Response of rat globus pallidus neurons to microintophoretically applied mu and kappa opioid receptor agonists

The effects of the microiontophoretic application of dynorphin A-(1-13) (DYN 13) and the benzomorphans ethylketocyclazocine (EKC), bremazocine and MRZ 2549, (kappa) opioid agonists, and of morphine and morphiceptin, (mu) opioid agonists, were compared on spontaneous or glutamate-evoked discharge of globus pallidus (GP) neurons in rat. Our results demonstrate that mu and kappa opioid agonists are able to depress the excitability of pallidal neurons, possibly by interacting with mu and kappa opioid receptor subtypes, respectively. In addition, the mu agonists and dynorphin A-(1-13), but not the benzomorphans, enhanced the excitability of a number of pallidal neurons. We have proposed a presynaptic site as the basis for this opioid-induced excitation, possibly also mediated by a mu opioid receptor. The selectivity of dynorphin A-(1-13) for benzomorphan kappa opioid receptors in the rat GP appears to be low and dynorphin A-(1-13) may elicit effects that are different from those produced by the benzomorphan kappa agonists by virtue of its ability to interact with other opioid receptor subtypes, for example mu opioid receptors.