Tonic desensitization of hippocampal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors regulates 5-hydroxytryptamine release in vivo

Extracellular Release of ILEI/FAM3C and Amyloid-β Is Associated with the Activation of Distinct Synapse Subpopulations

BACKGROUND

Brain amyloid-β (Aβ) peptide is released into the interstitial fluid (ISF) in a neuronal activity-dependent manner, and Aβ deposition in Alzheimer's disease (AD) is linked to baseline neuronal activity. Although the intrinsic mechanism for Aβ generation remains to be elucidated, interleukin-like epithelial-mesenchymal transition inducer (ILEI) is a candidate for an endogenous Aβ suppressor.

OBJECTIVE

This study aimed to access the mechanism underlying ILEI secretion and its effect on Aβ production in the brain.

METHODS

ILEI and Aβ levels in the cerebral cortex were monitored using a newly developed ILEI-specific ELISA and in vivo microdialysis in mutant human Aβ precursor protein-knockin mice. ILEI levels in autopsied brains and cerebrospinal fluid (CSF) were measured using ELISA.

RESULTS

Extracellular release of ILEI and Aβ was dependent on neuronal activation and specifically on tetanus toxin-sensitive exocytosis of synaptic vesicles. However, simultaneous monitoring of extracellular ILEI and Aβ revealed that a spontaneous fluctuation of ILEI levels appeared to inversely mirror that of Aβ levels. Selective activation and inhibition of synaptic receptors differentially altered these levels. The evoked activation of AMPA-type receptors resulted in opposing changes to ILEI and Aβ levels. Brain ILEI levels were selectively decreased in AD. CSF ILEI concentration correlated with that of Aβ and were reduced in AD and mild cognitive impairment.

CONCLUSION

ILEI and Aβ are released from distinct subpopulations of synaptic terminals in an activity-dependent manner, and ILEI negatively regulates Aβ production in specific synapse types. CSF ILEI might represent a surrogate marker for the accumulation of brain Aβ.

Direct enhancement of hippocampal dopamine or serotonin levels as a pharmacodynamic measure of combined antidepressant-anticonvulsant action

The neurobiological relationships between epilepsy and depression are receiving increased experimental attention. A key role for limbic monoamines in depression has been established and we recently showed the importance of hippocampal monoamines in limbic seizure control. We here studied whether anticonvulsant compounds are antidepressant and can elevate hippocampal dopamine (DA) or serotonin (5-HT) levels determined by in vivo microdialysis in rats. We used assessment of seizure severity in the focal pilocarpine model, antidepressant-like activity within the rat forced swim and the mouse tail suspension tests, and locomotor activity in an open field as behavioural tests. We studied the tricyclic antidepressant imipramine, the selective 5-HT reuptake inhibitor citalopram and the selective DA reuptake blocker GBR-12909. These compounds with combined antidepressant-anticonvulsant properties all directly enhanced extracellular hippocampal DA or 5-HT levels. Since glutamate-mediated hyperexcitability in temporal lobe regions seems to be involved in disturbed emotional behaviour, we next investigated possible antidepressant effects and hippocampal DA or 5-HT modulations exerted by selective ionotropic and metabotropic glutamate receptor ligands with anticonvulsant properties. Combined anticonvulsant-antidepressant activities of the NMDA antagonist MK-801 and the mGluR group I antagonists (AIDA, MPEP) were also associated with locally elicited increases in hippocampal DA and/or 5-HT levels. This study highlights that the hippocampus is an important site of action of combined anticonvulsant-antidepressant and monoamine enhancing effects.

Evidence that caspase-1 is a negative regulator of AMPA receptor-mediated long-term potentiation at hippocampal synapses

Best known for their pivotal role in a form of programmed cell death called apoptosis, caspases may also function in more subtle physiological processes. Caspases are present in synapses and dendrites of neurons where they can be activated in response to glutamate receptor stimulation and calcium influx. Here we tested the hypothesis that caspase-1 plays a role in modulating long-term potentiation (LTP) at hippocampal synapses. We provide evidence that caspase-1 plays a role in regulating alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated calcium influx and synaptic plasticity in the hippocampus. LTP of excitatory postsynaptic potentials at CA1 synapses was significantly enhanced when hippocampal slices were treated with either a pan-caspase inhibitor or a selective inhibitor of caspase-1, but not by an inhibitor of caspase-6. Inhibition of caspase-1 significantly enhanced the AMPA current-mediated component of LTP without affecting the N-methyl-D-aspartate current-mediated component. Calcium responses to AMPA were enhanced in hippocampal neurons treated with a caspase-1 inhibitor suggesting that caspase-1 normally functions to reduce AMPA receptor-mediated calcium influx. These findings suggest that, by selectively reducing AMPA currents and calcium influx, caspase-1 functions as a negative regulator of LTP at hippocampal synapses.

