Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Pharmacological Interaction Between Cannabidiol and Tramadol on Experimental Diabetic Neuropathic Pain: An Isobolographic Analysis

Introduction: Diabetic neuropathies are the most prevalent chronic complications of diabetes, characterized by pain and substantial morbidity. Although many drugs have been approved for the treatment of this type of pain, including gabapentin, tramadol (TMD), and classical opioids, it is common to report short-term results or potentially severe side effects. TMD, recommended as a second-line treatment can lead to unwanted side effects. Cannabidiol (CBD) has been gaining attention recently due to its therapeutic properties, including pain management. This study aimed to characterize the pharmacological interaction between CBD and TMD over the mechanical allodynia associated with experimental diabetes using isobolographic analysis. Materials and Methods: After diabetes induction by streptozotocin (STZ), diabetic rats were systemically treated with CBD or TMD alone or in combination (doses calculated based on linear regression of effective dose 40% [ED₄₀]) and had the mechanical threshold evaluated using the electronic Von Frey apparatus. Both experimental and theoretical additive ED₄₀ values (Z_mix and Z_add, respectively) were determined for the combination of CBD plus TMD in this model. Results: Acute treatment with CBD (3 or 10 mg/kg) or TMD (2.5, 5, 10, or 20 mg/kg) alone or in combination (0.38+1.65 or 1.14+4.95 mg/kg) significantly improved the mechanical allodynia in STZ-diabetic rats. Isobolographic analysis revealed that experimental ED₄₀ of the combination (Z_mix) was 1.9 mg/kg (95% confidence interval [CI]=1.2-2.9) and did not differ from the theoretical additive ED₄₀ 2.0 mg/kg (95% CI=1.5-2.8; Z_add), suggesting an additive antinociceptive effect in this model. Conclusions: Using an isobolographic analysis, these results provide evidence of additive pharmacological interaction between CBD and TMD over the neuropathic pain associated with experimental diabetes induced by STZ.

Cannabidiol enhances the antinociceptive effects of morphine and attenuates opioid-induced tolerance in the chronic constriction injury model

Neuropathic pain (NP) is a complex health problem that includes sensorial manifestations such as evoked and ongoing pain. Cannabidiol (CBD) has shown potential in the treatment of NP and the combination between opioids and cannabinoids has provided promising results on pain relief. Thus, our study aimed to investigate the effect of treatment combination between CBD and morphine on evoked and ongoing pain, and the effect of CBD on morphine-induced tolerance in the model of chronic constriction injury (CCI) of the sciatic nerve in rats. Mechanical thresholds (i.e., evoked pain) were evaluated before and 7 days after surgery. We also employed a 4-day conditioned place preference (CPP) protocol, to evaluate relief of ongoing pain (6-9 days after surgery). Treatment with morphine (2 and 4 mg/kg) or CBD (30 mg/kg) induced a significant antinociceptive effect on evoked pain. The combination of CBD (30 mg/kg) and morphine (1 mg/kg) produced an enhanced antinociceptive effect, when compared to morphine alone (1 mg/Kg). Treatment with morphine (1 and 2 mg/kg) or CBD (30 mg/kg) alone failed to induce significant scores in the CPP test. However, combined treatment of CBD (30 mg/kg) and morphine (1 mg/kg) provided significant positive scores, increased the number of entrances in the drug-paired chamber in the CPP test and did not alter locomotor activity in rats. Lastly, treatment with CBD partially attenuated morphine-induced tolerance. In summary, our results support the indication of CBD as an adjuvant to opioid therapy for the attenuation of NP and opioid-induced analgesic tolerance.

Using Corneal Confocal Microscopy to Identify Therapeutic Agents for Diabetic Neuropathy

Corneal confocal microscopy (CCM) is emerging as a tool for identifying small fiber neuropathy in both peripheral neuropathies and neurodegenerative disease of the central nervous system (CNS). The value of corneal nerves as biomarkers for efficacy of clinical interventions against small fiber neuropathy and neurodegenerative disease is less clear but may be supported by preclinical studies of investigational agents. We, therefore, used diverse investigational agents to assess concordance of efficacy against corneal nerve loss and peripheral neuropathy in a mouse model of diabetes. Ocular delivery of the peptides ciliary neurotrophic factor (CNTF) or the glucagon-like peptide (GLP) analog exendin-4, both of which prevent diabetic neuropathy when given systemically, restored corneal nerve density within 2 weeks. Similarly, ocular delivery of the muscarinic receptor antagonist cyclopentolate protected corneal nerve density while concurrently reversing indices of systemic peripheral neuropathy. Conversely, systemic delivery of the muscarinic antagonist glycopyrrolate, but not gallamine, prevented multiple indices of systemic peripheral neuropathy and concurrently protected against corneal nerve loss. These data highlight the potential for use of corneal nerve quantification by confocal microscopy as a bridging assay between in vitro and whole animal assays in drug development programs for neuroprotectants and support its use as a biomarker of efficacy against peripheral neuropathy.

