Chronic D1 and D2 dopaminomimetic treatment of MPTP-denervated monkeys: effects on basal ganglia GABA(A)/benzodiazepine receptor complex and GABA content.

The effect of various chronic dopaminergic treatments in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys on the brain gamma-aminobutyric acid type A (GABA(A)) /benzodiazepine receptor complex and GABA content was investigated in order to assess the GABAergic involvement in dopaminomimetic-induced dyskinesia. Three MPTP monkeys received for one month pulsatile administrations of the D1 dopamine (DA) receptor agonist SKF 82958 whereas three others received the same dose of SKF 82958 by continuous infusion. A long acting D2 DA receptor agonist, cabergoline, was given to another three animals. Untreated MPTP as well as naive control animals were also included. Pulsatile SKF 82958 relieved parkinsonian symptoms but was also associated with dyskinesia in two of the three animals whereas animals treated continuously with SKF 82958 remained as untreated MPTP monkeys. Chronic cabergoline administration improved motor response with no persistent dyskinesia. MPTP treatment induced a decrease of 3H-flunitrazepam binding in the medial anterior part of caudate-putamen and an increase in the internal segment of globus pallidus (GPi) which was in general unchanged by pulsatile or continuous SKF 82958 administration. Throughout the striatum, binding of 3H-flunitrazepam remained reduced in MPTP monkeys treated with cabergoline but was not significantly lower than untreated MPTP monkeys. Moreover, cabergoline treatment reversed the MPTP-induced increase in 3H-flunitrazepam binding in the GPi. GABA concentrations remained unchanged in the striatum, external segment of globus pallidus and GPi following MPTP denervation. Pulsatile but not continuous SKF 82958 administration decreased putamen GABA content whereas cabergoline treatment decreased caudate GABA. No alteration in GABA levels were observed in the GPe and GPi following the experimental treatments. These results suggest that: (1) D2-like receptor stimulation with cabergoline modulates GABA(A) receptor density in striatal subregions anatomically related to associative cortical afferent and (2) the absence of dyskinesia in dopaminomimetic-treated monkeys might be associated with the reversal of the MPTP-induced upregulation of the GABA(A)/benzodiazepine receptor complex in the Gpi.     

Stimulation of delta opioid receptors located in substantia nigra reticulata but not globus pallidus or striatum restores motor activity in 6‐hydroxydopamine lesioned rats: new insights into the role of delta receptors in parkinsonism

The delta opioid peptide (DOP) receptor has been proposed as a target in the symptomatic therapy of Parkinson’s disease. However, the circuitry underlying the antiparkinsonian action of DOP receptor agonists and their site of action have never been adequately investigated. Systemic administration of the DOP receptor agonist (+)‐4‐[(αR)‐α‐(2S,5R)‐allyl‐2,5‐dimethyl‐1‐piperazinyl)‐3‐methoxy‐benzyl]‐N‐N‐diethylbenzamide (SNC‐80) attenuated akinesia/bradykinesia and improved motor activity in 6‐hydroxydopamine hemilesioned rats. Opposite effects were produced by the selective DOP receptor antagonist naltrindole (NTD), suggesting that endogenous enkephalins tonically sustain movement under parkinsonian conditions. Microdialysis revealed that SNC‐80 reduced GABA release in globus pallidus (GP) while NTD elevated it. Moreover, SNC‐80 reduced GABA and glutamate release in substantia nigra reticulata (SNr) whereas NTD reduced GABA without affecting glutamate release. The bar test coupled to microdialysis showed that perfusion with NTD in SNr but not GP or striatum prevented the antiakinetic effect of systemic SNC‐80 and its neurochemical correlates. Consistently, microinjections of SNC‐80 into SNr or bicuculline in GP attenuated parkinsonian‐like symptoms while SNC‐80 microinjections in GP or striatum were ineffective. This study demonstrates that nigral DOP receptors mediate antiparkinsonian actions of SNC‐80 and challenges the common view that DOP receptor agonists solely attenuate parkinsonism via pallidal mechanisms.     

