Ryan T. LaLumiere

Infralimbic Prefrontal Cortex Is Responsible for Inhibiting Cocaine Seeking in Extinguished Rats

The rat prelimbic prefrontal cortex and nucleus accumbens core are critical for initiating cocaine seeking. In contrast, the neural circuitry responsible for inhibiting cocaine seeking during extinction is unknown. The present findings using inhibition of selected brain nuclei with GABA agonists show that the suppression of cocaine seeking produced by previous extinction training required activity in the rat infralimbic cortex. Conversely, the reinstatement of drug seeking by a cocaine injection in extinguished animals was suppressed by increasing neuronal activity in infralimbic cortex with the glutamate agonist AMPA. The cocaine seeking induced by inactivating infralimbic cortex resembled other forms of reinstated drug seeking by depending on activity in prelimbic cortex and the basolateral amygdala. A primary efferent projection from the infralimbic cortex is to the nucleus accumbens shell. Akin to infralimbic cortex, inhibition of the accumbens shell induced cocaine seeking in extinguished rats. However, bilateral inhibition of the shell also elicited increased locomotor activity. Nonetheless, unilateral inhibition of the accumbens shell did not increase motor activity, and simultaneous unilateral inactivation of the infralimbic cortex and shell induced cocaine seeking, suggesting that an interaction between these two structures is necessary for extinction training to inhibit cocaine seeking. The infralimbic cortex and accumbens shell appear to be recruited by extinction learning because inactivation of these structures before extinction training did not alter cocaine seeking. Together, these findings suggest that a neuronal network involving the infralimbic cortex and accumbens shell is recruited by extinction training to suppress cocaine seeking.

Glutamate Release in the Nucleus Accumbens Core Is Necessary for Heroin Seeking

Long-term changes in glutamate transmission in the nucleus accumbens core (NAcore) contribute to the reinstatement of drug seeking after extinction of cocaine self-administration. Whether similar adaptations in glutamate transmission occur during heroin and cue-induced reinstatement of heroin seeking is unknown. After 2 weeks of heroin self-administration and 2 weeks of subsequent extinction training, heroin seeking was induced by a noncontingent injection of heroin or by presentation of light/tone cues previously paired with heroin infusions. Microdialysis was conducted in the NAcore during reinstatement of heroin seeking in animals extinguished from heroin self-administration or in subjects receiving parallel (yoked) noncontingent saline or heroin. Reinstatement by either heroin or cue increased extracellular glutamate in the NAcore in the self-administration group, but no increase was elicited during heroin-induced reinstatement in the yoked control groups. The increase in glutamate during heroin-induced drug seeking was abolished by inhibiting synaptic transmission in the NAcore with tetrodotoxin or by inhibiting glutamatergic afferents to the NAcore from the prelimbic cortex. Supporting critical involvement of glutamate release, heroin seeking induced by cue or heroin was blocked by inhibiting AMPA/kainate glutamate receptors in the NAcore. Interestingly, although a heroin-priming injection increased dopamine equally in animals trained to self-administer heroin and in yoked-saline subjects, inhibition of dopamine receptors in the NAcore also blocked heroin- and cue-induced drug seeking. Together, these findings show that recruitment of the glutamatergic projection from the prelimbic cortex to NAcore is necessary to initiate the reinstatement of heroin seeking.

Glutamate transmission in addiction.

Cortico-striatal glutamate transmission has been implicated in both the initiation and expression of addiction related behaviors, such as locomotor sensitization and drug-seeking. While glutamate transmission onto dopamine cells in the ventral tegmental area undergoes transient plasticity important for establishing addiction-related behaviors, glutamatergic plasticity in the nucleus accumbens is critical for the expression of these behaviors. This information points to the value of exploring pharmacotherapeutic manipulation of glutamate plasticity in treating drug addiction.

