Dopamine D1 and D2 receptors regulate extracellular lactate and glucose concentrations in the nucleus accumbens

Optimized Fabrication of Carbon-Fiber Microbiosensors for Codetection of Glucose and Dopamine in Brain Tissue

Dopamine (DA) signaling is critically important in striatal function, and this metabolically demanding process is fueled largely by glucose. However, DA and glucose are typically studied independently and, as such, the precise relationship between DA release and glucose availability remains unclear. Fast-scan cyclic voltammetry (FSCV) is commonly coupled with carbon-fiber microelectrodes to study DA transients. These microelectrodes can be modified with glucose oxidase (GOx) to generate microbiosensors capable of simultaneously quantifying real-time and physiologically relevant fluctuations of glucose, a nonelectrochemically active substrate, and DA, which is readily oxidized and reduced at the electrode surface. A chitosan hydrogel can be electrodeposited to entrap the oxidase enzyme on the sensor surface for stable, sensitive, and selective codetection of glucose and DA using FSCV. This strategy can also be used to entrap lactate oxidase on the carbon-fiber surface for codetection of lactate and DA. However, these custom probes are individually fabricated by hand, and performance is variable. This study characterizes the physical nature of the hydrogel and its effects on the acquired electrochemical data in the detection of glucose (2.6 mM) and DA (1 μM). The results demonstrate that the electrodeposition of the hydrogel membrane is improved using a linear potential sweep rather than a direct step to the target potential. Electrochemical impedance spectroscopy data relate information on the physical nature of the electrode/solution interface to the electrochemical performance of bare and enzyme-modified carbon-fiber microelectrodes. The electrodeposition waveform and scan rate were characterized for optimal membrane formation and performance. Finally, codetection of both DA/glucose and DA/lactate was demonstrated in intact rat striatum using probes fabricated according to the optimized protocol. Overall, this work improves the reliable fabrication of carbon-fiber microbiosensors for codetection of DA and important energetic substrates that are locally delivered to the recording site to meet metabolic demand.

Dopamine signaling impairs ROS modulation by mitochondrial hexokinase in human neural progenitor cells

Dopamine signaling has numerous roles during brain development. In addition, alterations in dopamine signaling may be also involved in the pathophysiology of psychiatric disorders. Neurodevelopment is modulated in multiple steps by reactive oxygen species (ROS), byproducts of oxidative metabolism that are signaling factors involved in proliferation, differentiation, and migration. Hexokinase (HK), when associated with the mitochondria (mt-HK), is a potent modulator of the generation of mitochondrial ROS in the brain. In the present study, we investigated whether dopamine could affect both the activity and redox function of mt-HK in human neural progenitor cells (NPCs). We found that dopamine signaling via D₁R decreases mt-HK activity and impairs ROS modulation, which is followed by an expressive release of H₂O₂ and impairment in calcium handling by the mitochondria. Nevertheless, mitochondrial respiration is not affected, suggesting specificity for dopamine on mt-HK function. In neural stem cells (NSCs) derived from induced-pluripotent stem cells (iPSCs) of schizophrenia patients, mt-HK is unable to decrease mitochondrial ROS, in contrast with NSCs derived from healthy individuals. Our data point to mitochondrial hexokinase as a novel target of dopaminergic signaling, as well as a redox modulator in human neural progenitor cells, which may be relevant to the pathophysiology of neurodevelopmental disorders such as schizophrenia.

Simultaneous voltammetric detection of glucose and lactate fluctuations in rat striatum evoked by electrical stimulation of the midbrain

Glucose and lactate provide energy for cellular function in the brain and serve as an important carbon source in the synthesis of a variety of biomolecules. Thus, there is a critical need to quantitatively monitor these molecules in situ on a time scale commensurate with neuronal function. In this work, carbon-fiber microbiosensors were coupled with fast-scan cyclic voltammetry to monitor glucose and lactate fluctuations at a discrete site within rat striatum upon electrical stimulation of the midbrain projection to the region. Systematic variation of stimulation parameters revealed the distinct dynamics by which glucose and lactate responded to the metabolic demand of synaptic function. Immediately upon stimulation, extracellular glucose and lactate availability rapidly increased. If stimulation was sufficiently intense, concentrations then immediately fell below baseline in response to incurred metabolic demand. The dynamics were dependent on stimulation frequency, such that more robust fluctuations were observed when the same number of pulses was delivered at a higher frequency. The rates at which glucose was supplied to, and depleted from, the local recording region were dependent on stimulation intensity, and glucose dynamics led those of lactate in response to the most substantial stimulations. Glucose fluctuated over a larger concentration range than lactate as stimulation duration increased, and glucose fell further from baseline concentrations. These real-time measurements provide an unprecedented direct comparison of glucose and lactate dynamics in response to metabolic demand elicited by neuronal activation. Graphical abstract.

