Effects of recurrent hypoglycemia on brainstem function in diabetic BB rats: protective adaptation during acute hypoglycemia

Diet-induced diabetes is associated with lower hippocampal glycogen and reduced glycogenolysis following local exogenous insulin

Brain fuel (specifically, glucose) supply and metabolism are well-established to be limiting factors for cognitive performance, with the largest body of data being for hippocampally mediated tasks. Consistent with this, disease states such as Alzheimer's disease and insulin-resistant diabetes, that impair cognitive metabolism, impair cognition with this being shown again most prominently for hippocampally mediated processes. In addition to glucose supplied from the blood, brain oxidative metabolism can use local glycogen stores (within astrocytes) as a fuel source via conversion to lactate; both lactate and glycogen have been shown to be important contributors to regulation of cognitive metabolism. Insulin has been shown to be a key regulator of hippocampal cognitive and metabolic processes; in the periphery, insulin facilitates glycogen synthesis and storage, but the impact on brain glycogen is unclear. Furthermore, the impact of diet-induced diabetes on hippocampal glycogen levels and/or metabolism is unknown. Here, we show that in rats with high-fat diet-induced diabetes, hippocampal glycogen is reduced and is less responsive to acute intrahippocampal administration of insulin, which significantly reduces glycogen in the hippocampi of control animals: Our data suggest that impaired fuel availability from glycogen may be a contributing factor to the cognitive impairment seen in disease states that include central insulin resistance.

N-Hydroxyethyl-1-Deoxynojirimycin (Miglitol) Restores the Counterregulatory Response to Hypoglycemia Following Antecedent Hypoglycemia

Antecedent hypoglycemia suppresses the counterregulatory responses to subsequent hypoglycemic episodes, which can be prevented by normalizing portal-mesenteric vein (PMV) glycemia alone during the antecedent bout. Since the sodium-glucose transporter 3 receptor has been implicated in PMV glucosensing, we hypothesized that PMV infusion of the sodium-glucose cotransporter 3 receptor agonist N-hydroxyethyl-1-deoxynojirimycin (miglitol) would rescue the sympathoadrenal response to subsequent hypoglycemia. Rats underwent hyperinsulinemic-hypoglycemic clamps on 2 consecutive days without miglitol infusion (antecedent hypoglycemia without miglitol [HYPO]) or with miglitol infused upstream in the PMV, perfusing the glucosensors, or adjacent to the liver, bypassing PMV glucosensors, on day 1 or day 2. Control animals underwent day 1 euglycemic clamps, followed by hypoglycemic clamps on day 2. Peak epinephrine (EPI) responses for HYPO on day 2 were significantly blunted when compared with controls. Miglitol infusion on day 1 proved ineffective in restoring the EPI response following antecedent hypoglycemia, but day 2 miglitol infusion restored EPI responses to control levels. As norepinephrine and glucagon demonstrated similar responses, day 2 administration of miglitol effectively restored the counterregulatory response following antecedent hypoglycemia. In subsequent experiments, we demonstrate similar results with reduced miglitol infusion doses, approaching those currently prescribed for type 2 diabetes (correcting for rodent size), as well as the efficacy of oral miglitol administration in restoring the counterregulatory responses following antecedent hypoglycemia.

Recurrent Hypoglycemia Increases Anxiety and Amygdala Norepinephrine Release During Subsequent Hypoglycemia

Recurrent hypoglycemia (RH) is a common and debilitating side effect of therapy in patients with both type 1 and, increasingly, type 2 diabetes. Previous studies in rats have shown marked effects of RH on subsequent hippocampal behavioral, metabolic, and synaptic processes. In addition to impaired memory, patients experiencing RH report alterations in cognitive processes that include mood and anxiety, suggesting that RH may also affect amygdala function. We tested the impact of RH on amygdala function using an elevated plus-maze test of anxiety together with in vivo amygdala microdialysis for norepinephrine (NEp), a widely used marker of basolateral amygdala cognitive processes. In contrast to findings in the hippocampus and prefrontal cortex, neither RH nor acute hypoglycemia alone significantly affected plus-maze performance or NEp release. However, animals tested when hypoglycemic who had previously experienced RH had elevated amygdala NEp during plus-maze testing, accompanied by increased anxiety (i.e., less time spent in the open arms of the plus-maze). The results show that RH has widespread effects on subsequent brain function, which vary by neural system.

