Stressis one of the major factors in drug abuse, particularly in relapse and drug-seeking behavior. However, the mechanisms underlying the interactions between stress and drug abuse are unclear. For many years, studies have focused on the role of the dopaminergic reward system in drug abuse. Our results show that an increased dopaminergic activity is induced by drug sensitization and different stressors via potentiation of the ventral subiculum-nucleus accumbens (NAc) pathway. Although the role of the NE system in stress is well-known, its involvement in drug abuse has received less attention. This review explores the different mechanisms by which stressors can modulate the ventral subiculum-accumbens pathway, and how these modulations can induce alterations in the behavioral response to drug administration. In particular, we will focus on two main afferents to the NAc, the basolateral amygdala and the ventral subiculum of the hippocampus, and their interactions with the locus coeruleus-norepinephrine system.
Investigations into the pathophysiology of drug abuse have traditionally focused on the dopaminergic reward system in the development of addiction, with a particular emphasis on the neural changes in reward-sensitive regions that are induced during addiction, relapse and abstinence 1, 2. Repeated administration of drugs of abuse is believed to induce a pathological response in the neural circuitry involved in processing natural reward, and the mechanisms underlying these allostatic changes have been a topic of extensive research 3. The mesolimbic system formed in part by the ventral tegmental area (VTA) and the nucleus accumbens (NAc) is an integral part of the brain’s reward circuit. Mesolimbic dopamine is implicated in the processing of natural and drug-related rewards, mediates the hedonic aspects of rewarding stimuli 4, and acts as a learning signal for behavioral reinforcement 5. A model has been proposed for brain changes that occur during the development of addiction that explains the persistent vulnerability to relapse, even long after drug taking has ceased. Indeed, drug-induced modifications in synaptic plasticity in the mesolimbic system, especially the VTA and the NAc, and the possible role of dopamine receptors in the development of these neuroadaptations, have been the focus of several studies. The modulation of excitatory synaptic transmission in the limbic regions during and after drug exposure has been shown to play a significant role in drug relapse and reinstatement 1. Thus morphological as well as synaptic alterations of several neuronal cell types in the limbic regions of the brain may be responsible for long-term behavioral plasticity driving addiction 6. It is now well-established that the most difficult phase in treating drug addiction is not the drug withdrawal, but preventing relapse 7, 8. Relapse to drug addiction is usually associated with craving that accompanies drug-seeking behavior. It has been suggested that even after weeks, if not months of withdrawal, addicts become sensitized to drug-associated environmental cues that act as external stimuli for craving 9–11.
One of the many factors that are known to contribute to relapse to drug abuse is stress. Indeed, numerous clinical as well as animal studies have shown a predominant role of stress in drug abuse and relapse 12. However the mechanisms underlying this relationship remain unclear. Stress and psychostimulants cross-sensitize, with stress leading to heightened responsivity to psychostimulants and vice versa. One common feature that stress and drug sensitization share is their strong dependence on context. Indeed, animals exposed to a stressor in a specific context show behavioral changes in that same context 13, 14, and psychostimulant sensitization is greater when the animals are tested in the same environment in which the drug has been delivered 15, 16. A region that has been implicated in context-dependent processes is the ventral subiculum of the hippocampus (vSub). The vSub is involved in context-dependent fear conditioning 17 as well as other context-related processes 18–20. The vSub is also a key structure in stress-related physiological response 21 and hyperdopaminergic response to amphetamine 22, 23. Another key structure related to stress is the basolateral amygdala(BLA). Neuronal activity within the amygdala is strongly affected by acute stressors, chronic stress exposure, and conditioned aversive stimuli 24, 25. Synaptic plasticity within the amygdala is also affected by exposure to stress 26. Importantly, the BLA is also involved in drug relapse, in particular by integrating the influences of stress on drug-related memory 27. Moreover, the BLA provides a potent input to the vSub 28. The present review will focus on those two main afferents to the NAc and describe their possible role in drug relapse, drug seeking behavior and their relationship to stress.
