A method for concurrent measurement of picomole quantities of acetylcholine, choline, dopamine, norepinephrine, serotonin, 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, tryptophan, tyrosine, glycine, aspartate, glutamate, alanine, and gamma-aminobutyric acid in single tissue samples from different areas of rat central nervous system

Crystal structure of NAD+-dependent Peptoniphilus asaccharolyticus glutamate dehydrogenase reveals determinants of cofactor specificity

Glutamate dehydrogenases (EC 1.4.1.2-4) catalyse the oxidative deamination of l-glutamate to α-ketoglutarate using NAD(P) as a cofactor. The bacterial enzymes are hexamers and each polypeptide consists of an N-terminal substrate-binding (Domain I) followed by a C-terminal cofactor-binding segment (Domain II). The reaction takes place at the junction of the two domains, which move as rigid bodies and are presumed to narrow the cleft during catalysis. Distinct signature sequences in the nucleotide-binding domain have been linked to NAD(+) vs. NADP(+) specificity, but they are not unambiguous predictors of cofactor preferences. Here, we have determined the crystal structure of NAD(+)-specific Peptoniphilus asaccharolyticus glutamate dehydrogenase in the apo state. The poor quality of native crystals was resolved by derivatization with selenomethionine, and the structure was solved by single-wavelength anomalous diffraction methods. The structure reveals an open catalytic cleft in the absence of substrate and cofactor. Modeling of NAD(+) in Domain II suggests that a hydrophobic pocket and polar residues contribute to nucleotide specificity. Mutagenesis and isothermal titration calorimetry studies of a critical glutamate at the P7 position of the core fingerprint confirms its role in NAD(+) binding. Finally, the cofactor binding site is compared with bacterial and mammalian enzymes to understand how the amino acid sequences and three-dimensional structures may distinguish between NAD(+) vs. NADP(+) recognition.

Involvement of 5-hydroxytryptamine in the preventive effect of Armillariella tabescens against cisplatin-induced changes in gastric electromyographic activity in rats

AIM: To investigate the preventive effect of Armillariella tabescens against cisplatin-induced changes in gastric electromyographic activity in rats and to explore the role of 5-hydroxytryptamine (5-HT) in this process.

METHODS: Ninety male Sprague-Dawley rats were randomized into control group, model group, ondansetron group, low-, medium-, and high-dose Armillariella tabescens groups. Rats were injected intraperitoneally with cisplatin (6 mg/kg) to induce pica. The electrical activity of gastric antral smooth muscle was recorded and analyzed using the Biopac MP100-CE acquisition system. The concentration of 5-HT in gastric antrum tissue was assayed by ELISA. The effect of Armillariella tabescens on gastric electromyographic activity and its relationship with 5-HT were then examined.

RESULTS: During 24-72 h after cisplatin administration, the cycles per minute (CPM) and amplitude of vibration (AV) of slow-wave in the gastric antrum were significantly higher in the model group than in the control group (all P < 0.05), medium-, and high-dose Armillariella tabescens groups (all P < 0.01), and the differences were most significant between the model group (CPM: 7.33 ± 2.92, AV: 249.75 ± 79.09) and the control group (CPM: 3.00 ± 1.55, AV: 148.04 ± 63.51) and high-dose Armillariella tabescens group (CPM: 4.13 ± 1.14, AV: 163.46 ± 26.14) at 24 h after cisplatin administration. Compared with the control group, the concentrations of 5-HT (μg/L) in the other groups were dramatically increased (24 h: 389.7 ± 25.5, 354.5 ± 34.9, 314.5 ± 31.5, 282.2 ± 19.6, 271.0 ± 21.1 vs 244.6 ± 27.3, all P < 0.05 or 0.01). CMP and AV increased with the increase in the concentrations of 5-HT.

CONCLUSION: Armillariella tabescens could effectively inhibit cisplatin-induced changes in gastric electromyographic activity in rats possibly via mechanisms involving 5-HT.

Trypsinogen IV, serotonin transporter transcript levels and serotonin content are increased in small intestine of irritable bowel syndrome patients

Colorectal and small intestinal visceral hypersensitivity has been demonstrated in irritable bowel syndrome (IBS). Serine protease signalling via protease-activated receptor (PAR)-2 promotes hyperalgesia to mechanical distension. Furthermore, serotonergic pathways are involved in gastrointestinal visceral sensitivity. Abnormalities of serine protease and serotonergic signalling components have been identified in IBS colorectal mucosal biopsies. We determined the role of altered mucosal serine protease and serotonergic signalling in small intestine of IBS patients. Duodenal mucosal biopsies of 34 IBS patients (10 constipation-,11 diarrhoea-predominant and 13 alternating) and 20 healthy subjects (HS) were collected. Gene transcripts of PAR-2, trypsinogen IV, TPH-1, SERT (serotonin transport protein) and serotonin (5-HT(3)) subunits were quantified using real-time PCR and 5-HT content was measured by ELISA. Irritable bowel syndrome patients showed 1.5-fold higher trypsinogen IV mRNA level compared to HS (P = 0.016). SERT expression was 1.8-fold higher in IBS compared to HS (P = 0.007). Mucosal 5-HT content was 1.7-fold higher in IBS compared to HS (P = 0.015). The increase was 2.1-fold in IBS-C relative to HS (P = 0.018). Transcript levels of PAR-2, TPH-1 and 5-HT(3) receptor subunits did not differ between IBS and HS. In conclusion enhanced trypsinogen IV expression in IBS may cause increased PAR-2 activation. Increased SERT expression and mucosal 5-HT content in IBS suggest higher 5-HT availability. Both may contribute to small intestinal visceral hypersensitivity in IBS patients.

Neurosteroid modulation of the presynaptic NMDA receptors regulating hippocampal noradrenaline release in normal rats and those exposed prenatally to diazepam

Prenatal exposure to diazepam (DZ), a positive allosteric modulator of the gamma-aminobutyric acid(A) (GABA(A)) receptor complex, exerts profound effects that become more evident during puberty and in many cases are sex-specific, suggesting that such exposure interferes with the activity of steroid hormones. Apart from their well known effects on the genome, the reduced metabolites of many steroid hormones also interact directly with membrane receptors, including those for N-methyl-D-aspartate (NMDA). In this study, we compared the effects of several neurosteroids on NMDA receptors from normal rats and those exposed in utero to DZ (1.25mg/kg per day) from the 14th through the 20th day of gestation. In superfused rat hippocampal synaptosomes, activation of the NMDA receptor stimulates the basal release of [3H]noradrenaline ([3H]NA), which was used in our study as an index of receptor function. [3H]NA release was evoked in a concentration-dependent manner by NMDA (100 microM) plus glycine (GLY). The maximal increase (68.23+/-3.86%) with respect to basal release was achieved with a GLY concentration of 10 microM, and the EC(50) for GLY was 0.1 microM. Release stimulated by 100 microM NMDA + 0.1 microM GLY was not modified by any of the neurosteroids tested, with the exception of pregnenolone sulfate (PREG-S), which produced a 78.57+/-3.94% reduction in release at the maximal concentration used (0.3 microM). In synaptosomes from animals exposed in utero to DZ, the inhibitory effect of PREG-S was reduced by 46.55+/-2.33%. Given the important roles played by NMDA receptors in physiological and pathological processes within the central nervous system (CNS), characterization of NMDA receptor modulation is an important objective. The fact that this modulation can be altered by exposure in utero to DZ indicates that the behavioral abnormalities observed in exposed animals might be partially attributed to an altered sensitivity of NMDA receptors to the modulatory effects of neurosteroids.

