Role of Pharmacokinetic Effects in the Potentiation of Morphine Analgesia by L-Type Calcium Channel Blockers in Mice

Biased Opioid Receptor Agonists: Balancing Analgesic Efficacy and Side-Effect Profiles

Opioids are the most effective option for severe pain. However, it is well documented that the side effects associated with prolonged opioid use significantly constrain dosage in the clinical setting. Recently, researchers have concentrated on the development of biased opioid receptor agonists that preferentially activate the G protein signaling pathway over β-arrestin signaling. This approach is based on the hypothesis that G protein signaling mediates analgesic effects, whereas β-arrestin signaling is implicated in adverse side effects. Although certain studies have demonstrated that the absence or inhibition of β-arrestin signaling can mitigate the incidence of side effects, recent research appears to challenge these earlier findings. In-depth investigations into biased signal transduction of opioid receptor agonists have been conducted, potentially offering novel insights for the development of biased opioid receptors. Consequently, this review elucidates the contradictory roles of β-arrestin signaling in the adverse reactions associated with opioid receptor activation. Furthermore, a comparative analysis was conducted to evaluate the efficacy of the classic G protein-biased agonists, TRV130 and PZM21, relative to the traditional non-biased agonist morphine. This review aims to inform the development of novel analgesic drugs that can optimize therapeutic efficacy and safety, while minimizing adverse reactions to the greatest extent possible.

Essential role of P-glycoprotein in the mechanism of action of oliceridine

Mu opioid receptor (MOR) agonists comprise the most effective analgesics, but their therapeutic utility is limited by adverse effects. One approach for limiting such effects has been to develop "biased" MOR agonists that show preference for activating G protein over β-Arrestin signaling. However, the notion of biased agonism has been challenged by recent studies. Oliceridine (Olinvyk®, TRV-130, OLC) is a selective MOR agonist approved by the FDA in 2020 for pain management in controlled clinical settings. Oliceridine purportedly demonstrates diminished adverse effects compared to morphine or other MOR agonists, a profile attributed to its biased agonism. However, recent studies suggest that oliceridine does not display biased agonism but instead weak intrinsic efficacy for G protein and β-Arrestin activation. Nevertheless, these insights have been derived from in vitro studies. To better understand oliceridine's in vivo efficacy profile, we performed a comprehensive assessment of its in vitro and in vivo pharmacology using both cultured cells and rodents. In vitro, oliceridine displayed high MOR affinity and weak intrinsic efficacy. In vivo, oliceridine showed impaired brain penetrance and rapid clearance, effects we attributed to its interaction with the P-glycoprotein (P-gp) efflux transporter. Moreover, we found that P-gp was essential for oliceridine's in vivo efficacy and adverse effect profiles. Taken together with prior studies, our results suggest that oliceridine's in vivo efficacy and adverse effect profiles are not attributed solely to its weak intrinsic efficacy or biased agonism but, to a large extent, its interaction with P-gp as well.

L-Type Calcium Channel Blockers: A Potential Novel Therapeutic Approach to Drug Dependence

This review describes interactions between compounds, primarily dihydropyridines, that block L-type calcium channels and drugs that cause dependence, and the potential importance of these interactions. The main dependence-inducing drugs covered are alcohol, psychostimulants, opioids, and nicotine. In preclinical studies, L-type calcium channel blockers prevent or reduce important components of dependence on these drugs, particularly their reinforcing actions and the withdrawal syndromes. The channel blockers also reduce the development of tolerance and/or sensitization, and they have no intrinsic dependence liability. In some instances, their effects include reversal of brain changes established during drug dependence. Prolonged treatment with alcohol, opioids, psychostimulant drugs, or nicotine causes upregulation of dihydropyridine binding sites. Few clinical studies have been carried out so far, and reports are conflicting, although there is some evidence of effectiveness of L-channel blockers in opioid withdrawal. However, the doses of L-type channel blockers used clinically so far have necessarily been limited by potential cardiovascular problems and may not have provided sufficient central levels of the drugs to affect neuronal dihydropyridine binding sites. New L-type calcium channel blocking compounds are being developed with more selective actions on subtypes of L-channel. The preclinical evidence suggests that L-type calcium channels may play a crucial role in the development of dependence to different types of drugs. Mechanisms for this are proposed, including changes in the activity of mesolimbic dopamine neurons, genomic effects, and alterations in synaptic plasticity. Newly developed, more selective L-type calcium channel blockers could be of considerable value in the treatment of drug dependence. SIGNIFICANCE STATEMENT: Dependence on drugs is a very serious health problem with little effective treatment. Preclinical evidence shows drugs that block particular calcium channels, the L-type, reduce dependence-related effects of alcohol, opioids, psychostimulants, and nicotine. Clinical studies have been restricted by potential cardiovascular side effects, but new, more selective L-channel blockers are becoming available. L-channel blockers have no intrinsic dependence liability, and laboratory evidence suggests they reverse previously developed effects of dependence-inducing drugs. They could provide a novel approach to addiction treatment.

