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Increased Glutamatergic Synaptic Transmission in the Amygdala of Drug Abusers
Increased Glutamatergic Synaptic Transmission in the Amygdala of Drug Abusers Stephen Rayport tudies of the biology of addiction have advanced considerably because many aspects of the drug-taking process and indeed addiction can be modeled in animals. Animals can be given or taught to self-administer drugs. They show tolerance and dependence. When deprived of drugs, they show withdrawal. And when given access to further drug or presented with drug-associated cues, they resume drug-taking, in effect relapsing. In animals, ongoing drug use is associated with alterations in brain systems that foster further use, presumably manifesting as addiction. Although animal studies have revealed significant drug-associated alterations in neurotransmitter signaling at the synaptic level, there has been a relative dearth of comparable information in humans. In this issue of Biological Psychiatry, Ökvist et al. (1) examined markers of glutamatergic synaptic transmission in the amygdala of two samples of drug abuse subjects: one sample who used cocaine, heroin, or both; and a second larger sample of subjects who used heroin. In total, they studied 73 subjects, a number sufficient to arrive at significant conclusions. Molecular alterations shared amongst different drugs of abuse are arguably more related to the addictive process (2), although not many shared transcripts were found in a recent microarray study comparing expression in the nucleus accumbens of cocaine and heroin users (3). The strategy Ökvist et al. pursued was to look for correlations amongst functionally interacting proteins that might not manifest as alterations in their levels. A further innovation was to look in the amygdala, which has a role in the encoding of intense, emotionally charged memories and has also been strongly implicated in addictive behavior (4). Drug abuse subjects were identified by their drug-associated demise and measured postmortem blood drug levels. Although a drug-associated demise is a strong indicator of a significant drug abuse history, several issues must be considered in the interpretation of the data. Because of the lack of psychological autopsies and thus diagnostic information, the likely increased mental health burden in the drug-abuse subjects cannot be ascertained. All the drugabuse subjects died with significant drug levels, so alterations seen in the postmortem measurements could be due to antemortem drug use. Measurements could be affected by the presence of the abused drug in the brain. A heroin overdose could be neuroprotective, whereas a cocaine overdose could be neurotoxic, differentially affecting postmortem measurements. These issues could be ruled out by administering the drugs to animals in parallel to the presumptive antemortem drug use of the subjects. Short of that, Ökvist et al. used correlations amongst measurements of synaptic parameters and blood drug levels to parse antemortem drug effects from addiction-associated changes. Animal studies of addiction have focused on sensitization, self administration, and drug-induced reinstatement, which correlate with human initiation of drug-taking behavior, ongoing drug-tak-
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From the Department of Psychiatry, Columbia University, New York, New York. Address correspondence to Stephen Rayport, M.D., Ph.D., Department of Psychiatry/Molecular Therapeutics, Columbia University, 1051 Riverside Drive, NYSPI Unit 62, New York, NY 10032-2695; E-mail: sgr1@columbia. edu. Received Nov 22, 2010; revised Nov 28, 2010; accepted Nov 30, 2010.
0006-3223/$36.00 doi:10.1016/j.biopsych.2010.11.023
ing behavior, and relapse. Koob and Volkow (4) describe these stages in the addiction process as “binge/intoxication, withdrawal/ negative affect, and preoccupation/anticipation (craving).” A crucial focus of animal research on addiction has been on drug-induced reinstatement, because it models the craving addicts face when they come to treatment with the goal of averting relapse. Research on cocaine-induced reinstatement has shown the crucial involvement of dopamine and glutamate signaling in the nucleus accumbens as well as in other related structures, such as the amygdala (4). Cocaine and cues associated with cocaine use elicit dopamine increases that act through D1 receptors. Local infusion of D1 agonists into the accumbens can trigger reinstatement, and blockade of D1 receptors in either the accumbens or amygdala can block reinstatement (4). D1 receptor activation modulates glutamate receptor trafficking leading to increased glutamate receptor