Somatostatin and CCK-8 modulate release of striatal amino acids: role of dopamine receptors

The effects of somatostatin (SOM) and cholecystokinin octapeptide (CCK-8) on basal and potassium-evoked release of neurotransmitter amino acids were investigated in slices of rat caudate nucleus (CN) and, for comparison, cerebral cortex (CX). Endogenous aspartate (Asp), glutamate (Glu), glycine (Gly), and gamma-aminobutyric acid (GABA) were measured by high performance liquid chromatography. In both CN and CX, potassium (5-55 mM) produced a concentration-dependent increase in the release of Asp, Glu, Gly, and GABA in the presence of extracellular Ca2+. CCK-8 (1 microM) stimulated in CN the basal and K+-evoked release of Gly to 231% and 160% of control, respectively; this effect was blocked by sulpiride (SULP), a dopamine receptor antagonist. In contrast, SOM (1 microM) inhibited the K+-evoked release of Glu in CN by 26%, an effect that was not blocked by SULP. SOM and CCK-8 did not significantly affect the basal or K+ (35 mM)-evoked release of other amino acids in the CN or of any amino acids in CX. The results indicate that: CCK-8 facilitation of Gly release is dependent of Gly release is dependent on dopamine receptor activation, whereas the inhibition by SOM of Glu release is not: and the effects of SOM and CCK-8 are specific with respect to the brain region affected.     

Modulation of the neurotoxic effects of methamphetamine by the selective kappa-opioid receptor agonist U69593

Kappa-opioid receptor agonists prevent alterations in dopamine neurotransmission that occur in response to repeated cocaine administration. The present microdialysis study examined whether administration of the selective kappa-opioid receptor agonist U69593 with methamphetamine prevents alterations in dopamine levels produced by neurotoxic doses of methamphetamine. Swiss Webster mice were injected intraperitoneally with methamphetamine (10.0 mg/kg) or saline, four times in 1 day, at 2-h intervals. Prior to the first and third injection, they received U69593 (0.32 mg/kg s.c.) or vehicle. Microdialysis was conducted 3, 7, or 21 days later. Basal and K+-evoked (60 and 100 mM) dopamine overflow were reduced 3 days after methamphetamine administration. These effects were long-lasting in that they were still apparent 7 and 21 days after methamphetamine treatment. Intrastriatal (5.0 and 50 microM) or systemic (1.0-10.0 mg/kg) administration of methamphetamine increased dopamine concentrations in control animals. In mice preexposed to methamphetamine, methamphetamine-evoked dopamine overflow was reduced. In animals that had received methamphetamine with U69593, basal dopamine levels did not differ from those of vehicle-treated controls. U69593 treatment attenuated the decrease in K+-evoked dopamine produced by prior methamphetamine exposure. The reduction in methamphetamine-evoked dopamine levels was also attenuated. The administration of U69593 alone did not modify basal or stimulus-evoked dopamine levels. These data demonstrate that repeated methamphetamine administration reduces presynaptic dopamine neuronal function in mouse striatum and that co-administration of a selective kappa-opioid receptor agonist with methamphetamine attenuates these effects. U69593 treatment did not modify the hyperthermic effects of methamphetamine, indicating that this kappa-opioid receptor agonist selectively attenuates methamphetamine-induced alterations in dopamine neurotransmission.     

Modulation of γ-Aminobutyric Acid Release in Cerebral Cortex by Fluoride, Phorbol Ester, and Phosphodiesterase Inhibitors: Differential Sensitivity of Acetylcholine Release to Fluoride and K+ Channel Blockers

In this study we have used fluoride as a tool to investigate the involvement of G protein-coupled effector systems in the regulation of the depolarization-induced release of gamma-aminobutyric acid (GABA) from rat cerebral cortex. To distinguish among the activating effects of NaF on G proteins linked to different effectors, such as adenylate cyclase, polyphosphoinositide phospholipase C, and K+ channels, agents specific to these effectors have been used in parallel. NaF induced a marked dose-dependent facilitation of the K(+)-evoked release of [14C]GABA, with an EC50 of 1.26 mM, increasing release by 103% at 5 mM NaF. No effect on basal release was seen up to 3 mM NaF, and no modulation of [3H]acetylcholine (ACh) release was seen up to 5 mM NaF. Phorbol 12,13-diacetate (PDA) produced a similar dose-dependent facilitation of the K(+)-evoked release of [14C]GABA, potentiating the release of [14C]GABA by 50% at 10 microM PDA. The phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine (IBMX) and theophylline, inhibited the K(+)-evoked release of [14C]GABA, and IBMX reversed the NaF facilitation of GABA release in a dose-dependent manner (pA2 2.57). The K+ channel blocker (IA current) tetrahydroaminoacridine (THA), which markedly inhibits the K(+)-evoked release of [14C]GABA, also reversed the NaF facilitatory effect, but the release of [3H]ACh was less sensitive to the inhibitory effect of THA. On the other hand, the K+ channel blocker, tetraethylammonium, which has no effect on the release of [14C]GABA, caused a significant facilitation of K(+)-evoked release of [3H]ACh. From these studies, it is concluded that GABA release in cerebral cortex is subject to regulation by G protein-linked effector systems that are distinct from those affecting the release of [3H]ACh in cerebral cortex.     

