Prodynorphin and KOR immunoreactivity are abundant in the CeA (Mansour et al., 1996; Marchant et al., 2007). Activation of postsynaptic KOR in the CeA activates potassium currents in 8 to 17% of the recorded neurons (Chieng et al., 2006; Zhu and Pan, 2004), suggesting that only 1 out of 6 to 12 neurons express KOR. In our study, 80% of the neurons responded to dynorphin and 89% responded to U69 by a decrease of inhibitory transmission. Because the dynorphin effect occurred presynaptically, the presence of KOR on the recorded neuron is not necessary, whereas neurons displaying postsynaptic effects must express the receptor. Such KOR feature has also been reported in the globus pallidus where 25% of the recorded neurons showed a KOR-induced postsynaptic effect whereas 100% showed a KOR-induced decrease of GABAergic synaptic responses (Ogura and Kita, 2000). Thus CeA neurons possessing KOR influence the inhibitory synaptic network, and a large majority of neurons recorded in the medial division of the CeA were modulated by the dynorphin/KOR system, presumably on presynaptic terminals.
