The brain can flexibly adapt the physiology and behaviors of the body to specific life-stage demands. For example, sexual maturation is initiated when the maturation and nutritional state of the body are ready for reproduction. Parental caregiving behaviors to infants are facilitated around the time when their own young are expected. Specific neural activity patterns that regulate milk ejection only emerge in lactating mothers. These examples suggest the presence of neural mechanisms that plastically adjust neural input organizations and output functions based on an internal state of organisms. However, little is known as to how such adaptive plasticity is implemented at the level of neural circuits. Our research aims to elucidate the mechanisms of flexible neuronal functions associated with different life stages in mice by targeting a diverse array of brain regions, ranging from the frontal cortex to the sympathetic nervous system. We employ a combination of cutting-edge techniques, including single-cell RNAseq, virus-based circuit mapping, in vivo imaging of neural activity, and molecular and neural manipulation of specific neuron types. Through our work, we hope to provide novel insights into the plasticity of the nervous system that underlies the control of animal behavior and body physiology.

1. A Neural Basis of Malnutrition-mediated Pubertal Failure in Female Mice

Reproduction is a heavy burden, especially for mammalian females. Therefore, the acquisition of reproductive capacity is tightly regulated by the body's energy state. Both in women and rodent models, chronic energy deficiency, such as that induced by malnutrition, is associated with delayed puberty. The onset of puberty is governed by the pulsatile secretion of gonadotropin-releasing hormone (GnRH), the master regulator of gonadal functions. GnRH neurons are primarily controlled by kisspeptin neurons in the hypothalamic arcuate nucleus (ARC^kiss), the pulse generator of GnRH. Although malfunctions of ARC^kiss have been implicated in classical rodent studies under chronic energy deficiency, the neural circuit mechanisms by which fasting-related signals are transmitted to the activity dynamics of ARC^kiss remain poorly defined. Hence, we are currently exploring the activity dynamics of ARC^kiss, intending to elucidate how the systemic energy status modulates sexual maturation.

Dietary Availability Acutely Governs Puberty Onset via Hypothalamic Neural Circuit.
Goto T, Hagihara M, Irie S, Abe T, Kiyonari H, Miyamichi K. bioRxiv, (2023) doi: https://doi.org/10.1101/2023.09.15.558025

2. Regulation of Paternal Caregiving Behaviors

Male mice, before mating, exhibit aggressive behavior towards young; however, upon mating and becoming fathers, they manifest nurturing (caregiving) behaviors such as keeping the offspring warm and retrieving the offspring that have scattered from the nest. Our research focuses on investigating the role of oxytocin neurons in the paraventricular nucleus of the hypothalamus in the manifestation of these caregiving behaviors. Our findings indicate that inhibiting oxytocin synthesis in these neurons leads to neglect of the pups by the paternal mice. Conversely, activating the oxytocin neurons elicits caregiving behavior even in unmated males, suggesting that the oxytocin neurons are crucial in regulating paternal caregiving behavior. Furthermore, we discovered that paternal mice undergo changes in neural input that facilitate the activation of the oxytocin neurons, thus contributing to the emergence of caregiving behaviors.

Plasticity of Neural Connections Underlying Oxytocin-mediated Parental Behaviors of Male Mice.
Inada K, Hagihara M, Tsujimoto K, Abe T, Konno A, Hirai H, Kiyonari H, Miyamichi K. Neuron, (2022) doi: 10.1016/j.neuron.2022.03.033
See also wonderful preview by Dr. Kim: Fatherhood is life-changing: Uncovering structural and functional changes in the dad brain - PubMed (nih.gov)

2’ Satiety Formation and the Role of Oxytocin

While appetite is a fundamental need for animals, the brain also possesses the ability to prevent excessive food intake. When mice consume an appropriate amount of food, they halt further feeding, likely due to the activation of neural circuits that suppress appetite in the brain. Although the involvement of oxytocin in the control of feeding has been postulated, the precise mechanisms have remained unclear. In our research, we discovered that mice with a conditional knockout of the oxytocin gene in the paraventricular nucleus of the hypothalamus exhibited hyperphagic obesity. Furthermore, similar results were observed when oxytocin receptors were conditionally deleted in the arcuate nucleus. These findings demonstrate the existence of an oxytocin-mediated neural circuit that suppresses appetite and thus provides insights into the neural underpinnings of appetite control.

