Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

/pdf/cellular-and-synaptic-adaptations-mediating-opioid-3w7efxea4o.pdf

Cellular and synaptic adaptations mediating opioid dependence.

K etamine has a special position among anesthetic drugs. It was introduced into clinical practice >30 yr ago with the hope that it would function as a “monoanesthetic” drug: inducing analgesia, amnesia, loss of consciousness, and immobility. This dream was not fulfilled because significant side effects were soon reported. With the introduction of other IV anesthetic drugs, ketamine’s role diminished rapidly. However, it is still used clinically for indications such as induction of anesthesia in patients in hemodynamic shock; induction of anesthesia in patients with active asthmatic disease; IM sedation of uncooperative patients, particularly children; supplementation of incomplete regional or local anesthesia; sedation in the intensive care setting; and short, painful procedures, such as dressing changes in burn patients. However, recent insights into ketamine’s anesthetic mechanism of action and its neuronal effects, as well as a reevaluation of its profound analgesic properties, offer the potential of expanding this range of indications. In addition, studies with the S(+) ketamine isomer suggest that its use may be associated with fewer side effects than the racemic mixture. In this article, we review the mechanism of action of ketamine anesthesia, the pharmacologic properties of its stereoisomers, and the potential uses of ketamine for preemptive analgesia and neuroprotection. Several aspects discussed herein have been reviewed previously (l-4).

Ketamine: teaching an old drug new tricks.

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

Molecular Mechanisms and Regulation of Opioid Receptor Signaling

There is accumulating evidence to implicate the importance of N-methyl-d-aspartate (NMDA) receptors to the induction and maintenance of central sensitization during pain states. However, NMDA receptors may also mediate peripheral sensitization and visceral pain. NMDA receptors are composed of NR1, N

The role of N-methyl-D-aspartate (NMDA) receptors in pain: a review.

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine’s pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

https://pharmrev.aspetjournals.org/content/pharmrev/70/3/621.full.pdf

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine: its mechanism(s) of action and unusual clinical uses

BackgroundThe authors examined the interaction of ketamine with recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] opioid receptors and recombinant orphan opioid receptors expressed in Chinese hamster ovary cells (CHO-[micro sign], CHO-[Greek small letter kappa, CHO-[small delt

Stereoselective Interaction of Ketamine with Recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] Opioid Receptors Expressed in Chinese Hamster Ovary Cells

The rat μ-opioid receptor has recently been cloned, yet its second messenger coupling remains unclear. The endogenous μ-opioid receptor in SH-SY5Y cells couples to phospholipase C (PLC), increases [Ca2+]i and inhibits adenylyl cyclase (AC). We have examined the effects of μ-opioid agonists on inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), [Ca2+]i and adenosine 3′: 5′-cyclic monophosphate (cyclic AMP) formation in Chinese hamster ovarian (CHO) cells transfected with the cloned μ-opioid receptor.




Opioid receptor binding was assessed with [3H]-diprenorphine ([3H]-DPN) as a radiolabel. Ins(1,4,5)P3 and cyclic AMP were measured by specific radioreceptor assays. [Ca2+]i was measured fluorimetrically with Fura-2.




Scatchard analysis of [3H]-DPN binding revealed that the Bmax varied between passages. Fentanyl (10 pm–1 μm) dose-dependently displaced [3H]-DPN, yielding a curve which had a Hill slope of less than unity (0.6±0.1), and was best fit to a two site model, with pKi values (% of sites) of 9.97±0.4 (27±4.8%) and 7.68±0.07 (73±4.8%). In the presence of GppNHp (100 μm) and Na+ (100 mm), the curve was shifted to the right and became steeper (Hill slope=0.9±0.1) with a pKi value of 6.76±0.04.




Fentanyl (0.1 nm–1 μm) had no effect on basal, but dose-dependently inhibited forskolin (1 μm)-stimulated, cyclic AMP formation (pIC50=7.42±0.23), in a pertussis toxin (PTX; 100 ng ml−1 for 24 h)-sensitive and naloxone-reversible manner (Ki=1.7 nm). Morphine (1 μm) and [d-Ala2, MePhe4, gly(ol)5]-enkephalin (DAMGO, 1 μm) also inhibited forskolin (1 μm)-stimulated cyclic AMP formation, whilst [d-Pen2, d-Pen5], enkephalin (DPDPE, 1 μm) did not.




Fentanyl (0.1 nm–10 μm) caused a naloxone (1 μm)-reversible, dose-dependent stimulation of Ins(1,4,5)P3 formation, with a pEC50 of 7.95±0.15 (n=5). PTX (100 ng ml−1 for 24 h) abolished, whilst Ni2+ (2.5 mm) inhibited (by 52%), the fentanyl-induced Ins(1,4,5)P3 response. Morphine (1 μm) and DAMGO (1 μm), but not DPDPE (1 μm), also stimulated Ins(1,4,5)P3 formation. Fentanyl (1 μm) also caused an increase in [Ca2+]i (80±16.4 nm, n=6), reaching a maximum at 26.8±2.5 s. The increase in [Ca2+]i remained elevated until sampling ended (200 s) and was essentially abolished by the addition of naloxone (1 μm). Pre-incubation with naloxone (1 μm, 3 min) completely abolished fentanyl-induced increases in [Ca2+]i.