The sensitivity of catecholamine release to botulinum toxin C1 and E suggests selective targeting of vesicles set into the readily releasable pool

The impact of syntaxin and SNAP-25 cleavage on [3H]noradrenaline ([3H]NA) and [3H]dopamine ([3H]DA) exocytotic release evoked by different stimuli was studied in superfused rat synaptosomes. The external Ca2+-dependent K+-induced [3H]catecholamine overflows were almost totally abolished by botulinum toxin C1 (BoNT/C1), which hydrolyses syntaxin and SNAP-25, or by botulinum toxin E (BoNT/E), selective for SNAP-25. BoNT/C1 cleaved 25% of total syntaxin and 40% of SNAP-25; BoNT/E cleaved 40% of SNAP-25 but left syntaxin intact. The GABA uptake-induced releases of [3H]NA and [3H]DA were differentially affected: both toxins blocked the former, dependent on external Ca2+, but not the latter, internal Ca2+-dependent. BoNT/C1 or BoNT/E only slightly reduced the ionomycin-evoked [3H]catecholamine release. More precisely, [3H]NA exocytosis induced by ionomycin was sensitive to toxins in the early phase of release but not later. The Ca2+-independent [3H]NA exocytosis evoked by hypertonic sucrose, thought to release from the readily releasable pool (RRP) of vesicles, was significantly reduced by BoNT/C1. Pre-treating synaptosomes with phorbol-12-myristate-13-acetate, to increase the RRP, enhanced the sensitivity to BoNT/C1 of [3H]NA release elicited by sucrose or ionomycin. Accordingly, cleavage of syntaxin was augmented by the phorbol-ester. To conclude, our results suggest that clostridial toxins selectively target exocytosis involving vesicles set into the RRP.

Amino acid neurotransmitters: separation approaches and diagnostic value

Amino acids in the central nervous system can be divided into non-neurotransmitter or neurotransmitter depending on their function. The measurement of these small molecules in brain tissue and extracellular fluid has been used to develop effective treatment strategies for neuropsychiatric and neurodegenerative diseases and for the diagnosis of such pathologies. Here we describe the separation and detection techniques that have been used for the measurement of amino acids at trace levels in brain tissue and dialysates. An overview of the function of amino acid transmitters in the brain is given. In addition, the type of sampling techniques that are used for the determination of amino acid levels in the brain is described.

Selective blockade of mGlu5 metabotropic glutamate receptors is protective against methamphetamine neurotoxicity

Methamphetamine (MA), a widely used drug of abuse, produces oxidative damage of nigrostriatal dopaminergic terminals. We examined the effect of subtype-selective ligands of metabotropic glutamate (mGlu) receptors on MA neurotoxicity in mice. MA (5 mg/kg, i.p.; injected three times, every 2 hr) induced, 5 d later, a substantial degeneration of striatal dopaminergic terminals associated with reactive gliosis. MA toxicity was primarily attenuated by the coinjection of the noncompetitive mGlu5 receptor antagonists 2-methyl-6-(phenylethynyl)pyridine and (E)-2-methyl-6-styrylpyridine both at 10 mg/kg, i.p.). In contrast, the mGlu1 receptor antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (10 mg/kg, i.p.), and the mGlu2/3 receptor agonist (-)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (1 mg/kg, i.p.), failed to affect MA toxicity. mGlu5 receptor antagonists reduced the production of reactive oxygen species but did not reduce the acute stimulation of dopamine release induced by MA both in striatal synaptosomes and in the striatum of freely moving mice. We conclude that endogenous activation of mGlu5 receptors enables the development of MA neurotoxicity and that mGlu5 receptor antagonists are neuroprotective without interfering with the primary mechanism of action of MA.

Interaction of formamidine pesticides with the presynaptic alpha(2)-adrenoceptor regulating

The effects of the formamidine pesticides amitraz and chlordimeform on the alpha(2)-adrenergic receptor subtype that mediates the release of [(3)H]noradrenaline by synaptosomes from rat hypothalami were studied. We initially characterized the presynaptic autoreceptor on noradrenergic nerve endings using selective antagonists. Yohimbine (a nonselective alpha(2) antagonist) and BRL 44408 (selective for subtypes alpha(2A)/alpha(2D)) diminished the inhibitory effect of xylazine on K(+)-evoked release of [(3)H]noradrenaline; the K(B) values were 481 and 154 nM, respectively. In contrast, prazosin (a selective alpha(2B)/alpha(2C) antagonist) did not modify the inhibitory effect of xylazine. These results indicate that the release of noradrenaline by noradrenergic nerve endings in the rat hypothalamus is regulated by alpha(2D)-adrenoceptors, a species variation of the human alpha(2A) subtype. We then assessed the effects of the two pesticides on the K(+)-evoked release of [(3)H]noradrenaline. Amitraz reduced release in a dose-dependent manner; the effect observed at the maximal concentration tested (10 microM) was 13.0 +/- 2.0% and it was reversed by yohimbine. Amitraz also diminished the inhibitory effects of the alpha(2)-adrenergic agonists clonidine and xylazine. Chlordimeform displayed no effects, possibly because the true active compound of this insecticide is its demethylated metabolite. Based on these findings we conclude that the formamidine pesticides act as partial agonists of presynaptic alpha(2D)-adrenergic receptors in the rat hypothalamus. This interaction may be responsible for the in vivo alterations in catecholaminergic regulation of cyclic variations in gonadotropin-releasing hormone (GnRH) secretion, which can have grave functional repercussions on the reproductive system of mammals exposed to these xenobiotics.

Disruption of neuroendocrine control of luteinizing hormone secretion by aroclor 1254 involves inhibition of hypothalamic tryptophan hydroxylase activity

Mechanisms governing the effect of polychlorinated biphenyl (PCB) toxicity on hypothalamic serotonergic function and the neuroendocrine system controlling LH secretion were investigated in Atlantic croaker (Micropogonias unulatus) exposed to the PCB mixture Aroclor 1254 (1 microg x g body weight(-1) x day(-1)) in the diet for 30 days. PCB treatment caused a decrease in hypothalamic 5-hydroxytryptamine (5-HT) concentrations and significant inhibition of hypothalamic tryptophan hydroxylase (TPH), the rate-limiting enzyme in 5-HT synthesis, but did not alter the activity of monoamine oxidase, the catabolic enzyme. Further, PCB treatment caused significant decreases in GnRH content in the preoptic-anterior hypothalamic area. Significant decreases in pituitary GnRH receptor concentrations and the LH response to the GnRH analogue (GnRHa) were also observed in PCB-exposed fish, possibly as a consequence of a decline in GnRH release. The possible association between impaired serotonergic and neuroendocrine functions after PCB treatment was explored using serotonergic drugs. Treatment of croaker with p-chlorophenylalanine, an irreversible TPH inhibitor, mimicked the effects of PCB on the GnRH system and the LH response to GnRHa. Bypassing the TPH-dependent hydroxylation step with the administration of 5-hydroxytryptophan restored 5-HT to control levels and prevented the deleterious effects of PCB on the neuroendocrine parameters. Moreover, slow-release GnRH implants prevented the PCB-induced decline in GnRH receptors and restored the LH response to GnRHa, suggesting that GnRH therapy can reverse PCB-induced disruption of LH secretion. These results demonstrate that TPH is one of the targets of PCB neurotoxicity and indicate that a decrease in 5-HT availability in PCB-exposed croaker results in disruption of the stimulatory 5-HT/GnRH pathway controlling LH secretion.

