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

• Published in: Brain research Vol. 1423; pp. 17 - 29
• Main Authors: Mickley, G. Andrew, Wilson, Gina N, Remus, Jennifer L, Ramos, Linnet, Ketchesin, Kyle D, Biesan, Orion R, Luchsinger, Joseph R, Prodan, Suzanna
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 14-11-2011; Elsevier
• Subjects: amygdala; Analysis of Variance; Anatomical correlates of behavior; Animals; Avoidance Learning - physiology; Behavioral psychophysiology; Biological and medical sciences; c-Fos; Cell Count; Conditioned taste aversion; Conditioning, Psychological - physiology; cortex; CTA; Extinction; Extinction, Psychological - physiology; fearfulness; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation; immunohistochemistry; Learning; lithium chloride; Lithium Chloride - administration & dosage; Male; Memory; neocortex; Neurology; neurons; Olfactory system and olfaction. Gustatory system and gustation; PAG; Periaqueductal gray; Periaqueductal Gray - cytology; Periaqueductal Gray - metabolism; poisoning; Proto-Oncogene Proteins c-fos - metabolism; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; Rats; Rats, Sprague-Dawley; Reaction Time; saccharin; Saccharin - administration & dosage; Spontaneous recovery; Sweetening Agents - administration & dosage; taste; Taste - physiology; Time Factors; Vertebrates: nervous system and sense organs; Learning; CTA; Periaqueductal gray; Conditioned taste aversion; Extinction; Memory; PAG; c-Fos; Spontaneous recovery; Rat; Central nervous system; Gustative system; Recovery; Encephalon; Acquisition process; Protooncogene; fos Gene; Rodentia; Gene expression; Vertebrata; Mammalia; Periaqueductal gray matter; Gustation; Animal; C-Onc gene; Perception; Taste; Conditioned aversion; Immediate early gene
• ISSN: 0006-8993; 1872-6240
• DOI: 10.1016/j.brainres.2011.09.033

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 by these 2 extinction paradigms that produce very different behavioral outcomes. The findings are clinically relevant as we seek the development of treatments for deficits in fear extinction (e.g. PTSD, phobias).