Akihiro Yamanaka

Orexin neurons regulate physiological functions, such as energy homeostasis, wakefulness, and motivated behaviors. However, studies linking orexin neuron activity to behavior via selectively activating/inactivating these inputs in a temporally controlled manner in rats are scarce. Here, we examined the role that orexin neurons play in motivated behavior in transgenic rats using cell type–specific fiber photometry and optogenetic manipulation. Using chemogenetics, we found that motivation for a reward increased when orexin neurons were activated. Furthermore, during motivated behavior, orexin neuron activity changed dynamically: Activity increased during reward prediction and decreased after reward receipt. When an unexpected event occurred (i.e., not obtaining an expected reward), increased orexin activity was sustained. Notably, orexin activity strengthened with increasing effort. Optogenetic inhibition of orexin neuron activation during reward prediction and treatment with an orexin 1-receptor antagonist reduced reward-seeking behavior. Therefore, orexin is crucial for linking predicted expectations with motivated behavior. Moreover, optimization of orexin activity is necessary to overcome difficulties during motivated behaviors.

Orexin/hypocretin neurons regulate sleep/wakefulness, feeding, arousal, and motivated behavior. Genetic access to these neurons commonly relies on orexin-Cre mouse lines and local stereotaxic viral delivery, whereas compact promoter-based approaches could enable less invasive and more flexible targeting strategies in wild-type animals. Here, we engineered shortened human prepro-orexin promoter variants by deleting Alu-rich sequences between orexin-regulatory elements OE1 and OE2. Using systemic AAV2/PHP.eB delivery in adult C57BL/6J mice, we compared the original 3.2-kb promoter with three compact variants: 2.2hOX, 1.2hOX, and 0.4hOX. The original 3.2hOX promoter labeled 43.2% of orexin-immunoreactive neurons with 96.1% specificity. In contrast, 2.2hOX achieved 96.5% coverage with 97.6% specificity, while 1.2hOX and 0.4hOX also maintained high coverage and specificity. The 2.2hOX promoter further supported functional expression of jGCaMP8s after local LHA delivery, enabling miniature two-photon imaging of orexin-neuron calcium dynamics in freely moving mice. These compact human prepro-orexin promoters expand AAV cargo capacity while preserving selective targeting of orexin neurons, providing practical tools for orexin-neuron labeling and functional studies in adult mice.

Animals remain awake in unfamiliar environments to assess potential safety threats, a process involving changes in neuronal activity within sleep–wake regulatory brain regions. However, the specific circuits and neurotransmitters involved remain poorly understood. Here, we show that neurotensin (NTS) peptides in corticotropin-releasing factor (CRF) neurons of the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure (IPACL) play a key role in maintaining wakefulness in response to environmental changes. Activation of IPACL ^CRF neurons increased wakefulness, whereas their inhibition or deletion of NTS reduced wakefulness in novel environments. These neurons are activated in response to exposure to a novel environment and project primarily to the substantia nigra pars reticulata (SNr) and release NTS, which modulates wakefulness. These findings suggest that NTS signaling from IPACL ^CRF neurons to the SNr is essential for sustaining wakefulness in unfamiliar or changing environments.

Opsin 5 (OPN5), also known as neuropsin, is a violet/ultraviolet (UV) light-sensitive G protein-coupled receptor (GPCR) conserved across vertebrates. Most mammals possess a single OPN5 gene ( OPN5m ), whereas non-mammalian species also express OPN5L1 and OPN5L2 with distinct molecular properties. Mammalian OPN5 (OPN5m) functions as a non-visual photopigment, expressed in diverse extra-retinal tissues including the skin, testis, and brain. Recent studies reveal species-specific signaling: human OPN5m preferentially activates Gq-mediated Ca ² ⁺ signaling, mouse OPN5m couples with Gi to reduce cyclic adenosine monophosphate (cAMP), avian OPN5m engages either Gi or Gq depending on species and tissue, and amphibian/fish OPN5m homologs primarily signal through Gq pathways. These diverse signaling modes underlie a wide range of physiological functions, such as circadian photoentrainment, thermoregulation, vascular development, myopia suppression, corneal wound healing, seasonal reproduction in birds, and light-dependent hormone release in fish pituitary. As modern artificial lighting and indoor lifestyles limit violet light exposure, insufficient OPN5m activation may contribute to emerging health issues, particularly the global rise in myopia. This review provides an updated overview of the molecular diversity, expression patterns, signaling mechanisms, and physiological roles of OPN5m across species, and discusses its potential clinical relevance in the context of changing light environments.

Orexin neurons regulate physiological functions, including not only energy homeostasis and wakefulness but also motivated behaviors. These neurons play important roles in linking metabolic requirements to such behaviors. However, it is unknown whether these neurons are involved in reward-based choice behavior. We recently developed a transgenic rat line that expresses Cre recombinase exclusively in orexin neurons. Here, we examined the roles of orexin neurons in reward choice behavior under conditions of uncertainty in a gambling test in rats and analyzed the changes in choice behavior when orexin neurons were manipulated using chemogenetic approaches. In the gambling test, chemogenetic activation of orexin neurons resulted in risky arm choice. Positive, but not negative, reward-prediction error contributed to reward-based risky choice when orexin neurons were activated. These risky choice behaviors were canceled by suvorexant, an orexin receptor antagonist. In addition, suvorexant administered alone led to conservative choice behavior. The computational approach revealed that orexin/orexin neurons modulate the learning process and motivational value of rewards. Thus, our data suggest that orexin/orexin neurons affect the processing of reward-prediction error and alter strategy in reward-based decision-making.

