Recording and manipulation of the maternal oxytocin neural activities in mice

Probing neuronal activity with genetically encoded calcium and voltage fluorescent indicators

Monitoring neural activity in individual neurons is crucial for understanding neural circuits and brain functions. The emergence of optical imaging technologies has dramatically transformed the field of neuroscience, enabling detailed observation of large-scale neuronal populations with both cellular and subcellular resolution. This transformation will be further accelerated by the integration of these imaging technologies and advanced big data analysis. Genetically encoded fluorescent indicators to detect neural activity with high signal-to-noise ratios are pivotal in this advancement. In recent years, these indicators have undergone significant developments, greatly enhancing the understanding of neural dynamics and networks. This review highlights the recent progress in genetically encoded calcium and voltage indicators and discusses the future direction of imaging techniques with big data analysis that deepens our understanding of the complexities of the brain.

An oxytocin receptor gene polymorphism is associated with distinct neural responses to mating encounters in male prairie voles

Oxytocin is a conserved neuropeptide that regulates social and reproductive behaviors in diverse species. Genetic variation in Oxtr, the gene encoding the oxytocin receptor (OXTR), is associated with variation in social attachment behaviors in rodents and humans; however, it is unclear how genetic variation in Oxtr shapes the function of specific neural systems during social contexts. Here we address this question using the socially monogamous prairie vole (Microtus ochrogaster), a species that expresses an array of OXTR-dependent social behaviors and possesses Oxtr gene polymorphisms that predict individual variation in brain region-specific OXTR expression. We test the neural and behavioral effects of an Oxtr gene polymorphism that has previously been associated with brain region-specific OXTR expression and social attachment behaviors in male prairie voles. Our results suggest that, during brief mating encounters, Oxtr genotype is not associated with differences in mating behavior or in expression levels of the activity-dependent immediate early gene product FOS within brain regions, but it is associated with differences in correlated FOS expression patterns across brain regions.

Lateralized local circuit tuning in female mouse auditory cortex

Most offspring are born helpless, requiring intense caregiving from parents especially during the first few days of neonatal life. For many species, infant cries are a primary signal used by parents to provide caregiving. Previously we and others documented how maternal left auditory cortex rapidly becomes sensitized to pup calls over hours of parental experience, enabled by oxytocin. The speed and robustness of this maternal plasticity suggests cortical pre-tuning or initial bias for pup call stimulus features. Here we examine the circuit basis of left-lateralized tuning to vocalization features with whole-cell recordings in brain slices. We found that layer 2/3 pyramidal cells of female left auditory cortex show selective suppression of inhibitory inputs with repeated stimulation at the fundamental pup call rate (inter-stimulus interval ∼150 msec) in pup-naïve females and expanded with maternal experience. However, optogenetic stimulation of cortical inhibitory cells showed that inputs from somatostatin-positive and oxytocin-receptor-expressing interneurons were less suppressed at these rates. This suggested that disynaptic inhibition rather than monosynaptic depression was a major mechanism underlying pre-tuning of cortical excitatory neurons, confirmed with simulations. Thus cortical interneuron specializations can augment neuroplasticity mechanisms to ensure fast appropriate caregiving in response to infant cries.

Oxytocin induces embryonic diapause

Embryonic development in many species, including case reports in humans, can be temporarily halted before implantation during a process called diapause. Facultative diapause occurs under conditions of maternal metabolic stress such as nursing. While molecular mechanisms of diapause have been studied, a natural inducing factor has yet to be identified. Here, we show that oxytocin induces embryonic diapause in mice. We show that gestational delays were triggered during nursing or optogenetic stimulation of oxytocin neurons simulating nursing patterns. Mouse blastocysts express oxytocin receptors, and oxytocin induced delayed implantation-like dispersion in cultured embryos. Last, oxytocin receptor-knockout embryos transferred into wild-type surrogates had low survival rates during diapause. Our results indicate that oxytocin coordinates timing of embryonic development with uterine progression through pregnancy, providing an evolutionarily conserved mechanism for ensuring successful reproduction.

