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

2,3-Benzodiazepine-type AMPA receptor antagonists and their neuroprotective effects

AMPA receptors are fast ligand-gated members of glutamate receptors in neuronal and many types of non-neuronal cells. The heterotetramer complexes are assembled from four subunits (GluR1-4) in region-, development- and function-selective patterns. Each subunit contains three extracellular domains (a large amino terminal domain, an agonist-binding domain and a transducer domain), and three transmembrane segments with a loop (pore forming domain), as well as the intracellular carboxy terminal tail (traffic and conductance regulatory domain). The binding of the agonist (excitatory amino acids and their derivatives) initiates conformational realignments, which transmit to the transducer domain and membrane spanning segments to gate the channel permeable to Na+, K+ and more or less to Ca2+. Several 2,3-benzodiazepines act as non-competitive antagonists of the AMPA receptor (termed also negative allosteric modulators), which are thought to bind to the transducer domains and inhibit channel gating. Analysing their effects in vitro, it has been possible to recognize a structure-activity relationship, and to describe the critical parts of the molecules involved in their action at AMPA receptors. Blockade of AMPA receptors can protect the brain from apoptotic and necrotic cell death by preventing neuronal excitotoxicity during pathophysiological activation of glutamatergic neurons. Animal experiments provided evidence for the potential usefulness of non-competitive AMPA antagonists in the treatment of human ischemic and neurodegenerative disorders including stroke, multiple sclerosis, Parkinson's disease, periventricular leukomalacia and motoneuron disease. 2,3-benzodiazepine AMPA antagonists can protect against seizures, decrease levodopa-induced dyskinesia in animal models of Parkinson's disease demonstrating their utility for the treatment of a variety of CNS disorders.

Effects of 2,3-benzodiazepine AMPA receptor antagonists on dopamine turnover in the striatum of rats with experimental parkinsonism

Although levodopa is the current "gold standard" for treatment of Parkinson's disease, there has been disputation on whether AMPA receptor antagonists can be used as adjuvant therapy to improve the effects of levodopa. Systemic administration of levodopa, the precursor of dopamine, increases brain dopamine turnover rate and this elevated turnover is believed to be essential for successful treatment of Parkinson's disease. However, long-term treatment of patients with levodopa often leads to development of dyskinesia. Therefore, drugs that feature potentiation of dopamine turnover rate and are able to reduce daily levodopa dosages might be used as adjuvant in the treatment of patients suffering from Parkinson's disease. To investigate such combined treatment, we have examined the effects of two non-competitive AMPA receptor antagonists, GYKI-52466 and GYKI-53405, alone or in combination with levodopa on dopamine turnover rate in 6-hydroxydopamine-lesioned striatum of the rat. We found here that repeated administration of levodopa, added with the peripheral DOPA decarboxylase inhibitor carbidopa, increased dopamine turnover rate after lesioning the striatum with 6-hydroxydopamine. Moreover, combination of levodopa with GYKI-52466 or GYKI-53405 further increased dopamine turnover enhanced by levodopa administration while the AMPA receptor antagonists by themselves failed to influence striatal dopamine turnover. We concluded from the present data that potentiation observed between levodopa and AMPA receptor antagonists may reflect levodopa-sparing effects in clinical treatment indicating the therapeutic potential of such combination in the management of Parkinson's disease.

Characteristics of GABA release in mouse brain stem slices under normal and ischemic conditions

GABA is known to be the inhibitory neurotransmitter in the majority of brain stem nuclei. The release of GABA has been extensively studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release in the brain stem have not been systemically characterized. The properties of preloaded [3H]GABA were now investigated in mouse brain stem slices, using a superfusion system. The basal release was enhanced by K+ stimulation (50 mM K+) and under various cell-damaging conditions (ischemia, hypoglycemia, the presence of free radicals and metabolic poisons). No K+-stimulated release was discernible in the absence of Ca2+, indicating that the release was at least partly Ca2+-dependent. Moreover, the release was increased when Na+ or Cl- was omitted from the superfusion medium. GABA and beta-alanine stimulated the release, confirming the involvement of the reversed function of GABA transporters. Incubation of the slices with the anion channel inhibitors diisothiocyanostilbene and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate and with the Cl- uptake inhibitor 9-anthracenecarboxylic acid also reduced GABA release, demonstrating that a part of it comprises leakage through anion channels. All these mechanisms were involved in the ischemia-induced GABA release, which was over 4-fold greater than the release in normoxia. Contrary to the other brain areas, GABA release in the brain stem was not affected by ionotropic glutamate receptors but may be modulated by metabotropic receptors. This ischemia-induced GABA release might constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

