Increases In Hindpaw Withdrawal Latencies Research Articles

The activation of galanin receptor 2 plays an antinociceptive effect in nucleus accumbens of rats with neuropathic pain

Our previous research has shown that galanin plays an antinociceptive effect via binding to galanin receptors (GalRs) in nucleus accumbens (NAc). This study focused on the involvement of GalR2 in galanin-induced antinociceptive effect in NAc of neuropathic pain rats. The chronic constriction injury of sciatic nerve (CCI) was used to mimic neuropathic pain model. The hind paw withdrawal latency (HWL) to thermal stimulation and hind paw withdrawal threshold (HWT) to mechanical stimulation were measured as the indicators of pain threshold. The results showed that 14 and 28 days after CCI, the expression of GalR2 was up-regulated in bilateral NAc of rats, and intra-NAc injection of GalR2 antagonist M871 reversed galanin-induced increases in HWL and HWT of CCI rats. Furthermore, intra-NAc injection of GalR2 agonist M1145 induced increases in HWL and HWT at day 14 and day 28 after CCI, which could also be reversed by M871. Finally, we found that M1145-induced antinociceptive effect in NAc of CCI rats was stronger than that in intact rats. These results imply that the GalR2 is activated in the NAc from day 14 to day 28 after CCI and GalR2 is involved in the galanin-induced antinociceptive effect in NAc of CCI rats.

Galanin Receptor 2 Is Involved in Galanin-Induced Analgesic Effect by Activating PKC and CaMKII in the Nucleus Accumbens of Inflammatory Pain Rats.

It has been reported that galanin has an analgesic effect via activating galanin receptors (GALRs). This study focused on the involvement of GALR2 in the galanin-induced analgesic effect and its signaling mechanism in the nucleus accumbens (NAc) of inflammatory rats. Animal models were established through injecting carrageenan into the plantar of rats’ left hind paw. The results showed that GALR2 antagonist M871 weakened partially the galanin-induced increases in hind paw withdrawal latency (HWL) to thermal stimulation and hind paw withdrawal threshold (HWT) to mechanical stimulation in NAc of inflammatory rats. Moreover, the GALR2 agonist M1145 prolonged the HWL and HWT, while M871 blocked the M1145-induced increases in HWL and HWT. Western blotting showed that the phosphorylation of calcium/calmodulin-dependent protein kinase II (p-CaMKII) and protein kinase C (p-PKC) in NAc were upregulated after carrageenan injection, while p-PKC and p-CaMKII were downregulated after intra-NAc administration of M871. Furthermore, the CaMKII inhibitor KN93 and PKC inhibitor GO6983 attenuated M1145-induced increases in HWL and HWT in NAc of rats with inflammatory pain. These results prove that GALR2 is involved in the galanin-induced analgesic effect by activating CaMKII and PKC in NAc of inflammatory pain rats, implying that GALR2 agonists probably are potent therapeutic options for inflammatory pain.

Galanin plays a role in antinociception via binding to galanin receptors in the nucleus accumbens of rats with neuropathic pain

Galanin and galanin receptors (GalRs) play important roles in the transmission and modulation of nociceptive information. Our previous research has shown that the expression of GalR1 is upregulated and that GalR1 activation in the nucleus accumbens (NAc) of rats with neuropathic pain has an antinociceptive effect. However, the antinociceptive effect of NAc galanin in neuralgia remains unclear. The present study aimed to explore the antinociceptive effect induced by galanin in rats with neuropathic pain and the underlying mechanism. The results showed that the intra-NAc injection of galanin induced a dose-dependent increase in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in mononeuropathic rats and that this effect was stronger than that in intact rats. The intra-NAc injection of the non-selective GalR antagonist galantide reduced HWL in the rats with neuropathic pain, but there was no influence of galantide on HWL in intact rats. Moreover, galanin expression in the NAc was upregulated after sciatic nerve ligation. All of these results demonstrate that galanin plays a role in antinociception via binding to GalRs in the NAc of rats and that endogenous galanin is involved in the antinociception after peripheral nerve injury.

