Biomedical subjects
Publications and source records attributed to Peter W Kalivas.
Although many drug-induced neural changes are known, progress has been slow in identifying the ones that actually mediate addiction. Identifying changes that are specific to particular elements of the transition from initial to habitual to relapsing drug use may be a fruitful strategy for pinpointing which forms of drug-induced plasticity are critical for addiction.
BACKGROUND: An increased level of extracellular glutamate is a key neurochemical feature associated with ethanol exposure and withdrawal. METHODS: In the current study, extracellular levels of glutamate and glutamate transport in the nucleus accumbens were assayed 24 hr after repeated ethanol exposure (1 g/kg ip daily for 7 days) with use of in vivo no-net-flux microdialysis and in vitro [(3)H]glutamate uptake, respectively. RESULTS: Microdialysis revealed higher extracellular glutamate concentrations in the nucleus accumbens of rats that were given ethanol. The increase in basal extracellular glutamate levels was accounted for in part by a decrease in the in vivo probe recovery of glutamate. Moreover, an in vitro accumbens slice preparation measuring [(3)H]glutamate uptake revealed that Na(+)-dependent [(3)H]glutamate uptake was significantly reduced 24 hr after 7 days of repeated ethanol exposure. The ethanol-induced deficit in glutamate uptake was not associated with decreased total tissue levels of the transporters GLAST or GLT1. The in vivo and in vitro ethanol-induced changes in glutamate levels and uptake returned to control levels 14 days after discontinuing 7 days of repeated ethanol exposure. CONCLUSIONS: These results suggest that the previously reported increases in extracellular glutamate induced by ethanol exposure may be due in part to deficits in glutamate transport.
The expression of the transcriptional regulator NAC1 is increased in the nucleus accumbens of rats withdrawn from cocaine self-administration, and in vivo studies indicate that the up-regulation is a compensatory mechanism opposing the acute effects of cocaine. Both mammalian two-hybrid assay and punctate localization largely in the nucleus suggest NAC1 is a transcriptional regulator. However, in this report it is shown that in differentiated PC12 and Neuro2A cells, as well as in primary cortical neurons, NAC1 is diffusely expressed not only in the cell nucleus but also in cytoplasm. Blockade of spontaneous electrical activity by tetrodotoxin prevented the diffuse expression of NAC1, and depolarization with high potassium concentrations induced diffuse cellular localization in non-differentiating cells. The use of protein kinase C (PKC) inhibitors and activator, as well as the systematic mutation of potential PKC phosphorylation sites in NAC1, demonstrated that phosphorylation of residue S245 by PKC is a necessary event inducing diffuse NAC1 expression outside of the nucleus. These observations indicate a potential non-transcriptional role for NAC1 in the brain.
OBJECTIVE: A primary behavioral pathology in drug addiction is the overpowering motivational strength and decreased ability to control the desire to obtain drugs. In this review the authors explore how advances in neurobiology are approaching an understanding of the cellular and circuitry underpinnings of addiction, and they describe the novel pharmacotherapeutic targets emerging from this understanding. METHOD: Findings from neuroimaging of addicts are integrated with cellular studies in animal models of drug seeking. RESULTS: While dopamine is critical for acute reward and initiation of addiction, end-stage addiction results primarily from cellular adaptations in anterior cingulate and orbitofrontal glutamatergic projections to the nucleus accumbens. Pathophysiological plasticity in excitatory transmission reduces the capacity of the prefrontal cortex to initiate behaviors in response to biological rewards and to provide executive control over drug seeking. Simultaneously, the prefrontal cortex is hyperresponsive to stimuli predicting drug availability, resulting in supraphysiological glutamatergic drive in the nucleus accumbens, where excitatory synapses have a reduced capacity to regulate neurotransmission. CONCLUSIONS: Cellular adaptations in prefrontal glutamatergic innervation of the accumbens promote the compulsive character of drug seeking in addicts by decreasing the value of natural rewards, diminishing cognitive control (choice), and enhancing glutamatergic drive in response to drug-associated stimuli.
