Nitroglycerin (glycerol trinitrate, GTN) induces headache in migraineurs, an effect that has been used both diagnostically and in the study of the pathophysiology of this neurovascular pain syndrome. An important feature of this headache is a delay from the administration of GTN to headache onset that, because of GTN's very rapid metabolism, cannot be due to its pharmacokinetic profile. It has recently been suggested that activation of perivascular mast cells, which has been implicated in the pathophysiology of migraine, may contribute to this delay. We reported that hyperalgesia induced by intradermal GTN has a delay to onset of ∼ 30 min in male and ∼ 45 min in female rats. This hyperalgesia was greater in females, was prevented by pretreatment with the anti-migraine drug, sumatriptan, as well as by chronic pretreatment with the mast cell degranulator, compound 48/80. The acute administration of GTN and compound 48/80 both induced hyperalgesia that was prevented by pretreatment with octoxynol-9, which attenuates endothelial function, suggesting that GTN and mast cell-mediated hyperalgesia are endothelial cell-dependent. Furthermore, A-317491, a P2X3 antagonist, which inhibits endothelial cell-dependent hyperalgesia, also prevents GTN and mast cell-mediated hyperalgesia. We conclude that delayed-onset mechanical hyperalgesia induced by GTN is mediated by activation of mast cells, which in turn release mediators that stimulate endothelial cells to release ATP, to act on P2X3, a ligand-gated ion channel, in perivascular nociceptors. A role of the mast and endothelial cell in GTN-induced hyperalgesia suggests potential novel risk factors and targets for the treatment of migraine.

We have recently demonstrated a role of the vascular endothelium in peripheral pain mechanism by disrupting endothelial cell function using intravascular administration of octoxynol-9, a non-selective membrane active agent. As an independent test of the role of endothelial cells in pain mechanisms, we evaluated the effect of homocysteine, an agent that damages endothelial cell function. Mechanical stimulus-induced enhancement of endothelin-1 hyperalgesia in the gastrocnemius muscle of the rat was first prevented then enhanced by intravenous administration of homocysteine, but was only inhibited by its precursor, methionine. Both homocysteine and methionine significantly attenuated mechanical hyperalgesia in two models of ergonomic muscle pain, induced by exposure to vibration, and by eccentric exercise, and cutaneous mechanical hyperalgesia in an ischemia-reperfusion injury model of Complex Regional Pain Syndrome type I, all previously shown responsive to octoxynol-9. This study provides independent support for a role of the endothelial cell in pain syndromes thought to have a vascular basis, and suggests that substances that are endothelial cell toxins can enhance vascular pain.

Clinical studies have shown that agonist-antagonist opioid analgesics that produce their analgesic effect via action on the kappa-opioid receptor, produce a delayed-onset anti-analgesia in men but not women, an effect blocked by co-administration of a low dose of naloxone. We now report the same time-dependent anti-analgesia and its underlying mechanism in an animal model. Using the Randall-Selitto paw-withdrawal assay in male rats, we found that nalbuphine, pentazocine, and butorphanol each produced analgesia during the first hour followed by anti-analgesia starting at ∼90min after administration in males but not females, closely mimicking its clinical effects. As observed in humans, co-administration of nalbuphine with naloxone in a dose ratio of 12.5:1 blocked anti-analgesia but not analgesia. Administration of the highly selective kappa-opioid receptor agonist U69593 produced analgesia without subsequent anti-analgesia, and confirmed by the failure of the selective kappa antagonist nor-binaltorphimine to block nalbuphine-induced anti-analgesia, indicating that anti-analgesia is not mediated by kappa-opioid receptors. We therefore tested the role of other receptors in nalbuphine anti-analgesia. Nociceptin/orphanin FQ (NOP) and sigma-1 and sigma-2 receptors were chosen on the basis of their known anti-analgesic effects and receptor binding studies. The selective NOP receptor antagonists, JTC801, and J-113397, but not the sigma receptor antagonist, BD 1047, antagonized nalbuphine anti-analgesia. Furthermore, the NOP receptor agonist NNC 63-0532 produced anti-analgesia with the same delay in onset observed with the three agonist-antagonists, but without producing preceding analgesia and this anti-analgesia was also blocked by naloxone. These results strongly support the suggestion that clinically used agonist-antagonists act at the NOP receptor to produce anti-analgesia.