The effects of AMPA receptor antagonists in models of stroke and neurodegeneration

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists have been shown to have neuroprotective effects in stroke models and although clinical trials with some agents are still ongoing, published results have not been favourable. We therefore wished to compare the effects of GYKI 52466, GYKI 53405, EGIS-8332 and EGIS-10608, non-competitive AMPA receptor antagonists with homophthalazine chemical structures, in standard animal stroke models with effects in a neurodegenerative model--excitoxicity in newborn mice. All compounds inhibited the S-AMPA-induced spreading depression in the chicken retina, in vitro, and were potent anticonvulsants against maximal electroshock in mice, in vivo. The AMPA receptor antagonists prevented domoate-induced cell death of motoneurons, in vitro, and reduced infarct size in a dose-dependent manner in the permanent middle cerebral artery occlusion model in mice, in vivo. In newborn mice (P5, histopathology at P10), local injection of the AMPA receptor agonist S-bromo-willardiine at day 5 after birth induced cortical damage and white matter damage, which was reduced in a dose-dependent manner by the AMPA receptor antagonists. EGIS 10608 was a very powerful receptor antagonist of white matter damage. In contrast, GYKI 52466 did not antagonize cortical and white matter damage induced by ibotenic acid. These models allow quantification of the effects of AMPA receptor antagonists in vitro and in vivo.

Role of NMDA and AMPA glutamate receptors in the induction and the expression of dopamine-mediated sensitization in 6-hydroxydopamine-lesioned rats

Rats with unilateral 6-hydroxydopamine (6-OHDA) lesions exhibit behavioral sensitization following repeated treatment with dopamine agonists, a phenomenon called "priming." Priming has two distinct phases: induction and expression. Priming induction using three injections with D1/D2 agonist apomorphine (0.5 mg/kg) or D1 agonist SKF38393 (10 mg/kg) allows priming expression, robust contralateral rotational behavior and striatal Fos expression, following a challenge with the D2 agonist quinpirole (0.25 mg/kg). We examined the roles of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors on dopamine agonist priming. Administration of the NMDA antagonist (+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK801) (0.5 mg/kg) blocked apomorphine-priming of quinpirole-mediated responses, while MK801 dose-dependently attenuated SKF38393-priming of quinpirole-mediated striatal Fos expression and had no effect on SKF38393-priming of quinpirole-mediated rotational behavior. In contrast, administration of the AMPA antagonist 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo[f]quinoxaline (NBQX) (5 or 10 mg/kg) potentiated apomorphine- and SKF38393-priming of quinpirole-mediated striatal Fos expression, but had no effect on their priming of quinpirole-mediated rotational behavior. In SKF38393-primed 6-OHDA rats, administration of MK801 (0.5 mg/kg) blocked the expression of quinpirole-mediated responses, while administration of NBQX (10 mg/kg) or the noncompetitive AMPA antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI52466) (5 or 15 mg/kg) had no effect. These results suggest that NMDA and AMPA glutamate receptors have differing roles in dopamine agonist priming-with NMDA receptors required for D1/D2 priming induction and D2-mediated priming expression, and AMPA receptors inhibiting priming induction of D2-mediated immediate early gene expression in the striatum, but not affecting priming induction of D2-mediated rotational behavior or the expression of D2-mediated responses.

