These results demonstrate that the compensatory changes in DAT sensitivity to cocaine produced by self-administration are influenced by both the temporal pattern of cocaine consumption and the total amount of drug intake. When there was a sustained consumption pattern and intake was high (LgA), tolerance to cocaine’s effects at the DAT developed. Conversely, when intake was low and patterns of consumption were intermittent (IntA) sensitization of the DAT to cocaine occurred. Differences were also observed in baseline (pre-drug) DA measures, wherein IntA cocaine administration resulted in increased V max and stimulated DA release, and LgA and ShA did not produce any changes. This is the first comparison of the effects ShA and LgA on cocaine potency directly at the DAT. Previous work showed increased measures of drug seeking, motivation to administer cocaine, and craving in LgA vs ShA animals (Koob, 1996; Koob and Le Moal, 1997; Orio et al, 2009; Patterson and Markou, 2003); however, these measures are probably not due to the pharmacological effects of cocaine at the DAT, which we demonstrate are reduced. Further, we found that IntA cocaine self-administration produced a sensitized cocaine response at the DAT, demonstrating that the pattern of self-administration has an integral role in the compensatory effects of the DAT to cocaine effects following extended use.

Here we show that LgA results in reduced cocaine potency, indicative of a neurochemical tolerance to cocaine at the DAT, in comparison to both ShA and naive control animals. The decreased cocaine potency following LgA is consistent with previous reports demonstrating similar changes following a number of different extended-access paradigms. In our hands and others, LgA results in neurobiological compensations characterized by decreased cocaine effects and blunted DA system function in the NAc (Hurd et al, 1989; Meil et al, 1995; Macey et al, 2004; Ferris et al, 2011, 2012; Calipari et al, 2012; but see Hooks et al, 1994; Zapata et al, 2003). Also, immediately following a 21-day LgA cocaine self-administration while cocaine is still present, there are decreases in functional activity in the ventral and dorsal striatum, as well as a number of other brain regions, indicating that cocaine is less efficacious at activating these circuits (Macey et al, 2004). Previously, we examined a number of self-administration protocols, manipulating both frequency and duration of access, and regardless of time, animals given continuous cocaine access exhibited robust behavioral and neurochemical tolerance. Animals that administered cocaine for only 1 day of 40 injections and animals that underwent extended-access self-administration for 5 days (40 inj. per day) both exhibited the same effects as LgA self-administration (Ferris et al, 2011, 2012; Calipari et al, 2012). All of the previously tested procedures shared the property that animals could administer large amounts of cocaine for the duration of the session. Regardless of duration of testing, each of these groups that maintained high rates of responding over 5- to 6-h sessions showed neurochemical tolerance at the DAT when tested 24 h after the last self-administration session (Ferris et al, 2011, 2012; Calipari et al, 2012). Although ShA animals have a similar pattern of self-administration and can administer large amounts of drug, they can only do so for 2 h. This results in significantly less drug consumption, leading to no change in cocaine potency. This suggests that total intake within sessions, and not pattern, is what is driving the tolerance to the pharmacological effects of cocaine at the DAT.

In contrast, here we show that the IntA cocaine self-administration procedure enhances cocaine potency, indicative of a sensitized neurochemical response to cocaine at the DAT relative to both ShA and naive controls. The IntA results are opposite to the well-documented decrease in cocaine potency after LgA shown here and in previous research (Hurd et al, 1989; Ferris et al, 2011, 2012; Calipari et al, 2012). Although behavioral sensitization to cocaine has been reported previously (Schmidt and Pierce, 2010), to our knowledge, this is the first demonstration of a sensitized cocaine response directly at the DAT following cocaine self-administration. It is possible that the increased potency may promote increased rewarding and reinforcing effects of cocaine as well as an increase in motivation to self-administer the drug, potentially leading to greater risk of compulsive or addictive-like cocaine intake. Indeed, Zimmer et al (2012) showed that motivation to self-administer cocaine is significantly increased following IntA as compared with LgA and ShA cocaine self-administration.

