- Email: [email protected]
Effects of stress and aversion on dopamine neurons: Implications for addiction
Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Contents lists available at ScienceDirect
Neuroscience and Biobehavioral Reviews journal homepage: www.elsevier.com/locate/neubiorev
Review
Effects of stress and aversion on dopamine neurons: Implications for addiction Mark A. Ungless a , Emanuela Argilli b , Antonello Bonci b,∗ a b
Medical Research Council Clinical Sciences Centre, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK Ernest Gallo Clinic and Research Center, Department of Neurology, University of California, San Francisco, Emeryville, CA 94608, USA
a r t i c l e Keywords: Midbrain Aversive Addiction
i n f o
a b s t r a c t Stress plays a key role in modulating the development and expression of addictive behavior, and is a major cause of relapse following periods of abstinence. In this review we focus our attention on recent advances made in understanding how stress, aversive events, and drugs of abuse, cocaine in particular, interact directly with dopamine neurons in the ventral tegmental area, and how these interactions may be involved in stress-induced relapse. We start by outlining how dopamine neurons respond to aversive stimuli and stress, particularly in terms of firing activity and modulation of excitatory synaptic inputs. We then discuss some of the cellular mechanisms underlying the effects of cocaine on dopamine neurons, again with a selective focus on synaptic plasticity. Finally, we examine how the effects of stress and cocaine interact and how these cellular mechanisms in ventral tegmental area dopamine neurons may be engaged in stress-induced relapse. © 2010 Published by Elsevier Ltd.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute effects of aversive events on dopamine neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic effects of aversive events on dopamine neuron activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress-induced synaptic plasticity in dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cocaine-induced synaptic plasticity in dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress-induced relapse and dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Aversive events, and activation of stress systems, play key roles in modulating addictive behavior (Sinha, 2008). For example, early life stressors are associated with increased risk of substance abuse. Perhaps of most therapeutic significance is the observation that stressful events are a major cause of relapse in abstinent individuals. Therefore, understanding how aversive events and stress interact with the neural circuits underlying addiction is an important step in developing new therapeutic targets. An extensive literature has now emerged examining the neural circuits and cellular mechanisms through which stress modulates addictive behavior (Koob, 2008). In this review we do not attempt to provide a comprehensive overview of this large literature; instead we wish
∗ Corresponding author. Tel.: +1 510 985 3890; fax: +1 510 985 3101. E-mail address: [email protected] (A. Bonci). 0149-7634/$ – see front matter © 2010 Published by Elsevier Ltd. doi:10.1016/j.neubiorev.2010.04.006
151 152 153 154 155 155 155 156 156
to focus on recent progress concerning the direct effects of aversive events, stress, and drugs of abuse on midbrain dopamine neurons, and how these interactions might be involved in stress-induced relapse. It is well established that mesolimbic dopamine plays a central role in animal models of cue- and drug-induced relapse (Shaham et al., 2003), but it has been more difficult to determine a role for dopamine in stress-induced relapse. At first glance this may not appear surprising because dopamine is commonly associated with reward processing; however, it has long been known that aversive, stressful stimuli increase dopamine release in the striatum, as measured using microdialysis over minutes and hours (for a review see Joseph et al., 2003). Recent evidence suggests that stressinduced dopamine release does indeed play an important role in stress-induced relapse (Wang et al., 2005), and these findings raise some broad questions. For example, if dopamine neurons encode reward-related information, why does dopamine increase in projection targets? And, how do aversive events modulate activity in dopamine neurons? Further, how do these mechanisms interact
152
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 1. (A–C) An identified dopamine neuron that showed an inhibition at footshock onset, followed by an excitation at termination of the stimulus. (D–F) An identified dopamine neuron that showed a strong excitation at the onset of the footshocks. These neurons were located in more ventral parts of the VTA. Data are from Brischoux et al. (2009).
with drug-induced plasticity in dopamine neurons? In this selective review we present recent evidence that sheds light on these questions. We first provide an overview of how dopamine neurons respond acutely to aversive events in terms of firing activity and dopamine release. We then examine the longer-term, chronic effects of aversive events on dopamine neuron firing and dopamine release. We continue by exploring the cellular mechanisms that may underlie these chronic effects. Following this, we will outline how drugs of abuse, cocaine in particular, induce long-term synaptic plasticity in dopamine neurons. Lastly, we will discuss how these cellular processes engaged by aversive events, stress and cocaine interact and may be involved in stress-induced relapse during periods of abstinence. 2. Acute effects of aversive events on dopamine neuron It is well established that midbrain dopamine neurons play a role in processing reward-related information (Wise, 2004). In particular it has been suggested that dopamine neurons encode a reward-prediction-error signal (Schultz, 1998); they are excited by unexpected rewards, or reward-predicting stimuli. They are unresponsive to expected rewards, and they are inhibited by the omission of an expected reward. Although the literature concerning how these neurons respond to reward is relatively clear, how these neurons respond to stressful, aversive stimuli has been more difficult to establish. Many studies report that the majority of putative dopamine neurons are inhibited by aversive stimuli, which is consistent with their role in reward (e.g., Schultz and Romo, 1987; Mantz et al., 1989; Mirenowicz and Schultz, 1996; Guarraci and Kapp, 1999). However, a minority of putative dopamine neurons are excited by aversive stimuli. In these studies, dopamine neurons are identified as putatively dopaminergic on the basis of their electrophysiological characteristics. It appears, however, that not all ventral tegmental area (VTA) neurons with these characteristics are dopaminergic (Margolis et al., 2006). Indeed, when combined with single-cell labeling, we found that putative dopamine neurons that
were excited by a footpinch were in fact not dopaminergic (Ungless et al., 2004). Identified dopamine neurons were uniformly inhibited by the footpinch. Although this study was conducted with aneasthetised animals, to allow reliable single-cell labelling, responses seen in the VTA to stimuli such as footpinches and footshocks appear to be unaffected by aneasthesia (for discussion see Ungless et al., 2004; Brischoux et al., 2009). We therefore concluded that previous studies may have recorded from a mixture of dopaminergic and non-dopaminergic neurons. It is worth noting here also that although we believe that previous studies may have included some non-dopaminergic neurons, it seems likely that the majority of neurons recorded were dopaminergic and consequently the conclusions drawn regarding reward-related responses are still valid particularly where responses of all neurons sampled are relatively uniform (as appears to be the case for rewards in contrast to aversive stimuli). The finding that dopamine neurons are inhibited by aversive stimuli is consistent with reward theories of dopamine neuron function and with studies of dopamine release using fast-scan cyclic voltammetry (Roitman et al., 2008). However, this view is difficult to reconcile with a number of other findings (for discussion see Ungless, 2004). For example, it is well established that aversive events (such as footshocks) can increase dopamine release, as measured using microdialysis over several minutes (for a review see Joseph et al., 2003). How can this be explained? One possibility is that a subgroup of dopamine neurons are in fact excited by stressful, aversive stimuli, but that we did not record from them in our previous study. Indeed, we, like many others, target more dorsal regions of the ventral tegmental area (VTA). When we subsequently explored more ventral parts of the VTA we found a discrete population of dopamine neurons that are strongly excited by footshocks (Brischoux et al., 2009; Fig. 1). There is considerable behavioral evidence that the offset of an aversive stimulus can act as a reward (e.g., Tanimoto et al., 2004) and might therefore be expected to excite dopamine neurons (Daw et al., 2002). Consistent with this, of the neurons that were inhibited
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 2. Tonic activity in dopamine neurons. (A) Tonic, low-bursting, single-spike activity in a dopamine neuron that was inhibited by footshocks. (B) Bursting activity in a neuron that was excited by the footshocks (upper). An expanded view of five bursts (left lower) and a single burst (right lower). Data are from Brischoux et al. (2009).