Regional Differences in Striatal Dopamine Uptake and Release Associated with Recovery from MPTP-Induced Parkinsonism

Abstract : This study directly assessed striatal dopamine (DA) uptake rates and peak release in response to KCl in normal, symptomatic, and recovered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated cats using in vivo electrochemistry. DA uptake rates measured after direct application of known concentrations of DA to the striatum were slowed significantly in both dorsal and ventral striatum in symptomatic cats compared with rates recorded in normal animals. DA uptake rates remained significantly slowed in recovered cats and were not significantly different from the rates recorded in symptomatic animals. In symptomatic cats, both DA uptake rates and the signal recorded in response to KCl stimulation were significantly decreased from normal in all dorsal and ventral striatal regions sampled. Reduction/oxidation (redox) ratios recorded in response to KCl stimulation suggested DA to be the predominant electroactive species. In spontaneously recovered MPTP-treated cats, recordings in the ventral striatum subsequent to KCl stimulation again suggested DA to be the predominant electroactive species released, and peak levels were significantly higher than those recorded in symptomatic animals. In the dorsal striatum of recovered cats, redox ratios recorded subsequent to KCl stimulation suggested serotonin rather than DA to be the predominant electroactive species released. Peak levels of release in the dorsal striatum were not significantly greater than those recorded in symptomatic animals. These results suggest that in spontaneously recovered MPTP-treated cats, there is partial recovery of ventral striatal DAergic terminals, persistent loss of dorsal striatal DAergic terminals, and a down-regulation of DA transporter number/function throughout the striatum. These processes may contribute to volume transmission of DA in the striatum and promote functional recovery.     

In Vivo Release of Endogenous Amino Acids from the Rat Striatum: Further Evidence for a Role of Glutamate and Aspartate in Corticostriatal Neurotransmission

By means of the push-pull cannula method, the outflow of endogenous amino acids was studied in the striatum of halothane-anesthetized rats. Addition of K+ ions (30 mM for 4 min) to the superfusion fluid increased the release of aspartate (+116%), glutamate (+217%), taurine (+109%), and gamma-aminobutyric acid (GABA) (+429%) whereas a prolonged decrease in the outflow of glutamine (-28%) and a delayed reduction in the efflux of tyrosine (-25%) were observed. In the absence of Ca2+, the K+-induced release of aspartate, glutamate, and GABA was blocked whereas the K+-induced release of taurine was still present. Under these conditions, the decrease in glutamine efflux was reduced and that of tyrosine was abolished. Local application of tetrodotoxin (5 microM) decreased only the outflow of glutamate (-25%). One week following lesion of the ipsilateral sensorimotor cortex the spontaneous outflow of glutamine and of tyrosine was enhanced. Despite the lack of change in their spontaneous outflow, the K+-evoked release of aspartate and glutamate was less pronounced in lesioned than in control animals, whereas the K+-evoked changes in GABA and glutamine efflux were not modified. Our data indicate that the push-pull cannula method is a reliable approach for the study of the in vivo release of endogenous amino acids. In addition, they provide further evidence for a role for glutamate and aspartate as neurotransmitters of corticostriatal neurons.     

