Differential mediation of cold water swim stress-induced antinociception by delta-opioid receptor subtypes in diabetic mice

Role of delta-opioid receptor subtypes in anxiety-related behaviors in the elevated plus-maze in rats

RATIONALE

Recent studies have shown that endogenous opioid systems are associated with the regulation of emotional responses. In particular, it has been reported that delta-opioid receptors act naturally to inhibit stress and anxiety.

OBJECTIVE

The present study was designed to examine the possible involvement of opioid delta-receptor subtypes in the anxiety-related behavior in the elevated-plus-maze test.

METHODS

Six-week-old male Lewis rats were used. The total numbers of visits to the closed and open arms and the cumulative time spent and visits in the open arms were determined. Plasma corticosterone levels were measured by enzyme immunoassay.

RESULTS

Naltrindole (NTI), a delta-opioid receptor antagonist (3 mg/kg s.c.), induced a significant decrease in the percentages of time spent and visits in the open arms. Naltriben (NTB), a delta2-opioid receptor antagonist (3 mg/kg s.c.), but not 7-benzylidenenaltrexone, a delta1-opioid receptor antagonist, produced similar anxiety-related behaviors in the elevated plus-maze. These effects of NTI and NTB were antagonized by pretreatment with (+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), a delta-opioid receptor agonist. Furthermore, after exposure to the elevated plus-maze, the maximal increase in the plasma corticosterone level in NTI-treated rats was clearly higher than that in vehicle-treated rats. However, when NTI and SNC80 were coadministered, higher levels of plasma corticosterone were not seen after exposure to the elevated plus-maze.

CONCLUSION

These results suggest that endogenous delta2-opioid-receptor-mediated systems are involved in the regulation of anxiety-related behaviors and might play a physiologically important role in the regulation of adrenocortical activity.

delta-Opioid receptor agonist SNC80 elicits peripheral antinociception via delta(1) and delta(2) receptors and activation of the l-arginine/nitric oxide/cyclic GMP pathway

In this study, we characterized the role of delta(1) and delta(2) opioids receptors, as well the involvement of the l-arginine/NO/cGMP pathway in the peripheral antinociception induced by delta-opioid receptor agonist (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80). The paw pressure test was utilized, in which pain sensitivity is increased by intraplantar injection of prostaglandin E(2) (2 microg). Administration of SNC80 (20, 40 and 80 microg/paw) decreased the hyperalgesia induced by prostaglandin E(2) in a dose-dependent manner. The possibility that the higher dose of SNC80 (80 microg) has a central or systemic effect was excluded, since administration of the drug into the contralateral paw did not elicit antinociception in the right paw. 7-Benzylidenenaltrexone (BNTX), 5, 10 and 20 microg/paw, and 17-(Cyclopropylmethyl)-6,7-didehydro-3,14beta-dihydroxy-4,5alpha-epoxy-6,7-2',3'-benzo[b]furanomorphinan (naltriben), 2.5, 5 and 10 microg/paw, delta(1) and delta(2) opioid receptor antagonist respectively, elicited partial antagonism of the peripheral antinociceptive effect of the SNC80 (80 microg). The BNTX (10 microg/paw)-naltriben (5 microg/paw) combination completely antagonized the peripheral antinociception induced by SNC80 (80 microg). Further, blockers of the l-arginine/NO/cGMP pathway, N(G)-nitro-l-arginine (12, 18 and 24 microg/paw) and methylene blue (125, 250 and 500 microg/paw) were observed reverting the peripheral antinociceptive effect of SNC80. This study provides evidence that the peripheral antinociception induced by SNC80 occurs via delta(1) and delta(2) receptors and may result from l-arginine/NO/cGMP pathway activation.

