Integrated regulation of PKA by fast and slow neurotransmission in the nucleus accumbens controls plasticity and stress responses

Cortical glutamate and midbrain dopamine neurotransmission converge to mediate striatum-dependent behaviors, while maladaptations in striatal circuitry contribute to mental disorders. However, the crosstalk between glutamate and dopamine signaling has not been entirely elucidated. Here we uncover a molecular mechanism by which glutamatergic and dopaminergic signaling integrate to regulate cAMP-dependent protein kinase (PKA) via phosphorylation of the PKA regulatory subunit, RIIβ. Using a combination of biochemical, pharmacological, neurophysiological, and behavioral approaches, we find that glutamate-dependent reduction in cyclin-dependent kinase 5 (Cdk5)-dependent RIIβ phosphorylation alters the PKA holoenzyme autoinhibitory state to increase PKA signaling in response to dopamine. Furthermore, we show that disruption of RIIβ phosphorylation by Cdk5 enhances cortico-ventral striatal synaptic plasticity. In addition, we demonstrate that acute and chronic stress in rats inversely modulate RIIβ phosphorylation and ventral striatal infusion of a small interfering peptide that selectively targets RIIβ regulation by Cdk5 improves behavioral response to stress. We propose this new signaling mechanism integrating ventral striatal glutamate and dopamine neurotransmission is important to brain function, may contribute to neuropsychiatric conditions, and serves as a possible target for the development of novel therapeutics for stress-related disorders.     

I. Stress and hepatic inflammation

Stress is an ever-present part of modern life. The "stress response" constitutes an organism's mechanism for coping with a given stress and is mediated via the release of glucocorticoids and catecholamines. Patients often complain of stress-related worsening of their liver disease; however, the interrelationship between stress and hepatic inflammation is incompletely understood and has received little scientific attention. Considering the broad impact glucocorticoids and catecholamines have on immune cell function, it is very likely that stress has a significant impact on the hepatic inflammatory response. This themes article discusses studies of the stress response and its peripheral effectors (glucocorticoids and catecholamines) in liver disease and their impact on hepatic inflammation and outlines potential areas for future scientific investigation.     

Proposed glucocorticoid-mediated zinc signaling in the hippocampus

Corticosteroid hormones are secreted from the adrenal glands in hourly pulses and signal the hippocampus for the development and function. In contrast, the stress-induced rise in corticosteroid concentrations has a profound effect on emotional arousal, motivational processes and cognitive performance. This rise is required as the stress response to maintain homeostasis in the living body or restore it. However, abnormal rise in corticosteroid concentrations is a disadvantage to the hippocampus. Corticosteroid-glutamatergic interactions during information processing are proposed as a potential model to explain many of the diverse actions of corticosteroids in synaptic plasticity such as long-term potentiation and cognition. Because zincergic neurons are a subtype of glutamatergic neurons and release Zn(2+) and glutamate into the synaptic cleft, it is possible that homeostasis of synaptic Zn(2+), in addition to homeostasis of glutamate, is modified by glucocorticoids, followed by the changes in cognitive function and stress response. Zn(2+) signal participates in cognitive and emotional behavior in cooperation with signaling of glucocorticoids and glutamate, while can disadvantageously act on the hippocampus under sever stress circumstances. This paper analyzes the actions of glucocorticoid-mediated Zn(2+) signal in the hippocampus under stressful circumstances and its significance in both hippocampal function and dysfunction.     

Glucocorticoid-Induced Prenatal Modification of GABA-ergic Regulation of the Responsiveness of the Hypothalamo-Hypophyseal-Adrenocortical System to Stress in Rats

