Kappa-opioid antagonism impairs forebrain-dependent associative learning: A trace eyeblink conditioning study

The opioid receptor system is well known for its relationship to painful stimuli but has also been discovered to have a role in acquisition and consolidation of associative memories. Most opioid receptor specific studies have focused on, and attributed these findings to, modulation of the mu-opioid receptor (MOR); however, some studies have suggested that the kappa-opioid receptor (KOR) also plays in role in memory modulation. The following study set out to determine KOR involvement in acquisition for forebrain-dependent associations. Using the forebrain-dependent associative task whisker-trace eyeblink conditioning (WTEB), the current study demonstrated that KOR inhibition via NorBNI (10 mg/kg) significantly delayed acquisition. To explore the brain region mediating these NorBNI-induced learning impairments, subsequent experiments focused on primary somatosensory cortex (S1). S1 plays a pivotal role in the acquisition of WTEB with lesions either before or after conditioning inhibiting acquisition or retrieval respectively. NorBNI (10 μg or 20 μg) in S1 was found to significantly delay acquisition, similar to that observed following systemic injections. In support of these findings, studies have suggested a role for dynorphin (KOR's endogenous ligand) expressing GABAergic interneurons in cortical processing of whisker information. Although, additional studies will be required to determine the specific mechanism for KOR and these GABAergic interneurons; these findings strongly support previous studies suggesting KOR involvement in learning mechanisms, while elucidating an unexplored neocortical learning mechanism.