expression in postsynaptic zones (5). Reinstatement is associated with increased surface expression of ␣-amino-3-hydroxy-5-methylisoxazole propionate–type glutamate receptors (GluR), and blocking their insertion impedes reinstatement. Strengthening glutamatergic transmission presumably reinforces signaling associated with habitual drug use and continued addictive behavior. In addressing this in human postmortem material, one must look as Ökvist et al. did at subjects who died because of excessive drug use. Such a demise is arguably most likely to occur during a relapse, when subjects are then less tolerant, at a point of greater emotional extremis, or exposed to drugs of unexpected potency. Implicitly, the brain state during relapse is less likely to be confounded by changes associated with chronic drug use and so more likely to reveal synaptic alterations associated with addiction. Ökvist et al. have made several remarkable observations in the postmortem amygdala that argue for shared alterations in glutamatergic synaptic transmission in addiction. They measured both messenger RNA expression and protein levels, revealing a striking correlation between PSD-95, a major constituent of the postsynaptic membrane, and GluA1 glutamate receptors. PSD-95 mediates synaptic strengthening by recruiting GluR to the postsynaptic membrane (Figure 1). Although the levels of PSD-95 and GluA1 were not themselves altered, the correlation argues that there was a tighter association between the molecules, which would be expected if GluA1 receptors interact with PSD-95 during recruitment to the postsynaptic membrane. They found this association in both the mixed user sample and the heroin user sample, providing evidence for the association as a common mechanism in drug abuse, and at the same time providing an internal replication of their finding in the two different samples. With the impetus of the association, they extended their studies to other key molecules involved in glutamate receptor trafficking. They looked at Homer 1 b/c, a molecule involved in the recruitment of GluR to synaptic sites. Homer 1 b/c is of particular interest, because reductions in Homer 1b/c in the nucleus accumbens in animals have been associated with the development of behavioral sensitization to cocaine via a glutamate receptor-dependent mechanism (6). In contrast, Homer knockout mice show increased sensitization, and Homer overexpression in the nucleus accumbens attenuates sensitization (7). Although Ökvist et al. found significant increases in Homer 1b/c in the amygdala, counter to the reductions BIOL PSYCHIATRY 2011;69:199 –200 © 2011 Society of Biological Psychiatry
200 BIOL PSYCHIATRY 2011;69:199 –200
Commentary and dynamin-3 levels correlated. Moreover, they found by co-immunoprecipitation an increased interaction between Homer 1 b/c and dynamin-3. This interaction, however, correlated with tissue opioid levels, indicating that it could be due to antemortem opioid use. They also found that increases in GluA1 and Homer 1b/c correlated with cocaine levels in the cocaine subjects but not the heroin subjects, mitigating a further concern. And, although the GluA1 correlation did not affect the overall measurement of GluA1— which was not increased—the Homer correlation could explain their results in the cocaine sample; however, they again found the same increase in the heroin subjects, mitigating this concern. Although these concerns are acknowledged, the Homer 1b/c– dynamin-3 association shows not only a heightened interaction in drug users but provides further support for the correlation analysis as a proxy for direct demonstrations of molecular interactions. These multiple measures of closely interacting proteins involved in the trafficking of GluR to the postsynaptic membrane support the idea that strengthening of glutamatergic synaptic transmission in the amygdala is a shared mechanism in drug addiction.
Figure 1. Trafficking of glutamate receptors (GluR) in the postsynaptic membrane. In this schematic of an excitatory synapse, a presynaptic terminal is shown above with a dendritic spine below. In the dendritic spine, GluR are concentrated at the postsynaptic membrane, tethered by PSD-95. The GluR that escape from the synapse as well as other extrasynaptic GluR are captured and internalized via a Homer– and dynamin-3– dependent mechanism, driving them to endocytic compartments under the postsynaptic membrane. There they form a pool of GluR ready to be reinserted into the postsynaptic membrane (8). With synaptic activity or with drug abuse, these molecules interact more closely, presumably increasing receptor trafficking to the membrane and strengthening synaptic transmission.
