Acid-sensing ion channel 3 in the trigeminal mesencephalic neurons

• Main Authors: Yi-Hsien Su, 蘇怡嫻
• Other Authors: Min, Ming-Yuan
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/89021986034418267154

碩士 === 國立臺灣大學 === 動物學研究所 === 99 === Acid-sensing ion channels (ASICs) have been identified to be proton-gated cationic channels that can sense the extracellular acidification. Among them, ASIC3 is the most sensitive, detecting a slight pH change. ASIC3 is predominately expressed in the peripheral sensory neurons, where the channels respond to noxious and mechanical stimulation. In addition, it is the major subtype participating in pain caused by tissue acidosis such as inflammation and ischemia. Once activated, ASIC3 depolarizes the cell and increases its excitability. However, ASIC3 has not been reported in the central nervous system. Here, we provide evidence for the existence of functional ASIC3 in the trigeminal mesencephalic neurons (Me5), which innervate to masseter muscle spindles and are the only known primary afferent neurons in the brain. Local application (duration: 20 ms) of buffer of various pH induced fast inward currents in Me5 neurons in a dose-dependent manner (EC50 = pH 6.0). The acid-induced currents were reversely blocked by the pan-ASIC blocker, amiloride indicating the involvement of ASICs mediated currents (IASIC). In wild-type (WT) mice, IASIC was induced in all Me5 neurons tested for pH 5.0 (n=46) and pH 6.0 (n=7) with mean amplitudes 931±102pA and 334±90pA, respectively. In contrast, IASIC was only induced in 11 out of 14 cells tested for pH 5.0 and 5 out of 10 cells tested for pH 6.0 in Asic3 knock-out (Asic3-/-) with the respective mean amplitude being 465±103pA and 115±45pA, both of which were significantly smaller than the corresponding values of the WT mice. Furthermore, the desensitization time constant of IASIC was significantly faster in WT (0.89±0.05 s; n=12) than in Asic3-/- (1.21±0.10 s; n=9). These results support that IASIC of Me5 neurons involved ASIC3. Single-cell RT-PCR was used to investigate the composition of ASIC subtypes (1a, 1b, 2a, 2b and 3) in Me5 neurons. In WT mice, ASIC3 was detected in 17 out of 29 cells, with some of them (8 out of 17) co-expressing with other ASIC subtypes. Acid-induced muscle pain model established by injecting two separate doses of acidic saline into the masseter muscles was performed to study the function of ASIC3. In WT mice, the action potential (AP) threshold was significantly lower in the group injected pH 4.0 saline than that injected pH 7.4 saline which indicated that the mice subjected to acid-induced pain have neurons with higher excitability. In respect to the groups with acid-induced pain, AP threshold was significantly lower in WT than in Asic3-/- which revealed the role of ASIC3 in the acid-related masseter muscle pain in Me5 neurons. Moreover, in Asic3-/- mice, AP threshold was higher in the pH 4.0 saline-injected group. Together, our results show that the majority of acid-evoked currents in Me5 neurons are ASIC3-like. Furthermore, the physiological function of ASIC3 in Me5 neurons is probably related to the sensation of acid-induced pain.