<b><i>In vivo</i> patch-clamp analysis of dopaminergic antinociceptive actions on dorsal horn neurons in the spinal cord</b>

Abstract

Dopamine (DA) is well known as a neurotransmitter and neuromodulator in the brain, not only as a precursor in the synthesis of other catecholamines. DA controls a variety of functions including locomotor activity, cognition, emotion, positive reinforcement, food intake, and endocrine regulation. Compared with the enormous literature devoted to DA actions in the brain, little is known about the roles of DA in the spinal cord. To elucidate the mechanisms of antinociception mediated by the dopaminergic descending pathway in the spinal cord, we investigated the actions of DA on substantia gelatinosa (SG) neurons by in vivo whole-cell patch-clamp methods. In the voltage-clamp mode (VH = -70 mV), the application of DA induced outward currents in about 70% of SG neurons tested. DA also significantly suppressed the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic currents (EPSCs). DA-induced outward current was observed in the presence of TTX or a non-NMDA receptor antagonist, CNQX. DA also significantly decreased the frequency of miniature EPSCs in the presence of TTX. These results suggest that DA has both presynaptic and postsynaptic inhibitory actions on synaptic transmission in SG neurons. The DA-induced outward current was completely blocked by either GDP-β-S in the pipette solution or by perfusion of a non-selective K+ channel blocker, Ba2+. Moreover, the DA- induced outward currents were mimicked by a selective D2-like receptor agonist, quinpirole, but not by a D1-like receptor agonist, SKF 38393. In addition, the DA-induced outward current was attenuated by a selective D2-like receptor antagonist, sulpiride. These findings suggest that the DA- induced outward current is mediated by G-protein-activated K+ channels through D2-like receptors. Furthermore, we investigated whether DA has direct antinociceptive actions into noxious and innocuous stimuli on the receptive field skin of recording SG neuron. The results showed that DA produced direct analgesic effects in SG neurons to both noxious and innocuous stimuli to the skin. These findings suggest the dopaminergic descending pathway has an antinociceptive effect via D2-like receptors on SG neurons in the spinal cord.