Potentiation of acute opioid-induced respiratory depression and reversal of tolerance by the calcium antagonist nimodipine in awake rats

Mechanisms of Neurorespiratory Toxicity Induced by Fentanyl Analogs—Lessons from Animal Studies

In 2020, fentanyl and its analogs contributed to ~65% of drug-attributed fatalities in the USA, with a threatening increasing trend during the last ten years. These synthetic opioids used as potent analgesics in human and veterinary medicine have been diverted to recreational aims, illegally produced and sold. Like all opioids, central nervous system depression resulting from overdose or misuse of fentanyl analogs is characterized clinically by the onset of consciousness impairment, pinpoint miosis and bradypnea. However, contrasting with what observed with most opioids, thoracic rigidity may occur rapidly with fentanyl analogs, contributing to increasing the risk of death in the absence of immediate life support. Various mechanisms have been proposed to explain this particularity associated with fentanyl analogs, including the activation of noradrenergic and glutamatergic coerulospinal neurons and dopaminergic basal ganglia neurons. Due to the high affinities to the mu-opioid receptor, the need for more elevated naloxone doses than usually required in morphine overdose to reverse the neurorespiratory depression induced by fentanyl analogs has been questioned. This review on the neurorespiratory toxicity of fentanyl and analogs highlights the need for specific research focused on these agents to better understand the involved mechanisms of toxicity and develop dedicated strategies to limit the resulting fatalities.

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.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Increased opioid dependence in a mouse model of panic disorder

Panic disorder is a highly prevalent neuropsychiatric disorder that shows co-occurrence with substance abuse. Here, we demonstrate that TrkC, the high-affinity receptor for neurotrophin-3, is a key molecule involved in panic disorder and opiate dependence, using a transgenic mouse model (TgNTRK3). Constitutive TrkC overexpression in TgNTRK3 mice dramatically alters spontaneous firing rates of locus coeruleus (LC) neurons and the response of the noradrenergic system to chronic opiate exposure, possibly related to the altered regulation of neurotrophic peptides observed. Notably, TgNTRK3 LC neurons showed an increased firing rate in saline-treated conditions and profound abnormalities in their response to met⁵-enkephalin. Behaviorally, chronic morphine administration induced a significantly increased withdrawal syndrome in TgNTRK3 mice. In conclusion, we show here that the NT-3/TrkC system is an important regulator of neuronal firing in LC and could contribute to the adaptations of the noradrenergic system in response to chronic opiate exposure. Moreover, our results indicate that TrkC is involved in the molecular and cellular changes in noradrenergic neurons underlying both panic attacks and opiate dependence and support a functional endogenous opioid deficit in panic disorder patients.

Regulation of mu-opioid receptors, G-protein-coupled receptor kinases and beta-arrestin 2 in the rat brain after chronic opioid receptor antagonism

The aim of this study was to analyse the biochemical and behavioural consequences of chronic treatment with opioid receptor antagonists in rats. We have evaluated the respiratory depressant and antinociceptive effects of the mu-opioid agonist sufentanil, the density of brain mu-opioid receptors, and the expression of G-protein-coupled receptor kinases and beta-arrestin 2 in cerebral cortex and striatum, following sustained opioid receptor blockade. Our results demonstrate that 24 h after interruption of 7 days chronic infusion of naltrexone (120 microg/h), the respiratory depressant potency of the mu-opioid receptor agonist sufentanil was increased to a similar extent as the antinociceptive potency (about three-fold). This was accompanied by mu-opioid receptor up-regulation in several areas of the rat brain associated with opioid control of pain perception and breathing. Moreover, chronic treatment with either naltrexone (120 microg/h) or naloxone (120 microg/h) caused significant increases in the expression levels of G-protein-coupled receptor kinases types 2, 3, and 6, and of beta-arrestin 2 in brain cortex and striatum. Together our data suggest an increased constitutive receptor activity secondary to mu-opioid receptor up-regulation following chronic antagonist treatment.

