the insular cortex (ESI) on mechanical nociception, and general locomotor activity in rats subjected to chronic constriction injury (CCI) of the sciatic nerve. We also studied neuroplastic changes in central pain areas and the involvement of GABAergic signaling on ESI effects. CCI rats had electrodes implanted in the left agranular posterior insular cortex (pIC), and mechanical sensitivity was evaluated before and after one or five daily consecutive ESIs (15 min each, 60 Hz, 210 µs, 1 V). Five ESIs (repetitive ESI) induced sustained mechanical antinociception from the first to the last behavioral assessment without interfering with locomotor activity. A marked increase in Fos immunoreactivity in pIC and a decrease in the anterior and mid-cingulate cortex, periaqueductal gray and hippocampus were noticed after five ESIs. The intrathecal administration of the GABAA receptor antagonist bicuculline methiodide reversed the stimulation-induced antinociception after five ESIs. ESI increased GAD65 levels in pIC but did not interfere with GABA, glutamate or glycine levels. No changes in GFAP immunoreactivity were found in this work. Altogether, the results indicate the efficacy of repetitive ESI for the treatment of experimental neuropathic pain and suggest a potential influence of pIC in regulating pain pathways partially through modulating GABAergic signaling.

Intravenous doxapram administration as a potential model of panic attacks in rats.

Panic disorder can be categorized into the nonrespiratory or the respiratory subtypes, the latter comprising dyspnea, shortness of breath, chest pain, feelings of suffocation, and paresthesias. Doxapram is an analeptic capable of inducing panic attacks with respiratory symptoms in individuals diagnosed with the disorder; however, its neuroanatomical targets and its effects on experimental animals remain uncharacterized. One of the brain regions proposed to trigger panic attacks is the midbrain periaqueductal gray (PAG). Therefore, in this study, we evaluated the effects of doxapram in Fos (c-Fos) protein expression in the PAG and characterized its cardiorespiratory and behavioral effects on the elevated T maze and in the conditioned place aversion (CPA) paradigms. Doxapram increased Fos expression in different columns of the PAG, increased respiratory frequency, decreased heart rate, and increased arterial pressure when injected via intravenous route. Alprazolam, a panicolytic benzodiazepine, injected via intraperitoneal route, decreased respiratory frequency, whereas URB597, an anandamide hydrolysis inhibitor injected via intraperitoneal route, was ineffective. Doxapram injected via intraperitoneal route induced an anxiogenic-like effect in the elevated T-maze model; however, it failed to induce CPA. This study suggests that the cardiorespiratory and behavioral effects of doxapram in rodents serve as an experimental model that can provide insights into the neurobiology of panic attacks.

Topiramate inhibits offensive aggression through targeting ventrolateral periaqueductal gray.

Topiramate is an approved antiepileptic drug clinically used to treat epilepsy and prevent migraines. Currently, topiramate has been found to be effective in treating aggressive symptoms in neuropsychiatric patients. In preclinical studies, however, the effects and mechanisms of topiramate on offensive aggression are still largely uninvestigated. Our previous studies indicated that glutamatergic transmission in the ventrolateral periaqueductal gray (vlPAG) plays a crucial role in regulating elements of offensive aggressive behaviors. In the present work, we investigated the actions of topiramate on vlPAG glutamatergic transmission and aggressive behaviors in group-housed (GH) and socially isolated (SI) rats. The results suggested that a single injection of topiramate systemically and dose-dependently inhibited elements of offensive aggressive behaviors of both GH and SI rats in the resident-intruder test (RIT), with long-lasting effective time profiles in SI rats. Moreover, systemic single administration of topiramate reduced the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in the vlPAG. Bath perfusion of topiramate directly decreased the frequency and amplitude of mEPSCs and shortened the amplitude of evoked excitatory postsynaptic currents (EPSCs) in the vlPAG. Furthermore, intra-vlPAG single microinjection of topiramate dose-dependently inhibited offensive aggressive behaviors in GH and SI rats in a time-dependent manner. Additionally, both systemic and local topiramate inhibited offensive aggressive behaviors in a (2R,6R)-hydroxynorketamine (HNK)-dependent rat model. In conclusion, the present results suggest that topiramate exerts anti-aggressive roles through its inhibitory actions on glutamatergic activities in the vlPAG. These preclinical results support topiramate as a candidate drug to treat patients with heightened offensive aggression.