Inhibition of multiple voltage-gated calcium channels may contribute to spinally mediated analgesia by epipregnanolone in a rat model of surgical paw incision

Voltage-activated calcium channels play an important role in excitability of sensory nociceptive neurons in acute and chronic pain models. We have previously shown that low-voltage-activated calcium channels, or T-type channels (T-channels), increase excitability of sensory neurons after surgical incision in rats. We have also found that endogenous 5β-reduced neuroactive steroid epipregnanolone [(3β,5β)-3-hydroxypregnan-20-one] blocked isolated T-currents in dorsal root ganglion (DRG) cells in vitro, and reduced nociceptive behavior in vivo, after local intraplantar application into the foot pads of heathy rats and mice. Here, we investigated if epipregnanolone exerts an antinociceptive effect after intrathecal (i.t.) application in healthy rats, as well as an antihyperalgesic effect in a postsurgical pain model. We also studied if this endogenous neurosteroid blocks currents originating from high voltage-activated (HVA) calcium channels in rat sensory neurons. In in vivo studies, we found that epipregnanolone alleviated thermal and mechanical nociception in healthy rats after i.t. administration without affecting their sensory-motor abilities. Furthermore, epipregnanolone effectively reduced mechanical hyperalgesia after i.t application in rats after surgery. In subsequent in vitro studies, we found that epipregnanolone blocked isolated HVA currents in nociceptive sensory neurons with an IC₅₀ of 3.3 μM in a G-protein-dependent fashion. We conclude that neurosteroids that have combined inhibitory effects on T-type and HVA calcium currents may be suitable for development of novel pain therapies during the perioperative period.

Verapamil – L type voltage gated calcium channel inhibitor diminishes aggressive behavior in male Siamese fighting fish

Verapamil is a L-type voltage gated calcium channels inhibitor (VGCCI), which is a highly prescribed drug used in the treatment of hypertension, angina pectoris, cardiac arrhythmia and cluster headaches. Its common use caused its appearance in water environment. VGCC inhibit epinephrine release and cause many neuro-hormonal changes influencing also fish behavior. Siamese fighting fish was chosen to study the influence of verapamil given to the water on the beginning of experiment in 3 different concentrations of 0 (control), 8 and 160 μg · L−1, on aggressive behavior in these fish. The experimental fish were placed in individual glass containers for 3 weeks and the mirror test was used. The highest concentration led to a significant modulation of fish behavior after 1 week and the lower dose caused statistically significant behavioral changes after 2 weeks of verapamil treatment. Siamese fighting fish males exposed to verapamil had longer latencies to the first chase – 12.6 s (8 μg · L−1 of verapamil) and 18.8 s (160 μg · L−1 of verapamil) compared to 5.6 s in the control group, decreased attack frequency and shorter duration of these attacks. The number of attacks within 10 min was decreased from 38.3 in the control group to 27.1 and 16.1, respectively. Also the total duration of these attacks decreased from 354.8 (control) to 326.4 (decrease statistically insignificant) and to 194.8 s in verapamil treated groups. It was shown, that even relatively low concentrations of verapamil in water may have adverse effects on fish and probably other living organisms.