Involvement of galanin and galanin receptor 1 in nociceptive modulation in the central nucleus of amygdala in normal and neuropathic rats

The present study was performed to explore the role of galanin and galanin receptor 1 (GalR 1) in nociceptive modulation in the central nucleus of amygdala (CeA) in normal rats and rats with neuropathy, and the involvement of GalR 1 and PKC was also investigated. The hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulations were increased in a dose-dependent manner after intra-CeA injection of galanin in both normal rats and rats with neuropathy. The increased HWLs were significantly attenuated by intra-CeA injection of galanin receptor antagonist M40, indicating an involvement of galanin receptor in nociceptive modulation in CeA. Furthermore, intra-CeA administration of the GalR 1 agonist M 617 induced increases in HWLs in normal rats, suggesting that GalR 1 may be involved in galanin-induce antinociception in CeA. Additionally, intra-CeA injection of the PKC inhibitor inhibited galanin-induced antinociception, showing an involvement of PKC in galanin-induced antinociception in CeA of normal rats. Moreover, there was a significant increase in GalR1 content in CeA in rats with neuropathy than that in normal rats. These results illustrated that galanin induced antinociception in CeA in normal rats and rats with neuropathy, and there is an up-regulation of GalR1 expression in rats with neuropathy.

Involvement of galanin and galanin receptor 2 in nociceptive modulation in anterior cingulate cortex of normal rats and rats with mononeuropathy

The present study was performed to explore the role of galanin and galanin receptor 2 in nociceptive modulation in anterior cingulate cortex (ACC) of normal rats and rats with mononeuropathy. Intra-ACC injection of galanin induced significant increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulations in both normal rats and rats with mononeuropathy, the increased HWLs were attenuated significantly by intra-ACC injection of galanin receptor 2 antagonist M871, indicating an involvement of galanin receptor 2 in nociceptive modulation in ACC. Interestingly, the galanin-induced HWL was significant higher in rats with mononeuropathy than that in normal rats tested by Randall Selitto test. Furthermore, both the galanin mRNA expression and galanin content increased significantly in ACC in rats with mononeuropathy than that in normal rats. Moreover, both the mRNA levels of galanin receptor 2 and the content of galanin receptor 2 in ACC increased significantly in rats with mononeuropathy than that in normal rats. These results found that galanin induced antinociception in ACC in both normal rats and rats with mononeuropathy. And there may be plastic changes in the expression of galanin and galanin receptor 2 in rats with mononeuropathy, as well as in the galanin-induced antinociception.

Antinociceptive effects induced by intra-lateral habenula complex injection of the galanin receptor 1 agonist M617 in rats

The present study was performed to explore the antinociceptive effects of the galanin receptor 1 agonist M617 in lateral habenula complex in rats. Intra-lateral habenula injection of 0.1, 0.5, 1 or 2 nmol of galanin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, intra-lateral habenula injection of 0.1, 0.5, 1 or 2 nmol of the galanin receptor 1 agonist M617 also induced dose-dependent increases in HWLs to noxious thermal and mechanical stimulations in rats. Interestingly, there were no significant differences between the antinociceptive effects induced by intra-lateral habenula injection of 2 nmol of M617 and 2 nmol of galanin. The results indicate that galanin receptor 1 may be involved in the galanin-induced antinociceptive effects in the lateral habenula.

Pharmacologically inhibiting GluR2 internalization alleviates neuropathic pain.

Neuropathic pain is of serious clinical concern and only about half of patients achieve partial relief with currently-available treatments, so it is critical to find new drugs for this condition. Recently, the cellsurface trafficking of pain-related receptors has been suggested as an important mechanism underlying persistent neuropathic pain. Here, we used the short peptide GluA2-3y, which specifically inhibits the GluA2-dependent endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and tested its anti-nociceptive effect in the periaqueductal grey (PAG) of intact rats and rats with neuropathic pain. Intra-PAG injection of 0.15, 1.5, 7.5, and 15 pmol of GluA2-3y induced dose-dependent increases in hindpaw withdrawal latencies to noxious thermal and mechanical stimuli in intact rats, suggesting that GluA2 cell-surface trafficking in the PAG is involved in pain modulation. Furthermore, GluA2-3y had much stronger anti-nociceptive effects in rats with neuropathic pain induced by sciatic nerve ligation. Interestingly, the intra-PAG injection of 15 pmol GluA2-3y had an analgesic effect similar to 10 μg (35 nmol) morphine in rats with neuropathic pain. Taken together, our results suggested that GluA2 trafficking in the PAG plays a critical role in pain modulation, and inhibiting GluA2 endocytosis with GluA2-3y has potent analgesic effects in rats with neuropathic pain. These findings strongly support the recent hypothesis that targeting receptor trafficking could be a new strategy for the treatment of neuropathic pain.