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

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

Activation of Protein Kinase C Suppresses the γ-Aminobutyric AcidB Receptor-Mediated Inhibition of the Vesicular Release of Noradrenaline and Acetylcholine

Modulation of the gamma-aminobutyric acidB (GABAB) receptor-mediated response by protein kinase C (PKC) was examined with regard to inhibition by stimulation of the GABAB receptor of stimulation-evoked release of noradrenaline (NA) from slices of cerebellar cortex and of acetylcholine (ACh) from strips of ileum. 12-O-Tetradecanoylphorbol 13-acetate (TPA) potentiated the high K(+)-evoked Ca2+-dependent release of NA and ACh, but not the ouabain-evoked release, even in the presence of external Ca2+. The potentiating effect was antagonized by sphingosine, thereby suggesting that PKC participates in the exocytotic-vesicular release of neurotransmitters, but does not do so in case of a nonvesicular release. GABA inhibited the high K(+)-evoked release of NA and ACh, but not the ouabain-evoked Ca(2+)-independent release. The effect of GABA was mimicked by baclofen and was antagonized by phaclofen, thereby suggesting that stimulation of the GABAB receptor inhibits the vesicular but not the nonvesicular release of neurotransmitters. TPA suppressed the GABAB receptor-mediated inhibition of high K(+)-evoked release of NA and ACh. The effect of TPA was antagonized by sphingosine. These results indicate that stimulation of the GABAB receptor inhibits the stimulation-evoked Ca(2+)-dependent release of neurotransmitters and that activation of PKC suppresses the GABAB receptor-mediated response.     

Differential effects of zinc on native GABA(A) receptor function in rat hippocampus and cerebellum.

Hippocampal noradrenergic and cerebellar glutamatergic granule cell axon terminals possess GABA(A) receptors mediating enhancement of noradrenaline and glutamate release, respectively. The hippocampal receptor is benzodiazepine-sensitive, whereas the cerebellar one is not affected by benzodiazepine agonists, indicating the presence of an alpha6 subunit. We tested here the effects of Zn2+ on these two native GABA(A) receptor subtypes using superfused rat hippocampal and cerebellar synaptosomes. In the cerebellum, zinc ions strongly inhibited (IC50 approximately 1 microM) the potentiation of the K(+)-evoked [3H]D-aspartate release induced by GABA. In contrast, the GABA-evoked release of [3H]noradrenaline from hippocampal synaptosomes was much less sensitive to Zn2+ (IC50 > 30 microM). The effects of Zn2+ were then studied in two rat lines selected for high (ANT) and low (AT) alcohol sensitivity because granule cell GABA(A) receptors in ANT, but not AT, rats respond to benzodiazepine agonists due to a critical mutation in the alpha6 subunit. GABA increased the K(+)-evoked release of [3H]DCNS REGIONS-aspartate from cerebellar synaptosomes of AT and ANT rats, an effect prevented by the GABAA selective antagonist bicuculline. In AT rat cerebellum, the effect of GABA was strongly inhibited by Zn2+ (IC50 < or = 1 microM), whereas in ANT rats, the divalent cation was about 100-fold less potent. Thus, native benzodiazepine-sensitive GABAA receptors appear largely insensitive to functional inhibition by Zn2+ and vice versa. Changes in sensitivity to Zn2+ inhibition consequent to mutations in cerebellar granule cell GABA(A) receptor subunits may lead to changes in glutamate release from parallel fibers onto Purkinje cells and may play important roles in cerebellar dysfunctions.     

Modulation of Extracellular γ-Aminobutyric Acid in the Ventral Pallidum Using In Vivo Microdialysis

Intracranial microdialysis was used to investigate the origin of extracellular gamma-aminobutyric acid (GABA) in the ventral pallidum. Changes in basal GABA levels in response to membrane depolarizers, ion-channel blockers, and receptor agonists were determined. Antagonism of Ca2+ fluxes with high Mg2+ in a Ca(2+)-free perfusion buffer decreased GABA levels by up to 30%. Inhibition of voltage-dependent Na+ channels by the addition of tetrodotoxin also significantly decreased basal extracellular GABA concentrations by up to 45%, and blockade of Ca2+ and Na+ channels with verapamil reduced extracellular GABA by as much as 30%. The addition of either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, produced a 40% reduction in extracellular GABA. GABA release was stimulated by high K+ and the addition of veratridine to increase Na+ influx. High K(+)-induced release was predominantly Ca(2+)-dependent, whereas the effect of veratridine was potentiated in the absence of extracellular Ca2+. Both high K(+)- and veratridine-induced elevations in extracellular GABA were inhibited by baclofen, whereas only veratridine-induced release was antagonized by muscimol. These results demonstrate that at least 50% of basal extracellular GABA in the ventral pallidum is derived from Ca(2+)- or Na(+)-dependent mechanisms. They also suggest that Na(+)-dependent release of GABA via reversal of the uptake carrier can be shown in vivo.     