by aversive stimuli, around half also showed an excitation at the offset of the stimulus (Fig. 1). That is to say, that during the first 500 ms after removal of the footshocks, many neurons exhibited a phasic excitation above baseline. This offset excitation may also contribute to the increase in dopamine seen in response to aversive stimuli. It appears then that there are distinct subgroups of dopamine neurons that respond differentially to aversive events. One important feature of these responses is that they are extremely rapid, with an onset and duration of around 100 ms. It seems likely, therefore, that fast synaptic inputs are driving these changes in firing activity, rather than the slower effects elicited by stressrelated neurochemicals (see sections below). Recent studies of substantia nigra neurons in primates have further characterized substantia nigra dopamine neuron responses in tasks where both aversive and rewarding stimuli are presented. In these cases some putative dopamine neurons appear to selectively encode rewardprediction error rules, while others are also excited by aversive events (e.g., Joshua et al., 2008; Matsumoto and Hikosaka, 2009). Further, Matsumoto and Hikosaka (2009) found that these saliencyencoding neurons are in more lateral parts of the substantia nigra, whereas the reward encoding neurons are more medial. It should be noted that in the substantia nigra, in contrast to the VTA, the electrophysiological criteria used to identify putative dopamine neurons appear to selectively identify dopamine neurons (Brown et al., 2009). Taken together, these studies suggest that anatomically discrete dopamine neurons exhibit differential, rapid changes in activity in response to aversive stimuli. 3. Chronic effects of aversive events on dopamine neuron activity In the previous section we reviewed the rapid effects of aversive events on dopamine neuron activity. These effects are unlikely to involve activation of the classical stress-related systems, which have slower actions. In this section we review studies examining the long-term, or chronic, effects of aversive events on dopamine neuron activity. Single unit recordings in awake rats during 30 min restraint stress find an increase in firing rate and burst firing in putative VTA dopamine neurons (Anstrom and Woodward, 2005). Dopamine neurons can fire action potentials as single-spikes or bursts. Bursts are traditionally defined as starting with an inter-spike-interval (ISI) of <80 ms and finishing with an ISI >160 ms (Grace and Bunney, 1984; see Fig. 2). Bursts are thought to have important functional
153
consequences on dopamine release, because at these higher firing frequencies the dopamine transporter may become overwhelmed and extracellular dopamine increases supralinearly (Gonon, 1988). This increase in firing rate and bursting was still present 24 h following the restraint stress procedure, and a second exposure did not have any additional effect. The effects of restraint stress on bursting were most prominent in neurons that showed high levels of baseline bursting. This is particularly interesting in light of the fact that the ventral VTA dopamine neurons that were strongly excited by footshocks had higher baseline levels of bursting compared to those that were inhibited (Brischoux et al., 2009). One possibility is that high-bursting dopamine neurons in the ventral VTA are particularly sensitive to the acute and chronic effects of aversive, stressful stimuli. In a further study, Anstrom et al. (2009) examined the effects of social defeat on putative dopaminergic activity (it should be noted, as in the previous section, that although some caution should be observed it is likely that the majority of these neurons are dopaminergic). Rats were placed in the cage of an unfamiliar rat for 5 min, during which time a confrontation occurred. Following this confrontation, putative dopamine neurons exhibited an increase in burst number, but not firing rate. As for the effects of restraint stress, social defeat increased burst firing preferentially in those neurons that had higher baseline levels of bursting. There was also a significant increase in dopamine transients in the striatum following social defeat, consistent with the increased burst firing. These observations are consistent with the suggestion that slow changes in activity of dopamine neurons may encode average rate of punishment (Daw et al., 2002). It will be important to examine how these effects influence rapid, phasic events. These studies of action potential firing in dopamine neurons are consistent with a long-standing literature showing increases in dopamine release in response to stressful stimuli, as measured with microdialysis over periods of several minutes and more indirect measures of dopamine metabolism (Joseph et al., 2003). It has been difficult to reconcile these observations with the more traditional view that dopamine neuron firing is inhibited by aversive, stressful stimuli. However these recent studies suggest that there are three ways in which they can be reconciled. First, some dopamine neurons that are inhibited by aversive stimuli exhibit a phasic excitation at stimulus offset. Second, subgroups of dopamine neurons are phasically excited at the onset of aversive stimuli. And third, it appears that prolonged stress increases firing rates and bursting, and this leads to persistent increases in tonic activity. These changes in phasic and tonic activity no doubt contribute to the changes in dopamine release. One emerging theme is that dopamine neurons should not be viewed as a functionally homogenous group. In terms of responses to aversive stimuli and slower changes in response to prolonged or repeated stress, dopamine neurons in the VTA can be subdivided into at least two groups. One group appears to encode a reward-prediction error rule; a second group (with high baseline burst firing) exhibits phasic responses to aversive stimuli and slow changes in tonic activity. It is less clear how these neurons respond to rewards. One possibility is that they are also excited by unexpected rewards, in which case they may encode a saliency prediction error, but further studies will be required to examine this. It seems likely that these different groups will have distinct projection targets, but exactly how is unclear. Topographic organisation of the VTA suggests that ventral dopamine neurons (that are excited by footshocks) are more likely to project to medial targets (e.g., medial accumbens; Haber et al., 2000; Ikemoto, 2007). Similarly in the SNC there appear to be functionally distinct dopamine neurons (e.g., Joshua et al., 2008; Matsumoto and Hikosaka, 2009) that also exhibit some anatomical segregation (Matsumoto and Hikosaka, 2009), presumably related to projection targets (see Fig. 3).
154
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 3. Diagram illustrating the organisation of dopamine neuron projections in primate. Note, in particular, the medio-lateral topography, whereby medial dopamine neurons target medial regions of striatum and cortex, and more lateral dopamine neurons target more lateral regions. For more discussion and original figure see Haber et al. (2000). Similar medio-lateral topography has been observed in rodents (Ikemoto, 2007).
4. Stress-induced synaptic plasticity in dopamine neurons In this section we review recent progress made in understanding how stress interacts with dopamine neurons at a cellular level. In particular we focus on modulation of synaptic inputs onto dopamine neurons, which play a key role in regulating their activity and show experience-dependent plasticity in a number of situations, most notably in response to drugs of abuse. Saal et al. (2003) found that swim stress induced a form of long-term potentiation in dopamine neurons, similar to that seen in response to drugs of abuse, including cocaine, amphetamine and alcohol (Saal et al., 2003; Ungless et al., 2001). This stress-induced potentiation was blocked by glucocorticoid receptor antagonists, but not D1R antagonists, suggesting a different induction mechanism to cocaine-induced potentiation (Dong et al., 2004; Saal et al., 2003). Glucocorticoids directly modulate NMDAR function in the VTA (Cho and Little, 1999) and this may lead to the long-term potentiation (LTP)-like changes seen in response to swim stress (Saal et al., 2003). Corticotropin releasing factor (CRF), a 41 amino acid peptide, plays an essential role in the activation of the hypothalamicpituitary-adrenal axis by stress. In addition, CRF mediates many of the extrahypothalamic effects of stress, including activation of the dopamine system, which can lead to relapse in animal models of drug taking (for a review see Sarnyai et al., 2001; Shalev et al., 2009). Until recently, however, it was unclear how CRF modulated dopamine neuron activity. The VTA receives CRF inputs (co-released with GABA and glutamate) from the limbic forebrain and the paraventricular nucleus of the hypothalamus (Rodaros et al., 2007; Tagliaferro and Morales, 2008). VTA dopamine neurons express CRF Receptor 1 (CRF-R1) and CRF Receptor 2 (CRF-R2) (Ungless et al., 2003; Van Pett et al., 2000), and about 25% of
VTA dopamine neurons, mostly located in the parabrachial pigmented (PBP) region of the VTA, express the CRF binding protein (CRF-BP; Wang and Morales, 2008). CRF increases action potential firing rate in dopamine neurons (Korotkova et al., 2006; Wanat et al., 2008) via CRF-R1. This effect involves a protein-kinaseC-dependent enhancement of Ih (a hyperpolarization-activated inward current) (Wanat et al., 2008). CRF-R1 activation also facilitates slow, dopamine D2 - and GABAB -receptor-mediated inhibitory synaptic transmission (Beckstead et al., 2009). Both of these CRFR1-dependent effects do not involve CRF-BP. CRF can also induce a slowly developing, but transient, potentiation of NMDAR-mediated synaptic transmission (Ungless et al., 2003). This effect involves CRF-R2 and activation of the proteinkinase-C pathway; interestingly, CRF-BP is required for this effect. This suggests that CRF-BP can play an active role in promoting the effects of CRF, rather than as a buffer, although exactly how it does this is unclear. In addition to fast, excitatory glutamatergic synaptic transmission, dopamine neurons also exhibit slower, inhibitory glutamatergic synaptic transmission, mediated via metabotropic glutamate receptors (mGluRs; Fiorillo and Williams, 1998). CRF can enhance these mGluRs via a CRF-R2-PKA pathway that stimulates release of calcium from intracellular stores (Riegel and Williams, 2008). In this case, the CRF-BP inhibits the effect, presumably by ‘buffering’ CRF. Thus it is clear that CRF can modulate activity in dopamine neurons through a number of different mechanisms. One emerging theme is that stress-related neurochemicals, such as CRF and glucocorticoids, target NMDARs on dopamine neurons. This is particularly striking in light of the evidence to be discussed in the following section that cocaine can have a similar action.
