盤尼西林對蝸牛中樞神經元之影響

Abstract

本論文主要是利用藥理學及電生理學的方法，探討 penicillin 對於非洲大蝸牛 (Achatina fulica Ferussac) 食道下神經節中的 RP4 神經元之 (1) 自發性動作電位的影響；(2) 與 procaine 引發的猝發現象的相關性。 正常生理溶液灌流下，RP4 神經元會產生規則的自發性動作電位。細胞外給予 penicillin 對 RP4 神經元動作電位的影響具有劑量依賴性 (dose-dependence)。當給予低濃度的 penicillin (3 mM) 60 分鐘後，對於 RP4 神經元的自發性動作電位沒有影響；但將濃度提高至 10 mM、30 mM，會使得動作電位的膜電位去極化，且動作電位的振幅有抑制減小的情況。將濃度提高至 100 mM，自發性的動作電位則完全消失，靜止膜電位仍然呈現去極化狀態。根據膜電位箝制的實驗發現，penicillin 會抑制 RP4 神經元的總內向電流以及減少總外向電流。因此推測 penicillin 造成 RP4 神經元動作電位振幅的減小是來自於總內向電流的抑制，而膜電位去極化的現象可能跟總外向電流的減少有關。 灌流 10 mM procaine 60 分鐘能引起 RP4 神經元自發性動作電位的猝發現象，而此猝發現象在 procaine 與 penicillin (10 mM、30 mM) 共同存在時即不發生，顯示 penicillin 具有對於 procaine 引發的猝發現象有抑制的作用。 將細胞生理溶液改以低鈉生理溶液取代，在鈉離子 80% 濃度時對於 penicillin 抑制 procaine 產生猝發現象的能力並沒有影響，同樣需要 10 mM 的 penicillin 才能夠抑制猝發現象；但在鈉離子 50% 的濃度時，只需 3 mM 的 penicillin 就能夠抑制 procaine 猝發現象的產生，顯示 penicillin 抑制 procaine 產生猝發現象的作用與鈉離子有相關性。 將 phospholipase C 抑制劑 (neomycin、U73122) 與 penicillin 一起加入灌流池中，並無法加強 penicillin 抑制 procaine 產生猝發現象的能力，依舊需要 10 mM 的 penicillin 才能抑制 procaine 的猝發現象，顯示 penicillin 抑制 procaine 猝發現象的產生與 phospholipase C 可能無關。 利用 GABA、glycine 來誘導細胞的氯離子電流，將 GABA 或glycine 與 penicillin 一起投予至灌流池中，此時只需 5 mM 的penicillin 即可抑制 procaine 產生猝發現象。GABA 及glycine 能夠降低 penicillin 的抑制劑量，顯示氯離子電流的增加能夠加強 penicillin 的抑制能力。 使用 GABA、glycine 的拮抗劑 picrotoxin、strychnine 來阻斷氯離子電流，將 picrotoxin、strychnine 分別與 penicillin 共同加入灌流池後，penicillin 的抑制劑量仍然需要 10 mM 沒有改變，抑制氯離子的電流後即無法加強 penicillin 的抑制能力，表示氯離子的濃度與penicillin 的抑制能力相關。 本論文實驗結果發現在非洲大蝸牛之 RP4 神經元上，細胞外投予penicillin，高濃度時會減小動作電位的振幅並且會有靜止膜電位去極化的現象。動作電位振幅的減小應該與總內向電流的減少有關，而膜電位去極化的現象則應該與總外向電流的降低有關。Penicillin 能夠抑制procaine 產生猝發現象，此抑制作用可能來自於減少鈉離子電流、增加氯離子電流，而與 PLC 的活性較無相關。

Effects of penicillin on (1) spontaneous action potential and (2) the bursts elicited by procaine were investigated pharmacologically and electrophysiologically on dissociated RP4 neuron from subesophageal ganglion of giant African snails, Achatina fulica Ferussac. Extracellular application of peniciilin altered the properties of action potentials in a dose-dependant manner. Lower concentration of penicillin (3 mM) did not make any changes of resting membrane potential, amplitude and frequency on RP4 neuron. Higher concentration of penicillin (10 mM, 30 mM) depolarized resting membrane potential, and depressed amplitude of action potential. With a concentration up to 100 mM, action potentials were completely abolished, and resting membrane potential still appeared depolarization. Voltage clamp studies revealed that penicillin (10 mM, 30 mM) reduced the total fast inward currents (70 ms) and decreased the steady-state outward currents (5 sec).These results suggest that the depression of amplitude may come from the reduction of total fast inward currents while the depolarization of resting membrane potential may result from the decrease of the steady-state outward currents. Effects of penicillin on bursts of potential elicited by procaine were also investigated. Incubated RP4 neuron with procaine (10 mM) for 60 min, the spontaneous single spike action potential turned to bursts of potential reversibly. Treated procaine with penicillin (10 mM, 30 mM) for 60 min, instead of bursts, the spontaneous action potential remained one single spike. These results suggest that penicillin has the anti-convulsive effects which can abolish the convulsion-like bursts of potential evoked by procaine on RP4 neuron. The anti-convulsive effects of penicillin were enhanced by (1) perfusion with low sodium (50% Na+), (2) co-treatment with GABA and (3) co-treatment with glycine. In these conditions, the effective dose of penicillin anti-convulsion was reduced, less than 10 mM. However, the anti-convulsive effects of penicillin were not altered by (1) co-existence of neomycin, (2) co-existence of U73122, (3) co-existence of picrotoxin or (4) co-existence of strychnine. These results show that penicillin inhibit the bursts of potential elicited by procaine through sodium ion current, GABA-activated Cl- current and glycine-evoked Cl- current, while GABA receptor antagonist, picrotoxin, and glycine receptor antagonist, strychnine, did not show the enhancement of inhibitory effects. Phospolipase activity was neither involved, since the application of PLC inhibitor (neomycin, U73122) did not enhance the inhibitory effects of penicillin. We conclud that penicillin reduce the total inward currents result in the depression of amplitude, while decrease the steady-state outward currents result in the depolarization of resting membrane potential. The inhibitory effects of penicillin were associated with sodium currents and chloride currents.