Blockade of Astrocytic Glutamate Uptake in the Prefrontal Cortex Induces Anhedonia

Major depression is associated with both dysregulated glutamatergic neurotransmission and fewer astrocytes in limbic areas including the prefrontal cortex (PFC). These deficits may be functionally related. Notably, astrocytes regulate glutamate levels by removing glutamate from the synapse via the glutamate transporter (GLT-1). Previously, we demonstrated that central blockade of GLT-1 induces anhedonia and c-Fos expression in the PFC. Given the role of the PFC in regulating mood, we hypothesized that GLT-1 blockade in the PFC alone would be sufficient to induce anhedonia in rats. We microinjected the GLT-1 inhibitor, dihydrokainic acid (DHK), into the PFC and examined the effects on mood using intracranial self-stimulation (ICSS). At lower doses, intra-PFC DHK produced modest increases in ICSS thresholds, reflecting a depressive-like effect. At higher doses, intra-PFC DHK resulted in cessation of responding. We conducted further tests to clarify whether this total cessation of responding was related to an anhedonic state (tested by sucrose intake), a nonspecific result of motor impairment (measured by the tape test), or seizure activity (measured with electroencephalogram (EEG)). The highest dose of DHK increased latency to begin drinking without altering total sucrose intake. Furthermore, neither motor impairment nor evidence of seizure activity was observed in the tape test or EEG recordings. A decrease in reward value followed by complete cessation of ICSS responding suggests an anhedonic-like effect of intra-PFC DHK; a conclusion that was substantiated by an increased latency to begin sucrose drinking. Overall, these results suggest that blockade of astrocytic glutamate uptake in the PFC is sufficient to produce anhedonia, a core symptom of depression.     

Blockade of Astrocytic Glutamate Uptake in Rats Induces Signs of Anhedonia and Impaired Spatial Memory

Mood disorders are associated with regional brain abnormalities, including reductions in glial cell and neuron number, glutamatergic irregularities, and differential patterns of brain activation. Because astrocytes are modulators of neuronal activity and are important in trafficking the excitatory neurotransmitter glutamate, it is possible that these pathologies are interrelated and contribute to some of the behavioral signs that characterize depression and related disorders. We tested this hypothesis by determining whether depressive-like signs were induced by blocking central astrocytic glutamate uptake with the astrocytic glutamate transporter (GLT-1) inhibitor, dihydrokainic acid (DHK), in behavioral tests that quantify aspects of mood, including reward and euthymia/dysthymia: intracranial self-stimulation (ICSS) and place conditioning. We found that DHK elevated ICSS thresholds, a depressive-like effect that could reflect reduced sensitivity to reward (anhedonia) or increased aversion (dysphoria). However, DHK treatment did not establish conditioned place aversions, suggesting that this treatment does not induce dysphoria. To identify the brain regions mediating the behavioral effects of DHK, we examined c-Fos expression in areas implicated in motivation and emotion. DHK increased c-Fos expression in many of these regions. The dentate gyrus of the hippocampus was robustly activated, which led us to explore whether DHK alters hippocampal learning. DHK impaired spatial memory in the MWM. These findings identify disruption of astrocyte glutamate uptake as one component of the complex circuits that mediate anhedonia and cognitive impairment, both of which are common symptoms of depression. These finding may have implications for the etiology of depression and other disorders that share the features of anhedonia and cognitive impairment.     

Decreased CCK(B) receptor binding in rat amygdala in animals demonstrating greater anxiety-like behavior

