Descending forebrain projections targeting respiratory control areas in the midbrain and brainstem of rats.
Authors: Bau Pedro Trevizan , Dhingra Rishi R. , Furuya Werner I. , Mazzone Stuart B. , Dutschmann Mathias 
Published: April 2020

Breathing can be voluntarily modulated via descending inputs from the forebrain to evoke respiratory‐related behaviours, such as vocalization, sniffing, swallowing or breath‐holding. Such behaviors require controlled laryngeal adduction and thus, are conducted during the post‐inspiratory phase of respiratory cycle. However, descending pathways that connect forebrain regions with primary post‐inspiratory control areas such as the pontine Kölliker‐Fuse nucleus (KF) and the medullary Bötzinger complex (BötC) remain to be identified. Here, we investigated the topography of forebrain descending projection neurons to a variety of bulbar respiratory nuclei.We locally microinjected the conventional retrograde tracer cholera toxin subunit B (CT‐B, 100–150nL) into the BötC, KF, the pre‐Bötzinger complex (pre‐BötC), the midline raphé nuclei and the midbrain periaqueductal gray (PAG). Twelve days after unilateral CT‐B injections, brains were sectioned (40μm) and immunohistochemically stained with an anti‐CT‐B antibody. The strength of descending projections was qualitatively assessed: as strong (+++), moderate (++) or weak (+) numbers of CT‐B labeled cell bodies.Retrogradely labelled neurons after unilateral injections into the lateral PAG confirmed the predominantly ipsilateral location of strong and moderate descending projection neurons in the cingulate (+++), pre‐limbic (+++), ectorhinal (++), motor (+++) and insular (++) cortices, the lateral septum (++), amygdala (+++) and hypothalamus (+++). In comparison, retrogradely labeled neurons after unilateral KF injection were also found ipsilaterally in the motor (++), prelimbic (++) and insular cortices (+++), the amygdala (++) and hypothalamus (+++). However, amongst all analysed descending target areas, only the KF receives substantial inputs from the ectorhinal (+++) and endopiriform (++) cortices. In addition, the medullary BötC receives weaker inputs from prelimbic (+) and insular (+) cortices and receives moderate inputs from the amygdala and hypothalamus. Descending projection neurons to the pre‐BötC were in accordance with the literature: motor (+) and insular (+) cortices, amygdala (+++) and hypothalamus (++). Finally, descending inputs to the medullary raphé obscurus and raphé magnus nuclei also arose from motor, prelimbic and insular cortices, amygdala and hypothalamus. However, these projections were significantly weaker compared to KF or PAG.The results suggest that descending forebrain projections into respiratory control areas are organized in general pathways that originate from motor, prelimbic and insular cortices as well as the amygdala and hypothalamus. However, only the KF, a key area for the gating of post‐inspiratory activity and respiratory plasticity, receives projections arising from the endopiriform and ectorhinal cortex. The functional implications of these descending control pathways need to be explored in future studies.Support or Funding InformationMelbourne Research Scholarship (University of Melbourne) [181858] to PT‐B.