Interhemispheric regulation of the medial prefrontal cortical glutamate stress response in rats

Uninterrupted in vivo cerebral microdialysis measures of the acute neurochemical response to a single or repeated concussion in a rat model combining force and rotation

Altered extracellular amino acid concentrations following concussion or mild traumatic brain injury can result in delayed neuronal damage through overactivation of NMDA glutamatergic receptors. However, the consequences of repeated concussions prior to complete recovery are not well understood. In this study, we utilized in vivo cerebral microdialysis and a weight-drop model to investigate the acute neurochemical response to single and repeated concussions in adult rats that were fully conscious. A microdialysis probe was inserted into the hippocampus and remained in place during impact. Primary outcomes included concentrations of glutamate, GABA, taurine, glycine, glutamine, and serine, while secondary outcomes were righting times and excitotoxic indices. Compared to sham injury, the first concussion resulted in significant increases in glutamate, GABA, taurine, and glycine levels, longer righting times, and higher excitotoxic indices. Following the second concussion, righting times were significantly longer, suggesting cumulative effects of repeated concussion while only partial increases were observed in glutamate and taurine levels. GABA and glycine levels, and excitotoxic indices were comparable to sham injury. These findings suggest that single and repeated concussions may induce acute increases in several amino acids, while repeated concussions could exacerbate neurological symptoms despite less pronounced neurochemical changes.

Preventing prefrontal dysfunction by tDCS modulates stress-induced creativity impairment in women: an fNIRS study

Stress is a major external factor threatening creative activity. The study explored whether left-lateralized activation in the dorsolateral prefrontal cortex manipulated through transcranial direct current stimulation could alleviate stress-induced impairment in creativity. Functional near-infrared spectroscopy was used to explore the underlying neural mechanisms. Ninety female participants were randomly assigned to three groups that received stress induction with sham stimulation, stress induction with true stimulation (anode over the left and cathode over the right dorsolateral prefrontal cortex), and control manipulation with sham stimulation, respectively. Participants underwent the stress or control task after the transcranial direct current stimulation manipulation, and then completed the Alternative Uses Task to measure creativity. Behavioral results showed that transcranial direct current stimulation reduced stress responses in heart rate and anxiety. The functional near-infrared spectroscopy results revealed that transcranial direct current stimulation alleviated dysfunction of the prefrontal cortex under stress, as evidenced by higher activation of the dorsolateral prefrontal cortex and frontopolar cortex, as well as stronger inter-hemispheric and intra-hemispheric functional connectivity within the prefrontal cortex. Further analysis demonstrated that the cortical regulatory effect prevented creativity impairment induced by stress. The findings validated the hemispheric asymmetry hypothesis regarding stress and highlighted the potential for brain stimulation to alleviate stress-related mental disorders and enhance creativity.

Chronic hypothalamic-pituitary-adrenal axis disruption alters glutamate homeostasis and neural responses to stress in male C57Bl6/N mice

It is now well-established that stress elicits brain- and body-wide changes in physiology and has significant impacts on many aspects of health. The hypothalamic-pituitary-adrenal (HPA) axis is the major neuroendocrine system mediating the integrated response to stress. Appropriate engagement and termination of HPA activity enhances survival and optimizes physiological and behavioral responses to stress, while dysfunction of this system is linked to negative health outcomes such as depression, anxiety, and post-traumatic stress disorder. Glutamate signaling plays a large role in the transmission of stress-related information throughout the brain. Furthermore, aberrant glutamate signaling has negative consequences for neural plasticity and synaptic function and is linked to stress-related pathology. However, the connection between HPA dysfunction and glutamate signaling is not fully understood. We tested how HPA axis dysfunction (using low dose chronic corticosterone in the drinking water) affects glutamate homeostasis and neural responses under baseline and acute stress in male C57BL/6N mice. Using laser microdissection and transcriptomic analyses, we show that chronic disruption of the HPA axis alters the expression of genes related to glutamate signaling in the medial prefrontal cortex (mPFC), hippocampus, and amygdala. While neural responses to stress (as measured by FOS) in the hippocampus and amygdala were not affected in our model of HPA dysfunction, we observed an exaggerated response to stress in the mPFC. To further probe this we undertook in vivo biosensor measurements of the dynamics of extracellular glutamate responses to stress in the mPFC in real-time, and found glutamate dynamics in the mPFC were significantly altered by chronic HPA dysfunction. Together, these findings support the hypothesis that chronic HPA axis dysfunction alters glutamatergic signaling in regions known to regulate emotional behavior, providing more evidence linking HPA dysfunction and stress vulnerability.

Glutamate as a Stressoric Factor for the Ex Vivo Release of Catecholamines from the Rabbit Medial Prefrontal Cortex (mPFC)

One of the major roles of glutamic acid (Glu) is to serve as an excitatory neurotransmitter within the central nervous system (CNS). This amino acid influences the activity of several brain areas, including the thalamus, brainstem, spinal cord, basal ganglia, and pons. Catecholamines (CAs) are synthesized in the brain and adrenal medulla and by some sympathetic nerve fibers. CAs, including dopamine (DA), norepinephrine (NE), and epinephrine (E), are the principal neurotransmitters that mediate a variety of CNS functions, such as motor control, cognition, emotion, memory processing, pain, stress, and endocrine modulation. This study aims to investigate the effects of the application of various Glu concentrates (5, 50, and 200 µM) on CAs release from rabbit medial prefrontal cortex (mPFC) slices and compare any resulting correlations with CAs released from the hypothalamus during 90 min of incubation. Medial prefrontal cortex samples were dissected from decapitated, twelve-week-old female rabbits. The results demonstrated that Glu differentially influences the direct release of CAs from the mPFC and the indirect release of CAs from the hypothalamus. When under stress, the hypothalamus, a central brain structure of the HPA axis, induces and adapts such processes. Generally, there was an inhibitory effect of Glu on CAs release from mPFC slices. Our findings show that the effect arises from Glu's action on higher-order motivational structures, which may indicate its contribution to the stress response by modulating the amount of CAs released.

Efficacy of prophylactic versus therapeutic administration of the NMDA receptor antagonist MK-801 on the acute neurochemical response to a concussion in a rat model combining force and rotation

OBJECTIVE

Alterations in amino acid concentrations are a major contributor to the persistent neurological and behavioral effects induced by concussions and mild traumatic brain injuries (TBIs). Glutamate, the most abundant excitatory amino acid in the CNS, has a major role in the pathophysiological process of concussion. The indiscriminate liberation of glutamate immediately after a concussion triggers an excitotoxic response that leads to cell death, neuronal damage, and the dysfunction of surviving neurons, largely by overactivation of N-methyl-d-aspartate (NMDA) glutamatergic receptors. The aim of the present study was to investigate the efficacy of prophylactic versus therapeutic administration of MK-801, a promising NMDA receptor antagonist, on the acute changes in amino acid extracellular concentrations involved in excitotoxicity resulting from a concussive trauma.

