OROSENSORY STIMULATION OF FEEDING BY OILS IN PREWEANLING AND ADULT-RATS

To determine the orosensory control of ingestion by oils, we investigated the acceptance and preference of corn oil and of mineral oil in a series of experiments with preweanling rat pups, adult male, Sprague-Dawley rats, and adult female lean and obese (fa/fa) Zucker rats. Emulsions of corn oil in water (greater-than-or-equal-to 25%), administered by anterior intraoral catheter, elicited more intake than water at 14, but not 7, days of age. By 21 days of age, intake was a sensitive function of emulsified corn oil concentrations (greater-than-or-equal-to 6.25%). In experiments with adult rats, the sham feeding technique was used to maximize the relationship between orosensory stimulation and intake. Rats sham fed approximately similar volumes of 100% corn oil and 100% mineral oil. Maximal rate of intake was about 1 ml/min. The large intake of mineral oil is compelling evidence for the potency of the orosensory stimuli of oil to control intake in the absence of postingestive metabolic effects. The orosensory control is very sensitive in the adult rat -0.78% corn oil produced a larger intake than water. The orosensory control of acceptance (1-bottle tests) did not correlate with preference (2-bottle tests): 100% corn oil and 100% mineral oil were equally accepted, but rats showed a very strong preference for 100% corn oil. The peripheral sensory mechanism of this orosensory control is unknown. As a working hypothesis, we suggest that oral tactile mechanisms of the trigeminal afferent nerves provide the necessary sensory pathway. The central sensory and hedonic processing of the orosensory stimuli of oils is also unknown, although we obtained pharmacological evidence that central dopaminergic activity, particularly at D-2 receptors, is necessary for normal sensory and/or hedonic processing of corn oil.