μ-opioid-mediated inhibition of glutamate synaptic transmission in rat central amygdala neurons

The central nucleus of the amygdala (CeA) plays an important role both in stimulus-reward learning for the reinforcing effects of drugs of abuse and in environmental condition-induced analgesia. Both of these two CeA functions involve the opioid system within the CeA. However, the pharmacological profiles of its opioid receptor system have not been fully studied and the synaptic actions of opioid receptors in the CeA are largely unknown. In this study with whole-cell voltage-clamp recordings in brain slices in vitro, we examined actions of opioid agonists on glutamate-mediated excitatory postsynaptic currents (EPSCs) in CeA neurons. Opioid peptide methionine-enkephalin (ME; 10 μM) produced a significant inhibition (38%) in the amplitude of evoked EPSCs, an action mimicked by the μ-opioid receptor agonist [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO; 1 μM, 44%). Both effects of ME and DAMGO were abolished by the μ receptor antagonist CTAP (1 μM), suggesting a μ receptor-mediated effect. Neither δ-opioid receptor agonist [d-Pen2,d-Pen5]-enkephalin (1 μM) nor κ-opioid receptor agonist U69593 (300 nM) had any effect on the glutamate EPSC. ME significantly increased the paired-pulse ratio of the evoked EPSCs and decreased the frequency of miniature EPSCs without altering the amplitude of miniature EPSCs. Furthermore, the μ-opioid inhibition of the EPSC was blocked by 4-aminopyridine (4AP; 100 μM), a voltage-dependent potassium channel blocker, and by phospholipase A2 inhibitors AACOCF3 (10 μM) and quinacrine (10 μM). These results indicate that only the μ-opioid receptor is functionally present on presynaptic glutamatergic terminals in normal CeA neurons, and its activation reduces the probability of glutamate release through a signaling pathway involving phospholipase A2 and the presynaptic, 4AP-sensitive potassium channel. This study provides evidence for the presynaptic regulation of glutamate synaptic transmission by μ-opioid receptors in CeA neurons.