The Effect of Antiepileptic Drugs on 5-HT1A-Receptor Binding Measured by Positron Emission Tomography

PURPOSE

To study the effect of antiepileptic drugs (AEDs) on 5-HT(1A)-receptor binding in patients with temporal lobe epilepsy. 5-HT(1A)-receptor binding, measured by positron emission tomography, is reduced in patients with temporal lobe epilepsy. Antiepileptic drugs may act on the serotonergic system, as shown in animal models, and thus affect receptor-binding measurements.

METHODS

We analyzed the effect of AEDs on 5-HT(1A)-receptor binding in 31 patients and 10 normal controls. Patients with structural lesions, progressive neurologic disorders, or taking other medications were excluded. None had a seizure for >or=2 days before positron emission tomography (PET). [(18)F]FCWAY PET was performed on a GE Advance scanner with continuous EEG monitoring. Functional images of the distribution volume (V) were generated. Anatomic regions of interest were applied to co-registered PET images, after correction for partial-volume effect.

RESULTS

Patients had significantly higher [(18)F]FCWAY free fraction (f(1)) than did controls. No AED effects were observed on interictal [(18)F]FCWAY binding after correction for plasma free fraction. [(18)F]FCWAY V/f1 reduction in epileptic foci was not affected by AEDs.

CONCLUSIONS

5-HT(1A)-receptor binding is reduced in temporal lobe epileptic foci after partial-volume correction. AED plasma free fractions should be measured when PET receptor studies are performed in patients with epilepsy.

Mechanisms underlying domoic acid-induced dopamine release from striatum: an in vivo microdialysis study

The brain microdialysis technique has been used to examine the in vivo effects of the neurotoxin domoic acid (an ionotropic glutamate receptor agonist) on dopamine (DA) release in the striatum of conscious and freely moving rats. Local application of domoic acid (500 microM) through the microdialysis probe produced an increase in striatal DA content (597 +/- 96% with respect to basal levels). The release of DA induced by domoic acid was not attenuated in a Ca(+2)-free medium (469 +/- 59%) or after pretreatment with 10 mg/kg reserpine (533 +/- 79%). Intrastriatal infusion of 1 microM tetrodotoxin (TTX) partially reduced the domoic acid-evoked DA release (278 +/- 34%). Moreover, domoic acid perfusion had no effect on K+-evoked DA release. The results suggest that domoic acid increases the striatal DA release according to a reserpine-independent, calcium-independent and partially TTX-insensitive mechanism, suggesting that these effects probably involve a nonexocytotic process. On the other hand, the inhibitor of DA uptake nomifensine (10 microM) reduced the domoic acid-evoked DA release (356 +/- 59%), suggesting that a carrier-dependent mechanism could be involved in the effect of domoic acid on the striatal DA levels.

In vivo modulation of 5-hydroxytryptamine release in mouse prefrontal cortex by local 5-HT(2A) receptors: effect of antipsychotic drugs

