Effect of MK-801 on learning in rats: Evaluation of PK/PD relationship

Abstract

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 is widely used in animal research (over 3000 publications), however its extracellular brain concentration has never been reported. Here, we investigated the brain extracellular fluid (ECF) concentration of MK-801 and dopamine after systemic injection of different doses of MK-801 in freely moving rats using in vivo microdialysis. Furthermore, we were interested in: (1) comparing these data with results from locomotor activity in the open field and a 1-day Morris water maze task, (2) the effect of different injection intervalls of MK-801, and (3) reversal of MK-801 induced behavioural alterations by a histamine H3 receptor antagonist (ABT-239). In vitro brain microdialysis combined with LCMSMS analysis was used to assess the brain ECF concentration of MK-801 (0.05, 0.1, 0.2 mg/kg) after i.p. application. Following, data were recalculated based on in vivo recovery. Locomotor activity was measured in an open field for 2 h, while spatial learning and memory was assessed in the Morris water maze (5 consecutive trials on 1 day). MK-801 was injected 30, 60, 120, or 240 min before behavioral testing began. ABT-239 (3, 10 mg/kg) was injected i.p. 45 min prior to testing. Injection of MK-801 in doses of 0.05, 0.1, or 0.2 mg/kg resulted in peak brain ECF concentration of 5, 15 or 30 nM, repectively, at 60 min after application as well as in alterations of striatal dopamine levels. Interestingly, only one injection of 0.05 mg/kg MK-801 triggered an increase of baseline ECF dopamine concentration 24 h later. Moreover, MK-801 resulted in learning impairment in the water maze and ABT-239 at the dose of 3 mg/kg was able to reverse this deficit. These data demonstrate a clear PK/PD relationship of MK-801 at doses normally used for behavioural testing resulting in brain levels corresponding to the affinity at NMDA receptors. Moreover, the observation that MK-801-induced impairment of learning can be fully reversed by a H3 receptor antagonist suggests, that blockade of H3 receptors may reverse cognitive impairment resulting from glutamatergic hypofunction.