The neural mechanisms underlying memory regulation during sleep are not yet fully understood. We found that melanin concentrating hormone–producing neurons (MCH neurons) in the hypothalamus actively contribute to forgetting in rapid eye movement (REM) sleep. Hypothalamic MCH neurons densely innervated the dorsal hippocampus. Activation or inhibition of MCH neurons impaired or improved hippocampus-dependent memory, respectively. Activation of MCH nerve terminals in vitro reduced firing of hippocampal pyramidal neurons by increasing inhibitory inputs. Wake- and REM sleep–active MCH neurons were distinct populations that were randomly distributed in the hypothalamus. REM sleep state–dependent inhibition of MCH neurons impaired hippocampus-dependent memory without affecting sleep architecture or quality. REM sleep–active MCH neurons in the hypothalamus are thus involved in active forgetting in the hippocampus.

In mammals, the daily rhythms of physiological functions are timed by the central circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Although the importance of the SCN for the regulation of sleep/wakefulness has been suggested, little is known about the neuronal projections from the SCN, which regulate sleep/wakefulness. Here, we show that corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus mediate circadian rhythms in the SCN and regulate wakefulness. Optogenetic activation of CRF neurons promoted wakefulness through orexin/hypocretin neurons in the lateral hypothalamus. In vivo Ca2+ recording showed that CRF neurons were active at the initiation of wakefulness. Furthermore, chemogenetic suppression and ablation of CRF neurons decreased locomotor activity and time in wakefulness. Last, a combination of optical manipulation and Ca2+ imaging revealed that neuronal activity of CRF neurons was negatively regulated by GABAergic neurons in the SCN. Our findings provide notable insights into circadian regulation of sleep/wakefulness in mammals.

The stress response is a physiological system for adapting to various internal and external stimuli. Corticotropin-releasing factor-producing neurons in the paraventricular nucleus of the hypothalamus (PVN-CRF neurons) are known to play an important role in the stress response as initiators of the hypothalamic-pituitary-adrenal axis. However, the mechanism by which activity of PVN-CRF neurons is regulated by other neurons and bioactive substances remains unclear. Here, we developed a screening method using calcium imaging to identify how physiological substances directly affect the activity of PVN-CRF neurons. We used acute brain slices expressing a genetically encoded calcium indicator in PVN-CRF neurons using CRF-Cre recombinase mice and an adeno-associated viral vector under Cre control. PVN-CRF neurons were divided into ventral and dorsal portions. Bath application of candidate substances revealed 12 substances that increased and 3 that decreased intracellular calcium concentrations. Among these substances, angiotensin II and histamine mainly increased calcium in the ventral portion of the PVN-CRF neurons via AT 1 and H 1 receptors, respectively. Conversely, carbachol mainly increased calcium in the dorsal portion of the PVN-CRF neurons via both nicotinic and muscarinic acetylcholine receptors. Our method provides a precise and reliable means of evaluating the effect of a substance on PVN-CRF neuronal activity. Corticotropin-releasing factor (CRF)-producing neurons in the paraventricular nucleus of the hypothalamus (PVN-CRF neurons) are known to have various physiological functions. For example, these neurons play a central role in stress responses through the hypothalamic-pituitary-adrenal axis (HPA-axis) 1. In the HPAaxis, PVN-CRF neurons release CRF into the median eminence to enhance secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary into systemic circulation in response to stress. Reaching the adrenal cortex, ACTH induces secretion of glucocorticoids (GCs), such as corticosterone, to initiate various stress responses. Thus, PVN-CRF neurons are thought to serve as initiators of the HPA-axis. Recently, PVN-CRF neurons have also been implicated in the encoding of opposing valence, where increased and decreased neural activity encodes negative and positive valence, respectively 2. PVN-CRF neurons are also reported to partially co-express vasopressin 3-5 , oxytocin 6 , neurotensin 7 , enkephalin 7 and cholecystokinin 8. PVN-CRF neurons are thought to be primarily glutamatergic but also partially GABAergic 9 , they receive inputs from various brain regions such as the nucleus of the solitary tract 10 , and send projections to the median eminence 11 .