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TRANSLATIONAL PHYSIOLOGY
Department of Anesthesiology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania
Submitted 23 September 2005; accepted in final form 22 November 2005
The basolateral amygdala (BLA) is the major amygdaloid nucleus distributed with µ opioid receptors. The afferent input from the BLA to the central nucleus of the amygdala (CeA) is considered important for opioid analgesia. However, little is known about the effect of µ opioids on synaptic transmission in the BLA. In this study, we examined the effect of µ opioid receptor stimulation on the inhibitory and excitatory synaptic inputs to CeA-projecting BLA neurons. BLA neurons were retrogradely labeled with a fluorescent tracer injected into the CeA of rats. Whole cell voltage-clamp recordings were performed on labeled BLA neurons in brain slices. The specific µ opioid receptor agonist, (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAMGO, 1 µM), significantly reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in 77% of cells tested. DAMGO also significantly decreased the peak amplitude of evoked IPSCs in 75% of cells examined. However, DAMGO did not significantly alter the frequency of mEPSCs or the peak amplitude of evoked EPSCs in 90% and 75% of labeled cells, respectively. Bath application of the Kv channel blockers, 4-AP (Kv1.1, 1.2, 1.3, 1.5, 1.6, 3.1, 3.2), 
-dendrotoxin (Kv1.1, 1.2, 1.6), dendrotoxin-K (Kv1.1), or tityustoxin-K (Kv1.2) each blocked the inhibitory effect of DAMGO on mIPSCs. Double immunofluorescence labeling showed that some of the immunoreactivities of Kv1.1 and Kv1.2 were colocalized with synaptophysin in the BLA. This study provides new information that activation of presynaptic µ opioid receptors primarily attenuates GABAergic synaptic inputs to CeA-projecting neurons in the BLA through a signaling mechanism involving Kv1.1 and Kv1.2 channels.
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