Melanin-concentrating hormone-producing neurons (MCH neurons), found mainly in the lateral hypothalamus and surrounding areas, play essential roles in various brain functions, including sleep and wakefulness, reward, metabolism, learning, and memory. These neurons coexpress several neurotransmitters and act as glutamatergic neurons. The contribution of glutamate from MCH neurons to memory- and metabolism-related functions has not been fully investigated. In a mouse model, we conditionally knocked out Slc17a6 gene, which encodes for vesicular glutamate transporter 2 (vGlut2), in the MCH neurons exclusively by using two different methods: the Cre recombinase/loxP system and in vivo genome editing using CRISPR/Cas9. Then, we evaluated several aspects of memory and measured metabolic rates using indirect calorimetry. We found that mice with MCH neuron-exclusive vGlut2 ablation had higher discrimination ratios between novel and familiar stimuli for novel object recognition, object location, and three-chamber tests. In contrast, there was no significant change in body weight, food intake, oxygen consumption, respiratory quotient, or locomotor activity. These findings suggest that glutamatergic signaling from MCH neurons is required to regulate memory, but its role in regulating metabolic rate is negligible.

Alzheimer’s disease is a devastating disease that is accompanied by dementia, and its incidence increases with age. However, no interventions have exhibited clear therapeutic effects. We aimed to develop and characterize behavioural tasks that allow the earlier identification of signs preceding dementia that would facilitate the development of preventative and therapeutic interventions for Alzheimer’s disease. To this end, we developed a 3D virtual reality task sensitive to the activity of grid cells in the entorhinal cortex, which is the region that first exhibits neurofibrillary tangles in Alzheimer’s disease. We investigated path integration (assessed by error distance) in a spatial navigation task sensitive to grid cells in the entorhinal cortex in 177 volunteers, aged 20–89 years, who did not have self-reported dementia. While place memory was intact even in old age, path integration deteriorated with increasing age. To investigate the relationship between neurofibrillary tangles in the entorhinal cortex and path integration deficit, we examined a mouse model of tauopathy (P301S mutant tau-overexpressing mice; PS19 mice). At 6 months of age, PS19 mice showed a significant accumulation of phosphorylated tau only in the entorhinal cortex, associated with impaired path integration without impairments in spatial cognition. These data are consistent with the idea that path integration deficit is caused by the accumulation of phosphorylated tau in the entorhinal cortex. This method may allow the early identification of individuals likely to develop Alzheimer’s disease.

The enhancement of insulin secretion and of the proliferation of pancreatic β cells are promising therapeutic options for diabetes. Signals from the vagal nerve regulate both processes, yet the effectiveness of stimulating the nerve is unclear, owing to a lack of techniques for doing it so selectively and prolongedly. Here we report two optogenetic methods for vagal-nerve stimulation that led to enhanced glucose-stimulated insulin secretion and to β cell proliferation in mice expressing choline acetyltransferase-channelrhodopsin 2. One method involves subdiaphragmatic implantation of an optical fibre for the photostimulation of cholinergic neurons expressing a blue-light-sensitive opsin. The other method, which suppressed streptozotocin-induced hyperglycaemia in the mice, involves the selective activation of vagal fibres by placing blue-light-emitting lanthanide microparticles in the pancreatic ducts of opsin-expressing mice, followed by near-infrared illumination. The two methods show that signals from the vagal nerve, especially from nerve fibres innervating the pancreas, are sufficient to regulate insulin secretion and β cell proliferation.

Narcolepsy is a sleep disorder caused by deficiency of orexin signaling. However, the neural mechanisms by which deficient orexin signaling causes the abnormal rapid eye movement (REM) sleep characteristics of narcolepsy, such as cataplexy and frequent transitions to REM states, are not fully understood. Here, we determined the activity dynamics of orexin neurons during sleep that suppress the abnormal REM sleep architecture of narcolepsy. Orexin neurons were highly active during wakefulness, showed intermittent synchronous activity during non-REM (NREM) sleep, were quiescent prior to the transition from NREM to REM sleep, and a small subpopulation of these cells was active during REM sleep. Orexin neurons that lacked orexin peptides were less active during REM sleep and were mostly silent during cataplexy. Optogenetic inhibition of orexin neurons established that the activity dynamics of these cells during NREM sleep regulate NREM–REM sleep transitions. Inhibition of orexin neurons during REM sleep increased subsequent REM sleep in “orexin intact” mice and subsequent cataplexy in mice lacking orexin peptides, indicating that the activity of a subpopulation of orexin neurons during the preceding REM sleep suppresses subsequent REM sleep and cataplexy. Thus, these results identify how deficient orexin signaling during sleep results in the abnormal REM sleep architecture characteristic of narcolepsy.