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
5. Cocaine-induced synaptic plasticity in dopamine neurons In the previous sections we have reviewed recent progress in understanding how aversive events, stress, and stress-related neurochemicals directly regulate activity in dopamine neurons. However, to properly understand how aversive events and stress can induce relapse during periods of abstinence from drug use we need to understand what long-term changes drugs such as cocaine induce in this system and how those changes interact with stress. It has become clear that the direct actions of stress on dopamine are mediated, in large part, through excitatory, glutamatergic synaptic inputs. In this section we review recent progress in our understanding of how drugs of abuse, cocaine in particular, also target these synapses. It is striking that cocaine can enhance NMDAR-mediated synaptic transmission in dopamine neurons. This enhancement is slowly developing and transient, in a similar manner to CRF. Moreover, it is dependent upon the activation of D1 -like DA receptors, but is delayed by initial activation of D2 receptors (Schilström et al., 2006). This event seems to be the first necessary step in NMDA-dependent AMPAR LTP lasting for up to 5 days that has been observed just hours after a single or multiple cocaine injection in rats (Borgland et al., 2004; Ungless et al., 2001). Importantly, LTP of VTA glutamatergic synapses is observed after a single exposure to many other drugs of abuse and stress, demonstrating a convergence of cellular responses within the VTA (Saal et al., 2003). In contrast to passive administration of cocaine which lasts for a few days, voluntary selfadministration induces a form of potentiation that lasts for up to 3 months (Chen et al., 2008); this form of plasticity is not confined to drugs of abuse. Following training to associate a cue with a natural reward, these synapses are also transiently potentiated (Stuber et al., 2008). Cocaine-mediated potentiation of glutamatergic synapses involves insertion of GluR2-lacking AMPA receptors (Argilli et al., 2008; Bellone and Lüscher, 2005; Mameli et al., 2007). Importantly, these types of AMPA receptors are calcium permeable, which means they may play a role in establishing longer-term neuroadaptations (for discussion see Carlezon and Nestler, 2002). It is clear then that glutamatergic synaptic transmission onto dopamine neurons is a key site of plasticity, and that both stress and cocaine directly target NMDARs, which in turn can induce longterm potentiation of AMPAR-mediated synaptic transmission. In addition, two recent studies show that cocaine exposure can modify the effects of CRF on dopamine neurons. First, the enhancement of slow, dopamine D2 - and GABAB -receptor-mediated inhibitory synaptic transmission is reduced by repeated exposure to psychostimulants or stress (Beckstead et al., 2009). In contrast, following chronic cocaine, the effects of CRF on NMDARs are both larger and more prolonged (Hahn et al., 2009). In addition, CRF enhances AMPAR-mediated synaptic transmission following chronic cocaine (Hahn et al., 2009). Analysis of spontaneous miniature synaptic events revealed that following chronic cocaine, CRF increased glutamate release and modulated post-synaptic glutamate receptors. Taken together, these findings suggest that previous exposure to cocaine will enhance the excitatory effects, and reduce the inhibitory effects, of CRF on dopamine neurons, and in turn this might be expected to enhance stress-induced activation of the dopamine system and renewed cocaine seeking during periods of abstinence.
6. Stress-induced relapse and dopamine neurons In this section we discuss stress-induced relapse and how the interactions between stress and dopamine neurons discussed in
155
the previous sections may play a role. In terms of treating addiction, relapse is the primary target. Most addicts can stop taking drugs and remain abstinent for brief periods, but many relapse. Stress appears to be a major factor in causing relapse. There are broadly three types of relapse that have been examined in animal models of addiction: cue-induced, context-induced, and stress-induced (for a review see Shaham et al., 2003; Shalev et al., 2009). A commonly used model of relapse in animals involves training rats to press a lever for cocaine followed by a period of forced abstinence, during which time the animals learn that the lever no longer delivers the drug (Shaham et al., 2003). During this period of extinction (i.e., enforced abstinence) the animal will stop pressing the lever. However, if a stressor, such as footshock, is presented, then the animal will again start pressing the lever. Initial studies reported that although dopamine played an important role in cue- and context-induced relapse, it was not central to stress-induced relapse (for a review see Shaham et al., 2003). However, recent studies suggest that dopamine may indeed play a role and, moreover, that CRF in the VTA is involved (Wang et al., 2005, 2007). Footshock stress increases CRF release in the VTA and this drives increased dopamine release in the striatum, as measured using microdialysis (Wang et al., 2005). CRF-R1/2 antagonists perfused directly into the VTA block this stress-induced relapse (Wang et al., 2005). Strikingly, this effect is blocked by CRF-R2, but not CRF-R1 antagonists and requires CRF-BP (Wang et al., 2007). Moreover, the stress-induced increases in glutamate in the VTA (and dopamine in the striatum) were blocked by a CRF-R2 antagonist perfused into the VTA. This is consistent with the synaptic physiology described in the previous section which showed that after cocaine exposure CRF can modulate both pre- and post-synaptic glutamatergic transmission.
7. Conclusions Here we reviewed recent findings concerning the effects of stress on dopamine neurons and the role that those effects may play in relapse. There is a long-standing literature suggesting that stress increases dopamine release, but this has traditionally been difficult to integrate with studies of dopamine neuron firing, and the behavioral consequences of that dopamine release in relapse have not been made clear. Moreover, it has not been well understood how stress affects dopamine neurons at a cellular level. Recent studies have made progress regarding a number of these issues. First, it appears that stress has different effects on different dopamine neurons, and over differing timescales. Further research is needed to delineate more clearly how dopamine neurons respond to stressful stimuli and to relate these functionally distinct groups of dopamine neurons to their projection targets and functional microcircuitry. Second, it appears that a major target of stress, and stress-related hormones in particular, are fast excitatory and inhibitory synapses made onto dopamine neurons. For example, CRF and glucocorticoids can enhance NMDAR-mediated synaptic transmission, which may play a role in the induction of long-term potentiation of AMPAR-mediated synaptic transmission observed after stress. These effects on synaptic transmission are likely to be driving the changes in firing activity and dopamine release seen in response to stress. Third, it appears that stressinduced dopamine release plays a role in stress-induced relapse, and strikingly, some unexpected cellular mechanisms have been uncovered that have in turn suggested novel ways of interfering with relapse. The hope then is that by better understanding the effects of stress on dopamine neurons new avenues will be identified for therapeutic interventions in abstinent former drug addicts who are at risk of relapse.
156
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Acknowledgements MU is supported by grant U120085816 from the U.K. Medical Research Council (MRC) and a University Research Fellowship from The Royal Society. References Anstrom, K.K., Woodward, D.J., 2005. Restraint increases dopaminergic burst firing in awake rats. Neuropsychopharmacology 30, 1832–1840. Anstrom, K.K., Miczek, K.A., Budygin, E.A., 2009. Increased phasic dopamine signaling in the mesolimbic pathway during social defeat in rats. Neuroscience 161, 3–12. Argilli, E., Sibley, D.R., Malenka, R.C., England, P.M., Bonci, A., 2008. Mechanism and time course of cocaine-induced long-term potentiation in the ventral tegmental area. J. Neurosci. 28, 9092–9100. Beckstead, M.J., Gantz, S.C., Ford, C.P., Stenzel-Poore, M.P., Phillips, P.E., Mark, G.P., Williams, J.T., 2009. CRF enhancement of GIRK channel-mediated transmission in dopamine neurons. Neuropsychopharmacology (Epub ahead of print), doi:10.1038/npp.2009.25. Bellone, C., Lüscher, C., 2005. mGluRs induce a long-term depression in the ventral tegmental area that involves a switch of the subunit composition of AMPA receptors. Eur. J. Neurosci. 21, 1280–1288. Borgland, S.L., Malenka, R.C., Bonci, A., 2004. Acute and chronic cocaine-induced potentiation of synaptic strength in the VTA: electrophysiological and behavioral correlates in individual adolescent rats. J. Neurosci. 24 (34), 7482–7490. Brischoux, F., Chakraborty, S., Brierley, D.I., Ungless, M.A., 2009. Phasic excitation of dopamine neurons in ventral VTA by noxious stimuli. Proc. Natl. Acad. Sci. U.S.A. 106, 4894–4899. Brown, M.T., Henny, P., Bolam, J.P., Magill, P.J., 2009. Activity of neurochemically heterogeneous dopaminergic neurons in the substantia nigra during spontaneous and driven changes in brain state. J. Neurosci. 29, 2915–2925. Carlezon Jr., W.A., Nestler, E.J., 2002. Elevated levels of GluR1 in the midbrain: a trigger for sensitization to drugs of abuse? Trends Neurosci. 25, 610–615. Chen, B.T., Bowers, M.S., Martin, M., Hopf, F.W., Guillory, A.M., Carelli, R.M., Chou, J.K., Bonci, A., 2008. Cocaine but not natural reward self-administration nor passive cocaine infusion produces persistent LTP in the VTA. Neuron 59, 288–297. Cho, K., Little, H.J., 1999. Effects of corticosterone on excitatory amino acid responses in dopamine-sensitive neurons in the ventral tegmental area. Neuroscience 88, 837–845. Daw, N.D., Kakade, S., Dayan, P., 2002. Opponent interactions between serotonin and dopamine. Neural Netw. 15, 603–616. Dong, Y., Saal, D., Thomas, M., Faust, R., Bonci, A., Robinson, T., Malenka, R.C., 2004. Cocaine-induced potentiation of synaptic strength in dopamine neurons: behavioral correlates in GluRA(−/−) mice. Proc. Natl. Acad. Sci. U.S.A. 101, 14282–14287. Fiorillo, C.D., Williams, J.T., 1998. Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons. Nature 394, 78–82. Gonon, F.G., 1988. Nonlinear relationship between impulse flow and dopamine released by rat midbrain dopaminergic neurons as studied by in vivo electrochemistry. Neuroscience 24 (1), 19–28. Grace, A.A., Bunney, B.S., 1984. The control of firing pattern in nigral dopamine neurons: burst firing. J. Neurosci. 4 (11), 2877–2890. Guarraci, F.A., Kapp, B.S., 1999. An electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential Pavlovian fear conditioning in the awake rabbit. Behav. Brain Res. 99 (2), 169–179. Haber, S.N., Fudge, J.L., McFarland, N.R., 2000. Striatonigrostriatal pathways in primates form an ascending spiral from the shell to the dorsolateral striatum. J. Neurosci. 20, 2369–2382. Hahn, J., Hopf, F.W., Bonci, A., 2009. Chronic cocaine enhances corticotropinreleasing factor-dependent potentiation of excitatory transmission in ventral tegmental area dopamine neurons. J. Neurosci. 29, 6535–6544. Ikemoto, S., 2007. Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. Brain Res. Rev. 56, 27–78. Joseph, M.H., Datla, K., Young, A.M., 2003. The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition? Neurosci. Biobehav. Rev. 27, 527–541. Joshua, M., Adler, A., Mitelman, R., Vaadia, E., Bergman, H., 2008. Midbrain dopaminergic neurons and striatal cholinergic interneurons encode the difference between reward and aversive events at different epochs of probabilistic classical conditioning trials. J. Neurosci. 28, 11673–11684. Koob, G.F., 2008. A role for brain stress systems in addiction. Neuron 59, 11–34. Korotkova, T.M., Brown, R.E., Sergeeva, O.A., Ponomarenko, A.A., Haas, H.L., 2006. Effects of arousal- and feeding-related neuropeptides on dopaminergic and GABAergic neurons in the ventral tegmental area of the rat. Eur. J. Neurosci. 23, 2677–2685.
