Dextromethorphan analogs are neuroprotective in vitro and block glutamate-induced excitotoxic calcium signals in neurons

Inhibitory effects of dextrorotatory morphinans on the human 5-HT(3A) receptor expressed in Xenopus oocytes: Involvement of the N-terminal domain of the 5-HT(3A) receptor

We previously developed a series of dextromethorphan (DM, 3-methoxy-17-methylmorphinan) analogs modified at positions 3 and 17 of the morphinan ring system. Recent reports have shown that DM attenuates abdominal pain caused by irritable bowel syndrome, and multidrug regimens that include DM prevent nausea/vomiting following cancer surgery. However, little is known regarding the molecular mechanisms underlying the beneficial effects of DM. Here, we investigated the effects of DM, 3 of its analogs (AM, 3-allyloxy-17-methoxymorphian; CM, 3-cyclopropyl-17-methoxymorphinan; and DF, 3-methyl-17-methylmorphinan), and 1 of its metabolites (HM, 3-methoxymorphinan) on the activity of the human 5-HT(3A) receptor channel expressed in Xenopus laevis oocytes, using the 2-microelectrode voltage clamp technique. We found that intra-oocyte injection of human 5-HT(3A) receptor cRNAs elicited an inward current (I(5-HT)) in the presence of 5-HT. Cotreatment with AM, CM, DF, DM, or HM inhibited I(5-HT) in a dose-dependent, voltage-independent, and reversible manner. The IC(50) values for AM, CM, DF, DM, and HM were 24.5±1.4, 21.5±4.2, 132.6±35.8, 181.3±23.5, and 191.3±31.5μM, respectively. The IC(50) values of AM and CM were 7-fold lower than that of DM, and mechanistic analysis revealed that DM, DF, HM, AM, and CM were competitive inhibitors of I(5-HT). Point mutations of Arg241 in the N-terminal, but not amino acids in the pore region, to other amino acid residues attenuated or abolished DM- and DM-analog-induced inhibition of I(5-HT). Together, these results demonstrated that dextrorotatory morphinans might regulate 5-HT(3A) receptor channel activity via interaction with its N-terminal domain.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

Epilepsy surgery: perioperative investigations of intractable epilepsy

Recent advances in our understanding of the basic mechanisms of epilepsy have derived, to a large extent, from increasing ability to carry out detailed studies on patients surgically treated for intractable epilepsy. Clinical and experimental perioperative studies divide into three different phases: before the surgical intervention (preoperative studies), on the intervention itself (intraoperative studies), and on the period when the part of the brain that has to be removed is available for further investigations (postoperative studies). Before surgery, both structural and functional neuroimaging techniques, in addition to their diagnostic roles, could be used to investigate the pathophysiological mechanisms of seizure attacks in epileptic patients. During epilepsy surgery, it is possible to insert microdialysis catheters and electroencephalogram electrodes into the brain tissues in order to measure constituents of extracellular fluid and record the bioelectrical activity. Subsequent surgical resection provides tissue that can be used for electrophysiological, morphological, and molecular biological investigations. To take full advantage of these opportunities, carefully designed experimental protocols are necessary to compare the data from different phases and characterize abnormalities in the human epileptic brain.

Dextromethorphan attenuates morphine withdrawal syndrome in neonatal rats passively exposed to morphine

We had previously found that co-injection of dextromethorphan, an antitussive drug and a non-competitive NMDA receptor antagonist, with morphine into dam rats throughout the pregnancy period could attenuate the naloxone-precipitated morphine withdrawal syndrome in their offspring. In the present study, we further tested whether postnatal injection of dextromethorphan into the neonatal rats or a 3-day co-injection of dextromethorphan with morphine into the dam rats before delivery is also effective. Female Sprague-Dawley rats were bi-daily injected with escalating doses of morphine from a week before mating till the first postnatal week. Withdrawal syndrome of morphine in the offspring, manifested mainly as abdominal stretching, was generated by injection of naloxone on postnatal day 5. Direct injection of dextromethorphan into the offspring effectively reduced the severity of naloxone-precipitated abdominal stretching in a dose-dependent manner. A 3-day co-treatment with dextromethorphan given to the dam rat before delivery also had a similar attenuating effect, but the efficacy was lower than that produced by postnatal injection. Thus, the results from the present study support that dextromethorphan is of potential in treating or preventing neonatal morphine withdrawal syndrome.

Binding of dimemorfan to sigma-1 receptor and its anticonvulsant and locomotor effects in mice, compared with dextromethorphan and dextrorphan

