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The Journal of Neuroscience, August 1, 2002, 22(15):6321–6324 Caffeine Induces Dopamine and Glutamate Release in the Shell of
the Nucleus Accumbens

Marcello Solinas,1 Sergi Ferre´,1 Zhi-Bing You,2 Marzena Karcz-Kubicha,1 Patrizia Popoli,3 and
Steven R. Goldberg
1
Sections of 1Preclinical Pharmacology and 2Behavioral Neuroscience, Behavioral Neuroscience Branch, National Instituteon Drug Abuse, National Institutes of Health Intramural Research Program, Baltimore, Maryland 21224, and3Department of Pharmacology, Istituto Superiore di Sanita, 00161 Rome, Italy An increase in the extracellular concentration of dopamine in adenosine A1 receptor antagonist but not by a selective aden- the nucleus accumbens (NAc) is believed to be one of the main osine A2A receptor antagonist. This suggests that caffeine, mechanisms involved in the rewarding and motor-activating because of its ability to block adenosine A1 receptors, shares properties of psychostimulants such as amphetamines and neurochemical properties with other psychostimulants, which cocaine. Using in vivo microdialysis in freely moving rats, we could contribute to the widespread consumption of caffeine- demonstrate that systemic administration of behaviorally rele- vant doses of caffeine can preferentially increase extracellularlevels of dopamine and glutamate in the shell of the NAc. These Key words: caffeine; adenosine; dopamine; glutamate; ac- effects could be reproduced by the administration of a selective Caffeine is the most consumed psychoactive drug in the world.
for a dopamine-releasing effect of behaviorally relevant doses of Although it is a psychostimulant, it is not generally considered a caffeine in brain regions that may mediate its psychostimulant typical drug of dependence (Daly and Fredholm, 1998). Other actions has not yet been obtained. Only a previous study by psychostimulants, such as amphetamines and cocaine, elevate the Morgan and Vestal (1989), using in vivo voltammetry, indicated extracellular concentration of dopamine in the nucleus accum- that low doses of caffeine increase dopamine release in the rat bens (NAc); this is believed to be one of the main mechanisms caudate putamen. In the present study, by using in vivo microdi- involved in the rewarding and motor-activating properties of alysis in freely moving rats, we demonstrate that systemic admin- these drugs (Pontieri et al., 1995; Wise and Bozarth, 1987). In istration of behaviorally relevant doses of caffeine can preferen- addition, augmented extracellular levels of glutamate in the NAc tially increase the extracellular levels of dopamine and glutamate may be involved in the central effects of psychostimulants (Reid et in the shell of the NAc.
al., 1997). The main mechanism of action of caffeine in the brain seems to be a nonselective competitive blockade of adenosine MATERIALS AND METHODS
receptors, in particular adenosine A1 receptors and A2A recep- Subjects and drugs. Male Sprague Dawley rats, weighing 300 –350 gm, tors (Daly and Fredholm, 1998). In the striatum, adenosine plays were used in all experiments. Animals were maintained in facilities fully an important role as a modulator of both dopamine and gluta- accredited by the American Association for the Accreditation of Labo- mate neurotransmission. At a presynaptic level, adenosine, ratory Animal Care; all experimentation was conducted in accordance with the guidelines of the Institutional Care and Use Committee of the mostly by acting on adenosine A1 receptors localized in nerve Intramural Research Program, National Institute on Drug Abuse terminals, inhibits dopamine and glutamate release (Wood et al., (NIDA), National Institutes of Health, the directives of the Principles of 1989; Okada et al., 1996; Flagmeyer et al., 1997; Golembiowska Laboratory Animal Care (National Institutes of Health publication num- and Zylewska, 1997). At a postsynaptic level, adenosine decreases ber 85-23, revised 1985), and the Council of the European Communities (86/809/EEC). Caffeine, the adenosine A1 antagonist 8-cyclopentyl- dopaminergic neurotransmission by means of specific antagonis- theophylline (CPT), and the adenosine A2A antagonist 5-amino-7-(2- tic interactions between adenosine and dopamine receptors (Ferre´ et al., 1997). Thus, caffeine, by antagonizing the effects of (SCH 58261) were administered intraperitoneally in all experiments.
endogenous adenosine, can facilitate dopaminergic neurotrans- Caffeine was dissolved in warm saline, CPT was dissolved in saline with mission by stimulating dopamine release and by potentiating the a minimal amount of 1N NaOH, and the A2A receptor antagonist SCH 58261 was dissolved in dimethylsulfoxide.
effects of dopamine receptor stimulation (Ferre´ et al., 1997).
Motor activity experiments. Motor activity was measured in automated Although the latter mechanism is very well established, evidence activity meters (Automex II; Columbus Instruments, Columbus, OH) in habituated animals (30 min), as described in detail previously (Popoli et al., 1998). Total horizontal motor activity (total accumulated counts of a Received Feb. 11, 2002; revised May 2, 2002; accepted May 10, 2002.
single photocell interruption) was collected for 60 min after intraperito- This work was supported by the Intramural Research Program of the National neal administration of caffeine (3, 10, 30, and 100 mg/kg) or saline. A Institute on Drug Abuse, National Institutes of Health.