UCP2 and ANT differently modulate proton-leak in brain mitochondria of long-term hyperglycemic and recurrent hypoglycemic rats

A growing body of evidence suggests that mitochondrial proton-leak functions as a regulator of reactive oxygen species production and its modulation may limit oxidative injury to tissues. The main purpose of this work was to characterize the proton-leak of brain cortical mitochondria from long-term hyperglycemic and insulin-induced recurrent hypoglycemic rats through the modulation of the uncoupling protein 2 (UCP2) and adenine nucleotide translocator (ANT). Streptozotocin-induced diabetic rats were treated subcutaneously with twice-daily insulin injections during 2 weeks to induce the hypoglycemic episodes. No differences in the basal proton-leak, UCP2 and ANT protein levels were observed between the experimental groups. Mitochondria from recurrent hypoglycemic rats presented a decrease in proton-leak in the presence of GDP, a specific UCP2 inhibitor, while an increase in proton-leak was observed in the presence of linoleic acid, a proton-leak activator, this effect being reverted by the simultaneous addition of GDP. Mitochondria from long-term hyperglycemic rats showed an enhanced susceptibility to ANT modulation as demonstrated by the complete inhibition of basal and linoleic acid-induced proton-leak caused by the ANT specific inhibitor carboxyatractyloside. Our results show that recurrent-hypoglycemia renders mitochondria more susceptible to UCPs modulation while the proton-leak of long-term hyperglycemic rats is mainly modulated by ANT, which suggest that brain cortical mitochondria have distinct adaptation mechanisms in face of different metabolic insults.

Insulin-induced recurrent hypoglycemia exacerbates diabetic brain mitochondrial dysfunction and oxidative imbalance

Intensive insulin therapy can prevent or slow the progression of long-term diabetes complications but, at the same time, it increases the risk for episodes of severe hypoglycemia. In our study, we used a protocol intended to mimic the levels of blood glucose that occur in type 1 diabetic patients under an intensive insulin therapy. Streptozotocin (STZ)-induced diabetic rats were treated subcutaneously with twice-daily insulin injections for 2weeks to induce hypoglycemic episodes. Brain cortical and hippocampal mitochondria were isolated and mitochondrial bioenergetics (respiratory chain and phosphorylation system) and oxidative status parameters (malondialdehyde (MDA) levels, mitochondrial aconitase activity and enzymatic and non-enzymatic antioxidant defenses) were analyzed. The protein levels of synaptophysin, a marker of synaptic integrity, and caspase 9 activity were also evaluated in cortical and hippocampal homogenates. Brain cortical mitochondria isolated from hyper- and recurrent hypoglycemic animals presented higher levels of MDA and α-tocopherol together with an increased glutathione disulfide reductase activity, lower manganese superoxide dismutase (MnSOD) activity and glutathione-to-glutathione disulfide (GSH/GSSG) ratio. No significant alterations were found in cortical mitochondrial respiratory chain and oxidative phosphorylation system. Hippocampal mitochondria from both experimental groups presented an impaired oxidative phosphorylation system characterized by a decreased mitochondrial energization potential and ATP levels and higher repolarization lag phase. In addition, higher MDA levels and decreased GSH/GSSG, α-tocopherol levels, and aconitase, glutathione peroxidase and MnSOD activities were observed in both groups of animals. Hippocampal mitochondria from recurrent hypoglycemic animals also showed an impairment of the respiratory chain characterized by a lower state 3 of respiration, respiratory control ratio and ADP/O index, and a higher state 4 of respiration. Additionally, a non-statistically significant decrease in synaptophysin protein levels was observed in cortical homogenates from recurrent hypoglycemic rats as well as in hippocampal homogenates from hyperglycemic and recurrent hypoglycemic rats. An increase in caspase 9 activity was also observed in hippocampal homogenates from hyperglycemic and recurrent hypoglycemic animals. Our results show that mitochondrial dysfunction induced by long-term hyperglycemic effects is exacerbated by recurrent hypoglycemia, which may compromise the function and integrity of brain cells.