Mameli, M., Balland, B., Luján, R., Lüscher, C., 2007. Rapid synthesis and synaptic insertion of GluR2 for mGluR-LTD in the ventral tegmental area. Science 317, 530–533. Mantz, J., Thierry, A.M., Glowinski, J., 1989. Effect of noxious tail pinch on the discharge rate of mesocortical and mesolimbic dopamine neurons: selective activation of the mesocortical system. Brain Res. 476 (2), 377–381. Margolis, E.B., Lock, H., Hjelmstad, G.O., Fields, H.L., 2006. The ventral tegmental area revisited: is there an electrophysiological marker for dopaminergic neurons? J. Physiol. 577, 907–924. Matsumoto, M., Hikosaka, O., 2009. Two types of dopamine neuron distinctly convey positive and negative motivational signals. Nature 459, 837–841. Mirenowicz, J., Schultz, W., 1996. Preferential activation of midbrain dopamine neurons by appetitive rather than aversive stimuli. Nature 379, 449–451. Riegel, A.C., Williams, J.T., 2008. CRF facilitates calcium release from intracellular stores in midbrain dopamine neurons. Neuron 57, 559–570. Rodaros, D., Caruana, D.A., Amir, S., Stewart, J., 2007. Corticotropin-releasing factor projections from limbic forebrain and paraventricular nucleus of the hypothalamus to the region of the ventral tegmental area. Neuroscience 15, 8–13. Roitman, M.F., Wheeler, R.A., Wightman, R.M., Carelli, R.M., 2008. Real-time chemical responses in the nucleus accumbens differentiate rewarding and aversive stimuli. Nat. Neurosci. 11, 1376–1377. Saal, D., Dong, Y., Bonci, A., Malenka, R.C., 2003. Drugs of abuse and stress trigger a common synaptic adaptation in dopamine neurons. Neuron 37, 577–582. Sarnyai, Z., Shaham, Y., Heinrichs, S.C., 2001. The role of corticotropin-releasing factor in drug addiction. Pharmacol. Rev. 53, 209–243. Schilström, B., Yaka, R., Argilli, E., Suvarna, N., Schumann, J., Chen, B.T., Carman, M., Singh, V., Mailliard, W.S., Ron, D., Bonci, A., 2006. Cocaine enhances NMDA receptor-mediated currents in ventral tegmental area cells via dopamine D5 receptor-dependent redistribution of NMDA receptors. J. Neurosci. 26, 8549–8558. Schultz, W., 1998. Predictive reward signal of dopamine neurons. J. Neurophysiol. 80, 1–27. Schultz, W., Romo, R., 1987. Responses of nigrostriatal dopamine neurons to high-intensity somatosensory stimulation in the anesthetized monkey. J. Neurophysiol. 57 (1), 201–217. Shaham, Y., Shalev, U., Lu, L., De Wit, H., Stewart, J., 2003. The reinstatement model of drug relapse: history, methodology and major findings. Psychopharmacology (Berl.) 168, 3–20. Shalev, U., Erb, S., Shaham, Y., 2009. Role of CRF and other neuropeptides in stressinduced reinstatement of drug seeking. Brain Res. (Epub ahead of print), July 23. Sinha, R., 2008. Chronic stress, drug use, and vulnerability to addiction. Ann. NY Acad. Sci. 1141, 105–130. Stuber, G.D., Klanker, M., de Ridder, B., Bowers, M.S., Joosten, R.N., Feenstra, M.G., Bonci, A., 2008. Reward-predictive cues enhance excitatory synaptic strength onto midbrain dopamine neurons. Science 321, 1690–1692. Tagliaferro, P., Morales, M., 2008. Synapses between corticotropin-releasing factorcontaining axon terminals and dopaminergic neurons in the ventral tegmental area are predominantly glutamatergic. J. Comp. Neurol. 506, 616–626. Tanimoto, H., Heisenberg, M., Gerber, B., 2004. Experimental psychology: event timing turns punishment to reward. Nature 430 (7003), 983. Ungless, M.A., 2004. Dopamine: the salient issue. Trends Neurosci. 27, 702–706. Ungless, M.A., Magill, P.J., Bolam, J.P., 2004. Uniform inhibition of dopamine neurons in the ventral tegmental area by aversive stimuli. Science 303, 2040–2042. Ungless, M.A., Singh, V., Crowder, T.L., Yaka, R., Ron, D., Bonci, A., 2003. Corticotropinreleasing factor requires CRF binding protein to potentiate NMDA receptors via CRF receptor 2 in dopamine neurons. Neuron 39, 401–407. Ungless, M.A., Whistler, J.L., Malenka, R.C., Bonci, A., 2001. Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons. Nature 411, 583–587. Van Pett, K., Viau, V., Bittencourt, J.C., Chan, R.K., Li, H.Y., Arias, C., Prins, G.S., Perrin, M., Vale, W., Sawchenko, P.E., 2000. Distribution of mRNAs encoding CRF receptors in brain and pituitary of rat and mouse. J. Comp. Neurol. 428, 191–212. Wanat, M.J., Hopf, F.W., Stuber, G.D., Phillips, P.E., Bonci, A., 2008. Corticotropinreleasing factor increases mouse ventral tegmental area dopamine neuron firing through a protein kinase C-dependent enhancement of Ih. J. Physiol. 586, 2157–2170. Wang, H.L., Morales, M., 2008. Corticotropin-releasing factor binding protein within the ventral tegmental area is expressed in a subset of dopaminergic neurons. J. Comp. Neurol. 509, 302–318. Wang, B., Shaham, Y., Zitzman, D., Azari, S., Wise, R.A., You, Z.B., 2005. Cocaine experience establishes control of midbrain glutamate and dopamine by corticotropin-releasing factor: a role in stress-induced relapse to drug seeking. J. Neurosci. 25, 5389–5396. Wang, B., You, Z.B., Rice, K.C., Wise, R.A., 2007. Stress-induced relapse to cocaine seeking: roles for the CRF(2) receptor and CRF-binding protein in the ventral tegmental area of the rat. Psychopharmacology (Berl.) 193, 283–294. Wise, R.A., 2004. Dopamine, learning and motivation. Nat. Rev. Neurosci. 5, 483–494.
