Diffuse noxious inhibitory control (DNIC) is a phenomenon to reflect descending pain modulation in animals. Conditioned pain modulation (CPM) is the human counterpart of DNIC and is reduced in patients with several chronic pain conditions. Duloxetine is a serotonin and noradrenaline reuptake inhibitor that ameliorates CPM impairment in patients with diabetic neuropathy. Although some studies have reported the effects of different pharmacological agents on CPM, few studies have compared the effects of some analgesics in both humans and rodents. Therefore, we established a stable evaluation method for DNIC in rats and determined whether duloxetine and other specific analgesics affect DNIC impairment in rat models of peripheral neuropathic pain and osteoarthritis pain, two types of chronic pain. As a conditioning stimulus, capsaicin was injected into the forepaw of rats. The paw withdrawal threshold (PWT) in response to mechanical pressure was measured for the hindpaw. Peripheral neuropathic pain and osteoarthritis pain models were developed by partial sciatic nerve ligation (PSNL) and the intra-articular injection of 2 mg monoiodoacetate (MIA), respectively. Capsaicin (30-100 μg/site) increased the PWT, in a dose-dependent manner, in naive rats. The threshold significantly increased at 30 μg and reached its maximal level at 100 μg. The change in PWT following capsaicin injection was significantly reduced in PSNL-treated rats, but the threshold was increased by the subcutaneous administration of duloxetine (10 mg/kg). The oral administrations of pregabalin (10 mg/kg) and celecoxib (3 mg/kg) did not affect the PWT in PSNL-treated rats. Similarly, MIA-injected rats also showed a reduced change in PWT following capsaicin injection. Duloxetine, but not pregabalin and celecoxib, significantly increased the PWT in MIA-injected rats. These results suggested that duloxetine can directly ameliorate DNIC impairment in rat models of chronic pain. Duloxetine may be useful for modulating chronic pain by restoring function to the endogenous, descending, inhibitory pathway.

Glutamate transports (GTs), the only vehicle for removal of glutamate from the extracellular fluid, is reported to be related to chronic pain. To investigate whether the glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) participate in electroacupuncture (EA) analgesia, the EA effect was observed with paw withdraw threshold in a rat model of spared nerve injury. The expression levels of GLAST and GLT-1 were determined with Western Blot and RT-PCR. The results showed significantly upregulated GLAST and GLT-1, along with the relieved pain behaviors after EA treatment. In addition, intrathecal injection of GTs inhibitor, l-trans-pyrrolidine-2-4-dicarboxylate, attenuated the EA-induced analgesic effect. The experiment demonstrates that EA can increase the GTs of neuropathic pain rats, which might be one of the mechanisms underlying its effectiveness in the neuropathic pain.

The cyclic adenosine monophosphate-protein kinase A (cAMP-PKA) signaling acts a pivotal part in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels-mediated neuropathic and inflammatory pain. However, there has been no evidence of cAMP-PKA signaling is involved in regulation of spinal HCN channels function in the occurrence of diabetic neuropathic pain (DNP). The study aimed to elucidate the impact of HCN channels on neuropathic pain in a rat model of diabetes induced by streptozotocin, and whether cAMP-PKA signaling is involved in regulation of HCN channels function. In this report, we evaluated the effect of intrathecal administration of HCN channel blockers ZD7288, cAMP inhibitor SQ22536 and PKA inhibitor H-89 on nociceptive behavior in DNP rats. The mechanical withdrawal threshold (MWT) was measured to evaluate pain behavior in rats. Protein expression levels of HCN2, HCN4 channels and PKA in the spinal dorsal horn of rats were assessed. Furthermore, the levels of cAMP in rat spinal dorsal horn was analyzed. We discovered that DNP rats showed significant mechanical allodynia and are related to the increased HCN2 and HCN4 channels expression, enhanced cAMP production and elevated the expression of PKA protein in the spinal dorsal horn, which were attenuated by intrathecal ZD7288. Furthermore, intrathecal injection of SQ22536 and H-89 significantly reduced the HCN2 and HCN4 channels expression in the spinal dorsal horn of DNP rats. Our findings indicate that HCN channels of the spinal dorsal horn participate in the pathogenesis of allodynia in rats with DNP, which could be regulated by cAMP-PKA signaling. Therefore, HCN channels and cAMP-PKA signaling are potential targets for hyperalgesia treatment in DNP patients.

