Shaifali Bhalla

Determination of α2-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.

Determination of Adrenergic and Imidazoline Receptor Involvement in Augmentation of Morphine and Oxycodone Analgesia by Clonidine and BMS182874

Background: Numerous agents have been demonstrated to potentiate morphine analgesia, including clonidine (α2-adrenergic and I1-imidazoline receptor agonist) and BMS182874 (endothelin-A, ETA, receptor antagonist). ET has been shown to affect pharmacological actions of clonidine. The present study was conducted to determine whether α2-adrenergic and/or I1-imidazoline receptors are involved in the augmentation of morphine and oxycodone analgesia by clonidine and BMS182874. Methods: Analgesic tail flick latencies were measured in rats at various time intervals, and were converted to AUC₀→360 min. Results: It was found that clonidine produced mild analgesia, while BMS182874 did not have any analgesic effect. Clonidine (p = 0.015) and BMS182874 (p = 0.009) enhanced the analgesic action of morphine and oxycodone. Clonidine- or BMS182874-induced increases in the analgesic effect of morphine were not inhibited by idazoxan (I1-imidazoline receptor antagonist), while increases in the analgesic effect of oxycodone were blocked by idazoxan. Yohimbine (α2-adrenergic receptor antagonist) blocked the clonidine-induced potentiation of analgesic effect of morphine (p = 0.036) and oxycodone (p = 0.0167), while yohimbine did not affect BMS182874-induced potentiation of the analgesic effect of morphine or oxycodone. Conclusions: This is the first report showing that clonidine and BMS182874 augment oxycodone analgesia. Results suggest that α2-adrenergic receptors are involved in clonidine-induced, but not in the BMS182874-induced, potentiation of the analgesic effects of morphine or oxycodone, and that I1-imidazoline receptors are involved in the potentiation of oxycodone analgesia, but not morphine analgesia, by clonidine and BMS182874.

Effect of endothelin-A receptor antagonist on mu, delta and kappa opioid receptor-mediated antinociception in mice

We have previously shown the involvement of central endothelin (ET) mechanisms in morphine analgesia and tolerance. Here we investigated the interaction of centrally administered endothelin ET(A) receptor antagonist, BMS182874, with DAMGO (micro opioid receptor agonist), SNC80 (delta opioid receptor agonist), U50,488H (kappa opioid receptor agonist), and oxycodone (micro and kappa opioid receptor agonist) towards antinociception, tolerance to antinociception and body temperature. Antinociception was determined using tail-flick latency method. BMS182874 (50microg, i.c.v.) treatment alone did not produce analgesia or change in body temperature. However, BMS182874 significantly enhanced antinociception response of DAMGO (66.75%), SNC80 (62.40%), U50,488H (55.38%), and oxycodone (61.72%). Chronic treatment with DAMGO, SNC80, U50,488H or oxycodone, induced tolerance to antinociception. Treatment with BMS182874 restored antinociceptive effect in mice that were tolerant to DAMGO, SNC80, U50,488H as well as oxycodone. Antinociceptive response of DAMGO, SNC80, U50,488H, and oxycodone in tolerant mice treated with BMS182874 was significantly higher (44.55%, 37.48%, 43.02%, and 56.08%, respectively) compared to tolerant mice treated with vehicle. Body temperature decreased with DAMGO, SNC80, U50,488H, and oxycodone; tolerance did not develop to hypothermic effect and BMS182874 did not affect DAMGO, SNC80, U50,488H, or oxycodone induced changes in body temperature. Opioid-antagonist naloxone, completely blocked antinociceptive effect of DAMGO, SNC80, U50,488H or oxycodone and potentiation of antinociception by BMS182874. It is concluded that BMS182874 potentiated antinociception and restored antinociceptive effect in mice tolerant to micro, delta and kappa selective, as well as a non-selective opioid receptor agonist. Therefore, endothelin ET(A) receptor antagonists could be useful in the restoration of antinociceptive effect during tolerance to opiates.