Tandospirone, a 5-HT1A partial agonist, ameliorates aberrant lactate production in the prefrontal cortex of rats exposed to blockade of N-methy-D-aspartate receptors; Toward the therapeutics of cognitive impairment of schizophrenia

Rationale: Augmentation therapy with serotonin-1A (5-HT_1A) receptor partial agonists has been suggested to improve cognitive impairment in patients with schizophrenia. Decreased activity of prefrontal cortex may provide a basis for cognitive deficits of the disease. Lactate plays a significant role in the supply of energy to the brain, and glutamatergic neurotransmission contributes to lactate production.

Objectives and methods: The purposes of this study were to examine the effect of repeated administration (once a daily for 4 days) of tandospirone (0.05 or 5 mg/kg) on brain energy metabolism, as represented by extracellular lactate concentration (eLAC) in the medial prefrontal cortex (mPFC) of a rat model of schizophrenia.

Results: Four-day treatment with MK-801, an NMDA-R antagonist, prolonged eLAC elevation induced by foot-shock stress (FS). Co-administration with the high-dose tandospirone suppressed prolonged FS-induced eLAC elevation in rats receiving MK-801, whereas tandospirone by itself did not affected eLAC increment.

Conclusions: These results suggest that stimulation of 5-HT_1A receptors ameliorates abnormalities of energy metabolism in the mPFC due to blockade of NMDA receptors. These findings provide a possible mechanism, based on brain energy metabolism, by which 5-HT_1A agonism improve cognitive impairment of schizophrenia and related disorders.

Chronic treatment with tandospirone, a 5-HT(1A) receptor partial agonist, suppresses footshock stress-induced lactate production in the prefrontal cortex of rats

Serotonin 1A receptor (5-HT1A-R) agonists have been demonstrated to elicit antidepressant and anxiolytic effects. Lactate has been considered to play a major role in energy metabolism in the brain. Specifically, extracellular lactate concentrations (eLAC) have been suggested to reflect neural activity. Mild physical (e.g., handling) and non-physical (e.g., psychological) stressors have been shown to increase eLAC in several brain regions, including the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA). Using in vivo microdialysis technique, we measured eLAC in the mPFC and BLA of rats under electric footshock stress to clarify the effect of repeated injection procedure (saline, once daily for 14 days) as a stressor on brain energy metabolism. Then, we examined the effect of chronic treatment with tandospirone, a 5-HT1A-R partial agonist, on eLAC during footshock stress in the mPFC. Footshock stress led to an increase in eLAC both in the mPFC and BLA in rats without injections. Repeated saline injection increased basal eLAC in the BLA, while footshock-induced lactate increment was reduced. In the mPFC, repeated saline injection did not affect basal eLAC and footshock-induced eLAC increments. Chronic treatment with tandospirone, at 0.2 and 1.0 mg/kg/day, but not 2.0 mg/kg/day, attenuated footshock stress-induced eLAC elevation in the mPFC. These observations suggest that eLAC in the BLA is sensitive to repeated exposure to physical stress. Data also indicate chronic treatment with tandospirone diminishes acute energy demands during neural activation in the mPFC. The implications of the present findings in relation to clinical efficacy of 5-HT1A agonists are discussed.