Abstract Rationale. The potentiation of the acoustic startle response (ASR) by stimuli associated with aversive events is mediated via the amygdala and is used as an index of "anxiety" and "fear". In laboratory animals, cholecystokinin_B (CCK_B) agonists increase anxiety and fear and activation of amygdala CCK_B receptors potentiates ASR. Additionally, antagonism of CCK_B receptors attenuates fear-potentiated ASR. Objectives. To investigate the putative role of CCK_B receptors in individual differences in fear and anxiety, we examined individual differences in amygdala CCK_B receptor binding for animals demonstrating low versus high baseline and fear-potentiated ASR. Additionally, we examined individual differences in CCK_B binding for animals demonstrating low versus high anxiety-like behavior on the elevated plus-maze (EPM). Methods. Male Wistar rats were tested in the ASR, fear-potentiated ASR, and EPM paradigms. Following testing, brain slices were mounted and incubated with 50 pM ¹²⁵I-CCK8 (non-sulfated), a selective CCK_B receptor ligand, in the presence or absence of 1 μM non-radioactively labeled CCK and then exposed on tritium-sensitive film for 2–3 days. Results. Animals with high fear-potentiated ASR showed decreased CCK_B receptor binding in both the basolateral and central amygdaloid nuclei. Animals with high anxiety-like responses on the EPM showed decreased CCK_B binding in the basolateral, but not central, amygdala. There were no differences in amygdala CCK_B binding in animals demonstrating low versus high baseline ASR. None of the groups showed differences in CCK_B receptor binding in the nucleus accumbens. Conclusions. These results show that there is a down-regulation of amygdala CCK_B receptor binding in animals demonstrating greater anxiety-like responding in the fear-potentiated ASR and EPM models of anxiety, possibly as a compensation for increased CCK activity. Electronic Publication     

Inhibition of tetanically sciatic stimulation-induced LTP of spinal neurons and Fos expression by disrupting glutamate transporter GLT-1

Tetanic stimulation of the sciatic nerve produces spinal long-term potentiation (LTP) of C-fiber evoked field potentials, which is NMDA dependent and may be the substrate of inflammation- or nerve injury-produced central sensitization. Glial glutamate transporter GLT-1 has been considered as an important regulator of excitatory synaptic transmission and nociception. In the present study, we investigated the effects of GLT-1 on the spinal LTP and Fos expression induced by tetanically sciatic stimulation. Intrathecal administration of dihydrokainate (DHK), a GLT-1 selective inhibitor, partially inhibited (0.1 mM) or completely blocked (3.0 mM) the spinal LTP, which may be related to an accumulation of extracellular glutamate. Intrathecal DHK (3.0 mM) also suppressed tetanic stimulation-induced spinal Fos expression. Double immunofluorescence showed no Fos expression in glial fibrillary acidic protein (GFAP)-positive cells, and the cell DNA fragment study failed to detect a significant apoptosis of spinal neurons. These results suggest that disruption of GLT-1 may be associated with the inhibition of functional activation of spinal neurons expressing Fos, but not with glutamate excitotoxicity. In conclusion, glial GLT-1 may play an important role in tetanically sciatic stimulation-induced LTP of spinal nociceptive neurons via the regulation of extracellular levels of glutamate to an appropriate concentration.     

Glia mechanisms in mood regulation: a novel model of mood disorders

Introduction Recent evidence in clinical and preclinical studies has implicated glutamate neurotransmissions in pathophysiology of mood disorders. The regulation of amino acid neurotransmission, i.e., glutamate and gamma-aminobutyric acid (GABA) involves coordinated mechanisms of uptake and transport within a tripartite synaptic system that includes neurons and glia. Newly appreciated role of the glia, more specifically astrocytes on neuronal functions combined with reported postmortem abnormalities of glia in patients with mood disorders further supports the role of glia in mood disorders. Materials and methods This report presents some of our preliminary results utilizing glia-selective toxins and other pharmacological tools to suppress glial function within the limbic system to study the resulting behavioral abnormalities, and thus, elucidate glial involvement in the development of mood disorders. Results and discussion We demonstrate that chronic blockade of glutamate uptake by a glial/neuronal transporter antagonist l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) within the amygdala, a key area implicated in mood regulation, results in dose-dependent reduction in social exploratory behavior and disrupts circadian activity patterns consistent with symptoms of mood disorders. Similarly, the selective astrocytic glutamate transporter type 1 (GLT-1) blocker dihydrokainic acid (DHK) injected into the amygdala also results in reduced social interaction that is blocked by selective glutamate N-methyl-d-aspartate (NMDA) type receptor antagonist AP5. The results are discussed in the context of glial and glutamate mechanisms in mood disorders and potential therapeutic avenues to address these mechanisms.     