Characterization of Ca(2+)-channels responsible for K(+)-evoked [(3)H]noradrenaline release from rat brain cortex synaptosomes and their response to amyotrophic lateral sclerosis IgGs

The contribution of the different Ca(2+)-channel subtypes to the K(+)-evoked [(3)H]noradrenaline release from rat cerebral cortex synaptosomes has been investigated. In the same experimental model, it was also verified whether the calcium-mediated neurotransmitter release is influenced by IgGs purified from sera of seven patients affected by sporadic amyotrophic lateral sclerosis. Synaptosome treatment with 3.0 microM nifedipine or 2.0 microM calciseptine, which block L-type channels, slightly decreased [(3)H]noradrenaline release, the reduction being 7 and 13% of the control values, respectively. The blockade of N-type Ca(2+)-channels with omega-conotoxin-GVIA (0.001-1.0 microM) induced a concentration-dependent reduction of the neurotransmitter release, with maximum effect of 34%. omega-Agatoxin-IVA failed to significantly affect the studied release, which was instead markedly reduced by omega-conotoxin-MVIIC. After the blockade of N-type channels with maximal concentrations of omega-conotoxin-GVIA, 3.0 microM omega-conotoxin-MVIIC reduced the release by 58%. Synaptosome treatment with amyotrophic lateral sclerosis IgGs enhanced the K(+)-evoked [(3)H]noradrenaline release, which was mostly mediated by P/Q- and N-type Ca(2+)-channels. The increase induced by pathologic IgGs (0.2 mg/ml) ranged from 11 to 62% for the different patients, and it was concentration-dependent. The basal release was instead unaffected by IgG treatment. The results of the present study suggest that the K(+)-evoked [(3)H]noradrenaline release from brain cortex synaptosomes is mainly mediated by activation of P/Q- and N-type Ca(2+)-channels. Autoantibodies present in the sera of patients affected by sporadic amyotrophic lateral sclerosis may interact with these channels by producing an increased calcium influx, with consequent enhancement of the neurotransmitter release. Preliminary results of the present study have been published in abstract form (Martire et al., 1997, Pharmacol. Res. 35:9).

Evidence for calcium-dependent vesicular transmitter release insensitive to tetanus toxin and botulinum toxin type F

Whether exocytosis evoked by a given releasing stimulus from different neuronal families or by different stimuli from one neuronal population occurs through identical mechanisms is unknown. We studied the release of [3H]noradrenaline, [3H]acetylcholine and [3H]dopamine induced by different stimuli from superfused rat brain synaptosomes pretreated with tetanus toxin or botulinum toxin F, known to block exocytosis by cleaving VAMP/synaptobrevin. The external Ca2(+)-dependent [3H]transmitter overflows evoked by KCl were similarly inhibited by tetanus toxin or botulinum toxin F; the toxins cleaved similar amounts of synaptosomal synaptobrevin, as determined by western blot analysis, suggesting prevalent involvement of synaptobrevin-II. GABA uptake-mediated release of the three [3H]transmitters was that differentially sensitive to the toxins: only the release of [3H]noradrenaline, which is dependent on external Ca2+, but not of [3H]acetylcholine and [3H]dopamine was blocked. Neither toxin affected the [3H]transmitter overflows evoked by the Ca2(+) ionophore ionomycin. Cadmium blocked the K(+)-evoked release of all [3H]transmitters and the GABA-evoked release of [3H]noradrenaline; the GABA-evoked releases of [3H]acetylcholine and [3H]dopamine and those elicited by ionomycin were insensitive to cadmium. The results suggest that tetanus toxin and botulinum toxin F selectively affect exocytosis linked to activation of voltage-sensitive Ca2(+) channels; the Ca2(+)-dependent, exocytotic-like release induced by stimuli not leading to activation of voltage-sensitive Ca2+ channels seems insensitive to these clostridial toxins.

Effects of nitric oxide donors on basal and K+-evoked release of [3H]noradrenaline from rat cerebral cortex synaptosomes

We investigated the effects of nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside on basal and K+-evoked release of [3H]noradrenaline from superfused synaptosomes from the rat cerebral cortex. Both substances produced concentration-dependent increases in the release of the labeled transmitter under basal and depolarized conditions. The effects of the donors on basal release were Ca2+-independent but were not inhibited by the carrier-uptake blocker, desipramine; the effects were abolished by hemoglobin (an NO scavenger). Thirty-five minutes after stimulation with sodium nitroprusside, the synaptosomes were still responsive to KCl stimulation, indicating that the donor's effects were not caused by damage to the synaptosome membrane. The cGMP analogue, 8-bromo-cGMP, had no effect on basal release, and the enhanced release produced by sodium nitroprusside was not inhibited by the specific inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, indicating that NO's effects on basal release of the neurotransmitter are guanylate cyclase-independent. Both of the NO donors had more marked effects on release of [3H]noradrenaline during K+-stimulated depolarization. The NO-mediated increase in this case was partially antagonized by 10 microM LH-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, and 8-Br-cGMP was also capable of producing concentration-dependent increases in the K+-stimulated release of the transmitter. These findings indicate that the effects of the NO donors on [3H]noradrenaline release during depolarization are partially mediated by the activation of guanylate cyclase.

A new GABA-A receptor subtype coupled with Ca++/Cl- synporter modulates aminergic release from rat brain neuron terminals

The aim of the present study was to give a better characterization of GABA receptors that modulate aminergic release. GABA or muscimol (15 microM) increased basal noradrenaline (3H-NA) release but reduced the following K+-evoked 3H-NA release in the synaptosomes from rat cerebellar cortex. Bicuculline and picrotoxin counteracted these two effects. The same GABA modulation resulted to operate also on dopaminergic and serotoninergic neuron terminals. The increased basal noradrenaline release resulted to be both calcium and chloride dependent and associated with an increased entry of 45Ca++ into the synaptosomes. We therefore advance the hypothesis of an involvement of a Cl-/Ca++ synporter system coupled to the receptor. Baclofen also reduced the K+-evoked 3H-NA release, but did not increase basal 3H-NA release; moreover, the interaction of baclofen G with GABA-B receptors resulted to be associated with the inhibition of 45Ca++ entry into synaptosomes. GABA-B receptors resulted to be present also on serotoninergic but not on dopaminergic neuron terminals. The GABA-C receptor agonist cis-4-aminocrotonic acid (CACA) did not influence either basal or K+-evoked 3H-NA release. These results point to a new type of GABA functional role through a different A-family receptor subtype, coupled with calcium influx in aminergic neuron terminals, modulating aminergic release.

Nonsynaptic glycine receptor activation during early neocortical development

Glycine receptors (GlyRs) contribute to fast inhibitory synaptic transmission in the brain stem and spinal cord. GlyR subunits are expressed in the developing neocortex, but a neurotransmitter system involving cortical GlyRs has yet to be demonstrated. Here, we show that GlyRs in immature neocortex are excitatory and activated by a nonsynaptically released endogenous ligand. Of the potential ligands for cortical GlyRs, taurine is by far the most abundant in the developing neocortex. We found that taurine is stored in immature cortical neurons and that manipulations known to elevate extracellular taurine cause GlyR activation. These data indicate that nonsynaptically released taurine activates GlyRs during neocortical development. As fetal taurine deprivation can cause cortical dysgenesis, it is possible that taurine influences neocortical development by activating GlyRs.

Characterization of a selective antagonist of neuropeptide Y at the Y2 receptor. Synthesis and pharmacological evaluation of a Y2 antagonist

Neuropeptide Y (NPY) is a potent inhibitor of neurotransmitter release through the Y2 receptor subtype. Specific antagonists for the Y2 receptors have not yet been described. Based on the concept of template-assembled synthetic proteins we have used a cyclic template molecule containing two beta-turn mimetics for covalent attachment of four COOH-terminal fragments RQRYNH2 (NPY 33-36), termed T4-[NPY(33-36)]4. This structurally defined template-assembled synthetic protein has been tested for binding using SK-N-MC and LN319 cell lines that express the Y1 and Y2 receptor, respectively. T4-[NPY(33-36)]4 binds to the Y2 receptor with high affinity (IC50 = 67.2 nM) and has poor binding to the Y1 receptor. This peptidomimetic tested on LN319 cells at concentrations up to 10 microM shows no inhibitory effect on forskolin-stimulated cAMP levels (IC50 for NPY = 2.5 nM). Furthermore, we used confocal microscopy to examine the NPY-induced increase in intracellular calcium in single LN319 cells. Preincubation of the cells with T4-[NPY(33-36)]4 shifted to the right the dose-response curves for intracellular mobilization of calcium induced by NPY at concentrations ranging from 0.1 nM to 10 microM. Finally, we assessed the competitive antagonistic properties of T4-[NPY(33-36)]4 at presynaptic peptidergic Y2 receptors modulating noradrenaline release. the compound T4-[NPY(33-36)]4 caused a marked shift to the right of the concentration-response curve of NPY 13-36, a Y2-selective fragment, yielding a pA2 value of 8.48. Thus, to our best knowledge, T4-[NPY(33-36)]4 represents the first potent and selective Y2 antagonist.

