Effect of Tetanus Toxin on Transmitter Release from the Substantia Nigra and Striatum In Vitro

Abstract: The effect of tetanus toxin on the uptake and release of radiolabelled transmitters from slices prepared from substantia nigra (SN) and striatum of rats has been investigated. Tetanus toxin-500–750 mouse lethal doses (MLD)-injected into the SN 6 h before preparing the slices significantly reduced the calcium-dependent, potassium-evoked release of [³H]GABA. Endogenous GABA levels in the SN and [³H]GABA uptake by nigral slices were unaffected by pretreatment with the toxin. Injections of tetanus toxin (1000–2000 MLD) into the striatum significantly reduced the calcium-dependent, potassium-evoked release of [¹⁴C]GABA and also [³H]dopamine, but had no effect on the K⁺-evoked release of [³H]5-hydroxytryptamine or [¹⁴C]acetylcholine. It is concluded that tetanus toxin inhibits GABA release directly and not by interference with synthesis or inactivation processes.     

Induction of c-fos mRNA Expression in Rat Striatum by Neuroleptic Drugs

The effect of selective dopamine D2 receptor-acting drugs on striatal c-fos mRNA expression in the rat has been investigated by Northern hybridization and autoradiography to determine a possible role for c-fos in the initiation of adaptive changes in D2 receptor number by neuroleptic drugs. The neuroleptic drug haloperidol, a D2 receptor antagonist, was found to produce a rapid and transient induction of c-fos mRNA expression as compared with the expression in animals treated with saline. This induction by haloperidol was found to be dose dependent and D2 receptor mediated, inasmuch as a D2 agonist completely reversed the induction and the inactive isomer of the neuroleptic butaclamol, which does not produce an increase in D2 receptors, had no effect on c-fos mRNA expression. From these data, it can be concluded that c-fos expression in striatum is under dopamine D2 receptor-mediated inhibitory control. It is suggested that c-fos may play a role in the initiation of the increase in D2 receptor number produced by chronic neuroleptic drug treatment.     

Ghrelin amplifies the nicotine-induced dopamine release in the rat striatum

The orexigenic peptide ghrelin plays a prominent role in the regulation of energy balance and in the mediation of reward mechanisms and reinforcement for addictive drugs, such as nicotine. Nicotine is the principal psychoactive component in tobacco, which is responsible for addiction and relapse of smokers. Nicotine activates the mesencephalic dopaminergic neurons via nicotinic acetylcholine receptors (nAchR). Ghrelin stimulates the dopaminergic neurons via growth hormone secretagogue receptors (GHS-R1A) in the ventral tegmental area and the substantia nigra pars compacta resulting in the release of dopamine in the ventral and dorsal striatum, respectively. In the present study an in vitro superfusion of rat striatal slices was performed, in order to investigate the direct action of ghrelin on the striatal dopamine release and the interaction of ghrelin with nicotine through this neurotransmitter release. Ghrelin increased significantly the dopamine release from the rat striatum following electrical stimulation. This stimulatory effect was reversed by both the selective nAchR antagonist mecamylamine and the selective GHS-R1A antagonist GHRP-6. Nicotine also increased significantly the dopamine release under the same conditions. This stimulatory effect was antagonized by mecamylamine, but not by GHRP-6. Ghrelin further stimulated the nicotine-induced dopamine release and this effect was abolished by mecamylamine and was partially inhibited by GHRP-6. The present results demonstrate that ghrelin stimulates directly the dopamine release and amplifies the nicotine-induced dopamine release in the rat striatum. We presume that striatal cholinergic interneurons also express GHS-R1A, through which ghrelin can amplify the nicotine-induced dopamine release in the striatum. This study provides further evidence of the impact of ghrelin on the mesolimbic and nigrostriatal dopaminergic pathways. It also suggests that ghrelin signaling may serve as a novel pharmacological target for treatment of addictive and neurodegenerative disorders.     