Metabotropic glutamate receptors modulate ischemia-induced GABA release in mouse hippocampal slices

The involvement of glutamate receptors in GABA release in ischemia was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice. For in vitro ischemia, the slices were superfused in glucose-free media under nitrogen. Ionotropic glutamate receptor agonists failed to affect the ischemia-induced basal GABA release at either age. The K(+)-stimulated release in the immature hippocampus was potentiated by N-methyl-D-aspartate receptors, whereas in adults this release was reduced by both kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate receptor activation. The group I metabotropic receptor agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal ischemic GABA release in a receptor-mediated manner in adults, this being concordant with the positive modulation of GABAergic neurotransmission by group I metabotropic glutamate receptors. (1 +/-)-1-Aminocyclopentane-trans-1,3-dicarboxylate and (S)-3,5-dihydroxyphenylglycine also enhanced the K(+)-stimulated release in the developing hippocampus in a receptor-mediated manner. Because group I receptors generally increase neuronal excitability, the enhanced GABA release may attenuate hyperexcitation or strengthen inhibition, being thus neuroprotective, particularly under ischemic conditions. Group III metabotropic glutamate receptors were not at all involved in ischemic GABA release in the immature mice, but in adults their activation by O-phospho-L-serine potentiated the basal release and reduced the K(+)-stimulated release. These opposite effects were abolished by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Metabotropic glutamate receptors, namely group I and III receptors, are able to modify the release of GABA from hippocampal slices under ischemic conditions, both positive and negative effects being discernible, depending on the age and type of receptor activated.

Characteristics of GABA release modified by glutamate receptors in mouse hippocampal slices

The major part of hippocampal innervation is glutamatergic, regulated by inhibitory GABA-releasing interneurons. The modulation of [(3)H]GABA release by ionotropic and metabotropic glutamate receptors and by nitric oxide was here characterized in superfused mouse hippocampal slices. The ionotropic glutamate receptor agonists kainate, N-methyl-D-aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate potentiated the basal GABA release. These effects were blocked by their respective antagonists 6-nitro-7-cyanoquinoxaline-2,3-dione (CNQX), dizocilpine and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide (NBQX), indicating receptor-mediated mechanisms. The NO-generating compounds S-nitroso-N-acetylpenicillamine (SNAP), sodiumnitroprusside and hydroxylamine enhanced the basal GABA release. Particularly the sodiumnitroprusside-evoked release was attenuated by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA) and the inhibitor of soluble guanylyl cyclase 1H-(1,2,4)oxadiazolo(4,3a)quinoxalin-1-one (ODQ), indicating the involvement of the NO/cGMP pathway. This inference is corroborated by the enhancing effect of zaprinast, a phosphodiesterase inhibitor, which is known to increase cGMP levels. The K(+)-stimulated hippocampal GABA release was reduced by the groups I and III agonists of metabotropic glutamate receptors (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD) and L-(+)-2-amino-4-phosphonobutyrate (L-AP4), which effects were abolished by their respective antagonists (RS)-1-aminoindan-1,5-dicarboxylate (AIDA) and (RS)-2-cyclopropyl-4-phosphonophenylglycine (CPPG), again indicating modification by receptor-mediated mechanisms.

17beta-estradiol inhibits excitatory amino acid-induced activity of neurons of the nucleus tractus solitarius

The effects of 17beta-estradiol (17betaE2) on spontaneous and excitatory amino acid (EAA) induced nucleus tractus solitarius (NTS) neuronal activity were investigated by electrophysiological and immunohistochemical experiments in ovariectomized female Sprague-Dawley rats. Out of 62 NTS neurons tested, 42 were inhibited (68%) following iontophoretic application of 17betaE2 in a current-dependent manner. The averaged firing rate decreased from 3.06+/-0.40 to 0.78+/-0.17 Hz. The inhibitory responses were rapid in onset (within 1 min) and variable in duration (2-4 min). The inhibitory effects of 17betaE2 were blocked by simultaneously applied 17betaE2 antagonist ICI182,780, but not by GABA antagonist, bicuculline and phaclofen. L-Glutamate, AMPA or NMDA enhanced the activity of 71, 73 or 69% of NTS cells tested, respectively. The excitatory effects of EAA were significantly inhibited in the presence of 17betaE2. Fluorescent immunohistochemistry revealed that all subnuclei of the NTS contained high levels of estrogen receptors (ERs) immunoreactivity. These results suggest that 17betaE2 inhibits spontaneous and EAA-induced NTS neuronal activity through 17betaE2 activation of ERs.

