THE INHIBITION OF AUTONOMIC NEURO-EFFECTOR TRANSMISSION BY MORPHINE-LIKE DRUGS AND ITS USE AS A SCREENING TEST FOR NARCOTIC ANALGESIC DRUGS

DARK Classics in Chemical Neuroscience: Etonitazene and Related Benzimidazoles

Etonitazene and related 2-benzylbenzimidazoles are potent analgetics invented in the research laboratories of the Swiss pharmaceutical giant CIBA in the late 1950s. Though the unprecedented structure distinguishes this class of compounds from poppy-derived and other synthetic analgetics, a range of studies indicate that these drugs are selective μ opioid receptor agonists possessing morphine-like pharmacotoxicological properties in animals as well as humans. Several unscheduled members of this synthetically readily accessible class of opioids that are not controlled under the international and national drug control systems have recently emerged on the illicit drug market. Among them, isotonitazene has been implicated in at least 200 fatalities in Europe and North America. None of the 2-benzylbenzimidazole derivatives have been developed into medicines, but etonitazene and some of its derivatives have been used as receptor probes and in addiction behavior studies in animals. The unique structure has inspired research on such benzimidazoles and related benzimidazolones of which "brorphine" made its debut as one of the newest psychoactive substance to emerge on the illicit opioid drug market in mid-2019. This in-depth review provides a historical introduction, an overview on the chemistry, pharmacological profiles, adverse effects, addiction liability, regulatory status, and the impact on chemical neuroscience of the 2-benzylbenzimidazoles. Structurally related benzimidazoles with opioid and/or analgesic properties are also discussed briefly.

THE ACTION OF MORPHINE-LIKE DRUGS ON IMPULSE TRANSMISSION IN MAMMALIAN NERVE FIBRES

Experiments on nerves in situ and on isolated nerves provide no evidence that morphine interferes with impulse transmission in myelinated or nonmyelinated nerve fibres. The concentrations used in experiments on isolated nerves were 10- to 100-times as high as those required to depress transmission at autonomic nerve-effector cell junctions. Examination of the resting membrane potential, the action potential and the positive after-potential, the conduction velocity, the time courses of the recovery of the size of the action potential and of the excitability after a conditioning stimulus, the ability of the axons to sustain repetitive activity and the posttetanic hyperpolarization gave no indication that morphine affects either the mechanisms involved in the initiation of the propagated impulse or those leading to restoration of the resting state after activity. Analgesic drugs, such as pethidine and methadone which have a local anaesthetic action, may cause a reversible decrease in the size of the compound action potential and in the conduction velocity of A-B and C fibres.

Regulation of neural responses in the canine pyloric sphincter by opioids

1. Regulation of excitatory and inhibitory junction potentials (e.j.ps and i.j.ps) by opioid peptides was studied in isolated muscle strips from the pyloric sphincter of the dog. 2. Methionine enkephalin (MetEnk; 10(-10) to 10(-6) M) and [D-Ala2, D-Leu5] enkephalin (DADLE; 10(-11) to 10(-7) M), a delta-specific opioid agonist, inhibited i.j.ps and e.j.ps recorded from cells in the myenteric and submucosal regions of the circular muscle layer. These compounds had no effect on resting potential or slow wave activity suggesting that the effects on junction potentials were not due to direct effects on smooth muscle cells. 3. MetEnk and DADLE caused similar effects on junction potentials in preparations in which the myenteric plexus was removed, suggesting that opioids inhibit pre-junctional effects on nerve fibres within the muscularis externa. 4. Inhibition of junction potentials by MetEnk and DADLE was blocked by approximately the same extent by naloxone (10(-6) M) and ICI 174,864 (10(-6) M), a delta-specific antagonist. 5. MetEnk and DADLE blocked a portion of the i.j.p. that was sensitive to arginine analogues; after treatment with N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), MetEnk and DADLE had no further effect on i.j.ps. These data suggest that opioids regulate nitric oxide-dependent neurotransmission. 6. Naloxone (10(-6) M) alone had no effect on i.j.ps elicited by short trains of electrical field stimuli. 7. I.j.p. amplitude was reduced after a period of conditioning stimulation (2 min, 30 Hz, 30 V). Naloxone blocked the post-stimulation inhibition. Repetitive stimulation at high frequencies (30 Hz) resulted in sustained hyperpolarization. Naloxone increased the amplitude of the hyperpolarization responses elicited by high frequency stimulation.8. These results show that e.j.ps and i.j.ps in the canine pylorus are inhibited by opioids. A portion of the inhibitory effects appears to be mediated via delta receptors.9. Although pyloric muscles are richly innervated by nerves containing opioid peptides, brief trains of stimuli do not appear to release concentrations of opioids that are effective in regulating junction potentials. Higher frequency stimulation (or longer durations of stimulation) appear to be necessary to release concentrations of opioids that are effective in modulating the amplitude of junction potentials.