Diltiazem enhances the analgesic but not the respiratory depressant effects of morphine in rhesus monkeys

There is evidence that blockade of Ca(2+) channels can modify the analgesia and respiratory depression produced by opioid drugs. The interaction between Ca(2+) channel blockade and drug-induced analgesia and respiratory depression was examined by administration of the L-type Ca(2+) channel blocker diltiazem together with various analgesic drugs. The antinociceptive effects of the drugs were evaluated using a warm-water (50 degrees C) tail-withdrawal assay in rhesus monkeys, and the respiratory depressant effects were evaluated using a pressure-displacement plethysmograph. Pretreatment with diltiazem (10-40 mg/kg, i.m.) 30 min before administration of morphine (0.3 to 10 mg/kg) or heroin (0.03 to 1.0 mg/kg) produced a dose-dependent potentiation of the opioid-induced analgesia. The analgesic potency of morphine and heroin was increased by approximately 0.5 log unit in the presence of 40 mg/kg diltiazem. However, diltiazem failed to alter the analgesic potencies of the mu-opioid receptor agonists, fentanyl, etonitazene, nalbuphine, the kappa-opioid receptor agonist, U-50,488 [(trans)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide ], or the non-opioid, clonidine. Respiratory frequency, minute volume, and tidal volume were suppressed by morphine, heroin, and fentanyl, but these effects were not modified by pretreatment with diltiazem (40 mg/kg). These results suggest that diltiazem selectively potentiates morphine- and heroin-induced analgesia without modifying the effects of these opioids on respiration.

Effects of kappa- and mu-opioid receptor agonists on Ca2+ channels in neuroblastoma cells: involvement of the orphan opioid receptor

The effects of micro-, delta- and kappa-opioid receptor agonists, and orphanin FQ/nociceptin (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), on K+-induced [Ca2+]i increase were examined in SK-N-SH cells. Exposure to K+ (50 mM) resulted in a [Ca2+]i rise, which was blocked (-85%) by furaldipine (1 microM) and increased (63%) by BayK 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethyl-pyridine-5 -carboxylate) (0.5 microM), indicating the involvement of L-type Ca2+ channels. The kappa-opioid receptor agonists 3,4-dichloro-N-Methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U-50488H) (1-50 microM) and 5,7,8-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]benze neacetamide (U-69593) (25 microM), and the mu-opioid receptor agonist sufentanil (100 nM-3 microM) inhibited the amplitude of K+-induced [Ca2+]i increase. The agonist of the orphan opioid receptor, orphanin FQ/nociceptin (1 microM), induced dual excitatory and inhibitory effects on the depolarisation-induced Ca2+ influx. The effects of the opioid receptor agonists were not blocked by the kappa-opioid receptor antagonist nor-binaltorphimine (1 microM), only weakly prevented by naloxone (10-100 microM) and naltrexone (100 microM), and partially prevented by pertussis toxin (100 ng/ml, 24 h). The antagonist of the orphan opioid receptor, [Phe1psi(CH2-NH)Gly2]nociceptin(1-13)NH2 (1 microM), prevented the inhibitory effect of U-50488H, sufentanil and orphanin FQ. The present study provides pharmacological evidence for the presence of L-type Ca2+ channels in SK-N-SH cells, that are modulated by opioids through orphan opioid receptor activation.

Different mechanisms mediate development and expression of tolerance and dependence for peripheral mu-opioid antinociception in rat

The mu-opioid [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) exerts a peripheral antinociceptive effect against prostaglandin E2 (PGE2)-induced mechanical hyperalgesia in the hindpaw of the rat. Tolerance and dependence develop to this effect. We have shown previously that tolerance and dependence can be dissociated and are mediated by different second messenger systems. In the present study, we evaluated whether the same or different second messenger systems mediate the development of this peripheral opioid tolerance or dependence compared with the expression of the loss of antinociceptive effect or rebound opioid antagonist hyperalgesia (i. e., expression of tolerance and dependence). DAMGO-induced tolerance was prevented by pretreatment with the nitric oxide synthase inhibitor NG-methyl-L-arginine (NMLA) but not by the protein kinase C (PKC) inhibitor chelerythrine, the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (ddA), or the calcium chelators 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester (TMB-8) and 2-[(2-bis-[carboxymethyl]amino-5-methylphenoxy)-methyl]-6-methoxy-8-bis [carboxymethyl]aminoquinoline (Quin-2). Once established, however, expression of DAMGO tolerance was acutely reversed by TMB-8 or Quin-2 but not by chelerythrine or NMLA. In contrast, naloxone-precipitated hyperalgesia in DAMGO-tolerant paws, a measure of dependence, was blocked by pretreatment with chelerythrine but not by NMLA, ddA, TMB-8, or Quin-2. Naloxone-precipitated hyperalgesia in DAMGO-tolerant paws was acutely reversed by chelerythrine, ddA, TMB-8, or Quin-2 but not by NMLA. Taken together, these results provide the first evidence that different mechanisms mediate the development and expression of both tolerance and dependence to the peripheral antinociceptive effect of DAMGO. However, although the development of tolerance and dependence are entirely separable, the expression of tolerance and dependence shares common calcium-dependent mechanisms.