Spinal cannabinoid CB1 or CB2 receptors activation attenuates mechanical allodynia in streptozotocin-induced diabetic rats

Diabetes is a chronic disease associated with a high number of complications such as peripheral neuropathy, which causes sensorial disturbances and may lead to the development of diabetic neuropathic pain (DNP). The current treatment for DNP is just palliative and the drugs may cause severe adverse effects, leading to discontinuation of treatment. Thus, new therapeutic targets need to be urgently investigated. Studies have shown that cannabinoids have promising effects in the treatment of several pathological conditions, including chronic pain. Thus, we aimed to investigate the acute effect of the intrathecal injection of CB1 or CB2 cannabinoid receptor agonists N-(2-chloroethyl)-5Z, 8Z, 11Z, 14Z-eicosatetraenamide (ACEA) or JWH 133, respectively (10, 30 or 100 μg/rat) on the mechanical allodynia associated with experimental diabetes induced by streptozotocin (60 mg/kg; intraperitoneal) in rats. Cannabinoid receptor antagonists CB1 AM251 or CB2 AM630 (1 mg/kg) were given before treatment with respective agonists to confirm the involvement of cannabinoid CB1 or CB2 receptors. Rats with diabetes exhibited a significant reduction on the paw mechanical threshold 2 weeks after diabetes induction, having the maximum effect observed 4 weeks after the streptozotocin injection. This mechanical allodynia was significantly improved by intrathecal treatment with ACEA or JWH 133 (only at the higher dose of 100 μg). Pre-treatment with AM251 or AM630 significantly reverted the anti-allodynic effect of the ACEA or JWH 133, respectively. Considering the clinical challenge that the treatment of DPN represents, this study showed for the first time, that the intrathecal cannabinoid receptors agonists may represent an alternative for the treatment of DNP.

Bixin attenuates mechanical allodynia, anxious and depressive-like behaviors associated with experimental diabetes counteracting oxidative stress and glycated hemoglobin

Neuropathic pain, depression, and anxiety are common comorbidities in diabetic patients, whose pathophysiology involves hyperglycemia-induced increased oxidative stress. Bixin (BIX), an apocarotenoid extracted from the seeds of Bixa orellana, has been used in traditional medicine to treat diabetes and has been recognized by its antioxidant profile. We aimed to investigate the effect of the BIX over the mechanical allodynia, depressive, and anxious-like behaviors associated with experimental diabetes, along with its involved mechanisms. Streptozotocin-induced diabetic rats were treated for 17 days (starting 14 days after diabetes induction) with the corresponding vehicle, BIX (10, 30 or 90 mg/kg; p.o), or INS (6 IU; s.c.). Mechanical allodynia, depressive, and anxious-like behavior were assessed by electronic Von Frey, forced swimming, and elevated plus-maze tests, respectively. Locomotor activity was assessed by the open field test. Blood glycated hemoglobin (HbA1) and the levels of lipid peroxidation (LPO) and reduced glutathione (GSH) were evaluated on the hippocampus, pre-frontal cortex, lumbar spinal cord, and sciatic nerve. Diabetic animals developed mechanical allodynia, depressive and anxious-like behavior, increased plasma HbA1, increased LPO, and decreased GSH levels in tissues analyzed. Repeated BIX-treatment (at all tested doses) significantly attenuated mechanical allodynia, the depressive (30 and 90 mg/kg) and, anxious-like behaviors (all doses) in diabetic rats, without changing the locomotor performance. BIX (at all tested doses) restored the oxidative parameters in tissues analyzed and reduced the plasma HbA1. Thereby, bixin may represent an alternative for the treatment of comorbidities associated with diabetes, counteracting oxidative stress and plasma HbA1.

