Agmatine potentiates morphine-induced conditioned place preference in mice: modulation by alpha2-adrenoceptors.

The effects of agmatine, an endogenous polyamine metabolite formed by decarboxylation of L-arginine, and its combination with morphine on conditioned place preference (CPP) has been investigated in male mice. Our data show that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increases the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal administration of agmatine (1-40 mg/kg) alone does not induce either CPP or conditioned place aversion, while combination of agmatine and subeffective doses of morphine leads to potent rewarding effects. Lower doses of morphine (0.1, 0.05, and 0.01 mg/kg) are able to induce CPP in mice pretreated with agmatine 1, 5, and 10 mg/kg, respectively. Concomitant intraperitoneal administration of UK 14 304 (0.5 mg/kg), a highly selective alpha2-agonist, with per se noneffective dose of morphine (0.5 mg/kg) and also its combination with noneffective doses of agmatine (1 mg/kg) plus morphine (0.05 mg/kg) produces significant CPP. UK 14 304 (0.05, 0.5 mg/kg) alone, or in combination with agmatine (1, 5 mg/kg) have had no effect. We have further investigated the possible involvement of the alpha2-adrenoceptors in the potentiating effect of agmatine on morphine-induced place preference. Selective alpha2-antagonists, yohimbine (0.005 mg/kg) and RX821002 (0.1, 0.5 mg/kg), block the CPP induced by concomitant administration of agmatine (5 mg/kg) and morphine (0.05 mg/kg). Yohimbine (0.001-0.05 mg/kg) or RX821002 (0.05-0.5 mg/kg) alone or in combination with morphine (0.05 mg/kg) or agmatine (5 mg/kg) fail to show any significant place preference or aversion. Our results indicate that pretreatment of animals with agmatine enhances the rewarding properties of morphine via a mechanism which may involve alpha2-adrenergic receptors.     

Transcutaneous electrical acupoint stimulation potentiates analgesic effect of morphine

Pain is the major complaint of patients who choose acupuncture treatment. Transcutaneous electrical acupoint stimulation (TEAS) provides a safe, standardized technique without needle insertion. TEAS can be tested with the cold-pressor test, a simple, reliable, and widely used model in humansfor the induction of tonic pain. In this controlled study, the effects of TEAS on cold-pressor-induced pain were evaluated in 20 healthy human subjects. Electrical stimulation electrodes were applied to He-Gu (LI 4) and Nui-Guan (P 6) acupoints. The effects of saline plus no TEAS, 15-minute TEAS alone, 0.05 mg/kg morphine alone, and 15-minute TEAS plus morphine were assessed. Pain score ratings were evaluated at four time points from 30 to 170 seconds during the cold-pressor test. The authors observed analgesic effects in both TEAS-alone and morphine-alone sessions, and pain score rating reductions were statistically significant compared to unstimulated control (both p < 0.01). The degree of TEAS analgesia combined with 0.05 mg/kg morphine was significantly higher than TEAS alone (p < 0.01). The results support the efficacy of TEAS analgesia and suggest that combination of TEAS with low-dose morphine can achieve better pain control in a variety of clinical settings.     

Agmatine exerts anticonvulsant effect in mice: modulation by alpha 2-adrenoceptors and nitric oxide

The effect of agmatine, an endogenous polyamine metabolite, on seizure susceptibility was investigated in mice. Acute intraperitoneal administration of agmatine (5, 10, 20, 40 mg/kg) had a significant and dose-dependent inhibitory effect on pentylenetetrazole (PTZ)-induced seizures. The peak of this anticonvulsant effect was 45 min after agmatine administration. We further investigated the possible involvement of the alpha(2)-adrenoceptors and L-arginine/NO pathway in this effect of agmatine. The alpha(2)-adrenoceptor antagonist, yohimbine (0.5-2 mg/kg), induced a dose-dependent blockade of the anticonvulsant effect of agmatine. The nitric oxide synthase (NOS) substrate, L-arginine (60 mg/kg), inhibited the anticonvulsant property of agmatine and this effect was significantly reversed by NOS inhibitor N(G)-nitro-L-arginine (L-NAME, 30 mg/kg), implying an NO-dependent mechanism for L-arginine effect. We further examined a possible additive effect between agmatine (1 or 5 mg/kg) and L-NAME (10 mg/kg). The combination of L-NAME (10 mg/kg) with agmatine (5 but not 1 mg/kg) induced a significantly higher level of seizure protection as compared with each drug alone. Moreover, a combination of lower doses of yohimbine (0.5 mg/kg) and L-arginine (30 mg/kg) also significantly decreased the anticonvulsant effect of agmatine. In conclusion, the present data suggest that agmatine may be of potential use in seizure treatment.     

