1. Department of Physiology, Ponce School of Medicine, Ponce, Puerto Rico 00732

Correspondence to: Gregory J. Quirk Correspondence and requests for materials should be addressed to G.J.Q. (e-mail: Email: gjquirk@yahoo.com).

Abstract

Conditioned fear responses to a tone previously paired with a shock diminish if the tone is repeatedly presented without the shock, a process known as extinction. Since Pavlov¹ it has been hypothesized that extinction does not erase conditioning, but forms a new memory. Destruction of the ventral medial prefrontal cortex, which consists of infralimbic and prelimbic cortices, blocks recall of fear extinction^{2, 3}, indicating that medial prefrontal cortex might store long-term extinction memory. Here we show that infralimbic neurons recorded during fear conditioning and extinction fire to the tone only when rats are recalling extinction on the following day. Rats that froze the least showed the greatest increase in infralimbic tone responses. We also show that conditioned tones paired with brief electrical stimulation of infralimbic cortex elicit low freezing in rats that had not been extinguished. Thus, stimulation resembling extinction-induced infralimbic tone responses is able to simulate extinction memory. We suggest that consolidation of extinction learning potentiates infralimbic activity, which inhibits fear during subsequent encounters with fear stimuli.