Effects of MDMA alone and after pretreatment with reboxetine, duloxetine, clonidine, carvedilol, and doxazosin on pupillary light reflex

MDMA (3,4-methylenedioxymethamphetamine) produces marked sympathomimetic effects in humans, including increases in blood pressure, heart rate, body temperature and pupil diameter. Previous laboratory work documented MDMA-induced mydriasis, but it remained unclear whether MDMA alters the pupillary light reflex (dynamic response to a light stimulus), how static and dynamic pupillary changes relate to plasma MDMA exposure and other pharmacodynamic effects, and which adrenergic or serotonergic mechanisms mediate these effects. Hysek and colleagues set out to characterise MDMA effects on both static pupil size and the pupillary light reflex and to probe underlying mechanisms by testing five pharmacological pretreatments. Specifically, they pooled data from five placebo-controlled, double-blind, double-dummy, randomized crossover studies to examine MDMA alone and after pretreatment with reboxetine, duloxetine, clonidine, carvedilol or doxazosin, with the aims of describing time courses, pharmacokinetic–pharmacodynamic relationships, and interactions with monoaminergic and adrenergic manipulations.

The report is a pooled analysis of five separate but harmonised clinical trials, each enrolling 16 healthy volunteers (eight men, eight women) for a total of 80 subjects aged 18–44 years (mean ± SD 25±5 years). All studies used a double-blind, double-dummy, placebo-controlled, randomised crossover design with four experimental conditions per subject: placebo‑placebo, pretreatment‑placebo, placebo‑MDMA and pretreatment‑MDMA. Washout periods between sessions were at least 10 days. Ethics approval and regulatory authorisation for MDMA use were obtained and the trials were registered at ClinicalTrials.gov. MDMA was given as a single oral dose of 125 mg, chosen to approximate a typical recreational or psychotherapeutic adjunctive dose. The five pretreatments were: reboxetine (norepinephrine transporter inhibitor), duloxetine (serotonin–norepinephrine transporter inhibitor), clonidine (α2‑adrenergic agonist), carvedilol (α1/β antagonist with greater β activity) and doxazosin (α1 antagonist). Pretreatment regimens varied by drug to achieve peak plasma levels at or shortly before the expected peak MDMA effect; examples include repeated high-dose reboxetine or duloxetine dosing begun the evening before the test day and single-dose clonidine or carvedilol 1 h before MDMA. The continuous‑release doxazosin schedule involved dosing up to 40 h before MDMA to account for its long tmax. Pupillometry was performed with an infrared handheld pupillometer at baseline (1 h before dosing) and repeatedly up to 6 h after dosing (0, 0.33, 0.66, 1, 1.5, 2, 2.5, 3, 4, 5 and 6 h). Measurements under standardised dark–light conditions captured dark‑adapted pupil diameter (MAX), minimal diameter after a brief light stimulus (MIN), latency to pupillary constriction, constriction amplitude (MAX–MIN) and time to recover 75% of resting pupil size after constriction. Subjective drug effects were assessed with a single visual analogue scale (VAS) for “any subjective drug effects” at the same time points. Cardiovascular measures (blood pressure, heart rate, mean arterial pressure) and tympanic core temperature were also recorded. Plasma MDMA concentrations were sampled at matching time points up to 6 h and analysed noncompartmentally. Statistical analyses used repeated‑measures general linear models (two‑way ANOVA with factors MDMA and pretreatment) for each study and pooled analyses with MDMA as a within‑subject factor. Maximal (Emax) and minimal (Emin) values and area under the effect–time curves were derived. Spearman rank correlations were used for pharmacokinetic–pharmacodynamic and pharmacodynamic–pharmacodynamic associations, considering both between‑subjects correlations at individual time points (n=80) and correlations of mean changes over time (n=9 or 10 time points). A significance threshold of p<0.05 was applied.

