We also determined the effect of the highly selective synthetic KOR agonist U69593 (U69) (Raynor et al., 1994) on GABAA–mediated transmission. These experiments were designed to confirm KORs as the locus of action for dynorphin effects and as such, we utilized only two concentrations typical of recently published electrophysiological studies in brain slices (Lemos et al., 2012; Li et al., 2012). Upon superfusion of 0.5 μM U69, IPSP amplitude decreased to 78 ± 6% of control (n = 4; Fig. 1D). A concentration of 1 μM U69 did not elicit a larger effect, decreasing IPSPs to 76 ± 6% of control (n = 4; Fig. 1D, 2A). Combining those two concentrations, U69 significantly decreased IPSP amplitude to 77 ± 5% of control (n = 8, t = 4.478). Another neuron did not respond to U69. Thus, the synthetic (U69) and endogenous (dynorphin) KOR agonists reduced inhibitory transmission by 20–25% in CeA. Prolonged application of 1 μM U69 (25 minutes; n = 2) did not elicit a desensitization of the response.