Aquaporin-4 Deficiency Impairs Synaptic Plasticity and Associative Fear Memory in the Lateral Amygdala: Involvement of Downregulation of Glutamate Transporter-1 Expression

Astrocytes are implicated in information processing, signal transmission, and regulation of synaptic plasticity. Aquaporin-4 (AQP4) is the major water channel in adult brain and is primarily expressed in astrocytes. A growing body of evidence indicates that AQP4 is a potential molecular target for the regulation of astrocytic function. However, little is known about the role of AQP4 in synaptic plasticity in the amygdala. Therefore, we evaluated long-term potentiation (LTP) in the lateral amygdala (LA) and associative fear memory of AQP4 knockout (KO) and wild-type mice. We found that AQP4 deficiency impaired LTP in the thalamo-LA pathway and associative fear memory. Furthermore, AQP4 deficiency significantly downregulated glutamate transporter-1 (GLT-1) expression and selectively increased NMDA receptor (NMDAR)-mediated EPSCs in the LA. However, low concentration of NMDAR antagonist reversed the impairment of LTP in KO mice. Upregulating GLT-1 expression by chronic treatment with ceftriaxone also reversed the impairment of LTP and fear memory in KO mice. These findings imply a role for AQP4 in synaptic plasticity and associative fear memory in the amygdala by regulating GLT-1 expression.     

Repeated electroconvulsive shock attenuates the depressive-like effects of d-amphetamine withdrawal on brain reward function in rats

RATIONALE: The withdrawal of humans from high doses of psychostimulant drugs can result in a transient syndrome which appears isomorphic to endogenous depression. One of the more prominent symptoms is a loss of hedonic capacity; in animals, the anhedonia associated with amphetamine withdrawal has been measured objectively by decrements in responding for intracranial self-stimulation (ICSS). OBJECTIVE: To date, the effects of amphetamine withdrawal on ICSS responding have been reversed by different antidepressant drugs. In the present study, we sought to reverse withdrawal-induced anhedonia by administration of repeated electroconvulsive shocks (ECS). METHODS: Rats with electrodes in the lateral hypothalamus were trained on an ascending-series current intensity ICSS paradigm until stable levels of responding were attained. Half of the animals were then administered a 4-day escalating dose schedule of d-amphetamine, and tests for ICSS responding started 12 h after the final injection. During withdrawal, all animals received daily treatment with either ECS or sham-ECS. RESULTS: Amphetamine withdrawal was associated with reduced ICSS responding; animals treated with ECS exhibited a facilitated recovery compared to sham-ECS treated animals, and returned to control levels of ICSS responding 24 h earlier. CONCLUSIONS: ECS was able to mitigate the anhedonic effects of d-amphetamine withdrawal, and provides additional support for the use of psychostimulant withdrawal as a model of depression.     

Neurobiology of anxiety disorders and implications for treatment

The neurobiology of the anxiety disorders, which include panic disorder, post-traumatic stress disorder (PTSD), and specific phobias, among others, has been clarified by advances in the field of classical or Pavlovian conditioning, and in our understanding of basic mechanisms of memory and learning. Fear conditioning occurs when a neutral conditioned stimulus (such as a tone) is paired with an aversive, or unconditioned stimulus (such as a footshock), and then in the absence of the unconditioned stimulus, causes a conditioned fear response. Preclinical studies have shown that the amygdala plays a key role in fear circuitry, and that abnormalities in amygdala pathways can affect the acquisition and expression of fear conditioning. Drugs such as glutamate N-methyl-D-aspartate (NMDA) antagonists, and blockers of voltage-gated calcium channels, in the amygdala, may block these effects. There is also preliminary evidence for the use of centrally acting beta-adrenergic antagonists, like propranolol, to inhibit consolidation of traumatic memories in PTSD. Finally, fear extinction, which entails new learning of fear inhibition, is central to the mechanism of effective anti-anxiety treatments. Several pharmacological manipulations, such as D-cycloserine, a partial NMDA agonist, have been found to facilitate extinction. Combining these medication approaches with psychotherapies that promote extinction, such as cognitive behavioral therapy (CBT), may offer patients with anxiety disorders a rapid and robust treatment with good durability of effect.     