Pharmacological heterogeneity of release-regulating presynaptic AMPA/kainate receptors in the rat brain: study with receptor antagonists

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptors mediating hippocampal [(3)H]noradrenaline or [(3)H]serotonin release, striatal [(3)H]dopamine release and cortical [(3)H]acetylcholine release were pharmacologically characterized using several AMPA/kainate receptor antagonists. The releases of the four transmitters elicited by exposing synaptosomes to AMPA were antagonized by NBQX, indicating that they reflect AMPA/kainate receptor activation. GYKI52466 did not inhibit the AMPA-induced release of [(3)H]noradrenaline, [(3)H]dopamine or [(3)H]serotonin, while it weakly affected the AMPA-mediated release of [(3)H]acetylcholine. On the contrary, LY300164 and LY303070 were potent antagonists able to discriminate among AMPA/kainate receptor subtypes. Both compounds blocked the AMPA receptors mediating [(3)H]dopamine and [(3)H]acetylcholine release. However, LY303070, but not LY300164, inhibited the AMPA-induced release of [(3)H]noradrenaline, while the AMPA-mediated [(3)H]serotonin release was sensitive to LY300164 but not to LY303070. SYM2206 mimicked LY300164 and prevented the AMPA-induced release of [(3)H]dopamine, [(3)H]acetylcholine and [(3)H]serotonin, but not that of [(3)H]noradrenaline. NS102 failed to antagonize the AMPA-induced release of all four transmitters. LY293558 prevented the AMPA-mediated release of [(3)H]noradrenaline, [(3)H]dopamine, [(3)H]acetylcholine or [(3)H]serotonin. Differently, LY377770 did not inhibit the AMPA-mediated release of [(3)H]noradrenaline and [(3)H]acetylcholine, but it potently blocked the AMPA-induced release of [(3)H]serotonin and, less so, of [(3)H]dopamine. AMOA inhibited the AMPA-induced release of [(3)H]serotonin or [(3)H]acetylcholine, but not that of [(3)H]noradrenaline or [(3)H]dopamine. GAMS prevented the AMPA-mediated release of [(3)H]acetylcholine and, more weakly, that of [(3)H]dopamine, but it failed to inhibit the release of [(3)H]noradrenaline or [(3)H]serotonin elicited by AMPA. gamma-DGG did not affect the AMPA-mediated release of any of the four transmitters studied. In conclusion, based on the antagonist profiles obtained, the four receptors here analyzed all belong to the AMPA-preferring subclass of glutamate receptors; however, they appear to differ from each other, probably due to differences in subunit composition. The compounds LY300164, LY303070, LY377770, AMOA and GAMS may be useful to discriminate among AMPA-preferring receptor subtypes.

Negative allosteric modulation of AMPA-preferring receptors by the selective isomer GYKI 53784 (LY303070), a specific non-competitive AMPA antagonist

GYKI 53784 or LY303070 [(-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine] belongs to a new family of 2,3-benzodiazepine compounds (also called homophtalazines) selective and non-competitive antagonists at alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. These compounds include the original GYKI-52466, its more potent derivative GYKI 53655 and the active isomer of the latter, GYKI 53784. This review summarizes current knowledge of this novel AMPA antagonist: GYKI 53784. GYKI 53784 is the most potent of the compounds in the 2,3-benzodiazepine class, blocking AMPA receptor-mediated responses. In contrast to the compounds of the quinoxalinedione family, that block AMPA as well as kainate receptors, GYKI 53784 does not block the activation of kainate receptors. Furthermore, GYKI 53784 does not act at the same receptor site as positive AMPA modulators (i.e., cyclothiazide, BDP-12, 1-BCP or aniracetam). GYKI 53784 is a powerful neuroprotective agent in both in vitro and in vivo models of AMPA receptor-mediated excitotoxicity. In contrast to NMDA receptor antagonists, whose favorable clinical actions are compromised by important side effects such as the impairment of memory functions, the selective AMPA antagonist, GYKI 53784, may be of potential clinical value, both in acute (stroke and trauma) and chronic (Alzheimer's disease, epilepsy) neurological disorders.

