Effects of morphine on rat behaviour in the elevated plus maze: the role of central amygdala dopamine receptors.

The objective of the present study was to evaluate the possible role of dopamine D(1)/D(2) receptors of the central amygdala (CeA) on morphine-induced anxiolytic-like behaviour in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas the CeA and tested in an elevated plus maze (EPM) task. Intraperitoneal (i.p.) administration of morphine (5 and 6 mg/kg) increased the percentage of open arm time (%OAT) and open arm entries (%OAE), indicating an anxiolytic-like response. Intra-CeA administration of different doses of the dopaminergic agonist apomorphine (0.1-0.3 microg/rat) significantly increased %OAE, but not %OAT. Furthermore, co-administration of the same doses of apomorphine with an ineffective dose of morphine (4 mg/kg; i.p.) significantly increased %OAT and %OAE by the opioid. Single microinjection of the D(1) dopaminergic antagonist SCH23390 (0.5-1.5 microg/rat) or sulpiride (0.5-1.5 microg/rat) into the CeA caused no significant change for %OAT and %OAE. The obtained results also show that intra-CeA microinjection of the same doses of SCH23390 or sulpiride inhibits the anxiolytic-like effect of morphine (6 mg/kg; i.p.). Pre-treatment of animals with SCH23390 (intra-CeA) or sulpiride (intra-CeA) reversed the response induced by apomorphine (0.3 microg/rat) plus morphine (4 mg/kg; i.p.). It should be considered that the drugs also did not show any effect on locomotor activity in all experiments. In conclusion, these findings suggest that the central amygdala dopaminergic mechanisms, probably via D(1)/D(2) receptors, may be involved in the modulation of morphine-induced anxiolytic-like behaviour in rat.