Rapp KL, Khan AM, Watts AG (2006) Catecholaminergic afferents are required for hypothalamic parvicellular paraventricular neurons to transduce signals associated with hypoglycemia into p44/42 MAP kinase phosphorylation events. Neuroscience 2006 Abstracts 355.9. Society for Neuroscience, Atlanta, GA.
Summary: Hypoglycemia activates CRH neuroendocrine neurons in the hypothalamic paraventricular nucleus (PVH), thereby rapidly elevating plasma ACTH and corticosterone concentrations. Hindbrain-originating catecholaminergic (CA) neurons projecting to the PVH facilitate ACTH release after insulin-induced hypoglycemia; however, the intracellular signaling mechanisms that CRH neurons use to transduce CA signals into secretogogue function are unknown. We have previously shown that p44 and/or p42 MAP kinases (ERK1/2) are phosphorylated in CRH neurons after 2-deoxy-D-glucose challenge (Endoc. v145:351, 2004). We now ask: Are hindbrain-originating CA projections to the PVH essential for ERK1/2 phosphorylation in response to insulin-induced hypoglycemia? Male Sprague-Dawley rats received acute bilateral PVH microinjections of either saporin toxin conjugated (DSAP) to an antiserum against dopamine beta hydroxylase (DBH) or conjugated to a non-targeted mouse IgG (control mIgG-SAP), were fitted with jugular catheters, and allowed to recover. On the day of testing, plasma glucose was measured just before the onset of hypoglycemia (induced via insulin bolus [2U/kg, i.v.]) and then 10 and 30 min thereafter. Rats were then anesthetized and transcardially perfused. Brain tissue processed for dual immunofluorescence using antibodies raised against DBH and phospho-ERK1/2 was visualized by confocal microscopy. Baseline plasma glucose levels did not differ significantly between groups (5.66 ± 0.26 mM), but were significantly reduced in each group when compared to baseline after 10 (1.48 ± 0.12 mM) and 30 (1.92 ± 0.12 mM) minutes. DSAP rats displayed marked loss of DBH immunostaining in the PVH relative to controls, with concomitant loss of phospho-ERK1/2 immunostaining within the parvicellular PVH. Further analysis was performed to address the effect of DSAP on DBH staining and ERK1/2 recruitment in the hindbrain and arcuate nucleus. Our results confirm that phosphorylation of ERK1/2 in parvicellular PVH after insulin-induced hypoglycemia requires intact CA afferent innervation. These data contribute to our understanding of how intracellular signaling is regulated in parvicellular PVH neurons. They also underscore an emerging importance of CA afferents for mediating neuroendocrine responses to systemic alterations in glucose.
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