Withania somnifera root extract prolongs analgesia and suppresses hyperalgesia in mice treated with morphine

Previous studies demonstrated that Withania somnifera Dunal (WS), a safe medicinal plant, prevents the development of tolerance to the analgesic effect of morphine. In the present study, we investigated whether WS extract (WSE) (100 mg/kg, i.p.) may also modulate the analgesic effect induced by acute morphine administration (2.5, 5, 10 mg/kg, s.c.) in the tail-flick and in the hot plate tests, and if it may prevent the development of 2.5 mg/kg morphine-induced rebound hyperalgesia in the low intensity tail-flick test. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for opioid (μ, δ, k), cannabinoid (CB1, CB2), glutamatergic (NMDA), GABAergic (GABAA, GABAB), serotoninergic (5HT2A) and adrenergic (α2) receptors. The results demonstrated that (i) WSE alone failed to alter basal nociceptive threshold in both tests, (ii) WSE pre-treatment significantly protracted the antinociceptive effect induced by 5 and 10 mg/kg of morphine only in tail-flick test, (iii) WSE pre-treatment prevented morphine-induced hyperalgesia in the low intensity tail-flick test, and (iv) WSE exhibited a high affinity for the GABAA and moderate affinity for GABAB, NMDA and δ opioid receptors. WSE prolongs morphine-induced analgesia and suppresses the development of morphine-induced rebound hyperalgesia probably through involvement of GABAA, GABAB, NMDA and δ opioid receptors. This study suggests the therapeutic potential of WSE as a valuable adjuvant agent in opioid-sparing therapies.

In vivo study of ethanol-activated brain protein kinase A: manipulations of Ca2+ distribution and flux

BACKGROUND

The cAMP-dependent protein kinase (PKA) signaling transduction pathway has been shown to play an important role in the modulation of several ethanol (EtOH)-induced behavioral actions. In vivo, short-term exposure to EtOH up-regulates the cAMP-signaling cascade. Interestingly, different Ca(2+) -dependent cAMP-PKA cascade mediators play a critical role in the neurobehavioral response to EtOH, being of special relevance to the Ca(2+) -dependent adenylyl cyclases 1 and 8. We hypothesize an intracellular PKA activation elicited by EtOH administration, which may be regulated by a Ca(2+) -dependent mechanism as an early cellular response. Thus, the present work aims to explore the role of Ca(2+) (internal and external) on the EtOH-activated PKA cascade.

METHODS

Swiss male mice received an intraperitoneal injection of EtOH (0 or 4 g/kg), and brains were dissected following a temporal pattern (7, 15, 30, 45, 90, or 120 minutes). Either the enzymatic PKA activity or its fingerprint was analyzed on different brain areas (cortex, hypothalamus, hippocampus, and striatum). To explore the role of Ca(2+) on the EtOH-activated PKA cascade, mice were pretreated with diltiazem (0 or 20 mg/kg), dantrolene (0 or 5 mg/kg), or 3,7-Dimethyl-1-(2-propynyl)xanthine (0 or 1 mg/kg) 30 minutes before EtOH (4 g/kg) administration. After 45 minutes of EtOH administration, brains were removed and dissected to measure the PKA activity or its fingerprint.

RESULTS

Results from these experiments showed an EtOH-dependent activation of PKA in different brain areas. Manipulations involving a disruption of intracellular Ca(2+) release from the endoplasmic reticulum resulted in a decreased EtOH-induced activation of PKA. On the contrary, extracellular-to-cytoplasm Ca(2+) manipulations did not prevent the PKA activation by EtOH.

CONCLUSIONS

Altogether, these results show the critical role of stored Ca(2+) as an intracellular mediator of different neurobiological actions of EtOH and provide further evidence of a possible new target for EtOH within the central nervous system.

