The quantitative determination of R- and S-salsolinol in the striatum and adrenal gland of rats selectively bred for disparate alcohol drinking

Role of Metabolism on Alcohol Preference, Addiction, and Treatment

Studies presented in this chapter show that: (1) in the brain, ethanol is metabolized by catalase to acetaldehyde, which condenses with dopamine forming salsolinol; (2) acetaldehyde-derived salsolinol increases the release of dopamine mediating, via opioid receptors, the reinforcing effects of ethanol during the acquisition of ethanol consumption, while (3) brain acetaldehyde does not influence the maintenance of chronic ethanol intake, it is suggested that a learned cue-induced hyperglutamatergic system takes precedence over the dopaminergic system. However, (4) following a prolonged ethanol deprivation, the generation of acetaldehyde in the brain again plays a role, contributing to the increase in ethanol intake observed during ethanol re-access, called the alcohol deprivation effect (ADE), a model of relapse behavior; (5) naltrexone inhibits the high ethanol intake seen in the ADE condition, suggesting that acetaldehyde-derived salsolinol via opioid receptors also contributes to the relapse-like drinking behavior. The reader is referred to glutamate-mediated mechanisms that trigger the cue-associated alcohol-seeking and that also contribute to triggering relapse.

Quantitative Determination of Endogenous Tetrahydroisoquinolines, Potential Parkinson's Disease Biomarkers, in Mammals

Current diagnostic options for Parkinson's disease are very limited and primarily based on characteristic clinical symptoms. Thus, there are urgent needs for reliable biomarkers that enable us to diagnose the disease in the early stages, differentiate it from other atypical Parkinsonian syndromes, monitor its progression, increase knowledge of its pathogenesis, and improve the development of potent therapies. A promising group of potential biomarkers are endogenous tetrahydroisoquinoline metabolites, which are thought to contribute to the multifactorial etiology of Parkinson's disease. The aim of this critical review is to highlight trends and limitations of available traditional and modern analytical techniques for sample pretreatment (extraction and derivatization procedures) and quantitative determination of tetrahydroisoquinoline derivatives in various types of mammalian fluids and tissues (urine, plasma, cerebrospinal fluid, brain tissue, liver tissue). Particular attention is paid to the most sensitive and specific analytical techniques, involving immunochemistry and gas or liquid chromatography coupled with mass spectrometric, fluorescence, or electrochemical detection. The review also includes a discussion of other relevant agents proposed and tested in Parkinson's disease.

Mystic Acetaldehyde: The Never-Ending Story on Alcoholism

After decades of uncertainties and drawbacks, the study on the role and significance of acetaldehyde in the effects of ethanol seemed to have found its main paths. Accordingly, the effects of acetaldehyde, after its systemic or central administration and as obtained following ethanol metabolism, looked as they were extensively characterized. However, almost 5 years after this research appeared at its highest momentum, the investigations on this topic have been revitalized on at least three main directions: (1) the role and the behavioral significance of acetaldehyde in different phases of ethanol self-administration and in voluntary ethanol consumption; (2) the distinction, in the central effects of ethanol, between those arising from its non-metabolized fraction and those attributable to ethanol-derived acetaldehyde; and (3) the role of the acetaldehyde-dopamine condensation product, salsolinol. The present review article aims at presenting and discussing prospectively the most recent data accumulated following these three research pathways on this never-ending story in order to offer the most up-to-date synoptic critical view on such still unresolved and exciting topic.

Salsolinol, an endogenous compound triggers a two-phase opposing action in the central nervous system