Profile of minocycline neuroprotection in bilirubin-induced auditory system dysfunction

Excessive hyperbilirubinemia in human neonates can cause permanent dysfunction of the auditory system, as assessed with brainstem auditory evoked potentials (BAEPs). Jaundiced Gunn rat pups (jjs) exhibit similar BAEP abnormalities as hyperbilirubinemic neonates. Sulfadimethoxine (sulfa) administration to jjs, which displaces bilirubin from serum albumin into tissues including brain, exacerbates acute toxicity. Minocycline administered prior to sulfa in jjs protects against BAEP abnormalities. This study evaluates the neuroprotective capabilities of minocycline HCl (50 mg/kg) administered 30 or 120 min after sulfa (200 mg/kg) in 16 days old jjs. BAEPs are recorded at 6 or 24 h post-sulfa. Abnormal BAEP waves exhibit increased latency and decreased amplitude. The sulfa/saline treated jjs exhibited a significantly increased interwave interval between waves I and II (I-II IWI) and significantly decreased amplitudes of waves II and III compared to the saline/saline jjs. The minocycline 30 min post-sulfa (sulfa/mino+30) group was not significantly different from the saline/saline control group, indicating neuroprotection. The minocycline 120 min post-sulfa (sulfa/mino+120) group had a significantly decreased amplitude of wave III at both 6 and 24h. These studies indicate that minocycline has a graded neuroprotective effect when administered after acute bilirubin neurotoxicity.

Mini-review: impact of recurrent hypoglycemia on cognitive and brain function

Recurrent hypoglycemia (RH), the most common side-effect of intensive insulin therapy for diabetes, is well established to diminish counter-regulatory responses to further hypoglycemia. However, despite significant patient concern, the impact of RH on cognitive and neural function remains controversial. Here we review the data from both human studies and recent animal studies regarding the impact of RH on cognitive, metabolic, and neural processes. Overall, RH appears to cause brain adaptations which may enhance cognitive performance and fuel supply when euglycemic but which pose significant threats during future hypoglycemic episodes.

Recurrent antecedent hypoglycemia alters neuronal oxidative metabolism in vivo

OBJECTIVE

The objective of this study was to characterize the changes in brain metabolism caused by antecedent recurrent hypoglycemia under euglycemic and hypoglycemic conditions in a rat model and to test the hypothesis that recurrent hypoglycemia changes the brain's capacity to utilize different energy substrates.

RESEARCH DESIGN AND METHODS

Rats exposed to recurrent insulin-induced hypoglycemia for 3 days (3dRH rats) and untreated controls were subject to the following protocols: [2-(13)C]acetate infusion under euglycemic conditions (n = 8), [1-(13)C]glucose and unlabeled acetate coinfusion under euglycemic conditions (n = 8), and [2-(13)C]acetate infusion during a hyperinsulinemic-hypoglycemic clamp (n = 8). In vivo nuclear magnetic resonance spectroscopy was used to monitor the rise of(13)C-labeling in brain metabolites for the calculation of brain metabolic fluxes using a neuron-astrocyte model.

RESULTS

At euglycemia, antecedent recurrent hypoglycemia increased whole-brain glucose metabolism by 43 +/- 4% (P < 0.01 vs. controls), largely due to higher glucose utilization in neurons. Although acetate metabolism remained the same, control and 3dRH animals showed a distinctly different response to acute hypoglycemia: controls decreased pyruvate dehydrogenase (PDH) flux in astrocytes by 64 +/- 20% (P = 0.01), whereas it increased by 37 +/- 3% in neurons (P = 0.01). The 3dRH animals decreased PDH flux in both compartments (-75 +/- 20% in astrocytes, P < 0.001, and -36 +/- 4% in neurons, P = 0.005). Thus, acute hypoglycemia reduced total brain tricarboxylic acid cycle activity in 3dRH animals (-37 +/- 4%, P = 0.001), but not in controls.

