Periaqueductal gray c-Fos expression varies relative to the method of conditioned taste aversion extinction employed

Abstract

A conditioned taste aversion (CTA) is acquired when an animal consumes a novel taste (CS) and then experiences the symptoms of poisoning (US). Following CTA training, animals will avoid the taste that was previously associated with malaise. This defensive reaction to a learned fear can be extinguished by repeated exposure to the CS alone (CS-only; CSO-EXT). However, following a latency period in which the CS is not presented, the CTA will spontaneously recover (SR). Through the use of an explicitly unpaired extinction procedure (EU-EXT) we have shown that we can speed up extinction and attenuate SR of the CTA. Here we compared and contrasted the ability of CSO and EU extinction procedures to affect c-Fos expression in the periaqueductal gray (PAG). Fluid-deprived Sprague-Dawley rats acquired a strong CTA [via 3 pairings of 0.3% oral saccharin (SAC; the CS) and 81 mg/kg i.p. lithium chloride (LiCl; the US)] followed by extinction trials consisting of multiple exposures to either, (a) the CS every-other day (CSO-EXT), or (b) CS and US on alternate days (EU-EXT). A different group of rats did not receive multiple CS exposures and served as a “no extinction” (NE) control. Both extinction procedures resulted in ≥90% reacceptance of SAC (achieving asymptotic extinction). Some of the animals were sacrificed for c-Fos immunohistochemical analysis following asymptotic extinction. Other rats entered a 30-day latency period where they drank water only. These remaining animals were then tested for SR with a final exposure to SAC before being sacrificed for c-Fos immunohistochemistry. As reported previously, rats in the CS-only group exhibited a significant SR of the CTA. However, animals in the EU extinction group reached asymptotic extinction more rapidly than did CSO rats and they did not show SR of the CTA. As compared to rats that retained their CTA, both groups of extinguished rats showed suppression in the number of c-Fos-labeled neurons in all 4 longitudinal columns of the PAG. The number of c-Fos-labeled cells in the PAG was generally low but there was a reliable increase in c-Fos expression in dorsolateral PAG (dlPAG) following the SR test in the brains of rats that went through the EU-EXT procedure as compared with those that either went through the more-traditional CSO extinction procedure or experienced no extinction at all. The number of c-Fos-labeled neurons in the dlPAG was significantly correlated with the amount of SAC consumed at the SR test. Surprisingly, the brains of EU-extinguished rats and CSO extinguished rats did not differ in the number of c-Fos-labeled neurons in gustatory neocortex, medial prefrontal cortex, basolateral amygdala, or the central nucleus of the amygdala. Thus, behavioral differences in SR between the EU and CSO extinction animals were not represented by corresponding changes in the neural activity of several brain nuclei classically associated with extinction learning. However a detailed analysis of PAG c-Fos expression provided hints about some of the physiological changes evoked ...

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

“…Rats receiving dPAG stimulation exhibited a significantly enhanced SR of their CTA as compared to control animals that received comparable electrical stimulation outside the dPAG. Thus the correlation we originally observed between enhanced c-fos protein expression in dlPAG and reduced SR (Mickley et al, 2011) did not prove to reflect a cause-and-effect relationship. However, our data show that the dPAG is involved, directly or indirectly, in SR of a CTA.…”

“…Our previous studies (Mickley et al, 2011) indicated that enhanced c-fos expression in the dlPAG was correlated with a reduction in the SR of a previously extinguished CTA. However, we sought evidence that c-fos protein is a necessary, and specific, substrate of CTA SR (Lamprecht and Dudai, 1996).…”

“…These data lead us to evaluate the extent to which the increased c-fos protein expression we observed in the dlPAG (Mickley et al, 2011) caused the reduction of CTA SR. In the current study we used electrical brain stimulation to enhance c-fos protein expression in the dPAG.…”

“…Less is known about the role that the PAG plays in the extinction of conditioned fears although McNally et al (2004, 2005) have established the importance of endorphinergic neurons of the ventrolateral PAG (vlPAG) in the extinction of CERs. It is clear that the neural processes of fear extinction and those processes that allow the reoccurrence of once-extinguished fears must be related (Costanzi et al, 2011) and a recent study indicates that the PAG may be involved in the spontaneous recovery (SR) of a conditioned aversive response (Mickley et al, 2011). However, there remains a substantial gap in our knowledge of the means by which the brain produces SR of conditioned fears and the possible role that the PAG may play in this relapse phenomenon.…”

“…CTA extinction employs some of the same neural circuits as does the extinction of CERs. Specifically, we have reported changes in neural activity (as measured through c-fos immunoreactivity) in the amygdala, mPFC and PAG that correlate with various stages of extinction and SR of a CTA (Mickley et al, 2004, 2005, 2007, 2011). Several of these changes parallel alterations that are known to occur in these brain areas as a CER is extinguished (Quirk and Mueller, 2008).…”