Phasic versus tonic dopamine release and the modulation of dopamine system responsivity: a hypothesis for the etiology of schizophrenia

The current studies further investigated the effects, in animal models of anxiety, of novel putative anxiolytic and anxiogenic compounds believed to induce their effects by actions at the GABA-benzodiazepine receptor complex. It was expected that the results would also provide further validation for a novel test of anxiety based on the ratio of open to closed arm entries in an elevated plus maze in the rat. The novel putative anxiolytics CL 218,872 (10–20 mg/kg) and tracazolate (5 mg/kg) significantly elevated the percentage of time spent on the open arms of an elevated plus-maze, consistent with their anxiolytic activity in several other animal tests. Also consistent with results from other animal tests, no anxiolytic activity was observed for the phenylquinoline PK 8165 (10–25 mg/kg), the 3,4-benzodiazepine tofisopam (25–50 mg/kg), or buspirone (0.5–20 mg/kg). The benzodiazepine receptor inverse agonists FG 7142 (1–5 mg/kg) and CGS 8216 (3–10 mg/kg) had anxiogenic activity in this test, as did the atypical benzodiazepine Ro 5-4864 (1–5 mg/kg). Interestingly, however, the benzodiazepine receptor antagonists Ro 15-1788 (10–20 mg/kg) and ZK 93426 (5–10 mg/kg) had no anxiogenic activity in this test.

Anxiolytic and anxiogenic drug effects on exploratory activity in an elevated plus-maze: a novel test of anxiety in the rat

Regulation of noradrenaline release by presynaptic receptor systems.

The neurobiology and control of anxious states

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity.

DOPA and 5-HTP accumulated in vivo in rat brain after decarboxylase inhibition with NSD 1015 (3-hydroxybenzylhydrazine). This accumulation was linear for the first 30 min and occurred in several brain regions over a wide range of NSD 1015 doses. After a peripheral decarboxylase inhibitor much less, if any, DOPA or 5-HTP accumulated in the brain. The accumulation of DOPA was prevented by H 44/68 (methylester of α-methyl para-tyrosine), a tyrosine hydroxylase inhibitor. DOPA, which accumulated before H 44/68 was given, appeared stable for at least 20 min. There were no significant changes in the levels of NA, DA, 5-HT or tryptophan shortly after NSD 1015 administration, but a rise in tyrosine was noted. Increased brain tyrosine after l-tyrosine administration did not alter the DOPA accumulation, however. These data as well as the distribution of the accumulated amino acids suggest that the accumulation of DOPA and 5-HTP after decarboxylase inhibition occurs intraneuronally, that the decarboxylase enzyme is completely inhibited, and that the accumulated products are not appreciably metabolized or transported from the region studied. Amine synthesis rates and rate constants were calculated from the data and compare well with similar values determined by other methods. Thus this accumulation appears to be a reliable measure of the in vivo hydroxylation of tyrosine and tryptophan.

Simultaneous measurement of tyrosine and tryptophan hydroxylase activities in brain in vivo using an inhibitor of the aromatic amino acid decarboxylase.

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

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

The method is an extension of the chromatographic separation procedure for 5-HIAA and 5-HTP published by Lindqvist (1971). Brain tissue is extracted with perchloric acid. After adjustment of the pH to 2 the extract is passed through a Dowex 50W, X-4 column. 5-HIAA is eluted with 60% aqueous methanol, DOPA and tyrosine with 0.1 M sodium citrate buffer pH 4.5 and 5-HTP and tryptophan with 0.1 M sodium phosphate buffer pH 6.5. If the above procedure is combined with that published by Atack and Magnusson (1970) 9 different compounds can be estimated after a single column procedure. DOPA is assayed fluorimetrically using a modification of the trihydroxy-indole (THI) assay for noradrenaline (Bertler et al., 1958). The level of DOPA found in a normal rat brain was below the sensitivity of the method (10 ng/g). After inhibition of the aromatic amino acid decarboxylase a compound indistinguishable from authentic l-DOPA accumulated in the rat brain.

A method for the determination of 3,4-dihydroxyphenylalanine (DOPA) in brain.

We describe here biochemical and pharmacological effects of the β-carboline ZK 93426, a new and potent benzodiazepine (BZ) receptor antagonist. ZK 93426 was compared with Ro 15-1788 and CGS 8216, two compounds previously described as BZ receptor antagonists. Certain effects of ZK 93426, Ro 15-1788 and CGS 8216 were quite similar (e.g., 3H-FNM displacement, “GABA ratio”, “photo-shift”). In most pharmacological tests ZK 93426 and Ro 15-1788 lacked overt effects; Ro 15-1788 was a weak agonist in some paradigms, while ZK 93426 exhibited a potent proconflict effect but also a weak anticonvulsant effect. This interesting finding with ZK 93426 suggests that BZ receptor ligands may possess differential efficacy at BZ receptor subtypes. In contrast, CGS 8216 exhibited potent proconvulsant effects in several paradigms in addition to proconflict and pentylenetetrazol generalizing effects. ZK 93426, Ro 15-1788 and CGS 8216 were almost equally potent as antagonists of the effects of BZ receptor agonists, such as diazepam and lorazepam. However, ZK 93426 was the most potent inhibitor of the convulsions produced by the BZ receptor inverse agonist DMCM.

Wolfgang Kehr

Papers

Simultaneous measurement of tyrosine and tryptophan hydroxylase activities in brain in vivo using an inhibitor of the aromatic amino acid decarboxylase.

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

A method for the determination of 3,4-dihydroxyphenylalanine (DOPA) in brain.

Evaluation of the β-carboline ZK 93 426 as a benzodiazepine receptor antagonist

Effect of lisuride and other ergot derivatives on monoaminergic mechanisms in rat brain.