Post-Training Intra-Basolateral Amygdala Infusions of Norepinephrine Enhance Consolidation of Memory for Contextual Fear Conditioning

Post-training infusions of drugs, including noradrenergic agonists and antagonists, into the basolateral amygdala (BLA) influence the consolidation of memory for training in several tasks, including inhibitory avoidance. There is, however, conflicting evidence concerning whether post-training intra-BLA drug infusions modulate the consolidation of contextual fear conditioning (CFC). In the present study, norepinephrine (NE) was infused bilaterally into the BLA of male Sprague Dawley rats immediately after training on two CFC tasks: a Y-maze and a straight alley. Post-training intra-BLA infusions enhanced memory of CFC training in the Y-maze, as assessed by percentage of time spent freezing and shock arm entrance latencies. Post-training intra-BLA infusions of NE enhanced 48 hr retention of CFC training in the straight alley, as assessed by shock compartment entrance latencies and the number of shocks required to learn to avoid entering the shock compartment. These findings indicate that the consolidation of memory for CFC, like that for inhibitory avoidance training, is influenced by post-training neuromodulatory influences within the BLA. Thus, the findings provide additional evidence consistent with the hypothesis that the BLA has a general role in modulating memory consolidation.

Extinction Training after Cocaine Self-Administration Induces Glutamatergic Plasticity to Inhibit Cocaine Seeking

Learning to inhibit drug seeking can be an important strategy for inhibiting relapse, and this can be modeled by extinguishing drug seeking in response to a drug-paired context. Rats were either extinguished or withdrawn without extinction training (abstinence) from cocaine self-administration, and measurements of postsynaptic density proteins in the core and shell subcompartments of the nucleus accumbens were compared with yoked-saline controls. Only extinguished rats had elevations of PSD-95, Homer1b/c, and Narp in the postsynaptic density of the core, whereas no proteins measured were altered in the postsynaptic density of the shell in either extinguished or abstinent rats. Using a biotinylation strategy, it was found that surface expression of mGluR5 was reduced only in the core of extinguished animals. Although both extinguished and abstinent animals showed a reduction in long-term potentiation elicited in the core by stimulating prefrontal cortex, blunted long-term depression was observed only in extinguished rats. These data indicate that the elevation in Homer1b/c in the core may have sequestered mGluR5 away from the membrane surface and that the loss of surface mGluR5 inhibits long-term depression. Accordingly, when Homer1c was overexpressed in the core of cocaine-naive rats with an adenoassociated virus, long-term depression was inhibited. This mechanism may contribute to the inhibition of cocaine seeking by extinction training because overexpression of Homer1c in the core also inhibited cue-induced reinstatement of cocaine seeking. These data identify a cellular mechanism that may contribute to extinction-induced inhibition of cocaine seeking.

The infralimbic cortex regulates the consolidation of extinction after cocaine self-administration

The infralimbic cortex (IL) regulates the consolidation of extinction learning for fear conditioning. Whether the IL influences the consolidation of extinction learning for cocaine self-administration is unknown. To address this issue, male Sprague-Dawley rats underwent 2 wk of cocaine self-administration followed by extinction training. On the first 5 d of extinction, rats underwent brief (15- or 30-min) extinction sessions and received intra-IL microinjections immediately after each extinction session. On days 6-12 of extinction, rats underwent full-length (2-h) extinction sessions that were used to assess the retention of the extinction learning from the short sessions. IL inactivation via microinjections of the GABA agonists baclofen and muscimol (BM) immediately after the extinction sessions (days 1-5) impaired the retention of extinction learning. Control experiments demonstrated that this effect was not due to inactivation of the prelimbic cortex or due to effects of the drugs on the subsequent days behavior. In contrast, post-training intra-IL microinjections of the allosteric AMPA receptor potentiator 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide (PEPA) enhanced retention of the extinction learning. As evidence suggests a role for the beta-adrenergic receptors in memory consolidation, other rats received microinjections of the beta(2)-adrenergic receptor agonist clenbuterol or antagonist ICI-118,551 (ICI). Post-training intra-IL administration of clenbuterol or pre-training administration of ICI enhanced or impaired, respectively, the retention of extinction learning. These data indicate that the IL, and specifically the glutamatergic and beta-adrenergic systems in the IL, regulates the consolidation of extinction of cocaine self-administration and that the IL can be manipulated to influence the retention of extinction.

Optogenetic inhibition of cocaine seeking in rats

Inhibitory optogenetics was used to examine the roles of the prelimbic cortex (PL), the nucleus accumbens core (NAcore) and the PL projections to the NAcore in the reinstatement of cocaine seeking. Rats were microinjected into the PL or NAcore with an adeno‐associated virus containing halorhodopsin or archaerhodopsin. After 12 days of cocaine self‐administration, followed by extinction training, animals underwent reinstatement testing along with the presence/absence of optically induced inhibition via laser light. Bilateral optical inhibition of the PL, NAcore or the PL fibers in the NAcore inhibited the reinstatement of cocaine seeking.

