The role of the prefrontal cortex, amygdala and the claustrum in regulating fear through the perifornical lateral hypothalamus

Abstract

The perifornical lateral hypothalamus (PeF-LH) is a region in the hypothalamus that has been implicated in various physiological and cardiovascular functions. Traditionally, the PeF-LH has been recognised for having a role in feeding behaviour and cardiovascular responses to stressful situations, but in recent years the functions of the PeF-LH have been extended to include higher-order functions, including learning and memory. The neural circuitry that underlies fear learning is known to centre on the prefrontal cortex (PFC) and the amygdala which are responsible for learning and orchestrating fear expression. The aim of this thesis was to determine whether regions within the PFC and amygdala that send projections to the PeFLH are active during contextual control of fear learning in rats using a procedure called ABA fear renewal. The rats were first injected with CTb retrograde tracer into the PeF-LH, and after fear conditioning and extinction in two separate contexts, fear renewal tests were conducted. Rats showed fear renewal at test with significantly greater freezing for the fear group tested in the fear conditioning context (renewal group) than the rats tested in the extinction context (extinction group). The brains were then processed for CTb and c-Fos expression using standard immunohistochemical procedures. It was found that projection neurons from the PeFLH were present in all the regions analysed in the PFC (prelimbic cortex, infralimbic cortex, lateral (orbital and insular) cortex), the central amygdala and the claustrum. Analysis of c-Fos alone revealed that was greater c-Fos for the two tested groups (fear and extinction groups) compared to the control group that were not tested in the prelimbic area (and no differences for any other region). This suggests a general role for the prelimbic area in arousal. Analysis of double labelling revealed less c-Fos labelling in CTb cells in the orbital and insular regions and the claustrum for the two tested groups compared to the control group. This suggests that these lateral PFC projections to the PeF-LH are inhibited during arousal. A lack of difference between the fear and extinction groups in c-Fos expression was unexpected and perhaps suggests the parameters were not ideal (for instance, the tests could have been longer as c-Fos is stronger with longer and stronger stimuli). In any case, the current study suggests that lateral PFC may regulate levels of arousal in the PeF-LH region, which highlights the importance of this understudied projection.

Date of Thesis submission	2025
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Sam Merlin (Supervisor) & Teri Furlong (Supervisor)