Norepinephrine Depresses The Capsaicin-evoked Miniature Excitatory Postsynaptic Currents In Substantia Gelatinosa Of The Rat Spinal Cord

Substantia gelatinosa (SG; lamina II) of the spinal dorsal horn is the first station of the peripheral nociceptive information, which receives the termination of both non- and thin-myelinated primary afferent fibers and brainstem descending fibers, and plays an important role in peripheral nociceptive information transmission and modulation. Capsaicin has become a popular tool in the nociceptive study as it can selectively excite non- and thin-myelinated fibers that transmit nociceptive information. Our previous studies have demonstrated that capsaicin could obviously enhance the excitatory neurotransmitter release from primary afferent terminals in spinal dorsal hom. Norepinephirine (NE), one of the important neurotransmitters in the brainstem descending inhibitory system, plays a significant analgesic role at spinal level. The mechanisms of NE analgesic action in SG mainly include that: (1) NE directly acts on excitatory intemeurons and makes them hyperpolarization, and (2) NE acts on inhibitory intemeurons and facilitates the release of inhibitory neurotransmitters including glycine (Gly) and y-aminobutyric acid (GABA). In addition, NE might directly acts on primary afferent terminals. The purpose of our present study is to verify the effect of NE on primary afferent terminal neurotrasmitter release at the synaptic level in the SG of the rat spinal dorsal horn, which might contribute to better understanding the mechanism of NE analgesic effect at spinal level.Whole-cell voltage-clamp recordings were made on the SG neurons of 5-6 week old Sprague-Dawley rats. Spinal cord slice (500 urn) was perfused with fresh Krebs solution. Guanosine-5'-O-(2-thiodiphosphate)(GDP-p-S), the inhibitor of G protein, was added to the pipette solution to block postsynaptic effects induced by NE through G protein. The pipette was positioned on a neuron using a hydraulic micromanipulator by blind prick technique. After a giga fi seal formation, the whole-cell recording was performed by an Axopatch 200B amplifier. The holding potential was -70 mV. The currents were low filtered at 5 KHz and recorded and analyzed by software pClamp 6.0 and AxoGraph.The results showed that: ヽapsaicin could significantly increase the frequency of miniature excitatory postsynaptic currents (mEPSCs), in another word, capsaicin could enhance the excitatory neurotransmitter release from primary afferent terminals. This is coincidence with the results of our previous studies. ㎞E did not influence the spontaneous mEPSC in SG. However, after pretreatment with NE, the facilitatory effect of capsaicin on neurotransmitter release from primary afferent terminals was significantly depressed. ㏕he pretreatment of NE together with yohimbine, an a, adrenergic receptor antagonist, had no effect on the facilitatory action of capsaicin on neurotransmitter release from primary afferent terminals. This result suggests that the a2 adrenergic receptor on primary afferent terminals might be involved in NE depression on capsaicin-induced neurotransmitter release.The present results indicate that NE depresses capsaicin-induced primary afferent terminal neurotransmitter release by activating a2 adrenergic receptor. The selective action of capsaicin on primary afferent fibers that transmit nociceptive information simulates nociceptive stimulation in vivo, therefore, the NE depression of primary afferent terminal neurotransmitter release might be significant on nociceptive modulation at spinal level.