The present study identified a critical role for GLP-1 receptors in cocaine-seeking behavior. Specifically, we showed that systemically administered exendin-4 attenuated the reinstatement of drug-seeking behavior elicited by both cocaine and conditioned cues. Importantly, we identified doses of exendin-4 that selectively reduced cocaine seeking and did not affect food intake or body weight. Moreover, these behaviorally relevant doses of exendin-4 do not produce nausea/malaise-like adverse effects in rodents [21] further highlighting the selectivity of this behavioral response at these doses. We also showed that peripheral exendin-4 was able to cross the blood brain barrier and putatively bind receptors expressed on neurons and astrocytes in the VTA. Indeed, the suppressive effects of peripheral exendin-4 on cocaine seeking were due, in part, to activation of central GLP-1 receptors in the VTA. Moreover, cocaine self-administration and subsequent extinction may reduce endogenous GLP-1 expression in the brain, as evidenced by a significant decrease in PPG mRNA expression in the NTS following 7 days of extinction, a time point that coincides with drug-seeking behavior [23, 24]. Since the NTS sends direct monosynaptic GLP-1-expressing efferent projections to the VTA [22], these results suggest that decreased endogenous GLP-1 release in midbrain areas including the VTA may facilitate cocaine seeking. This hypothesis is supported by our data indicating that infusions of exendin-4 directly into the VTA were sufficient to attenuate the reinstatement of cocaine-seeking behavior. Collectively, these findings identify a novel neural mechanism focused on central GLP-1 receptors that could be targeted to prevent cocaine craving-induced relapse.

GLP-1 receptor agonists and cocaine addiction

A growing literature indicates that GLP-1 receptors play an important role in addiction-like behaviors and that systemic administration of GLP-1 receptor agonists reduces the rewarding and reinforcing effects of drugs of abuse [9,10,, 11]). For example, peripheral administration of the GLP-1 receptor agonist exendin-4 has been shown to attenuate cocaine-induced conditioned place preference (CPP) and cocaine self-administration in mice [25,26,27]. Our findings expand on these studies and identified an important role for GLP-1 receptors in an animal model of cocaine craving-induced relapse.

The present study also identified doses of peripheral exendin-4 that selectively attenuated addiction-like behaviors and did not produce adverse effects commonly associated with higher doses in rodents. Indeed, a significant limitation to interpreting results from previous studies of peripheral exendin-4 in preclinical models of drug addiction is the exceedingly high doses of exendin-4 tested. While previous studies have laid the foundation for our understanding of the role of GLP-1 in drug addiction, the doses of peripheral exendin-4 used to pretreat mice in these studies ranged from 3.0 to 100.0 µg/kg [25,26,27]. Doses of exendin-4 as low as 0.25 µg/kg have been shown to produce malaise-like adverse effects in rats, which can confound subsequent behavioral responses [20, 21]. Moreover, nausea and malaise are common adverse effects associated with high doses of peripherally administered GLP-1 receptor agonists in humans [28]. Since the doses of exendin-4 shown to reduce cocaine CPP and self-administration are likely producing malaise-like effects in mice, it is impossible to draw firm conclusions about the role of GLP-1 receptors in addiction-like behaviors from these previous studies. In contrast, we identified systemic doses of exendin-4 as low as 0.1 and 0.2 µg/kg that were sufficient to attenuate cocaine seeking in rats. Importantly, these doses were subthreshold for effects on feeding behavior in cocaine-experienced rats (present findings) and do not produce malaise-like effects in rats [20, 21]. The translational implications of these findings are profound in that they support potential therapeutic approaches toward the specific use of GLP-1 receptor agonists for the treatment of cocaine craving and relapse.

In addition to causing nausea and malaise-like effects, peripheral administration of exendin-4 at doses as low as 0.25 µg/kg have been shown to significantly decrease food intake and body weight in rats [20, 21]. These findings further confound prior studies that examined the effects of exendin-4 on drug-mediated behaviors because they indicate that doses of exendin-4 greater than 0.25 µg/kg reduce motivated behaviors generally and not drug reinforcement specifically. Furthermore, reductions in ad libitum food intake and body weight limit the therapeutic potential of GLP-1 receptor agonists for use in human addicts. Our novel findings clearly identified doses of exendin-4 (0.1 and 0.2 µg/kg) that selectively attenuated cocaine seeking and did not affect ad libitum food intake, meal patterns or body weight in cocaine-experienced rats. These data support selective effects of a peripherally administered GLP-1 receptor agonist on drug reinforcement. Consistent with our peripheral exendin-4 dose–response study, we also showed that exendin-4 reduced cocaine seeking when infused directly into the VTA at a dose (0.05 µg) that does not affect ad libitum chow intake or promote malaise-like effects in rats [22, 29, 30]. Moreover, intra-VTA infusion of exendin-4 did not alter sucrose seeking further supporting the selectivity of lower doses to reduce cocaine seeking. There is some evidence that administration of 0.05 µg exendin-4 into the VTA reduces operant responding for palatable food [30]. However, these effects are transient and do not persist with more prolonged operant sessions [17, 30]. Thus, the present study showed that a GLP-1 receptor agonist reduced cocaine seeking at doses not associated with common adverse effects in rodents.

