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.

Involvement of AMPA receptors in the antidepressant-like effects of dextromethorphan in mice

Dextromethorphan (DM) is an antitussive with rapid acting antidepressant potential based on pharmacodynamic similarities to ketamine. Building upon our previous finding that DM produces antidepressant-like effects in the mouse forced swim test (FST), the present study aimed to establish the antidepressant-like actions of DM in the tail suspension test (TST), another well-established model predictive of antidepressant efficacy. Additionally, using the TST and FST, we investigated the role of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors in the antidepressant-like properties of DM because accumulating evidence suggests that AMPA receptors play an important role in the pathophysiology of depression and may contribute to the efficacy of antidepressant medications, including that of ketamine. We found that DM displays antidepressant-like effects in the TST similar to the conventional and fast acting antidepressants characterized by imipramine and ketamine, respectively. Moreover, decreasing the first-pass metabolism of DM by concomitant administration of quinidine (CYP2D6 inhibitor) potentiated antidepressant-like actions, implying DM itself has antidepressant efficacy. Finally, in both the TST and FST, pretreatment with the AMPA receptor antagonist NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide) significantly attenuated the antidepressant-like behavior elicited by DM. Together, the data show that DM exerts antidepressant-like actions through AMPA receptors, further suggesting DM may act as a safe and effective fast acting antidepressant drug.

An extension of hypotheses regarding rapid-acting, treatment-refractory, and conventional antidepressant activity of dextromethorphan and dextrorphan

It was previously hypothesized that dextromethorphan (DM) and dextrorphan (DX) may possess antidepressant properties, including rapid and conventional onsets of action and utility in treatment-refractory depression, based on pharmacodynamic similarities to ketamine. These similarities included sigma-1 (σ(1)) agonist and NMDA antagonist properties, calcium channel blockade, muscarinic binding, serotonin transporter (5HTT) inhibition, and μ receptor potentiation. Here, six specific hypotheses are developed in light of additional mechanisms and evidence. Comparable potencies to ketamine for DM and DX are detailed for σ(1) (DX>DM>ketamine), NMDA PCP site (DX>ketamine>DM), and muscarinic (DX>ketamine>>>>DM) receptors, 5HTT (DM>DX≫ketamine), and NMDA antagonist potentiation of μ receptor stimulation (DM>ketamine). Rapid acting antidepressant properties of DM include NMDA high-affinity site, NMDR-2A, and functional NMDR-2B receptor antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation, μ potentiation, and 5HTT inhibition), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, μ potentiation, and 5HTT inhibition), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Those for dextrorphan include NMDA high-affinity site and NMDR-2A antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation and ß adrenoreceptor stimulation), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, ß stimulation, and μ antagonism), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Conventional antidepressant properties for dextromethorphan and dextrorphan include 5HTT and norepinephrine transporter inhibition, σ(1) stimulation, NMDA and PCP antagonism, and possible serotonin 5HT1b/d receptor stimulation. Additional properties for dextromethorphan include possible presynaptic α(2) adrenoreceptor antagonism or postsynaptic α(2) stimulation and, for dextrorphan, ß stimulation and possible muscarinic and μ antagonism. Treatment-refractory depression properties include increased serotonin and norepinephrine availability, PCP, NMDR-2B, presynaptic alpha-2 antagonism, and the multiplicity of other antidepressant receptor mechanisms. Suggestions for clinical trials are provided for oral high-dose dextromethorphan and Nuedexta (dextromethorphan combined with quinidine to block metabolism to dextrorphan, thereby increasing dextromethorphan plasma concentrations). Suggestions include exclusionary criteria, oral dosing, observation periods, dose-response approaches, and safety and tolerability are considered. Although oral dextromethorphan may be somewhat more likely to show efficacy through complementary antidepressant mechanisms of dextrorphan, a clinical trial will be more logistically complex than one of Nuedexta due to high doses and plasma level variability. Clinical trials may increase our therapeutic armamentarium and our pharmacological understanding of treatment-refractory depression and antidepressant onset of action.

Local anaesthetics have different mechanisms and sites of action at the recombinant N-methyl-D-aspartate (NMDA) receptors

(1) Although the principal pharmacological targets of local anaesthetics (LAs) are voltage-gated Na(+) channels, other targets have also been suggested. Here we examined the effects of LAs on the N-methyl-D-aspartate (NMDA) receptor, a receptor involved in the process of nociception. (2) LAs (bupivacaine, lidocaine, procaine, and tetracaine) reversibly and concentration-dependently inhibited recombinant epsilon1/zeta1 and epsilon2/zeta1 NMDA receptors expressed in Xenopus oocytes (IC(50)s for bupivacaine, lidocaine, procaine, and tetracaine were 1032.0, 1174.1, 642.1 and 653.8 micro M at the epsilon1/zeta1 receptor; and 1090.8, 1821.3, 683.0 and 662.5 micro M respectively (at the epsilon2/zeta1 receptor). Bupivacaine and procaine were non-competitive antagonists; bupivacaine possesses non-competitive and competitive actions when interacting with glycine, whereas procaine has only non-competitive action. (3) Mutation of asparagine residue at position 598 (Asp(598)) in the zeta1 subunit, a residue associated with the blockade site for Mg(2+) and ketamine, to glutamine or arginine reduced the sensitivity to procaine but not to bupivacaine. Thus, procaine may interact with sites of action that are closely related to those of Mg(2+) and ketamine blockade. (4) These results suggest that LAs inhibit the NMDA receptor by various mechanisms.

