Effects of dizocilpine [(+)-MK-801] on the expression of associative and non-associative sensitization to D-amphetamine.

Blockade of glutamate receptors of the NMDA type inhibits the sensitization to psychostimulant drugs, such as amphetamine, that occurs after repeated administration. Both associative (conditioning) and non-associative (pseudo-conditioning) mechanisms may contribute to sensitization phenomena. The aim of the present study was, thus, to determine which type of sensitization is influenced by blockade of NMDA-type receptors by examining the expression (manifestation) of sensitization. Locomotor activity was assessed and, in some experiments, extracellular dopamine in the nucleus accumbens was also assessed using in vivo microdialysis in non-anaesthetized, almost freely moving rats. Male albino Wistar rats of 225-250 g were given 1 mg/kg i.p. d-amphetamine every 2nd day for 7 days and with saline on the other days. Half the rats were exposed to d-amphetamine in the presence of conditioning stimuli (test cage, auditory and olfactory stimulus) and to saline in the home cage in absence of these stimuli, the other half were treated with saline and exposed to the conditioning stimuli and were placed into their home cages (without conditioning stimuli) after treatment with d-amphetamine. Ten days after the end of this treatment, both groups were exposed to the conditioning stimuli and half of each group were pretreated with dizocilpine [(+)-MK-801, 0.1 mg/kg i.p.], a blocker of NMDA receptors, 30 min before administration of 1 mg/kg d-amphetamine. (+)-MK-801 reduced the locomotor activity in rats sensitized associatively, but not in those sensitized non-associatively. It had no significant effect on spontaneous locomotor activity or that induced by acute administration of 1 mg/kg d-amphetamine. Similarly, (+)-MK-801 inhibited the increase in extracellular dopamine in the nucleus accumbens induced by the test dose of d-amphetamine in rats sensitized associatively but not non-associatively. The results suggest that the expression of both types of sensitization to d-amphetamine are dependent on glutamatergic NMDA mechanisms, although in different ways. Inhibition of sensitization, in particular of the associative type, might be of therapeutic value in drug dependence.