Etoposide modulates the effects of oral morphine analgesia by targeting the intestinal P-glycoprotein

Objectives

Opioids and anticancer compounds such as etoposide (ETP) are substrates of P-glycoprotein (P-gp), an ATP-dependent efflux pump. Chemotherapy compounds may impact on the analgesic effect of opioids such as morphine when the two drugs are co-administered. In this study, we used a mouse model to determine if there is a pharmacological interaction between ETP and morphine, focusing on the involvement of intestinal P-gp.

Methods

P-gp drug efflux activity was measured by an in-situ closed loop method with Rhodamine 123, a P-gp substrate. The analgesic effect of morphine was determined by the tail-flick test. Intestinal P-gp expression levels were determined by Western blot.

Key findings

ETP and morphine significantly decreased the intestinal Rhodamine 123 efflux activity of P-gp. Oral morphine analgesia was significantly enhanced when co-administered with ETP. However, repeated pretreatment (7 days) with oral ETP significantly decreased the oral morphine-induced analgesia, in a cyclosporine A (a P-gp inhibitor) reversible manner. Furthermore, repeated ETP significantly up-regulated intestinal P-gp expression.

Conclusions

It may be important to consider aspects of therapeutic design such as the administration route or scheduling of drugs in patients receiving concurrent chemotherapy and opioid therapy to avoid pharmacokinetic interactions between the two agents.

Influence of centrally administered diltiazem on behavioural responses, clinical symptoms, reticulo-ruminal contractions and plasma catecholamine level after experimentally induced duodenal distension in sheep

A different role of L-type antagonists for voltage-gated calcium channels (VGCC) has been previously identified in different types of experimental and clinical pain in man and animals. Present study examined the role of VGCC blocker - diltiazem administered icv (0.25, 0.5, 1.0 and/or 2.0mg in toto) on the development of pain related symptoms, clinical signs, plasma catecholamine level and the inhibition of reticulo-rumen motility caused by 5min lasting mechanical duodenum distension (DD) in the sheep. Experimental DD was conducted by insertion (during surgery) of rubber balloon into the duodenum and the distension by 40ml of warm water. Duodenal distension resulted in a significant increase of behavioural pain responses, tachycardia, hyperventilation, inhibition of reticulo-rumen contractions rate (from 85% to 45% during 15-20min), an increase of plasma catecholamine concentration (over sevenfold increase of epinephrine during 2h following DD, two-times norepinephrine and 84% increase of dopamine). Diltiazem infusion given 10min before DD decreased intensity of visceral nocifensive responses such as: behavioural changes, tachycardia, hyperventilation, reticulo-rumen motility and efficiently prevented appearance of catecholamine release. These data demonstrated that the development and persistence of acute duodenal pain depends on the activation of Ca(2+) ion flux leading to neurotransmitters release and modulation of membrane excitability. It seems that diltiazem given icv 10min prior to DD (as a source of acute visceral pain), inhibited specific receptors α(1) subunits of VGCCs in target tissues, prevent depolarization of cell membranes and release of neurotransmitters responsible for pain sensitivity in sheep. The observed antinociceptive action of VGCCs type-L blockers suggests that these channels play a crucial role in the modulation of acute visceral pain in sheep.

Participation of L-type calcium channels in ethanol-induced behavioral stimulation and motor incoordination: effects of diltiazem and verapamil

Calcium flux through voltage gate calcium channels (VGCC) is involved in many neuronal processes such as membrane depolarization, gene expression, hormone secretion, and neurotransmitter release. Several studies have shown that either acute or chronic exposure to ethanol modifies calcium influx through high voltage activated channels. Of special relevance is the L-type VGCC. Pharmacological manipulation of L-type calcium channels affects ethanol intake, ethanol discrimination and manifestations of withdrawal syndrome. The present study investigates the role of L-type channels on the psychomotor effects (stimulation and sedation/ataxia) of ethanol by testing the effects of different L-type calcium channel blockers (CCB) on such behaviors. Mice were pretreated intraperitoneally with the CCB, diltiazem (0-40 mg/kg) or verapamil (0-30 mg/kg) 30 min before ethanol (0-3.5 g/kg). Locomotion was measured in an open field chamber for 20 min immediately after ethanol. The two CCB tested prevented locomotor stimulation, but not locomotor suppression produced by ethanol. Doses of the two CCB which reduced ethanol stimulation, did not alter spontaneous locomotion. The ataxic effects of ethanol (1.25 g/kg), measured with an accelerating rotarod task, were not affected by diltiazem (20mg/kg) or verapamil (15 mg/kg). In addition, our results indicated that ethanol is more sensitive to the antagonism of L-type calcium channels than other drugs with stimulant properties; doses of the two CCB that reduced ethanol stimulation did not reduce the psychomotor effects of amphetamine, caffeine or cocaine. In conclusion, these data provide further evidence of the important involvement of L-type calcium channels in the behavioral effects produced by ethanol.

