Glycemic challenge is associated with the rapid cellular activation of the locus ceruleus and nucleus of solitary tract: Circumscribed spatial analysis of phosphorylated MAP kinase immunoreactivity in relation to catecholaminergic and cholinergic chemoarchitecture

Research using laboratory rats has shown that impaired glucose utilization or hypoglycemia is associated with cellular activation of neurons in the medulla (Winslow, 1733) (MY) that are implicated in the control of feeding behavior and hormonal counterregulatory responses. Changes in the activation of these neuronal populations are also associated with autonomic dysfunction that manifests itself in the form of blunted counterregulatory responses to repeated episodes of glucoprivation or hypoglycemia. An improved appreciation of the spatiotemporal dynamics of these medullary responses — and the phenotypes of the neurons responsible for them — could lead to the development of rational therapies or diagnostics to address patient complications associated with diabetes management, such as hypoglycemia-associated autonomic failure. At present, however, cellular-scale neural activation following glycemic challenge has been tracked in rodents primarily within hours of the challenge, rather than sooner, and these responses have been poorly mapped within standardized brain atlases in relation to the locations of cell types known to be present in the medulla and the larger rhombic brain (His, 1893) (RB). Here, we report that within 15 min of receiving the glycemic challenge, 2-deoxy-D-glucose (2-DG; 250 mg/kg, i.v.), which is known to produce intracellular glucopenia and trigger glucoprivic feeding behavior, marked elevations were observed in the numbers of RB neuronal cell profiles immunoreactive for the cellular activation marker(s), phosphorylated p44/42 MAP kinases (phospho-ERK1/2). Dual immunostaining experiments showed that some, but not all, of these neurons were catecholaminergic, as identified by immunostaining for dopamine β-hydroxylase (DBH). To begin the process of mapping the locations of these rapidly-activated neurons in relation to known cytoarchitecture with standardized neuroanatomical nomenclature, we performed Nissl- and chemoarchitecture-based plane-of-section analysis to map their distributions at discrete rostrocaudal levels, along with associated basally-activated cholinergic and other catecholaminergic cell groups, within an open-access rat brain atlas. This analysis revealed that neurons of the locus ceruleus (Wenzel & Wenzel, 1812) (LC) and the nucleus of solitary tract (>1840) (NTS) displayed elevated numbers of phospho-ERK1/2+ neurons within 15 min following 2-DG administration. Notably, while catecholaminergic neurons co-labeled with phospho-ERK1/2 immunoreactivity, many non-catecholaminergic NTS neurons also displayed such activation. Both 2-DG and saline-treated groups also displayed phospho-ERK1/2+ neurons in the dorsal motor nucleus of vagus nerve (>1840), identified both by Nissl criteria and immunostaining for the cholinergic marker, choline acetyltransferase. Our results show that 2-DG-activation of certain RB catecholaminergic neurons is more rapid than perhaps previously realized, engaging neurons that serve multiple functional systems and are of varying cellular phenotypes. They also place these and other activated populations within standardized maps of cytoarchitectonically-defined RB regions, thereby streamlining their precise targeting and/or comparable mapping in preclinical rodent models of disease.