| When the consumption of novel flavored food is followed by internal malaise, such as queasiness, diarrhea, bellyache and so on, animals will decrease or avoid ingesting the food with the same gustatory feature on the subsequent exposures. This type of behavioral change is defined as conditioned taste aversion (CTA).The animals acquired CTA for one gustatory stimulus, such as saccharin solution, showed aversive responses for other similar gustatory stimuli, such as sucrose, fructose, maltose. This phenomenon is the generalization of CTA. Then the rats were trained to discriminate those gustatory stimuli with differential conditioning paradigm that one taste stimulus combined with unconditioned stimulus but the other not, the rats showed aversion for reinforcer only and preference for nonreinforcers. This process is referred to as differential conditioning taste aversion.There have been many data indicated that parabrachial nucleus (PBN) was an important area for associating taste and general visceral afferent information. The FOS-like immunoreactive (FLI) neurons in PBN induced by different taste and visceral stimuli were differently distributed. However, little is known about the interaction in PBN between hedonics and quality of taste stimuli following generalization and differential conditioning taste aversion and the correlative brain area of generalization and differential of CTA. To elucidate this issue, in the present study, Sprague-Dawley rats were used, and were trained to establish CTA by intraoral infusion of either 0.1% saccharin or 0.2 mol/L sucrose paired with intraperitoneal injection of 0.15 mol/L LiCl. After animals showed aversion for both saccharin and sucrose, the rats were trained to discriminate saccharin and sucrose that injected with LiCl following intraoral infusion of either saccharin or sucrose until the rats displayed aversion only for reinforcer. Then, brain of the rats were took out and the expression of c-fos in the brain was observed, especially the distribution of FLI neurons induced by saccharin and sucrose in subdivisions of PBN was comparatively researched. Inaddition, the brain areas related to generalization and differentiation of CTA were investigated with the method of excitotoxic lesions by ibotenic acid.Results:1. All of the rats displayed strong aversion for both saccharin and sucrose after two treatments that intraoral infusion of either saccharin or sucrose paired with intraperitoneal injection of LiCl, but the rats experienced differential procedure of CTA showed aversive responses only for the taste solution associated with US.2. The FLI neurons induced by taste stimuli distributed extensively in the rat brain, such as gustatory cortex (GC), the amygdala complex (AMY), area postrema (AP), parabrachial nucleus (PBN), nucleus of the solitary tract (NTS) and so on.3. In contral group, different taste stimuli had no significant differences in PBN on c-fos expression except central lateral subnucleus (els), in which sucrose induced more FLI neurons than saccharin.4. When the rats established CTA, sucrose and saccharin induced c-fos expression had significant differences in external medial subnucleus (ems) and external lateral subnucleus (els) in Sac group, and in internal lateral subnucleus (ils), els, ems and central medial subnucleus (cms) in Sue group. And whether sucrose was conditioned stimulus or generalized stimulus, it induced more FLI neurons than saccharin.5. In generalization group, c-fos expression in PBN was changed. The number of FLI neurons in ems, els, cms and ventral lateral subnucleus (vis) was more than contral group, but in ils and els, contral group was more than generalization group.6. When the rats discriminate sucrose and saccharin, different stimuli had no influence in dorsal lateral subnucleus (dls) and vis on c-fos expression, and the number of FLI neurons induced by sucrose were more than saccharin in ils and ems, but in els and cms, saccharin were more than sucrose. Furthermore, hedonic stimulus induced higher c-fos expression in ils, cms and els, and aversive stimulus induced higher c-fos expression in ems and els.7. Bilateral microinjections of ibotenic acid in some nucleus coordinate according to Paxinos and Watson atlas. Rats lacking PBN and BLA almost completely failed to acquire CTA. Lesions of Rt, OFC and the foreside of AMY, had no effect on acquisition of CTA and generalization and differential conditioning taste aversion. Lesions of CeA and VPM madethe establish of CTA slower, but did not impact generalization and differential of CTA.Conclusions:1. The extensive distribution of FLI neurons in the brain areas which are concerned with the transmission of gustatory information and the formation of CTA, including AP, PBN, NTS, AMY, GC and so on, suggested that c-fos expression in the areas mentioned above may play an important role in acquisiton of CTA and the generalization and differential of CTA.2. The contral rats expressed a specific 'hobby-like' behaviour after they ingested saccharin and sucrose, and FLI neurons in PBN had no significant differences except els, suggested that c-fos expression do not sensitive to the different characters of the two sweet solutions.3. The rats having acquired generalization of CTA exhibited a series of aversive oral movements, when saccharin and sucrose solution were delivered directly into their mouths with an intraoral cannula. But FLI neurons in PBN induced by saccharin and sucrose was different, and sucrose induced more c-fos expression in all of the subnucleus which had significant differences. This suggested that when CTA was established, the response of these subnucleus to taste stimuli occured fictile change, and enhanced the ability of some neurons to discriminate similar tastes. The energy in taste stimulus may play an important role in taste discrimination.4. When the differential conditioning taste aversion was established, c-fos expression suggested, the effect of quality and hedonics of taste stimuli were separate in ils and cms, and they were interactional in els and ems. These results suggested that there were regions where related to taste discrimination and reward value in PBN, the two regions were partly overlapped, and ems and els had important role in integrating the information of taste quality and reward value.5. The modulation of acquisition of CTA and differentiation of CTA were differently. And the detailed neural mechanisms still need further study.
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