Adverse effects of dextromethorphan on the spatial learning of rats in the Morris water maze

Repeated dextromethorphan administration in adolescent rats produces long-lasting behavioral alterations

Initiation of non-medical dextromethorphan (DXM) use often occurs in adolescence, yet little is known about the consequences when use begins during this developmental period. The current experiments examined the acute response and the effects of repeated exposure to DXM in adolescence on behavior in adulthood. We examined locomotor activity, locomotor sensitization, and cognitive function, in rats that received repeated administration of DXM. Groups of adolescent (PND 30) and adult (PND 60) male rats were treated with DXM (60 mg/kg) once daily for 10 days. Locomotor activity in response to DXM was assessed following the first injection, on the 10th day of injection (adolescent - PND 39; adult - PND 69), and following 20 days of abstinence (adolescent - PND 60; adult - PND 90). Acute locomotor effects and locomotor sensitization were compared in adolescents and adults; cross-sensitization to ketamine, another dissociative with abuse potential, was also examined. In a separate group of rodents cognitive deficits were assessed following a 20 day abstinence period (adolescent - PND 60; adult - PND 90) in spatial learning and novel object recognition tasks. The locomotor stimulant effect of DXM was much greater in adolescents than adults. Also, only adolescent rats that were repeatedly administered DXM demonstrated locomotor sensitization at the end of 10 days of injection. However, sensitization occurred after the abstinence period in all rats regardless of age. Nonetheless, cross-sensitization to ketamine was only evident in adolescent-treated rats. DXM also led to an increase in perseverative errors in reversal learning only in the adolescent-treated group. We conclude that repeated use of DXM produces long-lasting neuroadaptations that may contribute to addiction. Deficits in cognitive flexibility occur in adolescents, although further work is necessary to confirm these findings. The results extend the understanding of potential long-term consequences of DXM use in adolescents and adults.

Deficits in motor and cognitive functions in an adult mouse model of hypoxia-ischemia induced stroke

Ischemic strokes cause devastating brain damage and functional deficits with few treatments available. Previous studies have shown that the ischemia-hypoxia rapidly induces clinically similar thrombosis and neuronal loss, but any resulting behavioral changes are largely unknown. The goal of this study was to evaluate motor and cognitive deficits in adult HI mice. Following a previously established procedure, HI mouse models were induced by first ligating the right common carotid artery and followed by hypoxia. Histological data showed significant long-term neuronal losses and reactive glial cells in the ipsilateral striatum and hippocampus of the HI mice. Whereas the open field test and the rotarod test could not reliably distinguish between the sham and HI mice, in the tapered beam and wire-hanging tests, the HI mice showed short-term and long-term deficits, as evidenced by the increased number of foot faults and decreased hanging time respectively. In cognitive tests, the HI mice swam longer distances and needed more time to find the platform in the Morris water maze test and showed shorter freezing time in fear contextual tests after fear training. In conclusion, this study demonstrates that adult HI mice have motor and cognitive deficits and could be useful models for preclinical stroke research.

Dextromethorphan: An update on its utility for neurological and neuropsychiatric disorders

Dextromethorphan (DM) is a commonly used antitussive and is currently the only FDA-approved pharmaceutical treatment for pseudobulbar affect. Its safety profile and diverse pharmacologic actions in the central nervous system have stimulated new interest for repurposing it. Numerous preclinical investigations and many open-label or blinded clinical studies have demonstrated its beneficial effects across a variety of neurological and psychiatric disorders. However, the optimal dose and safety of chronic dosing are not fully known. This review summarizes the preclinical and clinical effects of DM and its putative mechanisms of action, focusing on depression, stroke, traumatic brain injury, seizure, pain, methotrexate neurotoxicity, Parkinson's disease and autism. Moreover, we offer suggestions for future research with DM to advance the treatment for these and other neurological and psychiatric disorders.

