Scalp recorded direct current (DC) potential shifts associated with food intake in hungry humans

Euglycemic infusion of insulin detemir compared with human insulin appears to increase direct current brain potential response and reduces food intake while inducing similar systemic effects

OBJECTIVE

In the treatment of diabetic patients, the long-acting insulin analog insulin detemir is less prone to induce weight gain than other insulin formulations. Assuming that because of its pharmacologic properties, detemir displays stronger central nervous anorexigenic efficacy than human insulin, we compared acute effects of human insulin and detemir on electroencephalography (EEG) measures and food intake.

RESEARCH DESIGN AND METHODS

Frontocortical EEG direct current (DC) potentials were recorded in 15 healthy men during two hyperinsulinemic-euglycemic clamps that included an insulin bolus injection (human insulin, 17.75 mU/kg body wt; detemir, 90 mU/kg body wt) followed by a steady 90-min infusion (1.0 vs. 2.0 mU x kg(-1) x min(-1)). A higher dosage was chosen for detemir to compensate for its delay in impact relative to human insulin and to elicit similar systemic effects. At 20 min after infusion, subjects were allowed to eat ad libitum from a test buffet.

RESULTS

Mean glucose infusions to maintain euglycemia (P > 0.93) and blood glucose concentrations (P > 0.34) did not differ between conditions. Detemir infusion induced a negative DC-potential shift, averaging -372.2 microV from 21 to 90 min that was not observed during human insulin infusion (146.5 microV, P = 0.02). Detemir, in comparison with human insulin, reduced subsequent food intake by 303 kcal (1,257 vs. 1,560, P < 0.04).

CONCLUSIONS

While inducing comparable peripheral effects, detemir exerts stronger acute effects on brain functions than human insulin and triggers a relative decrease in food consumption, suggesting an enhanced anorexigenic impact of detemir compared with human insulin on central nervous networks that control nutrient uptake.

Food-Evoked Changes in Humans

Two experiments investigate effects related to food intake in humans. In Experiment 1, we measured startle response modulation while study participants ate ice cream, yoghurt, and chocolate. Statistical analysis revealed that ice cream intake resulted in the most robust startle inhibition compared to no food. Contrasting females and males, we found significant differences related to the conditions yoghurt and chocolate. In females, chocolate elicited the lowest response amplitude followed by yoghurt and ice cream. In males, chocolate produced the highest startle response amplitude even higher than eating nothing, whereas ice cream produced the lowest. Assuming that high response amplitudes reflect aversive motivation while low response amplitudes reflect appetitive motivational states, it is interpreted that eating ice cream is associated with the most appetitive state given the alternatives of chocolate and yoghurt across gender. However, in females alone eating chocolate, and in males alone eating ice cream, led to the most appetitive state. Experiment 2 was conducted to describe food intake-related brain activity by means of source localization analysis applied to electroencephalography data (EEG). Ice cream, yoghurt, a soft drink, and water were compared. Brain activity in rostral portions of the superior frontal gyrus was found in all conditions. No localization differences between conditions occurred. While EEG was found to be insensitive, startle response modulation seems to be a reliable method to objectively quantify motivational states related to the intake of different foods.

Periodic fasting alters neuronal excitability in rat neocortical and hippocampal tissues

Dietary restriction has been shown to be associated with marked changes in brain function. Periodic fasting was suggested to be beneficial in reducing both the incidence and severity of some neurological disorders. The aim of this investigation was to study the effect of periodic fasting on the neuronal network excitability in the neocortex and hippocampus and its possible influence on the brain under pathological conditions. Direct current (DC) recordings in the somatosensory neocortex of fasting rats (15 h water and food deprivation per day) during drinking revealed a negative potential shift. Using voltage sensitive dye imaging and tetanus-induced long-term potentiation (LTP) in ex vivo/in vitro experiments, neuronal network activities as well as synaptic efficacy were investigated in rat neocortical and hippocampal slices after 4 weeks of periodic fasting. Stimulus-induced patterns of bioelectric activity showed enhanced neuronal network excitability in the neocortex and decreased bioelectric activity in the hippocampus. LTP was significantly increased in neocortical slices and inhibited in hippocampal tissues. Both hippocampal and neocortical tissues exhibited a higher tolerance to hypoxic stress but not to 0-Mg(2+)-eliciting epileptiform field potentials. Neocortical slices also exhibited a higher threshold for the initiation of spreading depression. These experiments indicate that repetitive DC potential shifts occurring in fasting rats change the pattern of bioelectrical activities in cortical and subcortical regions. Through these alterations, the neocortex and hippocampus may become tuned for the efficient regulation of consummatory behaviour.