Drug addiction involves complex interactions between pharmacology and learning in genetically susceptible individuals. Members of the Homer gene family are regulated by acute and chronic cocaine administration. Here, we report that deletion of Homer1 or Homer2 in mice caused the same increase in sensitivity to cocaine-induced locomotion, conditioned reward, and augmented extracellular glutamate in nucleus accumbens as that elicited by withdrawal from repeated cocaine administration. Moreover, adeno-associated virus-mediated restoration of Homer2 in the accumbens of Homer2 KO mice reversed the cocaine-sensitized phenotype. Further analysis of Homer2 KO mice revealed extensive additional behavioral and neurochemical similarities to cocaine-sensitized animals, including accelerated acquisition of cocaine self-administration and altered regulation of glutamate by metabotropic glutamate receptors and cystine/glutamate exchange. These data show that Homer deletion mimics the behavioral and neurochemical phenotype produced by repeated cocaine administration and implicate Homer in regulating addiction to cocaine.
Chronic cocaine administration reduces G protein signaling efficacy. Here, we report that the expression of AGS3, which binds to GialphaGDP and inhibits GDP dissociation, was upregulated in the prefrontal cortex (PFC) during late withdrawal from repeated cocaine administration. Increased AGS3 was mimicked in the PFC of drug-naive rats by microinjecting a peptide containing the Gialpha binding domain (GPR) of AGS3 fused to the cell permeability domain of HIV-Tat. Infusion of Tat-GPR mimicked the phenotype of chronic cocaine-treated rats by manifesting sensitized locomotor behavior and drug seeking and by increasing glutamate transmission in nucleus accumbens. By preventing cocaine withdrawal-induced AGS3 expression with antisense oligonucleotides, signaling through Gialpha was normalized, and both cocaine-induced relapse to drug seeking and locomotor sensitization were prevented. When antisense oligonucleotide infusion was discontinued, drug seeking and sensitization were restored. It is proposed that AGS3 gates the expression of cocaine-induced plasticity by regulating G protein signaling in the PFC.
The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.
Addiction to cocaine results from changes in brain function arising from a combination of pharmacology, environmental circumstances, as well as genetic vulnerability. One change is a reduction in Homer protein in the nucleus accumbens. In this report we summarize the behavioral and neurochemical effects of Homer2 gene deletion in mice and compare this with the effects of chronic cocaine treatment in rats. It was shown that Homer2 KO mice demonstrate enhanced locomotor stimulant and conditioned place preference responses to cocaine. Homer2 deletion also caused mice to show reduced basal extracellular glutamate in the nucleus accumbens and a sensitized increase in extracellular glutamate in response to a cocaine injection. In contrast to glutamate, Homer2 KO mice showed a normal increase in extracellular dopamine following a cocaine challenge injection. The parallel between the effect of Homer2 gene deletion and chronic cocaine administration on behavioral and glutamatergic neurochemical responses to cocaine supports involvement of Homer proteins and glutamate transmission in the sensitization of behavior produced by repeated cocaine.
The rewarding effects of drugs of abuse have been linked to increases in dopamine transmission. However, changes in brain chemistry and morphology that are produced in addiction underlie the long-lasting vulnerability to relapse and are more closely linked with the adaptations in excitatory transmission. The drug-induced changes in excitatory transmission seem to be pathologic exacerbations of normal forms of brain plasticity, and they occur in the brain areas linked by neuroimaging studies in addicted patients to craving and relapse. This review describes the brain adaptations produced in excitatory transmission by addictive drugs and identifies new potential sites of pharmacotherapeutic intervention to ameliorate addiction.
All addictive drugs facilitate dopamine transmission, and determining the role of dopamine has been the predominant focus of biomedical research in addiction for 20 years. Newer data and hypotheses have begun to shift our focus to involvement of cortex and corticofugal glutamate projections. The rationale for shifting focus to glutamate ranges from evidence showing that cortical activity is altered in addicts to data from animal models demonstrating drug-induced changes in the function of proteins that regulate pre- and postsynaptic glutamate neurotransmission. Recent studies have particularly focused on involvement of a circuit that includes glutamate projections from the prefrontal cortex to the nucleus accumbens.