The dopamine reward system
It is now well-known that mesolimbic dopaminergic neurons have different states of activity. Dopaminergic neurons can be divided into two groups based on their activity: spontaneously active, corresponding to the population activity of DA neurons, and inactive neurons 29. Of the DA neurons that are firing spontaneously, the firing pattern is observed to exist in two activity patterns: a slow, irregular “tonic” firing pattern, and a bursting “phasic” pattern 30, 31. The bursting firing pattern is triggered by external reward-related stimuli in awake-behaving animals, or by stimulation of afferents 5, 22. One of the main efferents of the mesolimbic DA system is the NAc. Thus, irregular firing activity will modulate the tonic DA levels in the NAc whereas burst firing pattern mediates a large phasic, transient peak of dopamine in the synapse 32. These two firing patterns are induced by different types of afferents to the VTA. The bursting firing activity is driven by glutamate release in the VTA by the pedunculopontine tegmentum (PPTg) 32, 33, whereas population firing that mediates the tonic release of dopamine is induced by activation of an indirect pathway consisting of the vSub-NAc-ventral pallidum-VTA (Figure 1). This pathway has been confirmed by the ability of kynurenicacid injected into the NAc and local injection of muscimol/baclofen (specific agonists of GABAA/B receptors) in the ventral pallidum to block the effects of vSub activation on DA neuron firing 32. Interestingly, those two firing patterns have been shown to work synergistically to induce an appropriate behavioral response. Thus, we have shown that the number of DA neurons firing spontaneously determines the numbers of cells that can be driven into burst firing 22. Therefore, stimuli that increase vSub activity increase the amplitude of the DA system response to a particular phasic event.
The ventral subiculum and stress/drug relapse
The vSub is the primary output of the hippocampus, sending projections to many limbic-related regions, especially the NAc 34. The vSub is involved in different context-dependent processes such as fear-conditioning 17, 19, extinction 35, drug sensitization 12 and stress 36.
Studies have shown that vSub inactivation decreases cocaine and cue-induced reinstatement, highlighting the importance of the vSub in drug-seeking behavior 37. Drug sensitization has been proposed to model drug craving that occurs during the addiction process 38 and may play a significant role in reinstatement and relapse in drug-abstinent subjects. Indeed, context is known to play a large role in recidivism to drug-taking behavior 37. Drug sensitization is described as the repeated administration of psychostimulants, such as cocaine or amphetamine, resulting in a heightened response to a subsequent single drug administration 39. This behavioral sensitization has been compared to the increased drug craving observed in human drug abusers 38. Behavioral sensitization to amphetamine is attributable, at least in part, to an increased mesolimbic DA neuron drive, which is dependent on the vSub-NAc pathway. In fact, inactivation of the vSub in amphetamine-sensitized rats restores the DA population activity to basal levels, and eliminates the behavioral hyper-responsivity to amphetamine 23. Moreover, cocaine sensitization induces long-term potentiation in the vSub-NAc pathway dependent on D1-receptors activation 40. All of these studies support a substantial role of the vSub in drug sensitization.
Drugs of abuse engage similar brain areas as those involved in the response to stress. Stress can be defined as a threat to the maintenance of homeostatic balance, and the stress response inducing adaptive changes modulated by environmental factors 41. Many studies have shown an increasing role of the vSub in different stress responses 42. Thus, hippocampal lesion is linked to increased plasma levels of the adrenocorticotropic hormone (ACTH) and corticosterone under stressor conditions 43, and a decreased stress threshold in animals 44. One of the major stress responses of the vSub is to decrease, via multisynaptic pathways, the response of the hypothalamic-pituitary-adrenal (HPA) axis to stress 45. Moreover, other limbic system–associated regions, such as the prefrontal cortex, the amygdala and the NAc, have been shown to regulate the HPA axis 45. This suggests that limbic information may affect the activity of the homeostatic systems and dysfunctional stress integration may involve dysregulation in this circuitry.
The locus coeruleus-norepinephrine(LC-NE) system is one the major systems implicated in stress. In fact, corticotropin-releasing factor, a hormone that will induce release of ACTH during stress, has been shown to activate the LC-NE system in response to specific challenges 46. Thus, in vivo 47 and in vitro 48 studies have shown that administration of CRF induced an increase in LC firing rate simultaneously with an increase in NE efflux 47. The vSub receives a prominent NE innervation from the LC 49, and NE can produce an activation of vSub neurons 50. In rats, the vSub is described as having the highest density of beta-adrenergic receptors in the hippocampal formation 51. Thus, activation of beta adrenergic receptors by NE release into the vSub may induce a strong modulatory effect by increasing responses to glutamatergic afferent input to the vSub 52, 53.
Stress and drug abuse share many common features; in particular the ability to induce dopamine as well as norepinephrine release in limbic regions 54 and their strong association with context, implicating the vSub. Moreover, stress cross-sensitizes with psychostimulants. Thus an animal exposed to a stressor will show a heightened responsivity to amphetamine when exposed to a subsequent administration of the drug, and vice versa 16. We have shown that acute stress caused bya2h restraint stress protocol induced an increase in the population activity in the VTA, and that this increase is reversed by infusion of the sodium-channel inhibitor tetrodotoxin (TTX) in the vSub 55. The 2hr restraint protocol used in the previous study has been described to induce behavioral sensitization to amphetamine 56. Thus, the increased VTA population activity occurs in concert with a sensitized behavioral response to amphetamine; a response that is also reversed by vSub inactivation 55.
