Tor Magnusson

Evidence for a receptor-mediated feedback control of striatal tyrosine hydroxylase activity

WE have previously reported that cutting the nigrostriatal dopamine-carrying axons unexpectedly results in a transient increase in the rate of tyrosine hydroxylation in the rat forebrain (Carlsson, Kehr & others, 1972). It was suggested that striatal tyrosine hydroxylase activity is controlled via dopamine receptors at the synaptic cleft : when the impulse flow is interrupted by axotomy, the concentration of dopamine in the synaptic cleft decreases, and the ensuing reduction of dopamine receptor activity gives rise to a feedback activation of tyrosine hydroxylase, located in the striatal dopaminecarrying nerve terminals. The experiments now reported were made to test the above hypothesis. We argued that stimulation and blockade of dopamine receptors should result in inhibition and activation, respectively, of striatal tyrosine hydroxylase activity. Male Sprague-Dawley rats, 210-340 g, were used. Axotomy of the nigrostriatal dopamine fibres was performed under ether anaesthesia on the left side by means of a transverse cerebral hemisection, as previously described (BCdard, Carlsson&Lindqvist, 1972). At the same time (or in some experiments after 1 h) the aromatic amino-acid decarboxylase was inhibited by an intraperitoneal injection of NSD 101 5 (3-hydroxybenzylhydrazine HCl, 100 mg/kg). The animals were decapitated 30 min after the injection. The forebrains were analysed for dopa (Kehr, Carlsson & Lindqvist, 1972) and dopamine (Atack, to be published). For a direct activation of dopamine receptors, apomorphine HCl, 15 mg/kgf was injected intraperitoneally 7 min before the transection, and for blockade of these receptors haloperidol was given 1 h before the transection ( 5 mg/kg intraperitoneally, for references see AndCn, Carlsson & Haggendal, 1969). In some experiments both agents were given to the same animals. The levels of dopa in the forebrains are given in Fig. 1. As previously reported, inhibition of the aromatic amino acid decarboxylase causes the accumulation of dopa, which cannot be detected in the normal brain. This accumulation appears to be a useful indicator of the rate of tyrosine hydroxylation (Carlsson, Davis & others, to be published). In animals transected and treated simultaneously with NSD 101 5, the

Effect of chronic transection on dopamine, noradrenaline and 5-hydroxytryptamine in the rat spinal cord

SummaryFor intervals up to 15 days after transection of the rat spinal cord the level of noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and tryptophan were studied above and below the lesions. In the upper part NA, DA and 5-HT increased continuously, while 5-HIAA increased during the first 3 to 5 days and then returned to its original level. In the lower part NA had disappeared almost completely after 15 days, DA after 9 days, 5-HT and 5-HIAA after 5 days. During the first day after transection 5-HT showed an increase in the lower part as did DA for the first 3 days. The different time course for DA and NA suggests that part of the spinal DA serves an independent non-precursor role.

(+)-AJ 76 and (+)-UH 232: Central stimulants acting as preferential dopamine autoreceptor antagonists

SummaryThe biochemical and behavioral effects of the putative dopamine autoreceptor antagonists cis-(+)-5-methoxy-1-methyl-2-(n-propylamino)tetralin, (+)-AJ 76 and cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin, (+)-UH 232, were evaluated in various in vivo models in rats. Both compounds produced a marked elevation in brain dopamine synthesis and turnover with only slight effects on the synthesis and turnover of serotonin (5-HT) and noradrenaline being noted. (+)-AJ 76 and (+)-UH 232 also failed to antagonize the decrease in cortical noradrenaline synthesis rate caused by the alpha2 agonist clonidine. The apomorphine-induced decrease in dopamine synthesis rate in gamma-butyrolactone (GBL) treated animals was completely blocked by (+)-AJ 76 and (+)-UH 232 but not by d-amphetamine or methylphenidate. In activity experiments using habituated animals, (+)-AJ 76 and (+)-UH 232 produced locomotor stimulation and weak stereotypies and antagonized the sedative effects of low doses of apomorphine. Locomotor hyperactivity induced by apomorphine or the dopamine agonist DiPr-5,6-ADTN was antagonized by (+)-UH 232 and to a lesser degree by (+)-AJ 76. The locomotor hyperactivity produced by (+)-AJ 76, (+)-UH 232 and methylphenidate was completely prevented by reserpine pretreatment and partially blocked by the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (alpha-MT), whereas d-amphetamine-induced hyperactivity was only antagonized by alpha-MT pretreatment. It is concluded that (+)-AJ 76 and (+)-UH 232 produce behavioral stimulation via a preferential antagonism on central dopamine autoreceptors, an action different from that of all known stimulants including apomorphine, d-amphetamine and methylphenidate. (+)-AJ 76 and (+)-UH 232 possess but weak antagonistic effects on postsynaptic dopamine receptors and only the latter compound is able to induce sedation in rats.

