Adenosine A2A receptors (A2ARs) are enriched in the striatum but are also present at lower levels in the extra-striatal forebrain (i.e., hippocampus, cortex), integrating dopamine, glutamate, and brain-derived neurotrophic factor (BDNF) signaling, and are thus essential for striatal neuroplasticity and fear and anxiety behavior.
We tested two brain region-specific A2AR knockout lines with A2ARs selectively inactivated either in the striatum only (st-A2AR KO) or the entire forebrain (striatum, hippocampus, and cortex, fb-A2AR KO) on fear and anxiety-related responses. We also examined the effect of hippocampus-specific A2AR deletion by local injection of AAV5-Cre into conditional (floxed)-A2AR knockout mice.
Selective deletion of striatal A2ARs in st-A2AR KO mice increased Pavlovian fear conditioning (both context and tone), but when the A2AR deletion was extended to include extra-striatal regions in fb-A2AR KO mice, context fear conditioning was normalized and tone fear conditioning was attenuated. Moreover, focal deletion of hippocampal A2ARs by AAV5-Cre injection selectively attenuated context (but not tone) fear conditioning. Deletion of A2ARs in the entire forebrain in fb-A2AR KO mice also produced an anxiolytic phenotype in both the elevated plus maze and open field tests and increased the startle response. These extra-striatal forebrain A2AR behavioral effects were associated with reduced BDNF levels in the fb-A2AR KO hippocampus.
This study provides the first evidence that inactivation of striatal A2ARs facilitates Pavlovian fear conditioning while inactivation of extra-striatal A2ARs in the forebrain inhibits fear conditioning and also affects anxiety-related behavior.