Possible brain regions involved in parturition in mice

Parturition is a vital physiological process in the reproduction of female mammals, regulated by neurohumoral mechanisms coordinated by the central nervous system. The uterus is essential for this process; however, the neural pathways connecting the brain to the uterus remain poorly understood. In this study, we combined the pseudorabies virus (PRV) tracing tool with c-Fos immunofluorescence staining to identify brain regions that may regulate uterine muscle activity during parturition. We observed that the paraventricular nucleus (PVN), periaqueductal gray (PAG), and locus coeruleus (LC) were colabeled with PRV and c-Fos. Subsequently, we focused on the PVN to determine whether its activity correlated with parturition behavior. We used fiber photometry to record Ca2+ activity in the PVN during parturition in freely behaving mice and found a strong correlation between PVN activity and parturition behavior. Our results demonstrate that this method is both accessible and reliable for studying the roles of central-peripheral neural pathways involved in parturition behavior and suggest that PVN may be a key brain node for parturition.NEW & NOTEWORTHY Parturition is a vital physiological process in the reproduction of female mammals. Here, the authors established a method that combined retrograde tracing, c-Fos immunofluorescence staining, and fiber photometry to study the roles of central-peripheral neural pathways involved in parturition. Our method is simple and reliable to investigate the roles of central-peripheral neural pathways involved in a range of physiological processes in freely moving animals.

Oxytocin and neuroscience of lactation: Insights from the molecular genetic approach

In mammals, lactation is essential for the health and growth of infants and supports the formation of the mother-infant bond. Breastfeeding is mediated by the neurohormone oxytocin (OT), which is released into the bloodstream in a pulsatile manner from OT neurons in the hypothalamus to promote milk ejection into mammary ducts. While classical studies using anesthetized rats have illuminated the activity patterns of putative OT neurons during breastfeeding, the molecular, cellular, and neural circuit mechanisms driving the synchronous pulsatile bursts of OT neurons in response to nipple stimulation remain largely elusive. Only recently have molecular neuroscience techniques for imaging and manipulating specific genetically defined cells been applied to lactating mice. For instance, fiber photometry has revealed the temporal dynamics of the population pulsatile activity of OT neurons in freely moving dams across various lactation stages, while microendoscopy has provided single-cell level insights. In this review, we introduce the neuroscience of lactation with respect to OT neuron activity, discuss findings from molecular neuroscience approaches, and highlight key unresolved questions.

Flexible adjustment of oxytocin neuron activity in mouse dams revealed by microendoscopy

Oxytocin (OT) neurons in the hypothalamic paraventricular nucleus (PVH) play an important role in various physiological and behavioral processes, including the initiation of milk ejection and the regulation of maternal behaviors. However, their activity patterns at the single-cell level remain poorly understood. Using microendoscopic Ca2+ imaging in freely moving mouse dams, we demonstrate highly correlated pulsatile activity among individual OT neurons during lactation. The number of OT neurons engaged in the pulsatile activity significantly increased, along with a broadening of individual waveforms in the mid-lactation stage. Notably, only ~10% of the imaged OT neurons exhibited a significantly elevated response during pup retrieval, a hallmark of maternal behaviors, with a magnitude smaller than that observed during lactation. Collectively, these findings demonstrate the utility of microendoscopic imaging for PVH OT neurons and highlight the flexible adjustments of their individual activity patterns in freely behaving mouse dams.

Neurobiological mechanisms underlying oxytocin-mediated parental behavior in rodents

Parental behavior is essential for mammalian offspring to survive. Because of this significance, elucidating the neurobiological mechanisms that facilitate parental behavior has received strong interest. Decades of studies utilizing pharmacology and molecular biology have revealed that in addition to its facilitatory effects on parturition and lactation, oxytocin (OT) promotes the expression of parental behavior in rodents. Recent studies have also described the modulation of sensory processing by OT and the interaction of the OT system with other brain regions associated with parental behavior. However, the precise neurobiological mechanisms underlying the facilitation of caregiving behaviors by OT remain unclear. In this Review, I summarize the findings from rats and mice with a view toward integrating past and recent progress. I then review recent advances in the understanding of the molecular, cellular, and circuit mechanisms of OT-mediated parental behavior. Based on these observations, I propose a hypothetical model that would explain the mechanisms underlying OT-mediated parental behavior. Finally, I conclude by discussing some major remaining questions and propose potential future research directions.

Acquiring Social Safety Engages Oxytocin Neurons in the Supraoptic Nucleus: Role of Magel2 Deficiency

INTRODUCTION

Exposure to social trauma may alter engagement with both fear-related and unrelated social stimuli long after. Intriguingly, how simultaneous discrimination of social fear and safety is affected in neurodevelopmental conditions remains underexplored. The role of the neuropeptide oxytocin is established in social behaviors and yet unexplored during such a challenge post-social trauma.

