Interactions between the effects of endothelin-1, clonidine and yohimbine on electrically-induced contractions in rat vas deferens

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.

Involvement of imidazoline and opioid receptors in the enhancement of clonidine-induced analgesia by sulfisoxazole

Clonidine, an α2-adrenergic agonist, has been demonstrated to produce significant analgesia and potentiate morphine analgesia. Endothelin (ETA) receptor antagonists have also been found to potentiate the antinociceptive response to morphine. Clonidine and ET have been reported to have cardiovascular interactions involving the sympathetic nervous system, but it is not known whether ETA receptor antagonist affects clonidine analgesia. This study examined the influence of sulfisoxazole (ETA receptor antagonist) on clonidine analgesia. Male Swiss Webster mice were used to determine antinociceptive response of drugs by measuring tail-flick latency. The effect of clonidine (0.3, 1.0, and 3.0 mg/kg, i.p.) alone or in combination with sulfisoxazole (25, 75, and 225 mg/kg, p.o.) on analgesia and body temperature was determined. Clonidine produced a dose-dependent analgesia and hypothermia. Sulfisoxazole (25, 75, and 225 mg/kg), when administered with clonidine (0.3 mg/kg), significantly potentiated (31% increase in area under the curve (AUC)) the analgesic effect of clonidine. Yohimbine (α2-adrenergic receptor antagonist) did not affect analgesic effect of clonidine plus sulfisoxazole. Idazoxan (I1-imidazoline and α2-adrenergic receptor antagonist) reduced (47% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. Treatment with naloxone reduced (46% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. The effect of another ETA receptor antagonist, BMS-182874 (2, 10, and 50 µg, i.c.v.) was studied, and it was found that the dose of 10 µg significantly potentiated (26% increase in AUC) the analgesic effect of clonidine. These results indicate that sulfisoxazole, an ETA receptor antagonist, potentiates the analgesic effect of clonidine, which could be mediated through I1-imidazoline receptors and opioid receptors.

Inhibitory and facilitatory presynaptic effects of endothelin on sympathetic cotransmission in the rat isolated tail artery

1. The present study was undertaken to determine the modulatory effects of the endothelin peptides on the neurogenically-induced release of endogenous noradrenaline (NA) and the cotransmitter adenosine 5'-triphosphate (ATP) from the sympathetic nerves of endothelium-free segments of the rat isolated tail artery. The electrical field stimulation (EFS, 8 Hz, 0.5 ms, 3 min) evoked overflow of NA and ATP, in the absence of endothelins, was 0.035+/-0.002 pmol mg(-1) tissue and 0.026+/-0.002 pmol mg(-1) tissue, respectively. 2. Endothelin-1 (ET-1; 1-30 nM) significantly reduced the EFS evoked overflow of both NA and ATP. The maximum inhibitory effect was produced by a peptide concentration of 10 nM, the amount of NA overflow being 0.020+/-0.002 pmol mg(-1) and that of ATP overflow 0.015+/-0.001 pmol mg(-1). Higher peptide concentrations (100 and 300 nM) reversed the EFS-evoked overflow of NA to control levels and that of ATP to above control levels. The inhibitory effect of ET-1 (10 nM) was resistant to the selective ET(A) receptor antagonist cyclo-D-Trp-D-Asp(ONa)-Pro-D-Val-Leu (BQ-123) but was prevented by ET(B) receptor desensitization with sarafotoxin S6c (StxS6c) or by ET(B) receptor blockade with N, cis-2,6-dimethylpiperidinocarbonyl-L-gmethylleucyl-D-1-me thoxycarbonyltryptophanyl-D-norleucine (BQ-788). 3. StxS6c, upon acute application, exerted a dual effect on transmitter release. At concentrations of 0.001-0.3 nM the peptide significantly reduced the EFS-evoked NA overflow, whereas at concentrations of 1-10 nM it caused a significant increase in the evoked overflow of both ATP and NA. Both the maximum inhibitory effect of StxS6c at a concentration of 0.003 nM (approximately 85% reduction of NA overflow and 40% of ATP overflow) and the maximum facilitatory effect of the peptide at a concentration of 3 nM (approximately 400% increase of ATP overflow and 200% of NA overflow) were completely antagonized by either BQ-788 or by StxS6c-induced ET(B) receptor desensitization. 4. ET-3 (10-100 nM) did not affect the EFS evoked overflow of either ATP or NA, but at a concentration of 300 nM significantly potentiated the release of both transmitters (0.118+/-0.02 pmol mg(-1) tissue ATP overflow and 0.077+/-0.004 pmol mg(-1) NA overflow). This effect was prevented either by BQ-123 or by BQ-788. 5. In summary, the endothelin peptides exerted both facilitatory and inhibitory effects on the neurogenically-induced release of the sympathetic cotransmitters ATP and NA in the rat tail artery. Both transmitters were modulated in parallel indicating that the endothelins do not differentially modulate the release of NA and ATP in this tissue.

Modulatory effects of endothelin-1 on purinergic and adrenergic components of sympathetically-mediated contractile activity of rabbit saphenous artery

1. The present study has been performed to evaluate the modulatory effects of endothelin-1 (ET-1) on the purinergic and adrenergic components of sympathetically-mediated contractile responses of endothelium-free rabbit saphenous artery preparations. 2. ET-1 increased the smooth muscle tone, the pD2 value being 7.77 +/- 0.05. 3. Postjunctionally, ET-1 enhanced the responses to exogenous adenosine 5'-triphosphate (ATP) and did not influence those to exogenous noradrenaline (NA). 4. ET-1 increased the contractile responses to short-lasting and to long-lasting electrical field stimulation at a frequency of 5 or 10 Hz, showing a tendency towards decreasing the prazosin-sensitive component and increasing the mATP-sensitive component of the contractile responses. 5. In prazosin-treated preparations ET-1 increased the residual mATP-sensitive responses and this effect was more pronounced after yohimbine. 6. In mATP-treated preparations ET-1 increased the residual electrically-induced contractions and this increase was abolished after yohimbine. 7. It is suggested that ET-1 modulates co-transmission in the rabbit saphenous artery by potentiating postjunctionally the purinergic component of the contractile responses to both exogenous ATP or electrical stimulation.