Correspondence should be addressed to Dr. Steven R. Goldberg, National Insti- one-way ANOVA followed by a Dunnett's post hoc test was used for tute on Drug Abuse, National Institutes of Health Intramural Research Program, Behavioral Neuroscience Branch, Preclinical Pharmacology Section, 5500 Nathan In vivo microdialysis experiments. Concentric microdialysis probes Shock Drive, Baltimore, MD 21224. E-mail: sgoldber@intra.nida.nih.gov.
were prepared as described previously (Pontieri et al., 1995). Animals Copyright 2002 Society for Neuroscience 0270-6474/02/226321-04$15.00/0 were anesthetized with Equithesin (NIDA Pharmacy, Baltimore, MD),


6322 J. Neurosci., August 1, 2002, 22(15):6321–6324
Solinas et al. • Caffeine-Induced Dopamine and Glutamate Release Figure 1. Total horizontal motor activity after intraperitoneal adminis- tration of caffeine in habituated rats. The results represent means ⫾ SEM of the accumulated motor activity counts during the first 60 min period of observation (n ⫽ 6 per group). Significant motor activation was obtained with caffeine in a dose of 10 mg/kg (CAFF 10) and caffeine in a dose of 30 mg/kg (CAFF 30). **p ⬍ 0.01 compared with the group treated with saline (SAL).CAFF 3 and CAFF 100 indicate 3 and 100 mg/kg caffeine, and probes were implanted in the left brain hemisphere, either in the shell of the NAc [coordinates with respect to bregma: anterior (A), ⫹2.3; lateral (L), 1.0; ventral (V), 7.8 from the dura] or in the core of the NAc (A, ⫹1.6; L, 1.8; V, 7.6). The experiments were performed on freely moving rats 24 hr after the probe implantation. A solution (in mM) of 147 Ringer's, 4 KCl, and 2.2 CaCl2 was pumped through the dialysis probe at a constant rate of 0.4 ␮l/min from immediately after implantation until the beginning of the microdialysis experiment, when it was switched to 1 ␮l/min until the end of the experiment. Samples were collected at 20 min Figure 2. Extracellular concentrations of dopamine (DA) and glutamate intervals and split into two 10 ␮l fractions. One 10 ␮l fraction was assayed (Glu) in the shell of the NAc after intraperitoneal administration of saline for dopamine content and the other was assayed for glutamate content, or caffeine [3 (caff 3), 10 (caff 10), 30 (caff 30), or 100 (caff 100) mg/kg].
using HPLC systems with electrochemical (for dopamine) and fluores- The results represent means ⫾ SEM of the percentage of basal values of cence (for glutamate) detection, as described in detail previously (You et the extracellular concentrations of dopamine and glutamate (n ⫽ 6–8 per al., 1994; Pontieri et al., 1995). At the end of the experiment, rats were group). Basal values were the means of three values before drug admin- killed with an overdose of Equithesin and methylene blue was perfused istration. Caffeine at doses of 10 and 30 mg/kg but not at doses of 3 and through the probe. The brain was removed and placed in a 10% form- 100 mg/kg significantly increased the extracellular levels of dopamine and aldehyde solution, and coronal sections were cut to verify the probe glutamate (Student's paired t test; only significant results of pretreatment location. The statistical analysis used was the "summary measures" vs post-treatment are shown).
method (Matthews et al., 1990), using the mean of the three values previous to drug administration and the mean of the six values subse- quent to drug administration per animal as the summary measures.
concentrations of dopamine (up by ⬃100%) and glutamate (up by Pretreatment and post-treatment dopamine values obtained from the ⬃50%) in the shell of the NAc (Fig. 2). These effects of caffeine shell and core of the NAc in animals treated with 30 mg/kg caffeine were were dose- and region-specific. Thus, neither 3 nor 100 mg/kg of analyzed by repeated-measures ANOVA to assess treatment and brain area effects. Pretreatment versus post-treatment values were compared caffeine significantly modified dopamine or glutamate extracellu- using Student's paired t test to analyze treatment effects; p values shown lar levels in the shell of the NAc (Fig. 2); in the core of the NAc, in the figures refer in all cases to these differences.
30 mg/kg caffeine induced a small but significant increase in dopamine levels (⬃25%) and no significant changes in glutamate levels (Fig. 3). This effect was significantly different from that Determination of the motor-activating doses
produced by 30 mg/kg caffeine in the shell of the NAc (interac- of caffeine
tion between area and treatment factors: p ⬍ 0.01 by repeated- A dose–response study of the motor activity induced by intra- measures ANOVA). Basal extracellular levels of dopamine and peritoneal administration of caffeine was performed in male glutamate were 4.2 ⫾ 0.3 nM (n ⫽ 45) and 3.2 ⫾ 0.2 ␮M (n ⫽ 52), Sprague Dawley rats to establish behaviorally relevant doses for respectively, in the shell of the NAc and 4.6 ⫾ 0.6 nM (n ⫽ 9) and microdialysis studies. As reported previously (Daly and Fred- 3.4 ⫾ 0.6 ␮M (n ⫽ 10), respectively, in the core of the NAc.
holm, 1998), the motor-activation dose–response curve produced by caffeine had an inverted U shape, with 3 and 100 mg/kg doses Effects of caffeine on dopamine and glutamate release
being ineffective and 10 and 30 mg/kg producing a similar max- reproduced by a selective adenosine A1 receptor
antagonist

imal increase in motor activity (Fig. 1).
