Neurochemical phenotypes of cardiorespiratory neurons

Transcription factors define the neuroanatomical organization of the medullary reticular formation

The medullary reticular formation contains large populations of inadequately described, excitatory interneurons that have been implicated in multiple homeostatic behaviors including breathing, viserosensory processing, vascular tone, and pain. Many hindbrain nuclei show a highly stereotyped pattern of localization across vertebrates suggesting a strong underlying genetic organization. Whether this is true for neurons within the reticular regions of hindbrain is unknown. Hindbrain neurons are derived from distinct developmental progenitor domains each of which expresses distinct patterns of transcription factors (TFs). These neuronal populations have distinct characteristics such as transmitter identity, migration, and connectivity suggesting developmentally expressed TFs might identify unique subpopulations of neurons within the reticular formation. A fate-mapping strategy using perinatal expression of reporter genes within Atoh1, Dbx1, Lmx1b, and Ptf1a transgenic mice coupled with immunohistochemistry (IHC) and in situ hybridization (ISH) were used to address the developmental organization of a large subset of reticular formation glutamatergic neurons. All hindbrain lineages have relatively large populations that extend the entire length of the hindbrain. Importantly, the location of neurons within each lineage was highly constrained. Lmx1b- and Dbx1- derived populations were both present in partially overlapping stripes within the reticular formation extending from dorsal to ventral brain. Within each lineage, distinct patterns of gene expression and organization were localized to specific hindbrain regions. Rostro-caudally sub-populations differ sequentially corresponding to proposed pseudo-rhombomereic boundaries. Dorsal-ventrally, sub-populations correspond to specific migratory positions. Together these data suggests the reticular formation is organized by a highly stereotyped developmental logic.

Overview: the neurochemistry of respiratory control

This special issue of Respiratory Physiology and Neurobiology surveys a broad range of topics focused on the neurochemical control of breathing. A variety of approaches have integrated the neurochemistry of breathing with the physiology of individual neurons, with the neuroanatomy of brainstem and forebrain respiratory circuits, and with the clinical pathology of respiratory disorders all of which has been fueled by the ongoing explosion of information in the molecular biology of the nervous system. Accordingly, substantial progress has identified neurotransmitters, neuromodulators, receptors, signaling cascades, trophic factors, hormones, and genes mediating normal and pathological breathing. Dynamic changes in the neurochemistry of breathing are addressed with respect to brainstem development, environmental challenges such as intermittent or chronic hypoxia, and as a function of the sleep-wake cycle. Respiratory disruption has also been identified in an increasing variety of genetic-based disorders and remarkable progress has been made in determining the affected genes and their mutations that negatively impact respiration.