seen in animal studies in the accumbens, the difference might have to do with the Homer subtypes studied or the stage in the addiction process (because their postmortem studies were arguably being done in the relapse stage). Drug-associated alterations in Homer are heterogeneous across brain regions, which could also explain the difference. In any case, perturbation in Homer 1b/c levels supports the crucial involvement of Homer in addiction and thus in the trafficking of GluR. Ökvist et al. then examined dynamin-3, which interacts with Homer to maintain a dynamic pool of GluR in an endocytic recycling pool. This pool provides a ready reserve of GluR to be inserted in the postsynaptic membrane with synaptic strengthening (8). They found significant increases in dynamin-3. Increases in Homer 1 b/c
www.sobp.org/journal
The author reports no biomedical financial interests or potential conflicts of interest. 1. Ökvist A, Fagergren P, Whittard J, Garcia-Osta A, Drakenberg K, Horvath MCs, et al. (2011): Dysregulated postsynaptic density and endocytic zone in the amygdala of human heroin and cocaine abusers. Biol Psychiatry 69:245–252. 2. Knackstedt LA, Kalivas PW (2009): Glutamate and reinstatement. Curr Opin Pharmacol 9:59 – 64. 3. Albertson DN, Schmidt CJ, Kapatos G, Bannon MJ (2006): Distinctive profiles of gene expression in the human nucleus accumbens associated with cocaine and heroin abuse. Neuropsychopharmacology 31:2304 – 2312. 4. Koob G, Volkow N (2009): Neurocircuitry of addiction. Neuropsychopharmacology 35:217–238. 5. Chao SZ, Ariano MA, Peterson DA, Wolf ME (2002): D1 dopamine receptor stimulation increases GluR1 surface expression in nucleus accumbens neurons. J Neurochem 83:704 –712. 6. Ghasemzadeh MB, Permenter LK, Lake R, Worley PF, Kalivas PW (2003): Homer1 proteins and AMPA receptors modulate cocaine-induced behavioural plasticity. Eur J Neurosci 18:1645–1651. 7. Szumlinski KK, Dehoff MH, Kang SH, Frys KA, Lominac KD, Klugmann M, et al. (2004): Homer proteins regulate sensitivity to cocaine. Neuron 43:401– 413. 8. Newpher TM, Ehlers MD (2008): Glutamate receptor dynamics in dendritic microdomains. Neuron 58:472– 497.
S
From the Department of Psychiatry, Columbia University, New York, New York. Address correspondence to Stephen Rayport, M.D., Ph.D., Department of Psychiatry/Molecular Therapeutics, Columbia University, 1051 Riverside Drive, NYSPI Unit 62, New York, NY 10032-2695; E-mail: sgr1@columbia. edu. Received Nov 22, 2010; revised Nov 28, 2010; accepted Nov 30, 2010.
0006-3223/$36.00 doi:10.1016/j.biopsych.2010.11.023
ing behavior, and relapse. Koob and Volkow (4) describe these stages in the addiction process as “binge/intoxication, withdrawal/ negative affect, and preoccupation/anticipation (craving).” A crucial focus of animal research on addiction has been on drug-induced reinstatement, because it models the craving addicts face when they come to treatment with the goal of averting relapse. Research on cocaine-induced reinstatement has shown the crucial involvement of dopamine and glutamate signaling in the nucleus accumbens as well as in other related structures, such as the amygdala (4). Cocaine and cues associated with cocaine use elicit dopamine increases that act through D1 receptors. Local infusion of D1 agonists into the accumbens can trigger reinstatement, and blockade of D1 receptors in either the accumbens or amygdala can block reinstatement (4). D1 receptor activation modulates glutamate receptor trafficking leading to increased glutamate receptor expression in postsynaptic zones (5). Reinstatement is associated with increased surface expression of ␣-amino-3-hydroxy-5-methylisoxazole propionate–type glutamate receptors (GluR), and blocking their insertion impedes reinstatement. Strengthening glutamatergic transmission presumably reinforces signaling associated with habitual drug use and continued addictive behavior. In addressing this in human postmortem material, one must look as Ökvist et al. did at subjects who died because of excessive drug use. Such a demise is arguably most likely to occur during a relapse, when subjects are then less tolerant, at a point of greater emotional extremis, or exposed to drugs of unexpected potency. Implicitly, the brain state during relapse is less likely to be confounded by changes associated with chronic drug use and so more likely to reveal synaptic alterations associated with addiction. Ökvist et al. have made several remarkable observations in the postmortem amygdala that argue for shared alterations in glutamatergic synaptic transmission in addiction. They measured both messenger RNA expression and protein levels, revealing a striking correlation between PSD-95, a major constituent of the postsynaptic membrane, and GluA1 glutamate receptors. PSD-95 mediates synaptic strengthening by recruiting GluR to the postsynaptic membrane (Figure 1). Although the levels of PSD-95 and GluA1 were not themselves altered, the correlation argues that there was a tighter association between the molecules, which would be expected if GluA1 receptors interact with PSD-95 during recruitment to the postsynaptic membrane. They found this association in both the mixed user