Whole-Brain Wiring Diagram of Oxytocin System in Adult Mice

Abstract

Oxytocin (Oxt) neurons regulate diverse physiological responses via direct connections with different neural circuits. However, the lack of comprehensive input-output wiring diagrams of Oxt neurons and their quantitative relationship with Oxt receptor (Oxtr) expression presents challenges to understanding circuit-specific Oxt functions. Here, we establish a whole-brain distribution and anatomic connectivity map of Oxt neurons, and their relationship with Oxtr expression using high-resolution 3D mapping methods in adult male and female mice. We use a flatmap to describe Oxt neuronal expression in four hypothalamic domains including under-characterized Oxt neurons in the tuberal nucleus (TU). Oxt neurons in the paraventricular hypothalamus (PVH) broadly project to nine functional circuits that control cognition, brain state, and somatic visceral response. In contrast, Oxt neurons in the supraoptic (SO) and accessory (AN) nuclei have limited central projection to a small subset of the nine circuits. Surprisingly, quantitative comparison between Oxt output and Oxtr expression showed no significant correlation across the whole brain, suggesting abundant indirect Oxt signaling in Oxtr-expressing areas. Unlike output, Oxt neurons in both the PVH and SO receive similar monosynaptic inputs from a subset of the nine circuits mainly in the thalamic, hypothalamic, and cerebral nuclei areas. Our results suggest that PVH-Oxt neurons serve as a central modulator to integrate external and internal information via largely reciprocal connection with the nine circuits while the SO-Oxt neurons act mainly as unidirectional Oxt hormonal output. In summary, our Oxt wiring diagram provides anatomic insights about distinct behavioral functions of Oxt signaling in the brain. SIGNIFICANCE STATEMENT Oxytocin (Oxt) neurons regulate diverse physiological functions from prosocial behavior to pain sensation via central projection in the brain. Thus, understanding detailed anatomic connectivity of Oxt neurons can provide insight on circuit-specific roles of Oxt signaling in regulating different physiological functions. Here, we use high-resolution mapping methods to describe the 3D distribution, monosynaptic input and long-range output of Oxt neurons, and their relationship with Oxt receptor (Oxtr) expression across the entire mouse brain. We found Oxt connections with nine functional circuits controlling cognition, brain state, and somatic visceral response. Furthermore, we identified a quantitatively unmatched Oxt-Oxtr relationship, suggesting broad indirect Oxt signaling. Together, our comprehensive Oxt wiring diagram advances our understanding of circuit-specific roles of Oxt neurons.

Missing pieces in decoding the brain oxytocin puzzle: Functional insights from mouse brain wiring diagrams

“…In our recent publication, we quantified and compared Oxt projections and OxtR density in the whole mouse brain. Except for regions within the thalamus and medulla, our analysis revealed no significant quantitative correlation between Oxt projection and OxtR density in most brain areas ( Son et al, 2022 ). For example, most regions within the cerebral cortex of the mouse brain are enriched with OxtRs, but the Oxt projection fibers are sparse.…”

“…Oxt is synthesized mainly in the paraventricular hypothalamus (PVH) and the supra optic nucleus (SO) along with a smaller number of neurons in the accessory nuclei of the hypothalamus and extended amygdala ( Freda et al, 2022 ; Son et al, 2022 ). In addition, our recent study identified a dense cluster of Oxt neurons in the tuberal nucleus area (TU) of the hypothalamus in the mouse brain ( Son et al, 2022 ).…”

“…Oxt is synthesized mainly in the paraventricular hypothalamus (PVH) and the supra optic nucleus (SO) along with a smaller number of neurons in the accessory nuclei of the hypothalamus and extended amygdala ( Freda et al, 2022 ; Son et al, 2022 ). In addition, our recent study identified a dense cluster of Oxt neurons in the tuberal nucleus area (TU) of the hypothalamus in the mouse brain ( Son et al, 2022 ). Oxt functions mainly through a single Oxt receptor (OxtR) that is expressed on the target tissues throughout the body, and enriched in most parts of the brain ( Gimpl and Fahrenholz, 2001 ; Assinder, 2022 ).…”

“…For example, Oxt uptake to the MOB is postulated to occur via a trans ventricular pathway [cerebrospinal fluid (CSF)-Subarachnoid-lymphatic system] ( Veening et al, 2010 ). This is a very feasible pathway considering the distance between MOB to Oxt producing neurons, and the presence of Oxt fibers along the sides of the 3 rd ventricle ( Son et al, 2022 ). Further studies are required to confirm ventricular release of Oxt and the brain regions that uptake Oxt from the trans ventricular pathway.…”

Activation of oxytocin receptors in mouse GABAergic amacrine cells modulates retinal dopaminergic signaling

“…Though previous studies suggested that oxytocin system could act differently in males and females [ 13 , 73 – 75 ], the neuronal properties and projections of PVN oxytocin neurons are found to be sex invariance in mice [ 76 , 77 ], and the modulation function of oxytocin signal in visual cortex was reported to be similar in different genders [ 15 , 16 ]. In our study, we compared the differences in retinal oxytocin signals between males and females in the following four aspects: (1) Oxtr fluorescence intensities in the INL and GCL are not significantly different between males and females (Additional file 1 : Fig.…”