Short-Term Adaptation of Conditioned Fear Responses Through Endocannabinoid Signaling in the Central Amygdala

The cannabinoid receptor type 1 (CB1) and the central nucleus of the amygdala (CeA) are both known to have crucial roles in the processing of fear and anxiety, whereby they appear to be especially involved in the control of fear states. However, in contrast to many other brain regions including the cortical subregions of the amygdala, the existence of CB1 in the CeA remains enigmatic. In this study we show that CB1 is expressed in the CeA of mice and that CB1 in the CeA mediates short-term synaptic plasticity, namely depolarization-induced suppression of excitation (DSE) and inhibition (DSI). Moreover, the CB1 antagonist AM251 increased both excitatory and inhibitory postsynaptic responses in CeA neurons. Local application of AM251 in the CeA in vivo resulted in an acutely increased fear response in an auditory fear conditioning paradigm. Upon application of AM251 in the basolateral nucleus of the amygdala (BLA) in an otherwise identical protocol, no such acute behavioral effects were detected, but CB1 blockade resulted in increased fear responses during tone exposures on the subsequent days. Moreover, we observed that the efficacy of DSE and DSI in the CeA was increased on the day following fear conditioning, indicating that a single tone-shock pairing resulted in changes in endocannabinoid signaling in the CeA. Taken together, our data show the existence of CB1 proteins in the CeA, and their critical role for ensuring short-term adaptation of responses to fearful events, thereby suggesting a potential therapeutic target to accompany habituation-based therapies of post-traumatic symptoms.     

Central, but not basolateral, amygdala involvement in the anxiolytic-like effects of midazolam in rats in the elevated plus maze

The role of the amygdala in the mediation of fear and anxiety has been extensively investigated. However, how the amygdala functions during the organization of the anxiety-like behaviors generated in the elevated plus maze (EPM) is still under investigation. The basolateral (BLA) and the central (CeA) nuclei are the main input and output stations of the amygdala. In the present study, we ethopharmacologically analyzed the behavior of rats subjected to the EPM and the tissue content of the monoamines dopamine (DA) and serotonin (5-HT) and their metabolites in the nucleus accumbens (NAc), dorsal hippocampus (DH), and dorsal striatum (DS) of animals injected with saline or midazolam (20 and 30 nmol/0.2 μL) into the BLA or CeA. Injections of midazolam into the CeA, but not BLA, caused clear anxiolytic-like effects in the EPM. These treatments did not cause significant changes in 5-HT or DA contents in the NAc, DH, or DS of animals tested in the EPM. The data suggest that the anxiolytic-like effects of midazolam in the EPM also appear to rely on GABA-benzodiazepine mechanisms in the CeA, but not BLA, and do not appear to depend on 5-HT and DA mechanisms prevalent in limbic structures.     

Anxiogenic-like action of galanin after intra-amygdala administration in the rat.

The neuropeptide galanin is expressed in brain structures implicated in regulation of emotionality. The amygdala is known to play a central role in mechanisms of fear and anxiety. We therefore examined the effects of galanin (0.2 and 0.6 nmol/side) on experimental anxiety upon microinjection into the amygdala. Two established animal models of anxiety were used: a punished drinking test, and the elevated plus-maze. Punished responding was dose dependently suppressed by intra-amygdala galanin, whereas unpunished responding, drinking motivation, locomotor activity, and shock thresholds were unaffected. No effects on experimental anxiety were seen in the plus-maze following galanin injection. When injected into parietal cortex, no anxiety promoting properties of galanin were detected. These data suggest that activation of galanin receptors in amygdala modulates neurotransmission involved in fear and experimental anxiety.     

