Effect of suramin (Bayer 205) on renal ornithine decarboxylase activity and polyamine concentrations in rats.

After a single high dose (20 mg/100 g i.v.) of suramin ornithine decarboxylase activity was increased rapidly in the rat kidney. Enzyme kinetic measurements indicate that on the basis of K_m values renal ornithine decarboxylases from control or suramin treated rats are indistinguishable. Renal nucleic acid and polyamine levels were also enhanced in response to suramin. Changes observed in this study are considered as biochemical signs of induced renal growth.     

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.     

Reciprocal Innervation between Serotonergic and GABAergic Neurons in Raphe Nuclei of the Rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain in order to study serotonergic-GABAergic interaction. The slices were loaded with either [³H] serotonin or [³H]GABA, superfused and the electrically induced efflux of radioactivity was determined. The GABA_A receptor agonist muscimol (3 to 30 M) and the GABA_B receptor agonist baclofen (30 and 100 M) inhibited [³H]serotonin and [³H]GABA release. These effects of muscimol were reversed by the GABA_A antagonists bicuculline (100 M). The GABA_B antagonist phaclofen (100 M) also antagonized the baclofen-induced inhibition of [³H]serotonin and [³H]GABA release. Phaclofen by itself increased [³H]serotonin release but it did not alter [³H]GABA overflow. Muscimol (10 M) and baclofen (100 M) also inhibited [³H]serotonin release after depletion of GABAergic neurons by isoniazid pretreatment. These findings indicate the presence of postsynaptic GABA_A and GABA_B receptors located on serotonergic neurons. The 5-HT_1A receptor agonist 8-OH-DPAT (0.01 to 1 M) and the 5-HT_1B receptor agonist CGS-12066A (0.01 to 1 M) inhibited the electrically stimulated [³H]serotonin and [³H]GABA release. The 5-HT_1A antagonist WAY-100135 (1 M) was without effect on [³H]serotonin and [³H]GABA efflux by itself but it reversed the 8-OH-DPAT-induced transmitter release inhibition. During KCl (22 mM)-induced depolarization, tetrodotoxin (1 M) did not alter the inhibitory effect of CGS-12066A (1 M) on [³H]GABA release, it did blocked, however, the ability of 8-OH-DPAT (1 M) to reduce [³H]GABA efflux. After depletion of raphe serotonin neurons by p-chlorophenylalanine pretreatment, CGS-12066A (1 M) still inhibited [³H]GABA release whereas in serotonin-depleted slices, 8-OH-DPAT (1 M) was without effect on the release. We conclude that reciprocal influence exists between serotonergic projection neurons and the GABAergic interneurons or afferents in the raphe nuclei and these interactions may be mediated by 5-HT_1A/B and GABA_A/B receptors. Both synaptic and non-synaptic neurotransmission may be operative in the 5-HTergic-GABAergic reciprocal interaction which may serve as a local tuning in the neural connection between cerebral cortex and midbrain raphe nuclei.     

Neurochemical evidence for two types of presynaptic alpha2-adrenoceptors

Neurochemical and pharmacological evidence has been obtained that noradrenergic varicosities (in mouse and rat vas deferens) and cholinergic varicosities (in the Auerbach's plexus) contain heterogenous alpha₂-adrenoceptors through which the release of [³H]noradrenaline and [³H]acetylcholine can be modulated. The quantitative data also support the hypothesis that different noradrenaline and xylazine sensitive alpha₂-adrenoceptors are present prejunctionally in the vas deferens and Auerbach's plexus preparations. Prazosin, although it has a presynaptic inhibitory effect on alpha₂-adrenoceptors of noradrenergic axon terminals, has no effect on cholinergic axon terminals. These data suggest that there are two different types of alpha₂-adrenoceptors at the presynaptic axon terminals.Special Issue Dedicated to Dr. Abel Lajtha     