Contents lists available at ScienceDirect
Neuroscience and Biobehavioral Reviews journal homepage: www.elsevier.com/locate/neubiorev
Review
Effects of stress and aversion on dopamine neurons: Implications for addiction Mark A. Ungless a , Emanuela Argilli b , Antonello Bonci b,∗ a b
Medical Research Council Clinical Sciences Centre, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK Ernest Gallo Clinic and Research Center, Department of Neurology, University of California, San Francisco, Emeryville, CA 94608, USA
a r t i c l e Keywords: Midbrain Aversive Addiction
i n f o
a b s t r a c t Stress plays a key role in modulating the development and expression of addictive behavior, and is a major cause of relapse following periods of abstinence. In this review we focus our attention on recent advances made in understanding how stress, aversive events, and drugs of abuse, cocaine in particular, interact directly with dopamine neurons in the ventral tegmental area, and how these interactions may be involved in stress-induced relapse. We start by outlining how dopamine neurons respond to aversive stimuli and stress, particularly in terms of firing activity and modulation of excitatory synaptic inputs. We then discuss some of the cellular mechanisms underlying the effects of cocaine on dopamine neurons, again with a selective focus on synaptic plasticity. Finally, we examine how the effects of stress and cocaine interact and how these cellular mechanisms in ventral tegmental area dopamine neurons may be engaged in stress-induced relapse. © 2010 Published by Elsevier Ltd.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute effects of aversive events on dopamine neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic effects of aversive events on dopamine neuron activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress-induced synaptic plasticity in dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cocaine-induced synaptic plasticity in dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress-induced relapse and dopamine neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Aversive events, and activation of stress systems, play key roles in modulating addictive behavior (Sinha, 2008). For example, early life stressors are associated with increased risk of substance abuse. Perhaps of most therapeutic significance is the observation that stressful events are a major cause of relapse in abstinent individuals. Therefore, understanding how aversive events and stress interact with the neural circuits underlying addiction is an important step in developing new therapeutic targets. An extensive literature has now emerged examining the neural circuits and cellular mechanisms through which stress modulates addictive behavior (Koob, 2008). In this review we do not attempt to provide a comprehensive overview of this large literature; instead we wish
∗ Corresponding author. Tel.: +1 510 985 3890; fax: +1 510 985 3101. E-mail address: [email protected] (A. Bonci). 0149-7634/$ – see front matter © 2010 Published by Elsevier Ltd. doi:10.1016/j.neubiorev.2010.04.006
151 152 153 154 155 155 155 156 156
to focus on recent progress concerning the direct effects of aversive events, stress, and drugs of abuse on midbrain dopamine neurons, and how these interactions might be involved in stress-induced relapse. It is well established that mesolimbic dopamine plays a central role in animal models of cue- and drug-induced relapse (Shaham et al., 2003), but it has been more difficult to determine a role for dopamine in stress-induced relapse. At first glance this may not appear surprising because dopamine is commonly associated with reward processing; however, it has long been known that aversive, stressful stimuli increase dopamine release in the striatum, as measured using microdialysis over minutes and hours (for a review see Joseph et al., 2003). Recent evidence suggests that stressinduced dopamine release does indeed play an important role in stress-induced relapse (Wang et al., 2005), and these findings raise some broad questions. For example, if dopamine neurons encode reward-related information, why does dopamine increase in projection targets? And, how do aversive events modulate activity in dopamine neurons? Further, how do these mechanisms interact
152
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 1. (A–C) An identified dopamine neuron that showed an inhibition at footshock onset, followed by an excitation at termination of the stimulus. (D–F) An identified dopamine neuron that showed a strong excitation at the onset of the footshocks. These neurons were located in more ventral parts of the VTA. Data are from Brischoux et al. (2009).
with drug-induced plasticity in dopamine neurons? In this selective review we present recent evidence that sheds light on these questions. We first provide an overview of how dopamine neurons respond acutely to aversive events in terms of firing activity and dopamine release. We then examine the longer-term, chronic effects of aversive events on dopamine neuron firing and dopamine release. We continue by exploring the cellular mechanisms that may underlie these chronic effects. Following this, we will outline how drugs of abuse, cocaine in particular, induce long-term synaptic plasticity in dopamine neurons. Lastly, we will discuss how these cellular processes engaged by aversive events, stress and cocaine interact and may be involved in stress-induced relapse during periods of abstinence. 2. Acute effects of aversive events on dopamine neuron It is well established that midbrain dopamine neurons play a role in processing reward-related information (Wise, 2004). In particular it has been suggested that dopamine neurons encode a reward-prediction-error signal (Schultz, 1998); they are excited by unexpected rewards, or reward-predicting stimuli. They are unresponsive to expected rewards, and they are inhibited by the omission of an expected reward. Although the literature concerning how these neurons respond to reward is relatively clear, how these neurons respond to stressful, aversive stimuli has been more difficult to establish. Many studies report that the majority of putative dopamine neurons are inhibited by aversive stimuli, which is consistent with their role in reward (e.g., Schultz and Romo, 1987; Mantz et al., 1989; Mirenowicz and Schultz, 1996; Guarraci and Kapp, 1999). However, a minority of putative dopamine neurons are excited by aversive stimuli. In these studies, dopamine neurons are identified as putatively dopaminergic on the basis of their electrophysiological characteristics. It appears, however, that not all ventral tegmental area (VTA) neurons with these characteristics are dopaminergic (Margolis et al., 2006). Indeed, when combined with single-cell labeling, we found that putative dopamine neurons that
were excited by a footpinch were in fact not dopaminergic (Ungless et al., 2004). Identified dopamine neurons were uniformly inhibited by the footpinch. Although this study was conducted with aneasthetised animals, to allow reliable single-cell labelling, responses seen in the VTA to stimuli such as footpinches and footshocks appear to be unaffected by aneasthesia (for discussion see Ungless et al., 2004; Brischoux et al., 2009). We therefore concluded that previous studies may have recorded from a mixture of dopaminergic and non-dopaminergic neurons. It is worth noting here also that although we believe that previous studies may have included some non-dopaminergic neurons, it seems likely that the majority of neurons recorded were dopaminergic and consequently the conclusions drawn regarding reward-related responses are still valid particularly where responses of all neurons sampled are relatively uniform (as appears to be the case for rewards in contrast to aversive stimuli). The finding that dopamine neurons are inhibited by aversive stimuli is consistent with reward theories of dopamine neuron function and with studies of dopamine release using fast-scan cyclic voltammetry (Roitman et al., 2008). However, this view is difficult to reconcile with a number of other findings (for discussion see Ungless, 2004). For example, it is well established that aversive events (such as footshocks) can increase dopamine release, as measured using microdialysis over several minutes (for a review see Joseph et al., 2003). How can this be explained? One possibility is that a subgroup of dopamine neurons are in fact excited by stressful, aversive stimuli, but that we did not record from them in our previous study. Indeed, we, like many others, target more dorsal regions of the ventral tegmental area (VTA). When we subsequently explored more ventral parts of the VTA we found a discrete population of dopamine neurons that are strongly excited by footshocks (Brischoux et al., 2009; Fig. 1). There is considerable behavioral evidence that the offset of an aversive stimulus can act as a reward (e.g., Tanimoto et al., 2004) and might therefore be expected to excite dopamine neurons (Daw et al., 2002). Consistent with this, of the neurons that were inhibited
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 2. Tonic activity in dopamine neurons. (A) Tonic, low-bursting, single-spike activity in a dopamine neuron that was inhibited by footshocks. (B) Bursting activity in a neuron that was excited by the footshocks (upper). An expanded view of five bursts (left lower) and a single burst (right lower). Data are from Brischoux et al. (2009).