METHODS

The immediate neurochemical response to a concussion cannot be characterized in humans. Therefore, the authors used their previously validated combination of a weight-drop concussion rat model and in vivo cerebral microdialysis. The microdialysis probe was inserted inside the hippocampus and left inserted at impact to allow uninterrupted sampling of amino acids of interest immediately after concussion. The primary outcome included amino acid concentrations and the secondary outcome included righting time. Samples were taken in 10-minute increments for 60 minutes before, during, and 60 minutes after impact, and analyzed for glutamate, gamma-aminobutyric acid, taurine, glycine, glutamine, and serine using high-performance liquid chromatography. Righting time was acquired as a neurological restoration indicator. Physiological saline or 10 mg/kg MK-801 was administrated intraperitoneally 60 minutes before or immediately following induction of sham injury or concussion.

RESULTS

Following induction of concussion, glutamate, taurine, and glycine levels as well as righting times in cases from the MK-801 treatment group were comparable to those of vehicle-treated animals. In contrast, righting times and amino acid concentrations observed within the first 10 minutes after induction of concussion in cases assigned to the MK-801 prophylaxis group were comparable to those of sham-injured animals.

CONCLUSIONS

These results suggest that presynaptic actions and peak availability of MK-801 following prophylactic administration significantly inhibit the immediate and indiscriminate release of glutamate, taurine, and glycine in extracellular fluid after a concussion.

Mice with dopaminergic neuron-specific deletion of DTNBP-1 gene show blunted nucleus accumbens dopamine release and associated behaviors

Reduced expression of a schizophrenia-associated gene Dystrobrevin Binding Protein 1 (DTNBP1) and its protein product dysbindin-1, has been reported in the brains of schizophrenia patients. DTNBP1-null mutant Sdy (Sandy) mice exhibit several behavioral features relevant to schizophrenia. Changes in dopaminergic as well as glutamatergic and GABAergic neurotransmission in cortico-limbic regions have been reported in Sdy mice. Since dysbindin-1 is expressed in multiple brain regions, it is not known whether dopamine (DA) changes observed in Sdy null mutants are due to dysbindin-1 deficiency in DAergic neurons specifically. Here, using a mouse line with conditional knockout (cKO) of DTNBP1 in DA neurons, we studied the effects of dysbindin-1 deficiency on DA release and DA-dependent behaviors. Spontaneous locomotor activity of cKO mice in novel environment was significantly reduced initially but was comparable at later time points with littermate controls. However, the locomotion-enhancing effect of a low dose of d-amphetamine (d-AMPH; 2.5 mg/kg, ip) was significantly attenuated in the cKO mice suggesting a dampened mesolimbic DA transmission. Similarly, the prepulse inhibition disrupting effect of d-AMPH was found to be significantly reduced in the mutant mice. No significant differences between the cKO and control mice were observed in tests of anxiety, spatial learning and memory and social interaction. In- vivo microdialysis in the nucleus accumbens (NAc) showed a decrease in d-AMPH-induced extracellular DA release in the cKO mice. No significant alterations in protein levels of DA transporter, phosphorylated CaM kinase-II or Akt308 in the NAc were observed in the cKO mice. Taken together, our data suggest an important role of dysbindin-1 in maintaining mesolimbic DA tone and call for further investigations identifying mechanisms linking dysbindin-1, DA and schizophrenia.

Pharmacological mechanisms of interhemispheric signal propagation: a TMS-EEG study

Interhemispheric connections across the corpus callosum have a predominantly inhibitory effect. Previous electrophysiology studies imply that local inhibitory circuits are responsible for inducing transcallosal inhibition, likely through inhibitory GABA_B-mediated neurotransmission. We investigated the neurochemical mechanisms involved in interhemispheric connectivity by measuring transcranial magnetic stimulation (TMS)-induced interhemispheric signal propagation (ISP) in the motor cortex and dorsolateral prefrontal cortex (DLPFC) with electroencephalography (EEG) recordings under the pharmacological effects of baclofen, L-DOPA, dextromethorphan, and rivastigmine. We hypothesized that for both stimulated regions, GABA_B receptor agonist baclofen would decrease ISP when compared against baseline while drugs that target other neurotransmitter systems (dopaminergic, acetylcholinergic, and glutamatergic systems) would have no effect on ISP. Twelve right-handed healthy volunteers completed this study and underwent TMS across five sessions in a randomized order. In the motor cortex, participants showed a significant decrease in ISP under baclofen, but not in the other drug conditions. There were no drug-induced changes in ISP in the DLPFC and baseline ISP did not differ across experimental sessions for both brain regions. Together, our results suggest that the inhibitory effects observed with interhemispheric signal transmission are mediated by a population of interneurons involving GABA_B receptor neurotransmission. Inhibitory mechanisms of ISP may be more salient for motor-related functions in the motor cortex than for cognitive control in the DLPFC. These findings are a fundamental step in advancing our understanding of interhemispheric connectivity and may be used to identify treatments for disorders in which transcallosal transmission is dysfunctional.

Endogenous glutamate within the prelimbic and infralimbic cortices regulates the incubation of cocaine-seeking in rats

The incubation of cue-reinforced cocaine-seeking coincides with increased extracellular glutamate within the ventromedial prefrontal cortex (vmPFC). The vmPFC is comprised of two subregions that oppositely regulate drug-seeking, with infralimbic (IL) activity inhibiting, and prelimibic (PL) activity facilitating, drug-seeking. Thus, we hypothesized that increasing and decreasing endogenous glutamate within the IL would attenuate and potentiate, respectively, cue-reinforced drug-seeking behavior, with the converse effects observed upon manipulations of endogenous glutamate within the PL. Male Sprague-Dawley rats were trained to self-administer cocaine (0.25 mg/infusion; 6 h/day X 10 days), the delivery of which was signaled by a tone-light cue. Rats were then subdivided into 3 or 30 day withdrawal groups. For testing, rats were microinjected with vehicle, 20 mM of the mGlu2/3 agonist LY379268 (to lower endogenous glutamate), or 300 μM of the excitatory amino acid transporter inhibitor threo-β-benzyloxyaspartate (TBOA; to raise endogenous glutamate) into either the IL or PL (0.5 μl/side) and then given a 30-min test for cue-reinforced drug-seeking. Vehicle-infused rats exhibited incubated responding on the cocaine-associated lever. Neither LY379268 nor TBOA altered behavior at 3 days withdrawal, indicating that glutamate within neither subregion regulates cue-reinforced drug-seeking during early withdrawal. At 30 days withdrawal, intra-PL LY379268 microinjection significantly decreased drug-seeking behavior, while the effect was more modest when infused intra-IL. Interestingly, intra-IL TBOA attenuated incubated drug-seeking during protracted withdrawal, but did not affect behavior when infused intra-PL. These results argue that glutamate release within the PL in response to drug-seeking likely drives the manifestation of incubated cocaine-seeking during protracted withdrawal.