There is strong evidence showing that the activation of peripheral proteinase-activated receptors type 2 (PAR-2) can initiate hyperalgesic and inflammatory responses in the joint. However, to date, there is no report of functional spinal PAR-2 receptors in arthritis models. The primary aim of this study was to evaluate the activity of PAR-2 receptors at the spinal cord by using a potent agonist (FLIGRL) in naïve animals, and an antagonist (GB83) in different models of joint pain. Saline or FLIGRL (10 nmol) were injected intrathecally in naïve animals and nociceptive behaviour was evaluated over a 24 h time period by von Frey hair algesiometry. Paw withdrawal threshold decreased from 3 to 24 h and this allodynic effect was blocked by GB83 (90 nmol; i.p.). Acute inflammatory joint pain was induced by injecting 0.5 % kaolin/carrageenan (50 μL each) into the right knee joint of male Wistar rats (24 h recovery). Chronic inflammatory joint pain was modelled by intraarticular injection of Freund's complete adjuvant (FCA; 50 μL; 7 days recovery) or chronic osteoarthritis pain by sodium monoiodoacetate (MIA; 3 mg; 14 days recovery). Animals were then treated with either intrathecal vehicle or 10 nmol of GB83 (10 μL); joint pain was evaluated throughout the subsequent 3 h period. The acute inflammatory pain induced by kaolin/carrageenan was not affected by treatment with GB83. Conversely, both chronic arthritis models demonstrated increased hind paw withdrawal threshold after spinal injection of the PAR-2 antagonist. Based on these results, spinal PAR-2 receptors are involved in joint nociceptive processing in chronic but not acute arthritic conditions.

Transplantation of Schwann cells (SCs) can promote axonal regeneration and formation of the myelin sheath, reduce inflammation, and promote repair to the damaged nerve. Our previous studies have shown that transplantation of free or micro-encapsulated olfactory ensheathing cells can relieve neuropathic pain. There are no related reports regarding whether the transplantation of micro-encapsulated SCs can alleviate neuropathic pain mediated by P2X2/3 receptors. In the present study, we micro-encapsulated SCs in alginic acid and transplanted them into the region surrounding the injured sciatic nerve in the rat model of chronic constriction injury (CCI). The mechanical withdrawal threshold and thermal withdrawal latency were measured to assess changes in behavior 14 days after the surgery in CCI model rats. Ultrastructural changes in the injured sciatic nerve were assessed using transmission electron microscopy. Co-expression of P2X2/3 receptors with other markers in neurons in the L4-5 dorsal root ganglia (DRG) were assessed using double-label immunofluorescence 14 days after surgery. We determined P2X2/3 mRNA expression and protein level changes in the DRG using quantitative real-time polymerase change reaction technology and Western blotting analysis. We have investigated that the transplantation of micro-encapsulated SCs can alleviate pathological pain caused by P2X2/3 receptor stimulation and explored new methods for the prevention and treatment of neuropathic pain.

Neuropathic pain (NPP) is a clinically refractory disease that causes pain to patients, and its treatment has always been an urgent problem to be solved. P2X4 receptor (P2X4R) plays a key role in the pathogenesis of neuropathic pain. Therefore, the aim of this studies to explore the effect of olfactory ensheathing cells combined with chitosan (OECs+CS) transplantation on NPP caused by sciatic nerve injury in rats, and its relationship with the expression levels of P2X4R in the L4-5 dorsal root ganglion (DRG). In this study, olfactory ensheathing cells (OECs) were cultured, chitosan (CS) was prepared, and the compatibility of CS and OECs was detected by MTT method. Animal model of chronic constrictive sciatic nerve injury (CCI) was made, OECs and OECs+CS were transplanted to the region surrounding the chronic sciatic nerve injury, and the difference between the two groups in the treatment of NPP was compared. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were measured by using behavioral method. in situ hybridization and Western-blotting were used to detect the expression of P2X4R mRNA and protein in the DRG. These results showed that OECs had good biocompatibility with CS. Compared with the CCI, the MWT and TWL were significantly increased (P＜0.05), the expression levels of P2X4R mRNA and protein in the OECs and OECs+CS group were significantly reduced (P＜0.05). Compared with the OECs, the expression levels of P2X4R mRNA and protein in the OECs+CS group were significantly reduced (P＜0.05), the MWT and TWL were significantly increased (P＜0.05). We conclude that OECs+CS can better inhibit P2X4R over-expression-mediated NPP, and its therapeutic effect was superior to simple OECs transplantation, which may become another potential method for the treatment of NPP.

