Effects of Sensory Informations on the Ocellar L-neurons in American Cockroach, Periplaneta americana

[[abstract]]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 distributed on 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.