Phorbol 12,13-dibutyrate potentiates alpha 2-adrenoceptor-mediated contraction in the rat saphenous vein

The responses of the rat saphenous vein to potassium depolarization, noradrenaline and B-HT 920 were potentiated significantly by phorbol 12,13-dibutyrate (PDBu) (10 nM). In contrast, only the contraction induced by potassium depolarization was potentiated by PDBu in the rat tail artery. The responses to noradrenaline and methoxamine were slightly inhibited. These results suggest that PDBu may have a differential effect on the responses of the two subtypes of alpha-adrenoceptors-potentiation for the alpha 2-adrenoceptors in the rat saphenous vein and inhibition for the alpha 1-adrenoceptors in the rat tail artery.     

Phorbol ester potentiates alpha 1-adrenoceptor-mediated positive inotropic effect in rat papillary muscle

Tumor-promoting phorbol esters may alter alpha 1-adrenoceptor-mediated cardiac response by stimulating protein kinase C activity. We investigated the effect of phorbol-12,13-dibutyrate (PDBu) on the positive inotropic effect (PIE) in rat left ventricular papillary muscle. PDBu (1-100 nM) potentiated the phenylephrine (PE)-induced PIE in a dose- and time-dependent manner. The PIE induced by PE and PDBu was abolished by pretreatment with 3 x 10(-7) M prazosin. PDBu also enhanced PE-induced slow responses 2- to 3-fold. These results suggest that PDBu enhances alpha 1-adrenoceptor-mediated PIE by potentiating slow Ca2+ channels, presumably through the activation of protein kinase C.     

Induction by endogenous noradrenaline of an alpha 1-adrenoceptor-mediated positive inotropic effect in rabbit papillary muscles.

1. The possible involvement of alpha 1-adrenoceptors in the inotropic and electrophysiological responses to endogenous noradrenaline released by tyramine was examined in rabbit papillary muscles. 2. A concentration-dependent positive inotropic effect was produced by tyramine. This effect of tyramine was not observed in muscles from rabbits pretreated with reserpine. 3. The positive inotropic effect of tyramine was greatly inhibited by propranolol, but not altered by prazosin. However, when beta-adrenoceptors were blocked by pretreatment with propranolol, tyramine still produced a positive inotropic effect, an effect which was antagonized by prazosin. 4. Tyramine caused a decrease in action potential duration (APD) and an increase in action potential amplitude in a concentration-dependent manner. Isoprenaline also produced the same electrophysiological effects. These electrophysiological effects of both agents were inhibited by propranolol. 5. When beta-adrenoceptors were blocked by propranolol, the observed prazosin-sensitive positive inotropic effect of tyramine was not accompanied by any change in APD. In contrast, APD was markedly prolonged by alpha 1-adrenoceptor stimulation with phenylephrine in the presence of propranolol, in association with the positive inotropic effect. 6. It is concluded that in rabbit papillary muscles, endogenous noradrenaline causes a positive inotropic effect predominantly mediated by beta-adrenoceptors, but can still evoke a positive inotropic effect through alpha 1-adrenoceptors when beta-adrenoceptor stimulation is eliminated. This suggests that the alpha 1-adrenoceptor-mediated positive intropic mechanism(s) may be masked by simultaneous activation of beta-adrenoceptors. In addition, this study indicates that APD prolongation is not involved in the alpha 1-adrenoceptor-mediated inotropic responses to endogenous noradrenaline.     