The inhibition of experimentally induced visceral hyperalgesia by nifedipine - a voltage-gated Ca(2+) channels blocker (VGCCs) in sheep

Present study examined the effect of VGCC L-type blocker - nifedipine given i.c.v. (0.25, 0.5, 1 and/or 2mg in toto) on the development of nociceptive behavior, clinical symptoms, plasma catecholamin concentration and reticulo-rumen motility following 5 min lasting mechanical duodenal distension (DD) in sheep. After 24h of fasting, all animals received i.m. ketamine analgesia (20 mg kg(-1)B.W) and anesthetized with pentobarbital (20 mg kg(-1)B.W., i.v. infusion) The permanent stainless steel cannula 29 mm in length and 2mm in diameter was inserted into the lateral cerebral ventricle (controlled by cerebro-spinal efflux) 10mm above the bregma and 5mm laterally from the midline sutures using stereotaxic method. Under the same general anesthesia/analgesia a T-shaped silicon cannula (inside diameter of 21 mm), was inserted into the duodenum (12 cm from pylorus). Second identical cannule was inserted into the dorsal sac of the rumen, a previously described. After surgery each animal was kept in individual boxes for 10 days prior to experiment and was treated i.m. with benzyl procaine penicillin 30,000 I.U kg(-1)B.W.)+dihydrostreptomycine sulfate (10 g kg(-1)B.W.)+prednisolone acetate (1.2 mg kg(-1)B.W.) combination and i.m. ketamine (20 mg kg(-1)B.W.) every day by seven consecutive days. Experimental DD was conducted by insertion and then distension of rubber balloon (containing 40 ml of warm water) inserted into sheep duodenum. Duodenal distension produced a significant increase in behavioral pain manifestations, tachycardia, hyperventilation, inhibition of reticulo-ruminal contractions rate (from 87.2 to 38.0% during 15-20 min), an increase of plasma catecholamine concentration (over 6.4-fold increase of epinephrine during 2h following DD, 2-times norepinephrine and 84% increase of dopamine). Nifedipine infusion administered 10 min prior to DD decreased intensity of visceral pain manifestations such as: behavioral changes, hyperventilation, reticulo-rumen motility and efficiently prevent appearance of catecholamine release. These data demonstrated that the development and persistence of duodenal hyperalgesia depends on the activation of Ca(2+) ion flux leading to neurotransmitters release and modulation of membrane excitability. It seems that nifedipine given i.c.v. 10 min prior to DD (as a source of visceral pain), inhibited specific receptors 1 subunits of VGCCs in target tissues, prevented depolarization of cell membranes and release of neurotransmitters responsible for pain sensitivity in sheep. The observed antinociceptive action of VGCCs type L blockers suggest that these channels play a crucial role in the modulation of acute visceral hyperalgesia in sheep.