The marked differences between ShA and IntA cannot be attributed to intake, as the animals administer similar amounts of drug over sessions. In addition, the session length was different between IntA (360 min) and ShA (120 min); however, we do not think that effects are due to session length differences, because LgA and IntA both had 360-min sessions. These groups exhibited opposite effects on DAT pharmacology; therefore, it seems that session time is not a contributing factor to the neurochemical effects. The increased effects of cocaine at the DAT seem to be due to the intermittent intake pattern of cocaine that likely leads to intermittent cocaine–DAT interactions. The LgA and IntA groups differ substantially in the total cocaine self-administered over the course of the current experiment; however, we would predict that LgA would still produce tolerance when total intake is limited to equal the IntA condition. In fact, we have shown that a single day of LgA (60 mg total), which approximates the IntA group’s total intake of cocaine (70 mg total), produced robust tolerance (Ferris et al, 2011). Thus, the sensitized response was only seen in the group with intermittent cocaine access (IntA) and not in the matched intake groups that experienced sustained drug levels examined here (ie, ShA) and in previous work (ie LgA; Ferris et al, 2011).

Cocaine has been demonstrated to result in changes in DA release that are dependent on vesicular exocytosis (Venton et al, 2006); therefore, we wanted to confirm that changes in this process were not driving the observed changes in cocaine effects. DA release over the concentration–response curve for the compound was not changed in any of the groups, suggesting that it was cocaine-induced DAT inhibition, not increased/decreased DA release, causing the change in potency following IntA/LgA. It is possible that allosteric modifications to the DAT protein itself are mediating the decreased potency following LgA and the increased potency following IntA. Indeed, the DAT has been shown to undergo glycosylation and phosphorylation (Foster et al, 2008; Chen et al, 2009). Therefore, it is possible that changes in the interaction of the DAT protein with downstream effectors responsible for these post-translational modifications are altered following self-administration in a way that changes the ability of ligands to bind to the DAT and inhibit the clearance of DA. In addition, the DAT has been shown to form oligomer complexes (Hastrup et al, 2003) and be modulated by receptors such as the D2 autoreceptor (Chen et al, 2013), which are two additional mechanisms that could have a role in the observed changes in cocaine potency.

There appears to be many neurochemical adaptations that occur following cocaine exposure, and how they interact to influence cocaine-induced DA overflow and behavior is as yet unclear. In addition to the sensitized effect at the DAT reported here, other systems appear to have a role in DA system sensitization, including glutamate (for a review, see Vanderschuren and Kalivas, 2000), GABA (for a review, see Steketee, 2005; Filip et al, 2006), and serotonin (Neumaier et al, 2002; Filip et al, 2010). Owing to the convergence of these systems and factors, which lead to the expression of behavioral sensitization, it remains to be determined whether increased DA overflow and behavioral effects would be seen after IntA self-administration. Further, demonstrations of sensitized DA overflow are more robust after 1–2 weeks of drug abstinence (Kalivas and Duffy, 1990, 1993); thus, it is possible that the changes observed here could be more robust following acute and prolonged withdrawal.

Here we demonstrate that whether tolerance or sensitization develops to specific effects of cocaine appears to depend on access conditions and the intake or pattern of administration, respectively. Pattern, total dose, and abstinence periods need to be taken into account when modeling the behavioral and neurochemical processes involved in addiction. Cocaine sensitization has been difficult to demonstrate in humans, suggesting that either sensitization does not occur in humans or that it occurs during a part of the addiction process that has not been studied extensively (Leyton and Vezina, 2013). It is possible that early in the drug abuse process administration of cocaine is intermittent, leading to a sensitized cocaine response that facilitates continued drug use, while after long-term use there is tolerance to acute drug effects, an effect that has been reported in human subjects (Mendelson et al, 1998; Reed et al, 2009). It is suggested that in humans, drug use may be based on limited availability, which may lead individuals to administer drug in an intermittent pattern (Ahmed et al, 2013). However, it has also been suggested that LgA self-administration is a model of the escalation that is reported with long-term abuse of the compound in humans (Dackis and O’Brien, 2001). Here we demonstrate tolerance to the neurochemical effects of cocaine following LgA, suggesting that LgA may result in neurochemical changes consistent with human studies. In our previous work and currently accepted cocaine self-administration models, much emphasis has been placed on maximizing an animal’s intake, with the thought that more intake results in greater neurochemical effects. Here we demonstrate that greater intake is not necessary to produce robust neurochemical effects and, in fact, continuous high intake early in the animal’s self-administration history may produce marked tolerance to the drug. Thus, we highlight the importance of mimicking human patterns of administration in rodent models, as compensatory mechanisms associated with drug exposure are not only dependent on the drug and total intake but also on the pattern in which the drug is taken.