Low doses of dextromethorphan have a beneficial effect in the treatment of neuropathic pain

N-methyl-D-aspartate receptor (NMDAR) antagonists may be given in persistent neuropathic pain, but adverse events especially with ketamine may limit their clinical use. Less central and cognitive adverse events are described with dextromethorphan and memantine. These molecules have been explored in many preclinical and clinical studies, but data are conflicting as regards neuropathic pain alleviation. Dextromethorphan and memantine have been administered to animals after spinal nerve ligation (SNL) to evaluate their antinociceptive/cognitive effects and associated molecular events, including the phosphorylation of several tyrosine (pTyr(1336), pTyr(1472)) residues in the NR2B NMDAR subunit. Spinal nerve ligation and sham animals received dextromethorphan (10 mg/kg, i.p.), memantine (20 mg/kg, i.p.) or saline (1 mL/kg, i.p.). These drugs were administered once symptoms of allodynia and hyperalgesia had developed. Tests were carried out before and after surgery. Tactile allodynia, mechanical hyperalgesia and spatial memory were, respectively, evaluated by von Frey, Randall & Selitto and Y-maze tests and molecular events by Western blot analysis. Spinal nerve-ligated animals displayed nociception and impaired spatial memory. Dextromethorphan, but not memantine, reversed neuropathic pain (NP) symptoms, restored spatial memory integrity and decreased the expression of pTyr(1336)NR2B. Following postoperative administration of dextromethorphan, this study has demonstrated for the first time a concordance between behaviour, cognitive function and molecular events via pTyr(1336)NR2B for neuropathic pain alleviation. Confirmation of these findings in patients would constitute a major step forward in the treatment of neuropathic pain and in the improvement of cognitive function and quality of life.

Effect of a single dose of dextromethorphan on psychomotor performance and working memory capacity

BACKGROUND

Previous studies show that the prolonged use of dextromethorphan produces cognitive deterioration in humans.

AIM

The aim of this study was to investigate the effect of a single dose of dextroemthrophan on psychomotor performance and working memory capacity.

MATERIALS AND METHODS

This is a randomized, double-blind, controlled, and prospective study. Thirty-six (17 women, 19 men) medical students enrolled in the study; half of them (7 women, 11 men) were given placebo, while the other half (10 women, 8 men) received dextromethorphan. The choice reaction time, critical flicker fusion threshold, and N-back working memory task were measured before and after 2 h of taking the drugs.

RESULTS

Dextromethorphan showed a significant deterioration in the 3-back working memory task (P<0.05). No significant changes were seen as regards the choice reaction time components (total, recognition, motor) and critical flicker fusion threshold (P>0.05). On the other hand, placebo showed no significant changes as regards the choice reaction time, critical flicker fusion threshold, and N-back working memory task (P>0.05).

CONCLUSION

A single dose of dextromethorphan has no effect on attention and arousal but may significantly impair the working memory capacity.

REPETITIVE DEXTROMETHORPHAN AT ADOLESCENCE AFFECTS WATER MAZE LEARNING IN FEMALE RATS

Effects of repetitive dextromethorphan at adolescence on a spatial learning of rats were investigated. Rats received 10 daily injections of dextromethorphan (40 mg/kg) from postnatal day 28 thru 37, and were then subjected to the Morris water maze task from day 38. Significant impairments were found in the probe trial and the reversal training of the maze learning in the female rats, but not in males, treated with dextromethorphan. This result suggests that repetitive dextromethorphan, that is, abuse of dextromethorphan, at adolescence may induce deficits in the hippocampus-based memory function, perhaps more obviously in females.

Impairments in water maze learning of aged rats that received dextromethorphan repeatedly during adolescent period

RATIONALE

Dextromethorphan (DM), an over-the-counter cough suppressant, has been recently used as a drug of abuse by teenage groups in some countries, such as the United States, Canada, and Korea. We previously showed that repeated administration of DM, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors, impairs spatial learning performance in adolescent rats.

OBJECTIVES

In the present study, long-term adverse effects of repetitive DM use at adolescence were examined in rats.

