Involvement of AMPA receptors of lateral habenula in the expression and acquisition phases of morphine-induced place preference.

The lateral habenula (LHb), known as the brain structure of the epithalamic, plays the main role in depression and drug addiction. The glutamatergic system influences morphine reward. The effect of activation/inhibition of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) in the LHb on different phases of morphine-induced conditioned place preference (CPP) remains unknown. In this research, the effect of bilateral intra-LHb microinjection of AMPARs agonist and antagonist on the acquisition and expression phases of CPP in male rats has been investigated. Different doses of NBQX, the antagonist of AMPARs, in combination with the effective dose of morphine, increased the CPP score during the acquisition phase. While AMPA, the agonist of AMPARs, significantly reduced the conditioning scores in the acquisition phase. Pretreatment with NBQX (0.5 and 1 μg/rat) reversed the inhibitory effect of AMPA (1 μg/rat) on the acquisition phase of morphine-induced CPP. The antagonist (1 μg/rat) increased the effect of a high dose of agonist (2 μg/rat) on CPP. On the other hand, NBQX significantly increased CPP scores during the expression phase. AMPA did not significantly affect CPP scores in the expression phase, but significantly reduced locomotor activity in the test phase. These results confirmed the importance of AMPARs in the LHb in morphine reward. Our data also suggest that injection of an AMPARs antagonist into the LHb may alter the AMPA-induced morphine response in a dose-dependent manner.