探討感覺訊息對美洲蟑螂單眼巨大神經元的影響

Abstract

單眼、複眼及觸角、尾毛等都是美洲蟑螂（Periplaneta americana）身體表面重要的感覺器官。單眼及複眼都位於身體前方的頭部，分別會對光刺激產生反應，但就視細胞對光的敏感度而言，單眼是複眼的數百倍。觸角是頭部另一個重要的感覺器官，對機械刺激和化學刺激都有明顯的反應。尾毛是身體後方最重要的感覺構造，對周遭環境空氣流動的瞬間變化非常敏感。 單眼照光後，會誘發巨大神經元產生過極化反應，並藉由單眼神經將訊息傳入腦部；此時若再施以第二個刺激（例如：光照複眼、擺動觸角、拍動翅膀、移動後腳、風吹尾毛等），則會誘發單眼巨大神經元產生另一個明顯的去極化反應。利用離子電泳法注射GABA及GABAA受器的擬似劑 muscimol，都會引起去極化反應，但注射另一種GABAA受器的擬似劑THIP後，卻沒有任何影響。利用灌流法灌流GABA及GABAA受器的擬似劑imidazole acetic acid後，會使巨大神經元逐漸產生去極化反應，但若用其他的GABAA受器擬似劑3-APS及guanidine acetic acid來灌流，反而會使巨大神經元產生過極化反應。為了更加確認巨大神經元上GABA受器的屬性，另外再使用GABAB受器的擬似劑baclofen來灌流，但並未發現對該受器有任何影響。 picrotoxin、bicuculline、bicuculline methiodide和nipecotic acid等藥品都是傳統的GABAA受器拮抗劑，經由灌流測試的結果，只有picrotoxin才會有效抑制各種機械刺激（例如風吹尾毛、擺動觸角及拍動翅膀）及注射GABA所引起的去極化反應，且提高灌流濃度後，抑制的效果會更明顯。此外，在擺動觸角實驗中，還發現產生反應所需的延遲時間，會隨著灌流picrotoxin濃度的提高而逐漸增長。 灌流glycine受器的拮抗劑strychnine後，也會抑制由各種機械刺激所誘發的去極化反應，但抑制的效果不盡相同。對風吹尾毛及拍動翅膀所引起的去極化反應而言，灌流低濃度（10-7M）的strychnine即有明顯的抑制反應，抑制效果比灌流picrotoxin更明顯。但對擺動觸角所引起的去極化反應而言，灌流低濃度的strychnine後，抑制反應並不明顯；灌流同濃度的picrotoxin後，抑制的效果較顯著。 由上述結果可推測，單眼巨大神經元上確實存在著GABA受器，而實驗時所施加之來自前方頭部的擺動觸角刺激，與來自後方身體胸部的擺動翅膀及腹部末端的風吹尾毛刺激，這些訊息傳入中樞後，可能會再藉由不同的傳出神經元將訊息傳入單眼神經內，透過此種受器誘發巨大神經元產生去極化反應。而這些位於巨大神經元上的GABA受器，其特性雖然較類似脊椎動物的GABAA受器，活性也與氯離子孔道的通透性有關，但並不完全相同，可能是昆蟲所特有的一種受器。 除此之外，為了找出翅膀上的感覺接受器，是否如同觸角、尾毛一樣也是感覺毛，本研究特地針對美洲蟑螂的前翅和後翅，分別做了掃瞄式電子顯微鏡的觀察。結果發現美洲蟑螂翅膀表面分佈著許多形態各異的感覺毛，根據外形可分為硬棘型、剛毛型、細毛型、短毛型、短錐型、長錐型及腔錘型等七種。各種感覺毛在翅上的分佈情形不盡相同，其中有五種分佈在前翅的背面，三種分佈在前翅的腹面，六種分佈在後翅的背面，而在後翅腹面則幾乎沒有感覺毛。各種感覺毛的數量也有差異，前翅背面以剛毛型最多，前翅腹面以短毛型較多，而後翅背面則以長錐型最普遍。進一步使用甲基藍對翅膀染色後，發現前、後翅內分佈著許多感覺神經元，依據外形可分為單極、雙極和多極等三類。由此可推測，美洲蟑螂的翅膀不單只有飛行的功能，可能還可藉由翅膀上的感覺毛來感測周遭環境的變化，並經由感覺神經元將訊息傳入中樞以調控行為。

