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SummaryThe deep dorsal horn is a poorly characterized spinal cord region implicated in processing low-threshold mechanoreceptor (LTMR) information. We report an array of mouse genetic tools for defining neuronal components and functions of the dorsal horn LTMR-recipient zone (LTMR-RZ), a role for LTMR-RZ processing in tactile perception, and the basic logic of LTMR-RZ organization. We found an unexpectedly high degree of neuronal diversity in the LTMR-RZ: seven excitatory and four inhibitory subtypes of interneurons exhibiting unique morphological, physiological, and synaptic properties. Remarkably, LTMRs form synapses on between four and 11 LTMR-RZ interneuron subtypes, while each LTMR-RZ interneuron subtype samples inputs from at least one to three LTMR classes, as well as spinal cord interneurons and corticospinal neurons. Thus, the LTMR-RZ is a somatosensory processing region endowed with a neuronal complexity that rivals the retina and functions to pattern the activity of ascending touch pathways that underlie tactile perception.

Physiological homeostasis is essential for organism survival. Highly responsive neuronal networks are involved but constituent neurons are just beginning to be resolved. To query brain serotonergic neurons in homeostasis, we used a synthetic GPCR (Di)-based neuronal silencing tool, mouse RC∷FPDi, designed for cell type-specific, ligand (clozapine-N-oxide, CNO)-inducible and reversible suppression of action potential firing. In mice harboring Di-expressing serotonergic neurons, CNO administration by systemic injection attenuated the chemoreflex that normally increases respiration in response to tissue CO2 elevation and acidosis. At the cellular level, CNO suppressed firing rate increases evoked by CO2/acidosis. Body thermoregulation at room temperature was also disrupted following CNO triggering of Di; core temperatures plummeted, then recovered. This work establishes that serotonergic neurons regulate life-sustaining respiratory and thermoregulatory networks, and demonstrates a noninvasive tool for mapping neuron function.

The cochlear nuclear complex (CN) is the entry point for central auditory processing. Although constituent neurons have been studied physiologically, their embryological origins and molecular profiles remain obscure. Applying intersectional and subtractive genetic fate mapping approaches, we show that this complex develops modularly from genetically separable progenitor populations arrayed as rostrocaudal microdomains within and outside the hindbrain (lower) rhombic lip (LRL). The dorsal CN subdivision, structurally and topographically similar to the cerebellum, arises from microdomains unexpectedly caudal and noncontiguous to cerebellar primordium; ventral CN subdivisions arise from more rostral LRL. Magnocellular regions receive contributions from LRL and coaxial non-lip progenitors; contrastingly, ensheathing granule cells derive principally from LRL. Also LRL-derived and molecularly similar to CN granule cells are precerebellar mossy fiber neurons; surprisingly, these ostensibly intertwined populations have separable origins and adjacent but segregated migratory streams. Together, these findings provide new platforms for investigating the development and evolution of auditory and cerebellar systems.

