Tolerance to opioid narcotics, II. Cellular tolerance to levorphanol in mouse brain

Pharmacological Characterization of Levorphanol, a G-Protein Biased Opioid Analgesic

BACKGROUND

Levorphanol is a potent analgesic that has been used for decades. Most commonly used for acute and cancer pain, it also is effective against neuropathic pain. The recent appreciation of the importance of functional bias and the uncovering of multiple µ opioid receptor splice variants may help explain the variability of patient responses to different opioid drugs.

METHODS

Here, we evaluate levorphanol in a variety of traditional in vitro receptor binding and functional assays. In vivo analgesia studies using the radiant heat tail flick assay explored the receptor selectivity of the responses through the use of knockout (KO) mice, selective antagonists, and viral rescue approaches.

RESULTS

Receptor binding studies revealed high levorphanol affinity for all the μ, δ, and κ opioid receptors. In S-GTPγS binding assays, it was a full agonist at most µ receptor subtypes, with the exception of MOR-1O, but displayed little activity in β-arrestin2 recruitment assays, indicating a preference for G-protein transduction mechanisms. A KO mouse and selective antagonists confirmed that levorphanol analgesia was mediated through classical µ receptors, but there was a contribution from 6 transmembrane targets, as illustrated by a lower response in an exon 11 KO mouse and its rescue with a virally transfected 6 transmembrane receptor splice variant. Compared to morphine, levorphanol had less respiratory depression at equianalgesic doses.

CONCLUSIONS

While levorphanol shares many of the same properties as the classic opioid morphine, it displays subtle differences that may prove helpful in its clinical use. Its G-protein signaling bias is consistent with its diminished respiratory depression, while its incomplete cross tolerance with morphine suggests it may prove valuable clinically with opioid rotation.

Quantitative assessment of tolerance to and dependence on morphine in mice

1. Tolerance to morphine-induced analgesia (hot plate and acetic acid whrithing test), hypothermia and lethality can be quantified in mice by measuring the degree of parallel shifts of semilog. dose-response relationships induced by repeated opioid administration. 2. A similar procedure can be used for the quantification of naloxone-induced withdrawal as an indicator of dependence. 3. The intensity of tolerance development with respect to time of administration and dosage of morphine varies with the test procedure. It is closely parallel, however, in both analgesic tests during acquisition of tolerance. 4. Log-log-linear relationships exist between tolerance in analgesic tests and physical dependence as determined by naloxone-induced withdrawal. 5. The minimum tolerance-inducing dose of morphine in different tests could not be correlated to the ED50's in these tests. 6. Chronic opiate treatment leads to a decrease or an increase in motility response to morphine, depending on the time that has elapsed after the last morphine administration.

An electron spin resonance study of synaptosome opiate receptors. The preparation and use of a spin labeled morphine

Morphine spin labeled on the phenolic hydroxy group has been prepared using commercially available reagents and characterized by thin layer chromatography, mass spectroscopy, and electron spin resonance spectroscopy. It has been shown that morphine modified in this way retains some opiate activity, does not pass through the blood-brain barrier, and specifically binds to isolate rat brain synaptosomes. Spin labeled morphine has been shown to be an effective biophysical probe complementing radioactive tracer techniques in the study of the narcotic receptor site.

Partial purification of an opiate receptor from mouse brain

A proteolipid isolated from a lipid extract of mouse brain demonstrates stereospecific binding properties for levorphanol. It is present only in neuronal tissue and most abundant in the rhombencephalon. One component saturates at a concentration corresponding to maximum pharmacologic effect in vivo. The estimated mass is 60,000 daltons per bound opiate molecule.

Cellular and metabolic tolerance to an opioid narcotic in mouse brain

1. Running activity and brain levorphanol concentration were measured in nontolerant and tolerant mice given various doses of (3)H-levorphanol.2. The principal factor responsible for tolerance in the mouse is a loss of sensitivity to the narcotic drug at the cellular level in brain; despite adequate brain concentrations, the pharmacological effects are diminished or absent.3. There is also metabolic tolerance; a given dose establishes a lower brain concentration in tolerant than in non-tolerant animals.4. The two kinds of tolerance are distinguished here and the contribution of each is assessed.

Stereospecific and nonspecific interactions of the morphine congener levorphanol in subcellular fractions of mouse brain

A METHOD IS DESCRIBED FOR ANALYZING THE ASSOCIATION OF THE OPIATE NARCOTIC LEVORPHANOL WITH BRAIN TISSUE INTO THREE COMPONENTS: nonsaturable, saturable nonspecific, and saturable stereospecific. The method may be of general applicability for the study of the interaction of drugs with body tissues. In mouse brain the stereospecific binding of levorphanol represents only 2% of the total association of drug with tissue, and it was found only in certain membrane fractions. The material responsible for the stereospecific binding might be the opiate receptor.