Protective effect of the antiepileptic drug candidate talampanel against AMPA-induced striatal neurotoxicity in neonatal rats

2,3-Benzodiazepines represent a family of specific, noncompetitive AMPA receptor antagonists with anticonvulsant and neuroprotective properties. In this study, the antiexcitotoxic potency of the clinical antiepileptic drug candidate, talampanel (4 x 2 mg/kg), and that of two related 2,3-benzodiazepines, 5-(4-aminophenyl)-8-methyl-9H-1,3-dioxolo[4,5-h][2,3]-benzodiazepine (GYKI 52466) (4 x 10 mg/kg) and GYKI 53784 (4 x 2 mg/kg), was investigated in 7-day-old rats. The AMPA antagonists were applied in four consecutive i.p. injections at 1-h intervals, the first dosage was given shortly after the intrastriatal injection of (S)-alpha-amino-3-hydroxy-5,7-methylisoxazole-4-propionic acid (AMPA) (2.5 nmol). All tested compounds protected animals from brain damage induced by AMPA as assessed 5 days later by using a tissue volume determination method based on computer-aided serial section reconstruction. GYKI 53784 (56.1 +/- 5.0% protection) and talampanel (42.5 +/- 5.3% protection) were more potent neuroprotective agents than GYKI 52466 (21.8 +/- 2.8% protection). Furthermore, the three compounds attenuated the unilateral AMPA injection-induced turning behavior and seizure-like events.Our present findings are in agreement with those of other investigators who found talampanel neuroprotective in various in vivo experimental models. These data indicate that besides being a promising antiepileptic drug candidate talampanel may have a value in the pharmacotherapy of acute and chronic neurodegenerative diseases, including perinatal ischemia/hypoxia-induced brain injuries, as well.

Does extracellular calcium determine what pool of GABA is the target for alpha-latrotoxin?

Presynaptic neurotoxin alpha-latrotoxin, from the venom of Latrodectus mactans tredecimguttatus, causes massive [(3)H]GABA release from rat brain synaptosomes, irrespective of calcium presence in the extracellular medium. Whether the binding of alpha-latrotoxin to Ca(2+)-dependent (neurexin 1 alpha) or to Ca(2+)-independent (latrophilin) receptor triggers [(3)H]GABA release by the same mechanisms or different ones, inducing either exocytotic process or outflow by mobile membrane GABA transporter, is unknown. We examined alpha-latrotoxin-evoked [(3)H]GABA release from synaptosomes which cytosolic [(3)H]GABA pool was depleted either by applying competitive inhibitors of the GABA transporter, nipecotic acid and 2,4-diaminobutyric acid, or by permeation with digitonin. We also compared the effect of the GABA transporter inhibitors on depolarisation-evoked and alpha-latrotoxin-evoked [(3)H]GABA release using as depolarising agents 4-aminopyridine and high KCl in the Ca(2+)-containing and in Ca(2+)-free medium, respectively. Incubation of synaptosomes with nipecotic acid induced the essential acceleration of unstimulated [(3)H]GABA release and deep inhibition of high KCl-evoked Ca(2+)-independent [(3)H]GABA release. In contrast, at the similar conditions the effect of alpha-latrotoxin was greatly augmented with respect to the control response. Another way to assay what GABA pool was involved in alpha-latrotoxin-induced release lays in an analysis of the effects of depolarisation and alpha-latrotoxin in consecutive order. The preliminary 4-aminopyridine-stimulated [(3)H]GABA release attenuated the toxin effect. But when depolarisation occurred in Ca(2+)-free medium, no influence on alpha-latrotoxin effect was revealed. Employing digitonin-permeated synaptosomes, we have shown that alpha-latrotoxin could stimulate [3H]GABA release in the medium with 1mM EGTA, this effect of the toxin was blocked by concanavalin A and was ATP-dependent. The latter suggests that alpha-latrotoxin-released neurotransmitter has the vesicular nature. We assume that the type of the toxin membrane receptor does not determine the mechanisms of [(3)H]GABA release evoked by alpha-latrotoxin.

