Signal transduction mechanism of inhibitory responses in vertebrate olfactory receptor neurons

虽然许多研究已经报道了气味可以引起脊椎动物嗅觉受体神经元的抑制性反应以及兴奋性反应，但这种抑制的细胞机制尚不清楚。本研究采用全细胞膜片钳记录技术，观察了不同气味物质对蝾螈嗅感受器神经元的抑制作用。气味刺激引起超极化，并降低了所施加的超极化电压的幅度，表明超极化是由于去极化电导的降低而不是超极化电导的增加。不同气味物质对蝾螈嗅感受器神经元的抑制作用不同，但不同气味物质对神经元的抑制作用不同。（苯甲醚、乙酸异戊酯、桉叶素、柠檬烯和异戊酸）以剂量依赖性方式抑制去极化电流。剂量-反应关系可用Hill方程拟合：y = E_<max>[X^n /（X^n + IC <50>^n）]，其中E_<max>为最大抑制百分率，n为Hill系数。平均Hill系数约为1.6。加臭剂抑制的电导平均反转电位为6.64 ± 1.84mY，与IBMX诱导的电导非常接近（6.04 ± 2.23 mV，P&gt; 0.68，配对t检验），提示气味剂抑制IBMX诱导的电流，气味刺激抑制IBMX诱导的电流，这表明抑制不能仅仅是由于竞争效应，并且抑制的靶点是cAMP途径中受体结合的下游。细胞内注射cAMP诱导的电导也被气味剂抑制。通过注射cAMP诱导的电流的气味抑制表明，抑制不能仅由腺苷酸环化酶的抑制引起，并且表明抑制的靶点是在细胞内cAMP积累之后。因此，CNG通道的抑制似乎是cAMP诱导电流抑制的主要原因。

Although many studies have reported that odorants can elicit inhibitory responses as well as excitatory responses in vertebrate olfactory receptor neurons, the cellular mechanisms that underlie this inhibition are unclear. In this study, we have examined the inhibitory effect of odorants on newt olfactory receptor neurons using whole cell patch-clamp recording.Odor stimulation caused hyperpolarization and decreased the amplitude of the applied hyperpolarizing voltage, indicating that hyperpolarization was due to a decrease in depolarizing conductance rather than an increase in hyperpolarizing conductance.Various odorants (anisole, isoamyl acetate, cineole, limonene and isovaleric acid) suppressed the depolarizing current in a dose-dependent manner. The dose-response relation could be fitted by Hill equation : y = E_<max>[X^n / (X^n + IC_<50>^n], where E_<max> is the maximal percentage inhibition and n is the Hill coefficient. The average Hill coefficient was about 1.6.The mean reversal potential of the conductance suppressed by the odorant was 6.64 ± 1.84 my which was remarkably similar to the conductance induced by IBMX (6.04 ± 2.23 m V,P> 0.68, paired t-test), suggesting that odorant suppressed IBMX-induced current.Odor stimulation suppressed the IBMX -induced current, suggesting that suppression cannot only be due to competitive effects and that the target of suppression is downstream of receptor binding in the cAMP pathway. The conductance induced by intracellular injection of cAMP was also suppressed by odorants. Odor suppression of the current induced by injection of cAMP demonstrates that the suppression cannot only be caused by inhibition of adenylate cyclase and shows that the target of suppression is after accumulation of intracellular cAMP. Thus, inhibition of the CNG channel seems to be a major cause of suppression of the cAMP-induced current.