Effects of tetrahydropapaveroline and salsolinol on cerebral monoamine metabolism and their interactions with psychopharmacological drugs

Salsolinol: an Unintelligible and Double-Faced Molecule-Lessons Learned from In Vivo and In Vitro Experiments

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a tetrahydroisoquinoline derivative whose presence in humans was first detected in the urine of Parkinsonian patients on l-DOPA (l-dihydroxyphenylalanine) medication. Thus far, multiple hypotheses regarding its physiological/pathophysiological roles have been proposed, especially related to Parkinson’s disease or alcohol addiction. The aim of this review was to outline studies related to salsolinol, with special focus on in vivo and in vitro experimental models. To begin with, the chemical structure of salsolinol together with its biochemical implications and the role in neurotransmission are discussed. Numerous experimental studies are summarized in tables and the most relevant ones are stressed. Finally, the ability of salsolinol to cross the blood–brain barrier and its possible double-faced neurobiological potential are reviewed.

Biochemical basis of alcoholism: statements and hypotheses of present research

Experimental results and theoretical considerations on the biology of alcoholism are devoted to the following topics: genetically determined differences in metabolic tolerance; participation of the alternative alcohol metabolizing systems in chronic alcohol intake; genetically determined differences in functional tolerance of the CNS to the hypnotic effect of alcohol; cross tolerance between alcohol and centrally active drugs; dissociation of tolerance and cross tolerance from physical dependence; permanent effect of uncontrolled drinking behavior induced by alkaloid metabolites in the CNS; genetically determined alterations in the function of opiate receptors; and genetic predisposition to addiction due to innate endorphin deficiency. For the purpose of introducing the most important research teams and their main work, statements from selected publications of individual groups have been classified as to subject matter and summarized. Although the number for summary-quotations had to be restricted, the criterion for selection was the relevance to the etiology of alcoholism rather than consequences of alcohol drinking.

Effects of catecholamine-related mammalian alkaloids on spontaneous and vasopressin-induced behavior in mice

Heterocyclic catechol derivatives (tetrahydroisoquinoline alkaloids) are known to be formed endogenously via condensation of the catecholamines with carbonyl compounds. In this study, the effects of a variety of representative simple isoquinolines, benzyl isoquinolines, bicyclic isoquinoline-derived alkaloids (pavines and isopavines), an aporphine and berberine were investigated after intracerebroventricular injection in mice. Most (thirteen) of the alkaloids studied were found to induce significant alterations in three behavioral parameters (immobility, grooming and scratching behavior). In addition, the stereotypic scratching behavior elicited by central injection of arginine-vasopressin (AVP) was significantly antagonized by only one of these, 6-O-methyl-tetrahydropapaveroline (6-O-methyl-THP). To a lesser extent, (-)THP and the pavine, (+/-)bisnorargemonine, were also effective. That this rather specific effect did not involve opioid receptors was indicated by the failure of naloxone to reverse the antagonism by 6-O-methyl-THP.

Effects of ethanol and salsolinol on catecholamine function in LS and SS mice

Long Sleep (LS) and Short Sleep (SS) mice differ in duration of ethanol-induced sleep time because of differences in brain sensitivity to the depressant effects of alcohols. These lines of mice also differ in their sensitivity to salsolinol, the condensation product of acetaldehyde with dopamine. Some of ethanol's acute effects may be due to salsolinol interactions with catecholamine systems. In the present study, the half-lives of salsolinol were found to be 12.8 min (LS) and 12.3 min (SS). Salsolinol administration resulted in a decrease in brain norepinephrine content in LS but not SS mice. Dopamine levels were not altered by salsolinol. Ethanol or salsolinol, in vitro, inhibited dopamine uptake by striatal synaptosomes. The IC50 values for ethanol were 491 mM (LS) and 514 mM (SS), and for salsolinol, 300 microM (SS). Thus, the mouse line which is most sensitive to the behavioral effects of salsolinol is also most sensitive to salsolinol's effects on norepinephrine levels and inhibition of dopamine uptake. However, much higher concentrations are required to alter dopamine uptake in vitro than are required to alter behavior in vivo.

