THE OPIOID PEPTIDE DYNORPHIN MODULATES AMPA AND KAINATE RESPONSES IN ACUTELY ISOLATED NEURONS FROM THE DORSAL HORN

In freshly isolated spinal dorsal horn (DH) neurons (laminae I-IV) of the young rat, the effects of dynorphin A(1-17), U-50,488H and U-69,59 3 on inward currents induced by lpha-amino-3-hydroxy-5-methyl-4-isoxaz olepropionic acid (AMPA) and kainate (KA) were studied under whole-cel l voltage-clamp conditions. When the cells were clamped to a holding p otential of -60 mV, co-application of dynorphin A(1-17) (10(-6) M) and AMPA (2 x 10(-5) M) reversibly decreased the peak amplitude of the in itial transient component of the AMPA-induced current in 72% of the ex amined cells. In addition, dynorphin (10 mu M) in perforated patch-rec ordings consistently produced a decrease in the steady-state component of the AMPA response. The depressant effect was concentration-depende nt (IC50 = 86 nM) and reversible. The dynorphin A(1-17)-induced depres sion of the AMPA response was associated with slowing of the response kinetics, including bath a 10-90% rise-time and time constant of decay . The AMPA-induced currents were modulated by dynorphin not only durin g the co-administration but also after the removal of the peptide. Dyn orphin increased the initial peak AMPA current in 42% of the examined cells. Similar as with dynorphin A(1-17), the peak amplitude of the AM PA-induced current was reversibly suppressed in the presence of 1 mu M U-50,488H and U-69,593 in 75% and 86% of the examined cells, respecti vely. Naloxone and the kappa(1)-selective antagonist norbinaltorphimin e (nor-BNI) blocked the initial depressant but not late excitatory eff ects of dynorphin A(1-17) and U-50,488H. This antagonistic effect of n aloxone and norbinaltorphimine suggests that the depressant effect of dynorphin A(1-17) on the AMPA-activated conductance is a true opioid, probably kappa(1)-opioid receptor-mediated event. In contrast, the dyn orphin-induced late potentiation of AMPA/KA responses appears to be a non-opioid effect since it was not inhibited by nor-BNI, CTAP and nalt rindole, the selective kappa-, mu- and delta-opioid receptor blocking agents, respectively. Pretreatment of DH neurons with pertussis toxin blocked the depressant action of dynorphin A(1-17), indicating that a G(i)- or G(o)-type G protein was required for this effect on AMPA-acti vated currents. Intracellular dialysis with a highly specific peptide inhibitor (peptide 6-22) of the cAMP-activated protein kinase (PKA), a nd with Rp-cAMPS, prevented the depressant effect of dynorphin A(1-17) . In addition, staurosporine, a nonselective kinase inhibitor, blocked the dynorphin depression of the AMPA response. These results suggest the possibility that dynorphin, acting through a decrease in intracell ular cyclic AMP levels, can reduce the responses of DH neurons to exog enous AMPA. Besides modulating the AMPA responses of DH cells, co-appl ication of 1 mu M dynorphin and KA (2-5 x 10(-5) M) decreased the magn itude of the KA-induced current in 50% of the cells tested and increas ed it upon the removal of the peptide. Nor-BNI and Rp-cAMPs prevented the depression of KA responses, whereas the late potentiation was not modified. Our results suggest that dynorphin A(1-17), U-50,488H and U- 69,593 modulate the AMPA/KA receptors signaling function in a subset o f the rat spinal DH neurons. Possible mechanisms of action are discuss ed in relation to dynorphin-depressant regulation of excitatory amino acid-mediated primary afferent neurotransmission, including nociceptio n.