Insulin‐responsive autonomic neurons in rat medulla oblongata

Abstract

Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin-responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 U/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase-stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N-methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9 ± 0.3 mmol/L to 1.7 ± 0.2 mmol/L (mean ± SEM; n = 6); saline injection had no effect. In insulin-treated rats, many PNMT-immunoreactive C1 neurons had Fos-immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3% ± 1.4% (n = 8) of C1 neurons were Fos-positive. Insulin also induced Fos in 47.2% ± 2.0% (n = 5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT-positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2, or C3 neurons and no DMV neurons were Fos-positive. The high numbers of PNMT-immunoreactive and ChAT-immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.

Repetitive hypoglycemia reduces activation of glucose-responsive neurons in C1 and C3 medullary brain regions to subsequent hypoglycemia

“…Hypoglycemia initiates the activation of hypothalamic and medullary pathways that enhance pancreatic glucagon and adrenal medullary epinephrine secretion (24, 25, 58–60). Recently, Senthilkumaran et al (49) used a diaminobenzidine staining method to demonstrate the presence of glucose-responsive neurons in the C1 and C3 medullary regions as well as in the chromaffin cells of the adrenal medulla. Similar diaminobenzidine staining methodology revealed that Fos expression is also present in the C1 and C3 regions after systemic 2-deoxyglucose (2-DG)-induced glucoprivation (41, 45).…”

“…This does not discount the possibility that caudal C1 neurons projecting to the medial hypothalamus (paraventricular and arcuate nucleus) are also contributing to the downstream epinephrine response; however, the activation of caudal C1 cells is more closely linked to behavioral responses such as feeding following insulin-induced hypoglycemia (10, 44, 54, 61). Furthermore, Senthilkumaran et al (49) showed that both caudal and rostral C1 cells are activated after insulin-induced hypoglycemia, so it would not be surprising if repeated hypoglycemia also reduced the activation of caudal C1 neurons, since the behavioral response to hypoglycemia (i.e., feeding) is reduced in HAAF (47). The present study falls short in providing an answer for what the relative contribution of caudal C1 neuronal activation is in terms of hypoglycemia-induced epinephrine secretion.…”

“…Adrenergic neurons of the rostroventrolateral (C1) and dorsomedial (C3) medulla oblongata contain axonal projections to adrenal sympathetic preganglionic neurons (31, 37, 40, 51). Furthermore, hypoglycemia- or glucoprivation-induced activation of C1 and C3 neurons results in epinephrine release from chromaffin cells located in the adrenal medulla (29, 37, 44, 45, 49). These findings suggest that after hypoglycemia a positive association exists between the activation state of medullary catecholaminergic (C1 and C3) neurons and downstream epinephrine secretion.…”

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

“…Therefore, there have been intensive efforts to understand how the central nervous system (Carus, 1814) [ 16 ] (CNS) (see Section 2.1 for the formal (italicized) atlas nomenclature used in this study) senses circulating levels of glucose and initiates counterregulatory responses, and how CNS impairments contribute to defective glucose counterregulation [ 17 , 18 , 19 , 20 ]. A compelling body of evidence has established a critical role for various rhombic brain (His, 1893) [ 21 ] (RB) regions in glucosensing, feeding and/or counterregulatory responses to glycemic challenges [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 ], with much of this work focused on the role of catecholaminergic neurons in the medulla (Winslow, 1733) [ 42 ] (MY) in these functions [ 17 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. Certain neuronal populations in the hypothalamus (Kuhlenbeck, 1927) [ 52 ] (HY) also appear to be glucosensing and/or critical for counterregulation [ 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , …”

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

“…First, in terms of temporal dynamics, it is not fully understood how quickly RB neuronal networks track changes in glycemic status in vivo . Immunodetection of the transcription factor, Fos, has been used to track RB neuronal activation associated with glycemic challenge [30, 39, 42, 53, 75], but its protein expression typically peaks within 1–3 hours [77]. Thus, it is difficult to attribute its delayed expression to changes in glycemic status per se rather than to secondary effects set in motion by such changes.…”