Diurnal variations in the analgesic effectiveness of morphine in mice.

Response to thermal stimulation and the analgesic effectiveness of morphine during various phases of the diurnal cycle were assessed by the hotplate method. Saline treated controls exhibited shortest reaction times during the last quarter of the light-phase and first quarter of the dark phase. Longest reaction times were recorded during the last quarter of the dark phase. Doses of 4, 8, 16, and 32 mg/kg of morphine was administered IP at the peak and trough of the pain sensitivity rhythm. The ED50 (95% C.L.) during the last quarter of the light phase was found to be 14.60 (10.6-20.0) mg/kg while during the last quarter of the dark phase the ED50 was found to be 5.85 (4.5-7.7) mg/kg. In a second experiment, independent groups of ten mice each were injected SC with 8 mg/kg of morphine at three hr intervals over a 48 hr test session. Peak analgesic activity was obtained in the group injected during the last quarter of the dark phase while minimal analgesic effectiveness was obtained during the third quarter of the light phase. Central administration of morphine via the intraventricular route yielded the same relationship, i.e., maximal analgesic effectiveness during the last quarter of the dark phase.     

Interaction between beta 2-adrenoceptor-mediated vasodilation and alpha 2-adrenoceptor-mediated vasoconstriction in the pithed normotensive rat

With pithed normotensive rats we studied the interaction between beta 2-adrenoceptor-mediated vasodilation and pressor responses elicited by vasopressin, the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304, and the alpha 2-adrenoceptor-mediated pressor responses of (--)-norepinephrine, tyramine [via neuronally released (--)-norepinephrine], alpha-methylnorepinephrine, and (--)-epinephrine. Salbutamol was used as a selective agonist of beta 2-adrenoceptors. The selective beta 2-adrenoceptor antagonist ICI 118,551 was employed to reveal the intrinsic beta 2-adrenoceptor activation induced by alpha-methylnorepinephrine and (--)-epinephrine, measured as a potentiation of the increase in diastolic pressure. Two types of interaction between beta 2-adrenoceptor-mediated vasodilation and alpha 2-adrenoceptor-mediated vasoconstriction were found. The effect of the alpha 2-adrenoceptor agonists was attenuated in most cases. However, intravenously administered (--)-norepinephrine elicited an alpha 2-adrenoceptor-mediated vasoconstriction not attenuated by beta 2-adrenoceptor-mediated vasodilation. These results are interpreted as indications for two different populations of vascular alpha 2-adrenoceptors. Neuronally released (--)-norepinephrine activated alpha 2-adrenoceptors, and its effect was attenuated by beta 2-adrenoceptor-mediated vasodilation in contrast to that of intravenously administered (--)-norepinephrine. Therefore, an intrasynaptic and extrasynaptic population of vascular alpha 2-adrenoceptors as postulated. In contrast to (--)-norepinephrine, intravenously administered (--)-epinephrine seems to activate predominantly intrasynaptic alpha 2-adrenoceptors.     

Zopiclone potentiates the antinociceptive effect of morphine in rats

Summary The antinociceptive effect of morphine, as determined by the tail-flick test, was dose-dependently increased by the intraperitoneal injection of zopiclone. The benzodiazepine antagonists Ro 15-1788 (flumazepil) and Ro 15-3505, when intraperitoneally injected, significantly antagonized the effect of intraperitoneal injection of zopiclone on morphine antinociception. Intrathecal injection of zopiclone potentiated morphine antinociception, while the intracerebroventricular injection of zopiclone failed to enhance morphine antinociception and the intracerebroventricular injection of flumazepil to antagonize the intraperitoneal-zopiclone-induced increase in morphine antinociception. These results suggest that benzodiazepine sites are specifically involved in the potentiating effect of zopiclone on morphine antinociception. The anatomical locations of the receptors involved seem to be at the spinal level.This work was supported by MPI 60% grant Send offprint requests to F. Zambotti     

The effect of morphine pretreatment on hypothermia induced by morphine in mice.