We studied the effects of introduction of exogenous glucocorticoids within the prenatal period (seven subcutaneous injections of hydrocortisone acetate, 50 mg/kg, daily, on the 15th–21st pregnancy days, or two injections on the 16th and 18th days) on the state of the hippocampal GABA-ergic system and the hypothalamo-hypophyseal-adrenocortical system (HHAS) of adult rats under conditions of acute stress (1-h-long immobilization): effects of pre-stress injection of an agonist of GABA_B receptors, baclofen (10 mg/kg, 30 min before immobilization), were also examined. The activity of glutamate decarboxylase and binding of ³H-GABA were the indices characterizing the state of the former regulatory system, while the content of catecholamines in the hypothalamus and the level of hormones of the adrenal cortex characterized the state of the latter system. Prenatal introduction of hydrocortisone acetate resulted in weakening of the adrenocortical reaction to acute stress in adult offspring males; post-stress changes in the noradrenaline level in the hypothalamus and the activity of glutamate decarboxylase in the hippocampus, as well as stress-related activation of GABA_B receptors, were absent in these animals. Adult females subjected to the prenatal influence of hydrocortisone acetate, vice versa, demonstrated a greater reaction of the adrenal cortex to stress; this occurred against the background of suppression of the activity of glutamate decarboxylase in the hippocampus and preserved activity of GABA_B receptors. Our study shows that modifying influences, which exogenous glucocorticoids applied within the prenatal period exert on the GABA-ergic regulation of the responsiveness of the HHAS to stress, are characterized in adult offspring of rats by a significant sex-related dependence. Neirofiziologiya/Neurophysiology, Vol. 37, No. 3, pp. 244–249, May–June, 2005.     

The role of glutamate and nitric oxide in the reproductive neuroendocrine system.

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.     

NMDA receptor involvement in imipramine withdrawal-associated effects on swim stress,GABA levels and NMDA receptor binding in rat hippocampus

Abrupt antidepressant withdrawal after chronic treatment is associated with a stress response that may negatively affect the long-term outcome of depression, the neurochemical correlates, of which, remain undetermined. Prolonged depression involves the stress-related release of glucocorticoids and glutamate, while response to antidepressants involves gamma-amino butyric acid (GABA) and the glutamate N-methyl-D-aspartate (NMDA) receptor. Here, imipramine (IMI) was administered to rats for three weeks followed by acute withdrawal for seven days. Levels of GABA in the hippocampus (HC), and effects on swim stress immobility (SSI), were determined. Furthermore, glutamate/NMDA receptor binding properties were determined using [(3)H]-CGP-39653. Finally, the ability of dizocilpine (MK801), a glutamate NMDA antagonist, to reverse IMI withdrawal was determined. Chronic IMI (15 mg/kg ip) significantly reduced SSI together with a slight but insignificant decrease in HC GABA levels. However, IMI significantly reduced specific binding (B(max)) of [(3)H]-CGP-39653. Withdrawal of IMI for 7 days resulted in a loss of efficacy on SSI, a slight increase in GABA and a significant reversal of IMI effects on [(3)H]-CGP-39653 binding. MK801 (0.2 mg/kg ip) alone for seven days caused a significant decrease in SSI, a significant suppression of HC GABA, and significantly decreased [(3)H]-CGP-39653 B(max). MK801 during IMI-withdrawal significantly decreased GABA, prompted recovery on SSI, though not significantly, but significantly reversed withdrawal effects on [(3)H]-CGP-39653 B(max). In conclusion, acute antidepressant discontinuation is associated with subtle changes on HC GABA, a resurgence of NMDA receptor density and a loss of its anti-immobility response. These responses are reversed by a NMDA antagonist suggesting that abrupt antidepressant discontinuation mobilises glutamate activity.     

Metabolic control of vesicular glutamate transport and release

Fasting has been used to control epilepsy since antiquity, but the mechanism of coupling between metabolic state and excitatory neurotransmission remains unknown. Previous work has shown that the vesicular glutamate transporters (VGLUTs) required for exocytotic release of glutamate undergo an unusual form of regulation by Cl(-). Using functional reconstitution of the purified VGLUTs into proteoliposomes, we now show that Cl(-) acts as an allosteric activator, and the ketone bodies that increase with fasting inhibit glutamate release by competing with Cl(-) at the site of allosteric regulation. Consistent with these observations, acetoacetate reduced quantal size at hippocampal synapses and suppresses glutamate release and seizures evoked with 4-aminopyridine in the brain. The results indicate an unsuspected link between metabolic state and excitatory neurotransmission through anion-dependent regulation of VGLUT activity.     