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), an endogenous compound present in the brain, was suspected of participation in the etiopathogenesis of Parkinson’s disease, the most common serious movement disorder worldwide. In this study, we evaluated the effect of different (50, 100, and 500 µM) concentrations of salsolinol on markers of glutamate-induced apoptotic and neurotoxic cell damage, such as caspase-3 activity, lactate dehydrogenase (LDH) release, and the loss of mitochondrial membrane potential. Biochemical data were complemented with the cellular analysis, including Hoechst 33342 and calcein AM staining, to visualize apoptotic DNA-fragmentation and to assess cell survival, respectively. The assessment of all investigated parameters was performed in primary cultures of rat or mouse hippocampal and striatum cells. Our study showed that salsolinol had biphasic effects, namely, at lower concentrations (50 and 100 µM), it demonstrated a distinct neuroprotective activity, whereas in the highest one (500 µM) caused neurotoxic effect. Salsolinol in concentrations of 50 and 100 µM significantly antagonized the pro-apoptotic and neurotoxic effects caused by 1 mM glutamate. Salsolinol diminished the number of bright fragmented nuclei with condensed chromatin and increased cell survival in Hoechst 33342 and calcein AM staining in hippocampal cultures. Additionally, in the low 50 µM concentration, it produced a significant inhibition of glutamate-induced loss of membrane mitochondrial potential. Only the highest concentration of salsolinol (500 µM) enhanced the glutamate excitotoxicity. Ex vivo studies indicated that both acute and chronic administration of salsolinol did not affect the dopamine metabolism, its striatal concentration or α-synuclein and tyrosine hydroxylase protein level in the rat substantia nigra and striatum. Summarizing, the present studies exclude possibility that salsolinol under physiological conditions could be an endogenous factor involved in the neurogenerative processes; conversely, it can exert a protective action on nerve cells in the brain. These findings may have important implications for the development of the new strategies to treat or prevent neural degeneration.

Salsolinol modulation of dopamine neurons

Salsolinol, a tetrahydroisoquinoline present in the human and rat brains, is the condensation product of dopamine and acetaldehyde, the first metabolite of ethanol. Previous evidence obtained in vivo links salsolinol with the mesolimbic dopaminergic (DA) system: salsolinol is self-administered into the posterior of the ventral tegmental area (pVTA) of rats; intra-VTA administration of salsolinol induces a strong conditional place preference and increases dopamine release in the nucleus accumbens (NAc). However, the underlying neuronal mechanisms are unclear. Here we present an overview of some of the recent research on this topic. Electrophysiological studies reveal that DA neurons in the pVTA are a target of salsolinol. In acute brain slices from rats, salsolinol increases the excitability and accelerates the ongoing firing of dopamine neurons in the pVTA. Intriguingly, this action of salsolinol involves multiple pre- and post-synaptic mechanisms, including: (1) depolarizing dopamine neurons; (2) by activating μ opioid receptors on the GABAergic inputs to dopamine neurons – which decreases GABAergic activity – dopamine neurons are disinhibited; and (3) enhancing presynaptic glutamatergic transmission onto dopamine neurons via activation of dopamine type 1 receptors, probably situated on the glutamatergic terminals. These novel mechanisms may contribute to the rewarding/reinforcing properties of salsolinol observed in vivo.

Salsolinol stimulates dopamine neurons in slices of posterior ventral tegmental area indirectly by activating μ-opioid receptors

Previous studies in vivo have shown that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse. Although opioid receptors, especially the μ-opioid receptors (MORs), may be involved, the cellular mechanisms mediating the effects of salsolinol have not been fully explored. In the current study, we used whole-cell patch-clamp recordings to examine the effects of salsolinol on dopamine neurons of the ventral tegmental area (VTA) in acute brain slices from Sprague-Dawley rats. Salsolinol (0.01-1 μM) dose-dependently and reversibly increased the ongoing firing of dopamine neurons; this effect was blocked by naltrexone, an antagonist of MORs, and gabazine, an antagonist of GABA(A) receptors. We further showed that salsolinol reduced the frequency without altering the amplitude of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic currents in dopamine neurons. The salsolinol-induced reduction was blocked by both naltrexone and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, an agonist of MORs. Thus, salsolinol excites VTA-dopamine neurons indirectly by activating MORs, which inhibit GABA neurons in the VTA. This form of disinhibition seems to be a novel mechanism underlying the effects of salsolinol.

Revisiting the controversial role of salsolinol in the neurobiological effects of ethanol: old and new vistas

The possible involvement of salsolinol (Sal), an endogenous condensation product of ACD (the first metabolite of ethanol) and dopamine, in the neurochemical basis underlying ethanol action has been repeatedly suggested although it has not been unequivocally established, still being a controversial matter of debate. The main goal of this review is to evaluate the presumed contribution of Sal to ethanol effects summarizing the reported data since the discovery in the 1970s of Sal formation in vitro during ethanol metabolism until the more recent studies characterizing its behavioral and neurochemical effects. Towards this end, we first analyze the production and detection of Sal, in different brain areas, in basal conditions and after alcohol consumption, highlighting its presence in regions especially relevant in regulating ethanol-drinking behaviour and the importance of the newly developed methods to differentiate both enantiomers of Sal which could help to explain some previous negative findings. Afterwards, we review the behavioral and neurochemical studies. Finally, we present and discuss the previous and current enunciated mechanisms of action of Sal in the CNS.