Increase in brain dopamine after axotomy or treatment with gammahydroxybutyric acid due to elimination of the nerve impulse flow

SummaryBoth a unilateral, frontal section of the brain at the level of the caudal hypothalamus (hemisection) and systemic treatment with gammahydroxybutyric acid (GHBA, sodium form, 1.5 g/kg i.p.) increased the dopamine (DA) in the rat forebrain by about 70% in 1 h. Both procedures also markedly decelerated the α-methyltyrosine-induced DA disappearance. The brain noradrenaline was significantly lowered after the hemisection, but was not influenced by the treatment with GHBA given either alone or in combination with α-methyltyrosine.Intrastriatal injections of 25% KCl did not change the normal DA content significantly but prevented the increase in DA observed after hemisection or treatment with GHBA, probably due to a depolarization of the DA nerve terminals. Such a treatment with KCl also rapidly released the DA accumulated after hemisection. These effects were not seen after 20% NaCl.The same increase in forebrain DA, as produced by hemisection or treatment with GHBA, was also seen after injections of 25% KCl into the substantia nigra or injections of tetrodotoxin into the neostriatum.To judge from the turning of rats, unilateral injections of 25% KCl into the neostriatum depolarized the cells in this area, whereas stimulation of the DA receptors hyperpolarized them.The increases in brain DA described may be due to an inhibition of the nerve impulse flow to the DA nerve terminals.

5-HYDROXYTRYPTAMINE RELEASE BY NERVE STIMULATION OF THE SPINAL CORD.

Abstract In mice and frogs the spinal cords were dissected out and suspended in Ringer solution. Electrical stimulation resulted in release and increased synthesis of 5-hydroxytryptamine.

Effects of drugs influencing monoamine mechanisms on the increase in brain dopamine produced by axotomy or treatment with gammahydroxybutyric acid

SummaryThe influence of blockade or stimulation of dopamine (DA) receptors on the selective increase in brain DA seen after axotomy or injection of gammahydroxybutyric acid (sodium form, 1.5 g/kg i.p.) was studied in rats. The increases were not changed after blockade of the DA receptors by haloperidol but were slightly reduced after stimulation of these receptors by apomorphine. Since pretreatment with haloperidol counteracted this effect of apomorphine, a diminished stimulation of DA receptors may partially be responsible for the increase in brain DA seen when the nerve impulse flow has been blocked in the DA neurones by axotomy or treatment with gammahydroxybutyric acid. The NA content was usually somewhat lowered on the lesioned side and this reduction was not changed after treatment with haloperidol, apomorphine or amphetamine.The increase in brain DA usually observed after axotomy was not found when the rats were also treated with reserpine and nialamide. This effect indicates that the negative feed-back of cytoplasmic DA on the DA synthesis operates also in the absence of nerve impulses.Injection of amphetamine before or after axotomy or treatment with gammahydroxybytyric acid markedly inhibited the increase in brain DA, probably due to release of newly synthesized DA.