MDMA produced robust mydriasis and altered the pupillary light reflex. Specifically, compared with placebo MDMA increased resting pupil diameter and pupil diameter after the light stimulus, prolonged the latency to constriction, reduced constriction amplitude, and shortened the recovery time to 75% of resting size. The peak mydriatic effect occurred at 2.3±0.2 h after dosing and remained elevated over the 6‑hour observation window. In contrast, the maximal reduction in constriction amplitude occurred earlier (1.7±0.1 h) and returned to baseline within 6 h despite persistently high plasma MDMA levels. Subjective effects (VAS “any drug effect”) peaked at 1.5±0.1 h and likewise returned to baseline within 6 h while plasma MDMA remained elevated. Plasma pharmacokinetics showed a mean peak MDMA concentration of 243±6 ng/ml with a time to maximum concentration of 2.5±0.1 h. Pharmacokinetic–pharmacodynamic relationships differed by parameter. Group mean pupil size correlated with mean plasma MDMA over time (Spearman Rs=0.77, p<0.01, n=9) with moderate clockwise hysteresis, indicating a temporal offset between concentration and effect. By contrast, the MDMA‑induced reduction in constriction amplitude was not significantly correlated with plasma concentrations over time (Rs=0.43, p=0.24, n=9) because of pronounced clockwise hysteresis. Reductions in constriction amplitude were, however, tightly associated with subjective and autonomic effects over time: mean constriction amplitude correlated strongly with the VAS rating (Rs=0.96, p<0.001, n=10) and also with mean changes in mean arterial pressure (Rs=0.98), heart rate (Rs=0.92) and body temperature (Rs=0.87), all p<0.001. Between‑subject correlations at individual time points showed that MDMA‑induced reductions in constriction amplitude, pupil size after light, increases in MAP and subjective effects were significantly associated with plasma MDMA levels (reported in the paper’s table); associations with resting pupil diameter, latency or heart rate were weaker and mainly evident early in the time course. Pretreatment effects varied by drug. Reboxetine and duloxetine alone increased resting pupil diameter and pupil diameter after light; duloxetine alone also reduced constriction amplitude. Duloxetine administered before MDMA produced non‑additive, negatively synergistic effects: it largely prevented MDMA‑induced mydriasis and the impairment of the pupillary light reflex, and duloxetine also markedly attenuated MDMA’s cardiovascular, psychotropic and neuroendocrine responses in the same subjects. Reboxetine showed subadditive interactions with MDMA: pupil size after reboxetine plus MDMA was larger than after MDMA alone, but reboxetine did not prevent the MDMA‑induced impairment of the light reflex. Clonidine alone produced miosis and enhanced the light reflex consistent with sympatholytic action but did not significantly reduce the mydriatic effects of MDMA when co‑administered. Carvedilol did not alter the pupillary effects of MDMA, though it reduced MDMA’s cardiostimulant and thermogenic effects in these subjects; carvedilol alone slightly reduced pupil size but this reduction was not significant in post hoc testing. Doxazosin alone had no effect on pupil size and only tended to nonsignificantly attenuate MDMA‑induced mydriasis.

The investigators interpret their findings as evidence that MDMA produces both sympathetic activation (sustained mydriasis and faster redilation) and indirect central parasympathetic inhibition (prolonged latency and reduced constriction amplitude). They emphasise that the impaired pupillary light reflex is closely linked to MDMA’s subjective and cardiostimulant effects and exhibits acute pharmacological tolerance, normalising within 6 h despite continued high plasma MDMA concentrations. By contrast, mydriasis mirrors the plasma concentration–time curve more closely and is longer‑lasting. Results from the pretreatment manipulations inform mechanism. The ability of duloxetine (a serotonin–norepinephrine transporter inhibitor) to prevent MDMA’s effects on both static and dynamic pupillary measures supports a central serotonergic contribution to MDMA’s pupillary actions, potentially via serotonergic potentiation of noradrenergic outflow. Reboxetine’s subadditive interaction with MDMA suggests that transporter‑mediated norepinephrine release contributes to MDMA‑induced mydriasis, consistent with reboxetine’s attenuating effects on MDMA’s stimulant effects reported elsewhere. By contrast, blocking postsynaptic α1 receptors with doxazosin or α1/β receptors with carvedilol had little effect on MDMA‑induced pupillary changes, suggesting a limited role for direct iris α1‑adrenergic stimulation and a greater role for central parasympathetic inhibition. Clonidine’s sympatholytic, miotic effects when given alone did not translate into substantial antagonism of MDMA’s pupillary effects, indicating that α2‑mediated vesicular release of norepinephrine and α2 mechanisms are not primarily responsible for MDMA’s pupillary effects. The authors note that constriction amplitude (dynamic response) may be a more sensitive marker than static pupil size for estimating recent MDMA exposure and the magnitude of acute subjective and autonomic effects, given the tighter temporal association and apparent acute tolerance of the dynamic measures. They also acknowledge a likely ceiling effect for pupil size at the doses tested, which may explain why resting pupil diameter did not correlate strongly with plasma levels across subjects. Finally, pupillometry under the dark–light conditions used yielded latency and constriction amplitude values similar to those obtained under daylight in other work, suggesting these parameters are relatively robust to light‑condition differences. The authors conclude that both norepinephrine and serotonin mediate MDMA’s effects on pupillary function, with mydriasis tracking plasma MDMA and impaired light reflex reflecting the acute subjective and autonomic state induced by the drug.