Determination of salsolinol, norsalsolinol, and twenty-one biogenic amines using micellar electrokinetic capillary chromatography-electrochemical detection

Micellar electrokinetic chromatography coupled to amperometric electrochemical detection was used to resolve and then quantify biogenic amines and metabolites within the fruit fly Drosophila melanogaster. A new separation scheme was devised to allow resolution of 24 compounds of interest. This was accomplished by precisely controlling the amount of base added to the background buffer, optimizing the resolution of the separation, and then calculating the pH. Here we focused on measurements of six of the analytes that are thought to be involved in the response to alcohol, dopamine, salsolinol, norsalsolinol, N-acetyloctopamine, octopamine, and N-acetyldopamine. These were identified and quantified within the fly head. We believe that the identification of salsolinol and norsalsolinol in the fly brain is novel.

Locomotor stimulant effects of acute and repeated intrategmental injections of salsolinol in rats: role of mu-opioid receptors

RATIONALE

Microinjections of ethanol and acetaldehyde into ventral tegmental area (VTA) produce locomotor activation in rats through mechanisms dependent on the mu-opioid receptors. However, it is not clear how these drugs can interact with these receptors. It has been hypothesized that salsolinol could be the responsible for this interaction.

OBJECTIVES

The aim of the study was to investigate the ability of salsolinol to induce both motor activation and motor sensitization in rats after repeated intra-VTA administration.

MATERIALS

Rats received one microinjection into the posterior VTA of artificial cerebrospinal fluid (aCSF; 200 nL), salsolinol (0.3-3,000.0 pmol/200 nL), or salsolinol (30.0 pmol/200 nL) with either naltrexone (13.2 nmol/200 nL) or with the antagonist of the mu-opioid receptors, beta-funaltrexamine (beta-FNA; 2.5 nmol/300 nL). In the sensitization experiments, four microinjections of salsolinol (30.0 pmol/200 nL) or aCSF (200 nL) were performed over a 2-week period. This period was followed by a single challenge session, in which 0.3 pmol of salsolinol was microinjected to rats. Spontaneous activity was always monitored postinjection.

RESULTS

Intra-VTA salsolinol administration induces an increase of the spontaneous motor activity of the rats with the maximal effect at the dose of 30.0 pmol/200 nL. Salsolinol effects were blocked by the treatment with naltrexone or beta-FNA. Moreover, repeated injections of salsolinol produced locomotor sensitization.

CONCLUSIONS

Salsolinol induces locomotor activity and motor sensitization after intra-VTA administration. Moreover, the implication of the mu-opioid receptors was shown since the treatment with naltrexone or beta-FNA was able to suppress the salsolinol effects.

A critical evaluation of influence of ethanol and diet on salsolinol enantiomers in humans and rats

BACKGROUND

(R/S)-Salsolinol (SAL), a condensation product of dopamine (DA) with acetaldehyde, has been speculated to have a role in the etiology of alcoholism. Earlier studies have shown the presence of SAL in biological fluids and postmortem brains from both alcoholics and nonalcoholics. However, the involvement of SAL in alcoholism has been controversial over several decades, since the reported SAL levels and their changes after ethanol exposure were not consistent, possibly due to inadequate analytical procedures and confounding factors such as diet and genetic predisposition. Using a newly developed mass spectrometric method to analyze SAL stereoisomers, we evaluated the contribution of ethanol, diet, and genetic background to SAL levels as well as its enantiomeric distribution.

METHODS

Simultaneous measurement of SAL enantiomers and DA were achieved by high performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS). Plasma samples were collected from human subjects before and after banana (a food rich in SAL) intake, and during ethanol infusion. Rat plasma and brain samples were collected at various time points after the administration of SAL or banana by gavage. The brain parts including nucleus accumbens (NAC) and striatum (STR) were obtained from alcohol-non-preferring (NP) or alcohol-preferring (P) rats as well as P-rats which had a free access to ethanol (P-EtOH).

RESULTS

Plasma SAL levels were increased significantly after banana intake in humans. Consistently, administration of banana to rats also resulted in a drastic increase of plasma SAL levels, whereas brain SAL levels remained unaltered. Acute ethanol infusion did not change SAL levels or R/S ratio in plasma from healthy humans. The levels of both SAL isomers and DA were significantly lower in the NAC of P rats in comparison to NP rats. The SAL levels in NAC of P rats remained unchanged after chronic free-choice ethanol drinking. There were decreasing trends of SAL in STR and DA in both brain regions. No changes in enantiomeric ratio were observed after acute or chronic ethanol exposure.

