Involvement of spinal mu1-opioid receptors on the Tyr-d-Arg-Phe-sarcosine-induced antinociception

The involvement of spinal mu-opioid receptor subtypes on the antinociception induced by i.t.-administered Tyr-D-Arg-Phe-sarcosine (TAPS), a N-terminal tetrapeptide analog of dermorphin, was determined in mice tail-flick test. Intrathecal administration of TAPS produced the marked inhibition of the tail-flick response in a dose-dependent manner. The antinociception induced by TAPS was completely eliminated by i.t.-co-administration of Tyr-D-Pro-Phe-Phe-NH2 (D-Pro2-endomorphin-2), the mu1-opioid receptor antagonist, whereas i.t. co-treatment with Tyr-D-Pro-Trp-Phe-NH2 (D-Pro2-endomorphin-1) or Tyr-D-Pro-Trp-Gly-NH2 (D-Pro2-Tyr-W-MIF-1), the mu2-opioid receptor antagonists, did not affect the TAPS-induced antinociception. In contrast, the antinociception induced by i.t.-administered [D-Ala2,N-MePhe4,Gly-ol5]enkephalin was significantly attenuated by i.t.-co-administration of D-Pro2-endomorphin-1 or D-Pro2-Tyr-W-MIF-1, but not D-Pro2-endomorphin-2. These results suggest that TAPS may stimulate spinal mu1-opioid receptors to produce the antinociception.     

Differential involvement of mu 1-opioid receptors in dermorphin tetrapeptide analogues-induced antinociception

The involvement of putative μ₁-opioid receptors in the antinociception induced by the dermorphin tetrapeptide analogues Try-d-Arg-Phe-β-Ala (TAPA) and Tyr-d-Arg-Phe-β-Ala-NH₂ (TAPA-NH₂) was determined in mice, using a tail-pressure test and a formalin test. TAPA and TAPA-NH₂ injected i.c.v. and i.t. produced dose-dependent antinociception in both assays. In the tail-pressure test, the antinociception induced by i.c.v. or i.t. injected TAPA, but not TAPA-NH₂, was significantly attenuated by pretreatment with naloxonazine, a selective antagonist for putative μ₁-opioid receptors. Moreover, naloxonazine also significantly attenuated the antinociception induced by i.c.v. injected TAPA, but not TAPA-NH₂, in the formalin test. In contrast, the antinociception induced by both TAPA and TAPA-NH₂ given i.t. was significantly attenuated by pretreatment with naloxonazine in the formalin test. The present results suggest that TAPA and TAPA-NH₂ should be considered selective agonists for putative μ₁- and μ₂-opioid receptors, respectively. The C-terminal amidation of TAPA-NH₂ may be critical for distinguishing between putative μ₁- and μ₂-opioid receptors.     

Involvement of mu1-opioid receptor on oxycodone-induced antinociception in diabetic mice

The effect of naloxonazine, a selective μ₁-opioid receptor antagonist, on oxycodone-induced antinociception was examined in streptozotocin-induced diabetic mice. Oxycodone (5 mg/kg, s.c.) induced significant antinociception in both non-diabetic and diabetic mice. This antinociceptive effect of oxycodone was completely antagonized by pretreatment with naloxonazine (35 mg/kg, s.c.) in both non-diabetic and diabetic mice. The selective κ-opioid receptor antagonist nor-binaltorphimine (20 mg/kg, s.c.) also antagonized oxycodone-induced antinociception in diabetic mice, but only had a partial effect in non-diabetic mice. These results suggest that although primarily interacts with μ₁-opioid receptor, κ-opioid receptors are also strongly involved in oxycodone-induced antinociception.     

Supraspinal mu 2-opioid receptors mediate spinal/supraspinal morphine synergy.

TRIMU-5 (Tyr-D-Ala-Gly-NHC₂H₄CH(CH₃)₂) is a potent μ₂-opioid agonist/μ₁-opioid antagonist. A supraspinal dose (0.5 μg i.c.v.) of TRIMU-5 which is not analgesic when given alone antagonizes the analgesia produced by intracerebroventricular (i.c.v.) morphine, a μ₁ action. In contrast, in a synergy model consisting of the simultaneous administration of intrathecal morphine (0.1 μg) with multiple doses of i.c.v. morphine, the same supraspinal TRIMU-5 dose (0.5 μg i.c.v.) enhances analgesia. Supraspinal TRIMU-5 also potentiates spinal morphine directly, shifting its dose-response to the left. These results imply that within the brainstem μ₁ receptors mediate supraspinal analgesia while μ₂ receptors mediate the synergy with spinal μ systems.     

