Parabrachial-insular stimulation does not wake mice.

The parabrachial nucleus (PB) in the upper brainstem receives interoceptive information and sends a massive output projection directly to the cerebral cortex. Its glutamatergic axons primarily target the midinsular cortex, and we have proposed that this PB-insular projection promotes arousal. Here, we test whether stimulating this projection causes wakefulness. We combined optogenetics and video-electroencephalography (vEEG) in mice to test this hypothesis by stimulating PB axons in the insular cortex. Stimulating this projection did not alter the cortical EEG or awaken mice. Also, despite a tendency toward aversion, PB-insular stimulation did not significantly alter real-time place preference (RTPP). These results are not consistent with the hypothesis that the direct PB-insular projection is part of the ascending arousal system.NEW & NOTEWORTHY A brainstem region critical for wakefulness overlaps the medial parabrachial nucleus (PB) and has functional and direct axonal connectivity with the insular cortex. In this study, we hypothesized that this direct projection from the PB to the insular cortex promotes arousal. However, photostimulating PB axons in the insular cortex did not alter the cortical EEG or awaken mice. This information constrains the possible circuit connections through which brainstem neurons may sustain arousal.

A Century Searching for the Neurons Necessary for Wakefulness.

Wakefulness is necessary for consciousness, and impaired wakefulness is a symptom of many diseases. The neural circuits that maintain wakefulness remain incompletely understood, as do the mechanisms of impaired consciousness in many patients. In contrast to the influential concept of a diffuse "reticular activating system," the past century of neuroscience research has identified a focal region of the upper brainstem that, when damaged, causes coma. This region contains diverse neuronal populations with different axonal projections, neurotransmitters, and genetic identities. Activating some of these populations promotes wakefulness, but it remains unclear which specific neurons are necessary for sustaining consciousness. In parallel, pharmacological evidence has indicated a role for special neurotransmitters, including hypocretin/orexin, histamine, norepinephrine, serotonin, dopamine, adenosine and acetylcholine. However, genetically targeted experiments have indicated that none of these neurotransmitters or the neurons producing them are individually necessary for maintaining wakefulness. In this review, we emphasize the need to determine the specific subset of brainstem neurons necessary for maintaining arousal. Accomplishing this will enable more precise mapping of wakefulness circuitry, which will be useful in developing therapies for patients with coma and other disorders of arousal.

BoutonNet: an automatic method to detect anterogradely labeled presynaptic boutons in brain tissue sections.

Neurons emit axons, which form synapses, the fundamental unit of the nervous system. Neuroscientists use genetic anterograde tracing methods to label the synaptic output of specific neuronal subpopulations, but the resulting data sets are too large for manual analysis, and current automated methods have significant limitations in cost and quality. In this paper, we describe a pipeline optimized to identify anterogradely labeled presynaptic boutons in brain tissue sections. Our histologic pipeline labels boutons with high sensitivity and low background. To automatically detect labeled boutons in slide-scanned tissue sections, we developed BoutonNet. This detector uses a two-step approach: an intensity-based method proposes possible boutons, which are checked by a neural network-based confirmation step. BoutonNet was compared to expert annotation on a separate validation data set and achieved a result within human inter-rater variance. This open-source technique will allow quantitative analysis of the fundamental unit of the brain on a whole-brain scale.

Efferent projections of CGRP/Calca-expressing parabrachial neurons in mice.

The parabrachial nucleus (PB) is composed of glutamatergic neurons at the midbrain-hindbrain junction. These neurons form many subpopulations, one of which expresses Calca, which encodes the neuropeptide calcitonin gene-related peptide (CGRP). This Calca-expressing subpopulation has been implicated in a variety of homeostatic functions, but the overall distribution of Calca-expressing neurons in this region remains unclear. Also, while previous studies in rats and mice have identified output projections from CGRP-immunoreactive or Calca-expressing neurons, we lack a comprehensive understanding of their efferent projections. We began by identifying neurons with Calca mRNA and CGRP immunoreactivity in and around the PB, including populations in the locus coeruleus and motor trigeminal nucleus. Calca-expressing neurons in the PB prominently express the mu opioid receptor (Oprm1) and are distinct from neighboring neurons that express Foxp2 and Pdyn. Next, we used Cre-dependent anterograde tracing with synaptophysin-mCherry to map the efferent projections of these neurons. Calca-expressing PB neurons heavily target subregions of the amygdala, bed nucleus of the stria terminalis, basal forebrain, thalamic intralaminar and ventral posterior parvicellular nuclei, and hindbrain, in different patterns depending on the injection site location within the PB region. Retrograde axonal tracing revealed that the previously unreported hindbrain projections arise from a rostral-ventral subset of CGRP/Calca neurons. Finally, we show that these efferent projections of Calca-expressing neurons are distinct from those of neighboring PB neurons that express Pdyn. This information provides a detailed neuroanatomical framework for interpreting experimental work involving CGRP/Calca-expressing neurons and opioid action in the PB region.

Efferent projections of Vglut2, Foxp2, and Pdyn parabrachial neurons in mice.

The parabrachial nucleus (PB) is a complex structure located at the junction of the midbrain and hindbrain. Its neurons have diverse genetic profiles and influence a variety of homeostatic functions. While its cytoarchitecture and overall efferent projections are known, we lack comprehensive information on the projection patterns of specific neuronal subtypes in the PB. In this study, we compared the projection patterns of glutamatergic neurons here with a subpopulation expressing the transcription factor Foxp2 and a further subpopulation expressing the neuropeptide Pdyn. To do this, we injected an AAV into the PB region to deliver a Cre-dependent anterograde tracer (synaptophysin-mCherry) in three different strains of Cre-driver mice. We then analyzed 147 neuroanatomical regions for labeled boutons in every brain (n = 11). Overall, glutamatergic neurons in the PB region project to a wide variety of sites in the cerebral cortex, basal forebrain, bed nucleus of the stria terminalis, amygdala, diencephalon, and brainstem. Foxp2 and Pdyn subpopulations project heavily to the hypothalamus, but not to the cortex, basal forebrain, or amygdala. Among the few differences between Foxp2 and Pdyn cases was a notable lack of Pdyn projections to the ventromedial hypothalamic nucleus. Our results indicate that genetic identity determines connectivity (and therefore, function), providing a framework for mapping all PB output projections based on the genetic identity of its neurons. Using genetic markers to systematically classify PB neurons and their efferent projections will enhance the translation of research findings from experimental animals to humans.