| Summary: | 碩士 === 國立臺灣大學 === 動物學研究研究所 === 95 === Previous morphological studies showed that neurons in A7 catecholamine cell group receive nerve terminals releasing substance-P (SP) from several descending pain modulation nuclei (PAG, VMM and LH), A7 neurons are therefore believed to play important roles in regulating nociception. In this study, we provided physiological evidence for this argument. Firstly, two kinds of neurons in A7 nucleus were identified based on norepinephrine production. Physical contacts observed between these noradrenergic (NAergic) neurons and non-noradrenergic (non-NAergic) neurons displayed the complex regulation among neurons in A7 nucleus. Application of SP consistently increased the firing frequency of NAergic neurons and some of the non-NAergic neurons. The mechanism under SP function in A7 NAergic neurons was then explored. Holding current of these NAergic neurons were significantly elevated upon bath application of 0.2 μM SP. This response can be blocked by RP 67580 (NK1 receptor antagonist), by internally applied 10 μM suramine (uncoupling G protein from the receptor), and by U73122 (phospholipase C blocker), therefore showed that the effect of SP was mediated through NK1 receptor, G protein, and phospholipase C-dependent mechanism. BAPTA, heparin and inositol 1,4,5-triphosphate via the patch pipette had no effect on SP-induced current. Addition of DAG analogue (OAG) and DAG lipase inhibitor (RHC80267) had no effect on SP-induced response either. Notably, SP-induced current in the presence of OAG were larger than SP alone suggested that generation of DAG alone may not fully account for activation of SP-induced current. In addition, PKC inhibitor significantly attenuated the desensitization of SP-induced current; calcium influx was also an essential component of this PKC regulated desensitization. Three evidences showed that SP-induced current was caused by open a transient receptor potential (TRP)-like channel. The conductance was increased during SP application, the reversal potential of this current was around -2mV, and adding three TRP channel blockers could consistently depress this inward current. Taken together, the electrophysiological and pharmacological properties of the SP-induced inward current are consistent with the involvement of TRP-like channels whereas the identification and function of this TRP-like channel still needs further investigation.
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