Research Signpost 37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India Neural Mechanisms of Action of Drugs of Abuse and Natural Reinforcers, 2008: 89-105 ISBN: 978-81-308-0245-9 Editors: Milagros Méndez Ubach and Ricardo Mondragón-Ceballos Non-dopaminergic modulation of the discriminative properties 5 of amphetamine 1 2 David N. Velázquez-Martínez , Florencio Miranda Hugo Sánchez Castillo1 and Gabriela Orozco Calderón1 1Departamento de Psicofisiología, Facultad de Psicología, Universidad Nacional Autónoma de México, México, D.F. 04510, México; 2Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México, Av de los Barrios No.1, Los Reyes-Iztacala, Tlalnepantla, Estado de México, 54090 México Abstract Psychostimulants induce a direct or indirect increase in dopamine (DA) synaptic and extrasynaptic levels in mesocorticolimbic areas affecting the reward and appetitive system of the brain. Despite their therapeutical use, dependency, addiction and abuse to psychostimulants had become a worldwide problem that limits their usage. The modulation of the DA neurotransmission through non-DA mechanisms Correspondence/Reprint request: Dr. David N. Velázquez-Martínez, Departamento de Psicofisiología, Facultad de Psicología, Universidad Nacional Autónoma de México, México, D.F. 04510, México E-mail: [email protected] 90 David N. Velázquez-Martínez et al. may be of help in the treatment of addiction. Since the reinforcing properties of drugs have a concomitant subjective effect, the direct study of the stimulus properties of the drug has a predictive value for the abuse liability of the drug. The main objective here is to review evidence of the modulation of the discriminative properties of cocaine and amphetamine by serotonin, γ-aminobutyric acid (GABA) and glutamate that may give us some insight for the treatment of addiction. Introduction Psychostimulants induce excitation of the Central Nervous System (CNS) and a subjective experience of awareness and welfare [1]. Examples of natural psychostimulants are cocaine, nicotine, and caffeine, while synthetic ones are amphetamine (AMPH)-related drugs. AMPHs had been used to improve physical fitness, delay the onset of the sleep period, increase mental activity, reduce fatigue and have therapeutic use in obesity, narcolepsy, depression and attention deficit disorder [1]. However, tolerance and dependence is rapidly generated and dependency and addiction has become a world problem. Besides biological factors, social environment, subject’s personality and personal history are important in the determination of dependency and addiction. Therefore an effective treatment for addiction should include diverse factors and approaches, but basic knowledge of their actions is essential for therapeutics. Neurobiological mechanisms of psychostimulants Psychostimulants induce a direct or indirect increase in dopamine (DA) synaptic and extrasynaptic levels in mesocorticolimbic areas affecting the reward and appetitive system of the brain [2,3]; the indirect stimulation of DA receptors is particularly seen in the nucleus accumbens (NAcc) [4]. Cocaine and AMPH are indirect monoamine agonists with affinity for the reuptake transporters of DA (DAT), but are also able to inhibit the norepinephrine (NET) and serotonin (SERT) transporters increasing the extracellular levels of DA, NE and serotonin (5-HT) [5-8]. AMPH also blocks the transporters located in presynaptic vesicles inducing an increase in cytoplasmatic monoamine concentrations and being able to revert the membrane transport system [9-11]. Both natural reinforces (such as food, water, sex, among others) and psychostimulants increase extracellular levels of DA in NAcc. Other brain areas such as the hypothalamus, the frontal cortex, and the amygdala also contribute to the reward system, but the mesocorticolimbic pathway leading DA fibers from the ventral tegmental area (VTA) to the NAcc are of Non-DA modulation of amphetamine cue 91 particular importance [12]. Additionally, VTA neurons are regulated by a DA feedback loop from the NAcc and this loop is regulated, among several others, by 5-HT, γ-aminobutyric acid (GABA) and glutamate (Glu) projecting pathways [see 13]. There is no direct pharmacological treatment for addiction, since the use of DA antagonists is highly aversive in addicts, besides that their prolonged use leads to the development of parkinsonism [14]. It has been suggested that in order to diminish the reinforcement properties of psychostimulants an alternative strategy may be through non-DAergic afferents that may modulate the DA mesocorticolimbic pathway. The main objective here is to present evidence of the modulation of the discriminative properties of AMPH by 5-HT and Glu that may give us some insight