Occurrence and biosynthesis of endogenous cannabinoid precursor, N-arachidonoyl phosphatidylethanolamine, in rat brain

It has been suggested that anandamide (N-arachidonoylethanolamine), an endogenous cannabinoid substance, may be produced through Ca2+-stimulated hydrolysis of the phosphatidylethanolamine (PE) derivative N-arachidonoyl PE. The presence of N-arachidonoyl PE in adult brain tissue and the enzyme pathways that underlie its biosynthesis are, however, still undetermined. We report here that rat brain tissue contains both anandamide (11 +/- 7 pmol/gm wet tissue) and N-arachidonoyl PE (22 +/- 16 pmol/gm), as assessed by gas chromatography/mass spectrometry. We describe a N-acyltransferase activity in brain that catalyzes the biosynthesis of N-arachidonoyl PE by transferring an arachidonate group from the sn-1 carbon of phospholipids to the amino group of PE. We also show that sn-1 arachidonoyl phospholipids are present in brain, where they constitute approximately 0.5% of total phospholipids. N-acyltransferase activity is Ca2+ dependent and is enriched in brain and testis. Within the brain, N-acyltransferase activity is highest in brainstem; intermediate in cortex, striatum, hippocampus, medulla, and cerebellum; and lowest in thalamus, hypothalamus, and olfactory bulb. Pharmacological inhibition of N-acyltransferase activity in primary cultures of cortical neurons prevents Ca2+-stimulated N-arachidonoyl PE biosynthesis. Our results demonstrate, therefore, that rat brain tissue contains the complement of enzymatic activity and lipid substrates necessary for the biosynthesis of the anandamide precursor N-arachidonoyl PE. They also suggest that biosynthesis of N-arachidonoyl PE and formation of anandamide are tightly coupled processes, which may concomitantly be stimulated by elevations in intracellular Ca2+ occurring during neural activity.

Presynaptic A2-adrenoceptors and neuropeptide Y Y2 receptors inhibit [3H]noradrenaline release from rat hypothalamic synaptosomes via different mechanisms

Presynaptic receptors may reduce transmitter release with different mechanisms. Both the alpha 2-agonist, clonidine and the Y2-agonist, neuropeptide Y fragment 13-36 (NPY 13-36), induce a concentration-dependent inhibition of the 4-aminopyridine (4-AP)-evoked [3H]noradrenaline ([3H]NA) release from hypothalamic synaptosomes. Changes in alpha 2- and Y2-modulation of noradrenaline (NA) release were observed by lowering the calcium influx with the use of omega-conotoxin (omega-CgTx), a calcium-channel blocking agent. In these experimental conditions, clonidine was less active, whereas NPY 13-36 preserved its efficacy. It therefore seems possible that presynaptic alpha 2-adrenoceptors can primarily inhibit NA release by reducing calcium influx via voltage-sensitive calcium channels (VSCC), while Y2-receptors may inhibit the intracellular release process with a mechanism independent of the calcium entry.

Putative cognition enhancers reverse kynurenic acid antagonism at hippocampal NMDA receptors

Oxiracetam, aniracetam and D-cycloserine, three putative cognition enhancers, were examined in a functional assay for NMDA receptors. Rat hippocampal slices or synaptosomes were labeled with [3H]noradrenaline and exposed to NMDA or glutamate in superfusion. NMDA (100 microM) elicited a remarkable rise (about 500%) in the release of [3H]noradrenaline from slices. The effect of NMDA was antagonized by the glutamate receptor blocker, kynurenic acid. The antagonism by 100 microM kynurenate was reduced by submicromolar concentrations of oxiracetam and totally reversed by 1 microM of the drug. The concentration-antagonism curve for kynurenic acid was shifted to the right in the presence of 0.2 or 1 microM oxiracetam. Aniracetam and D-cycloserine, as well as glycine and D-serine, behaved similarly to oxiracetam: all compounds, tested at 1 microM, reversed the antagonism by 100 microM kynurenate of the NMDA-evoked [3H]noradrenaline release. In superfused hippocampal synaptosomes, 100 microM NMDA or glutamic acid elicited the release of [3H]noradrenaline; the evoked release was enhanced by glycine, but not by oxiracetam. In this preparation 1 microM glycine or 1 microM oxiracetam prevented the antagonism by kynurenate of the NMDA- or the glutamate-evoked [3H]noradrenaline release. As kynurenic acid is an endogenous glutamate receptor antagonist whose brain levels are known to increase in conditions associated to cognitive deficits, it is proposed that the putative cognition enhancers tested may act in vivo by relieving the antagonism produced by excessive endogenous kynurenate.

Age-related changes in rat brain monoamines release: peculiarity of dopamine release

Our aim has been to investigate the ability of the rat brain to retain its level of neurotransmitter release over life. We have investigated the neurotransmitter release from the rat brain synaptosomes prelabeled with 3H-DA, 3H-NA, or 3H-5HT, and perfused with Krebs-Ringer medium alone (basal release) or containing a high K+, calcium ionophore, tyramine or amphetamine (evoked release). Brain areas have been dissected of animals 45 days and 4, 6, and 11 months old. The results have shown a gradual reduction of the 3H-NA release evoked by a high K+ from 45 days to 6 months, which is stabilized until 11 months of age. The reduction rate has been relatively different from the brain areas investigated (36% for the frontal cortex and 26% for the hippocampus and cerebellar cortex). A similar reduction has been seen with 3H-5HT released from synaptosomes of the frontal cortex, hippocampus, and striatum. Surprisingly, the 3H-DA release that was evoked by high K+ was greater in rats 11 months old than in younger rats; this effect has been seen in synaptosomes from the caudate and the frontal cortex. The calcium ionophore A23187 has shown a releasing picture similar to a high K+. When we analyzed a nonexocitotic, but probably carrier mediated, release (evoked by tyramine or amphetamine), there was reduced release of all of the above neurotransmitters from 45 days to 11 months of age. We presume that there have been adaptive changes in neurotransmitter evoked release due to changes in Ca++ utilization, as inferred from the results from calcium ionophore experiments and carrier performance.(ABSTRACT TRUNCATED AT 250 WORDS)

5HT2-receptors and serotonin release: their role in human platelet aggregation

Involvement of 5HT2 receptors in human platelet aggregation was assessed by studying the effect of ADP, epinephrine and thrombin on 3H-5HT release from platelets. The release experiments were made with a perfusion method to preserve any compound, released or formed by platelet, from interacting with platelet itself. In these conditions, aggregation does not occur, as confirmed by Scanning Electron Microscopy. These release experiments showed that the platelet activation by such agents is coupled with 5-HT release. The aggregation experiments, made on different aliquots of the same platelet-rich plasma (PRP), showed that the released 5-HT, interacting with its own receptors on platelet activated surface, determines aggregation. In fact, although it is known that 5-HT added to PRP was only able to induce a moderate platelet aggregation, the 5-HT2 antagonist ketanserin counteracted the aggregation induced by ADP, epinephrine and thrombin. These results suggest that a 5HT2 antagonist could be therapeutically important in those pathological states in which serotonin, released by activated platelets, may increase aggregation.

Galanin increases potassium evoked release of [3H]5-hydroxytryptamine from rat hypothalamic synaptosomal preparations

The effects of galanin (GAL) have been evaluated on the depolarization evoked release of [3H]5-HT (serotonin or 5-hydroxytryptamine) from rat hypothalamic synaptosomal preparations, using low concentrations of potassium (15 mM). In the same preparation effects of GAL were also evaluated on [3H]5-HT uptake, using kinetic analysis to determine effects on Vmax values and Km values. GAL concentrations of 0.1-10 nM caused a concentration-related increase of the depolarization-evoked release of [3H]5-HT without influencing [3H]5-HT uptake. The results indicate the existence of high affinity GAL receptors on the hypothalamic 5-HT nerve terminals, exerting a facilitatory influence on depolarization-evoked [3H]5-HT release.