Involvement of spinal delta 1-opioid receptors in forced walking stress-induced antinociception in the tail-flick test in mice

The purpose of this study was to elucidate the involvement of spinal delta-opioid receptor subtypes in forced walking stress-induced antinociception mice. We first confirmed that forced walking stress produced walking duration-dependent antinociception in mice as determined by the tail-flick test. Intrathecal treatment with 7-benzylidenenaltrexone, a selective delta 1-opioid receptor antagonist, significantly attenuated forced walking stress-induced antinociception. In contrast, intrathecal treatment with naltriben, a selective delta 2-opioid receptor antagonist, had no significant effect on forced walking stress-induced antinociception. Intracerebroventricular treatment with either 7-benzylidenenaltrexone or naltriben had no effect on the forced walking stress-induced antinociception. These results suggest that forced walking stress-induced antinociception is mediated by spinal delta 1-opioid receptors in mice.     

Contribution of dopamine receptors to periaqueductal gray-mediated antinociception

Rationale  Morphine relieves pain, in part, by acting on neurons within the periaqueductal gray (PAG). Given that the PAG contains a subpopulation of dopamine neurons, dopamine may contribute to the antinociceptive effects mediated by the PAG. Methods  This hypothesis was tested by measuring the behavioral and electrophysiological effects of administering dopamine agonists and antagonists into the ventrolateral PAG (vPAG). An initial histological experiment verified the existence of dopamine neurons within the vPAG using dopamine transporter and tyrosine hydroxylase antibodies visualized with confocal microscopy. Results  Microinjection of cumulative doses of morphine into the vPAG caused antinociception that was dose-dependently inhibited by the dopamine receptor antagonist α-flupenthixol. α-Flupenthixol had no effect on nociception when administered alone. Injection of the dopamine receptor agonist (−) apomorphine into the vPAG caused a robust antinociception that was inhibited by the D2 antagonist eticlopride but not the D1 antagonist SCH-23390. The effects of dopamine on GABA_A-mediated evoked inhibitory post-synaptic potentials (eIPSCs) were measured in PAG slices. Administration of met-enkephalin inhibited peak eIPSCs by 20–50%. Dopamine inhibited eIPSCs by approximately 20–25%. Administration of α-flupenthixol (20 μM) attenuated eIPSC inhibition by dopamine but had no effect on met-enkephalin-induced inhibition. Conclusions  These data indicate that PAG dopamine has a direct antinociceptive effect in addition to modulating the antinociceptive effect of morphine. The lack of an effect of α-flupenthixol on opioid-inhibition of eIPSCs indicates that this modulation occurs in parallel or subsequent to inhibition of GABA release. This investigation was supported, in part, by funds provided for medical and biological research by the State of Washington Initiative Measure No. 171 and by NIH grant DA015498.     

The effect of ciprofloxacin and gentamicin on spinal morphine-induced antinociception in rats

This paper investigates the possible antinociceptive effect of systemically administered ciprofloxacin and gentamicin and its influence on intrathecal morphine-induced antinociception. Using thermal nociceptive tests (hot-plate test and tail-flick test) and a motor function test (catalepsy test) in male Sprague-Dawley rats (n=5-9/dose), the following observations were made: ciprofloxacin administered intraperitoneally in the dose range 4-64 mg/kg demonstrated a modest antinociceptive effect in both nociceptive tests. Solvent of ciprofloxacin (intraperitoneally) and saline (intraperitoneally), given as a control, showed no effect. Gentamicin, administered at a dose of 0.1-4 mg/kg intraperitoneally, demonstrated a significant (P<0.05) antinociceptive effect in the tail-flick test but not in the hot-plate test. However, opioid antagonists caused no significant change in the antibiotics. Furthermore, ciprofloxacin intraperitoneally produced a significant left-shift in the hot-plate test (ED50 saline-morphine=2.86 [CI 95%: 2.2, 4.32]microg; ED50 ciprofloxacin-morphine=0.87 (CI 95% 0.68, 1.21) microg, P<0.05) and in the tail-flick test (ED50 saline-morphine=1.98 (CI 95%: 1.21, 2.84) microg; ED50 ciprofloxacin-morphine=0.37 (CI 95%: 0.23, 0.44) microg; P<0.05) for intrathecal morphine-induced antinociception. From a comparison of these data with the predicted ciprofloxacin-morphine value (hot-plate test: 1.61 (CI 95%: 1.18, 2.51]microg; tail-flick test: 0.82 (CI 95%: 0.52, 1.92) microg) we estimate that ciprofloxacin and morphine produce at least additive effects (P>0.05). This was reversed with intraperitoneal naloxone (P<0.05). Gentamicin intraperitoneally did not influence the antinociception achieved with intrathecal administration of morphine (hot-plate test: ED50 gentamicin-morphine=2.71 (CI 95%: 2.35; 3.2) microg; tail-flick test: ED50 gentamicin-morphine=2.43 (CI 95%: 1.58; 5.22]microg; P>0.05). These data show that intraperitoneal administration of ciprofloxacin and gentamicin produces a modest antinociceptive effect in the hot-plate test and tail-flick test. Ciprofloxacin, but not gentamicin, can interact at least additively to increased naloxone-reversible morphine intrathecal antinociception. Differences in the ability to penetrate the blood-brain barrier between the two antibiotics could explain the lack of effect from gentamicin in the hot plate and on morphine-induced antinociception.     