METHODS

Using Magel2 knockout mice, an animal model of Prader-Willi syndrome (PWS) and Schaaf-Yang syndrome (SYS), we tested memory of social fear and safety after a modified social fear conditioning task. Additionally, we tracked the activity of oxytocin neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus by fiber photometry, as animals were simultaneously presented with a choice between fear and safe social cue during recall.

RESULTS

Male Magel2 KO mice trained to fear females with electrical footshocks avoided both unfamiliar females and males during recalls, lasting even a week post-conditioning. On the contrary, trained Magel2 WT avoided only females during recalls, lasting days rather than a week post-conditioning. Inability to overcome social fear and avoidance of social safety in Magel2 KO mice were associated with the reduced engagement of oxytocin neurons in the SON but not the PVN.

CONCLUSION

In a preclinical model of PWS/SYS, we demonstrated region-specific deficit in oxytocin neuron activity associated with behavioral generalization of social fear to social safety. Insights from this study add to our understanding of oxytocin action in the brain at the intersection of social trauma and PWS/SYS.

Lights, fiber, action! A primer on in vivo fiber photometry

Fiber photometry is a key technique for characterizing brain-behavior relationships in vivo. Initially, it was primarily used to report calcium dynamics as a proxy for neural activity via genetically encoded indicators. This generated new insights into brain functions including movement, memory, and motivation at the level of defined circuits and cell types. Recently, the opportunity for discovery with fiber photometry has exploded with the development of an extensive range of fluorescent sensors for biomolecules including neuromodulators and peptides that were previously inaccessible in vivo. This critical advance, combined with the new availability of affordable "plug-and-play" recording systems, has made monitoring molecules with high spatiotemporal precision during behavior highly accessible. However, while opening exciting new avenues for research, the rapid expansion in fiber photometry applications has occurred without coordination or consensus on best practices. Here, we provide a comprehensive guide to help end-users execute, analyze, and suitably interpret fiber photometry studies.

Neural basis for behavioral plasticity during the parental life-stage transition in mice

Parental care plays a crucial role in the physical and mental well-being of mammalian offspring. Although sexually naïve male mice, as well as certain strains of female mice, display aggression toward pups, they exhibit heightened parental caregiving behaviors as they approach the time of anticipating their offspring. In this Mini Review, I provide a concise overview of the current understanding of distinct limbic neural types and their circuits governing both aggressive and caregiving behaviors toward infant mice. Subsequently, I delve into recent advancements in the understanding of the molecular, cellular, and neural circuit mechanisms that regulate behavioral plasticity during the transition to parenthood, with a specific focus on the sex steroid hormone estrogen and neural hormone oxytocin. Additionally, I explore potential sex-related differences and highlight some critical unanswered questions that warrant further investigation.

Neural circuitry for maternal oxytocin release induced by infant cries

Oxytocin is a neuropeptide that is important for maternal physiology and childcare, including parturition and milk ejection during nursing1-6. Suckling triggers the release of oxytocin, but other sensory cues-specifically, infant cries-can increase the levels of oxytocin in new human mothers7, which indicates that cries can activate hypothalamic oxytocin neurons. Here we describe a neural circuit that routes auditory information about infant vocalizations to mouse oxytocin neurons. We performed in vivo electrophysiological recordings and photometry from identified oxytocin neurons in awake maternal mice that were presented with pup calls. We found that oxytocin neurons responded to pup vocalizations, but not to pure tones, through input from the posterior intralaminar thalamus, and that repetitive thalamic stimulation induced lasting disinhibition of oxytocin neurons. This circuit gates central oxytocin release and maternal behaviour in response to calls, providing a mechanism for the integration of sensory cues from the offspring in maternal endocrine networks to ensure modulation of brain state for efficient parenting.

The Role of Oxytocin in Domestic Animal’s Maternal Care: Parturition, Bonding, and Lactation

Oxytocin (OXT) is one of the essential hormones in the birth process; however, estradiol, prolactin, cortisol, relaxin, connexin, and prostaglandin are also present. In addition to parturition, the functions in which OXT is also involved in mammals include the induction of maternal behavior, including imprinting and maternal care, social cognition, and affiliative behavior, which can affect allo-parental care. The present article aimed to analyze the role of OXT and the neurophysiologic regulation of this hormone during parturition, how it can promote or impair maternal behavior and bonding, and its importance in lactation in domestic animals.

