Localization of glucokinase gene expression in the rat brain.

Abstract

The brain contains a subpopulation of glucosensing neurons that alter their firing rate in response to elevated glucose concentrations. In pancreatic ␤-cells, glucokinase (GK), the rate-limiting enzyme in glycolysis, mediates glucose-induced insulin release by regulating intracellular ATP production. A similar role for GK is proposed to underlie neuronal glucosensing. Via in situ hybridization, GK mRNA was localized to hypothalamic areas that are thought to contain relatively large populations of glucosensing neurons (the arcuate, ventromedial, dorsomedial, and paraventricular nuclei and the lateral area). GK also was found in brain areas without known glucosensing neurons (the lateral habenula, the bed nucleus stria terminalis, the inferior olive, the retrochiasmatic and medial preoptic areas, and the thalamic posterior paraventricular, interpeduncular, oculomotor, and anterior olfactory nuclei). Conversely, GK message was not found in the nucleus tractus solitarius, which contains glucosensing neurons, or in ependymal cells lining the third ventricle, where others have described its presence. In the arcuate nucleus, >75% of neuropeptide Y-positive neurons also expressed GK, and most GK + neurons also expressed KIR6.2 (the pore-forming subunit of the ATP-sensitive K + channel). The anatomic distribution of GK mRNA was confirmed in micropunch samples of hypothalamus via reverse transcription-polymerase chain reaction (RT-PCR). Nucleotide sequencing of the recovered PCR product indicated identity with nucleotides 1092-1411 (within exon 9 and 10) of hepatic and ␤-cell GK. The specific anatomic localization of GK mRNA in hypothalamic areas known to contain glucosensing neurons and the coexpression of KIR6.2 and NPY in GK + neurons support a role for GK as a primary determinant of glucosensing in neuropeptide neurons that integrate multiple signals relating to peripheral energy metabolism. Diabetes 49:693-700, 2000 M ammalian feeding behavior and general energy homeostasis appear to be regulated by circulating levels of nutrients (glucose) and peptides (e.g., leptin, insulin). Sensors to detect levels of these factors have been found to reside within specific nuclei of the hypothalamus (1-8), where central regulation of energy homeostasis is believed to be coordinated. For example, large changes in blood glucose are correlated with centrally mediated responses such as thermogenesis through activation of the sympathetic nervous system. These changes are monitored by the brain (9-11), and such responses are altered in obesity-prone animals (11-13). Moreover, lesions of the ventromedial hypothalamus (VMH) prevent the hypoglycemic activation of the sympathetic response (14). Thus, available data indicate that glucose detection by hypothalamic neurons may play an important role in regulating energy homeostasis.Glucosensing neurons are among the best characterized of such metabolic sensors. Unlike most neurons, they use glucose as a signaling molecule to alter their firing rate in response to changes in ambient glucose le...

Physiological and Molecular Characteristics of Rat Hypothalamic Ventromedial Nucleus Glucosensing Neurons

“…In keeping with its postulated role as a major regulator of neuronal glucosensing (8,10,21,27), GK mRNA was expressed in significantly more GE (P ϭ 0.001) and GI neurons (P ϭ 0.001) than NG neurons. In addition, GK was expressed in more GE than GI neurons (P ϭ 0.01) ( Table 3).…”

“…Finally, it is still unclear how GK, with its high K m , might mediate neuronal glucosensing at physiological brain glucose levels (4,10,11). Like the ␤-cell, which expresses a similar isoform of GK but not GKRP (27,32), glucosensing neuron GKRP expression was too infrequent for it to be a significant regulator of GK activity that might bring the functional K m of GK activity down to appropriate brain glucose levels (11,12,63). There are also several differences between GE neurons and pancreatic ␤-cells.…”

“…Most neurons express hexokinase-I (HK-I), which is unlikely to regulate the rate of glycolysis because it is both saturated at physiological brain glucose levels and inhibited by its product, glucose-6-phosphate (25,26). For this reason, GE neurons might use GK as a regulatory step in glucosensing (5,8,10,27). Inhibition of GK activity decreases intracellular Ca 2ϩ ([Ca 2ϩ ] i ) oscillations in dissociated hypothalamic GE neurons and decreases firing of GE neurons in hypothalamic slices (5,8,10,27).…”

“…For this reason, GE neurons might use GK as a regulatory step in glucosensing (5,8,10,27). Inhibition of GK activity decreases intracellular Ca 2ϩ ([Ca 2ϩ ] i ) oscillations in dissociated hypothalamic GE neurons and decreases firing of GE neurons in hypothalamic slices (5,8,10,27). On the other hand, GK inhibition increases [Ca 2ϩ ] i oscillations in GI neurons (10).…”

The effect of modafinil on counter-regulatory and cognitive responses to hypoglycaemia

“…Hypoglycaemia is associated with an increase in c-fos activity in this region of the brain [37]. A growing body of evidence suggests that the glucosesensing neurones of the hypothalamus and elsewhere in the brain respond to changes in plasma glucose using mechanisms similar to those used by the pancreatic beta cell [4,5,6,7,8]. In the latter, a decrease in glucose concentration suppresses insulin secretion.…”