The mechanisms of arousal from apneas during sleep in patients suffering from obstructive sleep apnea (OSA) are not well understood. However, respiratory chemosensory pathways converge on the parabrachial nucleus (PB), which sends glutamatergic projections to a variety of forebrain structures critical to arousal including the basal forebrain, lateral hypothalamus, midline thalamus, and cerebral cortex. We tested the role of glutamatergic signaling in this pathway by developing an animal model for repetitive CO2 arousals (RCA) and investigating the effect of deleting the gene for the vesicular glutamate transporter 2 (Vglut2) from neurons in the PB. We used mice with lox P sequences flanking exon2 of the Vglut2 gene, in which adeno-associated viral vectors containing genes encoding Cre recombinase and green fluorescent protein were microinjected into the PB to permanently and selectively disrupt Vglut2 expression while labeling the affected neurons. We recorded sleep in these mice and then investigated the arousals during RCA. Vglut2 deletions that included the external lateral and lateral crescent subdivisions of the lateral PB more than doubled the latency to arousal and resulted in failure to arouse by 30 s in over 30% of trials. By contrast, deletions that involved the medial PB subdivision had minimal effects on arousal during hypercapnia but instead increased NREM sleep by about 43% during the dark period, and increased delta power in the EEG during NREM sleep by about 50%. Our results suggest that glutamatergic neurons in the lateral PB are necessary for arousals from sleep in response to CO2, while medial PB glutamatergic neurons play an important role in promoting spontaneous waking.

The hippocampal formation and amygdala are responsible for regulating emotion, learning, and behavior. The hippocampal projection to the amygdala has been demonstrated to originate in the subiculum and adjacent portion of field CA1 of the Ammon's horn (Sub/CA1) in the rat; however, the topographical organization of this pathway is still understudied. To make it clear, we performed anterograde and retrograde tracing with biotinylated dextran amine (BDA) and cholera toxin B subunit (CTb), respectively, in the rat. A series of BDA experiments revealed that the temporal-to-septal axis of origin determined a medial-to-lateral axis of termination in the amygdala. Briefly, the temporal region of the Sub/CA1 projects preferentially to the medial amygdaloid region including the medial, intercalated, and basomedial nuclei and the amygdalohippocampal transition area, and progressively more septal portions of the Sub/CA1 distribute their efferents in more lateral regions of the amygdala. Sub/CA1 fibers distributed in the central amygdaloid nucleus were relatively few. Retrograde tracing with CTb confirmed this topography and revealed little hippocampal innervation of the central nucleus of the amygdala. These observations suggest that distinct Sub/CA1 regions arranged along the longitudinal hippocampal axis may influence distinct modalities of the amygdala function.

The Kölliker-Fuse nucleus (KF) is known primarily for its respiratory function as the “pneumotaxic center” or “pontine respiratory group.” Considered part of the parabrachial (PB) complex, KF contains glutamatergic neurons that project to respiratory-related targets in the medulla and spinal cord (Yokota, Oka, Tsumori, Nakamura, & Yasui, 2007). Here we describe an unexpected population of neurons in the caudal KF and adjacent lateral crescent subnucleus (PBlc), which are γ-aminobutyric acid (GABA)ergic and have an entirely different pattern of projections than glutamatergic KF neurons. First, immunofluorescence, in situ hybridization, and Cre-reporter labeling revealed that many of these GABAergic neurons express FoxP2 in both rats and mice. Next, using Cre-dependent axonal tracing in Vgat-IRES-Cre and Vglut2-IRES-Cre mice, we identified different projection patterns from GABAergic and glutamatergic neurons in this region. GABAergic neurons in KF and PBlc project heavily and almost exclusively to trigeminal sensory nuclei, with minimal projections to cardiorespiratory nuclei in the brainstem, and none to the spinal cord. In contrast, glutamatergic KF neurons project heavily to the autonomic, respiratory, and motor regions of the medulla and spinal cord previously identified as efferent targets mediating KF cardiorespiratory effects. These findings identify a novel, GABAergic subpopulation of KF/PB neurons with a distinct efferent projection pattern targeting the brainstem trigeminal sensory system. Rather than regulating breathing, we propose that these neurons influence vibrissal sensorimotor function.