Orosensory detection of fatty acids by obesity-prone and obesity-resistant rats: strain and sex differences.

A series of brief-access (15s) behavioral assays following the formation of a conditioned taste aversion (CTA) to linoleic acid were performed in order to follow up on observations showing differences in the chemosensory responses to dietary fat in obesity-prone (Osborne-Mendel [O-M]) and obesity-resistant (S5B/Pl) rat strains. Strong aversions to linoleic acid (conditioned stimulus 100 microM) were generated in both O-M and S5B/Pl rats to concentrations as low as 2.5 microM. Observed strain differences were in contrast to expectations based upon electrophysiological studies previously showing greater fatty acid-induced inhibition of delayed rectifying K+ channels in S5B/Pl rats. In the CTA assays, the O-M rats showed aversions at lower fatty acid concentrations with more resistance to extinction in brief-access orosensory tests, suggesting that the obesity-prone strain may be more sensitive in the detection and subsequent avoidance of linoleic acid than the obesity-resistant strain. The independent variable of sex produced even greater differences in the avoidance of linoleic acid following conditioning than the effects of strain. Female rats of both strains were significantly more sensitive to fatty acids, showed greater cross-generalization from linoleic to oleic acid, and showed greater avoidance of linoleic acid than male counterparts. These findings suggest genetic influences on yet to be identified mechanisms potentially within the gustatory system that affect the sensitivity to detect the fatty acid chemicals found in dietary fat during brief-access orosensory testing.

Chorda tympani nerve transection impairs the gustatory detection of free fatty acids in male and female rats.

Sprague-Dawley rats with intact (SHAM) and bilaterally transected chorda tympani nerves (CTX) received conditioned taste aversions (CTAs) to the free fatty acids (FFAs), linoleic and oleic acid, at micromolar quantities. Two-bottle preference tests showed that CTX eliminated avoidance of 88 muM linoleic acid but did not affect CTA avoidance of corn oil or 250 mM sucrose. Short-duration stimulus tests following single-pairing CTAs revealed that 8-s stimulus durations resulted in higher detection thresholds for linoleic acid than 30-s trials. In these short-duration tests, CTX rats showed 2-fold elevations in threshold for linoleic acid compared to the SHAM rats. A single-pairing CTA did not produce avoidance of oleic acid during the short-duration tests; however, 3 consecutive days of CTA pairings did produce avoidance of oleic acid in both male and female rats. Finally, both male and female rats received SHAM or CTX surgery after demonstrating successful CTAs to either 100 microM linoleic or oleic acid. The ability to detect and avoid linoleic and oleic acid was eliminated by CTX for both sexes. Differences in the ability of rats to form CTAs to linoleic and oleic acid suggest that linoleic acid is a more salient stimulus than oleic acid. Our results suggest that FFAs stimulate afferent taste signals in the chorda tympani nerve of male and female rats and that these signals play an important role in the gustatory behavior of accepting or avoiding taste stimuli following a conditioned taste aversion.