Effect of transient blockade of N-methyl-D-aspartate receptors at neonatal stage on stress-induced lactate metabolism in the medial prefrontal cortex of adult rats: role of 5-HT1A receptor agonism

Decreased activity of the medial prefrontal cortex (mPFC) has been considered a basis for core symptoms of schizophrenia, an illness associated with a neurodevelopmental origin. Evidence from preclinical and clinical studies indicates that serotonin (5-HT)1A receptors play a crucial role in the energy metabolism of the mPFC. This study was undertaken to determine (1) if transient blockade of N-methyl-D-aspartate receptors during the neonatal stage inhibit energy demands in response to stress, as measured by extracellular lactate concentrations, in the mPFC at the young adult stage, and (2) if tandospirone, a 5-HT1A partial agonist, reverses the effect of the neonatal insult on energy metabolism. Male pups received MK-801 (0.20 mg/kg) on postnatal days (PDs) 7-10. On PD 63, footshock stress-induced lactate levels were measured using in vivo microdialysis technique. Tandospirone (0.1, 1.0, and 5.0 mg/kg) was administered once daily for 14 days before the measurement of lactate levels. Neonatal MK-801 treatment suppressed footshock stress-induced lactate production in the mPFC, but not caudate-putamen, whereas basal lactate levels were not significantly changed in either brain region. The MK-801-induced suppression of footshock stress-induced lactate production in the mPFC was attenuated by tandospirone at 1.0mg/kg/day, but not 0.1 or 5.0 mg/kg/day, which is an effect antagonized by coadministration of WAY-100635, a selective 5-HT1A antagonist. These results suggest a role for impaired lactate metabolism in some of the core symptoms of schizophrenia, for example, negative symptoms and cognitive deficits. The implications for the ability of 5-HT1A agonism to ameliorate impaired lactate production in the mPFC of this animal model are discussed.

Levodopa Treatment Improves Functional Recovery After Experimental Stroke

Background and Purpose—

Delayed treatment of patients with stroke with levodopa/benserazide contributes to enhanced functional recovery, but the mechanisms involved are poorly understood. The present study was designed to investigate if levodopa/benserazide treatment improves recovery of lost neurological function and contributes to tissue reorganization in the rat brain after stroke.

Methods—

Male Wistar rats were subjected to transient occlusion of the middle cerebral artery (120 minutes) and treated with levodopa (1, 5, and 20 mg/kg)/benserazide (15 mg/kg) or saline for 12 consecutive days starting on Day 2 after transient occlusion of the middle cerebral artery. Infarct volume was determined and sensorimotor function was assessed using the rotating pole test, a 28-point neuroscore, and a cylinder test on Days 2, 7, and 14 after transient occlusion of the middle cerebral artery. The spatiotemporal expression pattern of dopamine-1 and dopamine-2 receptors and the dopamine- and cAMP-regulated neuronal phosphoprotein in reactive astrocytes were analyzed in the ischemic hemisphere as well as in cultured astrocytes.

Results—

Treatment with levodopa/benserazide significantly improved the recovery of sensorimotor function after transient occlusion of the middle cerebral artery without affecting the infarct volume. In addition, we found that different subpopulations of glial fibrillary acidic protein-positive astrocytes in the peri-infarct area express dopamine-1 receptors and dopamine-2 receptors as well as dopamine- and cAMP-regulated neuronal phosphoprotein.

Conclusions—

Our results strongly corroborate the concept of recovery enhancing actions of levodopa treatment after stroke. Also, astrocytes in the peri-infarct area may contribute to the dopamine enhanced recovery mechanisms.

The biphasic NAD(P)H fluorescence response of astrocytes to dopamine reflects the metabolic actions of oxidative phosphorylation and glycolysis

The NAD(+)/NADH redox pair constitutes an important metabolic node connecting catabolic pathways to energy production. We took advantage of the fluorescence of NADH to monitor changes in NADH levels by 2-photon laser scanning microscopy in cultured cortical astrocytes and acutely isolated brain slices in response to dopamine (DA), a major neurotransmitter involved in modulation of attention, motivation, and learning. DA induced a dose-dependent biphasic response of the NAD(P)H fluorescence signal, consisting of an initial decrease followed by a subsequent increase. This response was mediated by D1-receptors, protein kinase A, and 5'-AMP-activated protein kinase signaling. While the initial decrease could be inhibited by blocking mitochondrial respiratory chain, the increase was inhibited by blocking glycolysis. Finally, activation of DA receptors on astrocytes in acutely isolated mouse cortical brain slices also induced an increase in the NAD(P)H fluorescence signal. We conclude that DA activates two opposing components of astrocytic metabolism with different kinetics. This response of the astroglial metabolism might contribute to fine-tuned participation of astrocytes to neuronal activity and functional states of the brain.