Alterations in L-type calcium channels in the brain and spinal cord of acutely treated and morphine-tolerant mice

Opiate tolerance has been associated with changes in neuronal intracellular calcium levels. In vivo studies have indicated the involvement of dihydropyridine (DHP)-sensitive ('L-type') voltage-gated calcium channels in the morphine-tolerant state. In this study, we assessed how the morphine-tolerant state would affect the antinociception produced by intrathecal administration of two agents (Bay K 8644 and thapsigargin) that increase intracellular calcium. Morphine-tolerant mice displayed a 7-fold greater ED50 for Bay K 8644 antinociception than placebo-treated animals; no difference was found in the dose-response curve for thapsigargin. To further explore the role of the L-type channel in morphine tolerance development, we performed radiolabeled nitrendipine binding studies in the spinal cord and brain regions associated with pain modulation from acutely treated, tolerant, and control mice. Our treatment protocol produced a 4-fold shift in the ED50 for s.c. morphine antinociception, as determined by the tail-flick procedure. Binding site number and affinity were determined using Scatchard analysis for the following timepoints: 20 min and 60 min after s.c. injection of morphine or vehicle, as well as after 4 days of chronic administration, and after naloxone-precipitated withdrawal. Although some changes were observed in the affinity of nitrendipine for its receptor, a significant change in these measures was found only following naloxone administration, which produced increases in Bmax in both placebo- and morphine-treated mice.

Pharmacokinetic-pharmacodynamic relationships for opioids in balanced anaesthesia

The pure mu-receptor opioid agonists fentanyl, sufentanil and alfentanil are commonly used to provide the specific anti-nociceptive component of a balanced anaesthesia technique. Trefentanil and remifentanil are new opioids with a very short duration of action. Remifentanil has an ester structure and is very rapidly metabolised by blood and tissue esterases. Different perioperative stimuli require different plasma concentrations to suppress responses of the patient. The ability of the anaesthesiologist to select a precise dosage scheme for the individual patient is impeded by the large interindividual pharmacokinetic and pharmacodynamic variability. In addition, the combination of opioids and other drugs used to produce the desired components of balanced anaesthesia may exert additive, synergistic or antagonistic effects. Knowledge of factors influencing the pharmacokinetics and pharmacodynamics is still fragmentary and often controversial. Consequently, the opioid dose needs to be adjusted according to the responses of the patient during surgery to ensure adequate anaesthesia and rapid recovery. The duration of action is not predicted by the elimination half-life alone. The decline in effect-site concentration is dependent on the complex entity of infusion duration, and pharmacokinetic and pharmacodynamic parameters. Computer simulations of infusions of varying duration have been extremely useful when selecting an opioid for a specific clinical scenario on a rational basis. Traditionally, opioids are still administered by intermittent bolus injections. A disadvantage of this method of administration is that plasma concentrations fluctuate above and below the level required for adequate anaesthesia. Computer-assisted infusion pumps make it possible to target a particular drug concentration in plasma and to maintain or change this concentration as needed. This technique provides more stable anaesthesia and a more rapid recovery of the patient.

Kappa-opioid receptor mediated antinociception in rats is dependent on the functional state of dihydropyridine-sensitive calcium channels

The modulatory effect of the dihydropyridine Ca2+ channel antagonist nimodipine on the analgesic action of the kappa-opioid receptor agonist U-69,593 was analyzed using the tail-flick test in rats. The antinociceptive effect of U-69,593 (0.25-4 mg/kg) was antagonized by L-type Ca2+ channel blockade with nimodipine (200 microgram/kg, i.p.), the ED50 being increased from 1.4 to 7.3 mg/kg. On the contrary, when an increase in the density of these channels was induced by means of chronic and simultaneous treatment with nimodipine (1 microgram/h, 7 days) and sufentanil (2 micrograms/h, 8 days), the analgesic effect of U-69,593 was potentiated by 5-fold. Our results suggest a functional coupling between kappa-opioid receptors and L-type Ca2+ channels in nociception.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.