Explicit disassociation of a conditioned stimulus and unconditioned stimulus during extinction training reduces both time to asymptotic extinction and spontaneous recovery of a conditioned taste aversion

Abstract

Conditioned taste aversions (CTAs) may be acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). This aversion may be extinguished by repeated exposure to the CS alone. However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). In the current study we employed an explicitly unpaired extinction procedure (EU-EXT) to determine if it could thwart SR of a CTA. Sprague-Dawley rats acquired a strong CTA after 3 pairings of saccharin (SAC the CS) and Lithium Chloride (LiCl the US). CTA acquisition was followed by extinction (EXT) training consisting of either (a) CS-only exposure (CSO) or, (b) exposure to saccharin and Lithium Chloride on alternate days (i.e., explicitly unpaired: EU). Both extinction procedures resulted in >/= 90% reacceptance of SAC, although the EU extinction procedure (EU-EXT) significantly decreased the time necessary for rats to reach this criterion (compared to CSO controls). Rats were subsequently tested for SR of the CTA upon re-exposure to SAC following a 30-day latency period of water drinking. Rats that acquired a CTA and then underwent the CSO extinction procedure exhibited a significant suppression of SAC drinking during the SR test (as compared to their SAC drinking at the end of extinction). However, animals in the EU-EXT group did not show such suppression in drinking compared to CSO controls. These data suggest that the EU-EXT procedure may be useful in reducing both time to extinction and the spontaneous recovery of fears.

Figure 1

Mean volume of SAC consumption (± SEM) after either three CS+US pairings (CTA) or three explicitly unpaired CS/US exposures (NO CTA). The CTA group showed a significant decrease in the amount of SAC consumed over the three exposures. The NO CTA group showed a significant increase in SAC consumption over the same three periods. This indicates that the CTA groups had acquired the CTA, whereas the NO CTA group did not. * = significantly different from the CTA animals (α = 0.05).

Figure 2

Mean days (± SEM) for animals to reach asymptotic extinction. Animals either underwent the CTA+EU-EXT or CTA+CSO-EXT procedure. The CTA+EU-EXT group took significantly fewer days to extinguish the learned fear than the CTA+CSO-EXT group. * = significantly different from the CTA+CSO-EXT animals (α = 0.05).

Figure 3

Mean days (± SEM) spent in each phase of extinction (Nolan et al., 1997) for animals that underwent either CTA+EU-EXT or CTA+CSO-EXT procedures. Rats that experienced the CTA+EU-EXT procedure spent significantly fewer days in the static phase (SAC reacceptance of less than 10% of baseline) than the CTA+CSO-EXT group [* = significantly different from the CTA+CSO-EXT animals (α = 0.05)]. However, the CTA+EU-EXT and CTA+CSO-EXT groups spent comparable number of days in both the dynamic (SAC reacceptance greater than 10 % but less than 80% from baseline) and asymptotic phases (SAC reacceptance of greater than 80% from baseline).

Figure 4

Mean volume of SAC consumption (± SEM) on the day of asymptotic extinction and on the subsequent SR test day for both the CTA+EU-EXT and CTA+CSO-EXT animals. On the day of the final extinction test, the CTA+EU-EXT and CTA+CSO-EXT groups drank comparable amounts of SAC (p > 0.05; see text). The CTA+CSO-EXT group drank significantly more SAC on the last day of extinction than on the day of the SR test, indicating a spontaneous recovery of the CTA. However, the CTA+EU-EXT animals drank nearly the same amount of SAC on the day of extinction as they did on the SR test day, suggesting that the EU-EXT procedure may be effective in blocking SR. * = groups indicated are significantly different, α= 0.05.