Nifedipine suppresses morphine-induced thermal hyperalgesia: Evidence for the role of corticosterone

It has been shown that systemic administration of morphine induced a hyperalgesic response at an extremely low dose. We have examined the effect of nifedipine, as a calcium channel blocker, on morphine-induced hyperalgesia in intact and adrenalectomized rats and on hypothalamic-pituitary-adrenal axis activity induced by ultra-low dose of morphine. To determine the effect of nifedipine on hyperalgesic effect of morphine, nifedipine (2 mg/kg i.p. and 10 microg i.t.) that had no nociceptive effect, was injected concomitant with morphine (1 microg/kg i.p. and 0.01 microg i.t. respectively). The tail-flick test was used to assess the nociceptive threshold, before and 30, 60, 120, 180, 240 and 300 min after drug administration. The data showed that low dose morphine systemic administration could produce hyperalgesic effect in adrenalectomized rats equivalent to sham-operated animals while intrathecal injection of morphine only elicited hyperalgesia in sham-operated animals. Nifedipine could block morphine-induced hyperalgesia in sham and adrenalectomized rats and even a mild analgesic effect was observed in the adrenalectomized group which was reversed by corticosterone replacement. Systemic administration of low dose morphine produced significant increase in plasma level of corticosterone. Nifedipine has an inhibitory effect on morphine-induced corticosterone secretion. Thus, the data indicate that dihydropyridine calcium channels are involved in ultra-low dose morphine-induced hyperalgesia and that both the pattern of morphine hyperalgesia and the blockage of it by nifedipine are modulated by manipulation of the hypothalamic pituitary adrenal axis.

Intrathecal co-administration of morphine and nimodipine produces higher antinociceptive effect by synergistic interaction as evident by injecting different doses of each drug in rats

Earlier, we reported that morphine-nimodipine combination produces significantly higher antinociception after intrathecal but not after systemic administration in mice. Different doses of morphine and nimodipine (5 microg of morphine, 5 microg of nimodipine, 5 microg each of morphine and nimodipine, 10 microg of morphine, 10 microg of nimodipine, 10 microg morphine with 5 microg nimodipine and 5 microg of morphine with 10 microg of nimodipine) were now injected intrathecally in Wistar rats to further characterise this antinociceptive effect. The acute antinociceptive effect was measured by the tail-flick test between 15 min to 7 h. The onset of maximum antinociception (100% MPE) was earlier (by 15 min) in nimodipine (5 microg) than in morphine (5 microg) treated group (by 30 min). Though earlier in onset, 5 microg nimodipine produced transient antinociception, which was significantly higher than saline treated controls for the initial 30 min only. Morphine (5 microg) produced significantly higher antinociception between 15 min to 3:30 h in comparison to control animals. However, co-administration of both morphine and nimodipine led to significantly higher antinociception than morphine alone at 4:00 h and also between 5:00 to 6:30 h. Interestingly, the combined antinociceptive action of morphine and nimodipine was not significantly different from 10 microg of morphine, which indicated synergistic interaction. Naloxone (5 mg/kg) could reverse this antinociceptive effect of morphine-nimodipine combination though it failed to reverse nimodipine (5 microg)-mediated antinociception at 15 min. Increasing the dose of either morphine or nimodipine to 10 mug did not increase antinociception except between 6:30-7:00 h. No obvious side effect was noted after administration of either morphine or nimodipine or both.

Involvement of hypothalamic pituitary adrenal axis on the analgesic cross-tolerance between morphine and nifedipine

Bidirectional cross-tolerance develops between opioids and Ca(2+) channel blockers relating to their antinociceptive effects; however, the role of hypothalamic pituitary adrenal (HPA) axis on this action has not been elucidated yet. We examined the analgesic cross-tolerance between morphine and nifedipine, a dihydropyridine calcium channel blocker, in intact and adrenalectomized (ADX) rats and also evaluated modification of HPA activity during this phenomenon. The tail-flick test was used to assess the nociceptive threshold. The plasma level of corticosterone, as a marker of HPA function, was measured by radioimmunoassay. Our results showed that, in sham operated rats which were chronically treated with morphine, nifedipine failed to affect nociceptive threshold but it could induce significant antinociceptive effect in ADX morphine treated animals. This effect was reversed by corticosterone replacement. Furthermore, morphine could not induce analgesic effect either in sham operated or in ADX animals that received chronic nifedipine. Chronic morphine inhibited the effect of nifedipine on corticosterone secretion but nifedipine treatment had no effect on morphine-induced corticosterone secretion. Based on these results, we can conclude that HPA axis is involved in the induction of cross-tolerance between morphine and nifedipine due to chronic morphine and not nifedipine treatment.