by aversive stimuli, around half also showed an excitation at the offset of the stimulus (Fig. 1). That is to say, that during the first 500 ms after removal of the footshocks, many neurons exhibited a phasic excitation above baseline. This offset excitation may also contribute to the increase in dopamine seen in response to aversive stimuli. It appears then that there are distinct subgroups of dopamine neurons that respond differentially to aversive events. One important feature of these responses is that they are extremely rapid, with an onset and duration of around 100 ms. It seems likely, therefore, that fast synaptic inputs are driving these changes in firing activity, rather than the slower effects elicited by stressrelated neurochemicals (see sections below). Recent studies of substantia nigra neurons in primates have further characterized substantia nigra dopamine neuron responses in tasks where both aversive and rewarding stimuli are presented. In these cases some putative dopamine neurons appear to selectively encode rewardprediction error rules, while others are also excited by aversive events (e.g., Joshua et al., 2008; Matsumoto and Hikosaka, 2009). Further, Matsumoto and Hikosaka (2009) found that these saliencyencoding neurons are in more lateral parts of the substantia nigra, whereas the reward encoding neurons are more medial. It should be noted that in the substantia nigra, in contrast to the VTA, the electrophysiological criteria used to identify putative dopamine neurons appear to selectively identify dopamine neurons (Brown et al., 2009). Taken together, these studies suggest that anatomically discrete dopamine neurons exhibit differential, rapid changes in activity in response to aversive stimuli. 3. Chronic effects of aversive events on dopamine neuron activity In the previous section we reviewed the rapid effects of aversive events on dopamine neuron activity. These effects are unlikely to involve activation of the classical stress-related systems, which have slower actions. In this section we review studies examining the long-term, or chronic, effects of aversive events on dopamine neuron activity. Single unit recordings in awake rats during 30 min restraint stress find an increase in firing rate and burst firing in putative VTA dopamine neurons (Anstrom and Woodward, 2005). Dopamine neurons can fire action potentials as single-spikes or bursts. Bursts are traditionally defined as starting with an inter-spike-interval (ISI) of <80 ms and finishing with an ISI >160 ms (Grace and Bunney, 1984; see Fig. 2). Bursts are thought to have important functional
153
consequences on dopamine release, because at these higher firing frequencies the dopamine transporter may become overwhelmed and extracellular dopamine increases supralinearly (Gonon, 1988). This increase in firing rate and bursting was still present 24 h following the restraint stress procedure, and a second exposure did not have any additional effect. The effects of restraint stress on bursting were most prominent in neurons that showed high levels of baseline bursting. This is particularly interesting in light of the fact that the ventral VTA dopamine neurons that were strongly excited by footshocks had higher baseline levels of bursting compared to those that were inhibited (Brischoux et al., 2009). One possibility is that high-bursting dopamine neurons in the ventral VTA are particularly sensitive to the acute and chronic effects of aversive, stressful stimuli. In a further study, Anstrom et al. (2009) examined the effects of social defeat on putative dopaminergic activity (it should be noted, as in the previous section, that although some caution should be observed it is likely that the majority of these neurons are dopaminergic). Rats were placed in the cage of an unfamiliar rat for 5 min, during which time a confrontation occurred. Following this confrontation, putative dopamine neurons exhibited an increase in burst number, but not firing rate. As for the effects of restraint stress, social defeat increased burst firing preferentially in those neurons that had higher baseline levels of bursting. There was also a significant increase in dopamine transients in the striatum following social defeat, consistent with the increased burst firing. These observations are consistent with the suggestion that slow changes in activity of dopamine neurons may encode average rate of punishment (Daw et al., 2002). It will be important to examine how these effects influence rapid, phasic events. These studies of action potential firing in dopamine neurons are consistent with a long-standing literature showing increases in dopamine release in response to stressful stimuli, as measured with microdialysis over periods of several minutes and more indirect measures of dopamine metabolism (Joseph et al., 2003). It has been difficult to reconcile these observations with the more traditional view that dopamine neuron firing is inhibited by aversive, stressful stimuli. However these recent studies suggest that there are three ways in which they can be reconciled. First, some dopamine neurons that are inhibited by aversive stimuli exhibit a phasic excitation at stimulus offset. Second, subgroups of dopamine neurons are phasically excited at the onset of aversive stimuli. And third, it appears that prolonged stress increases firing rates and bursting, and this leads to persistent increases in tonic activity. These changes in phasic and tonic activity no doubt contribute to the changes in dopamine release. One emerging theme is that dopamine neurons should not be viewed as a functionally homogenous group. In terms of responses to aversive stimuli and slower changes in response to prolonged or repeated stress, dopamine neurons in the VTA can be subdivided into at least two groups. One group appears to encode a reward-prediction error rule; a second group (with high baseline burst firing) exhibits phasic responses to aversive stimuli and slow changes in tonic activity. It is less clear how these neurons respond to rewards. One possibility is that they are also excited by unexpected rewards, in which case they may encode a saliency prediction error, but further studies will be required to examine this. It seems likely that these different groups will have distinct projection targets, but exactly how is unclear. Topographic organisation of the VTA suggests that ventral dopamine neurons (that are excited by footshocks) are more likely to project to medial targets (e.g., medial accumbens; Haber et al., 2000; Ikemoto, 2007). Similarly in the SNC there appear to be functionally distinct dopamine neurons (e.g., Joshua et al., 2008; Matsumoto and Hikosaka, 2009) that also exhibit some anatomical segregation (Matsumoto and Hikosaka, 2009), presumably related to projection targets (see Fig. 3).
154
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Fig. 3. Diagram illustrating the organisation of dopamine neuron projections in primate. Note, in particular, the medio-lateral topography, whereby medial dopamine neurons target medial regions of striatum and cortex, and more lateral dopamine neurons target more lateral regions. For more discussion and original figure see Haber et al. (2000). Similar medio-lateral topography has been observed in rodents (Ikemoto, 2007).
4. Stress-induced synaptic plasticity in dopamine neurons In this section we review recent progress made in understanding how stress interacts with dopamine neurons at a cellular level. In particular we focus on modulation of synaptic inputs onto dopamine neurons, which play a key role in regulating their activity and show experience-dependent plasticity in a number of situations, most notably in response to drugs of abuse. Saal et al. (2003) found that swim stress induced a form of long-term potentiation in dopamine neurons, similar to that seen in response to drugs of abuse, including cocaine, amphetamine and alcohol (Saal et al., 2003; Ungless et al., 2001). This stress-induced potentiation was blocked by glucocorticoid receptor antagonists, but not D1R antagonists, suggesting a different induction mechanism to cocaine-induced potentiation (Dong et al., 2004; Saal et al., 2003). Glucocorticoids directly modulate NMDAR function in the VTA (Cho and Little, 1999) and this may lead to the long-term potentiation (LTP)-like changes seen in response to swim stress (Saal et al., 2003). Corticotropin releasing factor (CRF), a 41 amino acid peptide, plays an essential role in the activation of the hypothalamicpituitary-adrenal axis by stress. In addition, CRF mediates many of the extrahypothalamic effects of stress, including activation of the dopamine system, which can lead to relapse in animal models of drug taking (for a review see Sarnyai et al., 2001; Shalev et al., 2009). Until recently, however, it was unclear how CRF modulated dopamine neuron activity. The VTA receives CRF inputs (co-released with GABA and glutamate) from the limbic forebrain and the paraventricular nucleus of the hypothalamus (Rodaros et al., 2007; Tagliaferro and Morales, 2008). VTA dopamine neurons express CRF Receptor 1 (CRF-R1) and CRF Receptor 2 (CRF-R2) (Ungless et al., 2003; Van Pett et al., 2000), and about 25% of
VTA dopamine neurons, mostly located in the parabrachial pigmented (PBP) region of the VTA, express the CRF binding protein (CRF-BP; Wang and Morales, 2008). CRF increases action potential firing rate in dopamine neurons (Korotkova et al., 2006; Wanat et al., 2008) via CRF-R1. This effect involves a protein-kinaseC-dependent enhancement of Ih (a hyperpolarization-activated inward current) (Wanat et al., 2008). CRF-R1 activation also facilitates slow, dopamine D2 - and GABAB -receptor-mediated inhibitory synaptic transmission (Beckstead et al., 2009). Both of these CRFR1-dependent effects do not involve CRF-BP. CRF can also induce a slowly developing, but transient, potentiation of NMDAR-mediated synaptic transmission (Ungless et al., 2003). This effect involves CRF-R2 and activation of the proteinkinase-C pathway; interestingly, CRF-BP is required for this effect. This suggests that CRF-BP can play an active role in promoting the effects of CRF, rather than as a buffer, although exactly how it does this is unclear. In addition to fast, excitatory glutamatergic synaptic transmission, dopamine neurons also exhibit slower, inhibitory glutamatergic synaptic transmission, mediated via metabotropic glutamate receptors (mGluRs; Fiorillo and Williams, 1998). CRF can enhance these mGluRs via a CRF-R2-PKA pathway that stimulates release of calcium from intracellular stores (Riegel and Williams, 2008). In this case, the CRF-BP inhibits the effect, presumably by ‘buffering’ CRF. Thus it is clear that CRF can modulate activity in dopamine neurons through a number of different mechanisms. One emerging theme is that stress-related neurochemicals, such as CRF and glucocorticoids, target NMDARs on dopamine neurons. This is particularly striking in light of the evidence to be discussed in the following section that cocaine can have a similar action.