Oxytocin signaling in the posterior hypothalamus prevents hyperphagic obesity in mice.
Inada K, Tsujimoto K, Yoshida M, Nishimori K, Miyamichi K. eLife, (2022) doi: 10.7554/eLife.75718

3. Neuroscience of Parturition and Lactation

Oxytocin, a hormone that plays a crucial role in maternal physiology, exerts its effects on uterine contractions during labor and is often administered as a labor-inducing agent. In addition, oxytocin is necessary for the process of lactation, whereby milk is released from the mammary glands. Oxytocin secretion occurs in waves or pulses from the pituitary gland, with each wave reaching the uterus and mammary glands every few minutes. However, the neural circuit and molecular mechanisms that govern this rhythmic activity are not yet fully understood. Using advanced genetic tools in mice, we have been able to precisely record and analyze the activity of oxytocin neurons during parturition and lactation. Through our research, we have demonstrated that the pulsation of oxytocin in the mother can be artificially manipulated by selectively activating specific neurons based on mapping the neural inputs to oxytocin neurons.

Recording and manipulation of the maternal oxytocin neural activities in mice.
Yukinaga H, Hagihara M, Tsujimoto K, Chiang H-L, Kato S, Kobayashi K, Miyamichi K. Current Biology, (2022) doi: 10.1016/j.cub.2022.06.083

Based on this work, we are currently exploring the neural circuitry mechanisms that govern the generation of pulsatile activities of oxytocin neurons in the mother through the manipulation of upstream neural circuitry and gene functions. As a significant step towards facilitating this investigation, we have developed an uncomplicated method that does not require the use of Cre recombinase, which enables the efficient capture of oxytocin neuron pulsatile activity in wild-type animals and various genetically modified mice.

Dynamic modulation of pulsatile activities of oxytocin neurons in lactating wild-type mice.
Yaguchi K, Hagihara M, Konno A, Hirai H, Yukinaga H, Miyamichi K. PLoS One, (2023) doi: 10.1371/journal.pone.0285589

Studies on systemic (whole-body) oxytocin knockout mice have indicated that oxytocin may not be crucial for partition or caregiving behavior in mother mice. We have re-evaluated these findings with oxytocin conditional knockout mice. Our results suggest that oxytocin gene deletion from both the paraventricular (PVH) and supraoptic (SON) nucleus does not result in a significant phenotype in the partition or caregiving behavior of mother mice, supporting the classical studies. Interestingly, our examination of the classical function of oxytocin - lactation - revealed that deletion of the oxytocin gene from the PVH did not produce any significant defects, whereas severe lactation defects appeared when the number of oxytocin neurons in the SON fell below 200. These results indicate an essential role of oxytocin derived from SON for milk ejection, which may offer valuable insights into exploring the neural mechanisms of lactation.

The importance of oxytocin neurons in the supraoptic nucleus for breastfeeding in mice.
Hagihara M, Miyamichi K, Inada K. PLoS One, (2023) doi: 10.1371/journal.pone.0283152.