In conclusion, the cloned μ-opioid receptor when expressed in CHO cells stimulates PLC and inhibits AC, both effects being mediated by a PTX-sensitive G-protein. In addition, the receptor couples to an increase in [Ca2+]i. These findings are consistent with the previously described effector-second messenger coupling of the endogenous μ-opioid receptor.






British Journal of Pharmacology (1997) 120, 1165–1171; doi:10.1038/sj.bjp.0701012

The effects of recombinant rat μ‐opioid receptor activation in CHO cells on phospholipase C, [Ca2+]i and adenylyl cyclase

Previous studies have implicated the neuronal L-type voltage-sensitive Ca2+ channel (VSCC) as a target site for i.v. anaesthetic agents. It is unclear if these agents interact with the L-channel alpha-subunit 1,4-dihydropyridine (DHP) binding site. In this study, we have examined the interaction of thiopentone, pentobarbitone, ketamine, etomidate, propofol and alphaxalone, and the non-anaesthetic barbiturate, barbituric acid, with the DHP binding site on rat cerebrocortical membranes. Binding assays were performed in 1-ml volumes of Tris-HCl 50 mmol litre-1, pH 7.4, for 90 min at room temperature containing 200 micrograms of membrane protein with [3H]PN200-110 as a radiolabelled DHP. Non-specific binding was defined in the presence of nifedipine 10(-5) mol litre-1. The interaction of i.v. anaesthetics was determined by displacement of [3H]PN200-110 0.2 nmol litre-1. All i.v. anaesthetics showed some interaction with the DHP binding site. The concentrations of anaesthetic producing 25% inhibition of specific binding (corrected for the competing mass of [3H]PN200-110), K25 were (mumol litre-1): thiopentone 48 (SEM 2), pentobarbitone 95 (7), propofol 40 (2), etomidate 25 (2), alphaxalone 17 (3) and ketamine 198 (16). Barbituric acid was ineffective. With the exception of ketamine, there was a significant correlation between K25 and peak serum concentration during anaesthesia (P = 0.033) and serum concentrations on wakening (P = 0.018), suggesting that the L-channel DHP binding site may be a target for i.v. anaesthetic agents.

I.v. anaesthetic agents inhibit dihydropyridine binding to L-type voltage-sensitive Ca2+ channels in rat cerebrocortical membranes.

We have examined the interaction of procaine, prilocaine, lignocaine, bupivacaine, amylocaine and R(+) and S(-) ropivacaine with L-type voltage-sensitive Ca2+ channels in rat cerebrocortical membranes. Membranes were prepared in Tris HCl 50 mmol litre-1, pH 7.4, by homogenization and centrifugation. Binding assays were performed in 1-ml volumes of Tris HCl 50 mmol litre-1, pH 7.4, for 90 min at room temperature using approximately 200 micrograms of protein. Non-specific binding was defined in the presence of nifedipine 10(-5) mol litre-1, and bound and free radioactivity were separated by vacuum filtration. The effects of local anaesthetics were determined by displacement of [3H]PN200-110 (approximately 0.2 nmol litre-1), a radiolabelled 1,4- dihydropyridine (DHP) L-channel antagonist. The concentration of displacer producing 50% displacement was corrected for the competing mass of [3H]PN200-110 to yield the affinity constant, K50. All local anaesthetics displaced [3H]PN200-110 in a dose-dependent manner with a rank order potency of (K50, mmol litre-1) bupivacaine (0.48), amylocaine (0.74), lignocaine (1.09), prilocaine (2.06) and procaine (2.09). Ropivacaine enantiomers did not show stereo-selective displacement, with K50 values of 0.99 and 0.92 mmol litre-1 for R(+) and S(-) ropivacaine, respectively. There was a significant correlation between pK50 and p (octanol:buffer partition coefficient) (r2 = 0.872, P = 0.020), pK50 and p (local anaesthetic potency) (r2 = 0.816, P = 0.036), pK50 and p (relative conduction blocking potency) (r2 = 0.843, P = 0.028) and between pK50 and p (IC50 for inhibition of cardiac output) (r2 = 0.897, P = 0.015). These data suggest that DHP binding sites may be involved in both the mechanism of local anaesthesia and the cardiotoxicity of these agents.

K Hirota

Papers

Ketamine: its mechanism(s) of action and unusual clinical uses

Stereoselective Interaction of Ketamine with Recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] Opioid Receptors Expressed in Chinese Hamster Ovary Cells

The effects of recombinant rat μ‐opioid receptor activation in CHO cells on phospholipase C, [Ca2+]i and adenylyl cyclase

I.v. anaesthetic agents inhibit dihydropyridine binding to L-type voltage-sensitive Ca2+ channels in rat cerebrocortical membranes.

Do local anaesthetics interact with dihydropyridine binding sites on neuronal L-type Ca2+ channels?