[Abstract Title]. - Society for Neuroscience

Topic: F.02.h. Memory modulation, consolidation, and reconsolidation
Support: NSERC
CIHR
Title: Short and long term expression of extinction memory
Authors: *G. E. ARCHBOLD, K. NADER;
McGill Univ., Montreal, QC, Canada
Abstract: Evidence suggests that behavioral extinction, a process whereby a conditioned
response is diminished after presentations of the conditioned stimulus in the absence of
reinforcement, is new learning rather than unlearning of the original association. In the initial
acquisition of fear, there is both a short term memory (STM) and long term memory (LTM)
component. However, few studies have examined early and late phases of an extinction trace. In
conditioned taste aversion (CTA) Berman et al. (2003) reported that expression of an
extinguished CTA is only observed 2 hours after extinction and this latency for expression not
mediated by -adrenergic receptors, muscarinic cholinergic receptors, or protein synthesis. The
present study examines the expression of behavioral extinction of an auditory fear memory at
various time points after extinction training. Animals were conditioned via tone-shock pairings
to induce freezing behavior and later received non-reinforced tone presentations to extinguish
freezing. Testing for expression of extinction was conducted 1, 4, 8 or 24 hours after training.
Freezing remained high at both the 1 and 4 hour time points but was significantly reduced by 24
hours. We tested whether the absence of short term extinction was masked by reinstatement of
the original trace as result of the aversive experience of extinction itself. To address this issue a
second experiment was performed where animals were trained to fear two different auditory
stimuli, CS1 and CS2, on successive training sessions. Three days later CS1 was extinguished.
The following day CS2 was extinguished and animals were presented with a test for CS1 either 1
or 24 hours after this second extinction session (i.e. 25 or 48 hours after extinction of CS1). Only
animals tested 1 hour after CS2 extinction exhibited recovered freezing to CS1. Recently,
Westbrook et al. (2005) found that the β-adrenergic system mediated the reinstatement of
freezing to an extinguished CS by pre-exposure to a feared context 2 min before presentation of
the CS. We hypothesized that a similar mechanism may be mediating the return of fear to CS1 if
it is delivered shortly after CS2 extinction. To test this, 10mg/kg propranolol, a β-adrenergic
receptor antagonist, was injected 20 min prior to testing. Propranolol did not restore expression
of extinction 1 hour after CS2 extinction. However, in a renewal test conducted after a delay of 4
days, propranolol treated groups demonstrated reduced responding to CS1. Overall, the lack of
short term extinction expression is consistent with Berman et al. (2005). There may be no STM
mechanisms mediating extinction or extinction STM may be masked by other factors not tested
for.
Disclosures: G.E. Archbold, None; K. Nader, None.