扬子鳄纹状体AChE、SOMmRNA阳性神经元的分布

目的 观察扬子鳄纹状体内乙酰胆碱酯酶（acetylcholinesterase,AChE）和生长抑素信使核糖核酸（somatostatin messenger ribonucleic acid,SOMmRNA）阳性神经元的形态和分布.方法 采用亚铁氰化酮法和原位杂交法观察扬子鳄纹状体内AChE和SOMmRNA阳性神经元的分布和特征.结果 扬子鳄纹状体内有AChE和SOMmRNA阳性神经元分布,两种神经元均有大、中、小型细胞,以中、小型细胞为主,神经元胞体呈圆形、椭圆形、三角形、梭形和多角形.结论 扬子鳄纹状体内有AChE和SOMmRNA阳性神经元分布.     

Effects of Methylphenidate Analogues on Phenethylamine Substrates for the Striatal Dopamine Transporter

Methylphenidate (MPD) was found to inhibit competitively the striatal dopamine transporter (DAT) and bind at sites on the DAT in common with both cocaine (a non-substrate site ligand) and amphetamine (a substrate site ligand). Some methylphenidate analogues modified on the aromatic ring and/or at the nitrogen were tested to determine whether the profile of inhibition could be altered. None was found to stimulate the release of dopamine in the time frame (< or = 60 s) of the experiments conducted, and each of the analogues tested was found to noncompetitively inhibit the transport of dopamine. It was found that halogenating the aromatic ring with chlorine (threo-3,4-dichloromethylphenidate hydrochloride; compound 1) increased the affinity of MPD to inhibit the transport of dopamine. A derivative of MPD with simultaneous, single methyl group substitutions on the phenyl ring and at the nitrogen (threo-N-methyl-4-methylphenidate hydrochloride; compound 2) bound at a site in common with MPD. A benzyl group positioned at the nitrogen (threo-N-benzylmethylphenidate hydrochloride; compound 3) imparted properties to the inhibitor in which binding at substrate and non-substrate sites could be distinguished. This analogue bound at a mutually interacting site with that of methylphenidate and had a K(int) value of 4.29 microM. Furthermore, the N-substituted analogues (compounds 2 and 3), although clearly inhibitors of dopamine transport, were found to attenuate dramatically the inhibition of dopamine transport by amphetamine, suggesting that the development of an antagonist for substrate analogue drugs of abuse may be possible.     

NPY-mRNA expressions in the nucleus accumbens,caudate putamen and cerebral cortex of apomorphine-susceptible and apomorphine-unsusceptible rats

Using the apomorphine-induced stereotyped gnawing response as a selection criterion, two distinct groups of rats can be distinguished, apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) rats. These two lines differ in several components of both striatal and extrastriatal areas. This study deals with the expression of neuropeptide Y (NPY)mRNA-expressing neurons in the nucleus accumbens, caudate putamen and cerebral cortex of both rat lines, using non-radioactive in situ hybridisation. The morphology of the neurons in the three regions is similar, viz. oblong, rectangular or triangular, with two or three processes. The neurons are homogeneously distributed in all regions, and in the nucleus accumbens they are particularly numerous ventrally to the anterior commissure. Using automated image analysis, the mean numerical density of NPYmRNA-positive neurons per brain region and the mean NPYmRNA expression level per neuron per brain region were determined. No differences appear in the numerical densities of NPYmRNA-containing neurons in the nucleus accumbens, caudate putamen and cortex between APO-SUS and APO-UNSUS rats. However, distinct differences between the rat lines are present in the level of NPYmRNA expression per neuron in the nucleus accumbens and in the caudate putamen, showing that NPY contributes to the differential neurochemical make-up of these rat lines that is responsible for their obvious differences in behaviour, physiology and immune competence.     

Various synaptic activities and firing patterns in cortico-striatal and striatal neurons in vivo

It is commonly assumed that spontaneous activity of striatal output neurons is characterized by a two-state behavior. This assumption is mainly based on in vivo intracellular recordings under urethane and/or ketamine-xylazine anesthesia showing that striatal neurons oscillate between two preferred membrane potentials, a Down state (hyperpolarized level), resulting from an inwardly rectifying potassium conductance, and an Up state (depolarized level) caused by complex interactions between a barrage of cortical synaptic excitation and voltage-dependent potassium conductances. However, a recent comparative study using different anesthetics showed that striatal neurons can exhibit various shapes of synaptic activity depending on the temporal structure and the degree of synchronization of their cortico-striatal afferents. These new data demonstrate that the "classical" Up and Down states do not provide the unique spontaneous activity that can be encountered in striatal neurons in vivo. Rather we propose that striatal neurons should exhibit various synaptic activities and firing patterns depending on the states of vigilance. This hypothesis would be validated in further experiments in which the intracellular activity of striatal neurons will be recorded during the natural sleep-wake cycle.     