In the rat, postsynaptic 5-hydroxytryptamine2A receptors medial prefrontal cortex control the activity of the serotonergic system through changes in the activity of pyramidal neurons projecting to the dorsal raphe nucleus. Here we extend these observations to mouse brain. The prefrontal cortex expresses abundant 5- hydroxytryptamine2A receptors, as assessed by immunohistochemistry, Western blots and in situ hybridization procedures. The application of the 5-hydroxytryptamine2A/2C agonist DOI (100 microm) by reverse dialysis in the medial prefrontal cortex doubled the local release of 5-hydroxytryptamine. This effect was reversed by coperfusion of tetrodotoxin, and by the selective 5-hydroxytryptamine2A receptor antagonist M100907, but not by the 5-hydroxytryptamine2C antagonist SB-242084. The effect of DOI was also reversed by prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-hydroxytryptamine1A receptor agonist), NBQX (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate/kainic acid antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by dizocilpine (N-methyl-d-aspartate antagonist). alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate mimicked the 5-hydroxytryptamine elevation produced by DOI, an effect also reversed by BAY x 3702. Likewise, the coperfusion of classical (chlorpromazine, haloperidol) and atypical antipsychotic drugs (clozapine, olanzapine) fully reversed the 5-hydroxytryptamine elevation induced by DOI. These observations suggest that DOI increases 5-hydroxytryptamine release in the mouse medial prefrontal cortex through the activation of local 5-hydroxytryptamine2A receptors by an impulse-dependent mechanism that involves/requires the activation of local alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate receptors. This effect is reversed by ligands of receptors present in the medial prefrontal cortex, possibly in pyramidal neurons, which are involved in the action of antipsychotic drugs. In particular, the reversal by classical antipsychotics may involve blockade of alpha1-adrenoceptors, whereas that of atypical antipsychotics may involve 5-hydroxytryptamine2A receptors and alpha1-adrenoceptors.

Adaptation of monoaminergic responses to phencyclidine in nucleus accumbens and prefrontal cortex following repeated treatment with fluoxetine or imipramine

The adaptive neuronal changes that follow chronic administration of antidepressant drugs are thought to underlie clinical improvement in patient populations. Recent evidence suggests that alterations specific to N-methyl-D-aspartate (NMDA) receptors may be a final common pathway to antidepressant action. To investigate this possibility, we sought to establish the effects of chronic fluoxetine or imipramine treatment on the monoamine stimulating effect of the non-competitive NMDA antagonist phencyclidine. Male, Sprague-Dawley rats (n=9/group) were treated with saline (1 ml/kg, i.p.), imipramine (10 mg/kg, i.p.) or fluoxetine (10 mg/kg, i.p.) once daily for 14 consecutive days. After a 7-day drug-free period, animals given an acute challenge of either saline or phencyclidine (5 mg/kg, i.p.). One hour later, animals were killed, brains were removed, and the prefrontal cortex, striatum, and nucleus accumbens were dissected. Samples were assayed for the monoamines and their primary metabolites by HPLC. Repeated treatment with fluoxetine or imipramine did not alter baseline dopamine or serotonin turnover. Acute phencyclidine treatment increased prefrontal cortex and nucleus accumbens dopamine turnover in saline-treated animals (P<0.01); however, the effect in the nucleus accumbens was prevented in animals pretreated with imipramine or fluoxetine. Acute phencyclidine challenge also increased serotonin turnover in prefrontal cortex of saline- or imipramine-pretreated rats (P<0.01), though this effect was attenuated in animals pretreated with fluoxetine. Overall, the data suggest that repeated antidepressant treatment alters monoamine turnover in specific brain regions in response to blockade of NMDA receptors. The data highlight the importance of adaptive responses to NMDA receptors resulting from chronic antidepressant treatment.

Control of serotonergic function in medial prefrontal cortex by serotonin-2A receptors through a glutamate-dependent mechanism

We examined the in vivo effects of the hallucinogen 4-iodo-2,5-dimethoxyamphetamine (DOI). DOI suppressed the firing rate of 7 of 12 dorsal raphe (DR) serotonergic (5-HT) neurons and partially inhibited the rest (ED(50) = 20 microg/kg, i.v.), an effect reversed by M100907 (5-HT(2A) antagonist) and picrotoxinin (GABA(A) antagonist). DOI (1 mg/kg, s.c.) reduced the 5-HT release in medial prefrontal cortex (mPFC) to 33 +/- 8% of baseline, an effect also antagonized by M100907. However, the local application of DOI in the mPFC increased 5-HT release (164 +/- 6% at 100 microm), an effect antagonized by tetrodotoxin, M100907, and BAY x 3702 (5-HT(1A) agonist) but not by SB 242084 (5-HT(2C) antagonist). The 5-HT increase was also reversed by NBQX (AMPA-KA antagonist) and 1S,3S-ACPD (mGluR 2/3 agonist) but not by MK-801 (NMDA antagonist). AMPA mimicked the 5-HT elevation produced by DOI. Likewise, the electrical-chemical stimulation of thalamocortical afferents and the local inhibition of glutamate uptake increased the 5-HT release through AMPA receptors. DOI application in mPFC increased the firing rate of a subgroup of 5-HT neurons (5 of 10), indicating an enhanced output of pyramidal neurons. Dual-label fluorescence confocal microscopic studies demonstrated colocalization of 5-HT(1A) and 5-HT(2A) receptors on individual cortical pyramidal neurons. Thus, DOI reduces the activity of ascending 5-HT neurons through a DR-based action and enhances serotonergic and glutamatergic transmission in mPFC through 5-HT(2A) and AMPA receptors. Because pyramidal neurons coexpress 5-HT(1A) and 5-HT(2A) receptors, DOI disrupts the balance between excitatory and inhibitory inputs and leads to an increased activity that may mediate its hallucinogenic action.