Neuropathic pain is still one of the unsolved public health problems worldwide. Although the current reagents can attenuate neuropathic pain to a certain extent, their clinical application is very limited owing to larger toxicity and serious side effects. Trifluoro-icaritin (ICTF) has been documented to possess profound anti-inflammatory and neuroprotective activities, but whether ICTF exerts an anti-nociceptive effect on neuropathic pain remains unknown. Here, a rat model of spared nerve injury (SNI)-induced neuropathic pain was used. SNI rats were administrated with ICTF (i.p.) once daily lasting for 21 days, and subsequently the pain-related behaviors were evaluated by applying mechanical or thermal pain threshold, CatWalk gait parameter, and rotarod test on day 1 before and day 1, 3, 7, 10, 14, and 21 after SNI surgery, respectively. The results showed that ICTF (0.5 mg/kg, 1.5 mg/kg, and 5.0 mg/kg, i.p.) treatment alleviated SNI-induced mechanical allodynia but not thermal hyperalgesia in a dose-dependent manner. After administration of ICTF at the most effective dose of 5.0 mg/kg to SNI rats, CatWalk gait analysis revealed that ICTF not only significantly enhanced gait parameters including max contact max intensity, max intensity, print area, and stand time but also decreased the swing time; Rotarod test further exhibited that ICTF could effectively prolong the time on rod and increase the rotating speed in SNI rats. Additionally, following ICTF (5.0 mg/kg) treatment of SNI rats for 21 consecutive days, the max contact max intensity was found to be positively correlated with the rotating speed. Taken together, ICTF successfully ameliorates mechanical hypersensitivity and improves the motor coordination and balance in SNI rats, suggesting that ICTF may be exploited as a potential candidate in the management of neuropathic pain.

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.

Smokers have a higher incidence of chronic pain than non-smokers, but the neural mechanism is not yet fully understood. Nicotine is the main component of tobacco and acts as an agonist for nicotinic cholinergic receptors (nAChRs) in the nervous system. This study was approved by the IACUC of UM. The effects of chronic nicotine administration on mechanical sensitivity were studied using a rat model. The changes in the expression levels of the α7 isoform of nAChR (α7-nAChR), inflammatory cytokines TNFα and COX-2, as well as the density of neuro-immune cells (astrocytes and microglia) were measured concurrently. The results indicate that long-term nicotine administration induces hypersensitivity to mechanical stimuli, as demonstrated by a significant reduction in the pain perception threshold. In response to nicotine, the expression levels of α7-nAChR increased in the periaqueductal gray matter (PAG) and decreased in the spinal cord. Acute administration of the selective α7-nAChR agonist CDP-Choline reversed this hypersensitivity. Chronic nicotine administration led to an increase of microglial cells in the dorsal horn of the spinal cord and increased expression levels of the cytokines TNFα and COX-2. This study suggests that decreased α7-nAChR expression in the spinal cord, as a result of long-term exposure to nicotine, may be causatively linked to chronic pain. Simultaneously, the increase of neuro-immune factors in the spinal cord is also a potential factor leading to chronic pain.

