Hypothalamic Proopiomelanocortin Neurons Are Glucose Responsive and Express K_ATPChannels

Abstract

Hypothalamic proopiomelanocortin (POMC) neurons are critical for controlling homeostatic functions in the mammal. We used a transgenic mouse model in which the POMC neurons were labeled with enhanced green fluorescent protein to perform visualized, whole-cell patch recordings from prepubertal female hypothalamic slices. The mouse POMC-enhanced green fluorescent protein neurons expressed the same endogenous conductances (a transient outward K(+) current and a hyperpolarization-activated, cation current) that have been described for guinea pig POMC neurons. In addition, the selective micro -opioid receptor agonist DAMGO induced an outward current (maximum of 12.8 +/- 1.2 pA), which reversed at K(+) equilibrium potential (E(K+)), in the majority (85%) of POMC neurons with an EC(50) of 102 nM. This response was blocked by the opioid receptor antagonist naloxone with an inhibition constant of 3.1 nM. In addition, the gamma-aminobutyric acid(B) receptor agonist baclofen (40 micro M) caused an outward current (21.6 +/- 4.0 pA) that reversed at E(K+) in these same neurons. The ATP-sensitive potassium channel opener diazoxide also induced an outward K(+) current (maximum of 18.7 +/- 2.2 pA) in the majority (92%) of POMC neurons with an EC(50) of 61 micro M. The response to diazoxide was blocked by the sulfonylurea tolbutamide, indicating that the POMC neurons express both Kir6.2 and sulfonylurea receptor 1 channel subunits, which was verified using single cell RT-PCR. This pharmacological and molecular profile suggested that POMC neurons might be sensitive to metabolic inhibition, and indeed, we found that their firing rate varied with changes in glucose concentrations. Therefore, it appears that POMC neurons may function as an integrator of metabolic cues and synaptic input for controlling homeostasis in the mammal.

Kisspeptin Depolarizes Gonadotropin-Releasing Hormone Neurons through Activation of TRPC-Like Cationic Channels

“…Fluorescent cells or adjacent nonfluorescent cells were patched and then harvested into the patch pipette by applying negative pressure. The contents of the pipette were expelled into a siliconized microcentrifuge tube containing 1 l of 5ϫ Colorless GoTaq Flexi buffer (Promega, Madison, WI), 15 U of Rnasin, 0.5 l of 100 mM DTT, and DEPC-treated water in a total volume of 5 l. Each harvested cell was reverse transcribed as described previously (Ibrahim et al, 2003;Qiu et al, 2003). Briefly, the harvested cell solution and 25 ng of hypothalamic total RNA in 5 l were denatured for 5 min at 65°C, and then cooled on ice for 5 min.…”

Secrets of the lac Operon

“…In a series of studies we have found that hypothalamic ␣-melanocyte-stimulating hormone, which is produced from the proopiomelanocortin (POMC) precursor and is preferentially reduced during aging [68][69][70][71][72][73], is reduced by fasting and in genetic obesity [74], and is reduced by gold thioglucose [75]. POMC neurons are stimulated by glucose and other nutritional factors [76], and transgenic correction of reduced hypothalamic POMC corrects impairments in glucose homeostasis in obese mice [77]. Thus age-related impairments in hypothalamic POMC could plausibly contribute to age-related obesity and metabolic impairments.…”

Glucose Inhibition Persists in Hypothalamic Neurons Lacking Tandem-Pore K+Channels

“…Changing levels of extracellular glucose can lead to either excitation or inhibition of glucosesensitive hypothalamic neurons. Neurons excited by rising glucose behave similarly to the ␤ cells of the endocrine pancreas (Ibrahim et al, 2003). In these cells, an increase in glucose metabolism gives rise to elevated ATP levels that promote closure of ATP-sensitive K ϩ channels (K ATP ) and depolarization.…”