The role of delta-opioid receptor subtypes in neuropathic pain

A large body of evidence suggests an important role of delta-opioid receptor agonists in antinociception at the level of the spinal cord. Our study was undertaken to analyse the spinal antinociceptive and antiallodynic effects of delta(1)- and delta(2)-opioid receptor agonists and antagonist after their acute and chronic intrathecal administration in a neuropathic pain model in the rat. In rats with a crushed sciatic nerve, the delta(1)-opioid receptor agonist [D-Pen(2), D-Pen(5)]enkephalin (DPDPE, 5-25 microg i.t.) and the delta(2)-opioid receptor agonist deltorphin II (1.5-25 microg i.t.) dose dependently antagonized the cold-water allodynia which developed after sciatic nerve injury. These effects of DPDPE were antagonized by 7-benzylidenenaltrexon (BNTX, 1 microg i.t.) while the effects of deltorphin II were antagonized by 5'naltrindole izotiocyanate (5'NTII, 25 microg i.t.). Both agonists had a dose-dependent, statistically significant effect on the tail-flick latency in two tests, with focused light and cold water. Chronic administration of DPDPE (25 microg i.t.) and deltorphin II (15 microg i.t.) resulted in significant prolongation of the reaction time determined on days 2, 4 and 6 post-injury. In conclusion, our results show an antiallodynic and antinociceptive action of DPDPE and deltorphin II at the spinal cord level, which suggests that both delta-opioid receptor subtypes play a similar role in neuropathic pain. This indicates that not only delta(1)- but also delta(2)-opioid receptor agonists can be regarded as potential drugs for the therapy of neuropathic pain.

Algogenic mediator-induced nociceptive response in diabetic mice

The duration of the somatostatin-, bradykinin- or prostaglandin F2alpha-induced nociceptive response was significantly less in diabetic mice than in non-diabetic mice. Subcutaneous injection of 7-benzylidenenaltrexone (0.1, 0.3 and 1 mg/kg), an antagonist of delta1-opioid receptors, had no significant effect on either somatostatin-, bradykinin- or prostaglandin F2alpha-induced nociceptive responses in non-diabetic mice. 7-Benzylidenenaltrexone (0.1 and 0.3 mg/kg, s.c.) also had no significant effect on somatostatin- or prostaglandin F2alpha-induced nociceptive responses in diabetic mice. However, the bradykinin-induced nociceptive response in diabetic mice was dose-dependently and significantly increased when 7-benzylidenenaltrexone (0.1, 0.3 and 1 mg/kg, s.c.) was injected 10 min before the injection of bradykinin. These results suggest that a spinal delta1-opioid receptor-mediated endogenous antinociceptive system may inhibit the bradykinin-mediated nociceptive responses in the second phase of the formalin-induced nociceptive response in diabetic mice.

The role of spinal delta1-opioid receptors in inhibiting the formalin-induced nociceptive response in diabetic mice

Injection of formalin into the hindpaw of mice produced a biphasic nociceptive response consisting of immediate (first-phase) and tonic (second-phase) components. In diabetic mice, the flinching response of the first phase was increased while that in the second phase was decreased in diabetic mice relative to the results in non-diabetic mice. To examine the role of supraspinal and/or spinal endogenous delta1-opioid receptors in inhibiting the formalin-induced nociceptive response in diabetic mice, we assessed the effect of 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist, and naltriben, a selective delta2-opioid receptor antagonist, administered either i.c.v. or i.t., on the formalin-induced flinching response. The second-phase response appeared when diabetic mice were pretreated with 7-benzylidenenaltrexone (0.1 and 0.3 mg/kg, s.c.), but not with naltriben (0.3 and 1 mg/kg, s.c.). On the other hand, while 7-benzylidenenaltrexone (0.1, 0.3 and 1 microg/mouse) administered i.t. had no significant effect on the first phase, it significantly and dose-dependently increased the second phase of the formalin-induced flinching response in diabetic mice. 7-Benzylidenenaltrexone (1 and 3 microg/mouse) administered i.c.v. had no significant effect on either the first- or the second-phase response in both non-diabetic and diabetic mice. These results suggest that a spinal delta1-opioid receptor-mediated endogenous antinociceptive system may inhibit the formalin-induced second phase of the nociceptive response in diabetic mice.