Stress-Induced Reduction of Dorsal Striatal D2 Dopamine Receptors Prevents Retention of a Newly Acquired Adaptive Coping Strategy

The inability to learn an adaptive coping strategy in a novel stressful condition leads to dysfunctional stress coping, a marker of mental disturbances. This study tested the involvement of dorsal striatal dopamine receptors in the dysfunctional coping with the Forced Swim test fostered by a previous experience of reduced food availability. Adult male mice were submitted to a temporary (12 days) reduction of food availability [food-restricted (FR)] or continuously free-fed (FF). Different groups of FF and FR mice were used to evaluate: (1) dorsal striatal mRNA levels of the two isoforms of the dopamine D2 receptor (D2S, D2L). (2) Forced Swim-induced c-fos expression in the dorsal striatum; (3) acquisition and 24 h retention of passive coping with Forced Swim. Additional groups of FF mice were tested for 24 h retention of passive coping acquired during a first experience with Forced Swim immediately followed by intra-striatal infusion of vehicle or two doses of the dopamine D2/D3 receptors antagonist sulpiride or the D1/D5 receptors antagonist SCH23390. Previous restricted feeding selectively reduced mRNA levels of both D2 isoforms and abolished Forced Swim-induced c-fos expression in the left Dorsolateral Striatum and selectively prevented 24 h retention of the coping strategy acquired in a first experience of Forced Swim. Finally, temporary blockade of left Dorsolateral Striatum D2/D3 receptors immediately following the first Forced Swim experience selectively reproduced the behavioral effect of restricted feeding in FF mice. In conclusion, the present results demonstrate that mice previously exposed to a temporary reduction of food availability show low striatal D2 receptors, a known marker of addiction-associated aberrant neuroplasticity, as well as liability to relapse into maladaptive stress coping strategies. Moreover, they offer strong support to a causal relationship between reduction of D2 receptors in the left Dorsolateral Striatum and impaired consolidation of newly acquired adaptive coping.

Deficits in cognitive flexibility induced by chronic unpredictable stress are associated with impaired glutamate neurotransmission in the rat medial prefrontal cortex

Deficits in cognitive flexibility, the ability to modify behavior in response to changes in the environment, contribute to the onset and maintenance of stress-related neuropsychiatric illnesses, such as depression. Cognitive flexibility depends on medial prefrontal cortex (mPFC) function, and in depressed patients, cognitive inflexibility is associated with hypoactivity and decreased glutamate receptor expression in the mPFC. Rats exposed to chronic unpredictable stress (CUS) exhibit compromised mPFC function on the extradimensional (ED) set-shifting task of the attentional set-shifting test. Moreover, CUS-induced ED deficits are associated with dendritic atrophy and decreased glutamate receptor expression in the mPFC. This evidence suggests that impaired glutamate signaling may underlie stress-induced deficits in cognitive flexibility. To test this hypothesis, we first demonstrated that blocking NMDA or AMPA receptors in the mPFC during ED replicated CUS-induced deficits in naïve rats. Secondly, we found that expression of activity-regulated cytoskeleton-associated protein (Arc) mRNA, a marker of behaviorally induced glutamate-mediated plasticity, was increased in the mPFC following ED. We then showed that CUS compromised excitatory afferent activation of the mPFC following pharmacological stimulation of the mediodorsal thalamus (MDT), indicated by a reduced induction of c-fos expression. Subsequently, in vivo recordings of evoked potentials in the mPFC indicated that CUS impaired afferent activation of the mPFC evoked by MDT stimulation, but not the ventral hippocampus. Lastly, glutamate microdialysis showed that CUS attenuated the acute stress-evoked increase in extracellular glutamate in the mPFC. Together, these results demonstrate that CUS-induced ED deficits are associated with compromised glutamate neurotransmission in the mPFC.

Interhemispheric regulation of the rat medial prefrontal cortical glutamate stress response: role of local GABA- and dopamine-sensitive mechanisms

RATIONALE

We previously reported that stressors increase medial prefrontal cortex (PFC) glutamate (GLU) levels as a result of activating callosal neurons located in the opposite hemisphere and that this PFC GLU stress response is regulated by GLU-, dopamine- (DA-), and GABA-sensitive mechanisms (Lupinsky et al. 2010).

OBJECTIVES

Here, we examine the possibility that PFC DA regulates the stress responsivity of callosal neurons indirectly by acting at D₁ and D₂ receptors located on GABA interneurons.

METHODS

Microdialysis combined with drug perfusion (reverse dialysis) or microinjections was used in adult male Long-Evans rats to characterize D₁, D₂, and GABA_B receptor-mediated regulation of the PFC GABA response to tail-pinch (TP) stress.

RESULTS

We report that TP stress reliably elicited comparable increases in extracellular GABA in the left and right PFCs. SCH23390 (D₁ antagonist; 100 μM perfusate concentration) perfused by reverse microdialysis attenuated the local GABA stress responses equally in the left and right PFCs. Intra-PFC raclopride perfusion (D₂ antagonist; 100 μM) had the opposite effect, not only potentiating the local GABA stress response but also causing a transient elevation in basal (pre-stress) GABA. Moreover, unilateral PFC raclopride microinjection (6 nmol) attenuated the GLU response to TP stress in the contralateral PFC. Finally, intra-PFC baclofen perfusion (GABA_B agonist; 100 μM) inhibited the local GLU and GABA stress responses.

CONCLUSIONS

Taken together, these findings implicate PFC GABA interneurons in processing stressful stimuli, showing that local D₁, D₂, and GABA_B receptor-mediated changes in PFC GABA transmission play a crucial role in the interhemispheric regulation of GLU stress responsivity.

Brain Rewarding Stimulation Reduces Extracellular Glutamate Through Glial Modulation in Medial Prefrontal Cortex of Rats

Growing evidence implicates a critical involvement of prefrontal glial modulation of extracellular glutamate (GLU) in aversive behaviors. However, nothing is known about whether prefrontal glial cells modulate GLU levels in rewarding behaviors. To address this question, we measured GLU efflux in the medial prefrontal cortex (PFC) of rats associated with rewarding behaviors. We used intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB) as the rewarding behavior. GLU was indirectly measured using microdialysis combined with on-line fluorometric detection of NADH resulting from the reaction of GLU and NAD(+) catalyzed by GLU dehydrogenase with a time resolution of 1 min. ICSS caused a minute-by-minute change of extracellular GLU in the medial PFC, with a slight decrease during the stimulation, followed by an increase afterward. This bidirectional change was tetrodotoxin insensitive and abolished by the gliotoxin fluorocitrate. To confirm and extend the previous studies of aversion-induced increase of extracellular GLU in the medial PFC, we also measured prefrontal GLU efflux associated with an aversive stimulation, immobilization stress. The temporal change in extracellular GLU caused by this stress was markedly different from that observed during ICSS. A rapid increase in GLU was detected during the aversive stimulation, followed by a large increase afterward. This bimodal change was tetrodotoxin insensitive, similar to that detected for ICSS. These findings indicate a bidirectional regulation of extracellular GLU by prefrontal glial cells associated with rat ICSS behavior, and reveal that glial modulation of GLU neurochemistry in the medial PFC contributes to rewarding as well as aversive behaviors in rats.

