Receptor current fluctuation analysis in the labellar sugar receptor of the fleshfly.

Abstract

A B S T R A C T Fluctuations in the receptor current of the labellar sugar receptor of the fleshfly were analyzed. The receptor current was recorded extracellularly as a drop in potential between the tip and the base of the taste sensillum. After treatment with tetrodotoxin, the taste cells completely lost their impulses but retained their receptor currents, thus facilitating analysis of the receptor current without disturbance by impulses. The current fluctuation increased markedly when the sensillum was stimulated with effective sugars: maltose, sucrose, and fructose. The fluctuation increased in parallel with development of the receptor current, which indicates that it occurs as soon as the sugar reaches the apex of the sensory process. Analysis of fluctuations by computation of autocorrelation functions (ACFs) or power spectra (PS) revealed that: (a) the variance (mean square) of fluctuation vs. sugar concentration curve reached a maximum, in contrast to the monotonic increase shown by the receptor current; (b) the ACF was approximated by an exponential term, and its time constant differed according to the sugars used and their concentrations. The time constants for fructose and maltose decreased with increases in sugar concentration. At the concentrations of sugars evoking the same magnitude of receptor current, the time constant for fructose was the largest and that for maltose was the smallest. It was strongly suggested that transduction ion channels are present at the tip region of the sensory process of the sugar receptor cell and are operated directly by sugars.

Sugar-regulated cation channel formed by an insect gustatory receptor

“…However, there was no evidence for a second messenger-mediated pathway in the BmGr-9 response to D-fructose. Our results were consistent with previous electrophysiology experiments that suggested the presence of D-fructose-driven ion channel transduction in the flesh-fly sugar receptor neurons (28), and these sugar-activated currents showed nonselective cation conductance in vivo (29).…”

Adaptation-promoting effect of IP3, Ca2+, and phorbol ester on the sugar taste receptor cell of the blowfly, Phormia regina.

“…Previously, Amakawa et al (1990) showed that extracellularly applied dbcGMP evokes less adapting impulse discharge from the sugar receptor cell, suggesting that dbcGMP could act as an excitatory intracellular messenger in place ofcGMP. Even if, as argued by Kijima, Nagata, Nishiyama, and Morita (1988), the channel opening contributing to the receptor potential is primarily triggered by stimulus sugars without any cascadal reactions, cGMP could be involved in the transduction mechanism for sustaining the ion channels in the open state. At any rate, the adaptation cascade would link with the transduction process at an earlier step than the opening or sustaining of the ion channel by cGMP.…”

Transduction Ion Channels Directly Gated by Sugars on the Insect Taste Cell

“…Application of chemical modification reagents or enzymes to the sensory hair tip has revealed that the sensory process of the sugar receptor cell has several receptor sites (receptor molecules) with definite stimulant specificities: P sites activated by pyranose sugars, F sites for furanose sugars, R sites for aryl amino acids, A sites for alkyl amino acids, and the site for nucleotides (Shimada, 1978;Morita and Shiraishi, 1985;Amakawa et al, 1992;Ozaki et al, 1993;Furuyama et al, 1999). Little is known, however, about the transduction mechanisms of insect taste reception, either at the cell level or the ion channel level (Morita and Shiraishi, 1985;Kijima et al, 1988Kijima et al, , 1994. Here, we used patch-clamp techniques to study the responses of the labelar taste cells of the fleshfly to sugars and found transduction ion channels directly gated by sugars localized on the distal membrane of the taste cell's sensory process.…”