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.     

Presynaptic kainate receptors that enhance the release of GABA on CA1 hippocampal interneurons

We report that kainate receptors are present on presynaptic GABAergic terminals contacting interneurons and that their activation increases GABA release. Application of kainate increased the frequency of miniature inhibitory postsynaptic currents recorded in CA1 interneurons. Local applications of glutamate but not of AMPA or NMDA also increased GABA quantal release. Application of kainate as well as synaptically released glutamate reduced the number of failures of GABAergic neurotransmission between interneurons. Thus, activation of presynaptic kainate receptors increases the probability of GABA release at interneuron-interneuron synapses. Glutamate may selectively control the communication between interneurons by increasing their mutual inhibition.     

Adenosine receptor-mediated modulation of dopamine release in the nucleus accumbens depends on glutamate neurotransmission and N-methyl-D-aspartate receptor stimulation

Adenosine, by acting on adenosine A(1) and A(2A) receptors, exerts opposite modulatory roles on striatal extracellular levels of glutamate and dopamine, with activation of A(1) inhibiting and activation of A(2A) receptors stimulating glutamate and dopamine release. Adenosine-mediated modulation of striatal dopaminergic neurotransmission could be secondary to changes in glutamate neurotransmission, in view of evidence for a preferential colocalization of A(1) and A(2A) receptors in glutamatergic nerve terminals. By using in vivo microdialysis techniques, local perfusion of NMDA (3, 10 microm), the selective A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3, 10 microm), the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 300, 1000 microm), or the non-selective A(1)-A(2A) receptor antagonist in vitro caffeine (300, 1000 microm) elicited significant increases in extracellular levels of dopamine in the shell of the nucleus accumbens (NAc). Significant glutamate release was also observed with local perfusion of CGS 21680, CPT and caffeine, but not NMDA. Co-perfusion with the competitive NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (APV; 100 microm) counteracted dopamine release induced by NMDA, CGS 21680, CPT and caffeine. Co-perfusion with the selective A(2A) receptor antagonist MSX-3 (1 microm) counteracted dopamine and glutamate release induced by CGS 21680, CPT and caffeine and did not modify dopamine release induced by NMDA. These results indicate that modulation of dopamine release in the shell of the NAc by A(1) and A(2A) receptors is mostly secondary to their opposite modulatory role on glutamatergic neurotransmission and depends on stimulation of NMDA receptors. Furthermore, these results underscore the role of A(1) vs. A(2A) receptor antagonism in the central effects of caffeine.     

Glutamatergic Control of Dopamine Release During Stress in the Rat Prefrontal Cortex

Abstract: In vivo microdialysis was used to assess the hypothesis that the stress-induced increase in dopamine release in the prefrontal cortex is mediated by stress-activated glutamate neurotransmission in this region. Local perfusion of an α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, blocked the stress-induced increase in dopamine levels, whereas an NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid, at the dose tested, was not able to alter this response significantly. These data indicate that the effect of stress on dopamine release in the prefrontal cortex is mediated locally by activation of AMPA/kainate receptors, which modulate the release of dopamine in this region.     

Glutamate forward and reverse transport: from molecular mechanism to transporter-mediated release after ischemia

Glutamate transporters remove the excitatory neurotransmitter glutamate from the extracellular space after neurotransmission is complete, by taking glutamate up into neurons and glia cells. As thermodynamic machines, these transporters can also run in reverse, releasing glutamate into the extracellular space. Because glutamate is excitotoxic, this transporter-mediated release is detrimental to the health of neurons and axons, and it, thus, contributes to the brain damage that typically follows a stroke. This review highlights current ideas about the molecular mechanisms underlying glutamate uptake and glutamate reverse transport. It also discusses the implications of transporter-mediated glutamate release for cellular function under physiological and patho-physiological conditions.     

