Histamine in the central nervous system is produced mainly in the tuberomammillary nucleus (TMN) and is implicated in learning and memory as well as sleep and wakefulness, feeding and drinking, and neuroendocrine regulation. Previously, we demonstrated that and promote the recall of forgotten object memories. However, whether activation of histamine neurons influences memory recall and what kind of physiological stimulus activates histamine neurons are not clear. In this study, first, we examined whether chemogenetic activation of histamine neurons in the TMN affect retrieval of forgotten object and spatial and fear memories in mice. The pre-test activation of histamine neurons promoted retrieval of object and spatial memories. Next, we used fiber photometry to measure the activity of histamine neurons. We virally targeted GCaMP6, a genetically-encoded calcium indicator, was Cre-dependently expressed into histamine neurons in the TMN of HDC-Cre mice, and implanted an optic fiber was implanted in the upper part of the TMN. We found that various physiological stimuli, including auditory stimulus and sucrose presentation, increased activity of histamine neurons. These physiological stimuli may influence memory performance through activation of histamine neurons.
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