Because caffeine is a nonselective adenosine receptor antagonist, Caffeine induces a preferential release of dopamine
the selective adenosine A1 receptor antagonist CPT and the and glutamate in the shell of the NAc
selective adenosine A2A receptor antagonist SCH 58261 were In subsequent microdialysis experiments, 10 and 30 mg/kg doses used to investigate the adenosine receptors involved. Low motor- of caffeine induced similar significant increases in extracellular activating doses of both CPT (4.8 mg/kg) and SCH 58261 (2 Solinas et al. • Caffeine-Induced Dopamine and Glutamate Release J. Neurosci., August 1, 2002, 22(15):6321–6324 6323
Figure 3. Extracellular concentrations of dopamine (DA) and glutamate Figure 4. Extracellular concentrations of dopamine (DA) and glutamate (Glu) in the shell and core of the NAc after intraperitoneal administration (Glu) in the shell of the NAc after intraperitoneal administration of the of caffeine [30 mg/kg (caff 30)]. The results represent means ⫾ SEM of adenosine A1 receptor antagonist CPT (4.8 mg/kg) and the A2A recep- the percentage of basal values of the extracellular concentrations of tor antagonist SCH 58261 (2 mg/kg). The results represent means ⫾ SEM dopamine and glutamate (n ⫽ 6–8 per group). Basal values were the of the percentage of basal values of the extracellular concentrations of means of three values before drug administration. Caffeine (30 mg/kg) dopamine and glutamate (n ⫽ 6–7 per group). Basal values were the produced a significant increase in the extracellular concentration of means of three values before drug administration. CPT but not SCH dopamine, but not of glutamate, in the core of the NAc (Student's paired 58261 produced a significant increase in the extracellular concentrations t test; only significant results of pretreatment vs post-treatment are of dopamine and glutamate (Student's paired t test; only significant results shown). This effect was significantly different from that produced by 30 of pretreatment vs post-treatment are shown).
mg/kg caffeine in the shell of the NAc (interaction between area and treatment factors: p ⬍ 0.01 by repeated-measures ANOVA).
rats blocked the locomotor activation induced by amphetamines but failed to block the locomotor activation induced by caffeine.
mg/kg) were used (Popoli et al., 1998). It was found that CPT but Based on these results, the authors suggested that caffeine in- not SCH 58261 significantly increased extracellular levels of do- duces locomotor activity by acting independently of presynaptic pamine and glutamate (⬃100% in both cases) in the shell of the terminals in the mesolimbic dopaminergic system (Joyce and NAc (Fig. 4).
Koob, 1981; Swerdlow et al., 1986). However, in view of the demonstrated resistance to 6-hydroxydopamine-induced dopa- mine denervation in the shell versus the core of the NAc Dopamine release in either the core or the shell of the NAc has (Meredith et al., 1995; Boye et al., 2001), those studies cannot rule been suggested to be causally related to the locomotor stimulant out a preferential role of dopamine release in the shell of the NAc effects of psychostimulants such as amphetamine (Parkinson et on the motor effects induced by caffeine. Nevertheless, it must be al., 1999; Boye et al., 2001), whereas preferential release of pointed out that other striatal regions can also be involved, dopamine in the shell of the accumbens has been suggested because a significant although less pronounced effect of caffeine to be causally related to the rewarding effects of psychostimulants on dopamine release was also observed in the core of the NAc (Di Chiara and Imperato, 1988). The close correlations between (see the introductory remarks; Morgan and Vestal, 1989). Sur- the motor-activating effects and the previously described prisingly, the highest dose of caffeine (100 mg/kg) did not pro- discriminative-stimulus effects of caffeine (Mumford and Holtz- duce any effect on extracellular dopamine or glutamate levels in man, 1991) and the present microdialysis data are consistent with the shell of the NAc. Additional studies are needed to clarify the the possibility that the preferential release of dopamine and mechanisms involved in this lack of effect. However, high doses of glutamate in the shell of the NAc may also be involved in the caffeine are known to exert effects through mechanisms other psychostimulant effects of caffeine. Our hypothesis might seem to than adenosine receptor antagonism (e.g., phosphodiesterase in- be in conflict with the studies by Joyce and Koob (1981), who hibition and release of intracellular calcium) (Daly and Fred- found that 6-hydroxydopamine lesions in the region of the NAc of holm, 1998).
6324 J. Neurosci., August 1, 2002, 22(15):6321–6324
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