4. A Prefrontal Neural Circuit for Maternal Behavioural Learning in Mice

Maternal behavior is critical for mammalian infant survival. While it is often described as an instinctive behavior, it also contains various learning components. However, the neural circuit mechanisms that facilitate the efficient learning of maternal caregiving behaviors remain poorly understood. While prevailing circuit models of parental behaviors assume that a prefrontal cortical network integrates infant-related sensory signals to execute decision-making and motivation, these hypotheses have yet to undergo functional testing. As such, the detailed input-output neural circuit organization of the prefrontal cortex as it relates to maternal behavior is still unknown. Our research has revealed that the orbitofrontal cortex (OFC) plays a crucial role in facilitating the efficient learning of maternal behaviors when virgin female mice are cohabitated with lactating mothers. Chronic microendoscopy in freely behaving animals reveals robust representations of pup-directed anticipatory activities and ongoing sequential motions of pup retrieval that are innately sculpted and largely unaffected by learning. Through viral tracing and manipulations, we functionally identify the submedius thalamus as a prominent presynaptic partner of the OFC that shapes pup-related representations. Optogenetic inactivation of OFC reduces the pup retrieval-related activities of midbrain dopamine neurons that promote maternal behaviors. Collectively, these findings reveal a higher-order cognitive network that connects innately formed, pup-related integrated representation to the top-down control of motivation centers, thus enabling efficient maternal behavioral learning.

A Prefrontal Neural Circuit for Maternal Behavioural Learning in Mice.
Tasaka G, Hagihara M, Irie S, Kobayashi H, Inada K, Kihara M, Abe T, Miyamichi K. bioRxiv, (2023) doi: https://doi.org/10.1101/2023.02.03.527077

5. A Neural Activity during Reproductive Senescence

The transition to reproductive senescence significantly impacts the quality of life of women, but the underlying neural mechanisms remain poorly characterized. We establish long-term (from the reproductive to acyclic phase) chronic imaging of the central pacemaker activities of reproductive functions by fiber photometry in female mice. In particular, we focus on kisspeptin neurons in the arcuate nucleus (ARC^kiss) of the hypothalamus. Our data reveal that during the transition to reproductive senescence, the pulsatile activities of ARC^kiss show unexpected robustness in terms of frequency, but a tendency for the intensity to decline. Their findings exhibit the power of direct chronic visualization of hormonal regulators in the brain, which is generally applicable to facilitate studies of aging-associated malfunctions in neuroendocrine systems.

Dynamics of pulsatile activities of arcuate kisspeptin neurons in aging female mice.
Goto T, Hagihara M, Miyamichi K. eLife, (2023) doi: 10.7554/eLife.82533

6. Neural Basis of Social Dysfunctions Developed by Environmental Factors

The prenatal environment, including both maternal and external factors, is correlated with an increased risk of neurodevelopmental disorders which are often characterized by atypical social behavior. However, the specific mechanisms by which perturbations in the prenatal environment during the embryonic stage affect social behavior are not well understood. To address this question, we are employing mouse models to investigate the neural systems involved in regulating social behavior in the hypothalamus.

Selective Vulnerability of Parvocellular Oxytocin Neurons in Social Dysfunction.
Tsurutani M, Goto T, Hagihara M, Irie S, Miyamichi K. bioRxiv, (2023) doi: https://doi.org/10.1101/2023.12.02.569733

7. Sympathetic Nervous Efferents in the Downstream of the Hypothalamus

The hypothalamus regulates a wide range of body functions and homeostasis through two major output pathways: the neuroendocrine system and the autonomic nervous system. The autonomic nervous system controls diverse organs including the circulatory, respiratory, digestive, and reproductive systems. Unlike the skeletal muscles, which are controlled by somatic motor neurons through monosynaptic pathways, the autonomic nervous system regulates organs in parallel by the sympathetic and parasympathetic divisions, each with two layers of output neurons (preganglionic and postganglionic neurons). Owing to its complexity, the basic knowledge of the structural and functional diversity of the sympathetic efferent pathways has remained limited to classical neuroanatomy. We aim to elucidate the fundamental organization and function of the sympathetic nervous system, with a focus on thermoregulation and the digestive system, by analyzing the central medulla, cellular composition of preganglionic neurons in the spinal cord, and organ-specific projections of postganglionic neurons. These studies are supported by JST-CREST Multicellular Area and the Transformative Research Areas (A). We will share further particulars in due course, so please stay tuned for updates!