Many neuronal populations that release fast-acting excitatory and inhibitory neurotransmitters in the brain also contain slower acting neuropeptides. These facultative peptidergic cell types are common, but it remains uncertain whether obligate peptidergic neurons exist. Our fluorescence in situ hybridization, genetically-targeted electron microscopy, and electrophysiological characterization data strongly suggest that neurons of the non-cholinergic, centrally-projecting Edinger-Westphal nucleus in mice are fundamentally obligately peptidergic. We further show, using fiber photometry, monosynaptic retrograde tracing, anterograde projection mapping, and a battery of behavioral assays, that this peptidergic population both promotes fear responses and analgesia and activates in response to loss of motor control and pain. Together, these findings elucidate an integrative, ethologically relevant function for the Edinger-Westphal nucleus and functionally align the nucleus with the periaqueductal gray, where it resides. This work advances our understanding of the peptidergic modulation of fear and provides a framework for future investigations of putative obligate peptidergic systems.

Ubiquitous across all domains of life, tRNAs constitute an essential component of cellular physiology, carry out an indispensable role in protein synthesis, and have been historically the subject of a wide range of biochemical and biophysical studies as prototypical folded RNA molecules. Although conformational flexibility is a well-established characteristic of tRNA structure, it is typically regarded as an adaptive property exhibited in response to an inducing event, such as the binding of a tRNA synthetase or the accommodation of an aminoacyl-tRNA into the ribosome. In this study, we present crystallographic data of a tRNA molecule to expand on this paradigm by showing that structural flexibility and plasticity are intrinsic properties of tRNAs, apparent even in the absence of other factors. Based on two closely related conformations observed within the same crystal, we posit that unbound tRNAs by themselves are flexible and dynamic molecules. Furthermore, we demonstrate that the formation of the T-loop conformation by the tRNA TΨC stem-loop, a well-characterized and classic RNA structural motif, is possible even in the absence of important interactions observed in fully folded tRNAs.

Oxytocin and vasopressin are pleiotropic neuropeptides with well-established roles in the regulation of social behavior and homeostatic functions. Their structural similarity and conserved functions in vertebrate social behavior suggest that neurohypophyseal peptides may represent a single integrative neuromodulatory system, yet both peptides subserve sexually dimorphic functions at the behavioral level. The extent to which central oxytocin and vasopressin systems share similar circuit architecture has not been previously studied. Sex differences in the central circuitry of the oxytocin and vasopressin systems may underlie sex-variant behaviors, but it is currently unknown whether the synaptic inputs or outputs of each neuropeptidergic system vary across males and females. To close this gap, we generated quantitative anterograde and retrograde maps of the paraventricular oxytocin and vasopressin systems in mice. We observed that both oxytocinergic and vasopressinergic neurons share highly similar synaptic inputs that are sex-conserved. Projection patterns differed across systems and showed sex differences, more pronounced in the vasopressin neurons. Together our data represent the first comparative study of oxytocin and vasopressin input-output architecture highlighting how these neurohypopheseal peptides can play complementary and overlapping roles that are sex-dependent.