Imaging in vivo glutamate fluctuations with [(11)C]ABP688: a GLT-1 challenge with ceftriaxone

Molecular imaging offers unprecedented opportunities for investigating dynamic changes underlying neuropsychiatric conditions. Here, we evaluated whether [(11)C]ABP688, a positron emission tomography (PET) ligand that binds to the allosteric site of the metabotropic glutamate receptor type 5 (mGluR5), is sensitive to glutamate fluctuations after a pharmacological challenge. For this, we used ceftriaxone (CEF) administration in rats, an activator of the GLT-1 transporter (EAAT2), which is known to decrease extracellular levels of glutamate. MicroPET [(11)C]ABP688 dynamic acquisitions were conducted in rats after a venous injection of either saline (baseline) or CEF 200 mg/kg (challenge). Binding potentials (BP(ND)) were obtained using the simplified reference tissue method. Between-condition statistical parametric maps indicating brain regions showing the highest CEF effects guided placement of microdialysis probes for subsequent assessment of extracellular levels of glutamate. The CEF administration increased [(11)C]ABP688 BP(ND) in the thalamic ventral anterior (VA) nucleus bilaterally. Subsequent microdialysis assessment revealed declines in extracellular glutamate concentrations in the VA. The present results support the concept that availability of mGluR5 allosteric binding sites is sensitive to extracellular concentrations of glutamate. This interesting property of mGluR5 allosteric binding sites has potential applications for assessing the role of glutamate in the pathogenesis of neuropsychiatric conditions.

Chronic stress alters the dendritic morphology of callosal neurons and the acute glutamate stress response in the rat medial prefrontal cortex

We have previously reported that interhemispheric regulation of medial prefrontal cortex (PFC)-mediated stress responses is subserved by glutamate (GLU)- containing callosal neurons. Evidence of chronic stress-induced dendritic and spine atrophy among PFC pyramidal neurons led us to examine how chronic restraint stress (CRS) might alter the apical dendritic morphology of callosal neurons and the acute GLU stress responses in the left versus right PFC. Morphometric analyses of retrogradely labeled, dye-filled PFC callosal neurons revealed hemisphere-specific CRS-induced dendritic retraction; whereas significant dendritic atrophy occurred primarily within the distal arbor of left PFC neurons, it was observed within both the proximal and distal arbor of right PFC neurons. Overall, CRS also significantly reduced spine densities in both hemispheres with the greatest loss occurring among left PFC neurons, mostly at the distal extent of the arbor. While much of the overall decrease in dendritic spine density was accounted by the loss of thin spines, the density of mushroom-shaped spines, despite being fewer in number, was halved. Using microdialysis we found that, compared to controls, basal PFC GLU levels were significantly reduced in both hemispheres of CRS animals and that their GLU response to 30 min of tail-pinch stress was significantly prolonged in the left, but not the right PFC. Together, these findings show that a history of chronic stress alters the dendritic morphology and spine density of PFC callosal neurons and suggest a mechanism by which this might disrupt the interhemispheric regulation of PFC-mediated responses to subsequent stressors.

A highly tunable dopaminergic oscillator generates ultradian rhythms of behavioral arousal

Ultradian (∼4 hr) rhythms in locomotor activity that do not depend on the master circadian pacemaker in the suprachiasmatic nucleus have been observed across mammalian species, however, the underlying mechanisms driving these rhythms are unknown. We show that disruption of the dopamine transporter gene lengthens the period of ultradian locomotor rhythms in mice. Period lengthening also results from chemogenetic activation of midbrain dopamine neurons and psychostimulant treatment, while the antipsychotic haloperidol has the opposite effect. We further reveal that striatal dopamine levels fluctuate in synchrony with ultradian activity cycles and that dopaminergic tone strongly predicts ultradian period. Our data indicate that an arousal regulating, dopaminergic ultradian oscillator (DUO) operates in the mammalian brain, which normally cycles in harmony with the circadian clock, but can desynchronize when dopamine tone is elevated, thereby producing aberrant patterns of arousal which are strikingly similar to perturbed sleep-wake cycles comorbid with psychopathology.

DOI:http://dx.doi.org/10.7554/eLife.05105.001

The sleep-wake cycle of mammals is controlled by a ‘circadian clock’ within the brain, which is synchronized to the day–night cycle. However, other aspects of mammalian physiology including alertness and activity levels, as well as appetite and body temperature—fluctuate in cycles that repeat every few hours. These cycles are known as ultradian rhythms, and they may offer survival benefits by enabling potentially risky behaviors, such as foraging, to be coordinated between members of a group.

Despite their widespread nature and the fact that they appear to be conserved in evolution, virtually nothing is known about the molecular basis of ultradian rhythms. Blum et al. have now identified a second internal clock within the brain, which they name ‘the DUO’, and shown that this clock normally works in concert with the circadian clock to regulate daily patterns of activity and alertness.

Experiments in mice revealed that the DUO uses the brain chemical dopamine to generate bursts of activity roughly every four hours. Moreover, it continues to work when the circadian clock has been destroyed. Measurements of dopamine in freely moving mice showed that levels of the chemical fluctuate in synchrony with the animals' activity levels. Moreover, drugs that flood the brain with dopamine, such as methamphetamine, disrupt the 4-hour cycle by lengthening the period between bursts of activity, whereas drugs that block dopamine receptors have the opposite effect.

As well as revealing a mechanism by which the brain coordinates processes that repeat several times per day, the identification of the DUO could also provide insights into the biological basis of psychiatric disorders. Conditions such as schizophrenia and bipolar disorder are often accompanied by disturbances in patterns of activity and rest. While these have previously been attributed to the disruption of circadian rhythms, there is little direct evidence for this, which raises the possibility that these changes might instead reflect the disruption of ultradian rhythms.

DOI:http://dx.doi.org/10.7554/eLife.05105.002

Interactions between serotonin and glutamate-nitric oxide pathways in zebrafish scototaxis

NMDA receptors have been implicated in the acute response to stress, possibly mediated the nitric oxide pathway; serotonin has also been implicated in these responses, and has recently been shown to modulate the nitric oxide pathway via 5-HT1 and 5-HT2 receptors. In this work, we compare the effects of NMDA and a 5-HT1A receptor ligands on light/dark preference in adult zebrafish, and investigate whether nitric oxide mediates the effects of such drugs. The noncompetitive NMDA receptor antagonist MK-801 decreased dark preference (scototaxis), while NMDA increased it; the effects of NMDA were completely blocked by pretreatment with the nitric oxide synthase (NOS) antagonist L-NAME. SNP, a nitric oxide donor, produced a bell-shaped dose-response profile on scototaxis. Treatment with 5-HTP increased scototaxis, an effect which was potentiated by pre-treatment with NMDA, but not MK-801, and partially blocked by L-NAME. The 5-HT1A receptor antagonist WAY 100,635 decreased scototaxis, an effect which was completely blocked by L-NAME. These results suggest that tonic NOS inhibition is an important downstream effector of 5-HT1A receptors in the regulation of dark preference behavior in zebrafish, and that NOS is also under phasic independent control by NMDA receptors.