Our recent study revealed that spinal electromagnetic stimulation (sEMS) applied at low (0.2 Hz) frequencies may improve diminished transmission in damaged spinal cord in spinal cord injured (SCI) rats. We have recently begun a pilot study investigating the effects of sEMS in non-injured and SCI humans. One unexpected result was the reduction of chronic low back pain (CLBP), reported by some patients following sEMS treatment. Chronic low back pain is one of the main causes of disability affecting the general population. Opioids are the most common drugs prescribed to US adults with CLBP. To optimize parameters for sEMS for pain treatment, in this study we used the SCI animal model and examined effects of sEMS applied at lumbosacral level on parameters and frequency-dependent depression (FDD) of Hoffmann H-reflex responses, known as common neurophysiological measures for evaluation of sensorimotor condition and plasticity in humans. We have also examined the interactive effects of sEMS and the opiate partial agonist Buprenorphine on the parameters of H-reflex in naïve and SCI rats. Consistent with previous reports, chronic SCI resulted in a marked decrease of threshold intensity required to evoke H-reflex and a lesser rate of FDD of the H-response in adult rats. Our current study revealed the optimum parameters of spinal EMS for best recovery of the properties of the H-reflex in chronic SCI animals. Here we demonstrate that electro-magnetic stimulation applied at spinal L4-L5 level with a pulsed mode (pulse at 20 Hz frequency for 5 sec with 25 sec break between pulses, total 40 trains for 20 min; PSEMS) reversed effects of SCI on key parameters of H-reflex: i.e. (1) restored the threshold intensity of electric current applied at tibial nerve to evoke the H-reflex and (2) recovered FDD properties of the H-reflex in SCI rats. Importantly, subcutaneous injections of Buprenorphine, prior to PSEMS administration, abolished the ability of PSEMS to recover both threshold intensity and FDD of the H-reflex in chronic SCI animals. These results suggest that a semi-synthetic opioid Buprenorphine and PSEMS might share common sites of action. We thus conclude that PSEMS might carry potential as a non-invasive treatment approach for chronic low back pain.

Sensorimotor incongruence (SMI) is associated with pathological pain, such as phantom limb pain. Additionally, patients with pathological pain and brain dysfunction typically present with movement disorders, including diminished voluntary control and increased variability in bimanual movement performance. In healthy subjects, SMI leads to dysesthesia and bimanual movement motor dysfunction. However, the brain localization of this activity remains unclear, particularly in SMI-induced dysesthesia and decrease in movement accuracy. In this study, 17 healthy participants were asked to perform repetitive flexion/extension exercises with their wrists in a congruent/incongruent position while viewing the activity in a mirror. Indeed, SMI induced dysesthesia and decreased bimanual movement accuracy. Moreover, beta band activities of the bilateral presupplementary (P < 0.01) and bilateral cingulate (P < 0.05) motor areas were decreased. Collectively, our findings indicate that SMI induces dysesthesia and movement disorders and reduces beta band activities in motor-related areas.

Trigeminal neuralgia (TN) is a type of neuropathic pain characterized by intense pain; although anticonvulsants are used as an option to relieve pain, adverse side effects can decrease patient adherence. In this context, a low dose of naltrexone is effective in relieving pain in other pain conditions. Thus, the objective of the present study was to evaluate the analgesic effect of low-dose naltrexone on facial mechanical allodynia in a rat model of TN, as well as its effect(s) on biomarkers in the central nervous system (tumor necrosis factor-alpha, brain-derived neurotrophic factor [BDNF], interleukin [IL]-10, and toll-like receptor-4). Fifty-nine adult male Wistar rats (CEUA-HCPA#2017-0575) were allocated to following groups: control; sham-pain + vehicle; sham-pain + carbamazepine (100 mg/kg); sham-pain + naltrexone (0.5 mg/kg); pain + vehicle; pain + carbamazepine; and pain + naltrexone. TN was induced using chronic constriction of the infraorbital nerve. Facial allodynia was assessed using von Frey test. Drugs were administered by gavage 14 days after surgery for 10 days. At baseline, the mechanical threshold was similar between groups (P > 0.05; generalized estimating equation). Seven days after surgery, facial allodynia was observed in sham-TN and pain-TN groups (P < 0.05). Fourteen days after surgery, only pain-TN groups exhibited facial allodynia. The first dose of low-dose naltrexone or carbamazepine partially reversed facial allodynia. After 10 days of treatment, both drugs completely reversed it. Spinal cord levels of BDNF and IL-10 were modulated by low-dose naltrexone. Thus, low-dose naltrexone may be suitable to relieve TN; however, the exact mechanisms need to be clarified.