Reduced inhibitory effects of clonidine and neuropeptide Y on 3H-noradrenaline release from synaptosomes of the medulla oblongata of the spontaneously hypertensive rat

The release of 3H-noradrenaline (3H-NA) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto and spontaneously hypertensive male rat (14 weeks old) using a superfusion apparatus. Based on concentration-response curves clonidine was shown to have a reduced ability to inhibit 3H-NA release in synaptosomes isolated from the spontaneously hypertensive rat (SHR) versus the normotensive rat. Furthermore, only a high concentration of NPY (100 nM) had the ability to enhance the inhibitory effects of clonidine on 3H-NA release in synaptosomes isolated from the medulla oblongata of the SH male rat, while 1 nM of NPY was effective in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto rat. These results may indicate a reduced presynaptic alpha 2-adrenoceptor and NPY receptor function to inhibit 3H-NA release from NA and/or adrenaline (A) nerve terminals in the medulla oblongata of the adult 14 weeks old SHR.

On the mechanism by which extracellular sodium depletion causes 5-hydroxytryptamine release from rat brain synaptosomes

The release of 3H-labelled 5-hydroxytryptamine (5-HT) from preloaded and superfused rat forebrain synaptosomes in response to extracellular Na+ depletion was studied. In the absence of monoamine oxidase inhibitors, the release of [3H]-5-HT caused by Na+ depletion was not affected by immobilizers of the plasma membrane 5-HT carrier. The release of [3H]-5-HT in response to Na+ depletion was also either independent of, or inversely related to the concentration of extracellular Ca2+ depending on the degree to which extracellular Na+ was reduced. The efflux of 45Ca2+ from prelabelled synaptosomes was decreased by Na+ reduction but the amplitude of the changes in 45Ca2+ efflux did not totally correlate with the changes in [3H]-5-HT efflux under the same experimental conditions. These results suggest that the release of [3H]-5-HT caused by Na+ depletion in drug-free synaptosomes is not mediated by 5-HT efflux through the plasma membrane carrier, nor to changes in cytosolic Ca2+ consequent to changes in Ca2+ fluxes across the plasma membrane. The results have been tentatively explained as an elevation of spontaneous 5-HT efflux caused by an increase in membrane fluidity mediated by the ionic manipulations used to produce the Na+-depleted media.

Facilitation of [3H]acetylcholine and [3H]5-hydroxytryptamine release from rat cerebral cortex synaptosomes by a factor extracted from the skin of the Australian frog Pseudophryne coriacea

A partly purified extract of the skin of the Australian frog Pseudophryne coriacea (PsC) evoked the release of [3H]acetylcholine [( 3H]ACh) and of [3H]5-hydroxytryptamine [( 3H]5-HT) from superfused rat cerebral cortex synaptosomes prelabeled with [3H]choline or [3H]5-HT, respectively. The PsC-evoked release of both transmitters was sensitive to tetrodotoxin and was strictly Ca2+-dependent. The release of [3H]5-HT caused by PsC was unaffected by the 5-HT uptake inhibitor citalopram. Activation of muscarinic autoreceptors by ACh or of serotonin autoreceptors by 5-HT depressed the PsC-evoked release of [3H]ACh or of [3H]5-HT, respectively. It is concluded that PsC elicits a Ca2+-dependent exocytotic-like transmitter release, possibly by opening Na+ channels in the presynaptic membrane.

Different regulation of serotonin receptors following adrenal hormone imbalance in the rat hippocampus and hypothalamus

Adrenal influence on serotonin (5-HT) transmission in the hippocampal and hypothalamic areas was studied in adrenalectomized rats receiving or not corticosterone replacement. After adrenalectomy, the 5-HT presynaptic receptors were desensitized both in hippocampus and hypothalamus: a significant increase in 5-HT 1 and 5-HT 2 receptor binding numbers took place in membranes from the hippocampus, but not in hypothalamus, while no changes in affinity of receptors to radioligands were observed in either brain area. Corticosterone treatment restored the adrenalectomy-impaired 5-HT autoreceptor sensitivity in hippocampus and hypothalamus and 5-HT density receptor sites in the hippocampus. Serotonin autoreceptor down-regulation following adrenalectomy may increase 5-HT release to maintain the constancy of serotonergic transmission in the brain and 5-HT modulated CRH-ACTH release to compensate the plasma corticosteroid drop. Corticosterone seems to display a distinct tonic control on serotonin transmission in both hippocampus and hypothalamus, the diversity being due to the different roles played by the hormone in these brain regions.

Presynaptic mechanisms underlying the gamma-aminobutyric acid-evoked receptor-independent release of [3H]norepinephrine in rat hippocampus

The effects of gamma-aminobutyric acid (GABA) on the spontaneous efflux of [3H]norepinephrine ([3H]NE) were studied in synaptosomes prepared from rat hippocampus and prelabelled with [3H]NE. It had been observed previously that, when synaptosomes were exposed in superfusion to GABA, the basal release of the tritiated catecholamine was enhanced, apparently with no involvement of the known GABA receptors. The mechanisms underlying this effect have now been investigated. The potency of GABA as a releaser of [3H]NE was decreased by lowering the Na+ content of the superfusion medium, and its effect disappeared at 23 mM Na+. The GABA-induced [3H]NE release was counteracted by the GABA uptake inhibitor N-(4,4-diphenyl-3-butenyl)nipecotic acid (SKF 89976A), but it was unaffected by the NE uptake blockers desmethylimipramine and nisoxetine. The GABA-induced release of [3H]NE was Ca2+-dependent and tetrodotoxin-sensitive. The data support the hypothesis that GABA provoked [3H]NE release by a novel mechanism which involves penetration into the noradrenergic nerve terminals through a GABA carrier located on the NE terminals themselves. This uptake process might be electrogenic and provoke depolarization of the nerve terminals, causing an exocytotic release of [3H]NE.

Cobalt-sensitive and dihydropyridine-insensitive stimulation of dopamine release from tuberoinfundibular neurons by high extracellular concentrations of barium ions

Recently, it has been demonstrated that Ca2+ entrance into the neuronal cytoplasm can occur upon the activation of 3 different types of specific voltage-dependent channels which can be characterized according to the following criteria: (1) voltage threshold for activation; (2) tendency to inactivation; (3) bivalent cation permeability; and (4) drug sensitivity. In this study we investigated, in tuberoinfundibular dopaminergic (TIDA) hypothalamic neurons, the biochemical and pharmacological properties of Ca2+ channels, by comparing the effects of high extracellular concentrations of Ba2+ and Ca2+ ions on [3H]dopamine (DA) release from TIDA neurons. The results obtained show that extracellular Ba2+ ion concentrations dose-dependently (10-20 mM) stimulated [3H]DA release from superfused TIDA neurons and that this effect was prevented by Co2+ ions (2 mM). In addition, superfusion of TIDA neurons with a concentration of Ca2+ ions equimolar to that of Ba2+ ions (20 mM) failed to modify [3H]DA release. The fact that tetraethylammonium (10 mM), a blocker of K+ currents in excitable cells, did not mimick the stimulatory action of Ba2+ ions on [3H]DA release, seems to exclude that the effect of Ba2+ ions was dependent on the inhibition of K+ channels in TIDA neurons. The omission of Ca2+ ions from the extracellular medium did not prevent the stimulatory effect on [3H]DA release elicited by elevated concentrations of Ba2+ ions, but rather reinforced this effect. Finally, nitrendipine (50 microM) did not modify the stimulatory effect of high extracellular Ba2+ ions on [3H]DA release from TIDA neurons.