Prevention of Stress-Impaired Fear Extinction Through Neuropeptide S Action in the Lateral Amygdala

Stressful and traumatic events can create aversive memories, which are a predisposing factor for anxiety disorders. The amygdala is critical for transforming such stressful events into anxiety, and the recently discovered neuropeptide S transmitter system represents a promising candidate apt to control these interactions. Here we test the hypothesis that neuropeptide S can regulate stress-induced hyperexcitability in the amygdala, and thereby can interact with stress-induced alterations of fear memory. Mice underwent acute immobilization stress (IS), and neuropeptide S and a receptor antagonist were locally injected into the lateral amygdala (LA) during stress exposure. Ten days later, anxiety-like behavior, fear acquisition, fear memory retrieval, and extinction were tested. Furthermore, patch-clamp recordings were performed in amygdala slices prepared ex vivo to identify synaptic substrates of stress-induced alterations in fear responsiveness. (1) IS increased anxiety-like behavior, and enhanced conditioned fear responses during extinction 10 days after stress, (2) neuropeptide S in the amygdala prevented, while an antagonist aggravated, these stress-induced changes of aversive behaviors, (3) excitatory synaptic activity in LA projection neurons was increased on fear conditioning and returned to pre-conditioning values on fear extinction, and (4) stress resulted in sustained high levels of excitatory synaptic activity during fear extinction, whereas neuropeptide S supported the return of synaptic activity during fear extinction to levels typical of non-stressed animals. Together these results suggest that the neuropeptide S system is capable of interfering with mechanisms in the amygdala that transform stressful events into anxiety and impaired fear extinction.     

Blockade of kappa opioid receptors attenuates the development of depressive-like behaviors induced by cocaine withdrawal in rats

Drug dependence is characterized by dysregulation of brain reward systems and increased sensitivity to stress. Chronic exposure to drugs of abuse is associated with increased expression of the neuropeptide dynorphin, the endogenous ligand for kappa opioid receptors (KORs). Activation of KORs causes depressive- and aversive-like responses in rodents, raising the possibility that drug-induced upregulation of dynorphin plays a role independence-associated negative states. Here we used "binge" exposure to cocaine (3 daily intraperitoneal injections of 15 mg/kg for 14 days) to examine the development of dependence-like behavior in the intracranial self-stimulation (ICSS) test and the forced swim test (FST). When rats were tested 1 h before their first scheduled injection of each day-a period of drug withdrawal corresponding to 20 h after their last injection on the previous day-there were exposure-dependent increases in ICSS thresholds (a putative indicator of anhedonia) and decreases in latencies to immobility in the FST (a putative indicator of behavioral despair). Administration of the long-lasting KOR antagonist norBNI (20 μg, intracerebroventricular) before the beginning of the binge regimen attenuated the development of cocaine withdrawal-induced anhedonia in the ICSS test. In contrast, administration of norBNI in the midst of the binge regimen had no effect on expression of cocaine withdrawal-induced anhedonia in the ICSS test, although it did attenuate despair-like behavior in the FST. These data suggest that blockade of KORs before exposure to a stressor (in this case, cocaine withdrawal or forced swimming) can attenuate the development of stress-induced behavioral adaptations. This article is part of a Special Issue entitled 'Anxiety and Depression'.     

Effect of MS-153 on the acquisition and expression of conditioned fear in rats

Pavlovian fear conditioning is one of the most extensively studied and reliable behavioral paradigms used to investigate the mechanisms involved in fear and anxiety. Increased glutamatergic neurotransmission may play an important role in mediating fear conditioning. The present study assessed whether (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a novel cerebroprotective agent that inhibits the release of glutamate and enhances glutamate uptake, affects the acquisition and expression of conditioned fear. The rats received administration of MS-153 (i.p.) at 3, 10, and 30 mg/kg, 30 min before footshock and 24 h after footshock. Freezing behavior was measured in the chamber where they had previously received footshock for the acquisition experiments. For the expression experiments, the rats received MS-153 (i.p.) at the same doses 23.5 h after footshock and 30 min before expression testing. MS-153 significantly attenuated the acquisition and expression of freezing behavior. In addition, MS-153 administration did not affect locomotor activity. The present results suggest that extracellular glutamate is involved in fear conditioning, and that MS-153 has an anxiolytic effect by decreasing endogenous glutamate neurotransmission.     

