Novelty-induced arousal has motivational effects and can reinforce behavior. The mechanisms by which novelty acts as a reinforcer are unknown. Novelty-induced arousal can be either rewarding or aversive dependent on its intensity and the preceding state of arousal. The brain's histamine system has been implicated in both arousal and reinforcement. Histamine and histamine-1-receptor (H1R) agonists induced arousal and wakefulness in humans and rodents, e.g. by stimulating cortical acetylcholine (ACh) release. The H1R has also been implicated in processes of brain reward via interactions with the nigrostriatal- and mesolimbic dopamine (DA) systems. We asked whether the motivational effects of novelty-induced arousal are compromised in H1R knockout (KO) mice. The H1R-KO mice failed to develop a conditioned place-preference induced by novel objects. Even though they still explore novel objects, their reinforcing value is diminished. Furthermore, they showed impaired novelty-induced alternation in the Y-maze. Rearing activity and emotional behavior in a novel environment was also altered in H1R-KO mice, whereas object-place recognition was unaffected. The H1R-KO mice had higher ACh concentrations in the frontal cortex and amygdala (AMY). In the latter, the H1R-KO mice had also increased levels of DA, but a lower dihydrophenylacetic acid/DA ratio. Furthermore, the H1R-KO mice had also increased tyrosine hydroxylase immunoreactivity in the basolateral anterior, basolateral ventral and cortical AMY nuclei. We conclude that the motivational effects of novelty are diminished in H1R-KO mice, possibly due to reduced novelty-induced arousal and/or a dysfunctional brain reward system.
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