Midazolam reduces the selective activation of the rhinal cortex by contextual fear stimuli.
Independent brain circuits appear to underlie different forms of conditioned fear, depending on the type of conditioning used, such as a context or explicit cue paired with footshocks. Several clinical reports have associated damage to the medial temporal lobe (MTL) with retrograde amnesia. Although a number of studies have elucidated the neural circuits underlying conditioned fear, the involvement of MTL components in the aversive conditioning paradigm is still unclear. To address this issue, we assessed freezing responses and Fos protein expression in subregions of the rhinal cortex and ventral hippocampus of rats following exposure to a context, light or tone previously paired with footshock (Experiment 1). A comparable degree of freezing was observed in the three types of conditioned fear, but with distinct patterns of Fos distribution. The groups exposed to cued fear conditioning did not show changes in Fos expression, whereas the group subjected to contextual fear conditioning showed selective activation of the ectorhinal (Ect), perirhinal (Per), and entorhinal (Ent) cortices, with no changes in the ventral hippocampus. We then examined the effects of the benzodiazepine midazolam injected bilaterally into these three rhinal subregions in the expression of contextual fear conditioning (Experiment 2). Midazolam administration into the Ect, Per, and Ent reduced freezing responses. These findings suggest that contextual and explicit stimuli endowed with aversive properties through conditioning recruit distinct brain areas, and the rhinal cortex appears to be critical for storing context-, but not explicit cue-footshock, associations.
Albrechet-Souza
Lucas
L
Instituto de Neurociências & Comportamento, Campus USP, Ribeirão Preto, SP, Brazil. lucasas@pg.ffclrp.usp.br
Borelli
Karina G
KG
Almada
Rafael C
RC
Brandão
Marcus L
ML
D000704:Analysis of Variance; D000818:Animals; D001245:Association Learning / Q000187:drug effects; D001362:Avoidance Learning / Q000187:drug effects*; D001931:Brain Mapping; D003214:Conditioning, Classical / Q000187:drug effects*; D005239:Fear; D046352:Freezing Reaction, Cataleptic / Q000187:drug effects*; D018757:GABA Modulators / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D007150:Immunohistochemistry; D008297:Male; D008874:Midazolam / Q000494:pharmacology*; D009434:Neural Pathways / Q000187:drug effects / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013702:Temporal Lobe / Q000187:drug effects* / Q000378:metabolism; D014018:Tissue Distribution
D018757:GABA Modulators; D016760:Proto-Oncogene Proteins c-fos; D008874:Midazolam