Holmes Summary PDF: Basolateral Amygdala & Perirhinal Cortex

The Basolateral Amygdala Is Critical for Learning about Neutral Stimuli in the Presence of Danger, and the Perirhinal Cortex Is Critical in the Absence of Danger (Holmes, et al.) Main focus: the roles of the perirhinal cortex (PRh) and basolateral amygdala (BLA) in forming and extinguishing associa...

The Basolateral Amygdala Is Critical for Learning about Neutral Stimuli in the Presence of Danger, and the Perirhinal Cortex Is Critical in the Absence of Danger (Holmes, et al.) Main focus: the roles of the perirhinal cortex (PRh) and basolateral amygdala (BLA) in forming and extinguishing associations between two environmental stimuli in rats First-order conditioning: pairing a neutral stimulus (tone — S1) with an US (shock) to elicit a conditioned response (fear). The neutral stimulus becomes a CS that triggers the response on its own. Second-order conditioning: when a previously CS (tone from first-order conditioning — S1) is paired with another neutral stimulus (light). The new neutral stimulus (light — S2) eventually elicits the conditioned response, even without being directly paired with the US. Behavioural Procedures:: S1 = tone S2 = light Fear Pre-conditioning: Rats trained by pairing 2 neutral stimuli, light (S1) and tone (S2) to study associative learning under different emotional contexts First-order conditioning (danger contexts): Rats trained to associate danger (shock — US) to the tone (S1) Second-order conditioning (danger contexts): Rats trained to associate a danger cue (the tone — S1) to the to a neutral cue (light — S2) for it to also become a cue to predict danger Extinction Procedures: the tone (S1) was unpaired to the light (S2) to study the ability to extinguish learned associations Experimental Conditions: PP: CS2 (light) + CS1 (tone) > CS1 (tone) + shock = assoc. between light & danger PU: CS2 (light) + CS1 (tone) > CS1 (tone) w/o shock = no assoc. between light & danger UP: CS2 (light) W/O CS1 (tone) > CS1 (tone) + shock = no assoc. between light & danger Neuroscientific Procedures: GABAA Receptor Agonist (Muscimol) Infusion: temporarily inactivates either the perirhinal cortex (PRh) or the basolateral amygdala (BLA) to assess their roles in associative learning.  NMDA Receptor Antagonist (Ifenprodil) Infusion: block NMDA receptors in the PRh or BLA to determine the involvement of glutamatergic signaling in learning and extinction Results: Exp 1A: Sensory pre-conditioned fear Increased S2 freezing specifically due to learned associations to the shock Exp 1B: association between S2 and neutral S1 requires PRh but not the BLA PRh-MUS group froze less to S2 compared to controls = PRh crucial for forming S2-S1 associations when both stimuli are neutral Impairment specific to sensory preconditioning BLA-MUS did not impair freezing to S2 = BLA not necessary for S2-S1 association under neutral conditions PRh-IFEN group showed reduced freezing to S2 compared to the VEH group = NMDAR blocking impaired the S2-S1 association Impairment specific to sensory preconditioning Experiment 2: extinction of the association between S2 and neutral S1 requires NMDAR neurotransmission in the PRh PRh inactivation or NMDAR disruption impaired extinction of the S2–S1 association NMDAR neurotransmission in the PRh is critical for the extinction of the S2–S1 association, while BLA is not. Experiment 3A: demonstration of second-order conditioned fear Freezing to S2 driven by its association with S1 and the prior S1–US pairings Experiment 3B: the association between S2 and the conditioned S1 requires BLA but not the PRh 2nd order conditioning: PRh-MUS and BLA-MUS showed reduced freezing to S2 and S1 compared to the VEH VEH and PRh-MUS groups froze more to S2 than the BLA-MUS Freezing to S2 is dependent on BLA The PRh and BLA have distinct roles in forming S2–S1 associations: PRh: supports associations when S1 is neutral but not when conditioned BLA: critical associations when S1 is conditioned but not when neutral Experiment 4A: extinction of sensory preconditioned fear does not require the PRh or NMDAR neurotransmission in the PRh No impairment of extinction was observed due to PRh inactivation or disrupted NMDA neurotransmission, and retention of first-order conditioned fear to S1 remained unaffected. Experiment 4B: extinction of sensory preconditioned fear requires the BLA and NMDAR neurotransmission in the BLA BLA-MUS showed significantly reduced freezing to S2 compared to EXT-VEH and EXT-IFEN Both BLA-MUS and BLA-IFEN showed impaired extinction, with lower freezing to S2 than VEH The roles of the PRh and BLA in extinction are doubly dissociable PRh: supports extinction when S1 is neutral but not when conditioned BLA: supports extinction when S1 has been conditioned but not when neutral Experiment 5A: demonstration of sensory preconditioned fear in a dangerous context Freezing to S2: Group PP showed significantly more freezing to S2 compared to Groups PU and UP PP associates S2 to S1, which was associated to shocks Freezing to S1: Group PU froze significantly less to S1 than Groups PP and UP PU associates S2 to S1 only (no fear conditioning) Experiment 5B: in a dangerous context, the association between S2 and a neutral S1 requires the BLA but not the PRh S2–S1 Pairings: VEH froze more than PRh-MUS and BLA-MUS rats, whose freezing levels did not differ Testing with S2: BLA-MUS froze less to S2 than VEH and PRh-MUS Results: BLA-VEH froze more to S2 than BLA-IFEN group = NMDAR neurotransmission in the BLA is crucial for associating S2 and S1 in a dangerous context The roles of the PRh and BLA in extinction are doubly dissociable BLA is essential for forming S2–S1 associations in a dangerous context PRh is essential for forming S2–S1 associations in a neutral context Experiment 6: in a dangerous context, extinction of an association between S2 and a neutral S1 requires NMDAR neurotransmission in the BLA but does not require the PRh BLA-MUS and BLA-IFEN showed significantly more freezing to S2 compared to VEH and PRh-MUS Extinction of the S2–S1 association in a dangerous context required the BLA, specifically NMDAR neurotransmission PRh was not necessary for extinction in a dangerous context. The roles of the PRh and BLA in extinction are context-dependent: BLA is crucial in dangerous context PRh is not necessary in safe context Summary PRh and BLA Functions: PRh: Critical for forming associations between two neutral stimuli, S2 and S1, via NMDAR neurotransmission PRh: crucial for extinction of neutral S2–S1 associations BLA: Critical for associating S2 with a conditioned S1, via NMDAR neurotransmission BLA: crucial for extinction of fear responses triggered by second-order or sensory preconditioned S2 Emotional significance & Context PRh: Neutral S1 in Safe Context BLA: Fear-eliciting S1 or Dangerous Context via NMDAR transmission Distinct Neural Pathways: Cortical Pathway (PRh Involvement): Supports detailed sensory processing and associations between neutral stimuli Subcortical Pathway (BLA Involvement): Facilitates rapid defensive reactions and processing of fear-associated stimuli Chaining process: linking the separate S2-S1 and S1-US associations together Mediated conditioning: S1 makes you think about S2 while paired with US so it becomes linked to US The PRh is involved in associative learning S1-S2 when both are neutral and 2nd order includes a shock, the association shifts to the BLA in a dangerous context.