A Single Intra-PFC Infusion of BDNF Prevents Cocaine-Induced Alterations in Extracellular Glutamate within the Nucleus Accumbens

The glutamatergic pathway arising in the dorsomedial prefrontal cortex (dmPFC) and projecting to the nucleus accumbens (NAc) core is a critical component of the reward circuitry that underlies reinstatement to cocaine-seeking behavior. Brain-derived neurotrophic factor (BDNF) is expressed by and modulates PFC–NAc neurons. BDNF infusion into the dmPFC attenuates reinstatement to cocaine-seeking behavior, as well as some cocaine-induced molecular adaptations within the NAc. In the present study, it is demonstrated that a single intra-dmPFC infusion of BDNF prevents cocaine self-administration-induced reduction in basal extracellular glutamate, as well as cocaine prime-induced increases in extracellular glutamate levels within the NAc. These data suggest that intra-PFC BDNF attenuates reinstatement to cocaine-seeking behavior by normalizing cocaine-induced neuroadaptations that alter glutamate neurotransmission within the NAc.

Neural circuit competition in cocaine-seeking: roles of the infralimbic cortex and nucleus accumbens shell.

Following cocaine self‐administration and extinction training, activity in the infralimbic cortex (IL) suppresses cocaine‐seeking behavior. IL inactivation induces cocaine‐seeking whereas activation suppresses cocaine‐reinstated drug‐seeking. We asked how the suppression of cocaine‐seeking induced by IL activation integrates with the circuitry promoting reinstated cocaine‐seeking. Following cocaine self‐administration and extinction training, rats underwent cue‐induced reinstatement. In order to activate IL projections, microinjections of PEPA, a positive allosteric modulator of AMPA receptors, were made into the IL in combination with microinjections into a variety of nuclei known to regulate cocaine‐seeking. Intra‐IL PEPA administration suppressed cue‐induced reinstatement without affecting locomotor activity. The suppression of cocaine‐seeking was reversed by activating dopamine neurons in the ventral tegmental area with microinjections of the μ‐opioid receptor agonist DAMGO, and was partially reversed by dopamine microinjections into the prelimbic cortex or basolateral amygdala. Previous evidence suggests that the nucleus accumbens shell both promotes and suppresses cocaine‐seeking. The suppression of cue‐induced cocaine seeking by PEPA in the IL was reversed by intra‐shell microinjections of either dopamine or the AMPA receptor antagonist CNQX, suggesting that the accumbens shell bidirectionally regulates cocaine‐seeking depending on whether dopamine input is mimicked or glutamate input is inhibited. Together, these findings indicate that the IL acts ‘upstream’ from structures promoting cocaine‐seeking, including from the mesolimbic dopamine projections to the prelimbic cortex and basolateral amygdala, and that the accumbens shell may be a crucial point of integration between the circuits that promote (ventral tegmental area) and inhibit (IL) reinstated cocaine‐seeking.

Post-training intrabasolateral amygdala infusions of dopamine modulate consolidation of inhibitory avoidance memory: involvement of noradrenergic and cholinergic systems.

There is extensive evidence that several neurotransmitter systems within the basolateral amygdala (BLA) influence memory consolidation. The present study investigated the influence of dopamine (DA) in the BLA on the consolidation of memory for inhibitory avoidance (IA) training. Male Sprague–Dawley rats (≈300 g) were trained on a step‐through IA task and, 48 h later, tested for retention as indexed by their latencies to enter the shock compartment on the test day. Drugs were infused into the BLA or central amygdala nucleus (CEA) immediately or 3 h after training via bilateral cannulae. DA infused into the BLA immediately after training enhanced retention, whereas DA infused into the BLA 3 h after training or into the CEA did not affect retention. Infusions of the dopaminergic antagonist cis‐Flupenthixol together with DA blocked the DA‐induced memory enhancement. Immediate post‐training intra‐BLA infusions of the D1 receptor antagonist SCH 23390 or the D2 receptor antagonist sulpiride impaired retention. β‐adrenergic or muscarinic cholinergic receptor antagonists coinfused into the BLA with DA blocked the memory enhancing effects of DA. These findings indicate that dopaminergic activation within the BLA modulates memory consolidation and that the modulation involves activation of both D1 and D2 receptors and concurrent activation of β‐adrenergic and cholinergic influences within the BLA.