To study the neurocircuitry of the preoccupation/anticipation stage, animal models have been characterized extensively using drug-, cue-, and stress-induced reinstatement (see Animal Models). In animal models of cocaine-induced reinstatement, the neuropharmacological circuits have focused largely on dopaminergic and glutamatergic mechanisms (Figure 4.22; for further reading, see Shaham et al., 2003). Activation of the mesocorticolimbic dopamine system is clearly implicated in cocaine-induced reinstatement, based on studies of dopamine agonists that mimic the effects of cocaine and studies of dopamine receptor antagonists that block the effects of cocaine, depending on which brain region is targeted. Dopamine antagonism in the dorsal prefrontal cortex but not the nucleus accumbens blocked cocaine-induced reinstatement. Cocaine-induced reinstatement was facilitated by glutamatergic agonists and inhibited by glutamatergic antagonists at various levels of the mesocorticolimbic dopamine system, including the nucleus accumbens, prefrontal cortex, and ventral tegmental area.
Figure 4.21 Mean (± SEM) dialysate corticotropin-releasing factor (CRF) concentrations collected from the central nucleus of the amygdala of rats during baseline, a 12 h cocaine self-administration session, and a subsequent 12 h withdrawal period (cocaine group). CRF levels in animals with the same history of cocaine self-administration training and drug exposure, but not given access to cocaine on the test day, are shown for comparison (control group). Dialysate samples were collected over 2 h periods alternating with 1 h nonsampling periods. During cocaine self-administration, dialysate CRF concentrations in the cocaine group were decreased by about 25% compared with control animals. In contrast, termination of access to cocaine significantly increased CRF efflux, which began approximately 5 h post-cocaine and reached about 400% of presession baseline levels at the end of the withdrawal session. The asterisks (*) indicate significant differences between the cocaine and control groups, with multiple asterisks indicating more pronounced differences. [From: Richter RM, Weiss F. In vivo CRF release in rat amygdala is increased during cocaine withdrawal in self-administering rats. Synapse, 1999, (32), 254–261.]
Cue-induced reinstatement involves stimuli that were previously associated with cocaine reinforcement that can then elicit the reinstatement of cocaine self-administration. This cue-induced responding involves basal forebrain projections and connections with the origins and terminal areas of the mesocorticolimbic dopamine system. Cocaine-predictive stimuli activated Fos protein in the basolateral amygdala and medial prefrontal cortex (Box 4.15). With regard to cue-induced reinstatement, Fos activation in the amygdala in rats parallels findings in humans of neural activation within the amygdala and anterior cingulate during cue-induced craving for cocaine. The medial prefrontal cortex/nucleus accumbens glutamate connection appears to be critical for cue-induced reinstatement, in a similar way to cocaine-induced reinstatement (Figure 4.22; for further reading, see Kalivas and McFarland 2003; See et al., 2003).
Using in vivo electrophysiological measures, neurons in the nucleus accumbens also seem to respond to stimuli paired with cocaine reinforcement. Cells in the nucleus accumbens that exhibit a post-response change in firing within seconds of the reinforced response can be controlled by the stimulus that was paired with cocaine delivery. Similarly, neurons in the basolateral amygdala that exhibited increased firing immediately after the cocaine response can be activated by an audio/visual cue paired with cocaine. In a particularly intriguing study, neurons in both the nucleus accumbens and medial prefrontal cortex were recorded simultaneously in rats that self-administered cocaine. These neurons “anticipated” a cocaine infusion and increased their firing a few seconds before the lever press. Inter- and intraregional correlations in firing patterns were found between pairs of simultaneously recorded neurons in these two distinct brain areas.