Developmental Changes in the Calcium Dependency of γ-Aminobutyric Acid Release from Isolated Growth Cones: Correlation with Growth Cone Morphology

We have investigated the development of Ca2+-dependent gamma-[3H]aminobutyric acid [( 3H]GABA) release in superfused growth cone fractions isolated from rats between the postnatal ages of 1 and 11 days. We have compared this release with the overall morphology of the subcellular fractions, and identified those structures taking up [3H]GABA by electron microscopical autoradiography. In fractions isolated from rats between 1 and 5 days, K+-evoked [3H]GABA release was completely independent of extracellular Ca2+. After 5 days a Ca2+ dependency appeared, which increased with age, such that by 10 days approximately 50% of the K+-evoked release was Ca2+ dependent. Electron microscopical analysis showed that, at all ages, large numbers of GABAergic growth cones were present in the subcellular fractions. Up to postnatal day 5, the growth cones were synaptic vesicle sparse but, after this age, increasing numbers of synaptic vesicle-containing growth cones were seen. These results suggest that during maturation of GABAergic growth cones into synapses there is, initially, a mechanism for release that is independent of extracellular Ca2+ and that the appearance of a Ca2+-dependent [3H]GABA release from growth cones correlates with the appearance of synaptic vesicles.     

Effects of Antidepressant Drug Treatments on α2-Adrenoceptor Control of [3H]Noradrenaline Release from Hypothalamic Synaptosomes

KCl (16 mM) stimulated the release of [3H]noradrenaline ([3H]NA) from rat hypothalamic synaptosomes in a Ca2+-dependent manner; this release was attenuated by clonidine (0.01-100 microM). Changes in the release of [3H]NA and the functional status of alpha 2-adrenoceptors in the medial hypothalamus of rats treated acutely and chronically with clorgyline (1 mg/kg/day) or desipramine (DMI, 10 mg/kg/day) were assessed using superfused synaptosomes in which the attenuating effects of clonidine (1 microM) or the potentiating effects of yohimbine (1 microM) on K+-evoked release of [3H]NA were measured. After acute administration of DMI, significantly less [3H]NA was accumulated into synaptosomes. Although total (spontaneous + K+-evoked) [3H]NA release from these synaptosomes was unchanged, a significant reduction was apparent in the K+-evoked release from the DMI-treated tissue. Attenuation of K+-evoked release by clonidine was abolished in both these acute treatment groups. Following the chronic antidepressant drug regimens, [3H]NA uptake into DMI-treated tissue remained significantly reduced although total percent and K+-evoked [3H]NA release were unchanged. The K+-evoked release of [3H]NA in S1 was significantly enhanced (by 22%) in the clorgyline treatment group. Attenuation of K+-evoked [3H]NA release by clonidine in both chronic antidepressant-treated tissues was not significantly changed. It is concluded that the functional sensitivity of alpha 2-adrenoceptors on nerve endings in the medial hypothalamus is unchanged by these chronic antidepressant drug regimens. In synaptosomes from untreated tissue, yohimbine significantly potentiated K+-evoked release of [3H]NA; this effect was unchanged after acute regimens and reduced after chronic administration of both the antidepressants.(ABSTRACT TRUNCATED AT 250 WORDS)     

GABAA and GABAB receptors modulate the K(+)-evoked release of sensory CGRP from the guinea pig urinary bladder

Superfusion of mucosa-free muscle slices of guinea-pig urinary bladder with 40 mM K+ produced a remarkable increase in calcitonin gene-related peptide-like immunoreactivity (CGRP-LI), that in this organ is entirely contained in capsaicin-sensitive nerves. GABA (1 mM) did neither affect the basal nor the 40 mM K+ evoked CGRP-LI release. Baclofen (0.1 mM) or muscimol (1 mM) did not affect the basal CGRP-LI outflow. However, baclofen (0.1 mM) significantly reduced by 32% and muscimol (0.1-1 mM) significantly increased by 60% and 70%, respectively the K(+)-evoked CGRP-LI release. These findings add neurochemical evidence to the functional data suggesting the existence of GABAA and GABAB receptors which modulate the efferent function of capsaicin-sensitive afferents.     

