Structures And Neuron Identifications In The Gnathal Ganglion Of Helicoverpa Armigera Caterpillars

During the long-term evolution of insects,the gustatory system of insects have evolved as the "switch" of insect foraging activity,regulating multiple feeding behaviors including host selection,intaking and refusing feeding.Herbivorous insects can specially identify plant metabolite substances by converting chemical signals into electrical signals and transmitting them into the central nervous system(CNS)in the form of impulses through the peripheral nerve bundles.Then the information is integrated by the interneurons in the CNS and further is transmitted to the higher nervous center,such as to the prothoracic ganglion through projection neurons or to the motor neurons directly,which could regulate the entire process of feeding behavior.The cotton bollworm Helicoverpa armigera(Lepidoptera:Noctuidae)is a significant agricultural pest species with a wide host range and serious damages to host plants in China.Though the gustatory perception of peripheral nervous system and the central projection of peripheral gustatory sensilla in insects were elicited to an extent,the study of detailed structure of the neuromere in the CNS and the functions of the central gustatory neurons is not yet clear.In this research,the 5th instar larvae of H.armigera were used as the insect materials,the structure and the sub-region of the gnathal ganglion(GNG)were described by the techniques of the immunohistochemistry.The classification and the spacial distribution of the central gustatory neurons were classified by techniques of the intracellular recording combining the neuron staining.Furthermore,the responding patterns of different neurons in GNG were analyzed by the stimulation of one phagostimulant(sucrose)and one deterrent(sinigrin).The main results are as follows:(1)The gnathal ganglion neuromere of larval H.armigera contained three distinct layers,the external cell body layer(CBL),the internal neuromere,and the peripheral nerve bundles.The cell body layer was constituted by the ventral(CBLv),the anterior(CBLa),the posterior(CBLp),the lateral(CBL1),and the dorsal region(CBLd).According to the location of ventro-dorsal,the neuromere in the GNG were divided into 3 areas,6 sub-areas and 32 small areas by commissures and ventral bundles.(2)Five types of gustatory neurons in GNG were stained successfully:the gustatory receptor neurons(GRNs),the local neurons(LNs),the projection neurons(PNs),the motor neurons(MNs)and the descending neurons(DNs).The distributions of each type of neuron were diverse and their terminal arborizations projected into almost the entire GNG neuromere.Among the stained neurons,the PNs and the LNs took the largest proportion.All GRNs originated from the periphery sensilla and extended axons via periphery nerves to the GNG neuromere.PNs were classified into the ascending PNs(PNas)and the descending PNs(PNds)with axons projecting to the brain and the thoracic ganglion,respectively.The cell body and the neural fibers of LNs were restricted in the GNG.The branching patterns,the terminal targeting areas and the locations of cell bodies among LNs were quite diverse.MNs extended their axons to the periphery nerves,including MxN,LbN1 and LbN2.Axons of motor neurons in GNG innervate the muscles of mouthparts and neck.The cell bodies of the DNs were located in the brain,connecting the brain and the ventral nerve cord with swell terminal arborizations.(3)The gustatory responses and central projections of neurons in GNG to sucrose and sinigrin were recorded and traced by the combining techniques of intracellular recoding and neuron tracing.It shows that the response of central gustatory neurons exhibits multi-pattern,which varies dependent on the single gustatory neuron in the GNG.Combining the electrophysiological results that one stimulus could induce activities of multiple central taste neurons,it indicates the taste coding model for stimulus in CNS of H.armigera caterpillars might be the population coding pattern.This paper,for the first time,described the detailed structure of the gnathal ganglion of the lepidopteran caterpillars.The morphology and the function of the gustatory neurons in the gnathal ganglion were also identified,which could not only help us to understand the central taste coding patterns of herbivorous insects,but also could help us to learn the nervous circle of taste information from peripheral perception to central integration,and further to the information output of behavioral instructions.