for the treatment of addiction. Drug discrimination as a behavioral model of interoceptive effects of psychostimulants During training in the Drug Discrimination paradigm, reinforcement is correlated to responding to one alternative if drug A is injected, but when a different drug or saline is given, reinforcement is correlated with emitting an alternative response. That is, the drug is able to induce an internal “drug- state” that the subject uses as an indicator of which response will be reinforced. If administered with a similar drug, we expect subjects will respond in a similar way to the training drug, based on the assumption that similar drugs induce similar drug-states [15]. After training, during generalization or a substitution test, a different drug is given, while in combination tests another drug is given in conjunction with the training drug [16,17]. In has been described that AMPH is able to induce stimulus control in both humans [18] and animals [19,20]. Usually rats are trained in a discrimination test between 1 mg/kg of d-AMPH and saline in a two-lever operant task (Fig. 1A, C) or using a Conditioned Taste Aversion (CTA) procedure (Fig. 1B, D). Figure 1A presents data from our group. At the beginning of training, rats emitted random responses to either lever under both d-AMPH or saline, but as training progressed, and by the end of training, they emitted responses only to the d-AMPH lever when injected with d-AMPH and responded to the saline-lever under saline sessions. After training, different doses of d-AMPH were evaluated in generalization sessions. As shown on Figure 1C, after 0.1 mg/kg of d-AMPH, animals responded as if saline was injected, but as dose increased (up to 3 mg/kg) the preference for the d-AMPH-correlated lever increased, showing that d-AMPH induced a dose-dependent stimulus control. 92 David N. Velázquez-Martínez et al. Figure 1. A: Acquisition of d-amphetamine-saline discrimination using an operant procedure. Data are means ± SEM of 8-10 rats. A one-way ANOVA revealed significant differences (F (5,40)=48.409 p < 0.001) and the Duncan test confirmed significant (p < 0.01) differences for the last 5 sessions from each condition. C: Generalization test to different doses of amphetamine (Drug: D; Saline: S). A one- way ANOVA revealed significant differences (F(5,65)=75.878 p<0.001) and the Duncan test confirmed that performance after 0.1 and 0.3 mg/kg had significant (p<0.01) differences to drug-training sessions. B: Acquisition of d-amphetamine- saline discrimination using CTA procedure. A one-Way ANOVA revealed significant (F(5,35)=11.753 p<0.001) differences and the Duncan test confirmed that the last 3 sessions from each condition attained significant (p<0.01) differences. D: Results of substitution tests with amphetamine in the CTA procedure. A one-way ANOVA revealed significant (F(4,28)=12.566 p < 0.001) differences and the Duncan test confirmed significant (p<0.01) differences from 0.1 and 0.3 mg/kg performances to drug training sessions. Saccharin preference (discrimination index) was calculated according to the formula A/(A+B), where A is saccharin intake and B water intake. With this formula, a value close to 0 indicates a strong aversion to saccharin, while a value close to 1 indicates strong preference for saccharin. An alternative method of training is the use the CTA procedure. Water deprived rats are given 1 mg/kg of d-AMPH, then given access to a saccharin solution and, immediately after access, they are injected with LiCl to induce toxicosis. On alternating days, the same rats have access to saccharin Non-DA modulation of amphetamine cue 93 solution followed by administration of saline. In a few sessions rats learn to discriminate their pharmacological state and began to avoid saccharin after drug injections but showed an increased preference for saccharin when injected with saline. It has been shown that diverse drugs such as phencyclidine [21,22], 5-HT agonists [23], morphine [24], indorenate [25] and AMPH [20], among others, are able to induce stimulus control with the CTA procedure. Figure 1B shows data from our laboratory, on the acquisition of d-AMPH (1 mg/kg) versus saline discrimination using the CTA procedure [26]. d-AMPH is able to induce discriminative control over saccharin intake in a few trials. When the administration of d-AMPH preceded saccharin-LiCl pairing, the subjects decreased their saccharin intake, while saccharin intake increased after saline administration. Once rats learned this discrimination, different doses of d-AMPH were evaluated in generalization tests (Fig. 1C). It was observed that different doses of d-AMPH produced a dose-dependent stimulus control. 5-HT modulation of psychostimulants Observations that 5-HT neurons