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
5. Cocaine-induced synaptic plasticity in dopamine neurons In the previous sections we have reviewed recent progress in understanding how aversive events, stress, and stress-related neurochemicals directly regulate activity in dopamine neurons. However, to properly understand how aversive events and stress can induce relapse during periods of abstinence from drug use we need to understand what long-term changes drugs such as cocaine induce in this system and how those changes interact with stress. It has become clear that the direct actions of stress on dopamine are mediated, in large part, through excitatory, glutamatergic synaptic inputs. In this section we review recent progress in our understanding of how drugs of abuse, cocaine in particular, also target these synapses. It is striking that cocaine can enhance NMDAR-mediated synaptic transmission in dopamine neurons. This enhancement is slowly developing and transient, in a similar manner to CRF. Moreover, it is dependent upon the activation of D1 -like DA receptors, but is delayed by initial activation of D2 receptors (Schilström et al., 2006). This event seems to be the first necessary step in NMDA-dependent AMPAR LTP lasting for up to 5 days that has been observed just hours after a single or multiple cocaine injection in rats (Borgland et al., 2004; Ungless et al., 2001). Importantly, LTP of VTA glutamatergic synapses is observed after a single exposure to many other drugs of abuse and stress, demonstrating a convergence of cellular responses within the VTA (Saal et al., 2003). In contrast to passive administration of cocaine which lasts for a few days, voluntary selfadministration induces a form of potentiation that lasts for up to 3 months (Chen et al., 2008); this form of plasticity is not confined to drugs of abuse. Following training to associate a cue with a natural reward, these synapses are also transiently potentiated (Stuber et al., 2008). Cocaine-mediated potentiation of glutamatergic synapses involves insertion of GluR2-lacking AMPA receptors (Argilli et al., 2008; Bellone and Lüscher, 2005; Mameli et al., 2007). Importantly, these types of AMPA receptors are calcium permeable, which means they may play a role in establishing longer-term neuroadaptations (for discussion see Carlezon and Nestler, 2002). It is clear then that glutamatergic synaptic transmission onto dopamine neurons is a key site of plasticity, and that both stress and cocaine directly target NMDARs, which in turn can induce longterm potentiation of AMPAR-mediated synaptic transmission. In addition, two recent studies show that cocaine exposure can modify the effects of CRF on dopamine neurons. First, the enhancement of slow, dopamine D2 - and GABAB -receptor-mediated inhibitory synaptic transmission is reduced by repeated exposure to psychostimulants or stress (Beckstead et al., 2009). In contrast, following chronic cocaine, the effects of CRF on NMDARs are both larger and more prolonged (Hahn et al., 2009). In addition, CRF enhances AMPAR-mediated synaptic transmission following chronic cocaine (Hahn et al., 2009). Analysis of spontaneous miniature synaptic events revealed that following chronic cocaine, CRF increased glutamate release and modulated post-synaptic glutamate receptors. Taken together, these findings suggest that previous exposure to cocaine will enhance the excitatory effects, and reduce the inhibitory effects, of CRF on dopamine neurons, and in turn this might be expected to enhance stress-induced activation of the dopamine system and renewed cocaine seeking during periods of abstinence.
6. Stress-induced relapse and dopamine neurons In this section we discuss stress-induced relapse and how the interactions between stress and dopamine neurons discussed in
155
the previous sections may play a role. In terms of treating addiction, relapse is the primary target. Most addicts can stop taking drugs and remain abstinent for brief periods, but many relapse. Stress appears to be a major factor in causing relapse. There are broadly three types of relapse that have been examined in animal models of addiction: cue-induced, context-induced, and stress-induced (for a review see Shaham et al., 2003; Shalev et al., 2009). A commonly used model of relapse in animals involves training rats to press a lever for cocaine followed by a period of forced abstinence, during which time the animals learn that the lever no longer delivers the drug (Shaham et al., 2003). During this period of extinction (i.e., enforced abstinence) the animal will stop pressing the lever. However, if a stressor, such as footshock, is presented, then the animal will again start pressing the lever. Initial studies reported that although dopamine played an important role in cue- and context-induced relapse, it was not central to stress-induced relapse (for a review see Shaham et al., 2003). However, recent studies suggest that dopamine may indeed play a role and, moreover, that CRF in the VTA is involved (Wang et al., 2005, 2007). Footshock stress increases CRF release in the VTA and this drives increased dopamine release in the striatum, as measured using microdialysis (Wang et al., 2005). CRF-R1/2 antagonists perfused directly into the VTA block this stress-induced relapse (Wang et al., 2005). Strikingly, this effect is blocked by CRF-R2, but not CRF-R1 antagonists and requires CRF-BP (Wang et al., 2007). Moreover, the stress-induced increases in glutamate in the VTA (and dopamine in the striatum) were blocked by a CRF-R2 antagonist perfused into the VTA. This is consistent with the synaptic physiology described in the previous section which showed that after cocaine exposure CRF can modulate both pre- and post-synaptic glutamatergic transmission.
7. Conclusions Here we reviewed recent findings concerning the effects of stress on dopamine neurons and the role that those effects may play in relapse. There is a long-standing literature suggesting that stress increases dopamine release, but this has traditionally been difficult to integrate with studies of dopamine neuron firing, and the behavioral consequences of that dopamine release in relapse have not been made clear. Moreover, it has not been well understood how stress affects dopamine neurons at a cellular level. Recent studies have made progress regarding a number of these issues. First, it appears that stress has different effects on different dopamine neurons, and over differing timescales. Further research is needed to delineate more clearly how dopamine neurons respond to stressful stimuli and to relate these functionally distinct groups of dopamine neurons to their projection targets and functional microcircuitry. Second, it appears that a major target of stress, and stress-related hormones in particular, are fast excitatory and inhibitory synapses made onto dopamine neurons. For example, CRF and glucocorticoids can enhance NMDAR-mediated synaptic transmission, which may play a role in the induction of long-term potentiation of AMPAR-mediated synaptic transmission observed after stress. These effects on synaptic transmission are likely to be driving the changes in firing activity and dopamine release seen in response to stress. Third, it appears that stressinduced dopamine release plays a role in stress-induced relapse, and strikingly, some unexpected cellular mechanisms have been uncovered that have in turn suggested novel ways of interfering with relapse. The hope then is that by better understanding the effects of stress on dopamine neurons new avenues will be identified for therapeutic interventions in abstinent former drug addicts who are at risk of relapse.
156
M.A. Ungless et al. / Neuroscience and Biobehavioral Reviews 35 (2010) 151–156
Acknowledgements MU is supported by grant U120085816 from the U.K. Medical Research Council (MRC) and a University Research Fellowship from The Royal Society. References Anstrom, K.K., Woodward, D.J., 2005. Restraint increases dopaminergic burst firing in awake rats. Neuropsychopharmacology 30, 1832–1840. Anstrom, K.K., Miczek, K.A., Budygin, E.A., 2009. Increased phasic dopamine signaling in the mesolimbic pathway during social defeat in rats. Neuroscience 161, 3–12. Argilli, E., Sibley, D.R., Malenka, R.C., England, P.M., Bonci, A., 2008. Mechanism and time course of cocaine-induced long-term potentiation in the ventral tegmental area. J. Neurosci. 28, 9092–9100. Beckstead, M.J., Gantz, S.C., Ford, C.P., Stenzel-Poore, M.P., Phillips, P.E., Mark, G.P., Williams, J.T., 2009. CRF enhancement of GIRK channel-mediated transmission in dopamine neurons. Neuropsychopharmacology (Epub ahead of print), doi:10.1038/npp.2009.25. Bellone, C., Lüscher, C., 2005. mGluRs induce a long-term depression in the ventral tegmental area that involves a switch of the subunit composition of AMPA receptors. Eur. J. Neurosci. 21, 1280–1288. Borgland, S.L., Malenka, R.C., Bonci, A., 2004. Acute and chronic cocaine-induced potentiation of synaptic strength in the VTA: electrophysiological and behavioral correlates in individual adolescent rats. J. Neurosci. 24 (34), 7482–7490. Brischoux, F., Chakraborty, S., Brierley, D.I., Ungless, M.A., 2009. Phasic excitation of dopamine neurons in ventral VTA by noxious stimuli. Proc. Natl. Acad. Sci. U.S.A. 106, 4894–4899. Brown, M.T., Henny, P., Bolam, J.P., Magill, P.J., 2009. Activity of neurochemically heterogeneous dopaminergic neurons in the substantia nigra during spontaneous and driven changes in brain state. J. Neurosci. 29, 2915–2925. Carlezon Jr., W.A., Nestler, E.J., 2002. Elevated levels of GluR1 in the midbrain: a trigger for sensitization to drugs of abuse? Trends Neurosci. 25, 610–615. Chen, B.T., Bowers, M.S., Martin, M., Hopf, F.W., Guillory, A.M., Carelli, R.M., Chou, J.K., Bonci, A., 2008. Cocaine but not natural reward self-administration nor passive cocaine infusion produces persistent LTP in the VTA. Neuron 59, 288–297. Cho, K., Little, H.J., 1999. Effects of corticosterone on excitatory amino acid responses in dopamine-sensitive neurons in the ventral tegmental area. Neuroscience 88, 837–845. Daw, N.D., Kakade, S., Dayan, P., 2002. Opponent interactions between serotonin and dopamine. Neural Netw. 15, 603–616. Dong, Y., Saal, D., Thomas, M., Faust, R., Bonci, A., Robinson, T., Malenka, R.C., 2004. Cocaine-induced potentiation of synaptic strength in dopamine neurons: behavioral correlates in GluRA(−/−) mice. Proc. Natl. Acad. Sci. U.S.A. 101, 14282–14287. Fiorillo, C.D., Williams, J.T., 1998. Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons. Nature 394, 78–82. Gonon, F.G., 1988. Nonlinear relationship between impulse flow and dopamine released by rat midbrain dopaminergic neurons as studied by in vivo electrochemistry. Neuroscience 24 (1), 19–28. Grace, A.A., Bunney, B.S., 1984. The control of firing pattern in nigral dopamine neurons: burst firing. J. Neurosci. 4 (11), 2877–2890. Guarraci, F.A., Kapp, B.S., 1999. An electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential Pavlovian fear conditioning in the awake rabbit. Behav. Brain Res. 99 (2), 169–179. Haber, S.N., Fudge, J.L., McFarland, N.R., 2000. Striatonigrostriatal pathways in primates form an ascending spiral from the shell to the dorsolateral striatum. J. Neurosci. 20, 2369–2382. Hahn, J., Hopf, F.W., Bonci, A., 2009. Chronic cocaine enhances corticotropinreleasing factor-dependent potentiation of excitatory transmission in ventral tegmental area dopamine neurons. J. Neurosci. 29, 6535–6544. Ikemoto, S., 2007. Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. Brain Res. Rev. 56, 27–78. Joseph, M.H., Datla, K., Young, A.M., 2003. The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition? Neurosci. Biobehav. Rev. 27, 527–541. Joshua, M., Adler, A., Mitelman, R., Vaadia, E., Bergman, H., 2008. Midbrain dopaminergic neurons and striatal cholinergic interneurons encode the difference between reward and aversive events at different epochs of probabilistic classical conditioning trials. J. Neurosci. 28, 11673–11684. Koob, G.F., 2008. A role for brain stress systems in addiction. Neuron 59, 11–34. Korotkova, T.M., Brown, R.E., Sergeeva, O.A., Ponomarenko, A.A., Haas, H.L., 2006. Effects of arousal- and feeding-related neuropeptides on dopaminergic and GABAergic neurons in the ventral tegmental area of the rat. Eur. J. Neurosci. 23, 2677–2685.