Ocelli, compound eyes, antennas, and cerci are all important sense organs of american cockroach, Periplaneta americana. Ocelli and compound eyes are located on the head, and both of them can respond to the stimulation of light. The ocelli are more sensitive than compound eyes in response to illumination. Antenna is another important sense organs on the head and responds conspicuously to the mechanical and chemical stimulations. The cerci are the most important sense organs in the rear area of the abdomen and very sensitive to the air flow of surrounding environment. When the ocellus were illuminated, it induced a hyperpolarized membrane potential in the L-neuron and sent messages to the brain through ocellar nerves. At this moment, the second stimulation（such as illuminating compound eye, antenna-swinging, wing-raising, leg-moving, and cercal stimulation）induces another obviously depolarized membrane potential in the L-neuron. Iontophoretic injection of GABA and muscimol（a traditional GABAA receptor agonist）into the L-neurons produced a depolarization response. However, iontophoretic injection of THIP into the L-neurons（another GABAA receptor agonist）had no effects. Perfusion of GABA and imidazole acetic acid（a GABAA receptor agonist）depolarized, while 3-APS and guanidine acetic acid（both GABAA agonists）hyperpolarized the L-neurons. Perfusion of baclofen （a GABAB agonist）into the L-neurons failed to induce depolarization. Picrotoxin, bicuculline, bicuculline methiodide and nipecotic acid are all traditional GABAA receptor antagonists. Among them, only perfusion of picrotoxin effectively suppressed both mechanical stimulation- and GABA-induced depolarization. This inhibition would be more effective by perfusion of high concentration of picrotoxin. In the antenna-swinging test, we found that the delay time of depolarization response was extended progressively with increased picrotoxin through perfusion. Perfusion of strychnine, a glycine antagonist, was found to inhibit the depolarization induced by mechanical stimulation. This depolarization response induced by cercal stimulation and wing-raising was effectively suppressed with a relatively low concentration of strychnine. It seems that administration of strychnine produced a more effective blockade on GABA actions than that evoked by picrotoxin. In contrast, the depolarization induced by antenna-swinging was largely inhibited by perfusion of picrotoxin versus strychnine. These results strongly suggest that GABA receptor may present in the ocellar L-neurons. The messages from antenna-swinging, wing-raising or cercal stimulation may depolarize the ocellar L-neurons through GABA receptors. The characteristics of GABA receptors in ocellar L-neurons are similar to those GABAA receptors in the vertebrates. It is difficult to classify L-neuron GABA receptors as GABAA receptors seen in the vertebrates, although both types of GABA receptors being similar in term of chloride channel activation. It is very likely to be a unique GABA receptor in insect. The sensillae are main sense organs distributed on the surface of antenna and circus. In order to find the sense organs on the surface of wings, we investigated the forewings and hindwings by using a scanning electron microscope. There are many different types of sensillae distributedon the surface of wings in american cockroach. According to their morphology, these sensillae might be termed short-trichodeum, long-trichodeum, fine- trichodeum, chaeticum, short-basiconicum, long-basiconicum and coeloconicum. Each type of sensilla has a specific distribution pattern. There are five types of sensilla distributed on the dorsal surface and three types on the ventral surface of forewings. Six types of sensillae were distributed on the dorsal surface of hindwings, while only very few or no sensilla on the ventral surface of hindwings. The amount of long-trichodeum sensilla was much more than any other types on the dorsal surface of forewings, the chaeticum sensilla was more abundant on the ventral surface of forewings, whereas the long-basiconicum sensilla was the most abundant on the dorsal surface of hindwings. In addition, we used methylene blue to dye the wings and found that there were many sensory neurons distributed within fore- and hindwings. Based on morphological observation, the sensory neurons could be divided into three types: monopolar, bipolar and multipolar neurons. We suggest that the wings of american cockroach might not only function in flying, but also could detect the changes of surrounding environment using sensillae. The messages collected from sensillae may transmit to the central nervous system through the sensory neurons distributed on wings, and subsequently modulate behaviors.