| 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 referred to as conditioned taste aversion (CTA). CTA is based on the gustatory learning and memory after association of the taste of food with the visceral signals of poisoning. It is an important acquired adaptive behaviour, which may prevent animals from ingesting the harmful food again. To develop an experimental paradigm for CTA, a visceral malaise causing stimulus (unconditioned stimulus, US) such as intraperitoneal injection of LiCl is presented to the rat after its ingestion of the novel saccharin solution (conditioned stimulus, CS). Then the rat decreases or avoids ingesting the saccharin solution.The rats having acquired CTA exhibited a series of aversive oral movements, after the saccharin solution is delivered directly into their mouths with an intraoral catheter, while the rats in the control group expressed a specific 'hobby-like' behaviour after they ingested such a sweet tasted food. Thus, there are evidently emotional responses in acquisition of CTA. Enkephalin(ENK) is one of endogenous opiate like substance and could induce psychological euphoria and modulate function of learning and memory. However there has no report available about the role of ENK in acquisition ofSprague-Dawley rats as materials. 35 male SD rats (250-280g) were divided into 5 groups: control group, 0.9% NaCl (2% of body weight) intraperitoneal(i.p.) injected group, isotonic LiCl (0.15M, 2% of body weight) i.p. injected group, 0.1% saccharin (1.5~2ml/rat, in 5min) intraoral infused group and CTA group. The expression of endogenous Leucin-enkephalin(LEK) in the rat brain was observed and 5 parts of the thalamus including laterodorsal thalamic nucleus (LD), lateral part of mediodorsal thalamic nucleus (MDL), ventroposterolateral thalamic nucleus (VPL), ventroposteromedial thalamic nucleus (VPM) and reticular thalamic nucleus (Rt) were comparatively researched before and after the acquisition of CTA applying LEK-immunocytochemistry. In behavioral experiment, 18 adult male SD rats were divided into normal CTA group (control) and 2 naloxone i.p. injected groups. In the later two, naloxone (2mg/kg body weight) as one of effective opiate antagonists was i.p. injected into rats before or after the acquisition of CTA individually to research the effect of endogenous opiate like substance on the acquisition and retention of CTA. All of the data were analyzed for variance analysis and Newman-Keuls test between groups using software STAT. Results:1. LEK-immunoreactive neurons distributed extensively in the rat brain of each group, such as limbic system including the hypothalamus and the amygdala complex, area postrema (AP), parabrachial nucleus (PEN), the thalamus and so on.2. There were no significant differences between the control group and 0.9% NaCl i.p. injected group in every nucleus with LEK-immunoreactive neurons, thus both groups may be combined into as one control group.3. The numbers of LEK-immunoreactive neurons in LD and VPM of saccharin intraoral infused group were significantly more than those in control group while the numbers in LD and MDL of LiCl i.p. injected group were remarkably fewer than those in control group.4. The numbers of LEK-immunoreactive neurons in LD, MDL and VPM of CTA group were remarkably fewer than those in saccharin intraoral infusedof CTA group were remarkably fewer than those in saccharin intraoral infused group. The numbers in LD, MDL, VPL and Rt had no significant difference with those in LiCl i.p. injected group.5. Before the acquisition of CTA, the rats reflected hedonically ingestive responsiveness to 0.1% saccharin solution; After CTA, exposed to the solution again, the rats usually manifested strong aversive responses.6. Naloxone i.p. injected into rats before or after CTA had...
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