Gustatory behavior and electrophysiological responses to sucrose in fifth instar larvae of Spodoptera litura and activity-dependent labeling of sucrose-sensitive neurons in lateral styloconic sensilla

【Objective】To investigate the gustatory behavior and electrophysiological responses of fifth instar larvae of Spodoptera litura to sucrose，and to label the sucrose-sensitive neurons in the lateral styloconic sensilla using agmatine activity-dependent neuron labeling，which could provide reference for the study of function and activity pattern of gustatory neurons in larva of Lepidoptera.【Method】The behavioral responses of fifth instar larvae to sucrose at different concentrations（10，25，50 and 100 mmol/L）were studied by double selection method. The electrophysiological responses to agmatine（20 mmol/L）and sucrose at different concentrations（1，10，50 and 100 mmol/L）of fifth instar larvae of Spodoptera litura lateral styloconic sensilla were studied by electrophysiological apical recording technique，and 2 mmol/L KCl solution was as control. The distribution of sucrose-sensitive neurons in lateral styloconic sensilla was marked by agmatine activity-dependent labeling technique【. Result】Sucrose at concentrations of 10 and 25 mmol/L had no significant stimulatory effect on larval feeding behavior（P>0.05，the same below）. However，sucrose at concentrations of 50 and 100 mmol/L extremely significantly stimulated larval feeding behavior（P<0.001，the same below）. Lateral styloconic sensilla of Spodoptera litura contained sucrose-sensitive neurons，and the electrophysiological responses to 50 mmol/L sucrose were extremely significantly stronger than the control. The response intensity of sucrose-sensitive neurons in lateral styloconic sensilla of larvae was extremely significantly stronger than that in the medial styloconic sensilla. The lateral styloconic sensilla of larvae exhibited strong responses to 1，10，50 and 100 mmol/L sucrose，with extremely significant differences among these concentrations compared with control ，peaking at 50 mmol/L sucrose. There was no significant difference in the response of medial styloconic sensilla to 50 mmol/L sucrose. There was no significant difference in the electrophysiological response of lateral styloconic sensilla of the lateral Spodoptera litura to agmatine solution. Agmatine activity-dependent labeling technique could be used to label sucrose-sensitive neurons in lateral styloconic sensilla.【Conclusion】Sucrose exhibits significant feeding-stimulatory effect on Spodoptera litura larvae. However， no sucrose-sensitive neurons have been identified in the medial sensilla styloconica of Spodoptera litura larvae，whereas a single sucrose-sensitive neuron is present in the lateral styloconic sensilla. Using agmatine activity-dependent labeling， the sucrose-sensitive neurons in the lateral sensilla styloconica of Lepidopteran larvae can be effectively labeled，enabling clear visualization under a microscope.