NMDA GluN2A and GluN2B receptors play separate roles in the induction of LTP and LTD in the amygdala and in the acquisition and extinction of conditioned fear

Synaptic plasticity mediated by NMDA glutamate receptors is thought to be a primary mechanism underlying the formation of new memories. Activation of GluN2A NMDA receptor subunits may induce long-term potentiation (LTP), whereas low-frequency stimulation of GluN2B receptors induces long-term depression (LTD). In the present study, we show that blockade of GluN2A, but not GluN2B receptors with NVP-AAM077 and Ro25-6981 respectively, prevented LTP of auditory thalamic inputs to the lateral amygdala. Conversely, LTD induction in this pathway was prevented by blockade of GluN2B, but not GluN2A receptors. As this pathway plays a critical role in the acquisition, retrieval and extinction of a learned auditory-cue fear association, we next examined the effects of blockade of GluN2A and GluN2B receptors on the development and retention of a conditioned fear response. Administration of NVP-AAM077, but not Ro25-6981, prior to conditioning disrupted the expression of conditioned fear 24h later. Conversely, Ro25-6981 but not NVP-AAM077 impaired extinction of the conditioned fear response. These data expand on previous work showing that LTP/D in the thalamic-lateral amygdala pathway is dependent on NMDA receptors, by demonstrating selective roles for GluN2A and GluN2B NMDA receptor subunits in LTP and LTD respectively. Furthermore, GluN2A receptor activation and associated LTP may be involved specifically in the initial formation and/or stabilization of a learned fear response, whereas GluN2B receptor activation and associated LTD may facilitate the suppression of Pavlovian fear responses during extinction. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

The role of amygdala glutamate receptors in fear learning, fear-potentiated startle, and extinction

Using a paradigm known as fear-potentiated startle, we have examined the neurobiological substrates of Pavlovian fear conditioning. In these experiments, rats are trained to fear an initially neutral stimulus by pairing that stimulus with shock. The amount of fear elicited by the stimulus [i.e., now a conditioned stimulus (CS)] is later assessed by presenting startle-eliciting noise bursts both in the presence and also the absence of the CS. After training, startle responses are typically greater in the presence of the CS. Findings reviewed here suggest that amygdala N-methyl-D-aspartate (NMDA) receptors play a key role in triggering the neural changes that support fear learning and also the loss of fear that accompanies extinction training. Amygdala (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors also participate in fear learning. However, unlike NMDA receptor antagonists, AMPA receptor antagonists also block fear-potentiated startle when infused prior to testing. Very recent data indicate that glutamate metabotropic Group II receptor agonists also block fear learning when infused into the amygdala prior to training, and block fear-potentiated startle when infused prior to testing. A fuller understanding of the role of amygdala glutamate systems in fear and fear learning may suggest novel pharmacological approaches to the treatment of clinical anxiety disorders.     

Bromocriptine, an ergot alkaloid, inhibits excitatory amino acid release mediated by glutamate transporter reversal

Bromocriptine, a dopamine D₂ receptor agonist, has widely been used for patients with Parkinson's disease. The aim of the present study was to investigate the effect of bromocriptine on glutamate transporter. Since the astroglial glutamate transporter GLT-1 (EAAT2) is the predominant isoform in the forebrain, we generated EAAT2-expressing human embryonic kidney cells and immortalized mouse astrocytes. In the present studies, we observed a GLT-1-immunoreactive band and significant Na⁺-dependent d-[³H] aspartate uptake. Furthermore, the glutamate transporter inhibitors, dl-threo-β-benzyloxyaspartic acid (TBOA) and dihydrokainate (DHK), displayed a dose-dependent reduction of d-[³H] aspartate uptake in both types of cells. In contrast, cells exposed to either chemical anoxia or high KCl elicited a marked release of d-[³H] aspartate, and the release was inhibited by TBOA and DHK, implying the contribution of glutamate transporter reversal. Interestingly, we found that bromocriptine dose-dependently inhibits d-[³H] aspartate release elicited by chemical anoxia or high KCl, while no changes occurred in the uptake. The inhibitory action of bromocriptine was not affected by sulpiride, a dopamine D₂ receptor antagonist. On the other hand, bromocriptine had no effect on swelling-induced d-[³H] aspartate release, which is mediated by volume-regulated anion channels. In vivo studies revealed that bromocriptine suppresses the excessive elevation of glutamate levels in gerbils subjected to transient forebrain ischemia in a manner similar to DHK. Taken together, these results provide evidence that bromocriptine inhibits excitatory amino acid release via reversed operation of GLT-1 without altering forward transport.     