Neuroadaptive changes in the mesocortical glutamatergic system during chronic nicotine self-administration and after extinction in rats

Nicotine self-administration causes adaptation in the mesocorticolimbic glutamatergic system, including the up-regulation of ionotropic glutamate receptor subunits. We therefore determined the effects of nicotine self-administration and extinction on NMDA-induced glutamate neurotransmission between the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA). On day 19 of nicotine SA, both regions were microdialyzed for glutamate while mPFC was sequentially perfused with Kreb's Ringer buffer (KRB), 200 μM NMDA, KRB, 500 μM NMDA, KRB, and 100 mM KCl. Basal glutamate levels were unaffected, but nicotine self-administration significantly potentiated mPFC glutamate release to 200 μM NMDA, which was ineffective in controls. Furthermore, in VTA, nicotine self-administration significantly amplified glutamate responses to both mPFC infusions of NMDA. This hyper-responsive glutamate neurotransmission and enhanced glutamate subunit expression were reversed by extinction. Behavioral studies also showed that a microinjection of 2-amino-5-phosphonopentanoic acid (NMDA-R antagonist) into mPFC did not affect nicotine or sucrose self-administration. However, in VTA, NBQX (AMPA-R antagonist) attenuated both nicotine and sucrose self-administration. Collectively, these studies indicate that mesocortical glutamate neurotransmission adapts to chronic nicotine self-administration and VTA AMPA-R may be involved in the maintenance of nicotine self-administration.     

Characterization of the actions of AvTx 7 isolated from Agelaia vicina (Hymenoptera: Vespidae) wasp venom on synaptosomal glutamate uptake and release

It has previously been shown that the denatured crude extract of Agelaia vicina wasp venom inhibits glutamate and GABA uptake in rat cerebral cortex synaptosomes. To identify the components responsible for these effects, the neurotoxin AvTx 7 (molecular weight of 1210 Da) was isolated from A. vicina venom and its effects on glutamate neurotransmission investigated. AvTx 7 inhibits glutamate uptake in a dose-dependent and uncompetitive manner. AvTx 7 was found to stimulate the glutamate release in the presence of calcium and sodium channel blockers, suggesting that its action is not mediated through these channels. AvTx 7 potentiates glutamate release in the presence of K(+) channel blockers tetraethylammonium and 4-aminopyridine, indicating that the toxin may act through these drugs-sensible K(+) channels. We suggest that AvTx 7 can be a valuable tool to enhance our understanding of K(+) channels' involvement in the release of glutamate.     

Role of different voltage-gated Ca2+ channels in cortical spreading depression: specific requirement of P/Q-type Ca2+ channels

Gain-of-function mutations in CaV 2.1 (P/Q-type) Ca2+ channels cause familial hemiplegic migraine type 1 (FHM1), a subtype of migraine with aura. Knockin (KI) mice carrying FHM1 mutations show increased neuronal P/Q-type current and facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura and may activate migraine headache mechanisms. We recently studied cortical neurotransmission in neuronal microcultures and brain slices of FHM1 KI mice, and showed (1) gain-of-function of excitatory neurotransmission, due to increased action potential-evoked Ca2+ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at fast-spiking interneuron synapses, and (2) a causative link between enhanced glutamate release and facilitation of CSD induced by brief pulses of high K+ in cortical slices. Here, we show that after blockade of either the P/Q-type Ca2+ channels or the NMDA receptors, CSD cannot be induced in wild-type mouse cortical slices. In contrast, blockade of N- or R-type Ca2+ channels has only a small inhibitory effect on CSD threshold and velocity of propagation. Our findings support a model in which Ca2+ influx through presynaptic P/Q-type Ca2+ channels with consequent release of glutamate from recurrent cortical pyramidal cell synapses and activation of NMDA receptors are required for initiation and propagation of the CSD involved in migraine.     