Enhancement of guinea-pig intestinal peristalsis by blockade of muscarinic M1-receptors

1. The effects of pirenzepine and hyoscine on the peristaltic reflex were investigated in the guinea-pig isolated small intestine. Peristalsis was induced by raising the intraluminal pressure and the volume of fluid propelled was taken as a measure of the efficiency of peristaltic activity. 2. Low concentrations of pirenzepine (0.1-1 nM) and of hyoscine (0.01 nM) significantly enhanced peristalsis, whereas larger concentrations of both drugs caused inhibition. Pirenzepine was about 6 times less potent than hyoscine in increasing peristalsis, but was about 100 times less potent in inhibiting it. 3. Neither tolazoline (1 microM) nor naloxone (0.3 microM) affected the stimulatory action of pirenzepine on peristalsis. 4. Bicuculline increased the efficiency of peristalsis at concentrations of 1 microM and 10 microM; at 10 nM, bicuculline reduced significantly the increase of peristalsis by pirenzepine. gamma-Aminobutyric acid (GABA) did not affect peristaltic activity, but the stimulatory effect of pirenzepine was abolished in the presence of 100 microM GABA. 5. The results indicate that activation of neuronal M1-receptors causes inhibition of small intestinal peristalsis. Bicuculline-sensitive 'GABAergic' synapses are probably involved in this inhibition.

Action of morphine on the neuro-effector transmission in the guinea-pig ileum and in the mouse vas deferens

Effects of morphine on the neuro-effector junction of the guinea-pig ileum or mouse vas deferens were investigated by the micro-electrode and double sucrose gap methods. 1. Morphine (10(-8)-10(-5) M) did not change the membrane potential, membrane resistance and electrical threshold required to produce the action potential of smooth muscle cells of guinea-pig ileum or mouse vas deferens. 2. Morphine (10(-8)-10(-7) M) markedly suppressed the amplitude of excitatory junction potential (e.j.p.) of ileum or that of vas deferens. However the same concentration of morphine did not suppress the inhibitory junction potential recorded from the guinea-pig ileum or the facilitation phenomena observed with repetitive stimulation in the mouse vas deferens. In addition, this opiate (3.5 x 10(-5) M) did not alter the amplitude or the frequency of miniature excitatory junction potential recorded from the mouse vas deferens. 3. Naloxone (3.5 x 10(-7) M), itself, exerted no effect on the membrane potential and amplitude of the e.j.p. After pretreatment with naloxone, however, the inhibitory action of morphine on the e.j.p. was suppressed. 4. The extracellularly recorded action potential from the small nerve bundle innervating the mouse vas deferens was not affected by morphine (3.5 x 10(-5) M). 5. The amplitude of the e.j.p. of the guinea-pig ileum was dependent on the concentration of [Ca]o. When [Ca]o and the relative amplitude of e.j.p. were plotted on a double logarithmic scale, the above relation yielded a straight line with a slope of 1.7. Application of morphine resulted in a reduction in the slope of the straight line to 1.0. 6. These results indicate that morphine probably suppresses the influx of Ca during spike electrogenesis in the nerve terminal, however, there is no modification of the action of Ca in nerve terminals.

Effects of narcotic analgesics and their antagonists on the rabbit isolated heart and its adrenergic nerves

1. In isolated perfused hearts of rabbits, the effects of morphine, methadone, pethidine, fentanyl, levallorphan and naloxone on heart rate, the spontaneous outflow of noradrenaline, the uptake of infused noradrenaline and the overflow of noradrenaline in response to stimulation of the accelerans nerves were investigated.2. At concentrations of 10-100 muM, methadone, fentanyl, levallorphan and naloxone, but not morphine and pethidine, decreased the heart rate. Only pethidine and levallorphan (100 muM) augmented the spontaneous outflow of noradrenaline.3. With the exception of naloxone, all drugs diminished the neuronal uptake of noradrenaline from the perfusion fluid. Methadone and pethidine (1 muM) were the most effective inhibitors. The inhibitory effect of morphine was not antagonized by naloxone.4. All drugs increased the overflow of noradrenaline evoked by stimulation of the accelerans nerves at 5 Hz. Simultaneously, the positive chronotropic effect of stimulation was usually enhanced. Morphine also augmented the response to stimulation at 2.5 and 10 Hz. The effect of morphine was prevented by pre-infusion of cocaine. The response to stimulation was never depressed.5. It is concluded that the adrenergic nerves of the rabbit heart lack specific morphine receptors which in some other sympathetic nerves mediate an inhibition of the stimulation-induced secretion of noradrenaline. The mechanism of the enhancement of adrenergic neurotransmission by relatively high concentrations of the drugs is discussed.

Hyposensitivity to drugs induced by morphine and some related substances in the cat

1. Hyposensitivity to noradrenaline occurred concomitantly with morphine tachyphylaxis in the absence of any obvious depressant effect on the heart or peripheral circulation.2. The actions of adrenaline, noradrenaline and angiotensin and, occasionally, other drugs were tested before and after repeated injections of morphine or one of ten related substances in various preparations of the spinal cat.3. Tachyphylaxis in response to morphine could be related to a simultaneous decrease in the tissue responses to adrenaline, noradrenaline and angiotensin but such a parallelism has not been shown for the substances related to morphine.