Long-lasting incubation of conditioned fear in rats.
BACKGROUND
In 1937, Diven reported that human fear responses to cues previously paired with shock progressively increase or incubate over 24 hours. Since then, fear incubation has been demonstrated in both humans and nonhumans. However, the difficulty of demonstrating long-lasting fear incubation in rodents has hampered the study of the underlying mechanisms of this incubation. Here, we describe a rat procedure where fear reliably incubates over time.


METHODS
We trained food-restricted rats to lever-press for food pellets in daily 90-min sessions. We then gave each rat 100 30-sec tones co-terminating with a .5-sec .5-mA footshock over 10 days (10 pairings/day). Groups of rats (n = 10-15) were then given four presentations of the tone (the fear cue) 2, 15, 31, or 61 days after fear conditioning training and were assessed for conditioned suppression of lever-pressing.


RESULTS
We found that conditioned fear responses were significantly higher 31 and 61 days after fear training than after 2 or 15 days. In control experiments, we showed that extensive tone-shock pairing is necessary for the emergence of fear incubation and that it is unlikely that non-associative factors contribute to this incubation.


CONCLUSIONS
We describe a procedure for generating reliable and long-lasting conditioned fear incubation. Our procedure can be used to study mechanisms of fear incubation and might provide a model for studying the mechanisms of delayed-onset posttraumatic stress disorder that occur in a sub-population of people previously exposed to chronic stressors.
Pickens
Charles L
CL
Behavioral Neuroscience Branch, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Department of Health and Human Services, Baltimore, Maryland 21224, USA.
Golden
Sam A
SA
Adams-Deutsch
Tristan
T
Nair
Sunila G
SG
Shaham
Yavin
Y
D000818:Animals; D003214:Conditioning, Classical; D003216:Conditioning, Operant; D003463:Cues; D004597:Electroshock; D005239:Fear; D008297:Male; D051381:Rats; D020318:Rats, Long-Evans; D013997:Time Factors