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A distinct role for serotonin transmission from the dorsal and median raphé nuclei (DRN and MRN, respectively) was identified in regulating the behavioral and neurochemical effects of acute and repeated cocaine administration. Serotonin 1A (5-hydroxytryptophan (5-HT)1A) receptors were stimulated by intraraphé microinjection of 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT; 5 or 10 microg) and behavior, as well as extracellular neurotransmitter content in the nucleus accumbens was measured. Pretreatment of the DRN with DPAT caused a sensitization-like potentiation of acute cocaine-induced motor activity and an elevation in extracellular dopamine and glutamate. In contrast, DPAT microinjection into the MRN did not alter acute cocaine-induced motor activity or extracellular levels of dopamine or glutamate. Acutely, DPAT microinjection into either raphé nucleus reduced the basal and acute cocaine-stimulated levels of extracellular serotonin. Pretreatment with DPAT before systemic cocaine administration was continued for 5 days, and 3 weeks after the last injection, all rats were administered a cocaine challenge injection. The sensitized behavioral and neurochemical response produced by repeated cocaine in control subjects was unaffected by the intra-DRN administration of DPAT. However, in animals administered DPAT into the MRN, both the sensitized motor response and the increase in glutamate were augmented, while the sensitized serotonin response was blocked, without altering dopamine sensitization. These data show a differential role for 5-HT1A receptors in the DRN and MRN in the acute and sensitized effects of cocaine. While the DRN is involved in the acute effects of cocaine, neuroadaptations in the MRN may regulate the long-term consequences of repeated cocaine exposure.
Rearing rats in impoverished (IC) and enriched (EC) environmental conditions alters synaptic plasticity and cognitive processes. Metabotropic glutamate receptors (mGluRs) are known to play a key role in synaptic and behavioral plasticity. In the present study, the effect of rearing conditions on the expression of mGluR proteins in the prefrontal cortex (PFC) was assessed by immunoblotting. A significant difference in the content of prefrontal mGluR1 and mGluR5 (ie group I) and mGluR2/3 (ie group II) was observed between IC and EC rats. To functionally characterize this difference, in vivo microdialysis was used to verify differences in mGluR regulation of extracellular glutamate in the PFC. The results indicate that the capacity of group I and II mGluRs to elevate extracellular glutamate levels was significantly blunted in the PFC of IC rats compared to either EC subjects, or rats reared in normal environmental conditions (ie NIH standards). Group II mGluR receptors regulate performance in a forced T-maze spatial memory task that involves the PFC, and IC rats demonstrated deficits in this task relative to EC rats. These data suggest that reduced mGluR transmission in the PFC produced by impoverished, relative to enriched, rearing environments may contribute to cognitive deficits.
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Repeated cocaine causes enduring changes in dopamine and glutamate transmission in the nucleus accumbens, and dopamine and glutamate terminals synapse on GABAergic accumbens neurons. The present study demonstrates that there are changes in GABA transmission in the accumbens at 3 weeks after discontinuing daily cocaine injections. No-net flux microdialysis revealed a significant increase in the basal levels of extracellular GABA in the accumbens of cocaine-treated rats. The elevated extracellular GABA was normalized by blocking voltage-dependent Na+ channels and provided increased tone on GABA(B) presynaptic autoreceptors and heteroreceptors because blocking GABA(B) receptors produced a greater elevation in extracellular GABA, dopamine, and glutamate in cocaine-treated compared with control subjects. For many G-protein-coupled receptors, increased agonist can cause receptor desensitization. Consistent with GABA(B) receptor desensitization, baclofen-stimulated GTPgammaS binding was reduced, and the reduction in G-protein coupling was accompanied by reduced Ser phosphorylation of the GABA(B2) receptor subunit. No effect by repeated cocaine was found in the levels of total GABA(B1) or GABA(B2) protein. Together, these data demonstrate that withdrawal from repeated cocaine treatment produces an increase in the basal levels of extracellular GABA in the accumbens that depends on neuronal activity. The increase may be mediated in part by functional desensitization of GABA(B) receptors, likely the result of diminished Ser phosphorylation of the GABA(B2) receptor.