GABA Release Modified by Adenosine Receptors in Mouse Hippocampal Slices under Normal and Ischemic Conditions

The excitatory glutamatergic neurons in the hippocampus are modulated by inhibitory GABA-releasing interneurons. The neuromodulator adenosine is known to inhibit the presynaptic release of neurotransmitters and to hyperpolarize postsynaptic neurons in the hippocampus, which would imply that it is an endogenous protective agent against cerebral ischemia and excitotoxic neuronal damage. Interactions of the GABAergic and adenosinergic systems in regulating neuronal excitability in the hippocampus is of crucial importance, particularly under cell-damaging conditions. We now characterized the effects of adenosine receptor agonists and antagonists on the release of preloaded [³H]GABA from hippocampal slices prepared from adult (3-month-old) mice, using a superfusion system. The effects were tested both under normal conditions and in ischemia induced by omitting glucose and oxygen from the superfusion medium. Basal and K⁺-evoked GABA release in the hippocampus were depressed by adenosinergic compounds. Under normal conditions activation of both adenosine A₁ and A_2A receptors by the agonists R(-)N⁶-(2-phenylisopropyl)adenosine and CGS 21680 inhibited the K⁺-evoked release, which effects were blocked by their specific antagonists, 8-cyclopentyl-1,3-dipropyl-xanthine and 3,7-dimethyl-1-propargylxanthine, respectively. Under ischemic conditions the release of both GABA and adenosine is markedly enhanced. The above receptor agonists then depressed both the basal and K⁺-evoked GABA release, only the action of A_2A receptors being however receptor-mediated. The demonstrated depression of GABA release by adenosine in the hippocampus could be deleterious to neurons and contribute to excitotoxicity.     

Modulation of GABA Release by Second Messenger Substances and NO in Mouse Brain Stem Slices Under Normal and Ischemic Conditions

GABA is the inhibitory neurotransmitter in most brain stem nuclei. The properties of release of preloaded [³H]GABA were now investigated with slices from the mouse brain stem under normal and ischemic (oxygen and glucose deprivation) conditions, using a superfusion system. The ischemic GABA release increased about fourfold in comparison with normal conditions. The tyrosine kinase inhibitor genistein had no effect on GABA release, while the phospholipase inhibitor quinacrine reduced both the basal and K⁺-evoked release in normoxia and ischemia. The activator of protein kinase C (PKC) 4β-phorbol 12-myristate 13-acetate had no effects on the releases, whereas the PKC inhibitor chelerythrine reduced the basal release in ischemia. When the cyclic guanosine monophosphate (cGMP) levels were increased by superfusion with zaprinast and other phosphodiesterase inhibitors, GABA release was reduced under normal conditions. The NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA) enhanced the basal and K⁺-stimulated release by acting directly on presynaptic terminals. Under ischemic conditions GABA release was enhanced when cGMP levels were increased by zaprinast. This effect was confirmed by inhibition of the release by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The NO-producing agents SNAP, HA, and sodium nitroprusside potentiated GABA release in ischemia. These effects were reduced by the NO synthase inhibitor N^G-nitro-l-arginine, but not by ODQ. The results show that particularly NO and cGMP regulate both normal and ischemic GABA release in the brain stem. Their effects are however complex.     

Evidence for histamine as a neurotransmitter in the cardiac sympathetic nervous system

The colocalization of histamine (HA) and norepinephrine (NE) immunoreactivities was identified within the superior cervical ganglia neurons of the guinea pig. HA and NE immunoreactivity levels were significantly attenuated after chemical sympathectomy with 6-hydroxydopamine (6-OHDA). Coexistence of NE and HA was also visualized in the cardiac sympathetic axon and varicosities labeled with anterograde tracer biotinylated dextran amine. Depolarization of cardiac sympathetic nerve endings (synaptosomes) with 50 mM potassium stimulated endogenous HA release, which was significantly attenuated by 6-OHDA or a vesicular monoamine transporter 2 (VMAT2) inhibitor reserpine pretreatments. Compound 48/80, a mast cell releaser, did not affect cardiac synaptosome HA exocytosis. Furthermore, K+ -evoked HA release was abolished by the N-type Ca2+ -channel blocker omega-conotoxin but was not affected by the L-type Ca2+ -channel blocker lacidipine. Cardiac synaptosome HA exocytosis was augmented by the enhanced synthesis of HA or the inhibition of HA metabolism. HA H3-receptor activation by (R)-alpha-methylhistamine inhibited high K+ -evoked histamine release. The HA H3 receptor antagonist thioperamide enhanced K+ -evoked HA release and blocked the (R)-alpha-methylhistamine effect. The K+ -evoked endogenous NE release was attenuated by preloading the cardiac synaptosomes with L-histidine or quinacrine. These inhibitory effects were reversed by thioperamide or antagonized by alpha-fluoromethylhistidine. Our findings indicate that high K+ -evoked corelease of NE and HA may be inhibited by endogenous HA via activation of presynaptic HA H3-receptors. The H3-receptor may function as an autoreceptor, rather than a heteroreceptor, in the regulation of sympathetic neurotransmission and HA may be a novel sympathetic neurotransmitter.     