Modulation of pressor response to muscle contraction via monoamines following AMPA-receptor blockade in the ventrolateral medulla

We hypothesized that cardiovascular responses to static muscle contraction are mediated via changes in extracellular concentrations of monoamines (norepinephrine, dopamine and serotonin) following the administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an AMPA-receptor antagonist) into the rostral (RVLM) or caudal (CVLM) ventrolateral medulla. For the RVLM experiments (n= 8), a 2-min static muscle contraction increased the mean arterial pressure (MAP) and heart rate (HR) by 23 +/- 2 mmHg and 28 +/- 8 bpm, respectively. During this contraction, the concentrations of norepinephrine, dopamine, and serotonin within the RVLM increased by 278 +/- 52%, 213 +/- 23%, and 232 +/- 24%, respectively. Microdialysis of CNQX (1.0 microM) for 30 min into the RVLM attenuated the increases in MAP and HR ( 11 +/- 2 mmHg and 14 +/- 5 bpm) without a change in developed muscle tension. The levels of norepinephrine, dopamine, and serotonin within the RVLM were also attenuated. In contrast, microdialysis of CNQX into the CVLM (n= 8) potentiated the contraction-evoked responses in MAP ( 21 +/- 2 vs 33 +/- 5 mmHg) and HR ( 25 +/- 5 vs 46 +/- 8 bpm) without any effect on the monoamine levels within the CVLM region. These results suggest that AMPA-receptor blockade within the RVLM and CVLM has opposing effects on cardiovascular responses during static muscle contraction. In addition, such receptor blockade modulates extracellular concentrations of monoamines within the RVLM but not in the CVLM. These results provide evidence that AMPA receptors within the ventrolateral medulla play a role in exercise pressor reflex.

Repeated but not acute clomipramine decreases the effect of N-methyl-D-aspartate receptor activation on serotonergic transmission between the raphe nuclei and frontal cortex

The effect of acute or repeated treatment with the antidepressant clomipramine (CIM) on N-methyl-D-aspartate (NMDA) evoked changes in extracellular 5-hydroxytryptamine (5-HT) in the raphe nuclei and frontal cortex of the same rat has been studied using microdialysis. Acute injection of CIM (10 or 20 mg/kg) caused an increase in raphe extracellular 5-HT but did not significantly alter extracellular 5-HT in the frontal cortex. Infusion of 25 microM NMDA into the raphe decreased extracellular 5-HT in this region and increased terminal extracellular 5-HT in the frontal cortex. In contrast, infusion of 100 microM NMDA into the raphe was followed by an increase in local dialysate 5-HT and a decrease in 5-HT release in the cortex. When NMDA infusion, at either 25 or 100 microM was preceded by one acute injection of CIM the effects of NMDA on 5-HT release in both brain structures were generally more marked than in vehicle injected controls. Repeated (15 day) treatment with CIM (10 or 20 mg/kg) caused a dose-dependent increase in basal extracellular 5-HT in both raphe and frontal cortex. In these animals, however, the effects of infusion of both 25 and 100 microM NMDA on 5-HT release in raphe and frontal cortex were greatly attenuated or abolished. This suggests that adaptive functional changes occur in NMDA receptor function during treatment with an antidepressant. The possible significance of this in the aetiology and treatment of depression is discussed.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated stimulation of glutamate and GABA release in the rat striatum in vivo: a dual-label microdialysis study