Mameli, M., Balland, B., Luján, R., Lüscher, C., 2007. Rapid synthesis and synaptic insertion of GluR2 for mGluR-LTD in the ventral tegmental area. Science 317, 530–533. Mantz, J., Thierry, A.M., Glowinski, J., 1989. Effect of noxious tail pinch on the discharge rate of mesocortical and mesolimbic dopamine neurons: selective activation of the mesocortical system. Brain Res. 476 (2), 377–381. Margolis, E.B., Lock, H., Hjelmstad, G.O., Fields, H.L., 2006. The ventral tegmental area revisited: is there an electrophysiological marker for dopaminergic neurons? J. Physiol. 577, 907–924. Matsumoto, M., Hikosaka, O., 2009. Two types of dopamine neuron distinctly convey positive and negative motivational signals. Nature 459, 837–841. Mirenowicz, J., Schultz, W., 1996. Preferential activation of midbrain dopamine neurons by appetitive rather than aversive stimuli. Nature 379, 449–451. Riegel, A.C., Williams, J.T., 2008. CRF facilitates calcium release from intracellular stores in midbrain dopamine neurons. Neuron 57, 559–570. Rodaros, D., Caruana, D.A., Amir, S., Stewart, J., 2007. Corticotropin-releasing factor projections from limbic forebrain and paraventricular nucleus of the hypothalamus to the region of the ventral tegmental area. Neuroscience 15, 8–13. Roitman, M.F., Wheeler, R.A., Wightman, R.M., Carelli, R.M., 2008. Real-time chemical responses in the nucleus accumbens differentiate rewarding and aversive stimuli. Nat. Neurosci. 11, 1376–1377. Saal, D., Dong, Y., Bonci, A., Malenka, R.C., 2003. Drugs of abuse and stress trigger a common synaptic adaptation in dopamine neurons. Neuron 37, 577–582. Sarnyai, Z., Shaham, Y., Heinrichs, S.C., 2001. The role of corticotropin-releasing factor in drug addiction. Pharmacol. Rev. 53, 209–243. Schilström, B., Yaka, R., Argilli, E., Suvarna, N., Schumann, J., Chen, B.T., Carman, M., Singh, V., Mailliard, W.S., Ron, D., Bonci, A., 2006. Cocaine enhances NMDA receptor-mediated currents in ventral tegmental area cells via dopamine D5 receptor-dependent redistribution of NMDA receptors. J. Neurosci. 26, 8549–8558. Schultz, W., 1998. Predictive reward signal of dopamine neurons. J. Neurophysiol. 80, 1–27. Schultz, W., Romo, R., 1987. Responses of nigrostriatal dopamine neurons to high-intensity somatosensory stimulation in the anesthetized monkey. J. Neurophysiol. 57 (1), 201–217. Shaham, Y., Shalev, U., Lu, L., De Wit, H., Stewart, J., 2003. The reinstatement model of drug relapse: history, methodology and major findings. Psychopharmacology (Berl.) 168, 3–20. Shalev, U., Erb, S., Shaham, Y., 2009. Role of CRF and other neuropeptides in stressinduced reinstatement of drug seeking. Brain Res. (Epub ahead of print), July 23. Sinha, R., 2008. Chronic stress, drug use, and vulnerability to addiction. Ann. NY Acad. Sci. 1141, 105–130. Stuber, G.D., Klanker, M., de Ridder, B., Bowers, M.S., Joosten, R.N., Feenstra, M.G., Bonci, A., 2008. Reward-predictive cues enhance excitatory synaptic strength onto midbrain dopamine neurons. Science 321, 1690–1692. Tagliaferro, P., Morales, M., 2008. Synapses between corticotropin-releasing factorcontaining axon terminals and dopaminergic neurons in the ventral tegmental area are predominantly glutamatergic. J. Comp. Neurol. 506, 616–626. Tanimoto, H., Heisenberg, M., Gerber, B., 2004. Experimental psychology: event timing turns punishment to reward. Nature 430 (7003), 983. Ungless, M.A., 2004. Dopamine: the salient issue. Trends Neurosci. 27, 702–706. Ungless, M.A., Magill, P.J., Bolam, J.P., 2004. Uniform inhibition of dopamine neurons in the ventral tegmental area by aversive stimuli. Science 303, 2040–2042. Ungless, M.A., Singh, V., Crowder, T.L., Yaka, R., Ron, D., Bonci, A., 2003. Corticotropinreleasing factor requires CRF binding protein to potentiate NMDA receptors via CRF receptor 2 in dopamine neurons. Neuron 39, 401–407. Ungless, M.A., Whistler, J.L., Malenka, R.C., Bonci, A., 2001. Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons. Nature 411, 583–587. Van Pett, K., Viau, V., Bittencourt, J.C., Chan, R.K., Li, H.Y., Arias, C., Prins, G.S., Perrin, M., Vale, W., Sawchenko, P.E., 2000. Distribution of mRNAs encoding CRF receptors in brain and pituitary of rat and mouse. J. Comp. Neurol. 428, 191–212. Wanat, M.J., Hopf, F.W., Stuber, G.D., Phillips, P.E., Bonci, A., 2008. Corticotropinreleasing factor increases mouse ventral tegmental area dopamine neuron firing through a protein kinase C-dependent enhancement of Ih. J. Physiol. 586, 2157–2170. Wang, H.L., Morales, M., 2008. Corticotropin-releasing factor binding protein within the ventral tegmental area is expressed in a subset of dopaminergic neurons. J. Comp. Neurol. 509, 302–318. Wang, B., Shaham, Y., Zitzman, D., Azari, S., Wise, R.A., You, Z.B., 2005. Cocaine experience establishes control of midbrain glutamate and dopamine by corticotropin-releasing factor: a role in stress-induced relapse to drug seeking. J. Neurosci. 25, 5389–5396. Wang, B., You, Z.B., Rice, K.C., Wise, R.A., 2007. Stress-induced relapse to cocaine seeking: roles for the CRF(2) receptor and CRF-binding protein in the ventral tegmental area of the rat. Psychopharmacology (Berl.) 193, 283–294. Wise, R.A., 2004. Dopamine, learning and motivation. Nat. Rev. Neurosci. 5, 483–494.