Developmental exposure to concentrated ambient ultrafine particulate matter air pollution in mice results in persistent and sex-dependent behavioral neurotoxicity and glial activation

The brain appears to be a target of air pollution. This study aimed to further ascertain behavioral and neurobiological mechanisms of our previously observed preference for immediate reward (Allen, J. L., Conrad, K., Oberdorster, G., Johnston, C. J., Sleezer, B., and Cory-Slechta, D. A. (2013). Developmental exposure to concentrated ambient particles and preference for immediate reward in mice. Environ. Health Perspect. 121, 32-38), a phenotype consistent with impulsivity, in mice developmentally exposed to inhaled ultrafine particles. It examined the impact of postnatal and/or adult concentrated ambient ultrafine particles (CAPS) or filtered air on another behavior thought to reflect impulsivity, Fixed interval (FI) schedule-controlled performance, and extended the assessment to learning/memory (novel object recognition (NOR)), and locomotor activity to assist in understanding behavioral mechanisms of action. In addition, levels of brain monoamines and amino acids, and markers of glial presence and activation (GFAP, IBA-1) were assessed in mesocorticolimbic brain regions mediating these cognitive functions. This design produced four treatment groups/sex of postnatal/adult exposure: Air/Air, Air/CAPS, CAPS/Air, and CAPS/CAPS. FI performance was adversely influenced by CAPS/Air in males, but by Air/CAPS in females, effects that appeared to reflect corresponding changes in brain mesocorticolimbic dopamine/glutamate systems that mediate FI performance. Both sexes showed impaired short-term memory on the NOR. Mechanistically, cortical and hippocampal changes in amino acids raised the potential for excitotoxicity, and persistent glial activation was seen in frontal cortex and corpus callosum of both sexes. Collectively, neurodevelopment and/or adulthood CAPS can produce enduring and sex-dependent neurotoxicity. Although mechanisms of these effects remain to be fully elucidated, findings suggest that neurodevelopment and/or adulthood air pollution exposure may represent a significant underexplored risk factor for central nervous system diseases/disorders and thus a significant public health threat even beyond current appreciation.

Hemispheric differences in the mesostriatal dopaminergic system

The mesostriatal dopaminergic system, which comprises the mesolimbic and the nigrostriatal pathways, plays a major role in neural processing underlying motor and limbic functions. Multiple reports suggest that these processes are influenced by hemispheric differences in striatal dopamine (DA) levels, DA turnover and its receptor activity. Here, we review studies which measured the concentration of DA and its metabolites to examine the relationship between DA imbalance and animal behavior under different conditions. Specifically, we assess evidence in support of endogenous, inter-hemispheric DA imbalance; determine whether the known anatomy provides a suitable substrate for this imbalance; examine the relationship between DA imbalance and animal behavior; and characterize the symmetry of the observed inter-hemispheric laterality in the nigrostriatal and the mesolimbic DA systems. We conclude that many studies provide supporting evidence for the occurrence of experience-dependent endogenous DA imbalance which is controlled by a dedicated regulatory/compensatory mechanism. Additionally, it seems that the link between DA imbalance and animal behavior is better characterized in the nigrostriatal than in the mesolimbic system. Nonetheless, a variety of brain and behavioral manipulations demonstrate that the nigrostriatal system displays symmetrical laterality whereas the mesolimbic system displays asymmetrical laterality which supports hemispheric specialization in rodents. The reciprocity of the relationship between DA imbalance and animal behavior (i.e., the capacity of animal training to alter DA imbalance for prolonged time periods) remains controversial, however, if confirmed, it may provide a valuable non-invasive therapeutic means for treating abnormal DA imbalance.

Lactation reduces stress-caused dopaminergic activity and enhances GABAergic activity in the rat medial prefrontal cortex

We investigated the effect of restraint on the release of dopamine, GABA and glutamate in the medial prefrontal cortex (mPFC) of lactating compared with virgin Wistar female rats; besides the expression of D1, neuropeptide Y Y2, GABA receptors and corticotropin-releasing factor (CRF). Results from microdialysis experiments showed that basal dopamine and GABA, but not glutamate, concentrations were higher in lactating rats. In virgin animals, immobilization caused significant increase in dopamine, whereas GABA was unchanged and glutamate reduced. In lactating animals, restrain significantly decreased dopamine concentrations and, in contrast to virgin animals, GABA and glutamate concentrations increased. We found a higher expression of CRF, as well as the D1 and neuropeptide Y Y2 receptors in the left mPFC of virgin stressed rats; also, only stressed lactating animals showed a significant increase in immunopositive cells to GABA in the left cingulate cortex; meanwhile, a significant decrease was measured in virgin rats after stress in the left prelimbic region. The increased inhibition of the mPFC dopamine cells during stress and the down-regulated expression of the neuropeptide Y Y2 receptor may explain the lower CRF and hyporesponse to stress measured in lactating animals. Interestingly, participation of mPFC in stress regulation seems to be lateralized.

Exposure to high fat during early development impairs adaptations in dopamine and neuroendocrine responses to repeated stress

Perturbations in the perinatal environment have been shown to significantly alter mesolimbic dopamine (DA) and hypothalamic-pituitary-adrenal (HPA) responses to stressors in adulthood. We have previously demonstrated that adult offspring exposed to high fat during the last week of gestation and throughout lactation display permanent alterations in mesolimbic DA function and behavior. The goal of the present study was to investigate nucleus accumbens (NAc) DA and HPA responses to acute and repeated stress in high fat exposed (HFD, 30% fat) and control (CD, 5% fat) offspring. Using microdialysis to monitor extracellular DA, we report that adult HFD offspring show an enhanced NAc DA response to acute tail-pinch compared to CD offspring. With repeated tail-pinch, the response of the HFD animals remains unchanged while CD offspring exhibit a sensitized DA response. The pattern of the DA response to both acute and repeated stress is also significantly altered by early diet exposure with an earlier peak and faster return to baseline levels in CD compared with HFD offspring. Similarly, neuroendocrine adaptations to repeated tail-pinch are observed in CD animals, but not in HFD animals. While controls display a habituated adrenocorticotropic hormone (ACTH) response to repeated tail-pinch, and an exacerbated ACTH response to a novel stressor, this effect was not observed in the HFD offspring. Together, our data demonstrate that exposure to high fat during early development impairs adaptations in NAc DA and HPA responses usually observed with repeated stress.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