Neuropeptide Y increases the inhibitory effects of clonidine on potassium evoked 3H-noradrenaline but not 3H-5-hydroxytryptamine release from synaptosomes of the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat

The release of 3H-noradrenaline (3H-NA) and of 3H-5-hydroxytryptamine (3H-5-HT) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat using a superfusion apparatus. Based on concentration-response curves obtained by analyzing the full-time course of the inhibitory effects of clonidine on 3H-NA and on 3H-5-HT release neuropeptide Y (NPY) (1 nM) was shown to significantly increase the ability of clonidine to inhibit 3H-NA release in synaptosomes isolated from the hypothalamus and from the frontoparietal cortex. NPY (1 nM) alone had no effect on K+-evoked 3H-NA release from these regions. In contrast, NPY (1 nM) did not modulate the inhibitory effects of clonidine on 3H-5-HT release in the above mentioned regions. These results indicate that NPY can increase the sensitivity of the alpha 2-autoreceptors belonging to hypothalamic NA and/or to adrenaline nerve terminals and to cortical NA nerve terminals, while the alpha 2-heteroreceptors inhibiting 3H-HT release in the same brain regions appear not to be regulated by high affinity NPY receptors. Thus, alpha 2-autoreceptors and alpha 2-heteroreceptors appear to be differentially controlled by high affinity NPY receptors at least with regard to regulation of 3H-NA and 3H-5-HT release, respectively.

Heterogeneity of muscarinic autoreceptors and heteroreceptors in the rat brain: effects of a novel M1 agonist, AF102B

The effects of oxotremorine and AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a novel M1-selective muscarinic agonist, on acetylcholine (ACh) and dopamine (DA) release from superfused rat hippocampal and striatal synaptosomes were investigated. Synaptosomes that had been prelabeled with [3H]choline or [3H]DA were depolarized by high K+. Oxotremorine and AF102B decreased the K+-evoked [3H]ACh release from hippocampal synaptosomes and increased the K+-evoked [3H]DA release from striatal synaptosomes. The dose-response curves showed that AF102B was far less potent than oxotremorine at the hippocampal presynaptic muscarinic receptors (autoreceptors). On the other hand, AF102B was more potent than oxotremorine at the muscarinic receptors on the striatal dopaminergic terminals (heteroreceptors). Pirenzepine, a selective M1 antagonist, counteracted the effects of oxotremorine on [3H]DA release more potently than it did the effects of oxotremorine on [3H]ACh release. Our results suggest that AF102B and pirenzepine discriminate pharmacologically between muscarinic autoreceptors and heteroreceptors.

Alpha 2-adrenoceptor blocking properties of idazoxan stereoisomers: stereoselectivity for presynaptic alpha 2-adrenoceptors

The alpha 2-adrenoceptor blocking properties of idazoxan enantiomers were evaluated at pre- and postsynaptic level. The antagonism of the two idazoxan stereoisomers was assessed, at presynaptic level, by their ability to antagonize clonidine at the alpha 2-adrenoceptors regulating noradrenaline release. The antagonist (+)-idazoxan showed an affinity towards the alpha 2-autoreceptors 40 times higher than that showed by (-)-idazoxan. Binding studies revealed (+)-idazoxan to be 7-8 times more potent than (-)-idazoxan in inhibiting the p-[3H]aminoclonidine binding. These results indicate a different affinity of alpha 2-adrenoceptors for the two idazoxan stereoisomers, thus suggesting that the alpha 2-adrenoceptors located pre- and postsynaptically may be of two stereochemically different subtypes.

Studies on the mechanism by which tryptophan efflux from isolated synaptosomes is stimulated by depolarization

1. The efflux and influx of tryptophan across the synaptosomal plasma membrane has been studied under a variety of experimental conditions, in order to examine the mechanism by which depolarization enhances the efflux of tryptophan from superfused synaptosomes. 2. Efflux of [3H]-tryptophan from preloaded superfused synaptosomes was found to be enhanced by K+ depolarization in a Ca2+ and dose-dependent manner. In contrast, [3H]-phenylalanine efflux was only poorly stimulated by depolarization and only by very high concentrations of K+. 3. Tryptophan efflux was also enhanced by decreasing the extracellular Na+ concentration, but this effect was not dependent on extracellular Ca2+. 4. Influx of [3H]-tryptophan into synaptosomes was stimulated by extracellular Na+ removal, but the uptake of [3H]-phenylalanine was unaffected by this procedure. 5. Both the induced influx and efflux of tryptophan observed under these experimental conditions was inhibited by immobilizing the plasma membrane carrier with parachlorophenylalanine. This implied that both the enhanced influx and efflux arose as a consequence of the activation of the membrane tryptophan carrier, the direction of the observed effect being dependent upon the manner in which the experiments were conducted. 6. The relationship between depolarization, the activation of the membrane tryptophan carrier and the significance of this to the in vivo situation is discussed.

Changes in the content of glutamate and GABA in the cerebellar vermis and hemispheres of the Purkinje cell degeneration (pcd) mutant

The contents of glutamate and GABA, as well as aspartate, glycine, and alanine, were examined in the cerebellar vermis and hemispheres of normal and Purkinje cell degeneration (pcd) mutant mice at 6, 9, and 12 months of age. Relative to normal values, the content of glutamate was approximately 50% lower in the vermis for the 3 age groups. In the hemispheres, the content of glutamate was also lower than control values and showed a progressive loss from 30 to 47% with age. On the other hand, in the case of GABA in the vermis, the level was 39% lower in the pcd mutant at 6 months of age but no different from control values at 12 months. However, relative to data for normal mice, the content of GABA in the hemispheres was consistently lower (20%) for all age groups. The level of aspartate was approximately 60% lower in the cerebellar vermis and 45 to 55% lower in the hemispheres of the mutant with respect to control data for all three age groups. Likewise, alanine showed a reduced content in the hemispheres (36-46%) and vermis (24%) in the mutant relative to normal values at 6, 9, and 12 months of age. On the other hand, the level of glycine was 43-64% higher in the vermis and 77-100% greater in the hemispheres of the mutant than in the control group. The higher values for glycine were observed at the two oldest ages. In conclusions, the data are consistent with the idea that glutamate and GABA are present in high concentrations in granule and Purkinje cells, respectively, and provide additional support for a transmitter function for both amino acids in the cerebellum.

Digitonin-permeabilized cells are exocytosis competent

Release of norepinephrine from PC12 cells can be stimulated by free Ca2+ in micromolar concentrations after permeabilization with 10 micrograms/ml of digitonin. This release is time and temperature dependent, half-maximal at 0.3 microM Ca2+, and, after washing out of endogenous ATP, half-maximal at about 0.5 mM MgATP when exogenously added. Similar results were obtained with bovine adrenal chromaffin cells using the same protocol. Support for the idea that the mechanism of release from both permeabilized cell types is still exocytosis is demonstrated at the electron microscopic level by immunolabeling chromaffin granule membrane antigens that were introduced into the plasma membrane following stimulation. Electron micrographs furthermore demonstrate that chromaffin granules retain typical dense cores after permeabilization, indicating that leakiness of catecholamines from the granules was not a major factor. Pores, formed by digitonin in the plasma membranes, were utilized to introduce antibodies into such exocytosis-competent cells. Anti-actin and anti-chromaffin granule membrane antibodies show a staining pattern similar to conventionally fixed and stained preparations. Our results demonstrate that pores formed by digitonin do not impair the process of exocytosis although they are big enough to allow macromolecules to pass in both directions. The digitonin-permeabilized cell is therefore an ideal in vitro system with which to study the fusion process between chromaffin granules and the plasma membrane.