A novel combination of opiates and endothelin antagonists to manage pain without any tolerance development

Several neurotransmitter mechanisms have been proposed as playing a role in the development of morphine tolerance. We provide evidence for the first time that endothelin antagonists can restore morphine analgesia in morphine-tolerant rats and prevent the development of tolerance to morphine. Studies were carried out in rats and mice treated with implanted placebo or implanted morphine pellet. The maximal tail-flick latency in morphine pellet + vehicletreated rats (7.54 seconds) was significantly lower when compared with placebo pellet + vehicle-treated rats (10 seconds), indicating that tolerance developed to the analgesic effect of morphine. BQ123 potentiated tail-flick latency by 30.0% in placebo-tolerant rats and 94.5% in morphine-tolerant rats compared with respective controls. BMS182874 potentiated tail-flick latency by 30.2% in placebotolerant rats and 66.7% in morphine-tolerant rats. The enhanced analgesic effect of morphine after treatment with endothelin antagonists could be blocked by naloxone, indicating an opiatemediated effect; but naloxone binding to brain membranes was not affected by BQ123. Guanosine triphosphate binding was stimulated by morphine and endothelin-1 in non-tolerant mice and not in morphine-tolerant mice; however, guanosine triphosphate binding was stimulated by BQ123 in morphine-tolerant mice and was unaffected in non-tolerant mice. These results suggest that uncoupling of G-protein occurs in morphine tolerance and endothelin antagonist restores the coupling of G-protein to its receptors. A combination use of endothelin antagonist and opiates could provide a novel approach in improving analgesia and eliminating tolerance.

Centhaquin antinociception in mice is mediated by α2A- and α2B- but not α2C-adrenoceptors.

The use of clonidine as a primary and adjuvant analgesic is well-documented. It is known that imidazoline and α2-adrenoceptors are involved in clonidine antinociception. Clonidine also produces antihypertensive actions mediated through the central nervous system. We have reported that centhaquin, a centrally-acting anti-hypertensive drug produces its hypotensive effect through a mechanism of action similar to clonidine. Centhaquin has also been shown to possess significant antinociceptive activity. Centhaquin antinociception is partially blocked by yohimbine, idazoxan, and naloxone; however, the involvement of specific adrenoceptor subtypes (α2A, α2B, or α2C) in centhaquin antinociception is unknown. The present study was conducted to determine antinociceptive properties of centhaquin citrate, a water soluble salt of centhaquin, and involvement of α2A-, α2B-, or α2C-adrenoceptors in centhaquin citrate antinociception in mice. BRL-44408 (α2A-adrenoceptor antagonist), imiloxan (α2B-adrenoceptor antagonist) and JP-1302 (α2C-adrenoceptor antagonist) were used to determine the involvement of α2A-, α2B-, or α2C-adrenoceptors, respectively. Antinociceptive (tail-flick and hot-plate) latencies were determined in male Swiss-Webster mice treated with centhaquin citrate alone and in combination with BRL-44408, imiloxan, or JP-1302. Centhaquin citrate produced significant antinociception in mice (P<0.05) which was unaffected by JP-1302 (P>0.05) but blocked by BRL-44408 (tail-flick test: 49.75% decrease, P<0.05; hot-plate test: 49.12% decrease, P<0.05) and imiloxan (tail-flick test: 46.98% decrease, P<0.05; hot-plate test: 46.42% decrease, P<0.05). This is the first report demonstrating centhaquin citrate antinociception and its blockade by BRL-44408 and imiloxan. We conclude that α2A and α2B but not α2C adrenoceptors are involved in centhaquin antinociception in mice.

Study of adrenergic, imidazoline, and endothelin receptors in clonidine-, morphine-, and oxycodone-induced changes in rat body temperature.

