Elsevier

Neuroscience

Volume 171, Issue 1, 24 November 2010, Pages 125-133
Neuroscience

Cognitive, Behavioral, and Systems Neuroscience
Research Paper
Differential involvement of medial prefrontal cortex and basolateral amygdala extracellular signal-regulated kinase in extinction of conditioned taste aversion is dependent on different intervals of extinction following conditioning

https://doi.org/10.1016/j.neuroscience.2010.08.066Get rights and content

Abstract

Extinction reflects a decrease in the conditioned response (CR) following non-reinforcement of a conditioned stimulus. Behavioral evidence indicates that extinction involves an inhibitory learning mechanism in which the extinguished CR reappears with presentation of an unconditioned stimulus. However, recent studies on fear conditioning suggest that extinction erases the original conditioning if the time interval between fear acquisition and extinction is short. The present study examined the effects of different intervals between acquisition and extinction of the original memory in conditioned taste aversion (CTA). Male Long-Evans rats acquired CTA by associating a 0.2% sucrose solution with malaise induced by i.p. injection of 4 ml/kg 0.15 M LiCl. Two different time intervals, 5 and 24 h, between CTA acquisition and extinction were used. Five or 24 h after CTA acquisition, extinction trials were performed, in which a bottle containing 20 ml of a 0.2% sucrose solution was provided for 10 min without subsequent LiCl injection. If sucrose consumption during the extinction trials was greater than the average water consumption, then rats were considered to have reached CTA extinction. Rats subjected to extinction trials lasting 24 h, but not 5 h, after acquisition re-exhibited the extinguished CR following injection of 0.15 M LiCl alone 7 days after acquisition. Extracellular signal-regulated kinase (ERK) in the medial prefrontal cortex (mPFC) and basolateral nucleus of the amygdala (BLA) was examined by Western blot after the first extinction trial. ERK activation in the mPFC was induced after the extinction trial beginning 5 h after acquisition, whereas the extinction trial performed 24 h after acquisition induced ERK activation in the BLA. These data suggest that the original conditioning can be inhibited or retained by CTA extinction depending on the time interval between acquisition and extinction and that the ERK transduction pathway in the mPFC and BLA is differentially involved in these processes.

Section snippets

Animals

Male Long-Evans rats (8 weeks old) were purchased from the Animal Center of the National Science Council, Taipei, Taiwan, and weighed 350–450 g. The animals were housed individually in metal cages (30×30×20 cm3) maintained in a temperature (22±1°C)- and humidity (55±1%)-controlled room under a 12-h/12-h light/dark cycle (lights on at 5:00 h). Rodent chow was available ad libitum. All rats were subjected to 23 h water deprivation prior to adaptation training. All experimental procedures were

Adaptation training and extinction processes

Before performing the reinstatement experiment, the average fluid consumption of the five water trials in the 5H (n=11) and 24H (n=10) groups during the adaptation training was 7.9±0.4 ml and 7.7±0.3 ml, respectively (Fig. 1B). The average sucrose solution consumption during the first extinction trial was 3.8±0.6 ml in the 5H group and 3.3±0.7 ml in the 24H group. In our previous experiments, animals subjected to non-reinforced sucrose pre-exposure (n=11) drank approximately 14–16 ml of the

Discussion

The reinstatement experiment in the present study demonstrated that extinction training beginning 24 h after the conditioning trial did not affect the expression of extinguished conditioned responding in the 24H group, whereas no reappearance of conditioned responding was observed in the 5H group when the extinction procedure was initiated 5 h after acquisition. Our behavioral results with CTA are consistent with Myers et al. (2006) and Kimura et al. (2008), demonstrating that the time interval

Conclusion

In summary, our results indicate that the original conditioning can be retained or inhibited by CTA extinction depending on the time interval between acquisition and extinction. The ERK transduction pathway in the mPFC and BLA is differentially involved in these processes.

Acknowledgments

We are grateful to Dr. Linda Chia-Hui Yu for valuable discussion. This study was supported by a grant from the National Science Council (NSC 97-2410-H002-214), ROC.

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