The Hypocretin/Orexin System Mediates the Extinction of Fear Memories

Anxiety disorders are often associated with an inability to extinguish learned fear responses. The hypocretin/orexin system is involved in the regulation of emotional states and could also participate in the consolidation and extinction of aversive memories. Using hypocretin receptor-1 and hypocretin receptor-2 antagonists, hypocretin-1 and hypocretin-2 peptides, and hypocretin receptor-1 knockout mice, we investigated the role of the hypocretin system in cue- and context-dependent fear conditioning and extinction. Hypocretins were crucial for the consolidation of fear conditioning, and this effect was mainly observed in memories with a high emotional component. Notably, after the acquisition of fear memory, hypocretin receptor-1 blockade facilitated fear extinction, whereas hypocretin-1 administration impaired this extinction process. The extinction-facilitating effects of the hypocretin receptor-1 antagonist SB334867 were associated with increased expression of cFos in the basolateral amygdala and the infralimbic cortex. Intra-amygdala, but neither intra-infralimbic prefrontal cortex nor intra-dorsohippocampal infusion of SB334867 enhanced fear extinction. These results reveal a key role for hypocretins in the extinction of aversive memories and suggest that hypocretin receptor-1 blockade could represent a novel therapeutic target for the treatment of diseases associated with inappropriate retention of fear, such as post-traumatic stress disorder and phobias.     

Bromocriptine reverses the elevation in intracranial self-stimulation thresholds observed in a rat model of cocaine withdrawal.

Cocaine use frequently occurs in episodic prolonged binges. Following such a cocaine binge, the user suffers from severe depression mixed with irritability, anxiety, anergia and anhedonia. These symptoms constitute the cocaine withdrawal syndrome. Since cocaine's rewarding effects are mediated by enhanced dopaminergic neurotransmission in the mesocorticolimbic system, it is possible that a long-acting dopamine agonist might block the withdrawal effects associated with the termination of a prolonged bout of cocaine self-administration. An animal model of post-cocaine anhedonia was developed using the elevation in intracranial self-stimulation (ICSS) thresholds following the termination of prolonged periods of cocaine self-administration as a measure of an animal's "anhedonic" state. In the present study, an attempt was made to reverse the postcocaine elevation in ICSS thresholds with acute administration of a dopaminergic agonist, bromocriptine. Rats were allowed to self-administer cocaine for 24 hours continuously. Four hours after the termination of the self-administration session, animals were injected with either vehicle or bromocriptine (1, 2, or 4 mg/kg, IP). Two hours later (6 hours post cocaine), the animals' self-stimulation thresholds were assessed. Confirming previous work, treatment with the vehicle following a cocaine "binge" resulted in elevated ICSS thresholds compared to predrug baseline levels or to control rats' thresholds. Bromocriptine, at doses that had no effect on ICSS thresholds in control rats, reversed the postcocaine anhedonia in a dose-related manner. These results indicate that bromocriptine-like drugs (pharmacological agents that enhance dopaminergic neurotransmission) may be able to ameliorate some of the effects of cocaine withdrawal on mood and motivational state. In addition, the results of the present study indicate that the proposed animal model of cocaine withdrawal could be useful in the discovery and development of new pharmacotherapies for cocaine withdrawal.     

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.