Electrical response and function of a bitter substance receptor associated with the maxillary sensilla of the larva of the silkworm, Bombyx mori L.

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Abstract
This receptor was strongly stimulated by “bitter” substances such as strychnine nitrate, salicin, brucine, nicotine, etc. as well as by the aqueous extracts which were obtained from many normally unacceptable plant leaves for the larvae. Such bitter substances for human sense as phenylthiourea, betaine, and MgSO4 were inert.This maxillary hair associates a water receptor (W) and two salt receptors (N2 and N2′) as well as the R receptor.When a solution contained both bitter substance and NaCl, the respective substance had a synergistic effect on N2 + N2′ or R receptor activities in a certain concentration range of each compound. Responsiveness of the R or N2 + N2′ receptors seemed to depend on the the mixing ratio of the two compounds. CaCl2 had inhibitory effects on the activity of the R and W receptors, but not on those of the N2 and N2′ receptors. Sugars had no effect on R receptor activity.The inhibitory effect on the feeding was observed when the substance stimulating the R receptor was present in either filter paper or fresh leaves. The inhibitory effect was considerably lessened by inactivation of the R receptor neuron.Accordingly, it is supposed that the exclusion of botanical species from the food plant range of the silkworm larvae might be determined in some parts by a high frequency of occurrence of the specific stimuli for the R receptor in their leaves.