Central GLP-1 receptors and cocaine seeking

Mutant mice lacking GLP-1 receptors have enhanced cocaine CPP compared to wild-type controls [31]. These results suggest that activation of central GLP-1 receptors may function to reduce the rewarding effects of cocaine. Consistent with this hypothesis, we recently showed that direct activation of GLP-1 receptors in the VTA is sufficient to reduce cocaine self-administration in rats [17]. We expanded upon these studies here and showed that the intake suppressive effects of peripheral exendin-4 on cocaine seeking were due to activation of GLP-1 receptors in the VTA and that direct activation of VTA GLP-1 receptors was sufficient to reduce cocaine seeking.

The current data clearly support a role for VTA GLP-1 receptors in cocaine seeking, but it is possible and in fact likely that GLP-1 receptors expressed in other brain regions also play an important role in cocaine seeking. GLP-1 receptors are expressed ubiquitously throughout the rodent brain [15] including nuclei known to regulate drug-seeking behavior. Viral-mediated re-expression of GLP-1 receptors in the lateral septum of constitutive GLP-1 receptor knockout mice attenuates cocaine CPP [31] indicating that the rewarding effects of cocaine are mediated, in part, by enhanced GLP-1 signaling in this nucleus. GLP-1 receptors are also expressed in the nucleus accumbens, hippocampus, habenula and amygdala [15, 32] and future studies are required to define the exact role of GLP-1 signaling in these nuclei in addiction-like behaviors.

The exact mechanism(s) by which GLP-1 receptor activation in the VTA attenuates cocaine seeking are unknown. Doses of peripheral exendin-4 that reduce cocaine CPP and self-administration attenuate cocaine-mediated increases in extracellular dopamine in the nucleus accumbens [26, 27]. Since increased dopamine signaling in the accumbens promotes cocaine seeking [16, 33, 34], these results suggest that reduced cocaine seeking following peripheral exendin-4 administration may involve decreased extracellular dopamine levels in the nucleus accumbens. The present findings also indicated that peripheral exendin-4 bound putative GLP-1 receptors expressed on neurons and astrocytes in the VTA. However, it is not clear how exactly GLP-1 receptor activation in the VTA may reduce dopamine cell firing. GLP-1 receptors are predominantly Gs-coupled receptors (although they can activate Gq and Gi proteins as well) that are expressed on both pre- and post-synaptic sites in the VTA [4, 35]. While the exact phenotypes of GLP-1 receptor-expressing cells in the VTA must be further characterized, there is some evidence that GLP-1 receptor activation increases glutamate release in the VTA of drug-naïve rats [29]. GLP-1 receptor activation also enhances GABAA receptor-mediated currents in the drug-naïve brain [36,37,38], suggesting a possible GABA-mediated mechanism in the VTA that may underlie the suppressive effects of exendin-4 on cocaine seeking. Furthermore, a recent study showed that GLP-1 receptor agonists increase activation of astrocytes in the hindbrain [19]. It is intriguing to think that increased activation of GLP-1 receptors expressed on astrocytes in the VTA may regulate glutamate homeostasis and drug-seeking behavior. The effects of GLP-1 receptor activation on dopamine cell firing in the VTA are likely to be complex and may differ between drug-naïve and cocaine-experienced brains. Therefore, future studies are required to determine the cellular and neurophysiological mechanisms through which activation of GLP-1 receptors in the VTA suppresses cocaine-seeking behavior.

Extinction following cocaine self-administration dynamically regulated endogenous PPG mRNA expression in the NTS

To date no studies have examined the effects of cocaine self-administration and subsequent extinction on endogenous central GLP-1 signaling. We have previously reported that cocaine activates GLP-1-expressing neurons in the NTS and that increased VTA GLP-1 signaling in the brain may function as a homeostatic response to reduce cocaine self-administration [17]. To expand upon these data, we examined the effects of 1 day and 7 days of extinction following 21 days of cocaine self-administration on GLP-1 receptor mRNA expression in the VTA. We also quantified expression of PPG mRNA in the NTS. PPG is the protein precursor to GLP-1 in NTS neurons [4]. Using quantitative real-time PCR, we found no effects of extinction on VTA GLP-1 receptor mRNA expression. In contrast, we found a non-significant trend (P = 0.09) toward increased PPG mRNA expression in the NTS during acute withdrawal from cocaine self-administration. These results are consistent with our previous study showing that cocaine increases activation of PPG-expressing neurons in the NTS [17] and together suggest that increased endogenous PPG mRNA expression in the NTS may represent a homeostatic compensatory response to cocaine self-administration that serves to reduce further drug taking. Interestingly, we observed a significant decrease in NTS PPG mRNA expression following 7 days of extinction, a time point that coincides with maximal drug-seeking behavior [23, 24]. Since activation of central GLP-1 receptors may serve as a ‘brake’ on cocaine self-administration, this decrease in endogenous PPG mRNA expression in the caudal brain may facilitate drug seeking during extinction. Support for this hypothesis comes from the present study showing that direct activation of VTA GLP-1 receptors was sufficient to reduce cocaine priming-induced reinstatement of drug-seeking behavior. Our data are consistent with previous literature demonstrating that reduced PPG mRNA expression in the NTS increases the rewarding value of food. Viral-mediated knockdown of PPG mRNA expression in the NTS produces hyperphagia and exacerbates high fat diet-induced obesity [39]. Due to shared mechanisms mediating the rewarding properties of food and drugs of abuse [40, 41], our data further support a role for central GLP-1 signaling in regulating motivated behaviors.