Coexistence of carriers for dopamine and GABA uptake on a same nerve terminal in the rat brain

The ability of gamma-aminobutyric acid (GABA) to affect the release of [3H]-dopamine in rat brain synaptosomes prepared from corpus striatum, frontal cortex and hypothalamus and prelabelled with the radioactive catecholamine in the presence of desipramine was examined. GABA (10-300 microM) increased in a concentration-dependent way the basal release of [3H]-dopamine from striatum and cortical synaptosomes; however, its effect was much less pronounced in hypothalamic nerve terminals. 2,4-Diaminobutyric acid (DABA) mimicked GABA although less potently. Neutral amino acids such as leucine, valine or alpha-aminoisobutyric acid (100-300 microM) did not affect or increased minimally the release of [3H]-dopamine. The GABA-induced [3H]-dopamine release was not prevented by the GABAA-receptor antagonists, bicuculline or picrotoxin. The GABAA-receptor agonist, muscimol (10-300 microM), displayed only a very weak, not significant, enhancing effect on [3H]-dopamine release. The GABAB-receptor agonist (-)-baclofen (100 or 300 microM) had no effect. Three novel and selective inhibitors of GABA uptake, N-(4,4-diphenyl-3-butenyl)-nipecotic acid (SK&F 89976A), N-(4,4-diphenyl-3-butenyl)-guvacine (SK&F 100330A) and N-(4,4-diphenyl-3-butenyl)-homo-beta-proline (SK&F 100561) potently counteracted the enhancing effect of GABA on [3H]-dopamine release. Nipecotic acid also reduced the effect of GABA. It is concluded that carriers for the uptake of dopamine and GABA may coexist on the same nerve terminal in the rat brain.

Carriers for GABA and noradrenaline uptake coexist on the same nerve terminal in rat hippocampus

gamma-Aminobutyric acid (GABA; 1-300 microM) increased the basal release of [3H]noradrenaline ([3H]NA) from rat hippocampal synaptosomes. The effect of GABA at low concentrations (below 10 microM) was largely bicuculline-sensitive while the sensitivity to bicuculline decreased at higher concentrations. Muscimol mimicked GABA but only below 10 microM; bicuculline antagonized the effect of muscimol. Up to 300 microM (-)baclofen did not modify [3H]NA release. The effect of GABA was potently counteracted by SK & F 89976A, SK & F 100330A and SK & F 100561, three novel inhibitors of neuronal GABA uptake. These compounds could not entirely prevent the effect of GABA, being least effective at the lowest GABA concentrations (below 10 microM) and becoming progressively more effective when the concentrations of GABA were increased. The effect of muscimol was insensitive to SK & F 89976A. The effect of 100 microM GABA was totally prevented when bicuculline and uptake inhibitor were added together to the superfusion medium. The results suggest that the basal release of [3H]NA can be enhanced by GABA through two mechanisms: GABAA receptor activation and penetration into NA terminals by a GABA uptake process. Thus a carrier for the uptake of NA and a carrier for the uptake of GABA appear to coexist on the same nerve terminal in rat hippocampus.

Detergents inhibit exocytosis in PC 12 cells: evidence for an effect on ion fluxes

Membrane events in exocytosis were studied by examining the effect of different detergents on the K+-stimulated release of noradrenaline in the secretory cell line PC 12. The nonionic detergent Triton X-100 and the cationic detergent cetyltrimethylammonium bromide (CTAB) inhibit the noradrenaline release evoked by 55 mM K+ by 50% at very low concentrations (30 microM and 10 microM, respectively). These values are tenfold lower than the critical micellar concentrations (CMC). No such effect was seen with the anionic detergent sodium dodecyl sulphate (NaDodSO4). The inhibitory effect of 30 microM Triton X-100 is reversible, and the recovery from inhibition correlates with the loss of detergent from the cells as demonstrated by binding studies using [3H]Triton X-100. The possible relationship between this inhibition of secretion and the structural properties of the detergent was investigated. The inhibition in the presence of purified Triton X-100 subfractions turned out to be a function of the length of the oligometric ethyleneglycol chain (C6 to C26). The maximal effect was observed for Triton X-100 molecules having a chain length of 16 carbon atoms, which can penetrate just half of the lipid bilayer of the membrane. Additionally, the phase transition at 13-14 degrees C observed in an Arrhenius plot of noradrenaline release in stimulated cells was abolished. In the presence of 30 microM Triton X-100, 22Na+ uptake, 86Rb+ release, and 45Ca2+ uptake were reduced by 50-60%. These data suggest that the site of action of Triton X-100 is at the level of altering the movement of ions in PC 12 cells during the stimulatory phase of secretion.

A carrier for GABA uptake exists on noradrenaline nerve endings in selective rat brain areas but not on serotonin terminals

gamma-Aminobutyric acid (GABA) increased in a concentration-dependent way (3-300 microM) the basal release of tritium from rat cerebral cortex and hippocampus synaptosomes, prelabelled with 3H-noradrenaline (3H-NA); however, GABA was ineffective on hypothalamic nerve endings. The effect displayed by low concentrations (less than 10 microM) of GABA was largely bicuculline-sensitive. Muscimol mimicked GABA, while (-)baclofen was inactive. The releasing effects produced by concentrations of GABA higher than 10 microM were largely prevented by SK&F89976A, SK&F100330A and SK&F100561, three novel GABA uptake inhibitors. When present together, GABA uptake blocker and bicuculline counteracted entirely the GABA effects. The basal release of 3H-5-hydroxytryptamine (3H-5-HT) in synaptosomes from various CNS regions was not affected by GABA.

IN CONCLUSION

GABA can enhance 3H-NA release not only through GABA-A receptors but also by penetrating into NA terminals through a GABA uptake system. This implies coexistence of carriers for NA and GABA uptake on a same nerve terminal. The carrier coexistence occurs in selective CNS areas. The phenomenon appears to be transmitter-selective.

Release-regulating GABAA receptors are present on noradrenergic nerve terminals in selective areas of the rat brain

The effects of gamma-aminobutyric acid (GABA) on the spontaneous release of [3H]-norepinephrine ([3H]-NE) were investigated by means of superfused synaptosomes prepared from different areas of the rat brain and prelabeled with [3H]-NE. GABA increased in a concentration-dependent way (1-300 microM) the release of [3H]-NE in hippocampal synaptosomes. The effect of GABA was mimicked in part by muscimol. Similar effects were observed in cerebral cortex synaptosomes where GABA and muscimol were however less potent than in hippocampus. No effect could be observed in hypothalamic synaptosomes. Bicuculline antagonized the effect of muscimol and that of low concentrations of GABA (below 10 microM). Above 10 microM, the [3H]-NE releasing effects of GABA became progressively less sensitive to bicuculline. (-)-Baclofen did not affect the spontaneous release of [3H]-NE. It is concluded that release-regulating receptors of the GABAA subtype are present on NE nerve terminals in selective areas of the rat brain.

Simple method for the simultaneous determination of acetylcholine, choline, noradrenaline, dopamine and serotonin in brain tissue by high-performance liquid chromatography with electrochemical detection

A simple method for the simultaneous determination of acetylcholine, choline, noradrenaline, dopamine and serotonin in brain tissue was developed by using high-performance liquid chromatography with electrochemical detection. These compounds are analysed in a single chromatographic run within 30 min with a simple sample clean-up procedure. The detection system consists of two electrochemical detector cells aligned in series: a glassy-carbon electrode for catecholamines and serotonin, and a platinum electrode for acetylcholine and choline. For the detection of the latter compounds, they were converted enzymatically into hydrogen peroxide through a column reactor with immobilized acetylcholinesterase and choline oxidase. A column of boronic acid gel was placed just ahead of the immobilized enzyme column to remove catecholamines, which caused interfering responses on the platinum electrode. Two equivalent analytical columns and a column switching were employed to speed up the serotonin assay. Simultaneous determination of these major neurotransmitters in rat brain regions was successfully carried out with the system described.

Neuropeptide Y enhances the inhibitory effects of clonidine on 3H-noradrenaline release in synaptosomes isolated from the medulla oblongata of the male rat

In studies on superfused synaptosomes from the rat medulla oblongata, the inhibitory effects of the alpha 2-adrenergic agonist clonidine (0.1 microM) on potassium (15 mM K+) induced 3H-noradrenaline (NA) release was potentiated by 20%, when neuropeptide Y was added to the system. The effect of NPY was detectable at low concentrations (1 nM) and was not dose-dependent. Neuropeptide Y alone produced no significant effects on 3H-NA release. The results may indicate the existence of a presynaptic NPY receptor on the noradrenaline and/or adrenaline nerve terminals, which may enhance the presynaptic alpha 2-adrenoreceptor function to inhibit 3H-NA release.

