."."... ',f i L
r..,
"
Role of acetaldehyde in the rewarding effects of ethanol: behavioural data and neurochemical correlates. 1*
Alessandra_T. l
Peana
2
2
, Paolo Enrico, Donatella Sirca , Gian Fortunato l
l
]
Franchi, Alessandra Lintas , Francesca Panin , and Marco Diana (*
[email protected])
l "G.Minardi" Laboratory ofCognitive Neuroscience, Dipartimento di Scienze del Farmaco, 2 Dipàrtimento di Scienze Biomediche, University ofSassari
Introduction Acetaldehyde is the first metabolite of ethanol. lì is produced by an action of a1cohol dehydrogenase peripherally and primarily by catalase in the brain. After many years of debate and conflicting scientific data, acetaldehyde is now widely recognised as a potential psychoactive drug able to induce severai behaviorai effects ranging from locomotion to hypnosis and anxiolysis (1). In particular, experimental evidence shows that acetaldehyde may act as a pharmacological reinforcer at least in rodents (2), Ieading to the hypothesis that this drug (alone or acting in synergism with ethanol) might play a role in the pharmacological and behavioral effects of ethanol and possibly in its hedonistic effects (3). In support to this hypothesis recent in vivo eIectrophysiological data show that acetaIdehyde reinforcing effects, simiIarIy to ethanol, might be mediated by an activation of the mesolimbic dopaminergic pathway (4), a structure believed to play a key role in drug addiction (5). Despite the vast research effort in the field, the role of acetaIdehyde in the mediation of ethanoi effects is still a matter of debate. Ethanoi may well be a prodrug whose pharmacological properties are entirely mediated by acetaIdehyde (6), but it is also possibie that acetaIdehyde induces effects that are similar to ethanol, but by independent mechanisms, potentiating rather than mediating the effects of ethanol. In order to investigate the basis of the reinforcing effects of acetaldehyde in the rat and the relationships between the rewarding properties of ethanoi and those of its first metabolite, we decided to use a multidisciplinary approach combining the use of behaviorai (pIace preference conditioning) and neurochemical (in vivo microdialysis of dopamine (DA) in the nucleus accumbens (NAcb)) methods. Materials and methods Male Wistar rats weighting between 250-300 g were tested for the rewarding effects of acetaldehyde, ethanol and ethanol with 4-methylpyrazole (a competitive inhibitor of alcohol dehydrogenase), using a pIace preference conditioning procedure combined with a simultaneous microdialysis study of the dopaminergic transmission in the Nacb shell and core. PIace preference experiments were
539
...,....--
performed employing an unbiased procedure that is more sensitive to changes in either direction (preference or aversion). Each experiment consisted of thiee phases. During the preconditioning phase each rat was given access to both compartments of the apparatus and the time spent in each compartment was recorded for 30 min. The conditioning procedure (30 min) involved the administration of a total of 8 daily conditioning trials under 20 mg/kg acetaldehyde by gavage (Gv), or 15 daily conditioning trials under l g/kg (Gv) ethanol; 4methylpyrazole was administered, i.p. at the dose of 90 mg/kg in saline, at alternate days before ethanol treatment. During the postconditioning phase, 24h after the last treatment, the time spent by each rat in the drug-paired compartment was recorded during 30 min of observation. For microdialysis experiments, rats were implanted with a chronic guide cannula aimed at the NAcb shell or core and then subjected to the pIace preference procedure. The day before the behavioral test (post conditioning) a concentric dialysis probe was carefully inserted and locked in the guide cannula. During the experiment, the probe was perfused with a Ringer's solution, dialysates where collected every 15 minutes and assayed for DA with HPLC-ECD. Results and discussion
Subject conditioned using saline (Control) or 4-methylpyrazole showed no significant changes in pIace preference. 450
*
400
*
350 300 c: o
.~ 250 'c
~ ;>
~
200 150 100 50
T
O
Contro!
Contro!i.e,
ACD
ETOH
ETOH+ie
Fig. 1. Effects of acetaIdehyde, ethanoi and ethanoi with 4-methyIpyrazole on pIace preference conditioning. Data from the test (undrugged state) are shown as % variation in terms of time spent in the post conditioned side with respect to the time spent in the preconditioning side of the apparatus ::I::SEM. Significant differences from respective controi group are indicated by * (* P< 0.05, ANOVA t-test).
540
.....
On the other hand, animals conditioned with acetaldehyde and ethanol showed a c1ear-cut conditioned pIace preference effect of similar intensity (Fig. l), that lasted up to 30 days. In contrast, ethanol-administered rats when pre-treated with the enzymatic inhibitor, 4-methylpyrazole showed a significant reduction of conditioned pIace preference as compared with the ethanol group suggesting that acetaldehyde, is at least, partially involved in this effects (Fig. l). This observation is in line with recent in vivo electrophysiological data showing a reduction of the stimulating effect of ethanol on ventral tegmental area (VTA) dopamine neurones by pharmacological blockade of ethanol metabolism with 4-methylpyrazole (4). Further acute administration of 20 mg/kg acetaldehyde by Gv increases dopamine concentrations in the Nacb by 30-35%. The present results suggest that acetaldehyde induces pIace preference similar to ethanol, whose effect is blocked by pharmacological prevention of acetaldehyde formation through administration of 4-methylpyrazole. In addition, acetaldehyde increases DA microdialysate concentrations in the Nacb suggesting a possible neurochemical correlate for its behavioral effect. Our results support the hypothesis that acetaldehyde (1, 2) has centraI nervous system effects in its own right and is actively involved in the rewarding effects of a1cohol possibly by the activation of Nacb-projecting VTA dopamine neurons. References l. 2.
3. 4. 5.
6.
Quertemont E., Tambour S. and Tirelli E. (2005) The role of acetaldehyde in the neurobehavioral effects of ethanol: A comprehensive review of animai studies. Prog N eurobiol, 7 5(4), 247-74. Rodd-Henricks Z.A., Melendez R.I., Zaffaroni A., Goldstein A., McBride W.J. and Lu T.K. (2002) The reinforcing effects of acetaldehyde in the posterior ventral tegmental area of alcohol-preferring rats. Pharmacol. Biochem. Behav, 72, 55-64. Quertemont E. and Tambour S., Is ethanol a pro-drug? Role of acetaldehyde in the centrai effects of ethanol. (2004) Trends Pharmacol. Sci., 25,130-134. Foddai M., Dosia G., Spiga S. and Diana M. (2004) Acetaldehyde increases dopaminergic neuronal activity in the VTA. Neuropsychopharmacology, 29, 530-536. Di Chiara G., Bassareo V., Fenu S., De Luca M.A., Spina L., Cadoni c., Acquas E., Carboni E., Valentini V., Lecca D. (2004) Dopamine and drug addiction: the nucleus accumbens shell connection. Neuropharmacology, 47, 227-241. Quertemont, E., Tambour, S., (2004) Is ethanol a pro-drug ? Role of acetaldehyde in the centraI effects of ethanol. Trends Pharmacol. Sci. 25, 130-134.
541
