Papilio hospiton and Papilio machaon larvae discriminate bitter taste stimuli by different coding modalities

The mechanisms generally proposed for discriminative 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. Besides, rate, temporal and spatiotemporal codes also seem to contribute to taste processing. The taste discriminating capability of bitter compounds is particularly useful for herbivorous animals because some of these chemicals are toxic while others are only unpalatable. The spike activity of the medial and lateral maxillary styloconic taste sensilla in two genetically related oligophagous species of lepidopterous larvae (Papilio machaon and Papilio hospiton, generalist and specialist, respectively), was recorded by the tip-recording technique following stimulation with several bitter taste stimuli: nicotine, caffeine, salicin and quercitrin. In both species, the taste response to these stimuli is mediated by three pairs of bitter-sensitive cells, two of them are located in the lateral and the third one in the medial sensillum. The results show that nicotine and caffeine activate all bitter cells, while salicin and quercitrin only two of them, one in the lateral and one in the medial sensillum. This suggests that the discrimination of nicotine and caffeine could involve a labeled-line coding. Moreover, we examined the spike frequency time courses to assess the potential contribution of time-related codes to the discrimination process. The results also show that, both Papilio machaon and Papilio hospiton, are capable of discriminating among bitter taste stimuli by means of a time-related and a spatiotemporal coding modality. Acknowledgements: Supported by Regione Autonoma della Sardegna grant CRP-59859, and Fondazione Banco di Sardegna grant 2012/0245. FCOI Disclosure: Non