The Bed Nucleus of the Stria Terminalis Is Critically Involved in Enhancing Associative Learning After Stressfull Experience By Debra Bangasser, Jessica Santollo and Tracey Shors

Stressful events influence new memories –Exposure to acute stress facilitates classical eyeblink conditioning Stressors increase the release of corticosterone –Adrenalectomy prevents facilitation of eyeblink conditioning –Effect still seen 24 hrs after stressor event

Corticosterone response during a stressor is necessary to initiate a long process that mediates enhanced conditioning

Experiment 1: –Determine whether BNST is critically involved in enhanced conditioning Experiment 2: –Determine when BNST is involved During stressor During training

Experiment 1 Male Sprague-Dawley rats –60-90 days old –Housed singly –Food and water ad lib –12 hr. light dark cycle

Experiment 1 Cannula implanted into BNST to lesion or sham lesion 2 pairs of electrodes implanted through upper eyelid and attached to head stage –1 electrode in the muscle –1 electrode outside of the muscle CS = 83-dB white noise US = 0.40-mA eyelid shock

Eyeblinks defined as a change in electromyographic activity Stressor: –restraint tube in a dark sound attenuating chamber with 30 tail shocks at 1 per minute –24 h before training

Experiment 1 Eyeblink conditioning: –Establish baseline for msec. trials –10: 250 msec CS only and blinks recorded for ½ sec. Afterward Trace conditioning: 200 trials for 2 days –CS separated by periorbital US by 500 msec interval –Volume and intensity chosen to produce a slow rate of acquisition

Experiment 1 Trials occurred in sets of 10: –1: CS alone –4: paired trials –1: US alone –4: paired trials Conditioned Responses (CR) = –Eyeblinks occurring 500 msec after paired trials –Eyeblinks occurring 750 msec after CS

Experiment 1 Total rats: 27 –6 unstressed sham –7 stressed sham –7 unstressed lesioned –7 stressed lesioned

Experiment 2 Cannulae implanted to temporarily inactivate the BNST with Muscimol –GABA A receptor agonist

Experiment 2 Trained at 100 trials per day –Group 1= inactivation before stressor and vehicle before each training –Group 2 = vehicle before stressor and inactivation before training –Group 3 = vehicle before stressor and training –Group 4 = no stress, but muscimol on the day would have stress and vehicle before each training

Experiment 2 –Verify inactivation did not inhibit eyeblink conditioning, whether affected acquisition of CR Low responders chosen and given 100 additional trials with BNST inactivated

Experiment 2 Examining: –Effects of muscimol in BNST in those who already learned task Exposed to 100 additional trials If inactivation disrupts performance, the CR would decrease

Results Experiment 1 –Assess whether BNST lesions prevent stress- induced facilitation of eyeblink conditioning. Before training there was no effect on baseline eyeblinks with BNST lesion

Results – Experiment 1 Stressed with sham surgeries higher percentage of CRs than unstressed with sham Stressed with lesion to BNST had smaller percentage of CRs than stressed with sham –Not different from unstressed with sham Unstressed with lesions had fewer CRs than stressed after a sham surgery

Experiment 1 No group differences until 300 trials

Experiment 1 Sham surgery + stressor = more CR With BNST lesion and stressor similar to unstressed BNST neuronal activity necessary to facilitate trace conditioning after stress

Results - Experiment 2 Whether BNST is involved during the stressor or 24 h later during training. –BNST inactivation did not alter baseline blink rate or response to CS before training –No group differences until 200 trials

Experiment 2 Stressed rats infused with vehicle had more CR than unstressed, but not significant Exposed to stressor and injected with vehicle = more CR BNST inactivation during stressor = more CR BNST inactivation during training = no increase

Experiment 2 Suggests BNST necessary for enhanced conditioning after stress BNST inactivation prevented enhancement of eyeblink conditioning after stress but only when inactivation during training

Experiment 2 BNST inactivation during training with high intensity US rapidly acquire CR and similar to unstressed without inactivation Both groups acquire more rapidly with higher intensity

Experiment 2 BNST inactivation does not prevent expression of CR, but low responses could be from muscimol preventing the acquistion of eyeblink conditioning In the presence of muscimol allowed acquisiton of CR at higher US intensity and perform similarly to unstressed controls

Experiment 2 Another subset reexposed to stressor 5 days later and blood collected BNST inactivation not prevent corticosterone response to stressor

Experiment 2 BNST inactivation during stressor continue to release corticosterone and not different from stressed with vehicle infusion

Therefore, Professor Predator Says

Discussion BNST critically mediates enhanced conditioning BNST inactivation during stressor did not prevent stress-induced enhancement of conditioning, but BNST inactivation during training did prevent stress enhancement

Discussion BNST involved in stress-induced modulation of learning, but not learning itself Critically mediates persistent enhancement of conditioning after exposure to acute stress BNST does not prevent corticosterone response

Discussion Amygdala involved in enhanced conditioning after stressful experience BNST not involved during stressor, but necessary during training Critical but both involved at different time points

Discussion Additional structures: –Hippocampus –Cerebellum Both highly connected with BNST and both necessary for trace conditioning

Discussion Cerebellum –Connected to BNST by direct projections to Pons to relay sensory information about CS to cerebellum Stress exposure may alter the way CS is processed during training through connections of BNST to pontine nuclei

Cerebellum Pathway

Discussion BNST is also critically involved in other stress effects on learning –Lesions prevent the “learned helplessness” effect

Discussion Theorized the BNST mediates stress effects through anxiety –Exposure to acute stress induces long-lasting state of anxiety The state of anxiety may enhance an animal’s ability to form basic associative memories

The End