The existence of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate autoreceptors on glutamate nerve terminals in vitro has recently been demonstrated using synaptosomal and brain slice preparations. In the present study we have used a modification of a rapid dual-label intracerebral microdialysis method, previously developed by Young and co-workers(80,81) for the study of presynaptic mechanisms of neurotransmitter release, to investigate whether presynaptic AMPA receptors also play a role in the control of striatal glutamate release in vivo. For comparative purposes, the action of locally applied AMPA on striatal GABA release in vivo was also monitored. Local application of AMPA (0.01-100 microM), by reverse dialysis, into the striatum resulted in concentration-dependent increases in the Ca(2+)-dependent efflux of both [3H]L-glutamate and [14C]GABA. Maximum responses reached 142.0+/-6.5% and 166.8+/-7.7% of basal efflux for [3H]L-glutamate and [14C]GABA, respectively. No marked behavioural changes were observed at any dose of the agonist. Unexpectedly, the AMPA-evoked responses were not potentiated by the AMPA receptor desensitization inhibitors cyclothiazide (10-100microM) or aniracetam (1mM). Consistent with this finding, AMPA-stimulated [3H]L-glutamate and [14C]GABA efflux were significantly attenuated by co-perfusion with the selective, competitive AMPA receptor antagonist 6-nitro-7-sulphamoylbenzo(F)quinoxaline-2,3-dione (100microM) but not 1-(aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (100microM), a non-competitive AMPA receptor antagonist known to interact with the cyclothiazide site to control AMPA receptor function. The broad spectrum ionotropic glutamate receptor antagonist, kynurenic acid (100-1000microM) also markedly inhibited the AMPA-evoked responses in the striatum in vivo. None of the antagonists, when given alone, influenced basal efflux of [3H]L-glutamate suggesting a lack of tonic regulatory control of glutamate release via presynaptic AMPA-type autoreceptors in the rat striatum. These results demonstrate the presence of presynaptic AMPA receptors, of a novel cyclothiazide- and aniracetam-insensitive subtype, on presynaptic nerve terminals in the rat striatum in vivo, acting to enhance glutamate and GABA release. Our data support the concept of AMPA receptor heterogeneity in vivo, a finding which may facilitate the development of novel, more selective drugs for the treatment of a range of neurological disorders associated with abnormal cerebral glutamate release. The pharmacological profile of these novel presynaptic receptors is currently under investigation.

Site-specific activation of dopamine and serotonin transmission by aniracetam in the mesocorticolimbic pathway of rats

The effects of aniracetam on extracellular levels of dopamine (DA), serotonin (5-HT) and their metabolites were examined in five brain regions in freely moving stroke-prone spontaneously hypertensive rats (SHRSP) using in vivo microdialysis. Basal DA release in SHRSP was uniformly lower in all regions tested than that in age-matched control Wistar Kyoto rats. 3,4-Dihydroxyphenylacetic acid and homovanillic acid levels were altered in the basolateral amygdala, dorsal hippocampus and prefrontal cortex of SHRSP. While basal 5-HT release decreased in the striatum and increased in the basolateral amygdala, there was no associated change in 5-hydroxyindoleacetic acid levels. Systemic administration of aniracetam to SHRSP enhanced both DA and 5-HT release with partly associated change in their metabolite levels in the prefrontal cortex, basolateral amygdala and dorsal hippocampus, but not in the striatum and nucleus accumbens shell, in a dose-dependent manner (30 and/or 100 mg/kg p.o.). Microinjection (1 and 10 ng) of aniracetam or its metabolites (N-anisoyl-GABA and 2-pyrrolidinone) into the nucleus accumbens shell produced no turning behavior. These findings indicate that SHRSP have a dopaminergic hypofunction throughout the brain and that aniracetam elicits a site-specific activation in mesocorticolimbic dopaminergic and serotonergic pathways in SHRSP, possibly via nicotinic acetylcholine receptors in the ventral tegmental area and raphe nuclei. The physiological roles in the aniracetam-sensitive brain regions may closely link with their clinical efficacy towards emotional disturbances appearing after cerebral infarction.

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

Impulsivity and AMPA receptors: aniracetam ameliorates impulsive behavior induced by a blockade of AMPA receptors in rats

The study aimed to ascertain the involvement of central AMPA receptors in impulsive behaviors of aged rats and to examine the effects of aniracetam. Premature response in the two-lever choice reaction task was assessed as an index of impulsivity. Intracerebroventricular injection of 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX), an AMPA receptor antagonist, dose-dependently (10.1-1009 ng/rat) increased only premature response without altering responding speed and choice accuracy 30 min after the injection. Aniracetam (30 mg/kg p.o.), a positive allosteric modulator of AMPA receptors, or AMPA (55.9 ng/rat, co-injected with NBQX) completely restored the NBQX-induced increase in impulsivity. These results indicate that AMPA receptors are tonically involved in the regulation of impulsivity.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

