Nonopioid effect of morphine on electrically evoked acetylcholine release from Torpedo electromotor neurons

Levorphanol: revisiting an underutilized analgesic

Levorphanol (levo-3-hydroxy-N-methylmorphinan) is a step 3 opioid first developed in the 1940s as an alternative to morphine. Levorphanol belongs to the morphinan opioid series. Levorphanol has greater potency than morphine and is a potent N-methyl-d aspartate (NMDA) antagonist. Levorphanol interferes with the uptake of norepinephrine and serotonin, which makes it potentially useful for neuropathic pain. Glucuronidation changes Levorphanol to Levorphanol-3-glucuronide with excretion by the kidney. Levorphanol has a long half-life and may accumulate with repeated dosing. Levorphanol can be administered orally, intravenously, and subcutaneously. This article provides an update regarding the pharmacodynamics, pharmacology, and clinical efficacy of this often overlooked step 3 opioid.

Levorphanol: the forgotten opioid

BACKGROUND

Levorphanol (levo-3-hydroxy-N-methylmorphinan) is a strong opioid that is the only available opioid agonist of the morphinan series. Levorphanol was originally synthesized as a pharmacological alternative to morphine more than 40 years ago. It is considered a step-3 opioid by the World Health Organization (WHO) and has a greater potency than morphine. Analgesia produced by levorphanol is mediated via its interactions with mu, delta, and kappa opioid receptors. Levorphanol is also an N-methyl-D-aspartate (NMDA) receptor antagonist. There is evidence that levorphanol may inhibit uptake of norepinephrine and serotonin. Similar to morphine, levorphanol undergoes glucuronidation in the liver, and the glucuronidated products are excreted in the kidney. Levorphanol can be given orally, intravenously, and subcutaneously.

OBJECTIVE

This article reviews the pharmacodynamics, pharmacology, and clinical efficacy for this often overlooked step-3 opioid.

CONCLUSION

The long half-life of the drug increases the potential for drug accumulation. Levorphanol has clinical efficacy in neuropathic pain.

Methadone block of K+ current in squid giant fiber lobe neurons

Voltage-dependent ionic currents were recorded from squid giant fiber lobe neurons using the whole-cell patch-clamp technique. When applied to the bathing solution, methadone was found to block IK, I Na and I Ca. Both I Na and I Ca were reduced without apparent change in kinetics and exhibited IC(50)'s of 50-100 and 250-500 mu M, respectively, at +10 mV. In contrast, IK was reduced in a time-dependent manner that is well fit by a simple model of open channel block (K(D)= 32+/- or 2 mu M, +60 mV, 10 degrees Celsius). The mechanism of I(K) block was examined in detail and involves a direct action of methadone, a tertiary amine, on K channels rather than an opioid receptor-mediated pathway. The kinetics of I(K) block resemble those reported for internally applied long chain quaternary ammonium (QA) compounds; and recovery from I(K) block is QA-like in its slow time course and strong dependence on holding potential. A quaternary derivative of methadone (N-methyl-methadone) only reproduced the effects of methadone on I(K) when included in the pipette solution; this compound was without effect when applied externally. I(K) block thus appears to involve diffusion of methadone into the cytoplasm and occlusion of the open K channel at the internal QA blocking site by the protonated form of the drug. This proposed mode of action is supported by the pH and voltage dependence of block as well as by the observation that high external K+ speeds the rate of drug dissociation. In addition, the effect of methadone on I(K) evoked during prolonged (300 ms) depolarizations suggests that methadone block may interfere with endogenous K+ channel inactivation. The effects of temperature, methadone stereoisomers, and the methadone-like drugs propoxyphene and nor-propoxyphene on IK block were examined. Methadone was also found to block I(K) in GH3 cells and in chick myoblasts.

Dual regulation by opioids of 3H-norepinephrine release in the human neuroblastoma cell-line SK-N-SH

Depolarization-evoked 3H-norepinephrine release from SK-N-SH cells was found to be regulated by opioid ligands. Opioids exerted either inhibition or augmentation of 3H-norepinephrine release. Both effects were mediated by opioid receptors. In addition, a nonopioid inhibitory effect of opiates on release was observed. The SK-N-SH cell-line provides a suitable model for studying the various mechanisms underlying the opioid regulatory pathways within single cells.