Release of [3H]noradrenaline by α1-adrenoceptor agonists

Mouse isolated vas deferens preincubated with [³-H]noradrenaline was superfused and the effect of ₁-adrenoceptor agonists was studied on the release of total radioactivity ([³H]noradrenaline +³H-metabolites) and [³H]noradrenaline. Reverse phase high pressure liquid chromatography (HPLC) combined with scintillation spectrometry was used to separate [³H]noradrenaline from its metabolites. Among the ₁-adrenoceptor agonists (1-phenylephrine, ST-587(2-(2-chloro-5-trifluoromethyl phenylimino)-imidazole), (–)-amidephrine, methoxamine, cirazoline and l-noradrenaline) studied l-phenylephrine, ST-587 and l-noradrenaline were capable of releasing³H-noradrenaline. The effect of noradrenaline was stereospecific. As determined by HPLC combined with scintillation spectrometry the release of total radioactivity in response to l-noradrenaline is mainly due to [³H]noradrenaline. It is suggested that l-noradrenaline, l-phenylephrine, and ST-587 in addition to their direct effect on different receptors they also have indirect action through the release of noradrenaline which might be partly involved in the pharmacological responses. The mechanisms whereby l-noradrenaline and l-phenylephrine release noradrenaline would appear to involve a saturable Ca-independent and a cocaine and temperature sensitive process. On the basis of our findings among the ₁-adrenoceptor agonist studied (–)-amidephrine, methoxamine and cirazoline is a better choice than l-phenylephrine or ST-587 for selective stimulation of postjunctional ₁-adrenoceptor, they do not release noradrenaline.     

Effect of acetyl-l-carnitine on extracellular amino acid levels in vivo in rat brain regions

Acetyl-l-carnitine (ALCAR) was found to have beneficial effects in senile patients. In recent years many of its effects on the nervous system have been examined, but its mechanism(s) of action remains to be elucidated. We previously reported that it causes release of dopamine in the striatum. In the present paper we report that ALCAR, when administered at intracerebral sites via microdialysis, stimulates the release of amino acids in a concentration-dependent and regionally heterogeneous manner. The effect was strong in the striatum and cerebellum, less so in the frontal cortex, and weak in the thalamus. Seven amino acids were measured: the increase in the level of aspartate, glutamate, and taurine was substantial, and the increase in the level of glycine, serine, threonine, alanine, and glutamine in the microdialysate was minor. The stimulatory effect of ALCAR on the release of amino acids in the striatum was inhibited by the muscarinic antagonist atropine, but was not inhibited by the nicotinic antagonist mecamylamine. The effect of ALCAR on the levels of most of the amino acids tested was independent of the presence of Ca²⁺ in the perfused. These results indicate that ALCAR, when administered intracerebrally at fairly high concentrations, can affect the level and the release not only of such neurotransmitters as acetylcholine and dopamine, but also of amino acids. The mechanism of action of ALCAR on the release of cerebral amino acids may involve the participation of muscarinic receptors or may be mediated through the release of dopamine, but the lack of Ca²⁺ dependence indicates a release from the cytoplasmic amino acid pool, possibly through the effect of ALCAR on cell membrane permeability.     

The synaptic and nonsynaptic glycine transporter type-1 inhibitors Org-24461 and NFPS alter single neuron firing rate in the rat dorsal raphe nucleus. Further evidence for a glutamatergic-serotonergic interaction and its role in antipsychotic action

Single neuron firing rate was recorded from dorsal raphe nucleus of anesthetized rats. The firing rate of raphe neurons varied from 4 to 8 discharge per second before drug administration and this neuronal activity was decreased by L-701,324 (2 mg/kg i.v. injection), a competitive antagonist of glycineB binding site of N-methyl-D-aspartate (NMDA) receptors. The glycine transporter type-1 (GlyT1) antagonists Org-24461 (10 mg/kg i.v.) and NFPS (3 mg/kg i.v.) reversed the inhibitory effect of L-701,324 on single neuron activity recorded from dorsal raphe nucleus of the rat. Org-24461 and NFPS both tended to increase the raphe neuronal firing rate also when given alone but their effect was not significant. This finding serves further evidence that glutamate released from axon terminals of the cortico-striatal projection neurons stimulates serotonergic neurons in the raphe nuclei and this effect is mediated at least in part by postsynaptic NMDA receptors. Thus, GlyT1 inhibitors are able to reverse the hypofunctional state of NMDA receptors, suggesting that these drugs may have beneficial therapeutic effects in neurological and psychiatric disorders characterized with impaired NMDA receptor-mediated transmission.     