Morphine caused an apparently dose-dependent hypothermia in mice. Co-administration of naloxone antagonised this effect. Pretreatment with a single dose of morphine induced detectable tolerance to the hypothermic effect of a second dose of morphine given 3 h later and naloxone was more effective in antagonising the hypothermic effect of morphine in morphine-pretreated mice than in saline-pretreated animals. The present study has shown that morphine pretreatment can augment the antagonistic effect of naloxone towards the hypothermic action of morphine.     

Dopamine receptor mechanism(s) and morphine tolerance in mice

Based on our previous demonstration of the involvement of dopamine-2 (D2) dopamine receptors in morphine antinociception, we examined the role of D2 dopamine receptors in the expression and development of tolerance to morphine antinociception in mice. Tolerance to morphine antinociception was assessed by the tail-flick response after the administration of morphine (50 mg/kg) once daily for 3 days. The D2 dopamine receptor agonist, quinpirole (0.01, 0.02 and 0.03 mg/kg), but not the D2 dopamine receptor antagonist, sulpiride (12.5, 25 and 50 mg/kg), increased morphine antinociception in morphine non pre-exposed mice. The response of quinpirole was decreased by the lower doses of sulpiride. Both quinpirole and sulpiride decreased the expression and development of tolerance to antinociception induced by morphine (1.5, 3 and 6 mg/kg). The effect of quinpirole on the expression and development of tolerance, was reduced by a lower and per se non-effective dose of sulpiride. It was concluded that D2 dopaminergic receptors may play a part in the expression and development of tolerance to the antinociceptive effect of morphine.     

Intracellular alkalinization augments alpha(1)-adrenoceptor-mediated vasoconstriction by promotion of Ca(2+) entry through the non-L-type Ca(2+) channels.

Modulation by intracellular pH of the vasoconstriction induced by alpha-adrenoceptor agonists was investigated in isolated guinea pig aorta. NH(4)Cl (15 mM) increased intracellular pH of aortic smooth muscle cells by about 0.2 pH unit and significantly augmented KCl-induced contraction of aortic strips, whereas simultaneous administration of NH(4)Cl (15 mM) plus Na(+) propionate (30 mM) failed to affect intracellular pH or contractility. NH(4)Cl (15 mM) potentiated contractions induced by alpha-adrenoceptor agonists, norepinephrine, phenylephrine and clonidine. Contraction induced by alpha(1)-selective adrenoceptor agonist, phenylephrine, but not that induced by norepinephrine or clonidine, was insensitive to inhibition by verapamil (1 microM). Phenylephrine-induced contraction was not affected by NH(4)Cl in Ca(2+)-free medium whereas extracellular Ca(2+)-induced contraction of phenylephrine-stimulated aorta was significantly augmented by NH(4)Cl. Consistently, 45Ca(2+)uptake into phenylephrine 1 microM)-stimulated aortic strips was increased by incubation with NH(4)Cl. The potentiating effects of NH(4)Cl on both phenylephrine-induced Ca(2+) entry and contraction were antagonized by Na(+) propionate. These results suggest that intracellular alkalinization facilitates alpha(1)-adrenoceptor-mediated vasoconstriction by facilitation of an agonist-induced Ca(2+) entry pathway that is independent of L-type Ca(2+) channels.     