Objectives: The potentiation of morphine or oxycodone analgesia by endothelin-A (ETA) receptor antagonists and imidazoline/α2-adrenergic agonists is well documented. However, the effect of morphine or oxycodone in combination with an ETA receptor antagonist or an imidazoline/α2 adrenergic agonist on body temperature is not known. The present study was carried out to study the role of ETA and imidazoline/α2 adrenergic receptors in body temperature effects of morphine, oxycodone, and clonidine in rats. Methods: Body temperature was determined in male Sprague-Dawley rats treated with morphine, oxycodone, or clonidine. Yohimbine, idazoxan, and BMS182874 were used to determine the involvement of α2-adrenergic, imidazoline, and ETA receptors, respectively. Key Findings: Morphine and oxycodone produced hyperthermia which was not affected by α2-adrenergic antagonist yohimbine, imidazoline/α2-adrenergic antagonist idazoxan, or ETA receptor antagonist BMS182874. Clonidine alone produced hypothermia that was comparable to the hypothermia observed with clonidine plus morphine or oxycodone. The hypothermic effect of clonidine was blocked by idazoxan and yohimbine. The blockade by idazoxan was more pronounced compared to yohimbine. Clonidine hypothermia was not affected by BMS182874. Conclusions: This is the first report demonstrating that ETA receptors do not influence morphine- and oxycodone- induced hyperthermia or clonidine-induced hypothermia. Imidazoline receptors and α2-adrenergic receptors are involved in clonidine-induced hypothermia, but not in morphine- and oxycodone-induced hyperthermia.

Central endothelin-B receptor stimulation does not affect morphine analgesia in rats.

Several neurotransmitter mechanisms have been proposed to play a role in the actions of morphine. We reported that centrally administered endothelin A (ETA) receptor antagonists potentiate morphine analgesia in rats. It has also been reported that ETB agonist, IRL1620, has antinociceptive action mediated through opiate receptors in the periphery. The present study was conducted to determine if central ETB receptors are involved in analgesic actions of morphine. The effect of intracerebroventricular (i.c.v.) administration of ETB receptor agonist, IRL1620, on morphine-induced analgesia and hyperthermia was determined in the rat. Morphine (4 mg/kg, s.c.) produced a significant increase (84%) in tail-flick latency compared to the control group and the analgesic response lasted for 4 h. IRL1620 (30 µg, i.c.v.) did not produce any increase (16%) in tail-flick latency over the 5-hour observation period in vehicle-treated rats. Pretreatment with IRL1620 (3, 10, and 30 µg, i.c.v.) did not have any significant effect on the intensity and duration of morphine (4 mg/kg, s.c.)-induced analgesia. Morphine (4 mg/kg, s.c.) administration produced an increase in body temperature compared to the control group. In vehicle-pretreated rats, IRL1620 (30 µg, i.c.v.) did not produce any change in body temperature. The morphine-induced hyperthermic effect was not altered in IRL1620-pretreated rats. These studies demonstrate that IRL1620, a specific ETB receptor agonist, did not affect the morphine-induced analgesic and hyperthermic effect in rats. It can be concluded that central ETB receptors are not involved in modulation of pharmacological actions of morphine.

Endothelin ETA receptor antagonist reverses naloxone-precipitated opioid withdrawal in mice

Long-term use of opioids for pain management results in rapid development of tolerance and dependence leading to severe withdrawal symptoms. We have previously demonstrated that endothelin-A (ETA) receptor antagonists potentiate opioid analgesia and eliminate analgesic tolerance. This study was designed to investigate the involvement of central ET mechanisms in opioid withdrawal. The effect of intracerebroventricular administration of ETA receptor antagonist BQ123 on morphine and oxycodone withdrawal was determined in male Swiss Webster mice. Opioid tolerance was induced and withdrawal was precipitated by the opioid antagonist naloxone. Expression of ETA and ETB receptors, nerve growth factor (NGF), and vascular endothelial growth factor was determined in the brain using Western blotting. BQ123 pretreatment reversed hypothermia and weight loss during withdrawal. BQ123 also reduced wet shakes, rearing behavior, and jumping behavior. No changes in expression of vascular endothelial growth factor, ETA receptors, and ETB receptors were observed during withdrawal. NGF expression was unaffected in morphine withdrawal but significantly decreased during oxycodone withdrawal. A decrease in NGF expression in oxycodone- but not in morphine-treated mice could be due to mechanistic differences in oxycodone and morphine. It is concluded that ETA receptor antagonists attenuate opioid-induced withdrawal symptoms.