Glutaraldehyde cross-linked glutamate oxidase coated microelectrode arrays: selectivity and resting levels of glutamate in the CNS

Glutaraldehyde is widely used as a cross-linking agent for enzyme immobilization onto microelectrodes. Recent studies and prior reports indicate changes in enzyme activity and selectivity with certain glutaraldehyde cross-linking procedures that may jeopardize the performance of microelectrode recordings and lead to falsely elevated responses in biological systems. In this study, the sensitivity of glutaraldehyde cross-linked glutamate oxidase-based microelectrode arrays to 22 amino acids was tested and compared to glutamate. As expected, responses to electroactive amino acids (Cys, Tyr, Trp) were detected at both nonenzyme-coated and enzyme-coated microelectrodes sites, while the remaining amino acids yielded no detectable responses. Electroactive amino acids were effectively blocked with a m-phenylene diamine (mPD) layer and, subsequently, no responses were detected. Preliminary results on the use of poly(ethylene glycol) diglycidyl ether (PEGDE) as a potentially more reliable cross-linking agent for the immobilization of glutamate oxidase onto ceramic-based microelectrode arrays are reported and show no significant advantages over glutaraldehyde as we observe comparable selectivities and responses. These results support that glutaraldehyde-cross-linked glutamate oxidase retains sufficient enzyme specificity for accurate in vivo brain measures of tonic and phasic glutamate levels when immobilized using specific "wet" coating procedures.

Too much of a good thing: blocking noradrenergic facilitation in medial prefrontal cortex prevents the detrimental effects of chronic stress on cognition

Cognitive impairments associated with dysfunction of the medial prefrontal cortex (mPFC) are prominent in stress-related psychiatric disorders. We have shown that enhancing noradrenergic tone acutely in the rat mPFC facilitated extra-dimensional (ED) set-shifting on the attentional set-shifting test (AST), whereas chronic unpredictable stress (CUS) impaired ED. In this study, we tested the hypothesis that the acute facilitatory effect of norepinephrine (NE) in mPFC becomes detrimental when activated repeatedly during CUS. Using microdialysis, we showed that the release of NE evoked in mPFC by acute stress was unchanged at the end of CUS treatment. Thus, to then determine if repeated elicitation of this NE activity in mPFC during CUS may have contributed to the ED deficit, we infused a cocktail of α(1)-, β(1)-, and β(2)-adrenergic receptor antagonists into the mPFC prior to each CUS session, then tested animals drug free on the AST. Antagonist treatment prevented the CUS-induced ED deficit, suggesting that NE signaling during CUS compromised mPFC function. We confirmed that this was not attributable to sensitization of adrenergic receptor function following chronic antagonist treatment, by administering an additional microinjection into the mPFC immediately prior to ED testing. Acute antagonist treatment did not reverse the beneficial effects of chronic drug treatment during CUS, nor have any effect on baseline ED performance in chronic vehicle controls. Thus, we conclude that blockade of noradrenergic receptors in mPFC protected against the detrimental cognitive effects of CUS, and that repeated elicitation of noradrenergic facilitatory activity is one mechanism by which chronic stress may promote mPFC cognitive dysfunction.

Prefrontal/accumbal catecholamine system processes high motivational salience

Motivational salience regulates the strength of goal seeking, the amount of risk taken, and the energy invested from mild to extreme. Highly motivational experiences promote highly persistent memories. Although this phenomenon is adaptive in normal conditions, experiences with extremely high levels of motivational salience can promote development of memories that can be re-experienced intrusively for long time resulting in maladaptive outcomes. Neural mechanisms mediating motivational salience attribution are, therefore, very important for individual and species survival and for well-being. However, these neural mechanisms could be implicated in attribution of abnormal motivational salience to different stimuli leading to maladaptive compulsive seeking or avoidance. We have offered the first evidence that prefrontal cortical norepinephrine (NE) transmission is a necessary condition for motivational salience attribution to highly salient stimuli, through modulation of dopamine (DA) in the nucleus accumbens (NAc), a brain area involved in all motivated behaviors. Moreover, we have shown that prefrontal-accumbal catecholamine (CA) system determines approach or avoidance responses to both reward- and aversion-related stimuli only when the salience of the unconditioned stimulus (UCS) is high enough to induce sustained CA activation, thus affirming that this system processes motivational salience attribution selectively to highly salient events.

Green tea epigallocatechin-3-gallate (EGCG) promotes neural progenitor cell proliferation and sonic hedgehog pathway activation during adult hippocampal neurogenesis

SCOPE

Adult hippocampal neurogenesis is a lifelong feature of brain plasticity that appears to be critically involved in adult brain function and neurological disease. Recent studies suggest that (-)-epigallocatechin-3-gallate (EGCG), which is the main polyphenolic constituent of green tea, may be used for the prevention and treatment of various neurodegenerative diseases. We hypothesized that EGCG promotes adult neurogenesis, which may be beneficial to hippocampus-dependent learning and memory.

METHODS AND RESULTS

We show that EGCG treatment significantly increased the number of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in adult hippocampal neural progenitor cell (NPC) cultures and in the dentate gyrus of adult mice. Meanwhile, EGCG markedly improved spatial cognition in mice. These events are associated with the sonic hedgehog (Shh) signaling pathway. We observed that EGCG triggered a robust upregulation of Shh receptor (Patched) mRNA and protein expression in cultured NPCs as well as an upregulation of the downstream Shh transcriptional target Gli1. These changes were further confirmed in the hippocampus of mice administered EGCG. The blockage of the Shh signal with the pharmacological inhibitor cyclopamine attenuated EGCG-induced hippocampal neurogenesis.

CONCLUSION

Our results provide strong evidence that EGCG enhances adult hippocampal neurogenesis.

Role of projections from ventral medial prefrontal cortex to nucleus accumbens shell in context-induced reinstatement of heroin seeking

In humans, exposure to contexts previously associated with heroin use can provoke relapse. In rats, exposure to heroin-paired contexts after extinction of drug-reinforced responding in different contexts reinstates heroin seeking. This effect is attenuated by inhibition of glutamate or dopamine transmission in nucleus accumbens shell, or inactivation of ventral medial prefrontal cortex (mPFC). Here, we used an anatomical asymmetrical disconnection procedure to demonstrate that an interaction between glutamatergic projections from ventral mPFC to accumbens shell and local dopamine D(1) postsynaptic receptors contributes to context-induced reinstatement of heroin seeking. We also combined the marker of neuronal activity, Fos, with the retrograde tracer Fluoro-Gold to assess activation in this pathway during context-induced reinstatement. Rats were trained to self-administer heroin for 12 d; drug infusions were paired with a discrete tone-light cue. Lever pressing was subsequently extinguished in a nondrug-associated context in the presence of the discrete cue. Rats were then tested in the heroin- or extinction-associated contexts under extinction conditions. Injections of muscimol + baclofen into ventral mPFC in one hemisphere and D(1)-family receptor antagonist SCH 23390 into the contralateral or ipsilateral accumbens shell decreased context-induced reinstatement. Unilateral injections of muscimol + baclofen into ventral mPFC or SCH 23390 into the accumbens shell had no effect. Context-induced reinstatement was associated with increased Fos expression in ventral mPFC neurons, including those projecting to accumbens shell, with higher double-labeling in the ipsilateral projection than in the contralateral projection. Our results demonstrate that activation of glutamatergic projections from ventral mPFC to accumbens shell, previously implicated in inhibition of cocaine relapse, promotes heroin relapse.