Trace elemental signatures incorporated into calcified structures in fish and mollusk larvae have been used to reveal population connectivity and larval dispersal trajectories. To determine whether trace element analysis could be applied to larvae that do not permanently retain calcified structures, we raised larvae of the porcelain crab Petrolisthes cinctipes from multiple locations along the northern California, USA, coast in a common water source for up to 8 wk. We analyzed 20 elements in extractions of soft tissues from embryos and the 2 larval and 1 postlarval stages. Elemental signatures of individuals in each developmental stage were compared to an atlas of signatures from each collection site using discriminant analysis to determine whether larvae could be accurately assigned to their site of origin in 2 ways. First, postlarvae were classified using a natal site atlas constructed using embryonic signatures from each site, and this classification had poor success (average 39.7% correct). Second, larvae of each stage were classified using a natal site atlas constructed using signatures of larvae from that same stage. This yielded considerably better classification success (81.7% correct overall). Thus, the same trace element signatures were not consistently maintained from embryos to postlarvae, but differences in signatures among natal sites were maintained during the larval period. Trace element signatures in soft tissues could be useful in tracking dispersal between stages and determining how many sites, rather than which sites, contributed to a cohort of larvae or settlers. © Inter-Research 2013.

Natural trace element signatures have increasingly been used to track the dispersal of marine larvae, and these studies require an atlas of potential source populations with distinctive elemental signatures. To determine whether natal site atlases could be used repeatedly and to identify site characteristics that yield the best results, we built atlases in 5 consecutive years using embryos of the porcelain crab Petrolisthes cinctipes that were collected from 15 sites spanning 190 km of the open coast of northern California, USA. We analyzed the elemental composition of the embryos using a discriminant function optimization procedure to determine the suite of elements that resulted in the best reclassification success for individual sites and groups of sites each year. No single element or group of elements succeeded in discriminating the origins of embryos every year, and the reclassification success of the atlas varied at all spatial scales among years. Average reclassification success at the site level ranged annually from 39.5 to 54.3% correct, and combining sites into 2 or 3 areas improved the overall reclassification success to 72.5 to 97.7% correct. Sites with (1) distinctive elemental compositions of rocks, (2) unusual habitats, (3) consistent freshwater input, (4) consistent anthropogenic inputs, or (5) complex local oceano graphy had the highest reclassification success (up to 86.7% correct), but interannual variation in runoff reduced the temporal stability of the atlas. To improve population connectivity estimates, future trace element studies should consider these 5 key characteristics when selecting sites and anticipate temporal variation in natal site signatures. © Inter-Research 2013.

While chronic pain is a main symptom in endometriosis, the underlying mechanisms and effective therapy remain elusive. We developed an animal model enabling the exploration of ectopic endometrium as a source of endometriosis pain. Rats were surgically implanted with autologous uterus in the gastrocnemius muscle. Within two weeks, visual inspection revealed the presence of a reddish-brown fluid-filled cystic structure at the implant site. Histology demonstrated cystic glandular structures with stromal invasion of the muscle. Immunohistochemical studies of these lesions revealed the presence of markers for nociceptor nerve fibers and neuronal sprouting. Fourteen days after surgery rats exhibited persistent mechanical hyperalgesia at the site of the ectopic endometrial lesion. Intralesional, but not contralateral, injection of progesterone was dose-dependently antihyperalgesic. Systemic administration of leuprolide also produced antihyperalgesia. In vivo electrophysiological recordings from sensory neurons innervating the lesion revealed a significant increase in their response to sustained mechanical stimulation. These results are consistent with clinical and pathological findings observed in patients with endometriosis, compatible with the ectopic endometrium as a source of pain. This model of endometriosis allows mechanistic exploration at the lesion site facilitating our understanding of endometriosis pain.