CONCLUSIONS

SAL from dietary sources is the major contributor to plasma SAL levels. No significant changes of SAL plasma levels or enantiomeric distribution after acute or chronic ethanol exposure suggest that SAL may not be a biomarker for ethanol drinking. Significantly lower SAL and DA levels observed in NAC of P rats may be associated with innate alcohol preference.

Local salsolinol modulates dopamine extracellular levels from rat nucleus accumbens: shell/core differences

Salsolinol (SAL), a condensation product of dopamine and acetaldehyde that appears in the rat and human brain after ethanol ingestion, has been largely implicated in the aetiology of alcoholism. Although the behavioural consequences of systemic or intracerebral SAL administrations have been described, the neurochemical effects of pharmacologically relevant doses of SAL and other tetrahydroisoquinolines (THIQs) in the brain areas involved in alcohol addiction are practically unknown. To gain an insight into this topic, male Wistar rats were stereotaxically implanted with one concentric microdialysis probe in either the shell or the core of the nucleus accumbens (NAc). Treatments involved local administration of 0.1, 5 and 25 microM SAL for 20 min through the dialysis probe. Dopamine (DA) concentrations in the shell or core of the NAc were on-line analyzed every 20 min by HPLC with electrochemical detection. Implantation of the probe was histologically evaluated at the end of the experiments. Our results indicate that dialysis application of 5 and 25 microM SAL into the core increased the dialysate levels of DA. Conversely, the administration of the same doses of this drug into the shell significantly reduced the DA levels in this subregion. In conclusion, these data reveal that local application of SAL affects the DA levels in the NAc subterritories in a region-specific manner. These findings may prove useful in probing CNS sites and mechanisms involved in alcohol consumption.

Quantitative chiral analysis of salsolinol in different brain regions of rats genetically predisposed to alcoholism

A method to determine the catecholamine content in putamen (CPU) and midbrain (MB) regions of the brain of alcohol-preferring rats (P) is presented with a focus on the low-level detection of S,R-salsolinol, a metabolite of dopamine and a putative alcoholism marker. The developed strategy allows both quantitative profiling of related catecholamines and the enantiomeric separation and quantification of the S- and R-salsolinol isomers and their ratios. The described LC/MS strategy simplifies the current methodology that typically employs GC-MS by eliminating the need for derivatization. The data also suggest an increase in the non-enzymatic formation of salsolinol as a consequence of ethanol exposure.

The reinforcing properties of salsolinol in the ventral tegmental area: evidence for regional heterogeneity and the involvement of serotonin and dopamine

BACKGROUND

Salsolinol (SAL), the condensation product of acetaldehyde and dopamine, may be a factor contributing to alcohol abuse. Previous research indicated that both ethanol and acetaldehyde are self-administered into the posterior ventral tegmental area (VTA). The current study examined SAL self-infusions into the VTA, and determined the involvement of dopamine neurons and 5-HT3 receptors in this process.

METHODS

The intracranial self-administration technique was used to determine the self-infusion of SAL into the VTA of adult, male Wistar rats. The rats were placed in 2-lever (active and inactive) experimental chambers, and allowed to respond for the self-infusion of 0, 0.03, 0.1, 0.3, 1.0 or 3.0 microM SAL into the posterior or anterior VTA. In a second experiment, rats self-administered 0.3 microM SAL for the initial 4 sessions, co-administered SAL with ICS-205,930 (a 5-HT3 receptor antagonist) or quinpirole (a D(2,3) receptor agonist) for sessions 5 and 6, and then only 0.3 microM SAL for session 7.

RESULTS

Wistar rats, given 0.03 to 0.3 microM SAL, received more infusions per session than did the group given artificial cerebrospinal fluid (aCSF) alone (e.g., 41 infusions for 0.1 microM SAL versus 9 infusions for the aCSF group), and responded more on the active than inactive lever. These effects were observed in the posterior but not in anterior VTA. Co-infusion of 100 microM ICS-205,930, or quinpirole significantly reduced self-infusions and active lever responding.

CONCLUSIONS

SAL produces reinforcing effects in the posterior VTA of Wistar rats, and these effects are mediated by activation of DA neurons and local 5-HT3 receptors.