CONCLUSIONS

Our findings suggest that after antecedent hypoglycemia, glucose utilization is increased at euglycemia and decreased after acute hypoglycemia, which was not the case in controls. These findings may help to identify better methods of preserving brain function and reducing injury during acute hypoglycemia.

Effect of recurrent hypoglycemia on spatial cognition and cognitive metabolism in normal and diabetic rats

The effects of recurrent hypoglycemia (RH) on cognition in human subjects remain controversial, perhaps in part due to difficulty in completely controlling previous hypoglycemic history. We used a model of RH in nondiabetic and diabetic rats to examine the effects of short-term (3 h daily for 3 days) RH on subsequent hippocampally dependent spatial memory, tested either at euglycemia or under acute hypoglycemia. Hippocampal metabolism was simultaneously measured using microdialysis. Antecedent RH improved task performance (79 +/- 2% alternation in nondiabetic RH animals vs. 63 +/- 3% in controls; P < 0.001) at euglycemia, accompanied by reversal of the task-associated dip (20 +/- 1% below baseline) in hippocampal extracellular fluid (ECF) glucose seen in control animals. RH rats also had a larger rise in hippocampal ECF glucose, after intraperitoneal glucose injection, than did controls. However, RH animals tested at acute hypoglycemia ( approximately 2.8 mmol/l) performed significantly worse than control animals. Results were similar in diabetic and nondiabetic rats. Our data suggest that RH causes improvement in subsequent cognitive performance at euglycemia, accompanied by alterations in cognitive metabolism. When glucose availability is limited, complex cognitive functioning seems to be adversely effected in RH animals, perhaps to better maintain and preserve basic brain functions.

Extracellular glucose in rat ventromedial hypothalamus during acute and recurrent hypoglycemia

The activity of neurons in the ventromedial hypothalamus (VMH) important for initiating compensatory responses to hypoglycemia is influenced by ambient glucose concentration. In the present study, we used in vivo microdialysis to evaluate interstitial glucose concentrations in rat VMH under various glycemic conditions. Using the zero-net-flux method, steady-state glucose concentration in the VMH was approximately 20% of blood glucose (approximately 1.4 mmol/l) in fed rats but approximately 14% of blood glucose (approximately 0.7 mmol/l) in overnight-fasted rats. During moderate hypoglycemia VMH glucose declined in parallel with blood glucose; however, VMH glucose decreased to a greater degree than blood glucose during a more severe hypoglycemic episode, falling to 10 +/- 1.2% of blood levels (P < 0.01). To determine whether VMH glucose concentrations were influenced by recurrent episodes of hypoglycemia a second zero-net-flux study was conducted. Steady-state glucose concentrations in the VMH were approximately 20% lower after three episodes of recurrent hypoglycemia, a value 17.8 +/- 0.8% of blood glucose, although the relative change in VMH glucose levels during the first and fourth hypoglycemic episodes were similar. From these results, we conclude that interstitial glucose concentrations in the VMH are not maintained at a constant level and are more dynamic than previously proposed.

CNS sensing and regulation of peripheral glucose levels

It is clear that the brain has evolved a mechanism for sensing levels of ambient glucose. Teleologically, this is likely to be a function of its requirement for glucose as a primary metabolic substrate. There is no question that the brain can sense and mount a counterregulatory response to restore very low levels of plasma and brain glucose. But it is less clear that the changes in glucose associated with normal diurnal rhythms and feeding cycles are sufficient to influence either ingestive behavior or the physiologic responses involved in regulating plasma glucose levels. Glucosensing neurons are clearly a distinct class of metabolic sensors with the capacity to respond to a variety of intero- and exteroceptive stimuli. This makes it likely that these glucosensing neurons do participate in physiologically relevant homeostatic mechanisms involving energy balance and the regulation of peripheral glucose levels. It is our challenge to identify the mechanisms by which these neurons sense and respond to these metabolic cues.