METHODS

Male and female Sprague-Dawley rat pups received either intraperitoneal DM (40 mg/kg) or saline daily during postnatal days 28-37, and were then subjected to the Morris water maze task at the age of 18 months. Expression levels of NMDAR1, functional subunit of NMDA receptors, in the prefrontal cortex and the hippocampus were examined by Western blot analysis. Changes in plasma corticosterone levels responding to stress were determined by radioimmunoassay.

RESULTS

DM-experienced male rats exhibited deficits in the probe trial, and female rats in the initial learning and the reversal training, in water maze performance. Expression levels of NMDAR1 in the brain regions were significantly increased in DM-experienced rats, compared to control rats. Stress-induced increases in plasma corticosterone levels were blunted both in male and female DM rats.

CONCLUSIONS

The results suggest that repeated administration of DM at high doses during adolescent period may induce permanent deficits in cognitive function and that increased expression of NMDAR1 in the prefrontal cortex and the hippocampus may take a role in DM-induced memory deficits.

Ovariectomy ameliorates dextromethorphan--induced memory impairment in young female rats

We have previously found that dextromethorphan (DM), over-the-counter cough suppressant, impairs memory retention in water maze task, when it is repeatedly administrated to adolescent female rats at high doses. In this study we examined first if ovariectomy ameliorates the DM-induced memory impairment in female rats, and then whether or not the DM effect is revived by estrogen replacement in ovariectomized female rats. Female rat pups received bilateral ovariectomy or sham operation on postnatal day (PND) 21, and then intraperitoneal DM (40 mg/kg) daily during PND 28-37. Rats were subjected to the Morris water maze task from PND 38, approximately 24 h after the last DM injection. In probe trial, goal quadrant dwell time was significantly reduced by DM in the sham operated group, however, the reduction by DM did not occur in the ovariectomy group. When 17beta-estradiol was supplied to ovariectomized females during DM treatment, the goal quadrant dwell time was significantly decreased, compared to the vehicle control group. Furthermore, a major effect of estrogen replacement was found in the escape latency during the last 3 days of initial learning trials. These results suggest that ovariectomy may ameliorate the adverse effect of DM treatment on memory retention in young female rats, and that estrogen replacement may revive it, i.e. estrogen may take a major role in DM-induced memory impairment in female rats.

Mechanisms influencing acquisition and recall of motor memories

An internal model of the dynamics of a tool or an object is part of the motor memory acquired when learning to use the tool or to manipulate the object. Changes in synaptic efficacy may underlie acquisition and storage of memories. Here we studied the effect of pharmacological agents that interfere with synaptic plasticity on acquisition of new motor memories and on recall of a previously learned internal model. Forty-nine subjects, divided into six groups, made reaching movements while holding a robotic arm that applied forces to the hand. On day 1, all subjects learned to move in force field A. On day 2, each group of subjects was tested on their ability to recall field A and their ability to learn a new internal model in field B. Four groups participated in the experiments of day 2 under the effects of lorazepam (LZ; a GABA type A receptor-positive allosteric modulator), dextromethorphan [DM; an N-methyl-D-aspartate (NMDA) receptor blocker], lamotrigine (LG, a drug that blocks voltage-gated Na(+) and Ca(2+) channel), or scopolamine (SP; muscarinic receptor antagonist). Two control groups were tested in a drug-free condition: one group that was not exposed to additional experimental protocols (NP) and another group was tested under ~24 h of sleep deprivation between completion of learning on day 1 and start of testing on day 2 (SD). Recall of field A was normal in all groups. Learning of field B was reduced by LZ and DM but not by SP, LG, SD or in the NP condition. These results suggest that a 24-h sleep-deprivation period may have little or no effect on consolidation of this motor memory and that NMDA receptor activation and GABAergic inhibition are mechanisms operating in the acquisition but not recall of new motor memories in humans.

