Role of kinin receptors in skin pigmentation

Previous studies have shown that all kinin system is constitutively expressed in the normal and inflamed skin, with a potential role in both physiological and pathological processes. However, the understanding regarding the involvement of the kinin system in skin pigmentation and pigmentation disorders remains incomplete. In this context, the present study was designed to determine the role of kinins in the Monobenzone (MBZ)-induced vitiligo-like model. Our findings showed that MBZ induces higher local skin depigmentation in kinin receptors knockout mice (KOB1R, KOB2R and KOB1B2R) than in wild type (WT). Remarkably, lower levels of melanin content and reduced ROS generation were detected in KOB1R and KOB2R mice treated with MBZ. In addition, both KOB1R and KOB2R show increased dermal cell infiltrate in vitiligo-like skin, when compared to WT-MBZ. Additionally, lack of B1R was associated with greater skin accumulation of IL-4, IL-6, and IL-17 by MBZ, while KOB1B2R presented lower levels of TNF and IL-1. Of note, the absence of both kinin B1 and B2 receptors demonstrates a protective effect by preventing the increase in polymorphonuclear and mononuclear cell infiltrations, as well as inflammatory cytokine levels induced by MBZ. In addition, in vitro assays confirm that B1R and B2R agonists increase intracellular melanin synthesis, while bradykinin significantly enhanced extracellular melanin levels and proliferation of B16F10 cells. Our findings highlight that the lack of kinin receptors caused more severe depigmentation in the skin, as well as genetic deletion of both B1/B2 receptors seems to be linked with changes in levels of constitutive melanin levels, suggesting the involvement of kinin system in crucial skin pigmentation pathways.

B2-kinin receptors in the dorsal periaqueductal gray are implicated in the panicolytic-like effect of opiorphin

Reported results have shown that the pentapeptide opiorphin inhibits oligopeptidases that degrade brain neuropeptides, and has analgesic and antidepressant effects in experimental animals, without either tolerance or dependency after chronic administration. In a previous study we showed that opiorphin has a panicolytic-like effect in the dorsal periaqueductal gray (dPAG) electrical stimulation test (EST), mediated by the μ-opioid receptor (MOR). This study further analyzes the mechanism of opiorphin panicolytic action, using the EST and drug injection inside the dPAG. The obtained results showed that blockade of the 5-HT_1A receptors with WAY-100635 did not change the escape-impairing effect of opiorphin, and combined injection of sub-effective doses of opiorphin and the 5-HT_1A-agonist 8-OH-DPAT did not have a significant anti-escape effect. In contrast, the anti-escape effect of opiorphin was antagonized by pretreatment with the kinin B2 receptor blocker HOE-140, and association of sub-effective doses of opiorphin and bradykinin caused a significant anti-escape effect. The anti-escape effect of bradykinin was not affected by previous administration of WAY-100635. Therefore, the anti-escape effect of opiorphin in the dPAG seems to be mediated by endogenous bradykinin, acting on kinin B2 receptors, which previous results have shown to interact synergistically with MOR in the dPAG to restrain escape in two animal models of panic. Chemical compounds: Opiorphin (PubChem CID: 25195667); WAY100635 maleate salt (PubChem CID: 11957721); 8-OH-DPAT hydrobromide (PubChem CID: 6917794); Bradykinin (PubChem CID: 439201); HOE-140 (Icatibant) (PubChem CID: 6918173).

Panicolytic-like action of bradykinin in the dorsal periaqueductal gray through μ-opioid and B2-kinin receptors

A wealth of evidence has shown that opioid and kinin systems may control proximal defense in the dorsal periaqueductal gray matter (dPAG), a critical panic-associated area. Studies with drugs that interfere with serotonin-mediated neurotransmission suggest that the μ-opioid receptor (MOR) synergistically interacts with the 5-HT_1A receptor in the dPAG to inhibit escape, a panic-related behavior. A similar inhibitory effect has also been reported after local administration of bradykinin (BK), which is blocked by the non-selective opioid receptor antagonist naloxone. The latter evidence, points to an interaction between BK and opioids in the dPAG. We further explored the existence of this interaction through the dPAG electrical stimulation model of panic. We also investigated whether intra-dPAG injection of captopril, an inhibitor of the angiotensin-converting enzyme (ACE) that also degrades BK, causes a panicolytic-like effect. Our results showed that intra-dPAG injection of BK inhibited escape performance in a dose-dependent way, and this panicolytic-like effect was blocked by the BK type 2 receptor (B2R) antagonist HOE-140, and by the selective MOR antagonist CTOP. Conversely, the panicolytic-like effect caused by local administration of the selective MOR agonist DAMGO was antagonized by pre-treatment with either CTOP or HOE-140, indicating cross-antagonism between MOR and B2R. Finally, intra-dPAG injection of captopril also impaired escape in a dose-dependent way, and this panicolytic-like effect was blocked by pretreatment with HOE-140, suggesting mediation by endogenous BK. The panicolytic-like effect of captopril indicates that the use of ACE inhibitors in the clinical management of panic disorder may be worth exploring.