Transcortical direct current potential shift reflects immediate signaling of systemic insulin to the human brain

Circulating insulin is thought to provide a major feedback signal for the hypothalamic regulation of energy homeostasis and food intake, although this signaling appears to be slowed by a time-consuming blood-to-brain transport. Here we show, by recording direct current potentials, a rapid onset of the effects of circulating insulin on human brain activity. Recordings were obtained from 27 men who were intravenously injected with insulin (0.1 mU/kg body wt as bolus) and placebo. In a euglycemic condition, hypoglycemia was prevented, while in the hypoglycemic condition, plasma glucose reached a postinjection nadir of 43 mg/dl. Insulin injection induced a marked negative direct current (DC) potential shift starting within 7 min in all subjects. With euglycemic conditions, the DC potential at 10-60 min postinsulin injection averaged -621.3 microV (compared with preinjection baseline). Hypoglycemia reduced this potential to an average of -331.2 microV. While insulin per se did not affect oscillatory electroencephalographic activity, hypoglycemia peaking 25 min after insulin injection was accompanied by an immediate increase in theta activity. The rapid emergence of the DC potential shift, reflecting gross ionic changes in brain tissues, indicates that systemic insulin can serve as an immediate feedback signal in the control of hypothalamic and higher brain functions.

NPY attenuates positive cortical DC-potential shift upon food intake in man

Previous studies have shown that the regulation of hunger and satiety is accompanied by coordinate changes in cortical excitability. Starved subjects show a transient negative shift in the scalp-recorded cortical direct current (DC-)potential in the beginning of eating, indicating increased cortical excitability. With increasing satiety, the DC negativity becomes soon replaced by a reward related positive potential shift. Neuropeptide Y (NPY) is known from animal studies to increase food intake and induce weight gain, which might result from increasing hunger drive or reducing satiety. Here we investigated whether NPY affects the cortical sequelae of hunger and satiety regulation as reflected by cortical DC-potentials in man. DC-potentials were recorded over frontal (Fz, F3, F4), central (Cz, C3, C4) and parietal (Pz, P3, P4) electrode positions in 14 subjects who had abstained from eating for 15 h and who were intranasally administered 50 nmol of NPY and placebo 20 min prior to recordings. After a 3-min baseline epoch, subjects consumed 400 ml of liquid food within 5 min. Recordings ended 7 min after food consumption. In the placebo condition during food intake, with some delay a positive DC-potential shift developed which was most pronounced over frontal and central areas and reached maximum values 0-3 min after food consumption. NPY reduced this satiation associated positive shift (p<0.05) over all areas except P3 and Pz. Data suggest that NPY exerts its orexigenic influence by attenuating mechanisms of satiation.

Drinking related direct current positive potential shift in the human EEG depends on thirst

Scalp recorded direct current (DC)-potential shifts were examined in 11 human subjects who had either thirsted for 16 h or had quenched thirst before recordings. The recording epoch included a 3-min baseline, an interval of about 5 min during which subjects drank 400 ml of water, and a 7-min post-drinking interval. Consistent with previous data, when thirsty, subjects displayed a widespread negative DC-potential shift during drinking which was replaced by a positive DC shift at the transition to the post-drinking interval. The positivity after drinking lasted for about 2 min and averaged 146 microV at frontal recording sites. Quenching thirst before recordings reduced the positive DC-potential shift upon drinking, whereas changes in preceding drinking related DC negativity appeared to be secondary. The post-drinking positive DC-potential shift depending on the subject's motivational state can be considered an indicator of reward associated with quenching thirst, pointing to a lowered frontocortical excitability during reward.