Optogenetic inhibition of the basolateral amygdala influences hippocampal endocannabinoid modulation of fear memory retrieval in rats

Introduction: Cannabinoid receptors are highly expressed in cortico-limbic regions, where they modulate both excitatory and inhibitory signaling within specific neuronal circuits implicated in learning and memory processes for emotionally arousing experiences. Endocannabinoids are crucial modulators of the stress response, dampen excessive retrieval and facilitate extinction. Here, we aimed at evaluating whether the endocannabinoids anandamide (AEA) and 2-arachidonoylglycerol (2-AG), in the CA1 region of the dorsal hippocampus, differentially regulate fear memory retrieval depending on the environment-associated emotional arousal and if pharmacologic or optogenetic inactivation of the basolateral amygdala (BLA) influences the hippocampal endocannabinoid modulation of fear memory retrieval. Methods: Male adult Sprague-Dawley rats were divided in different cohorts and tested in a Contextual Fear Conditioning (CFC) task. We evaluated the effects of bilateral intra-CA1 administration of the AEA hydrolysis inhibitor URB597 or the 2-AG hydrolysis inhibitor KML29 on freezing behavior. To investigate whether the BLA plays a role in the regulation of hippocampal endocannabinoid effect, rats were given temporal lesions of the BLA by bilateral infusion of the GABAA receptor agonist muscimol. To further characterize the BLA role in the hippocampal endocannabinoid modulation of fear memory retrieval, rats were subjected to bilateral and intermittent optogenetic BLA inhibition during retrieval. Data were analyzed with one-way or repeated measures ANOVAs, when appropriate, followed by Tukey-Kramer post-hoc test. P values < 0.05 were considered statistically significant. Results: Enhancing endogenous levels of 2-AG, but not AEA, into the dorsal CA1 field of hippocampus impaired contextual fear memory retrieval. Hippocampal 2-AG modulation of memory retrieval required an intact BLA. The decrease of contextual fear memory retrieval, induced by the inhibition of 2-AG hydrolysis in the CA1, was abolished by BLA temporal inactivation. Accordingly, optogenetic BLA inhibition reverted the impairment of contextual fear memory retrieval induced by increased endogenous levels of 2-AG. Conclusions: These data demonstrate that bidirectional inputs between the CA1 and the BLA are critical for enabling 2-AG hippocampal effects on fear memory retrieval.