Antinociceptive roles of galanin receptor 1 in nucleus accumbens of rats in a model of neuropathic pain.

It has been reported that galanin and its receptors might be involved in the modulation and transmission of nociception in the central nervous system. Our previous research has also demonstrated that galanin induces antinociception in the nucleus accumbens (NAc) of intact rats. However, the interaction between galanin and its receptors in the NAc and the underlying mechanism of suppressing pain transmission remain unclear. The present study seeks to determine the antinociception induced by galanin receptor (GalR)-1 stimulation in the NAc of rats with neuropathic pain. The left sciatic nerve of rats was ligated to mimic a neuropathic pain model. Western blots showed that the expression of GalR1 was significantly upregulated in the NAc of rats with neuropathic pain. Intra-NAc injection of GalR1 agonist M617 induced a dose-dependent increase in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulations in rats with neuropathic pain. Also, the effect of M617 was attenuated by M35, a GalR1/2 antagonist; at the same time, M35 reduced the galanin-induced antinociception, suggesting that GalR1 mediates antinociception induced by galanin in the NAc of rats with neuropathic pain. Furthermore, we found that M617-induced antinociception in rats with neuropathic pain was stronger than the antinociception in intact rats. We also found that injections of M617 and galanin each induced significant increases in HWL, but the galanin-induced antinociception was stronger than that of M617. All these results suggest that GalR1 plays an important role in antinociception and that other GalRs also are involved in pain modulation induced by galanin in the NAc of rats with neuropathic pain.

Antinociceptive effects induced by intra-periaqueductal grey injection of the galanin receptor 1 agonist M617 in rats with morphine tolerance

The present study was performed to investigate the antinociceptive effects of M617, a selective galanin receptor 1 agonist, and M1145, a selective galanin receptor 2 agonist, in the periaqueductal grey (PAG) in rats with morphine tolerance. Intra-PAG injection of 0.1nmol, 0.5nmol and 1nmol of M617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats with morphine tolerance. Nevertheless, intra-PAG injection of 5nmol of the selective galanin receptor 2 agonist M1145 showed no significant influences on HWLs to noxious thermal and mechanical stimulations in rats with morphine tolerance. The results demonstrated that it is the selective galanin receptor 1 agonist M617, not the selective galanin receptor 2 agonist M1145, induced significant antinociceptive effects in morphine-tolerant rats, indicating that galanin receptor 1 is involved in nociceptive modulation in the PAG of morphine-tolerant rats.

Antinociceptive effects induced by injection of the galanin receptor 1 agonist M617 into central nucleus of amygdala in rats

The present study was performed to explore the antinociceptive effects of M617, a selective galanin receptor 1 agonist, in the central nucleus of amygdala (CeA) of rats. Intra-CeA injection of 0.1nmol, 0.5nmol and 1nmol of M617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, rats received intra-CeA administration of M617 and galanin. The HWL to noxious thermal and mechanical stimulations increased markedly, and there were no significant differences in HWLs of rats received intra-CeA administration of M617 and galanin. The results demonstrated that intra-CeA injection of M617 induced significant antinociceptive effects in CeA of rats, indicating that galanin receptor 1 may be involved in M617-induced antinociception in the CeA of rats.

Antinociceptive effects of intracerebroventricular injection of the galanin receptor 1 agonist M 617 in rats

Previous studies in our laboratory demonstrated that galanin and its receptors play important roles in nociceptive modulation in the central nervous system. The present study was performed to explore the antinociceptive effects of the galanin receptor 1 agonist M 617 in the central nervous system of rats. Intracerebroventricular injection of 0.1nmol, 0.5nmol, 1nmol or 2nmol of M 617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, both intracerebroventricular injection of M 617 and galanin induced significant increases in HWLs in rats. Interestingly, there were no significant differences between the antinociceptive effects induced by M 617 and galanin, indicating that galanin receptor 1 plays main roles in galanin-induced antinociceptive effects in the brain of rats.