Comparative Analysis of Superoxide Dismutase Activity between Acute Pharmacological Models and a Transgenic Mouse Model of Huntington's Disease

We examined the activity of striatal superoxide dismutase (SOD) in two acute pharmacological models of Huntington's disease (HD), and compared it with SOD activity in the striata of mice transgenic for the HD mutation. Total SOD, and Cu/ZnSOD activities increased in young transgenic mice, but decreased in older (35 week) mice. We consider that the increased enzyme activity represents a compensatory mechanism to protect cells from free radical-induced damage, but the system becomes insufficient in older animals. Major decreases in SOD activity were also observed both after quinolinic acid and 3-nitropropionic acid intrastriatal injections. The present results indicate that in both types of HD models striatal oxidative damage occurs, and that it is associated with alterations in the cellular antioxidant system.     

Methamphetamine rapidly decreases vesicular dopamine uptake

Vesicular sequestration is important in the regulation of cytoplasmic concentrations of monoamines such as dopamine. Moreover, recent evidence suggests that increases in cytoplasmic dopamine levels, perhaps attributable to changes in vesicular monoamine transporter function, contribute to methamphetamine-induced dopaminergic deficits. Hence, we examined whether striatal vesicular uptake is altered following methamphetamine treatment. Multiple administrations of methamphetamine rapidly (within 1 h) decreased vesicular dopamine uptake and dihydrotetrabenazine binding, an effect that (a) persisted at least 24 h, (b) was associated with dopamine and not serotonin neurons, and (c) was unrelated to residual drug introduced by the original methamphetamine treatment. These data suggest that methamphetamine rapidly decreases vesicular monoamine transporter function in dopaminergic neurons, a phenomenon that may be associated with the long-term damage caused by this stimulant.     

Region-specific enhancement of basal extracellular and cocaine-evoked dopamine levels following constitutive deletion of the Serotonin(1B) receptor

The behavioral effects of cocaine are enhanced following constitutive deletion of the serotonin(1B) receptor. The neural substrates mediating the enhanced response to cocaine are unknown. The present studies determined whether basal dopamine dynamics or cocaine-evoked dopamine levels are altered in projection areas of mesostriatal or mesoaccumbens dopamine neurons following serotonin(1B) receptor deletion. Male wild-type and serotonin(1B) knockout mice were implanted with microdialysis guide cannulas aimed at the dorsal striatum or nucleus accumbens. The zero net flux method of quantitative microdialysis was used to quantify basal extracellular dopamine concentrations (DA(ext)) and the extraction fraction of dopamine (E(d)), which provides an index of dopamine uptake. Conventional microdialysis techniques were used to quantify cocaine (0, 5.0, and 20.0 mg/kg)-evoked dopamine overflow. Basal DA(ext) and E(d) did not differ in striatum of wild-type and knockout mice. Similarly, cocaine-stimulated dopamine overflow did not differ between genotype. The basal E(d) did not differ in the nucleus accumbens of wild-type and knockout mice. However, DA(ext) was significantly elevated in the nucleus accumbens of knockout mice. Cocaine-evoked dopamine overflow (nM) was also enhanced in the nucleus accumbens of knockout mice. However, the cocaine-induced increase in dopamine levels, relative to basal values, did not differ between genotype. These data demonstrate that deletion of the serotonin(1B) receptor is associated with increases in basal DA(ext) in the nucleus accumbens. This increase is not associated with an alteration in E(d), suggesting increased basal dopamine release in these animals. It is hypothesized that these alterations in presynaptic neuronal activity are a compensatory response to constitutive deletion of the serotonin(1B) receptor and may contribute to the enhanced behavioral effects of psychostimulants observed in knockout mice.