The orexinergic neurons receive synaptic input from C1 cells in rats

Abstract

The C1 cells, located in the rostral ventrolateral medulla (RVLM), are activated by pain, hypoxia, hypoglycemia, infection and hypotension and elicit cardiorespiratory stimulation, adrenaline and ACTH release, and arousal. The orexin neurons contribute to the autonomic responses to acute psychological stress. Here, using an anatomical approach, we consider whether the orexin neurons could also be contributing to the autonomic effects elicited by C1 neuron activation. Phenylethanolamine N-methyl transferase-immunoreactive (PNMT-ir) axons were detected amongst orexin-ir somata and close appositions between PNMT-ir axonal varicosities and orexin-ir profiles were observed. The existence of synapses between PNMT-ir boutons labeled with diaminobenzidine and orexinergic neurons labeled with immunogold was confirmed by electron microscopy. We labeled RVLM neurons with a lentiviral vector that expresses the fusion protein ChR2-mCherry under the control of the catecholaminergic neuron-selective promoter PRSx8 and obtained light and ultrastructural evidence that these neurons innervate the orexin cells. Using a Cre-dependent adeno-associated vector and TH-Cre rats we confirmed that the projection from RVLM catecholaminergic neurons to the orexinergic neurons originates predominantly from PNMT-ir catecholaminergic (i.e. C1 cells). The C1 neurons were found to establish predominantly asymmetric synapses with orexin-ir cell bodies or dendrites. These synapses were packed with small clear vesicles and also contained dense core vesicles. In summary, the orexin neurons are among the hypothalamic neurons contacted and presumably excited by the C1 cells. The C1-orexin neuronal connection is probably one of several suprabulbar pathways through which the C1 neurons activate breathing and the circulation, raise blood glucose and facilitate arousal from sleep.

Catecholaminergic neurons projecting to the paraventricular nucleus of the hypothalamus are essential for cardiorespiratory adjustments to hypoxia

“…Although most CNS catecholaminergic cell groups are linked to cardiorespiratory regulation (5,18), the majority of catecholamine inputs to PVN arise from VLM and nTS (9,44,52). Catecholaminergic VLM neurons project heavily to and influence magnocellular vasopressinergic and oxytocinergic neurons and parvocellular CRH neurons in the PVN (10,52,55), suggesting a role in neuroendocrine function.…”

“…The nTS receives and integrates a variety of visceral inputs, including from chemoafferents, and has reciprocal projections to cardiorespiratory brain regions in addition to PVN (1,40). VLM C1 neurons also have multiple projections to cardiorespiratory regions and influence breathing (5,6,18). Thus, catecholaminergic inputs from VLM likely contribute to neuroendocrine responses to hypoxia, whereas both nTS and VLM may play a role in chemoreflex-mediated increases in ventilation.…”

Intrathecal Intermittent Orexin-A Causes Sympathetic Long-Term Facilitation and Sensitizes the Peripheral Chemoreceptor Response to Hypoxia in Rats

“…One recent study demonstrated that C1 neurons of the rostral ventrolateral medulla, which are intrinsically hypoxia-sensitive neurons, have afferent inputs to the orexinergic neurons (Bochorishvili et al, 2014). C1 neurons respond to severe reductions in brain PaO 2 (Sun and Reis, 1994).…”

“…A large proportion of C1 neurons express Fos protein in conscious mammals exposed to hypoxia (Erickson and Millhorn, 1994), and sympathetic nerve activation elicited by carotid body stimulation is severely depressed after selective lesions of these cells (Schreihofer and Guyenet, 2000). The C1 neurons establish asymmetric synapses with orexinimmunoreactive cell bodies or dendrites (Bochorishvili et al, 2014). Anterograde tracing with viral vectors has revealed C1 cell projections to the lateral hypothalamus (Bochorishvili et al, 2014); the rostral ventrolateral medulla may be a key nucleus regulating both descending presympathetic neurons and afferents projections to orexin nuclei to activate state-dependent control of sympathetic responses to hypoxia (Fig.…”

“…The C1 neurons establish asymmetric synapses with orexinimmunoreactive cell bodies or dendrites (Bochorishvili et al, 2014). Anterograde tracing with viral vectors has revealed C1 cell projections to the lateral hypothalamus (Bochorishvili et al, 2014); the rostral ventrolateral medulla may be a key nucleus regulating both descending presympathetic neurons and afferents projections to orexin nuclei to activate state-dependent control of sympathetic responses to hypoxia (Fig. 5).…”