Structure-activity studies of novel casomorphin analogues: binding profiles towards mu 1-, mu 2- and delta -opioid receptors.

The beta -casomorphin-5 sequence was systematically modified by substitution of the naturally occurring amino acids. The derivatives are compared on the basis of their affinities towards mu 1-, mu 2 and delta -opioid binding sites estimated by means of binding studies with [3H]dihydromorphine and [3H]D-Ala2-Leu5-enkephalin as labels and computer curve fitting. A C-terminal amide group which is known to increase mu-site affinity of beta -CM-4 and beta -CM-5 mainly enhances the mu 2-site affinity. Furthermore, the Pro4-amide structure, which seems to be responsible for the affinity enhancement can be substituted by the pyrrolidide ring structure. Modifications in position 2, 3 and 4 can lead to an increase in mu 1-, mu 2- or delta -site affinity and/or selectivity. The specificity of these effects might be dependent on the respective changes in the charge or hydrophobicity due to the introduction of other amino acids. The results suggest firstly that the alternating proline residues in the beta -CM-5 molecule are not absolutely necessary for its mu-site affinities, and secondly that both opioid receptor subtype affinity and selectivity may be modified by changing charge and/or hydrophobicity in the middle part of the beta -casomorphin molecule.     

Zimelidine attenuates the development of tolerance to morphine-induced antinociception

Objectives:

The aim of this study was to investigate effect of zimelidine (a serotonin reuptake inhibitor) on morphine-induced tolerance in rats.

Materials and Methods:

Male Wistar albino rats weighing 160–180 g were used in these experiments (n=72). A 3-day cumulative dosing regimen was used for the induction of morphine tolerance. To constitute of morphine tolerance, animals received morphine twice daily for 3 days. After the last dose morphine was injected on the fourth day, morphine tolerance was evaluated. The analgesic effects of zimelidine (15 mg/kg; i.p.) and morphine (5 mg/kg) were considered at 30-min time intervals (0, 30, 60, 90 and 120 min) by tail-flick and hot-plate analgesiometer (n=6 in each experimental group).

Results:

The results showed that zimelidine significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effect of zimelidine was obtained at the 60 minutes measurements in the zimelidine group and at the 30 minutes measurements in the morphine tolerant group by the tail-flick and hot-plate tests. Administration of zimelidine with morphine showed additive analgesic effect.

Conclusion:

In conclusion, our results show that zimelidine reduces the development of tolerance to morphine-induced antinociception in rats.KEY WORDS: Antinociception, hot-plate test, morphine, tail-flick test, zimelidine     

Contribution of supraspinal mu- and delta-opioid receptors to antinociception in the rat.

This study evaluated the contribution of supraspinal opioid receptors to the production of antinociception, in the rat. I.c.v. administration of a selective mu- (DAMGO) and a selective delta- (DPDPE), but not a selective kappa- (U50,488H) opioid receptor agonist, produced significant dose-dependent increase in mechanical nociceptive thresholds. ICI 174,864, a delta-opioid receptor antagonist, completely blocked the antinociceptive effects produced by DPDPE ([D-Pen2,D-Pen5]enkephalin) at a dose that had no effect on the increases in nociceptive thresholds produced by DAMGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin). The simultaneous i.c.v. administration of a low-antinociceptive dose of DAMGO or DPDPE given in combination with sequentially increasing doses of the other opioid agonist, produced synergy (i.e., a more than additive antinociceptive effect), at the lower doses tested. The results of these experiments provide evidence to support the suggestion that both supraspinal mu- and delta-opioid receptors contribute to the production of antinociception, in the rat.     