Antinociceptive effects of galanin in the central nucleus of amygdala of rats, an involvement of opioid receptors

The central nucleus of amygdala (CeA) is a very important brain structure involved in multiple physiological functions, especially in pain modulation. There are high densities of galanin and galanin receptors found in the CeA. The present study was performed to explore the antinociceptive effects of galanin in the CeA of rats, and possible involvements of opioid receptors in the galanin-induced antinociception. Intra-CeA injection of galanin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Interestingly, the amtinociceptive effect induced by intra-CeA injection of galanin was blocked by intra-CeA injection of naloxone, a common opioid receptor antagonist, indicating an involvement of opioid receptors in the galanin-induced antinociception in the CeA of rats. Moreover, intra-CeA injection of either selective mu-opioid receptor antagonist β-funaltrexamine (β-FNA) or delta-opioid receptor antagonist naltrindole, but not kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI), significantly attenuated the galanin-induced increases in HWLs in the CeA of rats. Taken together, the results demonstrate that galanin induces antinociceptive effects in the CeA of rats, and both mu- and delta-opioid receptors are involved in the galanin-induced antinociception.

The neural pathway of galanin in the hypothalamic arcuate nucleus of rats: Activation of beta‐endorphinergic neurons projecting to periaqueductal gray matter

We have previously shown that microinjection of galanin into the arcuate nucleus of hypothalamus (ARC) produced antinociceptive effects in rats (Sun et al., 2003a). In this study, the neural pathway of galanin from ARC to midbrain periaqueductal gray (PAG) in nociceptive modulation was investigated. The hindpaw withdrawal latencies (HWLs) with noxious thermal and mechanical stimulation were assessed by the hotplate and the Randall Selitto tests. Intra-ARC administration of 0.1, 0.5, or 1 nmol of galanin induced significant increases in HWLs of rats. The galanin-induced increases in HWLs were inhibited by injection of 10 microg of the opioid receptor antagonist naloxone or 1 nmol of the mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) into PAG, suggesting that the antinociceptive effects induced by intra-ARC injection of galanin occur via the neural pathway from ARC to PAG. Furthermore, our results demonstrate that the galaninergic fibers directly innervated the beta-endorphinergic neurons in ARC by immunofluorescent methods. Taken together, our results suggest that galanin produces antinociceptive effects in the ARC of rats by activating the beta-endorphinergic pathway from ARC to PAG.

It is AMPA receptor, not kainate receptor, that contributes to the NBQX-induced antinociception in the spinal cord of rats

Studies demonstrated that intrathecal 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo[f]quinoxaline-7-sulfonamide disodium (NBQX), an antagonist of AMPA/kainate receptors, induced antinociception in the spinal cord of rats. The present study demonstrated that the NBQX-induced increases in hindpaw withdrawal latencies (HWLs) were dose-dependently attenuated by intrathecal pretreatment of the AMPA receptor desensitization inhibitor, diazoxide. The effect was unrelated to the opening of K + channels by diazoxide. On the other hand, intrathecal pretreatment of concanavalin A, which selectively inhibits the desensitization of kainate receptor, produced no significant influence on the NBQX-induced antinociception. The results suggest that the NBQX-induced antinociception was mediated by AMPA receptors, not by kainate receptors, in the spinal cord of rats.

Involvement of neuropeptide Y and Y1 receptor in antinociception in the arcuate nucleus of hypothalamus, an immunohistochemical and pharmacological study in intact rats and rats with inflammation

Neuropeptide Y (NPY) plays an important role in pain modulation at different levels in the central nervous system. In the brain, NPY and NPY receptors distribute abundantly in the arcuate nucleus of hypothalamus (ARC), a structure involved in pain processing. The present study was undertaken to investigate the role of NPY in nociceptive modulation in the ARC of intact rats and rats with carrageenan-induced inflammation. Intra-ARC administration of NPY induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation in intact rats, which was attenuated by the Y1 receptor antagonist NPY28-36. Intra-ARC administration of NPY also induced dose-dependent increases in HWLs to noxious stimulation in rats with inflammation. Furthermore, intra-ARC injection of either the antiserum against NPY or NPY28-36 induced decreases in HWLs in rats with inflammation, while both of them produced no effects in intact ones. Additionally, there were marked increases of Y1 receptor in the bilateral ARC of rats with inflammation tested by immunohistochemistry, while no significant changes of NPY were observed, implicating that the increased Y1 receptor has an important effect in the NPY-induced antinociception. We also found that intra-ARC injection of Y2 receptor agonist NPY3-36 produced no significant antinociception in either intact rats or rats with inflammation. Together, we demonstrate that NPY exerts an antinociceptive effect in the ARC of intact rats and rats with inflammation. Both Y1 receptor and endogenous released NPY in the ARC are involved in the nociceptive modulation during inflammation.

Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, β-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both β-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective μ-opioid receptor antagonist β-funaltrexamine (β-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the δ-opioid receptor antagonist naltrindole or the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially μ-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.

Involvement of opioid receptors in the oxytocin-induced antinociception in the central nervous system of rats

Recent studies showed that oxytocin and opioid peptides play important roles in pain modulation at different levels in the central nervous system. The present study was performed to explore whether opioid system is involved in the oxytocin-induced antinociception in the brain of rats. The results showed that: (1) intracerebroventricular injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWL) to noxious thermal and mechanical stimulation in rats. (2) The antinociceptive effect of oxytocin was attenuated dose-dependently by intracerebroventricular injection of naloxone, indicating an involvement of opioid system in the oxytocin-induced antinociception. (3) It is interesting that the antinociceptive effect of oxytocin was attenuated by subsequent intracerebroventricular injection of the μ-opioid antagonist β-funaltrexamine (β-FNA) and the κ-opioid antagonist nor-binaltorphimine (nor-BNI), but not the δ-opioid antagonist naltrindole. The results indicate that oxytocin plays an antinociceptive role in the brain of rats; μ- and κ-opioid receptors, not δ-receptors, are involved in the oxytocin-induced antinociception in the central nervous system of rats.

Antinociceptive role of oxytocin in the nucleus raphe magnus of rats, an involvement of μ-opioid receptor

Recent studies showed that oxytocin plays an important role in nociceptive modulation in the central nervous system. The present study was undertaken to investigate the role of oxytocin in antinociception in the nucleus raphe magnus (NRM) of rats and the possible interaction between oxytocin and the opioid systems. Intra-NRM injection of oxytocin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulation in rats. The antinociceptive effect of oxytocin was significantly attenuated by subsequent intra-NRM injection of the oxytocin antagonist 1-deamino-2- d-Tyr-(Oet)-4-Thr-8-Orn-oxytocin. Intra-NRM injection of naloxone dose-dependently antagonized the increased HWLs induced by preceding intra-NRM injection of oxytocin, indicating an involvement of opioid receptors in oxytocin-induced antinociception in the NRM of rats. Furthermore, the antinociceptive effect of oxytocin was dose-dependently attenuated by subsequent intra-NRM injection of the μ-opioid antagonist β-funaltrexamine (β-FNA), but not by the κ-opioid antagonist nor-binaltorphimine (nor-BNI) or the δ-opioid antagonist naltrindole. The results demonstrated that oxytocin plays an antinociceptive role in the NRM of rats through activating the oxytocin receptor. Moreover, μ-opioid receptors, not κ and δ receptors, are involved in the oxytocin-induced antinociception in the NRM of rats.

Anti-nociceptive effects of diazepam binding inhibitor in the central nervous system of rats

The present study investigated the effect of diazepam binding inhibitor (DBI) on nociception in the central nervous system of rats. There were dose-dependent increases in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation after intrathecal injection of 1, 5 or 10 μg of DBI in rats, indicating a DBI-induced anti-nociceptive effect at the spinal levels of rats. Furthermore, it was found that there were no significant influences of intrathecal co-administration of γ-aminobutyric acid (GABA) on the intrathecal DBI-induced increases in HWLs of rats. Intracerebroventricular administration of 1, 10 or 20 μg of DBI also induced dose-dependent increases in HWL to thermal and mechanical stimulation in rats, suggesting an anti-nociceptive effect of DBI in the brain. Moreover, there were no significant influences of intracerebroventricular co-administration of 2 μg of GABA on the intracerebroventricular DBI-induced increases in HWL of rats. The results of the present study demonstrated that DBI played anti-nociceptive effects in the central nervous system of rats.

Blockade effect of mu and kappa opioid antagonists on the anti-nociception induced by intra-periaqueductal grey injection of oxytocin in rats

Intra-periaqueductal grey (PAG) injection of 1 nmol of oxytocin induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. The anti-nociceptive effect of oxytocin was attenuated significantly by subsequent intra-PAG injection of the mu opioid antagonist β-funaltrexamine (β-FNA) and the kappa opioid antagonist nor-binaltorphimine (nor-BNI), but not by the delta antagonist naltrindole. The results demonstrated that mu and kappa opioid receptors, not delta receptors, were involved in the oxytocin-induced anti-nociception in PAG of rats.