Pure uptake blockers of dopamine can reduce prolactin secretion: studies with diclofensine

The effects of diclofensine, a pure dopamine (DA) uptake inhibitor on 1) 3H-DA uptake in rat arcuate-periventricular nucleus-median eminence synaptosomes, 2) basal and K+-evoked endogenous DA release from tuberoinfundibular dopaminergic (TIDA) neurons and 3) in vivo prolactin (PRL) secretion were studied. Diclofensine, in concentrations of 0.01, 0.1 and 1 microM caused a marked decrease of 3H-DA uptake. In addition, it was unable to stimulate basal endogenous DA release which, on the contrary, was elicited by d-amphetamine in the same concentration (50 microM). On the other hand, diclofensine (50 microM) caused a 3 fold enhancement of K+-evoked DA release. Finally, the compound, when administered in vivo to male rats, significantly reduced basal serum PRL levels. The results of the present study seem to indicate that the pharmacological blockade of DA uptake in TIDA neurons is a condition sufficient to cause a reduction of PRL release.     

Sedimentation and release properties of P2 fractions derived from rat cerebral cortex slices incubated with radiolabeled GABA for a short or long time period

On homogenization of rat cerebral cortex slices previously incubated with [³H] GABA or [¹⁴C]GABA for 5 or 30 min, respectively, particles were recovered in P₂ fractions which exhibited similar buoyant density, but different sedimentation velocity on linear sucrose density gradient centrifugation. The K⁺-evoked release of [³H]GABA from particles isolated from slices previously incubated for 5 min with [³H]GABA was increased in the presence of exogenous Ca²⁺. In contrast, the K⁺-evoked release from particles isolated from slices previously incubated for 30 min with [³H]GABA, was not influenced by the presence of exogenous Ca²⁺.These results suggest that, depending on the incubation time of slices, exogenously applied GABA can be detected in differnnt pools. These pools not only seem to differ in their Ca²⁺ dependency of K⁺-evoked release but also in their subcellular localization.     

A parkinsonian syndrome induced in the goldfish by the neurotoxin MPTP.

Parkinson's disease has been modeled in humans, lower primates, and to a lesser extent in some other vertebrates by administration of the potent neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine). The MPTP model has thus drawn considerable attention as a system to search for anti-Parkinson's disease drugs, although the cost and scarcity of primates has limited extensive applications. We now report that a parkinsonian syndrome can be elicited in the common goldfish (Carassius auratus) by a single dose of MPTP. The syndrome is characterized by profound bradykinesia (slow movement), the full extent of which is reached 3 days after MPTP administration. The reduction in movement is paralleled by loss of dopamine and norepinephrine from the forebrain and midbrain and in other brain regions as well. The toxic oxidative product of MPTP, MPP+, is also accumulated predominantly in forebrain and midbrain, and pretreatment with the monoamine oxidase blocker tranylcypromine substantially reduces accumulation of the toxic metabolite. A barely perceptible coarseness in balance adjustment also occurs in treated animals. The MPTP-treated goldfish recover normal movement and normal brain monoamine levels within 10-13 days after administration of the drug. We interpret these and other data to indicate that MPTP can induce a Parkinson's disease-like syndrome in the goldfish that is similar in many aspects to the syndrome induced by MPTP in humans and other primates. This remarkable parallel may permit the goldfish to supplement expensive and scarce primates for the purpose of searching and screening neuroprotective drugs with specific relevance to Parkinson's disease.