Release of endogenous dopamine, 3,4-dihydroxyphenylacetic acid, and amino acid transmitters from rat striatal slices

The release of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) was measured in superfused striatal slices of the rat and the results compared with data obtained for the release of endogenous (a) DA and DOPAC in the cerebral cortex, nucleus accumbens and thalamus; (b) 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), GABA, and glutamate in the striatum; and (c) GABA, glutamate and 5-HT in the cerebral cortex. In superfused slices of all four CNS regions, there appeared to be a Ca2+-dependent, K+-stimulated release of endogenous DA. In addition, in slices of the striatum and nucleus accumbens there also appeared to be a Ca2+ -dependent, 60 mM K+ stimulated release of endogenous DOPAC. In the striatum, 16 mM Mg2+ was as effective as 2.5 mM Ca2+ in promoting the 60 mM K+-stimulated release of DOPAC. In addition, 16 mM Mg2+ appeared to function as a weak Ca2+ agonist since it also promoted the release of DA to approximately 40% of the level attained with Ca2+ in the presence of 60 mM K+. On the other hand, in the striatum, 16 mM Mg2+ inhibited the Ca2+-dependent, 60 mM K+-stimulated release of GABA and glutamate. Similar Mg2+-inhibition was observed in the cerebral cortex not only for GABA and glutamate but also for DA and 5-HT. With the use of alpha-methyl rho-tyrosine (tyrosine hydroxylase inhibitor), cocaine (uptake inhibitor) and pargyline (monoamine oxidase inhibitor), it was determined that most of the released DA and DOPAC was synthesized in the slices during the superfusion; DOPAC was not formed from DA which had been released and taken up; and DA and DOPAC were released from DA nerve terminals. In addition, the data indicate a difference in the release process between the amino acids and the monoamines from striatal slices since Mg2+ inhibited the Ca2+-dependent, K+-stimulated release of GABA and glutamate and appeared to promote the release of DA and 5-HT.

Rat brain alpha 2-pre- and postsynaptic receptors are different or differently modulated?

In order to get better characterization of alpha 2-pre- and postsynaptic noradrenergic receptors in the rat brain, we investigated the alpha 2-receptor changes which take place during a 12-day treatment with the alpha 2-antagonists yohimbine (4 mg/kg) and mianserin (10 mg/kg). These treatments caused a significant increase in the sensitivity of hypothalamic synaptosomes to the inhibitory action of the noradrenergic agonist clonidine on the 3H-noradrenaline release elicited by K depolarization. Frontal cortex alpha 2-autoreceptors were not affected by drug treatments. However, the 3H-p-aminoclonidine (3H-PAC) binding to membranes from hypothalamus or frontal cortex from treated animals was the same as in controls. Changes in neural firing, elicited by the alpha 2-antagonists on noradrenergic neurons, could explain our results. The presynaptic autoreceptors may thus become hypersensitive to counteract the enhanced neurotransmitter release in the hypothalamus, where the noradrenaline is accumulated at the synaptic cleft. In the frontal cortex, where it seems that only 5% of the noradrenergic terminals make synaptic contacts with postsynaptic elements, the alpha 2-autoreceptors are less sensitive to an enhanced neurotransmitter release. Alternatively, they have scarce functional importance because the noradrenaline release is effectively modulated by the inhibitory recurrent locus coeruleus collaterals. At the postsynaptic level, the receptor down-regulation might be prevented by chronic presence of the antagonist drug. Thus the different behavior between pre- and postysynaptic alpha 2-receptors and between alpha 2-receptors of different brain areas may be ascribed to a different modulation rather than to different molecular arrangements.

Identification of an acidic dipeptide, beta-aspartylglycine, in the CNS of Aplysia

A novel dipeptide, beta-aspartylglycine (beta-DG), has been isolated from tissues of the marine gastropod mollusc Aplysia californica. This compound was detected only in Aplysia and not in other molluscs, such as Helix or Mercenaria, or in lobster or frog. Among the Aplysia tissues, the highest levels of beta-DG were in nervous tissue and in the reproductive tract. beta-DG was assayed by HPLC as the o-phthaldialdehyde derivative and found to be present in all individual, identified neurons at a concentration of approximately 40 pmol/microgram protein. The peptide was identified as beta-DG by gas chromatography-mass spectrometry (GCMS) using trimethylsilyl derivatives prepared before and after acid hydrolysis. It was further characterized as the beta-isomer by TLC, including Rf, atypical blue-gray color with ninhydrin, and a violet color with Cu2+-ninhydrin. A fractionation scheme is described whereby acid-soluble tissue constituents can be divided into acidic, neutral, and basic components using mini ion-exchange columns. This partial purification prior to TLC analysis was necessary to remove compounds that interfered with the isolation of beta-DG.

Long-term treatment with clonidine and alpha-receptors in the brain of normotensive rats

The aim of the present study was to investigate whether or not changes in rat brain alpha-adrenoceptors take place during chronic treatment with a low dose of clonidine. Male Wistar normotensive rats were treated with clonidine (0.1 mg/kg)i.p. twice daily for 12 days. This treatment caused a significant increase in [3H]clonidine and in [3H]WB4101 binding, respectively, to alpha 2- and to alpha 1-adrenoceptors of the frontal cortex; the levels were 30% for [3H]clonidine and 20% for [3H]WB4101. The Scatchard analysis of data obtained in binding studies indicated that the enhanced binding of two ligands to membranes prepared from chronically clonidine-treated animals, was due to an apparent increase in the number of binding sites. These changes were seen 4 h after administration of the last treatment, before the appearance of the withdrawal syndrome. However, noradrenergic alpha 2-autoreceptors of synaptosomes, from the frontal cortex and hypothalamus of treated animals, were sensitive to the regulatory action of clonidine or of noradrenaline on the [3H]noradrenaline overflow elicited by high K+ as well as on the control animals. On the contrary, the alpha 2-receptors on the serotoninergic nerve terminals from the frontal cortex of treated animals were more sensitive than those of control animals to the action of clonidine or of noradrenaline in counteracting the [3H]5-hydroxytryptamine overflow elicited by high K+. These results suggest that during treatment with clonidine no autoreceptor hyposensitivity to the regulatory action of clonidine or noradrenaline on [3H]noradrenaline overflow elicited by high K+ takes place, but, as a consequence of the diminished noradrenaline availability at the synaptic cleft, the binding of [3H]WBA101 to alpha 1-receptors and of [3H]clonidine to pre- and postsynaptic alpha 2-receptors were significantly elevated in the frontal cortex, a brain areas where the alpha-2-receptors are mainly postsynaptic. Thus, the neurotransmitter concentration in the synaptic cleft may be responsible for the trans-synaptic modulation of the alpha 2-adrenoceptor postsynaptic population. In fact, the alpha 2-adrenoceptors which are presynaptically located on the serotoninergic terminals, but are postsynaptic in relation to the noradrenergic neurons, also show increased sensitivity after chronic clonidine treatment.

Noradrenaline uptake inhibitors do not reduce the presynaptic action of clonidine on 3H-noradrenaline release in superfused synaptosomes

The interaction between clonidine, as an agonist at the alpha 2-autoreceptors regulating noradrenaline release, and inhibitors of noradrenaline neuronal uptake was investigated in superfused synaptosomes, i.e. in conditions preventing accumulation of the released transmitter in the vicinity of presynaptic autoreceptors. Desipramine or cocaine did not release the inhibitory action of clonidine on the release of 3H-noradrenaline evoked by 15 mM KCl from rat cortex synaptosomes, even when the concentration ratio between uptake inhibitor and clonidine was very high. The present results do not support the hypothesis of an interaction between imidazolines and noradrenaline uptake inhibitors at the level of alpha 2-autoreceptors, or that of a functional coupling between presynaptic alpha 2-autoreceptors and noradrenaline uptake mechanism.