C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, United States.
Neuroscience (Impact Factor: 3.36). 10/2012; 227. DOI: 10.1016/j.neuroscience.2012.09.049
Source: PubMed

ABSTRACT C1 catecholamine neurons reside within the rostroventrolateral medulla (RVLM), an area that plays an integral role in blood pressure regulation through reticulospinal projections to sympathetic preganglionic neurons in the thoracic spinal cord. In a previous investigation we mapped the efferent projections of C1 neurons, documenting supraspinal projections to cell groups in the preautonomic network that contribute to the control of cardiovascular function. Light microscopic study also revealed putative local circuit connections within RVLM. In this investigation we tested the hypothesis that RVLM C1 neurons elaborate a local circuit synaptic network that permits communication between C1 neurons giving rise to supraspinal and reticulospinal projections. A replication defective lentivirus vector that expresses enhanced green fluorescent protein (EGFP) under the control of a synthetic dopamine beta hydroxylase (DβH) promoter was used to label C1 neurons and their processes. Confocal fluorescence microscopy demonstrated thin varicose axons immunopositive for EGFP and tyrosine hydroxylase that formed close appositions to C1 somata and dendrites throughout the rostrocaudal extent of the C1 area. Dual-labeled electron microscopic analysis revealed axosomatic, axodendritic and axospinous synaptic contacts with C1 and non-C1 neurons with a distribution recapitulating that observed in the light microscopic analysis. Labeled boutons were large, contained light axoplasm, lucent spherical vesicles, and formed asymmetric synaptic contacts. Collectively these data demonstrate that C1 neurons form an asynaptic network within the C1 area that may function to coordinate activity among projection-specific subpopulations of neurons. The data also suggest that the boundaries of RVLM should be defined on the basis of function criteria rather than the C1 phenotype of neurons.

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Available from: Khristofor Agassandian, Aug 09, 2014
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    • "Despite C1 neurons having a catecholaminergic phenotype, evidence suggests that they release glutamate as a primary neurotransmitter in adult rodents. PNMTcontaining terminals form asymmetric synaptic contacts, consistent with the ultrastructure of classic excitatory synapses (Milner et al., 1987, 1988, 1989; Agassandian et al., 2012; Depuy et al., 2013). "
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    ABSTRACT: Catecholaminergic neurons of the rostral ventrolateral medulla (RVLM-CA neurons; C1 neurons) contribute to the sympathetic, parasympathetic and neuroendocrine responses elicited by physical stressors such as hypotension, hypoxia, hypoglycemia, and infection. Most RVLM-CA neurons express vesicular glutamate transporter (VGLUT)2, and may use glutamate as a ionotropic transmitter, but the importance of this mode of transmission in vivo is uncertain. To address this question, we genetically deleted VGLUT2 from dopamine-β-hydroxylase-expressing neurons in mice [DβH(Cre/0) ;VGLUT2(flox/flox) mice (cKO mice)]. We compared the in vivo effects of selectively stimulating RVLM-CA neurons in cKO vs. control mice (DβH(Cre/0) ), using channelrhodopsin-2 (ChR2-mCherry) optogenetics. ChR2-mCherry was expressed by similar numbers of rostral ventrolateral medulla (RVLM) neurons in each strain (~400 neurons), with identical selectivity for catecholaminergic neurons (90-99% colocalisation with tyrosine hydroxylase). RVLM-CA neurons had similar morphology and axonal projections in DβH(Cre/0) and cKO mice. Under urethane anesthesia, photostimulation produced a similar pattern of activation of presumptive ChR2-positive RVLM-CA neurons in DβH(Cre/0) and cKO mice. Photostimulation in conscious mice produced frequency-dependent respiratory activation in DβH(Cre/0) mice but no effect in cKO mice. Similarly, photostimulation under urethane anesthesia strongly activated efferent vagal nerve activity in DβH(Cre/0) mice only. Vagal responses were unaffected by α1 -adrenoreceptor blockade. In conclusion, two responses evoked by RVLM-CA neuron stimulation in vivo require the expression of VGLUT2 by these neurons, suggesting that the acute autonomic responses driven by RVLM-CA neurons are mediated by glutamate.
    European Journal of Neuroscience 11/2013; 39(1). DOI:10.1111/ejn.12421 · 3.18 Impact Factor
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    ABSTRACT: The activity of neurons in the rostral ventrolateral medulla (RVLM) is critical for the generation of vasomotor sympathetic tone. Multiple pre-sympathetic pathways converge on spinally projecting RVLM neurons, but the origin and circumstances in which such inputs are active are poorly understood. We have previously shown that input from the contralateral brainstem contributes to the baseline activity of this population: in the current study we investigate the distribution, phenotype and functional properties of RVLM neurons with commissural projections in the rat. We firstly used retrograde transport of fluorescent microspheres to identify neurons that project to the contralateral RVLM. Labelled neurons were prominent in a longitudinal column that extended over 1 mm caudal from the facial nucleus and contained hybridisation products indicating enkephalin (27%), GABA (15%) and adrenaline (3%) synthesis and included 6% of bulbospinal neurons identified by transport of cholera toxin B. Anterograde transport of fluorescent dextran-conjugate from the contralateral RVLM revealed extensive inputs throughout the RVLM that frequently terminated in close apposition with catecholaminergic and bulbospinal neurons. In urethane-anaesthetised rats we verified that 28/37 neurons antidromically activated by electrical stimulation of the contralateral pressor region were spontaneously active, of which 13 had activity locked to central respiratory drive and 15 displayed ongoing tonic discharge. In six tonically active neurons sympathoexcitatory roles were indicated by spike-triggered averages of splanchnic sympathetic nerve activity. We conclude that neurons in the RVLM project to the contralateral brainstem, form synapses with sympathetic premotor neurons, and have functional properties consistent with sympthoexcitatory function.
    European Journal of Neuroscience 05/2013; 38(4). DOI:10.1111/ejn.12232 · 3.18 Impact Factor
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    ABSTRACT: The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brainstem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/ inflammation, hemorrhage and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity (SNA) associated with diseases such as hypertension.
    AJP Regulatory Integrative and Comparative Physiology 05/2013; DOI:10.1152/ajpregu.00054.2013 · 3.11 Impact Factor
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