Potentiation of oxycodone antinociception in mice by agmatine and BMS182874 via an imidazoline I2 receptor-mediated mechanism.

The potentiation of oxycodone antinociception by BMS182874 (endothelin-A (ET(A)) receptor antagonist) and agmatine (imidazoline receptor/α(2)-adrenoceptor agonist) is well-documented. It is also known that imidazoline receptors but not α(2)-adrenoceptors are involved in potentiation of oxycodone antinociception by agmatine and BMS182874 in mice. However, the involvement of specific imidazoline receptor subtypes (I(1), I(2), or both) in this interaction is not clearly understood. The present study was conducted to determine the involvement of imidazoline I(1) and I(2) receptors in agmatine- and BMS182874-induced potentiation of oxycodone antinociception in mice. Antinociceptive (tail flick and hot-plate) latencies were determined in male Swiss Webster mice treated with oxycodone, agmatine, BMS182874, and combined administration of oxycodone with agmatine or BMS182874. Efaroxan (imidazoline I(1) receptor antagonist) and BU224 (imidazoline I(2) receptor antagonist) were used to determine the involvement of I(1) and I(2) imidazoline receptors, respectively. Oxycodone produced significant antinociceptive response in mice which was not affected by efaroxan but was blocked by BU224. Agmatine-induced potentiation of oxycodone antinociception was blocked by BU224 but not by efaroxan. Similarly, BMS182874-induced potentiation of oxycodone antinociception was blocked by BU224 but not by efaroxan. This is the first report demonstrating that BMS182874- or agmatine-induced enhancement of oxycodone antinociception is blocked by BU224 but not by efaroxan. We conclude that imidazoline I(2) receptors but not imidazoline I(1) receptors are involved in BMS182874- and agmatine-induced potentiation of oxycodone antinociception in mice.

Involvement of α2-adrenoceptors, imidazoline, and endothelin-A receptors in the effect of agmatine on morphine and oxycodone-induced hypothermia in mice

Potentiation of opioid analgesia by endothelin‐A (ETA) receptor antagonist, BMS182874, and imidazoline receptor/α2‐adrenoceptor agonists such as clonidine and agmatine are well known. It is also known that agmatine blocks morphine hyperthermia in rats. However, the effect of agmatine on morphine or oxycodone hypothermia in mice is unknown. The present study was carried out to study the role of α2‐adrenoceptors, imidazoline, and ETA receptors in morphine and oxycodone hypothermia in mice. Body temperature was determined over 6 h in male Swiss Webster mice treated with morphine, oxycodone, agmatine, and combination of agmatine with morphine or oxycodone. Yohimbine, idazoxan, and BMS182874 were used to determine involvement of α2‐adrenoceptors, imidazoline, and ETA receptors, respectively. Morphine and oxycodone produced significant hypothermia that was not affected by α2‐adrenoceptor antagonist yohimbine, imidazoline receptor/α2 adrenoceptor antagonist idazoxan, or ETA receptor antagonist, BMS182874. Agmatine did not produce hypothermia; however, it blocked oxycodone but not morphine‐induced hypothermia. Agmatine‐induced blockade of oxycodone hypothermia was inhibited by idazoxan and yohimbine. The blockade by idazoxan was more pronounced compared with yohimbine. Combined administration of BMS182874 and agmatine did not produce changes in body temperature in mice. However, when BMS182874 was administered along with agmatine and oxycodone, it blocked agmatine‐induced reversal of oxycodone hypothermia. This is the first report demonstrating that agmatine does not affect morphine hypothermia in mice, but reverses oxycodone hypothermia. Imidazoline receptors and α2‐adrenoceptors are involved in agmatine‐induced reversal of oxycodone hypothermia. Our findings also suggest that ETA receptors may be involved in blockade of oxycodone hypothermia by agmatine.