Prior cocaine exposure disrupts extinction of fear conditioning Authors: Kathryn A. Burke, Theresa M. Franz, Nishan Gugsa, and Geoffrey Schoenbaum

Addiction Compulsive behavior Excessive seeking/taking of drugs in face of aversive consequences Drugs = long-lasting consequences Prefrontal cortex Medial prefrontal cortex (MPFC)

Role of Medial Prefrontal Cortex Not studied for effects w/ drugs but altered by them Suppresses inappropriate behavior Behavioral flexibility Needed to change behavior in response to uncertain/ambiguous events Not needed for initial learning Lesions (to the MPFC) Alters Attentional set-shifting Alters Extinction of fear conditioning

Hypothesis If psychostimulant-induced neuroadaptations in MPFC are functionally significant, then should observe changes in performance in these tasks in drug-experienced rats

Methods & Materials 23 3 mo old male Long-Evans rats Testing in behavioral boxes (12x10x12 inches) and recorded activity levels w/ photocell monitors For all training (except inflation) boxes were enclosed in sound and light-resistant shells Boxes configured with cues, food cups, response levers During conditioning & extinction, bar pressing rates calculated for pre-CS & CS periods for each trial & a suppression ratio was calculated from data for each at on each trial (bar pressingpre-tone – tone) / (bar pressingpre-tone + tone)

Methods & Materials (cont) Freezing scored manually (scored 1 sec increments using timed pulses) & averaged during pre-CS & CS periods Rats were deprived of food (85% baseline) then trained to bar press for sucrose pellets on a VI60 schedule until response of ~20 respones/min Saline-treated rats n=11, cocaine-treated rats n=12 14 days of 30 mg/kg, i.p. of cocaine for cocaine-treated rats & similar volume of saline for saline-treated rats After injection, each rat put into operant box to record activity for 1 hr

Methods & Materials (cont) 3 wks after cocaine sensitization retrained to bar press for food & then tested for FC and extinction Habituation 4 tones (conditioned stimulus) alone Then given 6 tones (CS) w/ mild footshock (unconditioned stimulus) After FC, divided into 4 groups: cocaine inflated (new environment; 3 US shocks @ 6 X Intensity of previous FS; shocks @ 4 min intervals and no tones), cocaine noninflated (no FS), saline inflated, saline noninflated 24 h after inflation began extinction training (2 sessions w/ 16 presentations of tone alone w/ no FS over 2 days)

Experimental Procedure Inflations procedure used to increase the incentive value of the shock stimulus and increases subsequent conditioned responding dependent on the basolateral amygdala (BLA).

Fig. 1 Effect of cocaine on locomotor activity Cocaine-treated rats exhibited sig. increase in locomotor activity after 14 day sensitization period (Fig. 1 left side). Cocaine-treated rats exhibited greater sensitivity to cocaine in the challenge test shown by increased activity to lower doses (30-45 and 45-60 min into the session) (Fig. 1 right side).

Fig. 2 Effect of cocaine on conditioned freezing & suppression at the end of habituation/during fear conditioning Freezing - % time during tone (CS) spent freezing Suppression - ratio comparing bar pressing during CS w/ bar pressing during pre-CS period

Results (Fig. 2) Cocaine- & saline-treated rats showed similar conditioning of both measures to the tone ANOVA’s indicated sig. main effects of trial for both freezing & conditioned suppression No sig. main effects or interactions involving treatment on either freezing or suppression behavior

Fig. 3 Effect of cocaine on extinction of conditioned freezing & suppression Cocaine-treated rats were sig. slower to extinguish both conditioned freezing and suppression and continued to show higher conditioned suppression on a 2nd day of extinction training.

Fig. 4 Effect of inflation of conditioned freezing in saline-treated (A) and cocaine-treated (C) rats Inflated rats in both groups froze sig. more than non-inflated rats (beginning of extinction); there was no effect of cocaine. Inflation affects previously acquired learning.

Fig. 4 Effect of inflation of conditioned suppression in saline-treated (B) and cocaine-treated (D) rats No effect of inflation on suppression in either group. The effect of cocaine treatment on extinction of conditioned freezing & suppression (shown in Fig. 2) was evident here in both inflated &non-inflated rats.

Discussion Associations b/w conditioned stimuli & fear-producing events are thought to be formed in the basolateral complex of the amygdala (BLA)  then expressed though excitatory output to central nucleus (CeA) CeA operates in parallel to encode motivating aspects of fear-producing conditioned stimuli Results may reflect effects of cocaine w/i BLA & CeA or downstream regions  make such learning resistant to extinction Difficult to separate effects of cocaine on locomotor activity from effects on conditioned behavior

Discussion Processing of fear associations w/in amygdala is largely intact after cocaine exposure Impaired extinction in amygdala would be selective More likely candidate for this process is the mPFC Due to alterations in mPFC or alterations in how mPFC interacts w/ amygdala Cocaine does cause alterations in MPFC structure and function Expression of fear memories in subcortical regions is modulated by output from the mPFC mPFC modulates encoding in the amygdala (BLA) Enhanced responding to fear-associated cues during extinction learning

Conclusion No effects of inflation on conditioned suppression in saline- or cocaine-treated rats Transfer of aversive properties of fear-producing US to the instrumental bar pressing response is insensitive to the value of the US predicted by the Pavlovian cue Similar to what has been reported for appetitive Pavlovian-to-instrumental transfer Dissociation of the effects of US conditioned suppression (remained unchanged) is consistent w/ the proposal that the properties of Pavlovian CS are mediated by different circuits in the amygdala