Dynamic modulation of pulsatile activities of oxytocin neurons in lactating wild-type mice

Breastfeeding, which is essential for the survival of mammalian infants, is critically mediated by pulsatile secretion of the pituitary hormone oxytocin from the central oxytocin neurons located in the paraventricular and supraoptic hypothalamic nuclei of mothers. Despite its importance, the molecular and neural circuit mechanisms of the milk ejection reflex remain poorly understood, in part because a mouse model to study lactation was only recently established. In our previous study, we successfully introduced fiber photometry-based chronic imaging of the pulsatile activities of oxytocin neurons during lactation. However, the necessity of Cre recombinase-based double knock-in mice substantially compromised the use of various Cre-dependent neuroscience toolkits. To overcome this obstacle, we developed a simple Cre-free method for monitoring oxytocin neurons by an adeno-associated virus vector driving GCaMP6s under a 2.6 kb mouse oxytocin mini-promoter. Using this method, we monitored calcium ion transients of oxytocin neurons in the paraventricular nucleus in wild-type C57BL/6N and ICR mothers without genetic crossing. By combining this method with video recordings of mothers and pups, we found that the pulsatile activities of oxytocin neurons require physical mother-pup contact for the milk ejection reflex. Notably, the frequencies of photometric signals were dynamically modulated by mother-pup reunions after isolation and during natural weaning stages. Collectively, the present study illuminates the temporal dynamics of pulsatile activities of oxytocin neurons in wild-type mice and provides a tool to characterize maternal oxytocin functions.

The importance of oxytocin neurons in the supraoptic nucleus for breastfeeding in mice

The hormone oxytocin, secreted from oxytocin neurons in the paraventricular (PVH) and supraoptic (SO) hypothalamic nuclei, promotes parturition, milk ejection, and maternal caregiving behaviors. Previous experiments with whole-body oxytocin knockout mice showed that milk ejection was the unequivocal function of oxytocin, whereas parturition and maternal behaviors were less dependent on oxytocin. Whole-body knockout, however, could induce the enhancement of expression of related gene(s), a phenomenon called genetic compensation, which may hide the actual functions of oxytocin. In addition, the relative contributions of oxytocin neurons in the PVH and SO have not been well documented. Here, we show that females with conditional knockout of oxytocin gene in both the PVH and SO undergo grossly normal parturition and maternal caregiving behaviors, while dams with a smaller number of remaining oxytocin-expressing neurons exhibit severe impairments in breastfeeding, leading to the death of their pups within 24 hours after birth. We also found that the growth of pups is normal even under oxytocin conditional knockout in PVH and SO as long as pups survive the next day of delivery, suggesting that the reduced oxytocin release affects the onset of lactation most severely. These phenotypes are largely recapitulated by SO-specific oxytocin conditional knockout, indicating the unequivocal role of oxytocin neurons in the SO in successful breastfeeding. Given that oxytocin neurons not only secrete oxytocin but also non-oxytocin neurotransmitters or neuropeptides, we further performed cell ablation of oxytocin neurons in the PVH and SO. We found that cell ablation of oxytocin neurons leads to no additional abnormalities over the oxytocin conditional knockout, suggesting that non-oxytocin ligands expressed by oxytocin neurons have negligible functions on the responses measured in this study. Collectively, our findings confirm the dispensability of oxytocin for parturition or maternal behaviors, as well as the importance of SO-derived oxytocin for breastfeeding.

Peripartum effects of synthetic oxytocin: The good, the bad, and the unknown

The first clinical applications of oxytocin (OT) were in obstetrics as a hormone to start and speed up labor and to control postpartum hemorrhage. Discoveries in the 1960s and 1970s revealed that the effects of OT are not limited to its peripheral actions around birth and milk ejection. Indeed, OT also acts as a neuromodulator in the brain affecting fear memory, social attachment, and other forms of social behaviors. The peripheral and central effects of OT have been separately subject to extensive scrutiny. However, the effects of peripheral OT-particularly in the form of administration of synthetic OT (synOT) around birth-on the central nervous system are surprisingly understudied. Here, we provide a narrative review of the current evidence, suggest putative mechanisms of synOT action, and provide new directions and hypotheses for future studies to bridge the gaps between neuroscience, obstetrics, and psychiatry.

Maternity: Oxytocin circuits during birth and lactation

Maternity transforms body, brain and behavior. A new study analyzing the activity of oxytocin neurons across birth and lactation revealed strengthening of suckling responses in mice. Although this did not involve major rewiring of inputs to oxytocin neurons, inhibition from the stria terminalis was found to pattern the suckling responses.