Changes of pHIMK-801 binding and nitric oxide synthase ACTTvTrY in rats rendered tolerant to and dependent on pentobarbital

Effects of continuous pentobarbital administration on binding characterisitcs of [³H]MK-801 in rats brain were examined by autoradiography. An experimental model of barbiturate tolerance and dependence was developed using i.c.v. infusion of pentobarbital (300 μg/10μl/hr for 7 days) by osmotic minipumps and abruptly withdrawn from pentobarbital. The levels of [³H]MK-801 binding were elevated in rats 24-hr after withdrawal from pentobarbital while there were no changes in tolerant rats. To define the activity of nitric oxide synthase (NOS) which is governed by NMDA receptor activity, changes of NADPH diaphorase were compared in tolerant and dependent rats. The NOS activity was significantly elevated in dependent rats and slightly elevated in tolerant rats. For assessing the role of NMDA receptor in barbiturate action, an NMDA antagonist (MK-801, 10 nM/day) was co-infused with pentobarbital. The pentobarbital-infused group had a shorter duration of pentobarbital (60 mg/kg, i.p.)-induced loss of righting reflex (sleeping time) than that of the control group, but MK-801 did not affect the righting reflex. However, MK-801 blocked hyperthermic effect in dependent rats. In addition, co-infusion of MK-801 blocked pentobarbital-induced up-regulation of NOS activity in dependent rats. These results suggest the involvement of NMDA receptor up-regulation in barbiturate withdrawl probably via activation of NOS.