The 5-HT2 antagonist ritanserin blocks dopamine re-uptake in the rat frontal cortex.

The effect of ritanserin on dopamine (DA) re-uptake and efflux was studied in rat frontal cortex synaptosomes. When compared to other 5HT2 receptor antagonists such as ketanserin and risperidone or DA D2 receptor antagonists such as haloperidol and raclopride, the effect of ritanserin proved to be more potent. Ritanserin blocked the DA transporter with a Ki of 0.18 +/- 0.06 microM, similar to cocaine (0.11 +/- 0.005 microM), while ketanserin had a Ki of 0.93 +/- 0.045; haloperidol of 2.07 +/- 0.12; risperidone of 18.01 +/- 0.62 and raclopride of 24.01 +/- 1.55. In addition, 15 min from its local application to the synaptosomes, ritanserin potently released [3]H-DA leaving only 29.6 +/- 1.6% of DA content, while ketanserin effect was equal to 46.5 +/- 0.9%; haloperidol to 70.4 +/- 2.2% and risperidone to 73.9 +/- 1.5%, all tested at the dose of 10 microM. Cocaine had no effect on DA efflux. These results suggest that ritanserin has a intrinsic dopaminergic effect which may help to explain its reported improvement on mood, cognition and negative symptoms of schizophrenia.

Lanthanides stimulate [3H]tyramine binding in the rat striatum.

Low (up to 100 nM) and high (approximately 100 microM) concentrations of lanthanides and Ca2+-ions, respectively, stimulated [3H]tyramine binding ([3H]TY) to rat striatal membranes, a putative marker for the vesicular transporter of dopamine. On the other hand, lanthanides (approximately 100 microM) inhibited or stimulated TY binding in striatal and extrastriatal (cortex, cerebellum) tissues, respectively. The binding increases by lanthanum (La3+) appeared to depend on endogenous Ca2+, whereas, those induced in EDTA-pretreated membranes were Ca2+-independent. The La3+-induced, apparent increase in the Bmax for [3H]TY binding seemed to reflect a retarded rate of dissociation of the ligand from its targets, rather than a larger availability of functionally-relevant, vesicular transport-related TY sites. This indicates uncertain mechanisms of present La3+ effects.

New benzamide-derived 5-HT3 receptor antagonists which prevent the effects of ethanol on extracellular dopamine, and fail to reduce voluntary alcohol intake in rats.

A set of substituted benzamides, characterized by the presence of a bulky quinolizidine moiety, were subjected to binding assays for 5-HT3 and D2 receptors on membranes obtained from the bovine area postrema ([3H]-GR65630) and the rat striatum ([3H]-spiperone) respectively. These benzamides resulted unsuitable for the recognition of D2 receptors, while a few of them, devoid of 5-HT4 receptor activity, had consistent affinity for central 5-HT3 receptors, inhibiting also potently the ethanol-induced dopamine efflux from the mesolimbic dopamine terminal region. However they failed in attenuating voluntary alcohol consumption in rats, as observed with several other chemically unrelated 5-HT3 antagonists. Thus the 5-HT3-mediated inhibition of alcohol-induced striatal release of dopamine by substituted benzamides is not a requisite for affecting ethanol intake.

Disulfiram and diethyldithiocarbamate intoxication affects the storage and release of striatal dopamine.

Acute intoxication and chronic therapy with the alcohol consumption deterrent dithiocarbamate disulfiram have been associated with several neurological complications perhaps involving the impairment of neurotransmitter pathways. In this study we have tested the hypothesis that dopaminergic malfunction is a critical component in disulfiram-evoked neurotoxicity. Disulfiram antagonized the in vitro striatal binding of [3H]tyramine, a putative marker of the vesicular transporter for dopamine, and the uptake of [3H]dopamine into striatal synaptic vesicles, with inhibitory constants (Ki) in the range of reported blood dithiocarbamate levels in treated alcoholics. Furthermore, disulfiram provoked a loss of radioactivity from [3H]dopamine-preloaded striatal vesicles, when added directly to the incubation mixture. Several metal-containing fungicide analogs were also potent displacers of specifically bound [3H]tyramine. Diethyldithiocarbamate (DDC), the major metabolite of disulfiram, had none of these effects. The intraperitoneal injection of a high dose of disulfiram and DDC into rats, mimicking acute intoxication, induced in vivo overflow of striatal dopamine from both a reserpine-sensitive (vesicular) and an alpha-methyl-p-tyrosine-sensitive (cytoplasmic) pool. The vesicular component of in vivo dopamine release resulted mainly from a direct activity of disulfiram, on the organelles (interaction with the carrier for dopamine plus membrane permeabilization) and indirectly through the mediation of serotonergic 5-HT3 receptors. DDC acted poorly at the vesicle membrane, and the in vivo releasing effect of dopamine was only partially prevented by the inhibition of 5-HT3 receptors, thus suggesting the role of additional mechanisms. It is concluded that disulfiram intoxication may acutely disrupt dopamine balance, an effect probably underlying some of the central neurotoxic, extrapyramidal symptoms associated with dithiocarbamate overdose.

Differential interaction of 1-methyl-4-phenylpyridinium ion with the putatively vesicular binding site of [3H]tyramine in dopaminergic and nondopaminergic brain regions.

The neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+) in the brain striatum has recently been shown to bind at a putatively vesicular site labeled by [3H]tyramine ([3H]TY). Whereas in the rat and mouse striatum MPP+ antagonized TY binding competitively, in the cerebellum there was a mixed-type antagonism, which suggests the simultaneous occupancy of two different sites. Ki values from displacement curves revealed a fourfold difference in the affinity of MPP+ for TY sites in the two brain regions. The degeneration of central noradrenergic terminals induced by an intraperitoneal injection of the toxin N-(2-chloroethyl)-N-ethyl-2- bromobenzylamine in rats decreased by 80% the maximal number of cerebellar TY binding sites, while not affecting striatal binding. Furthermore, guanethidine, a marker for noradrenaline (NA) vesicles, potently inhibited TY binding in NA-innervated regions, such as the cerebellum and the parietal cortex, and poorly in the striatum. It is concluded (a) that both MPP+ and TY may also label NA vesicles and (b) that the vesicular carriers for dopamine and NA have different characteristics, which may underlie a regional specificity in the rate of endovesicular sequestration of MPP+, with either neurodegenerative or neuroprotective consequences, depending on the brain area involved.