Stefania Vinci

Piecing together the puzzle of acetaldehyde as a neuroactive agent.

Mainly known for its more famous parent compound, ethanol, acetaldehyde was first studied in the 1940s, but then research interest in this compound waned. However, in the last two decades, research on acetaldehyde has seen a revitalized and uninterrupted interest. Acetaldehyde, per se, and as a product of ethanol metabolism, is responsible for many pharmacological effects which are not clearly distinguishable from those of its parent compound, ethanol. Consequently, the most recent advances in acetaldehydes psychopharmacology have been inspired by the experimental approach to test the hypothesis that some of the effects of ethanol are mediated by acetaldehyde and, in this regard, the characterization of metabolic pathways for ethanol and the localization within discrete brain regions of these effects have revitalized the interest on the role of acetaldehyde in ethanols central effects. Here we present and discuss a wealth of experimental evidence that converges to suggest that acetaldehyde is an intrinsically active compound, is metabolically generated in the brain and, finally, mediates many of the psychopharmacological properties of ethanol.

Ethanol-induced extracellular signal regulated kinase: role of dopamine D1 receptors.

BACKGROUND Addictive drugs activate extracellular signal regulated kinase (ERK) in brain regions critically involved in their affective and motivational properties. The aim of this study was to demonstrate the ethanol-induced activation of ERK in the nucleus accumbens (Acb) and in the extended amygdala [bed nucleus of the stria terminalis lateralis (BSTL) and central nucleus of the amygdala (CeA)] and to highlight the role of dopamine (DA) D(1) receptors in these effects. METHODS Ethanol (0.5, 1, and 2 g/kg) was administered by gavage and ERK phosphorylation was determined in the nucleus Acb (shell and core), BSTL, and CeA by immunohistochemistry. The DA D(1) receptor antagonist, SCH 39166 (SCH) (50 microg/kg), was administered 10 minutes before ethanol (1 g/kg). RESULTS Quantitative microscopic examination showed that ethanol, dose-dependently increased phospho-ERK immunoreactivity (optical and neuronal densities) in the shell and core of nucleus Acb, BSTL, and CeA. Pretreatment with SCH fully prevented the increases elicited by ethanol (1 g/kg) in all brain regions studied. CONCLUSIONS The results of this study indicate that ethanol, similar to other addictive drugs, activates ERK in nucleus Acb and extended amygdala via a DA D(1) receptor-mediated mechanism. Overall, these results suggest that the D(1) receptors/ERK pathway may play a critical role in the motivational properties of ethanol.

Role of Dopamine D1 Receptors and Extracellular Signal Regulated Kinase in the Motivational Properties of Acetaldehyde as Assessed by Place Preference Conditioning

BACKGROUND The role of dopamine D1 receptors and Extracellular signal Regulated Kinase (ERK) in the motivational properties of drugs can be studied by place-conditioning. Recent advances have shown that the motivational properties of ethanol, determined by place-conditioning, are mediated by its metabolic conversion into acetaldehyde. To date, the role of D1 receptors and ERK activation in acetaldehyde-elicited place preference has not been determined. The aim of this study was to assess the role of D1 receptors blockade and MEK inhibition in the acquisition of acetaldehyde-elicited conditioned place preference. METHODS Male Sprague-Dawley rats were subjected to repeated pairings with 1 compartment of the conditioning apparatus immediately following acetaldehyde (20 mg/kg i.g.) or ethanol (1 g/kg i.g.) administration. The D1 receptor antagonist, SCH 39166 (50 microg/kg s.c.), was administered 10 minutes before acetaldehyde or ethanol administration. In order to study the role of activated ERK in the acetaldehyde-elicited place preference, rats were administered the MEK inhibitor, PD98059 (1, 30, and 90 microg i.c.v.), 10 or 30 minutes before acetaldehyde. To verify the specificity of these effects, we also studied whether PD98059 pretreatment could affect morphine (1 mg/kg s.c.)-elicited place preference. RESULTS Both acetaldehyde and ethanol elicited significant place preferences and these were prevented by pretreatment with SCH 39166. In addition, pretreatment with PD98059, dose (30 and 90 but not 1 microg i.c.v.) and time (10 but not 30 minutes before) dependently, prevented the acquisition of acetaldehyde- and significantly reduced the acquisition of morphine-elicited conditioned place preference. CONCLUSIONS These results confirm that acetaldehyde and ethanol elicit conditioned place preference and demonstrate that D1 receptors are critically involved in these effects. Furthermore, the finding that PD98059 prevents the acquisition of acetaldehyde-elicited conditioned place preference highlights the importance of the D1 receptor-ERK pathway in its motivational effects.

