Abstract
BackgroundStress-induced analgesia (SIA) is an evolutionarily conserved phenomenon during stress. Neuropeptide S (NPS), orexins, substance P, glutamate and endocannabinoids are known to be involved in stress and/or SIA, however their causal links remain unclear. Here, we reveal an unprecedented sequential cascade involving these mediators in the lateral hypothalamus (LH) and ventrolateral periaqueductal gray (vlPAG) using a restraint stress-induced SIA model.MethodsMale C57BL/6 mice of 8–12 week-old were subjected to intra-cerebroventricular (i.c.v.) and/or intra-vlPAG (i.pag.) microinjection of NPS, orexin-A or substance P alone or in combination with selective antagonists of NPS receptors (NPSRs), OX1 receptors (OX1Rs), NK1 receptors (NK1Rs), mGlu5 receptors (mGlu5Rs) and CB1 receptors (CB1Rs), respectively. Antinociceptive effects of these mediators were evaluated via the hot-plate test. SIA in mice was induced by a 30-min restraint stress. NPS levels in the LH and substance P levels in vlPAG homogenates were compared in restrained and unrestrained mice.ResultsNPS (i.c.v., but not i.pag.) induced antinociception. This effect was prevented by i.c.v. blockade of NPSRs. Substance P (i.pag.) and orexin-A (i.pag.) also induced antinociception. Substance P (i.pag.)-induced antinociception was prevented by i.pag. Blockade of NK1Rs, mGlu5Rs or CB1Rs. Orexin-A (i.pag.)-induced antinociception has been shown previously to be prevented by i.pag. blockade of OX1Rs or CB1Rs, and here was prevented by NK1R or mGlu5R antagonist (i.pag.). NPS (i.c.v.)-induced antinociception was prevented by i.pag. blockade of OX1Rs, NK1Rs, mGlu5Rs or CB1Rs. SIA has been previously shown to be prevented by i.pag. blockade of OX1Rs or CB1Rs. Here, we found that SIA was also prevented by i.c.v. blockade of NPSRs or i.pag. blockade of NK1Rs or mGlu5Rs. Restrained mice had higher levels of NPS in the LH and substance P in the vlPAG than unrestrained mice.ConclusionsThese results suggest that, during stress, NPS is released and activates LH orexin neurons via NPSRs, releasing orexins in the vlPAG. Orexins then activate OX1Rs on substance P-containing neurons in the vlPAG to release substance P that subsequently. Activates NK1Rs on glutamatergic neurons to release glutamate. Glutamate then activates perisynaptic mGlu5Rs to initiate the endocannabinoid retrograde inhibition of GABAergic transmission in the vlPAG, leading to analgesia.
Highlights
Stress-induced analgesia (SIA) is an evolutionarily conserved phenomenon during stress
Neuropeptide S (NPS) (i.c.v.)-induced antinociception was antagonized by i.c.v. blockade of NPS receptors (NPSRs) To investigate if the central antinociceptive effect of NPS is mediated via NPSR, we co-administered [tBu-DGly5] NPS (10 nmol, i.c.v.), a selective and potent NPSR antagonist [47], along with NPS (0.3 or 1.0 nmol, i.c.v.) to mice before the hot-plate test. [tBu-D-Gly5] NPS at 10 nmol (i.c.v.) did not affect the nociceptive response in naïve mice, but completely blocked the antinociceptive effect of i.c.v
NPS (i.c.v.)-induced antinociception was antagonized by i.pag. blockade of OX1 receptors (OX1Rs), NK1 receptors (NK1Rs), mGlu5 receptors (mGlu5Rs) or CB1 receptors (CB1Rs) we investigated whether the established OX1R-NK1R-mGlu5R-CB1R cascade in the ventrolateral periaqueductal gray (vlPAG) is involved in the supraspinal antinociceptive effect of NPS
Summary
Stress-induced analgesia (SIA) is an evolutionarily conserved phenomenon during stress. We have shown that orexins can be released during stress and contribute to SIA, at least in part, via opioid-independent and endocannabinoid (eCB)dependent signaling [11, 12] in the ventrolateral periaqueductal gray (vlPAG), a crucial midbrain region for the initiation of descending pain inhibition [14, 15]. Orexins are released during stress [12], and orexins are known to induce antinociception by activating postsynaptic OX1Rs to generate 2arachidonoylglycerol (2-AG) [16, 17], an eCB, through a Gq protein-coupled enzymatic cascade mediated by phospholipase C (PLC) and diacylglycerol lipase (DAGL) [18], culminating in retrograde inhibition of GABA release (disinhibition) in the vlPAG [11, 12]
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