Influence of glycine on morphine-induced antinociception in mice

The effects of glycine on morphine-induced antinociception were investigated in mice, using a cutaneous thermal test (hot-plate), a visceral chemical test (acetylcholine writhing test), and a locomotor activity test. When glycine (200 mg/kg p.o.) and morphine (5 mg/kg s.c.) were given together during the first 30 min, glycine first antagonized the morphine-induced antinociception then this was followed by a synergistic effect. The two-phase influence of glycine on morphine-induced antinociception may be due to the interaction of glycine with different receptors.     

Corticosteroid effects on morphine-induced antinociception as a function of two types of corticosteroid receptors in brain

The antinociceptive effect of parenterally and intracerebroventricularly injected morphine and beta-endorphin in adrenalectomized rats and in adrenalectomized rats treated with adrenal steroids was examined employing the hot-plate method. (1) Adrenalectomy sensitized the rats to an analgesic effect of morphine and beta-endorphin. (2) Replacement therapy (chronic and acute) with corticosterone, dexamethasone or RU 28362 (glucocorticoid receptor agonist) effectively reversed the increase in the sensitivity to the analgesic effect of peripherally injected morphine (5 mg/kg i.p.) induced by adrenalectomy to the level of sham-operated animals. Glucocorticosteroids administered to non-adrenalectomized rats did not change the sensitivity to morphine. (3) Corticosterone had a biphasic, dose-dependent effect; the most significant attenuation of the hypersensitivity to morphine-induced antinociception in adrenalectomized rats was achieved after 0.01 mg and after 10 mg (per kg b.w.). Doses of corticosterone of 0.005 mg/kg and in a range of 0.05-0.30 mg/kg were ineffective. (4) Corticosterone in a dose of 0.01 mg/kg (s.c.) had suppressant effects on the adrenalectomy-induced increase in the sensitivity to antinociception induced by morphine when given prior to morphine (60, 30 and 5 min) as well as after the injection of morphine (before the first and the second testing on the hot-plate, 15 and 5 min, respectively). (5) Intracerebroventricularly (i.c.v.) injected morphine and beta-endorphin also displayed the hypersensitivity to the analgesic effect in adrenalectomized rats which in both cases could be suppressed by 0.01 mg/kg of corticosterone given subcutaneously 5 min prior to administration of the opiate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systemic and spinal administration of the mu opioid, remifentanil, produces antinociception in amphibians

Remifentanil is a relatively new opioid analgesic related to the fentanyl family of mu opioid receptor agonists and is used clinically for its unique property of having an ultra-short duration of action. However, there is little preclinical data on the analgesic (antinociceptive) effects of remifentanil and none obtained in non-mammalian animal models. The antinociceptive effects of remifentanil were assessed by using the acetic acid test in amphibians. Systemic and spinal administration of remifentanil was made by subcutaneous and intraspinal injections in the Northern grass frog, Rana pipiens. After administration, remifentanil produced dose-dependent and long-lasting antinociceptive effects which persisted for five hours after systemic administration but gave a shorter duration of action after spinal delivery. The antinociceptive effects of remifentanil were significantly blocked by pretreatment with systemic naltrexone. Systemic and spinal administration of remifentanil produced log dose-response curves which yielded ED50 values of 7.1 nmol/g and 3.2 nmol/animal respectively. The relative antinociceptive potency of remifentanil compared to other opioids administered to amphibians is similar to that found in mammalian models.     

Limited involvement of central noradrenergic pathways in morphine-induced antinociception

Lesioning of cerebral and spinal noradrenergic terminals by the neurotoxin DSP4 (50 mg/kg, 7 days prior to testing) significantly attenuated the effect of morphine (2.5–7.5 mg/kg) in rats tested with the hot-plate test. The effect of DSP4 was prevented by pretreatment with the selective inhibitor of uptake of NA, desipramine. Treatment with DSP4 did not attenuate the effect of morphine (5 mg/kg) in the flinch-jump and tail-flick tests, and did not by itself change the nociceptive thresholds in any of the tests. It is concluded that noradrenergic structures in the central nervous system play a limited role in analgesia induced by morphine.     

Use of antisense oligodeoxynucleotide to δ-opioid receptor mRNA in the study of turnover of δ-opioid receptors in the spinal cord of the mouse