Disruptions in the regulation of extracellular glutamate by neurons and glia in the rat striatum two days after diffuse brain injury

Disrupted regulation of extracellular glutamate in the central nervous system contributes to and can exacerbate the acute pathophysiology of traumatic brain injury (TBI). Previously, we reported increased extracellular glutamate in the striatum of anesthetized rats 2 days after diffuse brain injury. To determine the mechanism(s) responsible for increased extracellular glutamate, we used enzyme-based microelectrode arrays (MEAs) coupled with specific pharmacological agents targeted at in vivo neuronal and glial regulation of extracellular glutamate. After TBI, extracellular glutamate was significantly increased in the striatum by (∼90%) averaging 4.1±0.6 μM compared with sham 2.2±0.4 μM. Calcium-dependent neuronal glutamate release, investigated by local application of an N-type calcium channel blocker, was no longer a significant source of extracellular glutamate after TBI, compared with sham. In brain-injured animals, inhibition of glutamate uptake with local application of an excitatory amino acid transporter inhibitor produced significantly greater increase in glutamate spillover (∼ 65%) from the synapses compared with sham. Furthermore, glutamate clearance measured by locally applying glutamate into the extracellular space revealed significant reductions in glutamate clearance parameters in brain-injured animals compared with sham. Taken together, these data indicate that disruptions in calcium-mediated glutamate release and glial regulation of extracellular glutamate contribute to increased extracellular glutamate in the striatum 2 days after diffuse brain injury. Overall, these data suggest that therapeutic strategies used to regulate glutamate release and uptake may improve excitatory circuit function and, possibly, outcomes following TBI.

Stress, neurotransmitters, corticosterone and body-brain integration

Stress can be defined as a brain-body reaction towards stimuli arising from the environment or from internal cues that are interpreted as a disruption of homeostasis. The organization of the response to a stressful situation involves not only the activity of different types of neurotransmitter systems in several areas of the limbic system, but also the response of neurons in these areas to several other chemicals and hormones, chiefly glucocorticoids, released from peripheral organs and glands. Thus, stress is probably the process through which body-brain integration plays a major role. Here we review first the responses to an acute stress in terms of neurotransmitters such as dopamine, acetylcholine, glutamate and GABA in areas of the brain involved in the regulation of stress responses. These areas include the prefrontal cortex, amygdala, hippocampus and nucleus accumbens and the interaction among those areas. Then, we consider the role of glucocorticoids and review some recent data about the interaction of these steroids with several neurotransmitters in those same areas of the brain. Also the actions of other substances (neuromodulators) released from peripheral organs such as the pancreas, liver or gonads (insulin, IGF-1, estrogens) are reviewed. The role of an environmental enrichment on these same responses is also discussed. Finally a section is devoted to put into perspective all these environmental-brain-body-brain interactions during stress and their consequences on aging. It is concluded that the integrative perspective framed in this review is relevant for better understanding of how the organism responds to stressful challenges and how this can be modified through different environmental conditions during the process of aging. This article is part of a Special Issue entitled: Brain Integration.

The netrin receptor DCC is required in the pubertal organization of mesocortical dopamine circuitry

Netrins are guidance cues involved in neural connectivity. We have shown that the netrin-1 receptor DCC (deleted in colorectal cancer) is involved in the functional organization of the mesocorticolimbic dopamine (DA) system. Adult mice with a heterozygous loss-of-function mutation in dcc exhibit changes in indexes of DA function, including DA-related behaviors. These phenotypes are only observed after puberty, a critical period in the maturation of the mesocortical DA projection. Here, we examined whether dcc heterozygous mice exhibit structural changes in medial prefrontal cortex (mPFC) DA synaptic connectivity, before and after puberty. Stereological counts of tyrosine-hydroxylase (TH)-positive varicosities were increased in the cingulate 1 and prelimbic regions of the pregenual mPFC. dcc heterozygous mice also exhibited alterations in the size, complexity, and dendritic spine density of mPFC layer V pyramidal neuron basilar dendritic arbors. Remarkably, these presynaptic and postsynaptic partner phenotypes were not observed in juvenile mice, suggesting that DCC selectively influences the extensive branching and synaptic differentiation that occurs in the maturing mPFC DA circuit at puberty. Immunolabeling experiments in wild-type mice demonstrated that DCC is segregated to TH-positive fibers innervating the nucleus accumbens, with only scarce DCC labeling in mPFC TH-positive fibers. Netrin had an inverted target expression pattern. Thus, DCC-mediated netrin-1 signaling may influence the formation/maintenance of mesocorticolimbic DA topography. In support of this, we report that dcc heterozygous mice exhibit a twofold increase in the density of mPFC DCC/TH-positive varicosities. Our results implicate DCC-mediated netrin-1 signaling in the establishment of mPFC DA circuitry during puberty.

Tonic and phasic release of glutamate and acetylcholine neurotransmission in sub-regions of the rat prefrontal cortex using enzyme-based microelectrode arrays

The medial prefrontal cortex (mPFC) is an area of the brain critical for higher cognitive processes and implicated in disorders of the CNS such as drug addiction, depression and schizophrenia. Glutamate and acetylcholine are neurotransmitters that are essential for cortical functioning, yet little is known about the dynamic function of these neurotransmitters in subregions of the mPFC. In these studies we used a novel microelectrode array technology to measure resting levels (tonic release) of glutamate and acetylcholine as well as KCl-evoked release (stimulated phasic release) in the mPFC of the anesthetized rat to further our understanding of both tonic and phasic neurotransmission in the cingulate cortex, prelimbic cortex, and infralimbic cortex of the mPFC. Studies revealed homogeneity of tonic and phasic signaling among brain subregions for each neurotransmitter. However, resting levels of glutamate were significantly higher as compared to acetylcholine levels in all subregions. Additionally, KCl-evoked acetylcholine release in the cingulate cortex (7.1 μM) was significantly greater than KCl-evoked glutamate release in any of the three subregions (Cg1, 2.9 μM; PrL, 2.0 μM; IL, 1.8 μM). Interestingly, the time for signal decay following KCl-evoked acetylcholine release was significantly longer by an average of 240% as compared to KCL-evoked glutamate release for all three brain subregions. Finally, we observed a negative relationship between acetylcholine resting levels and KCl-evoked release in the Cg1. These data suggest a homogenous distribution of both glutamatergic and acetylcholinergic innervation in the mPFC, with alterations in tonic and phasic release regulation accounting for differences between these neurotransmitters.