瑞舒伐他汀部分恢复吗啡耐受大鼠的吗啡镇痛效能

目的探讨瑞舒伐他汀对吗啡耐受大鼠的吗啡镇痛效能的影响及其相关的分子机制。方法 48只♂SD大鼠随机分成6组(n=8):Ⅰ组为空白对照组;Ⅱ组为吗啡耐受组;Ⅲ组为10 mg.kg-1瑞舒伐他汀对照组;Ⅳ组、Ⅴ组、Ⅵ组分别为0.4、2、10 mg.kg-1瑞舒伐他汀处理组。Ⅱ、Ⅳ、Ⅴ、Ⅵ组皮下注射吗啡10 mg.kg-1,Ⅰ组和Ⅲ组皮下注射生理盐水,每天8∶00和16∶00各1次,连续10 d。d 6起,上午皮下注射前30 min,Ⅰ、Ⅱ组给予生理盐水灌胃,Ⅲ、Ⅳ、Ⅴ、Ⅵ组给予相应剂量的瑞舒伐他汀灌胃,连续5 d。d 6、11,测定大鼠基础热缩足潜伏期(PWTL)后,计算尾静脉注射吗啡30 min时各组大鼠的MPE值。d 11行为学检测后处死大鼠,留取腰5脊髓,比较各组脊髓内细胞外调节蛋白激酶(ERK)、磷酸化细胞外调节蛋白激酶(p-ERK)及星形胶质细胞表面标志物GFAP的表达水平。结果①d 6,6组的基础PWTL无明显差异。与Ⅰ组相比,Ⅱ、Ⅳ、Ⅴ和Ⅵ组MPE值明显降低(P<0.05)。d 11,6组的基础PWTL无差异。与Ⅰ组相比,Ⅱ、Ⅳ组尾静脉注射吗啡后30 min的MPE值明显降低(P<0.05);与Ⅱ组相比,Ⅴ、Ⅵ组的MPE值明显升高(P<0.05)。②d 11,6组大鼠腰5脊髓内总ERK表达水平差异无显著性。与Ⅰ组相比,Ⅱ、Ⅳ组腰5脊髓内p-ERK表达明显升高(P<0.05);与Ⅱ组相比,Ⅲ、Ⅴ、Ⅵ组腰5脊髓内p-ERK表达明显降低(P<0.05)。与Ⅰ组相比,Ⅱ组腰5脊髓内GFAP的荧光强度明显升高(P<0.05);与Ⅱ组相比,Ⅵ组腰5脊髓内GFAP的强度明显降低(P<0.05)。结论瑞舒伐他汀能够部分恢复吗啡耐受大鼠的吗啡镇痛效果,这可能与其抑制脊髓内ERK的磷酸化,减少星形胶质细胞活化有关。     

Interaction between beta 2-adrenoceptor-mediated vasodilation and alpha 1-adrenoceptor-mediated vasoconstriction in the pithed normotensive rat

With pithed normotensive rats we studied the interaction between beta 2-adrenoceptor-mediated vasodilation and alpha 1-adrenoceptor-mediated vasoconstriction. The selective beta 2-adrenoceptor agonist salbutamol was used to elicit vasodilatation. To induce alpha 1-adrenoceptor-mediated vasoconstriction, the selective alpha 1-adrenoceptor agonists cirazoline, St 587, and methoxamine were used. Furthermore, the alpha 1-adrenoceptor-mediated vasopressor effects of intravenously administered (--)-norepinephrine, and (--)-norepinephrine released from neurons by the nicotinic agonist 1,1-dimethyl-4-phenylpiperazine iodide (DMPP), the muscarinic ganglionic stimulant McN-A-343, electrical stimulation of the spinal cord, and the indirect sympathomimetic agent tyramine, were studied. By using the selective beta 2-adrenoceptor antagonist ICI 118,551, the interaction between the alpha 1-adrenoceptor-mediated vasoconstriction of (--)-epinephrine and alpha-methylnorepinephrine with their intrinsic beta 2-adrenoceptor agonistic effects was investigated. Two types of interaction between alpha 1-and beta 2-adrenoceptor-mediated vascular effects were found. Cirazoline and McN-A-343 activated alpha 1-adrenoceptors, inducing a vasoconstriction not affected by beta 2-adrenoceptor-mediated vasodilation. However, methoxamine at low doses, St 487, DMPP, electrical stimulation, intravenously administered (--)-norepinephrine, (--)-epinephrine, and alpha-methylnorepinephrine activated alpha 1-adrenoceptors, and their effect was attenuated by vasodilation. At low doses, tyramine stimulated alpha 1-adrenoceptors that were not sensitive to beta 2-adrenoceptor-mediated vasodilation, in contrast to the population of alpha 1-adrenoceptors activated at high doses of tyramine. It is hypothesized that there exist two different populations of alpha 1-adrenoceptors.     