Pretreatment of male ICR mice with an antisense oligodeoxynucleotide to δ-opioid receptor mRNA (DOR AS oligo, 163 pmol) given intrathecally (i.t.) once a day for 1–3 days produced a time-dependent attenuation of antinociception produced by i.t.-challenged [D-Ala²] deltorphin II (6.4 nmol), a δ-opioid receptor agonist. The attenuation of the [D-Ala²]deltorphin II-induced antinociception caused by pretreatment with DOR AS oligo given i.t. daily was blocked by co-pretreatment with naltriben (14.5 nmol), a δ-opioid receptor antagonist, but was markedly enhanced by concomitant pretreatment with thiorphan (19.7 nmol) or bestatin (14.5 nmol), which inhibits the degradation of endogenously released Met-enkephalin. Concomitant pretreatment with antiserum to Met-enkephalin, but not with antiserum to Leu-enkephalin, β-endorphin or dynorphin A (1–17), and DOR AS oligo given i.t. daily for 3 days prevented the attenuation of i.t.-challenged [D-Ala²]deltorphin II-induced antinociception caused by the DOR AS oligo pretreatment. Our results support the existence of a turnover of δ-opioid receptors in the mouse spinal cord caused by the release of Met-enkephalin. Received: 9 October 1996/Final version: 6 May 1997     

Classification of adenosine receptors mediating antinociception in the rat spinal cord.

Analogues of adenosine were injected intrathecally into rats implanted with chronic indwelling cannulae in order to determine a rank order of potency and hence characterize adenosine receptors involved in spinal antinociception. In the tail flick test L-N6-phenylisopropyl adenosine (L-PIA), cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamide adenosine (NECA) produced dose-related antinociception which attained a plateau level. NECA and CHA also produced an additional distinct second phase of antinociception. D-N6-Phenylisopropyl adenosine (D-PIA) and 2-chloroadenosine (CADO) had very little antinociceptive activity in this test. The rank order of potency in producing the plateau effect was L-PIA greater than CHA greater than NECA greater than D-PIA = CADO, while that for the second phase of antinociception was NECA greater than-CHA. Pretreatment with both theophylline and 8-phenyltheophylline (8-PT) antagonized antinociception produced by CHA, with 8-PT being at least an order of magnitude more potent than theophylline. Both antagonists produced a significant hyperalgesia in the tail flick test. L-PIA and CHA also produced methylxanthine-sensitive antinociception in the hot plate test. These results suggest that activation of A1-receptors in the spinal cord can produce antinociception. Activation of A2-receptors may produce an additional effect, but the relative activity of CHA in this component of activity is unusual.     

Vigabatrin attenuates the development and expression of tolerance to morphine-induced antinociception in mice

The efficacy of opioids is limited in chronic pain treatment, as a result of development of opioid tolerance. Based on previous demonstration of the effect of anticonvulsant drugs on morphine antinociception, the present study investigated the effects of vigabatrin (VGB) on the development and expression of morphine tolerance in mice. 101 male NMRI mice weighing 20-25 g were used in these experiments. To evaluate the VGB effects on the development or expression of morphine tolerance, animals received VGB (5, 10 or 20 mg/kg; i.p.), 30 min before morphine (50 mg/kg; s.c.) during induction period once daily for 3 days; or 30 min before challenge dose of morphine (5 mg/kg) before and after morphine-induced tolerance, respectively. The analgesic effect of VGB was evaluated at 30-time intervals (30, 60, 90 and 120 min) by tail-flick analgesiometer. The results showed that VGB at the dose of 20 mg/kg significantly attenuated the development and expression of morphine tolerance. Additionally, VGB alone did not affect the tail-flick latency times. Therefore, while VGB alone has no antinociceptive effect, it can prevent the development of morphine tolerance in mice.     

Role of gap junction in the expression of morphine-induced antinociception

The present study was undertaken to investigate whether gap junctional communication could be involved in morphine-induced antinociceptive response using blockers of the gap junctional channel, carbenoxolone and Gap27. Intrathecal pretreatment with either carbenoxolone or Gap27 caused a dose-dependent attenuation of morphine-induced antinociception. Furthermore, the dose-response line for morphine-induced antinociception was shifted to the right by 2.53-fold following intrathecal treatment with carbenoxolone. These findings suggest that gap-junctional-dependent communication in the mouse spinal cord may play, at least in part, a role in the expression of morphine-induced antinociception.     

Pertussis toxin abolishes the antinociception mediated by opioid receptors in rat spinal cord

Intrathecal injection of pertussis toxin (1 microgram) in rats produced a marked decrease in the antinociceptive effect of the intrathecally administered opioid agonists [D-Ala2,D-Leu5]enkephalin, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin and bremazocine. The effect of the toxin was time-dependent, since it was more pronounced at 6 than at 2 days after its injection. The pertussis toxin-catalyzed ADP ribosylation of a 40 KDa substrate in membranes prepared from the spinal cord of toxin-injected rats was strongly reduced as compared to controls. The data indicate that the antinociceptive effect produced by opioid agonists with different receptor preference is initiated at receptor sites which interact with G-protein substrates of pertussis toxin.