The mesoaccumbens dopamine in coping with stress

Mesoaccumbens dopamine (DA) is involved in the stress response. Although neural mechanisms involved in stress are of paramount importance for both clinical and preclinical research, the results of studies on the stress response by mesoaccumbens DA have received little attention. Therefore, we aimed to review these results and propose a role for mesoaccumbens DA in coping with stress. The data reviewed support the view that fluctuations of tonic levels characterize the mesoaccumbens DA stress response. Stress-induced increase of tonic DA levels in nucleus accumbens (NAc) supports expression of responses aimed at removing and avoiding the stressor through activation of DA D2 receptors, whereas inhibition of DA is associated with cessation of active defensive responses. In novel unescapable/uncontrollable stressful conditions tonic levels of DA in NAc show an initial increase followed by a decrease below pre-stress levels that lasts as long as the stressful situation. This biphasic response fits with the dynamics of the primary and secondary appraisal of a stressor that cannot be removed, escaped or controlled by the organism. In fact, NAc DA fluctuations are controlled by the medial pre-frontal cortex, which is involved in stress appraisal. We propose that enhanced mesoaccumbens DA supports expression of active coping strategies against an event appraised as a stressor and that inhibition of DA is required for passive coping with stressful situations appraised as unescapable/uncontrollable.

Functional brain asymmetry in adult novelty response: on fluidity of neonatal novelty exposure effects

Novelty and surprises differentially modify the left and right sides of the brain. Here we show that repeated brief exposures to the novelty of a non-home environment during infancy and early adulthood lead to long-lasting changes in adulthood in the global bi-lateralization organization of the brain as indexed by a transiently detectable right-sided orientating bias upon the initial encounter with the novel environment. Most surprisingly, we show that in the same individuals, the short-term effect of the combined neonatal and adulthood novelty exposures on functional brain asymmetry measured at young adulthood (5 months of age) is distinctively different from the long-term effect measured at late adulthood (15 months of age). These results suggest that long-lasting, cumulative effects of early life experience on brain and behavior organization are not necessarily permanent, but continue to unfold, presumably via interactions with a multitude of unmonitored intervening life events.

Aging and stress: past hypotheses, present approaches and perspectives

Brain aging has been suggested to be conditioned by an excessive glucocortioid secretion leading to damages on brain areas involved not only in cognitive and emotional processes but also in the control of the activity of the hypothalamic-pituitary adrenal axis. This review describes some of the hypothesis that try to explain the relation between the dysregulation of the stress response and brain aging, focusing on corticosterone but also on neurotransmission in the hippocampus, the prefrontal cortex and the amygdala. Moreover, different molecular factors can account for an enhanced vulnerability of the aged brain to stress exposure, specially for resilience. Among them, good candidates could be those mechanisms determining the levels of corticosterone in the brain, several molecules downstream glucocorticoid receptor activation (ie: heat shock proteins, BAG-1) or even the epigenetic programming of the HPA axis in early stages. In conclusion, genetic and environmental factors (early life stress, chronic stress during adulthood) can produce an enhanced vulnerability and a reduced resilience of the brain to subsequent stress exposures or to metabolic challenges leading, in turn, to an unsuccessful aging of the brain. However, results obtained with the use of the environmental enrichment model in animals, added to several results in humans also described in this review suggest that positive environmental factors (cognitive-demanding tasks or physical exercise) can help to maintain neuronal plasticity during aging and to protect the brain against the damaging effects of stress exposure.

Rapid microelectrode measurements and the origin and regulation of extracellular glutamate in rat prefrontal cortex

Glutamate in the prefrontal cortex (PFC) plays a significant role in several mental illnesses, including schizophrenia, addiction and anxiety. Previous studies on PFC glutamate-mediated function have used techniques that raise questions on the neuronal versus astrocytic origin of glutamate. The present studies used enzyme-based microelectrode arrays to monitor second-by-second resting glutamate levels in the PFC of awake rats. Locally applied drugs were employed in an attempt to discriminate between the neuronal or glial components of the resting glutamate signal. Local application of tetrodotoxin (sodium channel blocker), produced a significant (∼ 40%) decline in resting glutamate levels. In addition significant reductions in extracellular glutamate were seen with locally applied ω-conotoxin (MVIIC; ∼ 50%; calcium channel blocker), and the mGluR(2/3) agonist, LY379268 (∼ 20%), and a significant increase with the mGluR(2/3) antagonist LY341495 (∼ 40%), effects all consistent with a large neuronal contribution to the resting glutamate levels. Local administration of D,L-threo-β-benzyloxyaspartate (glutamate transporter inhibitor) produced an ∼ 120% increase in extracellular glutamate levels, supporting that excitatory amino acid transporters, which are largely located on glia, modulate clearance of extracellular glutamate. Interestingly, local application of (S)-4-carboxyphenylglycine (cystine/glutamate antiporter inhibitor), produced small, non-significant bi-phasic changes in extracellular glutamate versus vehicle control. Finally, pre-administration of tetrodotoxin completely blocked the glutamate response to tail pinch stress. Taken together, these results support that PFC resting glutamate levels in rats as measured by the microelectrode array technology are at least 40-50% derived from neurons. Furthermore, these data support that the impulse flow-dependent glutamate release from a physiologically -evoked event is entirely neuronally derived.

Depression and the hyperactive right-hemisphere

Depression is associated with an inter-hemispheric imbalance; a hyperactive right-hemisphere (RH) and a relatively hypoactive left-hemisphere (LH). Nevertheless, the underlying mechanisms which can explain why depression is associated with a RH dominance remain elusive. This article points out the potential links between functional cerebral asymmetries and specific symptoms and features of depression. There is evidence that the RH is selectively involved in processing negative emotions, pessimistic thoughts and unconstructive thinking styles--all which comprise the cognitive phenomenology of depression and in turn contribute to the elevated anxiety, stress and pain associated with the illness. Additionally, the RH mediates vigilance and arousal which may explain the sleep disturbances often reported in depression. The RH had also been linked with self-reflection, accounting for the tendency of depressed individuals to withdraw from their external environments and focus attention inward. Physiologically, RH activation is associated with hyprecortisolemia, which contributes to the deterioration of the immune system functioning and puts depressed patients at a greater risk of developing other illnesses, accounting for depression's high comorbidity with other diseases. Conversely, the LH is specifically involved in processing pleasurable experiences, and its relative attenuation is in line with the symptoms of anhedonia that characterize depression. The LH is also relatively more involved in decision-making processes, accounting for the indecisiveness that is often accompanied with depression.