Muscimol Infusions to Infralimbic Cortex Impair Extinction but Not Acquisition of a Trace Eyeblink Conditioned Response in Rabbits

This study assessed the effects of reversible lesions with microinfusions of the GABAA agonist muscimol (MUSC) to infralimbic cortex (IL; Brodmann’s area 25) of the medial prefrontal cortex (mPFC) on trace eyeblink conditioning and extinction in rabbits (Oryctolagus cuniculus). Four groups were tested: rabbits receiving MUSC infusions 5 min before acquisition and extinction sessions (MUSC/MUSC group), rabbits receiving vehicle (VEH) 5 min before acquisition and extinction sessions (VEH/VEH group), rabbits receiving MUSC before acquisition and VEH before extinction sessions (MUSC/VEH group), and rabbits receiving VEH before acquisition and MUSC before extinction sessions (VEH/MUSC). Results revealed that MUSC infusions to IL had no effects on acquisition but retarded extinction when injected before either acquisition or extinction sessions. These findings are among the first to demonstrate that MUSC infusions to the IL disrupt extinction of an EB CR in rabbits, and are in agreement with data from rodent studies noting the critical role of IL for the extinction of aversive fear-conditioned responses.