Central serotonin (5-HT, 5-hydroxytryptamine)-producing neurons are heterogeneous, differing in location, morphology, neurotoxin sensitivity and associated developmental disorders such as sudden infant death syndrome, fetal alcohol syndrome, and autism. Yet the molecular underpinnings associated with such heterogeneity and differential disease vulnerability are largely unknown, as are molecular markers capable of identifying physiological subtypes of 5-HT neurons. Here we redefine subtypes within the mature 5-HT system based on genetic programs differentially enacted and differentially required in progenitor cells. We present a molecular framework for the 5-HT neural system that, having genetic lineages as its basis, is likely to have physiological relevance and will provide a means for selectively accessing 5-HT neurons for in vivo manipulations.Abnormalities in 5-HT-producing neurons are increasingly implicated in a broad spectrum of developmental disorders, including sudden infant death syndrome 1 , fetal alcohol syndrome 2 , and autism (reviewed in 3 ). Each disorder differs in clinical feature, and mounting evidence suggests that different 5-HT neuron subtypes are selectively affected. Heterogeneity within the 5-HT neuron population is further demonstrated by differences in anatomical distribution, cell morphology and axonal trajectory, neurotoxin sensitivity and physiological properties (reviewed in 4 ). Mechanisms that determine these differences are largely unknown and presently few molecular markers have been identified which are capable of distinguishing individual 5-HT neuron subtypes. Such knowledge is central to understanding etiological differences among 5-HT neuron disorders and for gaining genetic access to select 5-HT neuron subgroups for experimental study.While markers capable of distinguishing mature 5-HT neuron subtypes are wanting, at hand are markers that, when viewed in combinations, can resolve 5-HT progenitor cells into discrete subsets. From these subsets may arise physiologically relevant groupings of mature 5-HT neurons; this is because developmental programs that define the fate and function of neurons are often set in motion by the action of factors differentially expressed among their antecedent * to whom correspondence should be addressed: Telephone: 617-432-4812, Facsimile: 617-432-7595, dymecki@genetics.med.harvard.edu. † these authors contributed equally Competing Interests StatementThe authors declare that they have no competing financial interests. NIH Public AccessAuthor Manuscript Nat Neurosci. Author manuscript; available in PMC 2010 July 6. This progenitor territory can be subdivided along the AP axis into molecularly distinct subsets based on the broader partitioning of the hindbrain into segments (rhombomeres) with distinguishing gene expression profiles (reviewed in 5 ). Thus, aspects of 5-HT neuron subtype identity may be determined through the action of rhombomere(r)-specific genetic programs on resident 5-HT progenitor and precursor cell subsets. We have se...

As conditional genetic strategies advance, the need for multiple site-specific recombinase systems has emerged. To meet this need in part, we have targeted the constitutive ROSA26 locus to create a mouse strain with generalized expression of the enhanced version of the site-specific recombinase FLP (FLPe). This strain is designated FLPeR ("flipper"). Using this strain, extensive target gene recombination can be achieved in most tissue types, including cells of the developing germ line. FLPeR mice therefore serve two important functions: as a source of many different FLPe-expressing primary cell lines and as a deleter strain. Moreover, because the FLPeR mouse is a 129-derived strain, a 129 genetic background can be preserved when crossed to most ES cell-derived mice. This enables conditional genetic alterations to be maintained on a standard background, a feature important for obtaining reproducible results and genetically defined controls.

The lower rhombic lip (LRL) is a germinal zone in the dorsal hindbrain productive of tangentially migrating neurons, streaming extramurally (mossy fiber neurons) or intramurally (climbing fiber neurons). Here we show that LRL territory, operationally defined by Wnt1 expression, is parceled into molecular subdomains predictive of cell fate. Progressing dorsoventrally, Lmx1a and Gdf7 expression identifies the primordium for hindbrain choroid plexus epithelial cells; Math1, for mossy fiber neurons; and immediately ventral to Math1 yet within Wnt1(+) territory, a climbing fiber primordium dominated by Ngn1-expressing cells. Elimination of Pax6 results in expansion of this Ngn1(+) progenitor pool and reduction in the Math1(+) pool, with accompanying later enlargement of the climbing fiber nucleus and reductions in mossy fiber nuclei. Pax6 loss also disrupts Msx expression cell-nonautonomously, suggesting Pax6 may influence LRL progenitor identity indirectly through potentiating BMP signaling. These studies suggest that underlying the diversity and proportions of fates produced by the LRL is a precise suborganization regulated by Pax6.

The sudden infant death syndrome (SIDS) is the sudden death of an infant under one year of age that is typically associated with sleep and that remains unexplained after a complete autopsy and death scene investigation. A leading hypothesis about its pathogenesis is that many cases result from defects in brainstem-mediated protective responses to homeostatic stressors occurring during sleep in a critical developmental period. Here we review the evidence for the brainstem hypothesis in SIDS with a focus upon abnormalities related to the neurotransmitter serotonin in the medulla oblongata, as these are the most robust pathologic findings to date. In this context, we synthesize the human autopsy data with genetic, whole-animal, and cellular data concerning the function and development of the medullary serotonergic system. These emerging data suggest an important underlying mechanism in SIDS that may help lead to identification of infants at risk and specific interventions to prevent death.