Role of the central amygdala GABA-A receptors in morphine state-dependent memory

In the present experiments, the effects of bilateral microinjections of the GABA-A receptor agonist and/or antagonist into the central amygdala (CeA) on morphine state-dependent memory were examined.

In order to assess memory retrieval, a step-through passive avoidance task was used in adult male Wistar rats.

Subcutaneous (s.c.) administration of morphine (5 and 7.5mg/kg) immediately after training (post-training) decreased the memory retrieval. Pre-test administration of the opioid (7.5mg/kg) also induced amnesia. The response induced by post-training morphine (7.5mg/kg) was significantly reversed by pre-test administration of the drug (5 and 7.5mg/kg), indicating morphine state-dependent memory. Pre-test intra-CeA microinjection of muscimol, a GABA-A receptor agonist (0.01, 0.02 and 0.03microg/rat) reduced morphine state-dependent memory. However, the same doses of muscimol by itself had no effect on memory retrieval. Furthermore, pre-test intra-CeA microinjection of bicuculline, a GABA-A receptor antagonist by itself did not alter memory retrieval. The antagonist also did not change post-training morphine (7.5mg/kg)-induced amnesia, but in combination with a lower dose of morphine (0.5mg/kg), improved memory performance. Moreover, muscimol's ability to interfere with morphine state-dependent memory was reversed by co-injection of bicuculline.

The results suggest that GABA-A receptor mechanism of the CeA may influence morphine state-dependent memory.