Insulin‐responsive Adrenergic Neurons in Rat Medulla Oblongata

Abstract

Adrenergic neurons in the brainstem play critical roles in glucose homeostasis. When glucose availability is low, these neurons drive secretion of adrenaline from the adrenal medulla and food intake. Although glucoprivation‐sensitive adrenergic neurons in the brainstem have been mapped, the distribution of adrenergic neurons that respond to insulin has not been reported. To fill this gap in knowledge, we fasted rats overnight and then gave them intraperitoneal injections of either insulin (10 IU/kg) or saline after 10 days of twice‐daily handling. Tail vein blood samples were collected immediately before injection and at 30, 60 and 90 min after injection, at which time the rats were perfused with 4% formaldehyde. Coronal sections of perfused medulla (1:4 series) from the spinomedullary junction to mid‐facial nucleus were immunoperoxidase stained to show Fos‐immunoreactivity (IR) with a black reaction product plus phenylethanolamine N‐methyltransferase (PNMT) or choline acetyltransferase (ChAT)‐immunoreactivity with a brown reaction product. Insulin treatment lowered blood glucose from 4.9+0.3 mmol/L at baseline to 1.7+0.2 mmol/L at 90 min (mean + SEM; n=6) whereas saline treatment had no effect (4.9+0.2 mmol/L at baseline vs 5.2+0.4 at 90 min; n=3). In insulin‐treated rats, many PNMT‐IR C1 neurons had intensely‐stained Fos‐IR nuclei, indicating that they had been activated. The proportion of Fos‐IR C1 neurons was higher in the caudal than in the rostral part of the C1 cell column. Insulin treatment also induced Fos‐IR in almost all of the rostral C2 neurons and in many of the midline C3 neurons. The insulin‐responsive C2 neurons may be catecholaminergic dorsal vagal motor neurons since ChAT‐positive neurons with Fos‐positive nuclei occurred in the same locations and numbers. In saline‐treated rats, many fewer C1 neurons were stained for Fos and the staining was much less intense. Fos‐IR occurred in almost no C2 or C3 neurons in control animals. These observations indicate that all three groups of medullary adrenergic neurons are activated by insulin. The high numbers of PNMT‐positive neurons that strongly express Fos after insulin treatment support the important roles that these neurons play in the central response to a decrease in glucose bioavailability.Support or Funding InformationDiabetes Australia Research Trust; National Health and Medical Research Council of Australia