N-methyl-d-aspartate receptors regulate 5-HT release in the raphe nuclei and frontal cortex of freely moving rats: differential role of 5-HT1A autoreceptors

The effects of infusing N-methyl-d-aspartate (NMDA) into the raphe nuclei on release of 5-HT in this brain region and also the frontal cortex of the same animal were studied using in vivo microdialysis in freely moving rats. Infusion of 25 microM NMDA into the raphe led to a substantial decrease in dialysate 5-HT in this region and a prolonged increase in terminal 5-HT release in the frontal cortex. These effects were blocked by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (D-AP5; 100 microM). When 25 microM NMDA was co-infused into the raphe with the selective 5-HT1A receptor antagonist (N-¿2-¿4-(2-methoxyphenyl)-1-piperazinyl¿ethyl-N-(2-pyridinyl) cyclohexanecarboxamide) (WAY-100635; 1.0 microM) the effect of NMDA infusion was unaltered. WAY-100635 infused alone into the raphe did not alter local 5-HT or extracellular 5-HT in the cortex. Infusion of 100 microM NMDA into the raphe was followed by an increase in local dialysate 5-HT and a decrease in 5-HT release in the cortex. These changes were reversed by D-AP5. Following infusion of 100 microM NMDA with 1.0 microM WAY-100635 into the raphe local 5-HT release was still increased, however, the decrease in 5-HT observed in the frontal cortex was abolished. These data suggest that the degree of NMDA receptor activation leads to dramatically different outcomes with regard to serotonergic transmission to the frontal cortex. Furthermore, there appears to be a differential role of the 5-HT1A autoreceptor in regulating these effects. These data are discussed in relation to other studies on the regulation of serotonergic transmission in ascending pathways.

The glutamate receptor/NO/cyclic GMP pathway in the hippocampus of freely moving rats: modulation by cyclothiazide, interaction with GABA and the behavioural consequences

Monitoring of extracellular cGMP during intracerebral microdialysis in freely moving rats permits the study of the functional changes occurring in the glutamate receptor/nitric oxide (NO) synthase/guanylyl cyclase pathway and the relationship of these changes to animal behaviour. When infused into the rat hippocampus in Mg2+-free medium, cyclothiazide, a blocker of desensitization of the AMPA-preferring receptor, increased cGMP levels. The effect of cyclothiazide (300 microM) was abolished by the NO synthase inhibitor L-NARG (100 microM) or the soluble guanylyl cyclase inhibitor ODQ (100 microM). During cyclothiazide infusion the animals displayed a pre-convulsive behaviour characterized by frequent "wet dog shakes" (WDS). Neither L-NARG nor ODQ decreased the WDS episodes. Both cGMP and WDS responses elicited by cyclothiazide were prevented by blocking NMDA receptor function with the glutamate site antagonist CGS 19755 (100 microM), the channel antagonist MK-801 (30 microM) or Mg2+ ions (1 mM). The AMPA/kainate receptor antagonists DNQX (100 microM) and NBQX (100 microM) abolished the WDS episodes but could not inhibit the cyclothiazide-evoked cGMP response. DNQX or NBQX (but not MK-801) elevated, on their own, extracellular cGMP levels. The cGMP response elicited by the antagonists appears to be due to prevention of a glutamate-dependent inhibitory GABAergic tone, since infusion of bicuculline (50 microM) caused a strong cGMP response. The results suggest that (a) AMPA/kainate receptors linked to the NO/cGMP pathway in the hippocampus (but not NMDA receptors) are tonically activated and kept in a desensitized state by endogenous glutamate; (b) blockade of AMPA/kainate receptor desensitization by cyclothiazide leads to endogenous activation of NMDA receptors; (c) the hippocampal NO/cGMP system is under a GABAergic inhibitory tone driven by non-NMDA ionotropic receptors; (d) the pre-convulsive episodes observed depend on hippocampal NMDA receptor activation but not on NO and cGMP production.

