The Inhibitory Effect of Opioid on the Hyperpolarization-Activated Cation Currents in Rat Substantia Gelatinosa Neurons

The Inhibitory Effect of Opioid on the Hyperpolarization-Activated Cation Currents in Rat Substantia Gelatinosa Neurons

<P> The action of opioid on the hyperpolarization-activated cation current (I<SUB>h</SUB>) in substantia gelatinosa neurons were investigated by using whole-cell voltage-clamp recording in rat spinal brain slices. Hyperpolarizing voltage steps revealed slowly activating currents in a subgroup of neurons. The half-maximal activation and the reversal potential of the current were compatible to neuronal I<SUB>h</SUB>. DAMGO (1μM), a selective- opioid agonist, reduced the amplitude of I<SUB>h</SUB> reversibly. This reduction was dose-dependent and was blocked by CTOP (2μM), a selective μ-opioid antagonist. DAMGO shifted the voltage dependence of activation to more hyperpolarized potential. Cesium (1 mM) or ZD 7288 (100μM) blocked I<SUB>h</SUB> and the currents inhibited by cesium, ZD 7288 and DAMGO shared a similar time and voltage dependence. These results suggest that activation of μ-opioid receptor by DAMGO can inhibit I<SUB>h</SUB> in a subgroup of rat substantia gelatinosa neurons.