Monoamine oxidase -A and -B activity in the rat brain after hemitransection

Abstract The activity of MAO-A and MAO-B in four different brain regions (striatum, limbic system, occipito-temporal cortex and hemispheres) was determined after hemitransection of the left side. There was no difference in the MAO-A activities of either the left or right sides of the brain in either control or hemitransected rats. The activity of MAO-B was the same for both sides in control rats, but there was an increased MAO-B activity in the left side of the hemitransected rats with respect to the right side in all brain regions investigated, with the possible exception of the limbic system. The increase was due to a change in the V max rather than to a changed K m of the MAO-B. The interaction of the MAO-B with oxygen was unchanged after hemitransection.

Neurotensin and its amide analogue [Gln4]-neurotensin: effects on brain monoamine turnover.

SummaryIntracerebroventricularly administered neurotensin and [Gln4]-neurotensin (50–200 μg) increased the formation of Dopa in different brain regions of rats after inhibition of the aromatic l-amino acid decarboxylase. For both neuropeptides these increases were dose dependent (20–150%). In the corpus striatum [Gln4]-neurotensin was twice as active as neurotensin and it tended to be more active also in other brain regions. The brain tyrosine concentrations were also increased. [Gln4]-neurotensin (100–200 μg) following inhibition of the aromatic l-amino acid decarboxylase, increased the accumulation of 5-hydroxytryptophan in all brain regions by 30–60%. In contrast, neurotensin was completely inactive. In both cases the brain tryptophan concentrations were increased. Both neurotensin and [Gln4]-neurotensin also accelerated the disappearance of dopamine, noradrenaline and 5-hydroxytryptamine after inhibition of monoamine synthesis. These results show an increased brain monoamine turnover induced by both neuropeptides.

Increase in rat brain glutathione following intracerebroventricular administration of γ-glutamylcysteine

The effects of intracerebroventricularly (i.c.v.) administered gamma-glutamylcysteine (gamma-GC) and glutathione (GSH) monoethyl ester, subcutaneously (s.c.) injected L-2-oxo-4-thiazolidinecarboxylic acid (OTC) and intraperitoneally (i.p.) administered cysteine on the concentration of GSH in rat brain were investigated. The brain content of GSH, cysteine and gamma-GC was determined by HPLC with electrochemical detection (gold/mercury electrode) using N-acetylcysteine as internal standard. A dose-dependent increase in the GSH concentration (145-170% of controls) was found in the substantia nigra (SN) and in the rest of the brain stem after injection of gamma-GC, whereas no significant alterations in GSH were observed in the striatum and in the cerebral cortex. High levels of gamma-GC could be detected in the brain tissue after the administration, and the concentration of cysteine did also increase markedly after gamma-GC injection in all brain regions assessed. I.c.v. administration of L-buthionine sulfoximine (L-BSO) reduced the brain concentration of GSH by 50-70% within 24 hr. Injection of gamma-GC 24 hr after L-BSO resulted in an increase in GSH up to control values within 1-3 hr in the SN and the rest of the brain stem, whereas only a slight increase in GSH was observed in the striatum and the cerebral cortex. The concentration of GSH in the striatum and SN did not change after i.p. injection of cysteine, but a slight increase in the GSH concentration in the limbic region was observed. GSH monoethyl ester (i.c.v.) and OTC (s.c.) did not produce any significant increase in the GSH concentration in the brain. When the GSH concentration had been reduced by administration of L-BSO (i.c.v.; 24 hr) subsequent injection of GSH monoethyl ester led to a slight increase in the striatal and limbic GSH levels. These data show that, of the drugs studied, gamma-GC was the most effective in increasing brain GSH. It could thus serve as a valuable tool in future studies regarding metabolism and function of GSH in the brain. The observed difference in the effects of gamma-GC in different brain regions indicate that the brain tissue is not homogeneous with regard to GSH synthesizing capacity.

Noradrenaline release by nerve stimulation of the spinal cord

Abstract In mice pretreated with the monoamine oxidase inhibitor nilamide the spinal cords were dissected out suspended in Ringer solution. Electrical stimulation resulted in release and increased synthesis of nonadrenaline.