Anxiety disorders are often associated with an inability to extinguish learned fear responses. The hypocretin/orexin system is involved in the regulation of emotional states and could also participate in the consolidation and extinction of aversive memories. Using hypocretin receptor-1 and hypocretin receptor-2 antagonists, hypocretin-1 and hypocretin-2 peptides, and hypocretin receptor-1 knockout mice, we investigated the role of the hypocretin system in cue- and context-dependent fear conditioning and extinction. Hypocretins were crucial for the consolidation of fear conditioning, and this effect was mainly observed in memories with a high emotional component. Notably, after the acquisition of fear memory, hypocretin receptor-1 blockade facilitated fear extinction, whereas hypocretin-1 administration impaired this extinction process. The extinction-facilitating effects of the hypocretin receptor-1 antagonist SB334867 were associated with increased expression of cFos in the basolateral amygdala and the infralimbic cortex. Intra-amygdala, but neither intra-infralimbic prefrontal cortex nor intra-dorsohippocampal infusion of SB334867 enhanced fear extinction. These results reveal a key role for hypocretins in the extinction of aversive memories and suggest that hypocretin receptor-1 blockade could represent a novel therapeutic target for the treatment of diseases associated with inappropriate retention of fear, such as post-traumatic stress disorder and phobias.

Background: Rats emit 50-kHz ultrasonic vocalizations (USVs) in response to either pleasurable natural or pharmacological stimuli, and these USVs have emerged as a new behavioral measure for investigating the motivational properties of drugs. Earlier studies have indicated that activation of the dopaminergic system is critically involved in 50-kHz USVs emission. However, evidence also exists that non-dopaminergic neurotransmitters participate in this behavioral response. Methods: To ascertain whether glutamate transmission plays a role in 50-kHz USVs emission stimulated by amphetamine, rats received five amphetamine (1–2 mg/kg, i.p.) administrations on alternate days in a test-cage, alone or combined with the glutamate N­methyl-D-aspartate (NMDA) receptor antagonist MK-801 (0.1–0.5 mg/kg, i.p.). Seven days after treatment discontinuation, rats were re-exposed to the test-cage to assess drug-conditioning, and afterwards received a drug challenge. USVs and locomotor activity were evaluated, along with immunofluorescence for Zif-268 in various brain regions and spontaneous alternation in a Y maze. Results: Amphetamine-treated rats displayed higher 50-kHz USVs emission and locomotor activity than vehicle-treated rats, and emitted conditioned vocalizations on test-cage re-exposure. Rats coadministered amphetamine and MK-801 displayed lower and dose-dependent 50-kHz USVs emission, but not lower locomotor activity, during repeated treatment and challenge and scarce conditioned vocalization compared with amphetamine-treated rats. These effects were associated with lower levels of Zif-268 after amphetamine challenge and spontaneous alternation deficits. Conclusions: These results indicate that glutamate transmission participates in the acute, long-term, and conditioned effects of amphetamine on 50-kHz USVs, possibly by influencing amphetamine-induced long-term neuronal changes and/or amphetamine-associated memories.