Taken together, these data demonstrate that the DA hyperactivity described after a stress exposure or psychostimulant sensitization is due to an increase in the tonic firing of VTA DA neurons and is dependent on hyperactivity in the vSub-NAc pathway. Activation of the vSub by norepinephrine could be one possible mechanism that underlies the hyperactivity in the vSub efferent pathway to the NAc.
Norepinephrine and stress/drug relapse
Norepinephrine (NE) is one of the most abundant neurotransmitters in the brain and it plays a significant role in selective attention 57 general arousal 58, and stress59, 60. The norepinephrine system originates mostly in the locus coeruleus and, as described above, has a central role in the response to stressors. Thus, a large variety of stressors will increase the firing activity of LC neurons 61 as well as increase the turnover of NE in many projection regions of the LC 62. The role of NE in drug abuse has long been neglected, as the dopamine-reward system has been the focus of most of the studies in this field. Nonetheless, NE release is reported to affect the reinstatement of drug-seeking behavior 63. Thus, the LC-NE system has been shown to be activated during withdrawal from drugs 64 and it has been suggested that part of the reinforcing properties of the addictive drug morphine derive in part from its ability to decrease stress-induced NE release and the anxiety associated with this release 65. Moreover, pharmacological studies using alpha2 adrenergic autoreceptor agonists have highlighted the role of NE in stress-induced reinstatement of drug-seeking 66, and alpha-2adrenergic antagonists induce an increase in dopamine-dependent locomotor activity 67.
Besides directly activating the LC-NE system, stressors can activate other structures that project to the LC, such as the BLA. Importantly, one structure that plays a major role in the emotional component of the stress response is the BLA 68. Thus, stressful stimuli such as footshock or tail-pinch induce activation of the amygdala 69, 70. In addition, synaptic plasticity within the amygdala is also affected by different stressors 24, 26. Moreover, chronic, as well as acute, stressors induced an increase in the activity of BLA neurons 71. However, the modulation of LC neuron activity by the BLA is indirect, via activation of the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) that will induce release of CRF in the dendritric pericoerulear regions 72. Thus, the BLA sends excitatory inputs to the CeA 70, a structure that will then activate the LC-NE system by releasing CRF 48. The relation between the LC-NE system and the BLA is reciprocal. Thus, besides sending an indirect projection to the LC, the BLA receives direct afferent projections from the locus coeruleus, and the NE release by the LC modulates the activity of BLA neuronsvia alpha-and beta-adrenergic receptors 73 (Figure 2).
The BLA plays also an important role in relapse to drug-seeking behavior, since inactivation of this nucleus affects the conditioned-cued reinstatement without modulating the effect of drug administration 74. Moreover, a disconnection study has shown that a strong interaction exists between the dopaminergic system and the BLA inducing cue-evoked firing of neurons from the NAc that will promote reward-seeking behavior 75.
Inputs from the BLA and the vSub have been described to converge on the same NAc neurons 28. Reciprocal connections between the BLA and the vSub have also been described 28 suggesting that BLA and vSub can interact with each other independent of their connectivity in the NAc. As mentioned above, the vSub is proposed to mediate the effects of stress in part via the vSub-NAc pathway. Moreover, the vSub receives numerous inputs from stress-related regions such as the LC-NE system, as well as the BLA 28. We have recently found that stimulation of the LC-NE system and the BLA activate vSub neuronal activity 50, and both acute and chronic stressors induce an increase of activity in those two inputs 24, 76. Thus, one hypothesis underlying drug sensitization and its modulation by stressors may involve the activation of the vSub-NAc pathway by the LC-NE system and/or the BLA leading to an increase in DA population activity, which mediates the increased behavioral response to psychostimulants.
Relapse to drug-seeking behavior depends on a complex array of factors: environmental context engaging the vSub, cue-induced reinstatement involving the BLA, and stressful events activating a widespread neural circuit including the vSub and BLA. Stressful events and drug abuse have common substrates. They both induce sensitization, which is a context-dependent event that involves hyperactivation of the mesolimbic DA system. The vSub is a pivotal structure that plays a major role in coordinating the response to stressful events and in drug-seeking behavior. We haves hown that the vSub, in particular the vSub-NAc pathway, is responsible for the hyperactivity of the DA system in response to a stressor and to drug sensitization. This structure receives two main inputs known to be activated by different stressors and involved in drug-seeking behavior: the LC-NE system and the BLA.
To better understand how drug administration can induce drug-relapse and drug-seeking behavior, it is important to study the pathophysiological alterations occurring in the stress-vSub-limbic system circuit. Such information is important in guiding future pharmacotherapy and treatment for addiction, via pharmacological intervention in one or several structures of this circuitry, such as the vSub or the BLA.