Wang X, Robertson D
Department of Physiology, The Auditory Laboratory, The University of Western Australia, Nedlands, Western Australia 6907, Australia.
J Neurophysiol. 1998 Jul;80(1):218-29. doi: 10.1152/jn.1998.80.1.218.
The effects of substance P (SP) on whole cell currents were studied in neurons of the medial olivocochlear efferent system (MOCS) in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of SP (0.1-10 microM) reversibly induced an apparent inward current in 49 of 63 labeled neurons when voltage clamped at near resting voltages. This apparent inward current was consistent with the SP-induced membrane depolarization observed in current-clamp mode. The SP-induced change in current was dose dependent with a half-maximal response dose of 200 nM. It was mimicked by [Cys3,6, Tyr8, Pro9]-SP, a neurokinin (NK1) receptor selective agonist, whereas [Succinyl-Asp6, MePhe8]-SP 6-11 (Senktide), a NK3 receptor agonist, had no detectable effect. The SP effect was not blocked by 10(-6) M tetrodotoxin (TTX) and persisted when the perfusate contained 30 mM tetraethylammonium (TEA) or 100 microM Cd2+ or was in a 0-Ca solution. In a TTX-containing solution, SP caused a voltage-dependent decrease of membrane conductance, and the SP-evoked current reversed at a potential at around -105 mV. The predicted K+ equilibrium potential was -93.8 mV under the experimental conditions. The SP-induced inward current was attenuated by 66% when the perfusate contained 3 mM Cs+. We conclude that the apparent inward current is partly caused by SP decreasing an outward current normally maintained by the inward rectifier K+ channels in these cells. In the presence of Cs solution in the recording pipette and with a perfusate containing 3 mM Cs+, 0.1 mM Cd2+ and 10(-6) M TTX, a residual SP-induced inward current was observed at test voltages ranging from -120 to 40 mV. This subcomponent reversed its polarity at approximately 20 mV. This inward current was reduced substantially (but not abolished) when all NaCl in the external solution was replaced by TEA-Cl. The results indicate that SP also opens an unknown cation channel, which the available data suggests may be relatively nonselective. The results suggest that MOCS neurons are subject to modulation by SP, which depolarizes the cell membrane by decreasing the activity of inward rectifier K+ channels as well as concurrently activating a separate cation conductance. It also was found that in MOCS neurons responsive to both SP and norepinephrine, the norepinephrine effect was abolished by TTX, suggesting that an interneuronal population excited by norepinephrine converges selectively onto SP-sensitive MOCS neurons in the VNTB.
在新生大鼠脑干切片的梯形核腹侧部（VNTB）内侧橄榄耳蜗传出系统（MOCS）的神经元中，研究了P物质（SP）对全细胞电流的影响。通过向耳蜗注射快蓝逆行标记来识别每个神经元。当在接近静息电压下进行电压钳制时，浴加SP（0.1 - 10 microM）可在63个标记神经元中的49个中可逆地诱导出明显的内向电流。这种明显的内向电流与在电流钳模式下观察到的SP诱导的膜去极化一致。SP诱导的电流变化呈剂量依赖性，半数最大反应剂量为200 nM。它可被神经激肽（NK1）受体选择性激动剂[Cys3,6, Tyr8, Pro9]-SP模拟，而NK3受体激动剂[Succinyl-Asp6, MePhe8]-SP 6 - 11（速激肽）则无明显作用。10^(-6) M河豚毒素（TTX）不能阻断SP的作用，当灌流液中含有30 mM四乙铵（TEA）或100 microM Cd2+或处于无钙溶液中时，SP的作用仍然持续。在含TTX的溶液中，SP导致膜电导呈电压依赖性降低，且SP诱发的电流在约 - 105 mV的电位处反转。在实验条件下预测的K+平衡电位为 - 93.8 mV。当灌流液中含有3 mM Cs+时，SP诱导的内向电流减弱了66%。我们得出结论，这种明显的内向电流部分是由于SP降低了这些细胞中内向整流K+通道通常维持的外向电流所致。在记录电极内含有Cs溶液且灌流液中含有3 mM Cs+、0.1 mM Cd2+和10^(-6) M TTX的情况下，在 - 120至40 mV的测试电压范围内观察到了残余的SP诱导的内向电流。该亚成分在约20 mV处反转其极性。当外部溶液中的所有NaCl被TEA-Cl取代时，这种内向电流大幅降低（但未消除）。结果表明，SP还可打开一个未知的阳离子通道，现有数据表明该通道可能相对非选择性。结果提示，MOCS神经元受到SP的调制，SP通过降低内向整流K+通道的活性以及同时激活一个单独的阳离子电导使细胞膜去极化。还发现，在对SP和去甲肾上腺素均有反应的MOCS神经元中，TTX可消除去甲肾上腺素的作用，这表明由去甲肾上腺素兴奋的中间神经元群体选择性地汇聚到VNTB中对SP敏感的MOCS神经元上。