Improved temporal resolution for in vivo microdialysis by using segmented flow

Microdialysis sampling probes were interfaced to a segmented flow system to improve temporal resolution for monitoring concentration dynamics. Aqueous dialysate was segmented into nanoliter plugs by pumping sample stream into the base of a tee channel structure microfabricated on a PDMS chip that had an immiscible carrier phase (perfluorodecalin) pumped into the cross arm of the tee. Varying the oil flow rate from 0.22 to 6.3 microL/min and sample flow rate from 42 to 328 nL/min allowed control of plug volume, interval between plugs, and frequency of plug generation between 6 and 28 nL, 0.6 and 10 s, and 0.1 and 1.7 Hz, respectively. Temporal resolution of the system, determined by measuring fluorescence in individual sample plugs following step changes of fluorescein concentration at the sampling probe surface, was as good as 15 s. Temporal resolution was independent of both sampling flow rate and distance that samples were pumped from the sampling probe. This effect is due to the prevention of Taylor dispersion of the sample as it was transported by segmented flow. In contrast, without flow segmentation, temporal resolution was worsened from 25 to 160 s as the detection point was moved from the sampling probe to 40 cm downstream. Glucose was detected by modifying the chip to allow enzyme assay reagents to be mixed with dialysate as sample plugs formed. The resulting assay had a detection limit of 50 microM and a linear range of 0.2-2 mM. This system was used to measure glucose in the brain of anesthetized rats. Basal concentration was 1.5 +/- 0.1 mM (n = 3) and was decreased 60% by infusion of high-K(+) solution through the probe. These results demonstrate the potential of microdialysis with segmented flow to be used for in vivo monitoring experiments with high temporal resolution.

Lactate production and neurotransmitters; evidence from microdialysis studies

Recent studies have found that lactate metabolism plays a significant role in energy supply during acute neural activation in the brain. We will review evidence from microdialysis studies for a relationship between neurotransmitters and lactate production, as revealed in studies of the effects of psychotropic drugs on stress-induced enhancement of extracellular lactate concentrations. Glutamate enhances stress-induced lactate production via activation of N-methyl-D-asparate receptors, and is affected by uptake of glutamate through glutamate transporters. Findings from microdialysis studies suggest that major neurotransmitters, including norepinephrine, dopamine, serotonin, and GABA (via benzodiazepine-receptors) affect lactate production, depending on brain areas, especially during stress. Among these neurotransmitters, glutamate may principally contribute to the regulation of lactate production, with other neurotransmitter systems affecting the extracellular lactate levels in a glutamate-mediated manner. The role for anaerobic metabolism in the supply of energy, as represented by lactate dynamics, deserves further clarification. Monitoring with intracerebral microdialysis is a reliable method for this purpose. Research into this area is likely to provide a novel insight into the mode of action of psychotropic drugs, and the pathophysiology of some of the stress-related mental disorders as well.

Role of glutamate transporters in the modulation of stress-induced lactate metabolism in the rat brain

RATIONALE

Lactate, like glucose, has recently been found to be an energy substrate for neural activity. It is indicated that lactate is produced by astrocytes under the regulation of glutamatergic tone.

OBJECTIVES

Using in vivo microdialysis technique, we measured extracellular lactate concentrations in the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) of rats. To investigate the role of the glutamate transporter in the modulation of footshock stress-induced energy demands in both brain regions, we attempted to determine whether the footshock stress-induced changes of extracellular lactate concentrations are attenuated by local perfusion of the glutamate uptake inhibitor dihydrokainate (DHK).

RESULTS

Perfusion of 1.0 mM DHK produced an increase in basal extracellular lactate levels in the mPFC and BLA, whereas 0.1 mM DHK did not affect lactate concentrations in either region. DHK also attenuated stress-induced increment of extracellular lactate concentrations in the mPFC, and completely prevented it in the BLA.

CONCLUSIONS

These results suggest that glutamate transporters regulate lactate availability in astrocytes and indicate that the rapid energy demand induced by glutamate contributes to local lactate production.