Mechanism of the dynorphin-induced dualistic effect on free intracellular Ca2+ concentration in cultured rat spinal neurons

In order to study the different mechanisms of dynorphin spinal analgesia and neurotoxicity at low and high doses, the effects of various concentrations of dynorphin A-(1-17) on the free intracellular Ca2+ concentration ([Ca2+](i)) in the cultured rat spinal neurons were studied using single cell microspectrofluorimetry. While dynorphin A-(1-17) 0.1-100 mu M had no significant effect on basal [Ca2+](i), dynorphin A-(1-17) 0.1 and 1 mu M significantly decreased the high KCl-evoked peak [Ca2+](i) by 94% and 83% respectively. Dynorphin A-(1-17) 10 and 100 mu M did not affect the peak [Ca2+](i) following K+ depolarization, but in all these neurons there was a sustained and irreversible rise in [Ca2+](i) following high-K+ challenge. Pretreatment with the specific kappa-opioid receptor antagonist nor-binaltorphimine 10 mu M, but not the competitive NMDA receptor antagonist, DL-2-amino-5-phosphonovalerate (APV) 10 mu M, significantly blocked the inhibitory effect of dynorphin A-(1-17) 0.1 mu M on peak [Ca2+](i). However, APV 10 mu M and nor-binaltorphimine 10 mu M significantly antagonized the sustained rise in [Ca2+](i) induced by a high concentration of dynorphin A-(1-17) 10 mu M. Furthermore, in the presence, and following the addition, of increasing concentrations of dynorphin A-(1-17) (0.1, 1, 10 and 100 mu M), the high concentrations of dynorphin A-(1-17) failed to produce a sustained rise in peak [Ca2+](i). These results suggested that dynorphin exerted a dualistic modulatory effect on [Ca2+](i) in cultured rat spinal neurons, inducing a sustained and irreversible intracellular Ca2+ overload via activation of both NMDA and kappa-opioid receptors at higher concentrations, but inhibiting depolarization-evoked Ca2+ influx via kappa-opioid but not NMDA receptors at lower concentrations. Serial addition of graded concentrations of dynorphin A-(1-17) prevented the effect of high concentrations of dynorphin A-(1-17) on [Ca2+](i). (C) 1998 Elsevier Science B.V.