The relative contributions of glutamate and dopamine within the nucleus accumbens to cocaine-induced reinstatement of drug-seeking behavior were assessed. When extinguished cocaine self-administration behavior was reinstated by a cocaine-priming injection, extracellular levels of both dopamine and glutamate were elevated in the nucleus accumbens. However, when yoked cocaine or saline control subjects were administered a cocaine prime, only dopamine levels were elevated. Thus, glutamate increased only when animals reinstated lever pressing, whereas dopamine increased regardless of behavior. The increase in glutamate was not accounted for simply by the act of lever pressing itself, because the cocaine self-administration group still demonstrated elevated glutamate when the levers were withdrawn from the operant chamber. Moreover, reinstatement of lever pressing for food did not elevate extracellular glutamate, indicating that increased glutamate initiated responding selectively for a drug reinforcement. The source of glutamate was shown to be glutamatergic afferents from the prefrontal cortex because inhibiting prefrontal cortical glutamatergic neurons that project to the accumbens prevented the rise in glutamate. Together, these data demonstrate that activation of a glutamatergic projection from the prefrontal cortex to the nucleus accumbens underlies cocaine-primed reinstatement of drug-seeking behavior.
RATIONALE: Recent studies have attempted to identify the neuroanatomical substrates underlying primed reinstatement of drug-seeking behavior. Identification of neuronal substrates will provide a logical rationale for designing pharmacological interventions in treating drug relapse. OBJECTIVE: The objective was to identify brain circuitry that is shared between cue-, drug- and stress-primed reinstatement, as well as identifying aspects of brain circuitry that are distinct for each stimulus modality. The resulting circuit offers theoretical interpretations for consideration in future studies. RESULTS: Aspects of the circuitry mediating reinstatement can be identified with reasonable confidence. The role of the basolateral amygdala in cue-primed reinstatement, the role of the ventral tegmental area in drug-primed reinstatement and the role of adrenergic innervation of the extended amygdala in stress-primed reinstatement are well characterized. Also, all three modes for priming reinstatement may converge on the anterior cingulate cortex and have a final common output through the core of the nucleus accumbens. Lacunae in our understanding of the circuit were identified, especially with regard to how stress priming is conveyed from the extended amygdala to the shared anterior cingulate accumbens core circuit. CONCLUSIONS: The proposed convergence of priming stimuli into the glutamatergic projection from anterior cingulate to the accumbens core combined with the changes in glutamate transmission and signaling that accompany repeated psychostimulant administration points to the potential value of pharmacological agents that manipulate glutamate release or postsynaptic glutamate receptor signaling and trafficking in treating primed relapse in addicts.
Repeated cocaine treatment and withdrawal produces changes in brain function thought to be involved in relapse to drug use. Withdrawal from repeated cocaine reduced in vivo extracellular glutamate in the nucleus accumbens of rats by decreasing the exchange of extracellular cystine for intracellular glutamate. In vivo restoration of cystine/glutamate exchange by intracranial perfusion of cystine or systemically administered N-acetylcysteine normalized the levels of glutamate in cocaine-treated subjects. To determine if the reduction in nonvesicular glutamate release is a mediator of relapse, we examined cocaine-primed reinstatement of drug seeking after cocaine self-administration was stopped. Reinstatement was prevented by stimulating cystine/glutamate exchange with N-acetylcysteine and restoring extracellular glutamate. Thus, withdrawal from repeated cocaine increases susceptibility to relapse in part by reducing cystine/glutamate exchange, and restoring exchanger activity prevents cocaine-primed drug seeking.