Synaptic adaptation to repeated hypoglycemia depends on the utilization of monocarboxylates in Guinea pig hippocampal slices

This report provides in vitro evidence that synaptic activity becomes resistant to repeated hypoglycemia, i.e., hypoglycemic synaptic adaptation occurs. Synaptic function was estimated by the amplitude of the postsynaptic population spike (PS) recorded in the granule cell layer of guinea pig hippocampal slices. ATP, phosphocreatine (PCr), glycogen, and glucose concentrations were measured to investigate energy metabolism homeostasis. Glucose deprivation produced a complete elimination of the PS amplitude, with a 50% inhibition by 10.6 min, and a approximately 15% reduction in ATP and PCr concentrations. Low-glucose (0.5-1 mmol/l) medium gradually depressed the PS. After recovery from glucose depletion, repeated glucose deprivation produced a slowly developing depression of PS, with a 50% inhibition by 36.5 min. However, ATP and PCr concentrations were maintained. Incubation in secondary low-glucose medium maintained PS amplitude. Hippocampal glycogen and glucose concentrations promptly decreased during repeated glucose deprivation, indicating that glycogenolysis does not fuel synaptic adaptation to repeated hypoglycemia. Synaptic function during repeated glucose depletion was reversibly depressed by addition of alpha-cyano-4-hydroxycinnamic acid or 3-isobutyl-1-methylxanthine, inhibitors of the monocarboxylate transporter. Replacement of extracellular glucose with Na-lactate or Na-pyruvate sustained synaptic transmission after transient glucose depletion. These results indicate that synaptic utilization of monocarboxylates sustains hypoglycemic synaptic adaptation.

Blood-to-brain glucose transport, cerebral glucose metabolism, and cerebral blood flow are not increased after hypoglycemia

Recent antecedent hypoglycemia has been found to shift glycemic thresholds for autonomic (including adrenomedullary epinephrine), symptomatic, and other responses to subsequent hypoglycemia to lower plasma glucose concentrations. This change in threshold is the basis of the clinical syndromes of hypoglycemia unawareness and, in part, defective glucose counterregulation and the unifying concept of hypoglycemia-associated autonomic failure in type 1 diabetes. We tested in healthy young adults the hypothesis that recent antecedent hypoglycemia increases blood-to-brain glucose transport, a plausible mechanism of this phenomenon. Eight subjects were studied after euglycemia, and nine were studied after approximately 24 h of interprandial hypoglycemia ( approximately 55 mg/dl, approximately 3.0 mmol/l). The latter were shown to have reduced plasma epinephrine (P = 0.009), neurogenic symptoms (P = 0.009), and other responses to subsequent hypoglycemia. Global bihemispheric blood-to-brain glucose transport and cerebral glucose metabolism were calculated from rate constants derived from blood and brain time-activity curves-the latter determined by positron emission tomography (PET)-after intravenous injection of [1-(11)C]glucose at clamped plasma glucose concentrations of 65 mg/dl (3.6 mmol/l). For these calculations, a model was used that includes a fourth rate constant to account for egress of [(11)C] metabolites. Cerebral blood flow was measured with intravenous [(15)O]water using PET. After euglycemia and after hypoglycemia, rates of blood-to-brain glucose transport (24.6 +/- 2.3 and 22.4 +/- 2.4 micromol. 100 g(-1). min(-1), respectively), cerebral glucose metabolism (16.8 +/- 0.9 and 15.9 +/- 0.9 micromol. 100 g(-1). min(-1), respectively) and cerebral blood flow (56.8 +/- 3.9 and 53.3 +/- 4.4 ml. 100 g(-1). min(-1), respectively) were virtually identical. These data do not support the hypothesis that recent antecedent hypoglycemia increases blood-to-brain glucose transport during subsequent hypoglycemia. They do not exclude regional increments in blood-to-brain glucose transport. Alternatively, the fundamental alteration might lie beyond the blood-brain barrier.