Influence of AMPA/kainate receptors on extracellular 5-hydroxytryptamine in rat midbrain raphe and forebrain

1. The regulation of 5-hydroxytryptamine (5-HT) release by excitatory amino acid (EAA) receptors was examined by use of microdialysis in the CNS of freely behaving rats. Extracellular 5-HT was measured in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), nucleus accumbens, hypothalamus, frontal cortex, dorsal and ventral hippocampus. 2. Local infusion of kainate produced increases in extracellular 5-HT in the DRN and MRN. Kainate infusion into forebrain sites had a less potent effect. 3. In further studies of the DRN and nucleus accumbens, kainate-induced increases in extracellular 5-HT were blocked by the EAA receptor antagonists, kynurenate and 6,7-dinitroquinoxaline-2,3-dione (DNQX). 4. The effect of infusing kainate into the DRN or nucleus accumbens was attenuated or abolished by tetrodotoxin (TTX), suggesting that the increase in extracellular 5-HT is dependent on 5-HT neuronal activity. In contrast, ibotenate-induced lesion of intrinsic neurones did not attenuate the effect of infusing kainate into the nucleus accumbens. Thus, the effect of kainate in the nucleus accumbens does not depend on intrinsic neurones. 5. Infusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate (AMPA) into the DRN and nucleus accumbens induced nonsignificant changes in extracellular 5-HT. Cyclothiazide and diazoxide, which attenuate receptor desensitization, greatly enhanced the effect of AMPA on 5-HT in the DRN, but not in the nucleus accumbens. 6. In conclusion, AMPA/kainate receptors regulate 5-HT in the raphe and in forebrain sites.

Desensitization of AMPA receptors and AMPA-NMDA receptor interaction: an in vivo cyclic GMP microdialysis study in rat cerebellum

1. Desensitization is an important characteristic of glutamate receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type. 2. Stimulation of N-methyl-D-aspartate (NMDA) or AMPA receptors in cerebellum results in increased production of cyclic GMP. We have investigated AMPA receptor desensitization in vivo by monitoring extracellular cyclic GMP during intracerebellar microdialysis in conscious unrestrained adult rats. 3. Local infusion of AMPA (10 to 100 microM) caused dose-related elevations of cyclic GMP levels. The effect of AMPA was prevented by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and by the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NOARG). 4. In the absence of AMPA, DNQX lowered the basal levels of cyclic GMP whereas the NMDA receptor channel antagonist dizocilpine (MK-801) was ineffective. 5. Cyclothiazide, a blocker of AMPA receptor desensitization, potentiated the cyclic GMP response to exogenous AMPA. Moreover, cyclothiazide (100-300 microM) produced on its own dose-dependent elevations of extracellular cyclic GMP. The cyclothiazide-induced response was prevented not only by DNQX but also by MK-801. 6. While the cyclic GMP response elicited by AMPA was totally insensitive to MK-801, the response produced by AMPA (10 microM) plus cyclothiazide (30 microM) was strongly attenuated by the NMDA receptor antagonist (30 microM). 7. The results suggest that (a) AMPA receptors linked to the NO-cyclic GMP pathway in the cerebellum can undergo desensitization in vivo during exposure to exogenous AMPA; cyclothiazide inhibits such desensitization; (b) AMPA receptors (but not NMDA receptors) are 'tonically' activated and kept in a partly desensitized state by endogenous glutamate; (c) if cyclothiazide is present, activation of AMPA receptors may permit endogenous activation of NMDA receptors.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors modulate dopamine release in rat hippocampus and striatum

The effects of infusing the glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) on the release of dopamine (DA) has been studied in rat hippocampus and striatum in vivo. In hippocampus, AMPA (1-10 microM) produced a dose related increase in dialysate DA, but at 100 microM AMPA a sustained decrease in extracellular DA was observed. However, when samples were collected at 5-min intervals 100 microM AMPA infusion revealed a brief increase in hippocampal dialysate DA. Infusion of 100 microM AMPA and 500 microM diazoxide, which blocks AMPA receptor desensitization, led to a marked increase in extracellular DA, as did diazoxide alone, although to a lesser extent. The AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3,-dione (CNQX; 200 microM) reversed the effect of AMPA and/or diazoxide infusion on dialysate DA and when infused alone, CNQX also decreased hippocampal dialysate DA. AMPA (50-500 microM) increased striatal DA release. The effect of AMPA on extracellular DA was reversed by CNQX (200 microM). Diazoxide infusion caused a decrease in striatal DA release, and this was not affected by CNQX. These data suggest that hippocampal, but not striatal AMPA receptor desensitization may play a role in regulating DA release.