Novel approaches to the study of viscosity discrimination in rodents

Discrimination of cellulose microparticles in rats

Oral perception of food particles is important in mastication and swallowing. However, the mechanism underlying particle perception remains poorly understood because of the lack of suitable experimental systems. We evaluated microparticle perception in rats utilizing insoluble cellulose particles of varying diameters (20-170 μm). The cellulose additives have polycrystalline morphologies and contain smaller crushed particles. The filtrate containing 20 μm particles at a concentration of 1.6% was passed through 3 μm pore-size filter paper, and numerous small particles equivalent to a 0.25 mM soluble solution were observed. In two-bottle preference tests, rats showed no innate preference or avoidance of particles of any size at concentrations ranging from 0.05-1.6%. Next, conditioned preference learning tests employing 8% glucose and fructose solutions were performed. After being repeatedly presented with glucose and fructose solutions containing particles of different sizes (170 and 20 μm particles or 20 μm filtrate) at a concentration of 1.6%, the rats preferred particles in glucose solution even without glucose presentation. Intriguingly, rats preferred the filtrate following repeated presentations of glucose-containing filtrate and water containing fructose. These results suggest that rats can distinguish microparticles in water. The preference learning test is useful for analyzing particle perception mechanisms in mammals.