Ingestion and diet destination in the mosquito Culiseta inornata: effects of carbohydrate configuration

Effects of 17 different carbohydrates and a 1:1 mixture of alpha-glucose and fructose on ingestion and diet destination in female Culiseta inornata were determined. Sucrose was the most potent phagostimulant. The trisaccharides melezitose and raffinose and the dissacharides furanose, trehalose, maltose and a 1:1 mixture of alpha-glucose and fructose were also very potent (ED50 for gorging < 180 mM). The monosaccharides fructose, alpha-glucose, and methyl-D-alpha-glucopyranoside, and the dissacharides isomaltose, melibiose and cellobiose had ED50 values for gorging between 200 and 400 mM. The disaccharides lactose, lactulose and gentiobiose stimulated little gorging while beta-glucose and methyl-D-beta-glucopyranoside evoked responses similar to the water controls. P-chloromercuribenzoate (PHMB), an inhibitor of pyranose receptor sites, nearly abolished the stimulatory effects of maltose and alpha-glucose and diminished the potency of both fructose and a 1:1 mixture of fructose and alpha-glucose. The effectiveness of 0.5 M sucrose was only slightly diminished. Diet destination is affected by differential, chemosensory-based control of the crop and midgut valves. This control is separate from the regulation of ingestion. Two categories of sugars can be distinguished on the basis of their effects on diet destination: those which induce crop-only filling and those which evoke primarily midgut filling. Sucrose, fructose, alpha-glucose, maltose, isomaltose, methyl-D-alpha-glucopyranoside and melezitose induce crop-only filling. The results indicate that there is a mosaic of different types of specific receptor sites on the dendritic membrane. The results provide evidence for a pyranose site with specific requirements for at least two adjacent equatorial hydroxyl groups on carbons C2 and C3 and an axial hydroxyl group on C1. The pyranose site is blocked by PHMB and appears to be specific for alpha-glucopyranose and fructopyranose. There is also a fructofuranose site which is not blocked by PHMB. This site has a higher threshold than the pyranose site(s) and appears to be insensitive to D-galactose. In addition a trehalose site may be present. Many sites of each type must occur on each receptor membrane. The fructofuranose sites appear to be interspersed amid the pyranose sites in a way that allows sucrose molecules to simultaneously interact with both types of site.