Trigeminal neuropathic pain causes changes in affective processing of pain in rats.

Trigeminal neuropathic pain has been modeled in rodents through the constriction of the infraorbital nerve (CCI-ION). Sensory alterations, including spontaneous pain, and thermal and mechanical hyperalgesia are well characterized, but there is a notable lack of evidence about the affective pain component in this model. Evaluation of the emotional component of pain in rats has been proposed as a way to optimize potential translational value of non-clinical studies. In rats, 22 and 50 kHz ultrasonic vocalizations (USVs) are considered well-established measures of negative and positive emotional states, respectively. Thus, this study tested the hypothesis that trigeminal neuropathic pain would result, in addition to the sensory alterations, in a decrease of 50 kHz USV, which may be related to altered function of brain areas involved in emotional pain processing. CCI-ION surgery was performed on 60-day-old male Wistar rats. 15 days after surgery, von Frey filaments were applied to detect mechanical hyperalgesia, and USV was recorded. At the same timepoint, systemic treatment with d,l-amphetamine (1 mg/kg) allowed investigation of the involvement of the dopaminergic system in USV emission. Finally, brain tissue was collected to assess the change in tyrosine hydroxylase (TH) expression in the nucleus accumbens (NAc) and c-Fos expression in brain areas involved in emotional pain processing, including the prefrontal cortex (PFC), amygdala, and NAc. The results showed that CCI-ION rats presented mechanical hyperalgesia and a significant reduction of environmental-induced 50 kHz USV. Amphetamine caused a marked increase in 50 kHz USV emission in CCI-ION rats. In addition, TH expression was lower in constricted animals and c-Fos analysis revealed an increase in neuronal activation. Taken together, these data indicate that CCI-ION causes a reduction in the emission of environmental-induced appetitive calls concomitantly with facial mechanical hyperalgesia and that both changes may be related to a reduction in the mesolimbic dopaminergic activity.

Melatonin Delivery: Transdermal and Transbuccal Evaluation in Different Vehicles.

PURPOSE: Melatonin (MLT) could be candidate drug for treatment of several diseases because of its high antioxidant and anticarcinogenic activity and its important biological roles. The aim of this study was to assess the influence of different vehicles on the permeation of MLT through buccal and skin tissues. METHODS: Formulations were characterized in terms of rheology, drug release and permeation through human skin as well as porcine buccal mucosa. Irradiation experiments were also performed. RESULTS: The lowest amount of MLT released was from oral adhesive paste Orabase® (OB) and the highest from the emulsion system Montanov® 68 (M68). Skin permeation revealed high pattern for Carbopol® 940 (C940) and M68, and poor for poloxamer 407 (P407) and Pluronic® lecithin organogel (PLO). Statistical differences of MLT remaining in skin between M68 vs C940 (p < 0.05) and M68 vs PLO (p < 0.05) were observed. Transmucosal results showed that sodium carboxymethylcellulose (NaCMC) was the best and OB the worst vehicle. P407 and PLO followed similar behaviour. Photostability studies revealed high percentage of degradation of MLT in solution which was also similar when was loaded in OB. The rest of formulations showed low rates of degradation. CONCLUSIONS: C940 or M68 and NaCMC can be proposed as formulations for a potential systemic effect of MLT by skin and buccal mucosa routes, respectively. However, if the intended objective is to obtain local action in the skin and buccal mucosa, the proposed formulations are M68 or P407 and PLO.

Sex Dimorphism in Resolvin D5-induced Analgesia in Rat Models of Trigeminal Pain.

Resolvin D5 (RvD5) is a specialized pro-resolving lipid mediator with potent anti-inflammatory and analgesic properties. Orofacial pain conditions, especially those that are chronic, present clinical challenges in terms of pharmacological management. Thus, new therapeutic options are clearly warranted. Herein, we investigated the antinociceptive effect of RvD5 in the chronic constriction injury of the infraorbital nerve (CCI-ION) model and in the orofacial formalin test in female and male Wistar rats. Our results indicated that repeated subarachnoid medullary injections of RvD5 at 10 ng resulted in a significant reduction of heat and mechanical hyperalgesia induced by the CCI-ION in male and female rats, but males were more sensitive to RvD5 effects. In addition, after CCI-ION, interleukin-6 (IL-6) level was increased in the trigeminal nucleus caudalis of male, but not female rats, which was reduced by RvD5 repeated treatment. No changes in the levels of IL-1ß were found. Minocycline blocked the effect of RvD5 in male rats but failed to affect RvD5 antinociceptive effect in females. Moreover, a single medullary injection of RvD5 caused a significant reduction of formalin-induced facial grooming, in phases I and II of the test, but only in male rats. This study demonstrated for the first time the analgesic effect of RvD5 in trigeminal pain models, and corroborated previous evidence of sex dichotomy, with a greater effect in males. This article presents a translational potential of RvD5 for targeted therapies aiming at the control of acute and chronic trigeminal pain, but further studies are needed to elucidate its sex-related mechanisms. PERSPECTIVE: This study demonstrated that RvD5 may provide the benefits for trigeminal neuropathic pain treatment in male and female rats, but its effect on inflammatory orofacial pain seems to be restricted only to males. Also, it provided the evidence for sex dichotomy in the mechanisms related to the antinociceptive effect of RvD5.