The non-competitive AMPA receptor antagonist (GYKI 52466) blocks quisqualate-induced acetylcholine release from the rat hippocampus and striatum: an in vivo microdialysis study

The effects of the non-N-methyl-D-aspartate (NMDA) agonist quisqualate (QUIS) and selective AMPA/kainate receptor antagonist 1-(aminophenyl)-methyl-7, 8-methyilendioxy-5H-2,3-benzodiazepine (GYKI 52466) on the release of acetylcholine (ACh) from the hippocampus and striatum of freely moving rats were studied by transversal microdialysis. Acetylcholine level in the dialisate was measured by the high performance liquid chromatography (HPLC) method with an electrochemical detector. The QUIS (100 microM) perfused through the striatum induced an increase of extracellular ACh level (250%) which lasted for over 1h and gradually returned to basal values. Local perfusion of GYKI 52466 (10-100 microM) to the striatum did not change the basal release of ACh. GYKI 52466 (10 microM) administered together with QUIS (100 microM) in he striatum antagonized the stimulant effect of QUIS on the ACh release. Local administration of the QUIS (100 microM) through the microdialysis fiber implanted in the hippocampus, caused a long lasting increase of extracellular hippocampal ACh level (360%) which was reversed when the drug was withdrawn from the perfusion solution. The stimulant effect of QUIS was antagonized by concomitant perfusion of GYKI (10 microM). No effect was seen on the basal ACh release when GYKI (10-100 microM) was perfused through the hippocampus. Local perfusion with tetrodotoxin (1 microM) decrease the basal release of ACh and prevented the QUIS-induced increase of ACh both in the hippocampus and striatum. Our in vivo neurochemical results indicate that hippocampal and striatal cholinergic systems are regulated by non-NMDA (probably AMPA) glutamatergic receptors located in the hippocampus and striatum.

Somatostatin stimulates striatal acetylcholine release by glutamatergic receptors: an in vivo microdialysis study

The modulation of striatal cholinergic neurons by somatostatin (SOM) was studied by measuring the release of acetylcholine (ACh) in the striatum of freely moving rats. The samples were collected via a transversal microdialysis probe. ACh level in the dialysate was measured by the high performance liquid chromatography method with an electrochemical detector. Local administration of SOM (0.1, 0.5 and 1 microM) produced a long-lasting and concentration-dependent increase in the basal striatal ACh output. The stimulant effect of SOM was antagonized by the SOM receptor antagonist cyclo(7-aminopentanoyl-Phe-D-Trp-Lys-Thr[BZL]) (1 microM). In a series of experiments, we studied the effect of 6,7-dinitroquinoxaline-2, 3-dione (DNQX), a selective non-NMDA (N-methyl-D-aspartate) glutamatergic antagonist, on the basal and SOM-induced ACh release from the striatum. DNQX, 2 microM, perfused through the striatum had no effect on the basal ACh output but inhibited the SOM (1 microM)-induced ACh release. The non-NMDA glutamatergic receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3- benzodiazepine (GYKI-52466), 10 microM, antagonized the SOM (1 microM)-induced release of ACh in the striatum. Local administration of the NMDA glutamatergic receptor antagonist, 2-amino-5-phosphonopentanoic acid (APV), 100 microM, blocked SOM (1 microM)-evoked ACh release. Local infusion of tetrodotoxin (1 microM) decreased the basal release of ACh and abolished the 1 microM SOM-induced increase in ACh output suggesting that the stimulated release of ACh depends on neuronal firing. The present results are the first to demonstrate a neuromodulatory role of SOM in the regulation of cholinergic neuronal activity of the striatum of freely moving rats. The potentiating effect of SOM on ACh release in the striatum is mediated (i) by SOM receptor located on glutamatergic nerve terminals, and (ii) by NMDA and non-NMDA glutamatergic receptors located on dendrites of cholinergic interneurones of the striatum.