The role of various calcium and potassium channels in the regulation of somatodendritic serotonin release

We prepared slices from midbrain containing the raphe nuclei and from hippocampus of rats. The brain slices were loaded with [³H]serotonin and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. No difference was observed in the resting and stimulated fractional release of tritium in the somatodendritic and axon terminal parts of serotonergic neurons. The selective 5-HT_1A receptor agonist 8-OH-DPAT decreased the electrically induced tritium effux from raphe nuclei slices preloaded with [³H]serotonin, and this inhibition was reversed by 5-HT_1A receptor antagonist (+)WAY-100135. The 5-HT_1B receptor agonist CGS-12066B but not 8-OH-DPAT, inhibited the stimulation-evoked tritium efflux from hippocampal slices after labeling with [³H]serotonin. The electrical stimulation-evoked tritium efflux in raphe nuclei slices incubate with [³H]serotonin was completely external Ca²⁺-dependent, and omega-conotoxin GVIA and Cd²⁺, but not diltiazem, inhibited the tritium overflow. In raphe nuclei slices 4-aminopyridine enhanced the electrical stimulation-induced trititum release in a concentration-dependent manner. The inhibition of tritium efflux by 8-OH-DPAT was abolished with 4-aminopyridine. Glibenclamide or tolbutamide proved to be ineffective. These data indicate that (1) different 5-HT receptor subtypes (5-HT_1A and 5-HT_1B) regulate dendritic and axon terminal 5-HT release; (2) serotonin release from the dendrites may be regulated by the voltage-sensitive N-type Ca²⁺ channels; (3) the 5-HT_1A receptor-mediated inhibition of serotonin release may be due to opening of voltage-sensitive K⁺ channels.     

Dopamine Efflux from Striatum After Chronic Nicotine: Evidence for Autoreceptor Desensitization

We examined the effect of chronic nicotine treatment on dopaminergic activity by measuring the effects of D1 and D2 dopamine (DA) receptor agonists and antagonists on tritium release from mouse striatum preloaded with [3H]DA. The radioactivity released during superfusion was separated on alumina columns and the distribution and efflux of [3H]DA and its main 3H-labeled metabolites were quantified. After preloading by incubation with [3H]DA, the electrical stimulation-evoked tritium overflow was higher in striatum prepared from nicotine-treated mice, whereas in vitro addition of nicotine caused a similar increase in tritium release from striatum of untreated and chronic nicotine-treated mice. The overflow of [3H]DA and its 3H-metabolites exhibited similar distribution patterns in [3H]DA-preloaded striatum dissected from untreated and chronic nicotine-pretreated mice, indicating that repeated injections with nicotine did not alter the metabolism of [3H]DA taken up by the tissue. (-)-Quinpirole, a selective agonist for D2 DA receptors, and apomorphine, a nonselective D1/D2 agonist, inhibited the electrical stimulation-induced tritium efflux from striatum of untreated mice, whereas (+/-)-sulpiride, a D2 DA receptor antagonist, enhanced the evoked release of tritium. These changes in tritium efflux effected by (-)-quinpirole and (+/-)-sulpiride reflected changes in [3H]DA release and not in DA metabolism, as shown by separation of the released radioactivity on alumina columns. The D1 receptor agonist (+/-)-SKF-38393 did not affect the tritium overflow, whereas the D1 receptor antagonist (+)-SCH-23390 exerted a stimulatory action but only at a high concentration.(ABSTRACT TRUNCATED AT 250 WORDS)