The dorsal hippocampal group III metabotropic glutamate receptors are involved in morphine effect on memory formation in male mice

This study investigated the role of dorsal hippocampal (CA1) group III metabotropic glutamate (mGlu) receptors in impairment of memory formation and state-dependent memory induced by morphine. For this purpose, the CA1 area was cannulated and one-trial passive avoidance task was selected to assess the memory function. Morphine was administrated subcutaneously (s.c.) and mGlu receptors agonist or antagonist were microinjected into CA1 regions. The obtained results indicated that pre-training administration of morphine (5 mg/kg; s.c.) decreased memory retention. Moreover, pre-test administration of morphine (5 mg/kg; s.c.) induced morphine state-dependent memory retention under pre-training morphine effect (5 mg/kg; s.c.). Although, intra-CA1 microinjection of lower doses of group III mGlu receptors agonist, L-AP4, (10 and 20 mmol/mouse) and antagonist, CPPG, (10 and 15 mmol/mouse) did not affect memory retention, but higher dose of the drugs (30 mmol/mouse) decreased memory retention. Pre-test microinjection of L-AP4 (10, 20 and 30 mmol/mouse; intra-CA1) had no effect on morphine-induced amnesia, but same doses of L-AP4 plus an effective dose of morphine (1 mg/kg; s.c.) reversed morphine-induced amnesia. Interestingly, amnesia induced by pre-training morphine (5 mg/kg; s.c.) was significantly reversed by pre-test administration of CPPG (30 mmol/mouse; intra-CA1). On the other hand, pre-test co-administration of CPPG (10 and 15 mmol/mouse; intra-CA1) and morphine (5 mg/kg; s.c.) following pre-training morphine (5 mg/kg; s.c.) decreased memory retention. Taken together, our results suggested that morphine effects on memory formation might be mediated via the activity of dorsal hippocampal metabotropic glutamate receptors.