Effect of different doses of cyanamide on striatal salsolinol formation after ethanol treatment

To assess the dose-dependent effect of cyanamide (CY, a potent aldehyde dehydrogenase inhibitor) on salsolinol release in the striatum, rats were treated with CY (25, 50 and 100 mg/kg) plus ethanol (EtOH,1 g/kg) intraperitoneally. Striatal salsolinol was detected using in vivo microdialysis coupled with high-performance liquid chromatography with an electrochemical detector in free-moving rats, and blood acetaldehyde (AcH) and EtOH were detected using the head-space gas chromatographic method. With the increase in the doses of CY following EtOH, the peak concentrations of striatal salsolinol and blood AcH were increased significantly. Our study indicated that the magnitude of striatal salsolinol levels may depend on the concentration of blood AcH, and that there is a correlation between the blood AcH and striatal salsolinol.

Salsolinol produces reinforcing effects in the nucleus accumbens shell of alcohol-preferring (P) rats

BACKGROUND

The formation of salsolinol (SAL) has been hypothesized to be a factor contributing to alcoholism and alcohol abuse. If SAL is formed under chronic alcohol-drinking conditions, then it may contribute to alcohol addiction by being rewarding itself. Because SAL can be formed by the nonenzymatic condensation of acetaldehyde with dopamine, the reinforcing effects of SAL were tested in the nucleus accumbens shell, a dopamine-rich site considered to be involved in regulating alcohol-drinking behavior.

METHODS

The intracranial self-administration technique was used to test the reinforcing properties of SAL. Adult, female alcohol-preferring (P) rats were stereotaxically implanted with guide cannulae aimed at the nucleus accumbens shell. After 7 to 10 days to allow recovery from surgery, P rats were attached to the electrolytic microinfusion transducer system, placed in two-lever experimental chambers, and allowed to respond for the self-infusion of 100 nl of modified artificial cerebrospinal fluid (aCSF) or 0.03, 0.3, 3.0, or 12.5 microM SAL (3-1250 fmol/100 nl). Sessions were 4 hr in duration and were conducted in the dark cycle every 48 hr. The effects of coinfusing 10 to 400 microM sulpiride (given in sessions 5 and 6 after four acquisition sessions) on the intracranial self-administration of 3.0 microM SAL were tested in a separate experiment.

RESULTS

P rats given 0.3 to 12.5 microM SAL received significantly more infusions per session than did the group given aCSF alone (e.g., 50 infusions for 3.0 microM SAL versus 10 or fewer infusions for the aCSF group) and responded significantly more on the active than inactive lever. Coinfusion of 100 or 400 microM sulpiride reduced the responding on the active lever (80-100 responses/session without sulpiride) to levels observed for the inactive lever (fewer than 10 responses/session with sulpiride). This effect was reversible because giving SAL alone in session 7 reinstated responding on the active lever.

CONCLUSIONS

SAL is reinforcing in the nucleus accumbens shell of P rats at concentrations that are pharmacologically possible, and these reinforcing actions are mediated in part by D2/D3-like receptors.

Assay of salsolinol in peripheral blood mononuclear cells of alcoholics and healthy subjects by gas chromatography-mass spectrometry

Endogenous 6,7-dihydroxy-1-methyl-1,2,3,4-tetrahydroisoquinoline (salsolinol) could be a potential marker involved in the aetiology of alcoholism. Whereas the amount of salsolinol in plasma and urine depends on several dietary conditions, the salsolinol from peripheral mononuclear cells should be formed endogenously. Salsolinol was quantified in lymphocytes of 10 controls and 11 alcoholics (after 1 and 13 weeks of abstinence) using solid phase extraction and gas chromatography/mass spectrometry. Alcoholics showed significantly lower salsolinol concentration than the controls. After 13 weeks of abstinence a further significant decrease of SAL levels could be seen in the lymphocytes of alcoholics. The findings of this study support the theory that salsolinol might be a trait marker in alcoholism.

In vitro pharmacological activity of the tetrahydroisoquinoline salsolinol present in products from Theobroma cacao L. like cocoa and chocolate

Cocoa and chocolate contain the tetrahydroisoquinoline alkaloid salsolinol up to a concentration of 25 microg/g. Salsolinol is a dopaminergic active compound which binds to the D(2) receptor family, especially to the D(3) receptor with a K(i) of 0.48+/-0.021 micromol/l. It inhibits the formation of cyclic AMP and the release of beta-endorphin and ACTH in a pituitary cell system. Taking the detected concentration and the pharmacological properties into account, salsolinol seems to be one of the main psychoactive compounds present in cocoa and chocolate and might be included in chocolate addiction.