Papilio hospiton larvae discriminate bitter and sweet stimuli by the “labeled lines” coding modality

In herbivorous animals, food selection may depend on sensitivity to specific chemical stimuli as well as to secondary metabolites (bitter compounds) and sugars (phagostimulant compounds). Bitter compounds are noxious, unpalatable or both and evoke an aversive feeding response. Instead, sugars and sugar alcohols play a critical role in determining and enhancing the palatability of foods. It is suggested that whether one animal eats or not a given food depends on the total “sensory impression” which results from the integrated response to phagostimulant and deterrent compounds at level of the CNS. The coding mechanisms generally proposed for neural processing of sensory information are: 1) a “labeled lines” code, by which the stimulus identity is represented by the activity in a subset of afferent taste neurons, and 2) an “across fiber patterning” code, where tastant identity is represented by activity across large populations of afferent gustatory neurons. We investigated the discriminating modalities in the larvae of lepidopterous species (Papilio hospiton), that has the advantage of possessing a limited number of gustatory receptor neurons, the axons of which project directly to the CNS. The spike activity of the medial and lateral maxillary styloconic taste sensilla was recorded following stimulation with several carbohydrates, nicotine and NaCl. With the aim of evaluating the discrimination capability between different compounds and the coding mechanism used, we calculated the neural discrimination (d) and the labeled line (l) indexes. The results show that P. hospiton is capable of discriminating bitter from sugar tastants and different bitter and sugar compounds from one another: the neural code principally used is of the “labeled lines” type.