Pharmacological induction of ischemic tolerance in hippocampal slices by sarcosine preconditioning

Brain ischemic tolerance is a protective mechanism induced by a preconditioning stimulus, which prepare the tissue against harmful insults. Preconditioning with N-methyl-d-aspartate (NMDA) agonists induces brain tolerance and protects it against glutamate excitotoxicity. Recently, the glycine transporters type 1 (GlyT-1) have been shown to potentiate glutamate neurotransmission through NMDA receptors suggesting an alternative strategy to protect against glutamate excitotoxicity. Here, we evaluated the preconditioning effect of sarcosine pre-treatment, a GlyT-1 inhibitor, in rat hippocampal slices exposed to ischemic insult. Sarcosine (300mg/kg per day, i.p.) was administered during seven consecutive days before induction of ischemia in hippocampus by oxygen/glucose deprivation (OGD). To access the damage caused by an ischemic insult, we evaluated cells viability, glutamate release, nitric oxide (NO) production, lactate dehydrogenase (LDH) levels, production of reactive oxygen species (ROS), and antioxidant enzymes as well as the impact of oxidative stress in the tissue. We observed that sarcosine reduced cell death in hippocampus submitted to OGD, which was confirmed by reduction on LDH levels in the supernatant. Cell death, glutamate release, LDH levels and NO production were reduced in sarcosine hippocampal slices submitted to OGD when compared to OGD controls (without sarcosine). ROS production was reduced in sarcosine hippocampal slices exposed to OGD, although no changes were found in antioxidant enzymes activities. This study demonstrates that preconditioning with sarcosine induces ischemic tolerance in rat hippocampal slices submitted to OGD.     

Glutamate uptake in synaptic plasticity: from mollusc to mammal

A great deal of research has been directed toward understanding the cellular mechanisms underlying synaptic plasticity and memory formation. To this point, most research has focused on the more "active" components of synaptic transmission: presynaptic transmitter release and postsynaptic transmitter receptors. Little work has been done characterizing the role neurotransmitter transporters might play during changes in synaptic efficacy. We review several new experiments that demonstrate glutamate transporters are regulated during changes in the efficacy of glutamatergic synapses. This regulation occurred during long-term facilitation of the sensorimotor synapse of Aplysia and long-term potentiation of the Schaffer-collateral synapse of the rat. We propose that glutamate transporters are "co-regulated" with other molecules/processes involved in synaptic plasticity, and that this process is phylogenetically conserved. These new findings indicate that glutamate transporters most likely play a more active role in neurotransmission than previously believed.     

Acute Stress Increases Depolarization-Evoked Glutamate Release in the Rat Prefrontal/Frontal Cortex: The Dampening Action of Antidepressants

Background

Behavioral stress is recognized as a main risk factor for neuropsychiatric diseases. Converging evidence suggested that acute stress is associated with increase of excitatory transmission in certain forebrain areas. Aim of this work was to investigate the mechanism whereby acute stress increases glutamate release, and if therapeutic drugs prevent the effect of stress on glutamate release.

Methodology/Findings

Rats were chronically treated with vehicle or drugs employed for therapy of mood/anxiety disorders (fluoxetine, desipramine, venlafaxine, agomelatine) and then subjected to unpredictable footshock stress. Acute stress induced marked increase in depolarization-evoked release of glutamate from synaptosomes of prefrontal/frontal cortex in superfusion, and the chronic drug treatments prevented the increase of glutamate release. Stress induced rapid increase in the circulating levels of corticosterone in all rats (both vehicle- and drug-treated), and glutamate release increase was blocked by previous administration of selective antagonist of glucocorticoid receptor (RU 486). On the molecular level, stress induced accumulation of presynaptic SNARE complexes in synaptic membranes (both in vehicle- and drug-treated rats). Patch-clamp recordings of pyramidal neurons in the prefrontal cortex revealed that stress increased glutamatergic transmission through both pre- and postsynaptic mechanisms, and that antidepressants may normalize it by reducing release probability.

Conclusions/Significance

Acute footshock stress up-regulated depolarization-evoked release of glutamate from synaptosomes of prefrontal/frontal cortex. Stress-induced increase of glutamate release was dependent on stimulation of glucocorticoid receptor by corticosterone. Because all drugs employed did not block either elevation of corticosterone or accumulation of SNARE complexes, the dampening action of the drugs on glutamate release must be downstream of these processes. This novel effect of antidepressants on the response to stress, shown here for the first time, could be related to the therapeutic action of these drugs.