Baclofen-induced neuro-respiratory toxicity in the rat: contribution of tolerance and characterization of withdrawal syndrome


Baclofen, a γ-amino-butyric acid type-B receptor agonist with exponentially increased use at high-dose to facilitate abstinence in chronic alcoholics, is responsible for increasing poisonings. Tolerance and withdrawal syndromes have been reported during prolonged treatment but their contribution to the variability of baclofen-induced neurotoxicity in overdose is unknown. We studied baclofen-induced effects on rat sedation, temperature and ventilation and modeled baclofen pharmacokinetics and effect/concentration relationships aiming to investigate the consequences of repeated baclofen pretreatment and to characterize withdrawal syndrome. Baclofen-induced dose-dependent sedation (P<.01), hypothermia (P<.001) and respiratory depression (P<.01) were altered in repeatedly baclofen-pretreated rats (P<.05). Repeatedly baclofen-pretreated rats did not exhibit respiratory depression following baclofen overdose due to limitations on baclofen-induced increase in inspiratory (P<.01) and expiratory times (P<.01). Only slight hypoxemia without respiratory acidosis was observed. Baclofen discontinuation resulted in hyperlocomotion and non-anxiogenic withdrawal symptoms. Regarding pharmacokinetics, repeated baclofen pretreatment increased the peak concentration (P<.05) and absorption constant rate (P<.05) and reduced the distribution volume (P<.0001) and elimination half-life (P<.05). Analysis of the effect/concentration relationships indicated that plasma baclofen concentration decreases more rapidly than all studied neuro-respiratory effects, in tolerant and non-tolerant rats. Taken together, our findings supported the role of brain distribution in baclofen-induced neurotoxicity expression and its probable involvement in tolerance-related attenuation in addition to physiological adaptations of ventilation. In conclusion, repeated pretreatment attenuates baclofen-attributed neurotoxicity in overdose and results in post-discontinuation withdrawal syndrome. Our findings suggest both pharmacodynamic and pharmacokinetic mechanisms whose relative contributions to the variability of baclofen-induced neurotoxicity in overdose remain to be established.

Keywords : Baclofen, Tolerance, Respiratory depression, Pharmacokinetics, Poisoning, Withdrawal