sample and the heroin user sample, providing evidence for the association as a common mechanism in drug abuse, and at the same time providing an internal replication of their finding in the two different samples. With the impetus of the association, they extended their studies to other key molecules involved in glutamate receptor trafficking. They looked at Homer 1 b/c, a molecule involved in the recruitment of GluR to synaptic sites. Homer 1 b/c is of particular interest, because reductions in Homer 1b/c in the nucleus accumbens in animals have been associated with the development of behavioral sensitization to cocaine via a glutamate receptor-dependent mechanism (6). In contrast, Homer knockout mice show increased sensitization, and Homer overexpression in the nucleus accumbens attenuates sensitization (7). Although Ökvist et al. found significant increases in Homer 1b/c in the amygdala, counter to the reductions BIOL PSYCHIATRY 2011;69:199 –200 © 2011 Society of Biological Psychiatry
200 BIOL PSYCHIATRY 2011;69:199 –200
Commentary and dynamin-3 levels correlated. Moreover, they found by co-immunoprecipitation an increased interaction between Homer 1 b/c and dynamin-3. This interaction, however, correlated with tissue opioid levels, indicating that it could be due to antemortem opioid use. They also found that increases in GluA1 and Homer 1b/c correlated with cocaine levels in the cocaine subjects but not the heroin subjects, mitigating a further concern. And, although the GluA1 correlation did not affect the overall measurement of GluA1— which was not increased—the Homer correlation could explain their results in the cocaine sample; however, they again found the same increase in the heroin subjects, mitigating this concern. Although these concerns are acknowledged, the Homer 1b/c– dynamin-3 association shows not only a heightened interaction in drug users but provides further support for the correlation analysis as a proxy for direct demonstrations of molecular interactions. These multiple measures of closely interacting proteins involved in the trafficking of GluR to the postsynaptic membrane support the idea that strengthening of glutamatergic synaptic transmission in the amygdala is a shared mechanism in drug addiction.
Figure 1. Trafficking of glutamate receptors (GluR) in the postsynaptic membrane. In this schematic of an excitatory synapse, a presynaptic terminal is shown above with a dendritic spine below. In the dendritic spine, GluR are concentrated at the postsynaptic membrane, tethered by PSD-95. The GluR that escape from the synapse as well as other extrasynaptic GluR are captured and internalized via a Homer– and dynamin-3– dependent mechanism, driving them to endocytic compartments under the postsynaptic membrane. There they form a pool of GluR ready to be reinserted into the postsynaptic membrane (8). With synaptic activity or with drug abuse, these molecules interact more closely, presumably increasing receptor trafficking to the membrane and strengthening synaptic transmission.
seen in animal studies in the accumbens, the difference might have to do with the Homer subtypes studied or the stage in the addiction process (because their postmortem studies were arguably being done in the relapse stage). Drug-associated alterations in Homer are heterogeneous across brain regions, which could also explain the difference. In any case, perturbation in Homer 1b/c levels supports the crucial involvement of Homer in addiction and thus in the trafficking of GluR. Ökvist et al. then examined dynamin-3, which interacts with Homer to maintain a dynamic pool of GluR in an endocytic recycling pool. This pool provides a ready reserve of GluR to be inserted in the postsynaptic membrane with synaptic strengthening (8). They found significant increases in dynamin-3. Increases in Homer 1 b/c
www.sobp.org/journal
The author reports no biomedical financial interests or potential conflicts of interest. 1. Ökvist A, Fagergren P, Whittard J, Garcia-Osta A, Drakenberg K, Horvath MCs, et al. (2011): Dysregulated postsynaptic density and endocytic zone in the amygdala of human heroin and cocaine abusers. Biol Psychiatry 69:245–252. 2. Knackstedt LA, Kalivas PW (2009): Glutamate and reinstatement. Curr Opin Pharmacol 9:59 – 64. 3. Albertson DN, Schmidt CJ, Kapatos G, Bannon MJ (2006): Distinctive profiles of gene expression in the human nucleus accumbens associated with cocaine and heroin abuse. Neuropsychopharmacology 31:2304 – 2312. 4. Koob G, Volkow N (2009): Neurocircuitry of addiction. Neuropsychopharmacology 35:217–238. 5. Chao SZ, Ariano MA, Peterson DA, Wolf ME (2002): D1 dopamine receptor stimulation increases GluR1 surface expression in nucleus accumbens neurons. J Neurochem 83:704 –712. 6. Ghasemzadeh MB, Permenter LK, Lake R, Worley PF, Kalivas PW (2003): Homer1 proteins and AMPA receptors modulate cocaine-induced behavioural plasticity. Eur J Neurosci 18:1645–1651. 7. Szumlinski KK, Dehoff MH, Kang SH, Frys KA, Lominac KD, Klugmann M, et al. (2004): Homer proteins regulate sensitivity to cocaine. Neuron 43:401– 413. 8. Newpher TM, Ehlers MD (2008): Glutamate receptor dynamics in dendritic microdomains. Neuron 58:472– 497.