Rodrigo A. España and Sara R. Jones
Wake Forest University School of Medicine, Medical Center Blvd, Winston Salem, NC 27157



Over the past 20 years an extensive literature has focused on elucidating the neural correlates responsible for the reinforcing effects of cocaine. This work has resulted in the central tenet that the reinforcing properties of cocaine are associated with cocaine’s ability to block the dopamine transporter. Despite the general acceptance of this hypothesis, there remains a debate as to whether blockade of the dopamine transporter is, in fact, directly related to the rewarding properties of cocaine.
In the article entitled “Cocaine serves as a peripheral interoceptive conditioned stimulus for central glutamate and dopamine release” published in PLOS One, volume 3 issue 8, Wise et al. (2008) investigate the extent to which peripheral actions of cocaine participate in the reinforcing properties of this drug. Specifically, they examine the ability of a quaternary cocaine analogue, cocaine methiodide, which does not cross the blood-brain barrier, to elicit glutamate and dopamine release within the ventral tegmental area (VTA). In addition to several other findings, Wise and colleagues (2008) show convincingly that in cocaine-experienced, but not naïve rats, intraperitoneal (i.p.) injections of cocaine-methiodide increased both glutamate and dopamine levels in the VTA.
Based on their results, the authors conclude that in naïve rats, cocaine does not exert its initial effects via direct actions on the dopamine transporter, but rather via unknown peripheral actions that produce glutamatergic excitation of the VTA and lead to dopamine release. This interpretation conflicts with our previously published work indicating that a 2 sec i.v. bolus of cocaine (1.5 mg/kg) inhibits dopamine uptake within 5 sec of the injection (Mateo et al. 2004). Wise and colleagues (2008) acknowledge this discrepancy, yet question the validity of the interpretations of our data. However, their attempt to re-interpret our data as increased dopamine release rather than inhibited uptake is hampered by inaccurate assumptions regarding the methods used for enzyme kinetic analysis.
1) Regarding the latency for observable extracellular increases in dopamine after i.v. injection of cocaine, Wise and colleagues (2008) state that “This very short latency effect of cocaine HCL is not detected in freely moving animals without prior cocaine experience”. Despite this comment, rapid increases in extracellular dopamine are readily measured in naïve, freely moving rats, sometimes within 3 sec after the beginning of an i.v. cocaine infusion (see Aragona et al. 2008, Figure 4A; Wightman et al. 2007, Figure 3D; Heien et al. 2005, Figure 4).
2) Wise and colleagues (2008) also state that the interpretation of the data in Mateo et al. (2004) “…is based on visual inspection of the decay functions.” This statement is inaccurate. The data analysis, consistent with many others in the voltammetry field, utilizes a computational assessment of transporter-mediated uptake kinetics using a Michaelis–Menten-based model (Wu et al. 2001).
3) In the Wise et al., 2008 paper, it is suggested that the analysis of cocaine effects on dopamine uptake is based on a portion of the dopamine overflow curves where dopamine cannot be differentiated from electrical noise. Using kinetic modeling, both dopamine release parameters and uptake parameters such as Vmax, (related to the number and turnover rate of the DAT and unaffected by cocaine) and Km (inversely related to the affinity of the transporter for dopamine and increased in the presence of cocaine) are empirically derived and consistently fit the curves with r2 values greater than 0.9. Uptake parameters are obtained from analysis of the entire dopamine overflow curve, rising, peak, and falling portions, and not only from the potentially noisy portion near the baseline that Wise and colleagues (2008) describe. In fact, the data in Mateo et al. (2004) and España et al. (2008) is not particularly noisy (≤ 0.25 nA noise). Using the modeling program, we are able to consistently detect changes in dopamine overflow and inhibition of dopamine uptake following application of cocaine in both in vitro and in vivo preparations.
4) Wise and colleagues (2008) report that if “the upper portions of the peaks are superimposed… there is no apparent difference in the rate of dopamine clearance” (see Mateo et al., 2004, Fig 1 and España et al., 2008, Fig 1). This is correct given that the “upper portions of the peaks” are dominated by Vmax kinetics which are not altered by cocaine. Thus, the contribution of cocaine-induced Km changes in that portion of the curve is minimal and therefore difficult to detect by visual inspection, as Wise and colleagues (2008) attempt to do.
One final consideration involves our recent publication (España et al., 2008) which extends the Mateo et al. (2004) observations by testing multiple doses of cocaine (0.75, 1.5 and 3.0 mg/kg), cocaine-methiodide (1.97 mg/kg), and examining the effects of another dopamine uptake inhibitor, GBR-12909 (0.75, 1.5, and 3.0 mg/kg). In these studies, it was confirmed that a 2 sec i.v. bolus of cocaine, at all doses examined, significantly inhibited dopamine uptake within 5 sec of injection. A similar effect was also observed following injections of GBR-12909. Most importantly, however, when the tested effects of cocaine-methiodide were tested, no changes in dopamine uptake or peak height were observed. Taken together, these observations demonstrate unambiguously that the central dopamine uptake inhibiting effects of cocaine occur within the time frame of the rapid behavioral effects observed following i.v. drug delivery.
The observations presented in Wise et al., 2008 are important and interesting, and contribute to our understanding of the addiction process. Nevertheless, we are confident that our published data, which uses voltammetry techniques designed to provide information about dopamine transporter dynamics with a high temporal and spatial resolution, are better suited to make claims regarding the temporal profile of cocaine actions on the dopamine transporter. Consequently, we believe that the data presented by Wise and colleagues (2008) does not address the issue of how quickly cocaine can interact with the dopamine transporter.

References

1. Aragona BJ, Cleaveland NA, Stuber GD, Day JJ, Carelli RM, Wightman RM (2008). Preferential enhancement of dopamine transmission within the nucleus accumbens shell by cocaine is attributable to a direct increase in phasic dopamine release events. J Neurosci 28: 8821-8831.
2. España RA, Roberts DC, Jones SR (2008). Short-acting cocaine and long-acting GBR-12909 both elicit rapid dopamine uptake inhibition following intravenous delivery. Neuroscience 155: 250-257.
3. Heien ML, Khan AS, Ariansen JL, Cheer JF, Phillips PE, Wassum KM, et al. (2005). Real-time measurement of dopamine fluctuations after cocaine in the brain of behaving rats. Proc Natl Acad Sci U S A 102: 10023-10028.
4. Mateo Y, Budygin EA, Morgan D, Roberts DC, Jones SR (2004). Fast onset of dopamine uptake inhibition by intravenous cocaine. Eur J Neurosci 20: 2838-2842.
5. Wightman RM, Heien ML, Wassum KM, Sombers LA, Aragona BJ, Khan AS, et al. (2007). Dopamine release is heterogeneous within microenvironments of the rat nucleus accumbens. Eur J Neurosci 26: 2046-2054.
6. Wise RA, Wang B, You ZB (2008). Cocaine serves as a peripheral interoceptive conditioned stimulus for central glutamate and dopamine release. PLoS ONE 3: e2846.
7. Wu Q, Reith ME, Wightman RM, Kawagoe KT, Garris PA (2001). Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry. J Neurosci Methods 112: 119-133.