Chemotherapy-induced peripheral neuropathy (CIPN) has a adverse impact to the living quality of cancer patients. This side effect of CIPN limit the dose of drug used in many chemotherapies, such as vincristine (VCR). The activation of microglia in the spinal dorsal horn is involved in the occurrence and development of neuropathic pain induced by VCR. Recent study has demonstrated that hypoxia induced microglia activation depends on Notch signaling, and it is involved in the release of many inflammatory related factors in microglia. In this work, we aimed to study that the role of Notch signaling pathway in microglia activation on a VCR-induced neuropathy rat model. Our results showed that the mechanical, thermal and cold pain threshold of rats was decreased by treatment of VCR, but N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT), a γ-secretase inhibitor, relieved the hyperalgesia. Molecular analysis showed that activation of Notch signaling pathway increased after nerve injury and that DAPT could significantly inhibit the upregulation of Notch signaling pathway, the activation of microglia, and the release of pro-inflammatory cytokines in the spinal. Taking together, Notch signaling pathway could be a potential therapeutic target to alleviate neuropathic pain.

Increased opioid synthesis and release, and enhanced alpha-2 adrenoceptor signaling have been suggested to mediate repeated oxytocin-induced long-lasting effects including elevated pain threshold in rats. This study evaluated whether oxytocin pretreatment would influence development of dependence and tolerance to the nociceptive and body temperature responses to morphine and enhance effects of alpha-2 adrenergic agonist clonidine on nociceptive threshold, body temperature and morphine withdrawal signs. Rats injected subcutaneously with saline or 1 mg/kg oxytocin for 5 days were implanted with placebo or morphine pellets 24 h after the treatment period. Body temperature and nociception were assessed, with nociception determined via by hot plate and tail immersion tests, before and 4, 24 and 48 h after pellet implantation, and following a challenge dose of morphine. Withdrawal signs were determined after naloxone administration. Oxytocin produced analgesia, as evidenced by increased paw withdrawal latency in the hot plate test. Morphine increased body temperature and nociceptive threshold which declined over time. Morphine challenge could not demonstrate tolerance to the body temperature response. Analgesic tolerance was observed in the hot plate test in saline and in both tests in oxytocin pretreated rats. Naloxone-precipitated withdrawal appeared to be less severe in oxytocin pretreatment. Clonidine was ineffective on the withdrawal signs but decreased body temperature and increased tail flick latency in the tail immersion test in oxytocin pretreated animals. These results, while producing evidence for a hyperresponsiveness in alpha-2 adrenoceptors, provide contrasting effects on morphine tolerance and dependence, and their partial mediation by opioidergic and adrenergic activation in repeated oxytocin treatment.

We previously demonstrated that sodium channel 1.7 (Nav1.7) in trigeminal ganglion (TG) was a critical factor in temporomandibular joint (TMJ) inflammation-induced hypernociception, but the mechanism underlying inflammation-induced upregulation of Nav1.7 remained unclear. Glial-neuron interaction plays a critical role in pain process and connexin 43 (Cx43), a gap junction protein expressed in satellite glial cells (SGCs) has been shown to play an important role in several pain models. In the present study, we investigate the role of Cx43 in TMJ inflammation-induced hypernociception and its possible impact on neuronal Nav1.7. We induced TMJ inflammation in rats by injecting complete Freund's adjuvant (CFA) into TMJ and observed a decrease in head withdraw threshold after 24 h. Electron microscopy showed morphological alterations of SGCs in TMJ-inflamed rats. The expression of Cx43, glial fibrillary acidic protein (GFAP), and Nav1.7 increased greatly compared with controls. In addition, pretreatment with Cx43 blockers in TMJ-inflamed rats could alleviate mechanical hypernociception, inhibit SGCs activation and IL-1βrelease, and thus block the upregulation of Nav1.7. These findings indicate that the propagation of SGCs activation via Cx43 plays a critical role in Nav1.7-involved mechanical hypernociception induced by TMJ inflammation.

We have recently demonstrated that orexin acts centrally in the brain to induce antinociceptive action against colonic distension through orexin 1 receptors in conscious rats. Although the dopaminergic system can induce antinociceptive action for somatic pain, the association between changes in the dopaminergic system and visceral pain perception has not been investigated. In the present study, we hypothesized that the dopaminergic system may be involved in visceral nociception, and if so, the dopaminergic system may mediate the orexin-induced visceral antinociception. Visceral sensation was evaluated using the colonic distension-induced abdominal withdrawal reflex (AWR) in conscious rats. Intracisternal injection of D1 (SKF38398) or D2 (quinpirole) dopamine receptor agonist increased the threshold volume of colonic distension-induced AWR in a dose-dependent manner. Pretreatment with either the D1 or D2 dopamine receptor antagonist (SCH23390 or sulpiride, respectively) potently blocked the centrally injected orexin-A-induced antinociceptive action against colonic distension. These results suggest for the first time that dopaminergic signaling via D1 and D2 dopamine receptors in the brain may induce visceral antinociception and that the dopaminergic signaling may be involved in the central orexin-induced antinociceptive action against colonic distension.