Gender differences in limbic responsiveness, by SPECT, following a pharmacologic challenge in healthy subjects

Limbic system functioning is integral to the control and modulation of affect, motivation, reward, and memory. Neuropsychiatric disturbances involving disruptions in these cognitive and emotional dimensions exhibit different prevalence rates for men and women. Gender-specific differences in this integrated brain area may therefore be important in understanding both normal behavioral functioning and the etiologic underpinnings of neuropsychiatric disorders. To further explore such differences in limbic system function, we assessed regional cerebral blood flow, by SPECT, in men and women following the administration of procaine. Procaine is a local anesthetic that preferentially stimulates limbic structures. Psychiatrically and medically healthy, age-matched women (n = 15, 33.2 +/- 6.9 years) and men (n = 15, 32.8 +/- 6.9 years) were administered 1.38 mg/kg procaine or saline intravenously in two separate sessions. Using voxel-based analyses (P < 0.001), males significantly activated the bilateral insular cortex following procaine, whereas females more strongly activated the bilateral anterior and mesial temporal cortex. Both groups demonstrated significant anterior cingulate activation. Subjective responses to procaine did not significantly differ between the men and women. To our knowledge, this is the first report demonstrating gender-specific responses in limbic activation following a pharmacologic challenge. These findings suggest that men and women can activate different limbic structures following the same provocative pharmacologic stimulus, despite sharing a similar subjective experience. Studies assessing pharmacologic challenges of limbic system structures should consider gender as a critical variable in assessing biologic responsiveness.

The inhibition of the N-methyl-D-aspartate receptor channel by local anesthetics in mouse CA1 pyramidal neurons

Although the effects of local anesthetics on sodium channels and various other channels and receptors have been intensively investigated, there is little information available about their effects on N-methyl-D-aspartate (NMDA) receptors. We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Procaine and tetracaine produced a reversible and concentration-dependent inhibition of NMDA-induced currents, but lidocaine showed little inhibition at 1 mM or less. The half-maximal inhibition values (mM; mean +/- SEM) for procaine, tetracaine, bupivacaine, and lidocaine at -60 mV were 0.296 +/- 0.031, 0.637 +/- 0.044, 2.781 +/- 0.940 (extrapolated data), and 7.766 +/- 14.093 (extrapolated data), respectively. Procaine 0.2 mM reduced the maximal NMDA-induced currents without affecting the 50% effective concentration values for NMDA. The inhibition by procaine exhibited voltage dependence and was more effective at negative potentials. These results indicate a noncompetitive antagonism of procaine on NMDA receptors and suggest that the inhibition is the result of a channel-blocking mechanism.

IMPLICATIONS

We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Both procaine and tetracaine produced a reversible and concentration-dependent inhibition of the NMDA-induced currents.

Interaction of local anaesthetics with recombinant mu, kappa, and delta-opioid receptors expressed in Chinese hamster ovary cells

Local anaesthetics potentiate epidural or intrathecal opioid analgesia via a poorly defined mechanism. In this study, we have examined the interaction of local anaesthetics (lidocaine, bupivacaine and its optical isomers, tetracaine, procaine and prilocaine) with recombinant mu-, kappa-, and delta-opioid receptors expressed in Chinese hamster ovary cells (CHO-mu, kappa, and delta, respectively). Lidocaine produced a concentration-dependent displacement of radiolabelled opioid antagonist [3H]diprenorphine ([3H]DPN) binding with the following rank order of inhibitor constant (Ki): kappa (210 microM) > mu (552 microM) > delta (1810 microM). Procaine, prilocaine, tetracaine and bupivacaine also displaced [3H]DPN binding in CHO-mu with Ki values of 244, 204, 43 and 161 microM respectively. Lidocaine produced a concentration-dependent and naloxone-insensitive inhibition of cAMP formation in all cell lines including untransfected cells. Concentration producing 50% inhibition of maximum was mu, 1.32 mM; kappa, 2.41 mM; delta, 1.27 mM; untransfected, 2.78 mM. When lidocaine (300 microM) was co-incubated with spiradoline (kappa-selective) and [D-Ala2, MePhe4, Gly(ol)5] enkephalin (DAMGO mu-selective) in CHO-kappa and mu cells we did not observe an additive interaction for cAMP formation. In contrast, there was an apparent inhibitory action of the combination at the kappa receptor. This study suggests that clinical concentrations of local anaesthetics interact with mu and kappa but not delta opioid receptors. As there was no synergism between local anaesthetics and opioids we suggest that the interaction of these agents in the clinical setting does not occur at the cellular level.

Vagal afferent transmission in the NTS mediating reflex responses of the rat esophagus

In urethan-anesthetized rats, esophageal distension evoked volume-dependent reflex contractions with phase-locked multiunit discharges in the central subnucleus of the solitary tract complex (NTSC) and the nucleus ambiguus. During blockade of solitarial, but not peripheral, muscarinic cholinoceptors, the volume-response relationship of reflex contractions was shifted rightward with a depression in pressure wave amplitude. Concurrently, premotor NTSC responses were attenuated and nucleus ambiguus activity was abolished during esophagomotor inhibition. Both NTSC discharges and reflex responses were eliminated, or strongly inhibited, during blockade of excitatory amino acid receptors (EAARs) with 6-cyano-7-nitroquinoxaline-2,3-dione, gamma-glutamylglycine or 2-amino-7-phosphonoheptanoate. In brain stem slice preparations, whole cell recordings in the NTSC region revealed fast excitatory postsynaptic potentials (EPSPS) with spikes in response to electrical stimulation of the solitary tract. Although spiking was facilitated by muscarine, EPSPS were resistant to cholinoceptor antagonists but sensitive to EAAR blockers. We conclude that esophageal vagal afferents excite ipsilateral NTSC interneurons via activation of glutamate receptors of the DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid and N-methyl-D-aspartate subtypes. Cholinergic input to the NTSC probably derives from propriobulbar sources and serves to modulate the responsiveness of reflex interneurons.