Dextromethorphan ((+)-3-methoxy-N-methylmorphinan, DM) has been shown to have both anticonvulsant and neuroprotective effects. The mechanisms of these CNS effects of DM have been suggested to be associated with the low-affinity, noncompetitive, N-methyl-d-aspartate (NMDA) antagonism of DM and/or the high-affinity DM/sigma receptors. DM is largely O-demethylated into the phencyclidine (PCP)-like compound dextrorphan (DR), which may limit its therapeutic use by producing PCP-like adverse effects, such as hyperlocomotion. Dimemorfan ((+)-3-methyl-N-methylmorphinan, DF), an analog of DM, which has been safely used as an antitussive for more than 20 years, is also known not to form DR. This study therefore characterized the binding of DF to the sigma receptors and NMDA-linked PCP sites and examined the anticonvulsant as well as locomotor effects of DF in mice in comparison with those of DM and DR. We found that DF, DM, and DR were relative high-affinity ligands at sigma-1 receptors (Ki=151, 205, 144 nM, respectively) while all of them were with low affinity at sigma-2 receptors (Ki=4-11 microM). Only DR exhibited moderate affinity for PCP sites (Ki=0.9 microM), whereas DF (Ki=17 microM) and DM (Ki=7 microM) were much less active. DF, DM and DR produced prominent anticonvulsant effects in mice as measured by the supramaximal electroshock test with comparable potency (ED50 approximately 70 micromol/kg, i.p.). At the tested doses (20-260 micromol/kg, i.p.), DM and DR exhibited biphasic effects on the locomotor activity whereas DF produced a consistent dose-dependent decrease. These results revealed that, unlike DM and DR, DF did not cause a PCP-like hyperlocomotion adverse effect that is parallel with the PCP sites binding data. Furthermore, since they have equipotent anticonvulsant effects and similar binding affinities to sigma-1 receptors, the very low affinity of DF at PCP sites may suggest that acting on the PCP sites may not be the requisite for mediating the anticonvulsant activity of these DM analogs. With the history of safety and relative less adverse effects, DF appears to be worth further studying on its CNS effects other than the antitussive effect.

Characteristics of the NMDA receptor modulating hypoxia/hypoglycaemia-induced rat striatal dopamine release in vitro

We investigated the functional characteristics of the NMDA receptor that modulates hypoxia/hypoglycaemia-induced striatal dopamine release. Dopamine release was detected by fast cyclic voltammetry in rat neostriatal slices. Four variables were measured: T(on) -- time from initiation of hypoxia/hypoglycaemia to the onset of dopamine release, Tpk -- time from onset to maximum, deltaDA/delta(t) -- rate of dopamine release and DAmax -- maximum extracellular dopamine concentration. In controls, T(on) = 164.9 +/- 1.7 s, Tpk = 20.9 +/- 0.9 s, deltaDA/delta(t) = 5.31 +/- 0.44 microM/s and DAmax = 79.1 +/- 2.5 microM (means +/- S.E.M., n = 203). Cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755, 20 microM) lengthened, while N-methyl-D-aspartate (NMDA) (100 microM) shortened T(on). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK 801, 1 and 10 microM) and dextromethorphan (10 and 100 microM) increased Tpk and decreased DAmax. Neither glycine (100 microM), 7-chlorokynurenic acid (50 microM) nor 5-nitro-6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione (ACEA 1021, 100 microM) had any effect although 7-chlorokynurenic acid blocked the effect of NMDA. Increasing [Mg2+] from 1.3 to 3.7 mM, increased Tpk and decreased deltaDA/delta(t). Dithiothreitol (1 mM) accelerated T(on) while 5.5-dithio-bis-(2-nitrobenzoic acid) (1 mM) delayed T(on). Neither drug affected Tpk, DAmax or deltaDA/delta(t). Neither spermidine (100 microM) nor arcaine (100 microM) affected T(on), Tpk or deltaDA/delta(t) although arcaine decreased DAmax. In conclusion, hypoxia/hypoglycaemia-induced dopamine release was influenced by an NMDA receptor although modulation of the glycine recognition site of the receptor was ineffective, as were agents acting at polyamine modulatory zones. These findings highlight differences between recombinant and native NMDA receptors and suggest caution in extrapolating molecular biology to functional studies.

Dextromethorphan protects against cerebral injury following transient, but not permanent, focal ischemia in rats

Dextromethorphan (DM) has been observed to afford neuroprotection in a variety of in vitro and in vivo experimental models of CNS injury. We have evaluated the neuroprotective activity of DM following both transient (2 h) and permanent focal cerebral ischemia in the rat. Middle cerebral artery occlusion (MCAO) was produced in male Sprague-Dawley rats using the intraluminal filament technique. Animals were dosed s.c with 20 mg/kg DM at 0.5, 1, 2, 4, and 6 hours post occlusion. Analysis of brain injury was performed 24 hours after permanent occlusion or reperfusion. Following transient MCAO, vehicle treated rats exhibited a total infarct volume of 203 +/- 33 mm3. DM produced a 61% reduction in infarct volume to 79 +/- 13 mm3. Permanent MCAO produced a larger infarct volume (406 +/- 44 mm3) which was not significantly reduced in size by treatment with DM (313 +/- 58 mm3). Infarcted hemispheric oedema was not different in vehicle treated rats following transient or permanent MCAO and was not reduced by DM in either group. Following transient MCAO, rectal temperature was elevated 1,2 and 5 hours post occlusion. While not inducing hypothermia or altering physiological parameters such as blood pressure and blood gases, DM attenuated this injury-related increase in temperature, an effect which appeared to correlate with its ability to protect neurons in temperature regulating hypothalamic centres. The DM-induced reduction in infarction demonstrated in our model of transient focal cerebral ischemia provides further support for the in vivo neuroprotective activity of this compound. Importantly, these data demonstrate the limited neuroprotective efficacy of DM when attempting to combat more severe focal ischemic injuries and imply that drug-induced hypothermia is not ultimately responsible for its protective action.