Widespread Neonatal Brain Damage following Calcium Channel Blockade

An abundance of evidence exists that shows calcium channel blockade promotes injury in cultured neurons. However, few studies have addressed the in vivo toxicity of such agents. We now show that the L-type calcium channel antagonist nimodipine promotes widespread and robust injury throughout the neonatal rat brain, in an age-dependent manner. Using both isolated neuronal as well as brain slice approaches, we address mechanisms behind such injury. These expanded studies show a consistent pattern of injury using a variety of agents that lower intracellular calcium. Collectively, these observations indicate that postnatal brain development represents a transitional period for still developing neurons, from being highly sensitive to reductions in intracellular calcium to being less vulnerable to such changes. These observations directly relate to current therapeutic strategies targeting neonatal brain injury.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Nifedipine potentiates antinociceptive effects of morphine in rats by decreasing hypothalamic pituitary adrenal axis activity

It has been shown that nifedipine, as a calcium channel blocker can potentiate the antinociceptive effect of morphine; however, the role of Hypothalamic-Pituitary-Adrenal (HPA) axis on this action has not been elucidated. We examined the effect of nifedipine on morphine-induced analgesia in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. To determine the effect of nifedipine on morphine analgesia, nifedipine (2 mg/kg i.p.) that had no antinociceptive effect, was injected concomitant with sub-effective dose of morphine (1 and 2 mg/kg). The tail-flick test was used to assess the nociceptive threshold, before and 15, 30, 60, 90, 120 and 180 min after drug administration. Our results showed that, nifedipine could potentiate the antinociceptive effect of morphine and this effect of nifedipine in ADX was greater than sham operated rats which, was reversed by corticosterone replacement. Nifedipine has an inhibitory effect on morphine -induced corticosterone secretion. Thus, the data indicate that the mechanism underlying the potentiation of morphine analgesia by nifedipine involves mediation, at least in part, by attenuating the effect of morphine on HPA axis.

Enhanced analgesic effect of morphine-nimodipine combination after intraspinal administration as compared to systemic administration in mice

Calcium plays an important role in the pathophysiology of pain. A number of studies have investigated the effect of L-type calcium channel blockers on the analgesic response of morphine. However, the results are conflicting. In the present study, the antinociceptive effect of morphine (2.5 microg) and nimodipine (1 microg) co-administered intraspinally in mice was observed using the tail flick test. It was compared to the analgesic effect of these drugs (morphine - 250 microg subcutaneously; nimodipine - 100 microg intraperitoneally) after systemic administration. Nimodipine is highly lipophilic and readily crosses the blood brain barrier. Addition of nimodipine to morphine potentiated the analgesic response of the latter when administered through the intraspinal route but not when administered through systemic route. It may be due to direct inhibitory effect of morphine and nimodipine on neurons of superficial laminae of the spinal cord after binding to mu -opioid receptors and L-type calcium channels respectively.

Involvement of Peripheral Mechanism in the Verapamil-Induced Potentiation of Morphine Analgesia in Mice

Morphine's analgesic actions are thought to be mediated through both the central and peripheral nervous systems. L-type calcium channel blockers have been reported to potentiate the analgesic effects of morphine, but the locus of this interaction is not known. In this experiment, we examined the site of verapamil-induced potentiation of morphine analgesia in mice using the quaternary opioid receptor antagonist naloxone-methiodide (NLX-M). Subcutaneous injections of morphine increased locomotor activity and serum corticosterone level, which are mediated by the central nervous system. These central effects were not antagonized by 0.1 mg/kg of NLX-M, whereas this dose of NLX-M partially antagonized the analgesic effect of morphine. Treatment with verapamil potentiated morphine analgesia in a dose-dependent manner. The verapamil-induced potentiation of morphine analgesia was abolished by pretreatment with NLX-M (0.1 and 1 mg/kg). These findings suggest that peripheral mechanisms partially contribute to morphine analgesia and mediate the potentiation of morphine analgesia by verapamil.