Effect of fluoxetine on tolerance to the analgesic effect of morphine in mice with skin cancer

Adjuvant drugs that attenuate or inhibit the development of tolerance to morphine may lead to improved management of pain in chronic diseases such as cancer. The aim of this study was to investigate effect of fluoxetine, a specific 5-HT (5-hydroxytryptamine, serotonin) reuptake inhibitor, on tolerance induced to the analgesic effect of morphine in mice with skin cancer. The study was carried out on female Swiss albino mice. For skin tumorigensis, mice were initiated with a single dose of 7,12-dimethylbenz(a)anthracene (DMBA) and promoted by multiple doses of croton oil. Tolerance to morphine analgesia was induced by daily subcutaneous (sc) injections of morphine (5 mg/kg for 30 days) and assayed using the hot plate method. Results obtained from this study showed that pain thresholds in mice with skin cancer were significantly lower. Tolerance to the analgesic effect of morphine (5 mg/kg, sc) appeared at day 15, whereas in normal and skin tumor bearing mice co-treated daily with morphine (5 mg/kg, sc) and three different intraperitoneal (ip) doses of fluoxetine (0.16, 0.32 and 0.64 mg/kg) tolerance was observed at days 20, 25 and 30, respectively. In conclusion, our data indicate that concurrent use of morphine with fluoxetine may produce good cancer pain control and attenuate the development of tolerance.     

Differential inhibition of alpha 2-adrenoceptor-mediated pressor responses by (+)- and (-)-verapamil in pithed rats

The inhibitory effects of (+)- and (-)-verapamil on the hypertensive responses brought about by the alpha 2-adrenoceptor agonist B-HT 920 were investigated in pithed normotensive rats. (-)-Verapamil was found to be about 4 times more potent with respect to depressing alpha 2-pressor responses compared with (+)-verapamil. To rule out the effect of 'unspecific' vasodilatation after the administration of the stereoisomers of verapamil, vasopressin was continuously infused into the carotid artery of pithed rats in a separate series of experiments. In the course of this vasopressin infusion, new inhibitory activities of the stereoisomers of verapamil on alpha 2-adrenoceptor-mediated pressor responses were determined. Under these circumstances, the potency ratio of (-)- vs (+)-verapamil was about 7. With the aid of a radioligand binding assay using [3H]clonidine to identify alpha 2-adrenoceptors, low affinities were measured for the stereoisomers of verapamil. A Ki = 6170 nM for (-)-verapamil and a Ki = 41700 nM for (+)-verapamil were calculated. The results indicate that the interaction between alpha 2-adrenoceptor-mediated pressor responses and calcium entry blockers, such as verapamil, is a stereoselective event.     

Influence of klofelin on the analgesic effect of morphine

It has been revealed on conscious rats that it is possible to decrease the dose of morphine and to facilitate the vegetocorrective component of the drug effect without analgesia changes using the combination with klofelin . The effect of the combined use of morphine and klofelin has been found to be mediated by the common adrenergic but not opioidergic substrate.     

Restoration of analgesic response of C57BL/6J-bgJ (beige-J) mice to morphine by carbachol

C57BL/6J-bgJ (beige-J) mice respond poorly to the analgesic effects of centrally administered (intracerebroventricular; i.c.v.) morphine in the tail-flick test. In the present study C57BL/6J-bgJ mice, their littermate (heterozygous) controls, and normal Swiss mice were given carbachol (200 micrograms/ml) in their drinking water for three weeks. Carbachol had no effect on control animals. However, the carbachol-treated beige-J mice responded to i.c.v.-administered morphine (1 microgram) significantly better than untreated beige-J mice and nearly as well as the normal mice.