Antinociceptive and Anti-Inflammatory Effects of Bixin, a Carotenoid Extracted from the Seeds of Bixa orellana

Bixin is the main natural apocarotenoid extracted from the seeds of Bixa orellana, widely used as a cosmetic and textile colorant. Despite the description of several pharmacological properties of B. orellana extracts, little has been studied regarding the pharmacological properties of bixin. Then we aimed to investigate the potential anti-inflammatory and antinociceptive effect of bixin in preclinical models of inflammation and acute pain. The anti-inflammatory activity of bixin (15 or 30 mg/kg, orally) was determined using carrageenan-induced paw edema and the myeloperoxidase (MPO) activity in male Wistar rats. The antinociceptive effect of bixin was assessed in the formalin and hot plate tests in rats (at same doses) and in the acetic acid-induced writhing test in Swiss albino male mice (at doses of 27 or 53 mg/kg). General locomotor activity was evaluated in the open field test. Only the higher dose of bixin significantly decreased the carrageenan-induced paw edema and the MPO activity and increased the latency time in the hot plate. Both doses of bixin significantly reduced the number of flinches in both phases of the formalin test and the number of acetic acid-induced writhings without changing the locomotor performance in the open field test. This study validates the use of bixin as an anti-inflammatory trough mechanism related to the reduction of neutrophil migration. Furthermore, this is the first report showing the antinociceptive property of bixin, which does not appear to be related to the sedative effect. Further studies are necessary to characterize the mechanisms involved in these effects.

Sub-chronic treatment with cannabidiol but not with URB597 induced a mild antidepressant-like effect in diabetic rats

Depression associated with diabetes has been described as a highly debilitating comorbidity. Due to its complex and multifactorial mechanisms, the treatment of depression associated with diabetes represents a clinical challenge. Cannabidiol (CBD), the non-psychotomimetic compound derived from Cannabis sativa, has been pointed out as a promising compound for the treatment of several psychiatric disorders. Here, we evaluated the potential antidepressant-like effect of acute or sub-chronic treatment with CBD in diabetic rats using the modified forced swimming test (mFST). Also, to better understand the functionality of the endocannabinoid system in diabetic animals we also evaluated the effect of URB597, a fatty acid amide hydrolase inhibitor. Four weeks after the treatment with streptozotocin (60 mg/kg; i.p.; diabetic group-DBT) or citrate buffer (i.p.; normoglycemic group-NGL), DBT animals received an acute intraperitoneal injection of CBD (0, 0.3, 3, 10, 30 or 60 mg/kg), 1 h before the mFST, or URB597 (0, 0.1, 0.3 or 1 mg/kg) 2 h before the mFST. In another set of experiments, animals were sub-chronically treated with CBD (0, 0.3, 3, 30 or 60 mg/kg i.p.), 24, 5 and 1 h before the mFST or URB597 (0, 0.1, 0.3 or 1 mg/kg i.p.) 24, 5 and 2 h before the mFST. The NGL group was acutely treated with CBD (0, 30 mg/kg i.p.) or URB597 (0, 0.3 mg/kg; i.p.). Acute treatment with either CBD or URB induced an antidepressant-like effect in NGL rats, but not in DBT rats. However, sub-chronic treatment with CBD (only at a dose of 30 mg/kg), but not with URB597, induced a mild antidepressant-like effect in DBT animals. Neither body weight nor blood glucose levels were altered by treatments. Considering the importance of the endocannabinoid system to the mechanism of action of many antidepressant drugs, the mild antidepressant-like effect of the sub-chronic treatment with CBD, but not with URB597 does not invalidate the importance of deepening the studies involving the endocannabinoid system particularly in DBT animals.

Orofacial sensory changes after streptozotocin-induced diabetes in rats

Peripheral neuropathy is a common complication of diabetes and is often accompanied by episodes of pain. There is evidence that diabetic neuropathy may affect the trigeminal nerve, altering the transmission of orofacial sensory information. Structural changes in the trigeminal ganglia may be involved in the development of these sensory alterations. Herein, we evaluate the development of orofacial sensory changes after streptozotocin-induced diabetes in rats, and their sensitivity to pregabalin and morphine treatments. Furthermore, stereological analysis of the trigeminal ganglia was performed. Diabetic rats showed similar responses to 1% formalin applied into the upper lip compared to normoglycemic rats on weeks 1, 2 and 4 after streptozotocin. Additionally, there was no difference in the facial mechanical threshold of normoglycemic and diabetic rats, on weeks 1 up to 5 after streptozotocin, while the paw mechanical threshold of diabetic rats was significantly reduced. In contrast, diabetic rats developed long-lasting orofacial heat and cold hyperalgesia. Moreover, stereological analyses revealed significant neuronal loss in the trigeminal ganglia of diabetic compared to normoglycemic rats. Pregabalin treatment (30mg/kg, p.o.) of diabetic rats resulted in marked and prolonged (up to 6h) reduction of heat and cold orofacial hyperalgesia. Likewise, morphine treatment (2.5mg/kg, s.c.) abolished orofacial heat and cold hyperalgesia, but its effect was significant only up to 1h after the administration. In conclusion, the results of the present study demonstrated that streptozotocin-treated rats developed long-lasting orofacial heat and cold hyperalgesia, which is more amenable to reduction by pregabalin than morphine.