The neuronal tyrosine kinase receptor ligand ALKAL2 mediates persistent pain

The anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase known for its oncogenic potential that is involved in the development of the peripheral and central nervous system. ALK receptor ligands ALKAL1 and ALKAL2 were recently found to promote neuronal differentiation and survival. Here, we show that inflammation or injury enhanced ALKAL2 expression in a subset of TRPV1⁺ sensory neurons. Notably, ALKAL2 was particularly enriched in both mouse and human peptidergic nociceptors, yet weakly expressed in nonpeptidergic, large-diameter myelinated neurons or in the brain. Using a coculture expression system, we found that nociceptors exposed to ALKAL2 exhibited heightened excitability and neurite outgrowth. Intraplantar CFA or intrathecal infusion of recombinant ALKAL2 led to ALK phosphorylation in the lumbar dorsal horn of the spinal cord. Finally, depletion of ALKAL2 in dorsal root ganglia or blocking ALK with clinically available compounds crizotinib or lorlatinib reversed thermal hyperalgesia and mechanical allodynia induced by inflammation or nerve injury, respectively. Overall, our work uncovers the ALKAL2/ALK signaling axis as a central regulator of nociceptor-induced sensitization. We propose that clinically approved ALK inhibitors used for non–small cell lung cancer and neuroblastomas could be repurposed to treat persistent pain conditions.

Peroxynitrite is a positive inotropic agent in atrial and ventricular fibres of the frog heart

1. We report opposite inotropic effects of NO donors in frog cardiac fibres. The negative effect, elicited by either 3-morpholino-sydnonimine (SIN-1) or S-nitroso-N-acetyl-penicillamine (SNAP), involved cyclic GMP (cGMP) production. However, SIN-1, unlike SNAP, could elicit a positive effect, in a superoxide dismutase (SOD)-sensitive manner. SIN-1, unlike SNAP, can release both NO and superoxide anion, the precursors of peroxynitrite (OONO-). The role of these messengers was examined. 2. Catalase did not reduce the positive inotropic effect of SIN-1. Thus, a conversion of superoxide anion into hydrogen peroxide was not involved in this effect. In addition, catalase did not modify the negative effects of SIN-1 plus SOD, or SNAP plus SOD. 3. LY 83583, a superoxide anion generator, elicited a positive inotropic effect, like SIN-1. The effect of LY 83583 was additive to the negative effects of SIN-1 or SNAP, and to the positive effect of SIN-1. Thus, superoxide anion generation, per se, did not account for the positive effect of SIN-1. 4. Authentic peroxynitrite (OONO-), but not mock-OONO- (negative control plus decomposed OONO-), exerted a dramatic positive inotropic effect in cardiac fibres. The effect of OONO- was larger in atrial fibres, as compared with ventricular fibres. 5. The positive effect of OONO- was not additive with that of SIN-1, suggesting a common mechanism of action. In contrast, the effects of either OONO- or SIN-1 were additive with the negative inotropic effect of SNAP. Furthermore, the effect of OONO-, like that of SIN-1, was not antagonized by 1H-[1,2,4]xidiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM), the guanylyl cyclase inhibitor. 6. The positive inotropic effects of SIN-1 and OONO- were not modified by hydroxyl radical scavengers, such as dimethyl-thio-urea (DMTU; 10 mM). 7. The positive inotropic effect of SIN-1 (100 microM) was abolished in sodium-free solutions, a treatment that eliminates the activity of the sodium-calcium exchanger. In contrast, the effect of SIN-1 was unchanged by a potassium channel inhibitor (tetraethyl-ammonium, 20 mM), or a sodium-potassium pump inhibitor (ouabain 10 microM). 8. We conclude that OONO- is a positive inotropic agent in frog cardiac fibres. The generation of OONO- accounts for the positive inotropic effect of SIN-1. OONO- itself was responsible for the positive inotropic effect, and appeared to modulate the activity of the sodium-calcium exchanger.

Methylene blue is a muscarinic antagonist in cardiac myocytes

We studied the mechanism of action of methylene blue (Mblue), a putative guanylyl cyclase inhibitor, on the L-type calcium current (ICa) and the muscarinic activated K+ current (IK,ACh) in rat ventricular and atrial myocytes, respectively, and on the binding of [3H]quinuclidinyl benzylate in rat ventricular membranes. Superfusion, but not internal dialysis, with 30 microM Mblue antagonized the inhibitory effect of acetylcholine (ACh, 1 microM) on beta-adrenergic stimulation of ICa with isoprenaline (Iso, 10 nM or 1 microM). However, Mblue had no effect on the basal ICa or on the stimulation of ICa by Iso in the absence of ACh. The activation of IK,ACh by 3 microM ACh was also antagonized by Mblue in a dose-dependent manner. In contrast, Mblue had no effect on the activation of IK,ACh by either guanosine-5'-O-(3-thio)triphosphate or guanosine-5'-(beta,gamma-imido)triphosphate. Chlorpromazine (CPZ), a piperazine derivative like Mblue, also inhibited the muscarinic activation of IK,ACh in a dose-dependent manner. The specific binding of [3H]QNB, a muscarinic ligand, to rat ventricular membranes was displaced in a dose-dependent manner by Mblue and CPZ. The piperazine derivatives behaved like competitive antagonists of [3H]QNB binding, exhibiting equilibrium dissociation constant (Ki) values of 187 nM for Mblue and 366 nM for CPZ. In conclusion, Mblue exerts antimuscarinic effects on ICa and IK,ACh in rat cardiac myocytes that are best explained by the binding of Mblue to the M2 subtype of muscarinic receptors. This property probably contributes to the antimuscarinic effect of the putative guanylyl cyclase inhibitor reported in previous studies.