Endometriosis of sciatic nerve is a common gynecological disease. Here we aimed to study the anti-inflammatory and anti-nociceptive role of sulforaphane on sciatic nerve endometriosis. The sciatic nerve endometriosis rat model was constructed by autologous implantation of uterine tissue. Sulforaphane was administered intraperitoneally at the dose of 5, 15, 30 and 60 mg/kg/day for 28 days. Behavioral testing was performed at day 7, 14, 21 and 28. At day 28, rats were sacrificed, followed by collecting superficial dorsal horn tissues and lesions. Quantitative real-time PCR and Western blot were performed to assess COX2, Keap1, Nrf2 expression in collected tissues. Enzyme-linked immunosorbent assay was conducted to assess the expression of pro-inflammatory cytokines. Sulforaphane alleviated pain of sciatic endometriosis as evidenced by the increase in paw withdrawal threshold and paw withdrawal latency. Sulforaphane also inhibited ectopic endometrial tissue growth in sciatic endometriosis rat, shown as the shrinkage of lesion size and decreased VEGF levels. IL6, IL-1β and TNF-α levels were decreased by sulforaphane. Sulforaphane induced DOX2 and INOS suppression and Keap1 and Nrf2 upregulation. Sulforaphane alleviates pain induced by sciatic endometriosis, which is mediated by inhibiting inflammation.

Opioid agonists are used in clinic for pain management, however this application is challenged by development of tolerance and dependence following prolonged exposure. Various approaches have been suggested to address this concern, however, there is still no consensus among the researchers. Neural processing of sleep and nociception are co-regulated through shared brain regions having bidirectional interplays. Thus, we aimed to investigate whether application of REM sleep deprivation (REM-SD) could affect morphine analgesic tolerance and dependence. To this end, adult male rats underwent sleep deprivation during light and dark phases (LSD and DSD, respectively) using the inverted flower pot method and then tolerance and dependence was induced by repeated injection of morphine for 7 days (10 mg/kg, daily, i.p.). Results indicated that REM-SD delays the development of tolerance to morphine during both phases; however this effect was more potent following LSD. Moreover, LSD decreased the baseline thermal threshold and total withdrawal score. One possible hypothesis for our observations is REM-SD-induced attenuation of orexin system which is still controversial among the researchers. Other stronger possibilities might be down-regulation of opioid receptors in response to sleep loss experience. Finally, it seems that modification of sleep periods may assist to decrease the severity of opioid tolerance and dependence.

This study evaluated the immediate and persistent behavioral and molecular consequences (i.e., emotional signs emerging during withdrawal and signs of neurotoxicity) following adolescent morphine exposure with a sex perspective. Basally, and prior to drug treatment, adolescent female rats showed smaller body weight and lower pain threshold than their male counterparts. Adolescent morphine treatment induced some sex-specific differences, while morphine impaired normal weight gain in male rats, no effects were observed for female rats. Plus, morphine produced an attenuated antinociceptive response in female rats. Moreover, following adolescent morphine treatment some emotional signs of withdrawal emerged exclusively in male rats in conjunction with signs of neurotoxicity, while female rats were not affected. In particular, an anxiolytic-like effect in adolescence during early withdrawal was followed by the development of a depressive-like phenotype (i.e., behavioral despair, anxiety-like behavior and anhedonia) in adulthood during prolonged withdrawal and paired with decreased contents of neurofilaments proteins in the prefrontal cortex. In conclusion, morphine administration during adolescence induced persistent changes in negative affect and brain toxicity selectively in male rats, suggesting female rats were resilient to these harmful effects. Given the widespread availability and use of opiate-based painkillers, the interplay between addiction, analgesia and emotional behaviors, and since adolescents and young adult humans are the age group with the highest abuse potential, these results add to the current literature by reporting distinct sex-specific opioid actions when administered in adolescence.