Neuronal mechanisms regulating ethanol effects on the dopaminergic system

Chronic ethanol consumption induces an increase in striatal 3H-Spiroperidol and 3H(-)Sulpiride specific binding by enhancing the affinity between the different dopaminergic recognition sites and the labelled ligands. Dopamine (DA) receptor supersensitivity is also suggested by the enhanced effect of neuroleptics in inducing hypomotility in rats treated with ethanol. The results, obtained by means of the administration of neuroleptics in comparison to ethanol treated rats, indicate a lack of cross tolerance between ethanol and other drugs acting on the dopaminergic recognition sites. These data suggest that ethanol effects on the dopaminergic system are mediated by events involving other neurotransmitter systems.

Ethanol metabolism and striatal dopamine turnover

In recent reports it has been indicated that acute and chronic ethanol treatments affect the central dopaminergic system. In particular, after acute ethanol administration it has been detected an increase of dopamine (DA) turnover measured as dihydroxyphenylacetic acid (DOPAC) content in rat corpus striatum. In order to verify the correlation between these neuronal events and the metabolism of ethanol, we measured striatal DA activity after different experimental manipulations of liver function. Ethanol metabolic rate has been stimulated by administering phenobarbital sodium, while liver ethanol metabolism was decreased with a subtotal hepatectomy. In these conditions we found a shift to the left of the time curve for DOPAC levels and a significant reduction of the peak of DOPAC increase respectively. In this paper we report that acetaldehyde induces modifications of the striatal DOPAC content, which become significant after a shorter latency period in comparison with the acute ethanol injection. Our data suggest the hypothesis that the neurochemical effects of ethanol may be mediated by the formation of specific metabolic products.

The route and significance of endogenous synthesis of alkaloids in animals

There is now substantial evidence that several TIQs and beta-carbolines are present in vivo and increase during certain pathological conditions. It still remains to be determined, however, precisely what roles they play in endogenous functions and whether or not they are critical for the expression of these pathological conditions. Accumulating biochemical information continues to support the notion that these compounds can act as false transmitters. The exciting new findings, which will certainly receive a great deal more attention, concern the interaction of some of the beta-carbolines with the benzodiazepine receptor. Determining if a beta-carboline is an endogenous receptor ligand will attract further research interest on the theoretical and specifically clinically-directed levels. Biochemical, morphological, and behavioral data indicate that some of the condensation products can act as neurotoxins. Very few experiments have included an examination of long-term effects of exposure to one of these alkaloids, so the amount of information on this issue is limited. Chronic rather than acute administration of an alkaloid is more likely to mimic the pathological states in which these compounds are hypothesized to play a role. Biochemically, both the dopaminergic and serotonergic systems have been shown to be affected by chronic treatments with certain alkaloids. Progressive and long-term behavioral alterations also have been reported. Such changes may reflect an adaptation to an increase or decrease in activity of particular systems or a neurotoxic action.

A critical evaluation of tetrahydroisoquinoline induced ethanol preference in rats

It has been reported that certain tetrahydroisoquinoline compounds, especially salsolinol and tetrahydropaveroline (THP) when infused into the lateral ventricle of rats' brains results in increased preference for alcohol solutions. The effect is reported to be long-term, in that animals do not return to baseline drinking even months later. The current report provides a replicatin of the original experiments and also an extension of the work to complete dose-response curves for salsolinol and THP. Generally we have confirmed that rats of the Sprague-Dawley and Long-Evans strains do increase their alcohol intake in response to infused THP or salsolinol and that the effect is long lasting, up to 10 months. Such animals consume less alcohol at concentrations above 20% than below, in contrast to the previous reports where drinking was maintained at high concentrations of alcohol. While the animals will select alcohol in the face of a saccharin choice, they will not drink alcohol adulterated with quinine. We have failed to observe signs of dependence or withdrawal by these techniques and suggest that the original reports of these signs may have been a result of cellular damage caused by the long-term infusions. Additionally we have carried out extensive dose-response experiments with both salsolinol and THP. Doses of THP of 104 nmoles/day were inhibitory to alcohol drinking. We conclude that these compounds do shift these animals preference for alcohol relatively permanently, but not to the point of gross intoxication nor into the highly aversive range of alcohol concentration. We cannot confirm the reports that salsolinol or THP produce withdrawal symptoms when infused.