Effect of opioid receptor blockade on acetaldehyde self-administration and ERK phosphorylation in the rat nucleus accumbens

We have previously shown that acetaldehyde (ACD), the first metabolite of ethanol, regulates its motivational properties and possesses reinforcing effects by itself. A large and still growing body of evidence indicates that the endogenous opioidergic system plays a critical role in the motivational effects of ethanol and suggests a role for extracellular signal-regulated kinase (ERK) in these effects of both ethanol and ACD. The present study was undertaken to examine if opioid-mediated mechanisms are involved in the reinforcing properties of ACD and in ACD-elicited ERK activation. To this end, Wistar rats were trained to orally self-administer ACD (0.2%) by nose poking. Responses on active nose poke caused delivery of ACD solution, whereas responses on inactive nose poke had no consequences. The effect of pretreatment with a nonselective opioid receptor antagonist, naltrexone (NTX), was evaluated during (1) maintenance of ACD self-administration, (2) deprivation effect after ACD extinction, and (3) ACD self-administration under a progressive-ratio schedule of reinforcement. Additionally, we tested the effect of NTX on saccharin (0.05%) reinforcement, as assessed by oral self-administration, and on ACD-elicited ERK phosphorylation in the nucleus accumbens (Acb), as assessed by immunohistochemistry. Finally, we examined the effect of a μ(1)-selective opioid receptor antagonist, naloxonazine (NLZ), on the maintenance phase of ACD and saccharin self-administration. The results indicate that NTX (0.4-0.8mg/kg) reduced the maintenance, the deprivation effect, and the break points of ACD self-administration without suppressing saccharin self-administration. Moreover, NTX decreased ACD-elicited ERK activation in the Acb shell and core. NLZ (10-15mg/kg) reduced the maintenance phase of ACD self-administration without interfering with saccharin self-administration, whereas both NTX and NLZ failed to modify responses on inactive nose poke indicating the lack of a nonspecific behavioral activation. Overall, these results indicate that the opioid system is implicated in the reinforcing properties of ACD and suggest an involvement of ERK. The finding that NTX and NLZ reduce ACD but not saccharin self-administration indicates that these effects are specific to ACD.

Acetaldehyde elicits ERK phosphorylation in the rat nucleus accumbens and extended amygdala.

Recent advances suggest that acetaldehyde mediates some of the neurobiological properties of ethanol. In a recent study, we have shown that ethanol elicits the phosphorylation of extracellular signal‐regulated kinase (pERK) in the nucleus accumbens and extended amygdala, via a dopamine D1 receptor‐mediated mechanism. The aim of this study was to determine whether acetaldehyde and ethanol‐derived acetaldehyde elicit the activation of ERK in the nucleus accumbens and extended amygdala. The effects of acetaldehyde (10 and 20 mg/kg) and ethanol (1 g/kg), administered to rats intragastrically, were assessed by pERK peroxidase immunohistochemistry. To establish the role of ethanol‐derived acetaldehyde, the alcohol dehydrogenase inhibitor, 4‐methylpyrazole (90 mg/kg), and the acetaldehyde‐sequestering agent, D‐penicillamine (50 mg/kg), were administered before ethanol. Acetaldehyde increased pERK immunoreactivity in the nucleus accumbens and extended amygdala. Inhibition of ethanol metabolism and sequestration of newly synthesized acetaldehyde completely prevented ERK activation by ethanol. In addition, to establish the role of D1 receptors stimulation in acetaldehyde‐elicited ERK phosphorylation, we studied the effect of the D1 receptor antagonist, SCH 39166. Pretreatment with the D1 receptor antagonist (50 μg/kg) fully prevented acetaldehyde‐elicited ERK activation. Overall, these results indicate that ethanol activates ERK by means of its metabolic conversion into acetaldehyde and strengthen the view that acetaldehyde is a centrally acting compound with a pharmacological profile similar to ethanol. Synapse 64:916–927, 2010.

Role of dopamine D(1) receptors in caffeine-mediated ERK phosphorylation in the rat brain.