Figure 4.22 The role of glutamate (GLU) and dopamine (DA) transmission in the relapse to drug-seeking behavior. During baseline neurotransmission, tonic dopamine and glutamate transmission equally modulate the output of the nucleus accumbens (NAc) to allow normal locomotor activity. Following acute cocaine administration, dopamine levels in the nucleus accumbens are elevated, with little effect on glutamatergic tone, to increase locomotor activity and stimulate reward processes. After withdrawal from chronic drug intake, a single cocaine administration may induce relapse to drug taking or paranoia through increased dopamine release associated with and dependent on increased glutamate transmission which may be a consequence of interoceptive cues (that is, internal cues within the body or brain) associated with drug taking. However, in the absence of cocaine administration, an environmental cue may induce craving and relapse through enhanced glutamate transmission with little dopamine involvement. [From: Cornish JL, Kalivas PW. Cocaine sensitization and craving: differing roles for dopamine and glutamate in the nucleus accumbens. Journal of Addictive Diseases, 2001, (20), 43–54.]
FOS PROTEIN AND THE C-FOS GENEFos protein and its gene, c-fos, are common markers of brain activation used in neurobiological research. Fos detection tells researchers which brain regions have neurons that are activated by a particular manipulation, so this procedure is often used to narrow the field of regions that are likely candidates for further investigation.
Stress-induced reinstatement involves activation of the brain stress neurotransmitter CRF outside of the HPA axis in the extended amygdala. CRF receptor antagonists block the footshock-induced reinstatement of cocaine-seeking behavior in rats. The bed nucleus of the stria terminalis, an area rich in CRF receptors, terminals, and cell bodies, appears to play a significant role. An asymmetric lesion technique functionally dissected a critical, but not exclusive, role of the CRF pathway that originates in the central nucleus of the amygdala and projects to the bed nucleus of the stria terminalis. Similarly, microinjection studies revealed a role for the ventral noradrenergic (norepinephrine) pathway that projects to the bed nucleus of the stria terminalis in the stress-induced reinstatement of cocaine seeking behavior (Figure 4.23). These observations are consistent with major reciprocal connections of CRF and norepinephrine in the basal forebrain and brainstem, in which CRF activates brainstem norepinephrine, and norepinephrine activates forebrain CRF (for further reading, see Aston-Jones and Harris, 2004; Koob and Kreek, 2007). Such an hypothesized loop might explain the intensified stress responses that occur with repeated drug exposure that could lead to psychopathology and addiction.
Vulnerability to relapse continues for weeks, months, and even years after acute withdrawal, and long-term molecular changes in the brain circuits described above are thought to mediate such vulnerability. One hypothesized mechanism for long-term molecular changes in cellular function is the induction of transcription factors (proteins that bind to specific parts of DNA and control the transfer or transcription of genetic information from DNA to RNA). Acute administration of cocaine induces the expression of the transcription factor c-fos, but the expression is short-lived and returns to normal levels within 12 h of drug exposure. Chronic cocaine administration reduced the ability of cocaine to induce c-fos expression, suggesting tolerance to this effect. However, chronic cocaine caused an accumulation of another transcription factor, activator protein 1 (AP-1), which is composed of proteins from the Fos family, specifically ΔFosB. The induction of ΔFosB in the nucleus accumbens is long-lived after chronic cocaine exposure, and ΔFosB overexpression increases the sensitivity to the locomotor-activating and rewarding effects of cocaine, increases cocaine self-administration, and increases progressive-ratio responding for cocaine. ΔFosB, therefore, has a potential role in initiating and maintaining an addictive state by increasing the drive for drug reward, even weeks and months after the last drug exposure (for further reading, see Nestler, 2005).
In parallel, during protracted abstinence, imaging studies have reported that enhanced sensitivity to conditioned cues also occurs during detoxification. The brain sites that mediate such responses include basal forebrain projections and connections with the mesocorticolimbic dopamine system and hippocampus, including the basolateral amygdala, medial prefrontal cortex, nucleus accumbens, and ventral pallidum. These conditioned responses can help sustain the cycle of abstinence and relapse that is a hallmark of substance use disorders. Imaging studies that evaluated markers of brain function have also shown that drug abusers who are tested during protracted detoxification have disrupted activity of frontal brain regions, including dorsolateral prefrontal regions, the cingulate gyrus, and the orbitofrontal cortex. This dysfunction may underlie impaired inhibitory control and impulsivity and contribute to relapse (for further reading, see Jentsch and Taylor 1999; Goldstein and Volkow, 2002; Volkow et al., 2003).