The opioid peptide dynorphin modulates AMPA and kainate responses in acutely isolated neurons from the dorsal horn.

Abstract In freshly isolated spinal dorsal horn (DH) neurons (laminae I–IV) of the young rat, the effects of dynorphin A1–17, U-50,488H and U-69,593 on inward currents induced by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate (KA) were studied under whole-cell voltage-clamp conditions. When the cells were clamped to a holding potential of −60 mV, co-application of dynorphin A1–17 (10−6 M) and AMPA (2 × 10−5 M) reversibly decreased the peak amplitude of the initial transient component of the AMPA-induced current in 72% of the examined cells. In addition, dynorphin (10 μM) in perforated patch-recordings consistently produced a decrease in the steady-state component of the AMPA response. The depressant effect was concentration-dependent (IC50 = 86 nM) and reversible. The dynorphin A1–17-induced depression of the AMPA response was associated with slowing of the response kinetics, including both a 10–90% rise-time and time constant of decay. The AMPA-induced currents were modulated by dynorphin not only during the co-administration but also after the removal of the peptide. Dynorphin increased the initial peak AMPA current in 42% of the examined cells. Similar as with dynorphin A1–17, the peak amplitude of the AMPA-induced current was reversibly suppressed in the presence of 1 μM U-50,488H and U-69,593 in 75% and 86% of the examined cells, respectively. Naloxone and the κ1-selective antagonist norbinaltorphimine (nor-BNI) blocked the initial depressant but not late excitatory effects of dynorphin A1–17 and U-50,488H. This antagonistic effect of naloxone and norbinaltorphimine suggests that the depressant effect of dynorphin A1–17 on the AMPA-activated conductance is a true opioid, probably κ1-opioid receptor-mediated event. In contrast, the dynorphin-induced late potentiation of AMPA/KA responses appears to be a non-opioid effect since it was not inhibited by nor-BNI, CTAP and naltrindole, the selective κ-, μ- and δ-opioid receptor blocking agents, respectively. Pretreatment of DH neurons with pertusis toxin blocked the depressant action of dynorphin A1–17, indicating that a Gi- or G0-type G protein was required for this effect on AMPA-activated currents. Intracellular dialysis with a highly specific peptide inhibitor (peptide 6–22) of the cAMP-activated protein kinase (PKA), and with Rp-cAMPS, prevented the depressant efect of dynorphin A1–17. 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 intracellular cyclic AMP levels, can reduce the responses of DH neurons to exogenous AMPA. Besides modulating the AMPA response of DH cells, co-application of 1 μM dynorphin and KA (2–5 × 10−5 M) decreased the magnitude of the KA-induced current in 50% of the cells tested and increased 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 A1–17, U-50,488H and U-69,593 modulate the AMPA/KA receptors signaling function in a subset of the rat spinal DH neurons. Possible mechanisms of action are discussed in relation to dynorphin-depressant regulation of excitatory amino acid-mediated primary afferent neurotransmission, including nociception.