The aim of this research was to study the role of dopamine D1 receptors in caffeine elicited ERK phosphorylation in the prefrontal and other cortical (cingulate and motor) and subcortical (shell and core of the nucleus accumbens) regions. To this end, caffeine (3 and 10 mg/kg) was administered before phosphoERK immunohistochemistry. Caffeine dose‐dependently increased the number of phosphoERK‐positive neurons in the prefrontal and cingulate cortices but not in the secondary motor cortex and in the nucleus accumbens shell and core. The dopamine D1 receptor antagonist, SCH 39166 (50 μg/kg), fully prevented phosphoERK activation by caffeine (10 mg/kg) in the superficial and deep layers of the prefrontal cortex but failed to prevent it in the cingulate cortex. Given that phosphoERK can be regarded as a postsynaptic marker of neuronal activation, the present results indicate that psychotropic properties of caffeine may result from the activation of prefrontal, via dopamine D1 receptors, and cingulate cortices. Failure of caffeine to activate ERK in the nucleus accumbens further supports, indirectly, the observation that caffeine fails to activate dopamine transmission in this structure and is consistent with the tenet that caffeine lacks of true addictive properties. Synapse 64:341–349, 2010.

Acquisition and expression of conditioned taste aversion differentially affects extracellular signal regulated kinase and glutamate receptor phosphorylation in rat prefrontal cortex and nucleus accumbens

Conditioned taste aversion (CTA) can be applied to study associative learning and its relevant underpinning molecular mechanisms in discrete brain regions. The present study examined, by immunohistochemistry and immunocytochemistry, the effects of acquisition and expression of lithium-induced CTA on activated Extracellular signal Regulated Kinase (p-ERK) in the prefrontal cortex (PFCx) and nucleus accumbens (Acb) of male Sprague-Dawley rats. The study also examined, by immunoblotting, whether acquisition and expression of lithium-induced CTA resulted in modified levels of phosphorylation of glutamate receptor subunits (NR1 and GluR1) and Thr34- and Thr75-Dopamine-and-cAMP-Regulated PhosphoProtein (DARPP-32). CTA acquisition was associated with an increase of p-ERK-positive neurons and phosphorylated NR1 receptor subunit (p-NR1) in the PFCx, whereas p-GluR1, p-Thr34- and p-Thr75-DARPP-32 levels were not changed in this brain region. CTA expression increased the number of p-ERK-positive neurons in the shell (AcbSh) and core (AcbC) but left unmodified p-NR1, p-GluR1, p-Thr34- and p-Thr75-DARPP-32 levels. Furthermore, post-embedding immunogold quantitative analysis in AcbSh revealed that CTA expression significantly increased nuclear p-ERK immunostaining as well as p-ERK-labeled axo-spinous contacts. Overall, these results indicate that ERK and NR1, but not GluR1 and DARPP-32, are differentially phosphorylated as a consequence of acquisition and expression of aversive associative learning. Moreover, these results confirm that CTA represents an useful approach to study the molecular basis of associative learning in rats and suggest the involvement of ERK cascade in learning-associated synaptic plasticity.

Caffeine-mediated ERK phosphorylation in the rat brain

Caffeine is the most consumed psychotropic compound worldwide due to its presence in tea and coffee. As a direct consequence of such presence in everyday life, significant efforts have been devoted in pre-clinical and clinical research toward a better understanding of its actions on human health and, in particular, on brain function. It is nowadays established that, depending on its plasma concentrations, caffeine exerts its central effects by blocking adenosine receptors, stimulating ryanodine receptors and inhibiting phosphodiesterases. These mechanisms cover a quite wide range of effects, from motor to cognitive. Compelling evidence has emerged, in addition, to suggest that caffeine exerts some of its central effects also via activation of ERK, a kinase critical in physiological effects of neurotransmitters and in the mechanism of action of many psychoactive compounds. In fact, recent data suggest that ERK activation by caffeine in brain regions such as the mPFCx and Cg 1 -Cg 2 , may represent a hallmark of caffeine-elicited neuronal activation, whereas failure to activate ERK by caffeine in the mesolimbic DA system has become instrumental to sustain the tenet that caffeine lacks addictive properties. Overall, given the arousing properties of caffeine and the prominent role of ERK in synaptic plasticity, it is concluded that caffeine-elicited ERK activation in different cerebral regions may represent a molecular index of synaptic plasticity subserving arousal, attention and cognition.