Subchronic cocaine produces training paradigm-dependent learning deficits in laboratory rats

The effect of cocaine on spatial learning was investigated by exposing male Sprague-Dawley rats to 0, 20, or 40 mg/kg cocaine prior to and during training on a water maze task. Half the animals were pretrained on cued trials prior to hidden platform trials, while the remaining animals completed hidden platform trials immediately. Escape latencies for all animals improved with training, but pretrained animals located the hidden platform faster than untrained animals (P<.001). Pretraining also decreased the effect of cocaine. In pretrained animals, only the high dose of cocaine caused significant increases in escape latency (P<.001), while in the untrained group the lower dose of cocaine also caused a significant increase (P<.001). On working memory measures, cocaine affected both the pretrained (P<.01) and untrained (P<.001) groups. Dwell ratio measurements indicated unaffected reference memory in both pretrained (P<.001) and untrained (P<.001) animals, and no significant differences were detected among the treatment conditions in either group (P>.05). Thus, while cocaine did not abolish learning, the efficiency with which the task was learned was compromised. However, this effect was reduced by pretraining.

Effects of oral administration of the competitive N-methyl-D-aspartate antagonist, CGP 40116, on passive avoidance, spatial learning, and neuromotor abilities in mice

The effects were investigated of the potent competitive N-methyl-D-aspartate (NMDA) receptor antagonist CGP 40116[D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] on the performance of mice in water maze and passive avoidance tasks, and in wire suspension, rotarod, and cage activity tests. The drug was administered per os (p.o.) in its anticonvulsant dose range. CGP 40116 dose-dependently impaired passive avoidance learning when given before, but not when given after training. The antagonist (5, 10, and 20 mg/kg, administered 4 h before each training session) dose-dependently affected water maze acquisition, and impaired retention test performance in both hidden- and visible-platform water maze tasks. In addition, the drug dose-dependently decreased swimming speed during water maze acquisition. Repeated administration of CGP 40116 (20 mg/kg, p.o.) persistently decreased cage activity and wire suspension test performance, whereas motor coordination and equilibrium on the rotarod apparatus remained unimpaired. In our administration protocol, no tolerance was found to the effects of the drug on passive avoidance learning and neuromotor abilities. The parallel effects of CGP 40116 on memory and motor performance are discussed, and it was concluded that the antagonist impairs neuromotor abilities and also induces memory impairments which cannot be entirely reduced to motor interference.

Dextromethorphan and dextrorphan in rats: common antitussives--different behavioural profiles

Dextromethorphan (DM), a widely used and well-tolerated centrally acting antitussive, has been tested in several clinical trials for its antiepileptic and neuroprotective properties. However, the use of DM in these new clinical indications requires higher doses than antitussive doses, which may therefore induce phencyclidine (PCP)-like side-effects (memory and psychotomimetic disturbances) through its metabolic conversion to the active metabolite dextrorphan (DX), a more potent PCP-like non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor than DM. Thus, we compared the behavioural effects in rats of intraperitoneal administration of DM and DX on motor activity in an open field and on learning and memory in the Morris water maze. DM (20, 30, 40 mg/kg) produced a dose-dependent decrease in both locomotion and stereotyped behaviour with a slight ataxia for the highest dose. DX (20, 30, 40 mg/kg) induced a dose-dependent increase in locomotion and stereotypies (swaying, turning) with moderate ataxia. Assessments of learning and memory were performed with lower doses of DM (10, 20, 30 mg/kg) and DX (5, 10, 15 mg/kg) because of motivational deficits (40 mg/kg of DM, 20-40 mg/kg of DX) and motor disorders (30, 40 mg/kg of DX) in the cue learning procedure. DX (10, 15 mg/kg) impaired spatial learning with a long-lasting effect for the highest dose whereas 5 mg/kg of DX and DM (10-30 mg/kg) did not. Only 15 mg/kg of DX appeared to slightly impair working memory. DM (10-30 mg/kg) and DX (5-15 mg/kg) did not impair reference memory. Thus, the two antitussives DM and DX induced different behavioural effects suggesting sedative effects for DM and PCP-like effects for DX. However, PCP-like side-effects with DM remain possible through its metabolic conversion to DX, with very high doses and/or in extensive metabolizers and/or in aged subjects prone to cognitive dysfunction. Therefore, the identification of DM metabolism phenotype, an adapted prescription and a pharmacological modulation of the DM metabolism may avoid adverse effects.