AMPA Receptor Plasticity in Accumbens Core Contributes to Incubation of Methamphetamine Craving

Abstract

Background: The incubation of cue-induced drug craving in rodents provides a model of persistent vulnerability to craving and relapse in human addicts. After prolonged withdrawal, incubated cocaine craving depends on strengthening of nucleus accumbens (NAc) core synapses through incorporation of Ca²⁺-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (CP-AMPARs). Through metabotropic glutamate receptor 1 (mGluR1)-mediated synaptic depression, mGluR1 positive allosteric modulators remove CP-AMPARs from these synapses and thereby reduce cocaine craving. This study aimed to determine if similar plasticity accompanies incubation of methamphetamine craving.

Methods: Rats self-administered saline or methamphetamine under extended-access conditions. Cue-induced seeking tests demonstrated incubation of methamphetamine craving. After withdrawal periods ranging from 1 to >40 days, rats underwent one of the following procedures: 1) whole-cell patch clamp recordings to characterize AMPAR transmission, 2) intra-NAc core injection of the CP-AMPAR antagonist 1-naphthyl acetyl spermine followed by a seeking test, or 3) systemic administration of a mGluR1 positive allosteric modulator followed by a seeking test.

Results: Incubation of methamphetamine craving was associated with CP-AMPAR accumulation in NAc core, and both effects were maximal after ~1 week of withdrawal. Expression of incubated craving was decreased by intra-NAc core 1-naphthyl acetyl spermine injection or systemic mGluR1 positive allosteric modulator administration.

Conclusions: These results are the first to demonstrate a role for the NAc in the incubation of methamphetamine craving and describe adaptations in synaptic transmission associated with this model. They establish that incubation of craving and associated CP-AMPAR plasticity occur much more rapidly during withdrawal from methamphetamine compared with cocaine. However, a common mGluR1-based therapeutic strategy may be helpful for recovering cocaine and methamphetamine addicts.

Keywords: Ca(2+)-permeable AMPA receptors; Extended-access drug self-administration; Incubation of craving; Metabotropic glutamate receptor 1 (mGluR1); Methamphetamine; Nucleus accumbens.

Figure 1

Incubation of methamphetamine craving. (A) Timeline of the experimental procedure. Rats were trained to self-administer methamphetamine for 6 h/day for a total of 10 days, and tested for cue-induced methamphetamine craving on either WD1 or WD45. (B) Training phase: Mean ± SEM number of methamphetamine infusions (top) and number of active hole (AH) and inactive hole (IH) nose-pokes (bottom) over the ten, 6-h daily self-administration training sessions (n=26 total rats). (C) Seeking tests: Data are mean ± SEM nose-pokes in the previously active hole and in the inactive hole during the seeking tests on WD1 (n=13 rats) and WD45 (n=13 rats). During the seeking tests, active hole nose-pokes led to contingent presentation of a 20-sec light cue previously paired with each methamphetamine injection. Meth, methamphetamine; SA, self-administration; WD, withdrawal day.

Figure 2

The incubation of methamphetamine craving is accompanied by accumulation of CP-AMPARs in NAc core synapses and emergence of robust mGlu1-mediated synaptic depression. (A) Timeline of the experimental procedure (see legend to Figure 1 for details). (B) Bath application of the CP-AMPAR antagonist naspm (100 μM) produced a significantly greater reduction in EPSC_−70mV in MSN from methamphetamine rats (WD>40) as compared to saline controls (24.3% vs. 8.8%; calculated from min 9–12 after naspm application, as shown in shaded region; p<0.05) (methamphetamine, n=8 cells/5 rats; saline, n=6 cells/4 rats). (C) In MSN from methamphetamine rats (>WD40), application of the mGlu1 positive allosteric modulator SYN119 (SYN; 1μM) induced a robust depression of the EPSC_−70mV (28%), and subsequent application of naspm had no further effect (based on comparison of shaded regions: last 4 min of SYN alone versus last 4 min of naspm). Scale bars: 40 ms × 100 pA.

Figure 3

Systemic administration of the mGlu1 positive allosteric modulator SYN119 reduces incubated cue-induced methamphetamine seeking. (A) Timeline of the experimental procedure (see legend to Figure 1 for details). (B) Training phase: Mean ± SEM number of methamphetamine infusions, active hole and inactive hole nose-pokes over the ten, 6-h daily self-administration training sessions (n=20 total rats). (C) Seeking test: Data are mean ± SEM nose-pokes in the previously active hole and in the inactive hole during seeking tests on or after WD40. SYN119 or vehicle was injected (10 mg/kg, i.p.) 20 min prior to the seeking test. SYN119 significantly reduced cue-induced methamphetamine seeking compared to vehicle-injected controls (*p<0.05).

Figure 4

Blockade of CP-AMPARs in the NAc core with naspm reduces incubated cue-induced methamphetamine seeking. (A) Timeline of the experimental procedure (see legend to Figure 1 for details). (B) Training phase: Mean ± SEM number of methamphetamine infusions (top) and number of active hole (AH) and inactive hole (IH) nose-pokes (bottom) over the ten, 6-h daily self-administration training sessions (n=26 total rats). (C) Seeking test: Shown are nose-pokes in the previously AH and in the IH during a seeking test on WD45. Naspm (n=14 rats) or vehicle (n=12 rats) was injected into the NAc core 15 min prior to the seeking test. Naspm significantly reduced cue-induced methamphetamine seeking compared to vehicle-injected controls (*p<0.05). Data are expressed as percent of vehicle group (mean ± SEM); note that both AH and IH responses for the vehicle group are set at 100%. Data are presented in this manner because we combined results from two different experiments (done years apart) with substantially different raw values for AH and IH nose-pokes during the seeking test [Experiment 1: veh group (n=7), AH: 40.4 ± 9.1, IH: 10.0 ± 3.1; naspm group (n=10), AH: 23.4 ± 3.2, IH: 9.8 ± 1.7; Experiment 2: veh group (n=5), AH: 16.4 ± 1.3, IH: 4.8 ± 0.6; naspm group (n=4), AH: 11.8 ± 3.2, IH: 4 + 0.9].

Figure 5

Incubation of methamphetamine craving and CP-AMPAR accumulation in NAc core synapses reach maximal levels by withdrawal day 7–8. (A) Timeline of experimental procedures for results shown in panel B (see legend to Figure 1 for details). (B) The reduction in EPSC₋₇₀ produced by bath application of the CP-AMPAR antagonist naspm (100 μM) was used to define the contribution of CP-AMPARs to synaptic transmission in methamphetamine rats recorded on different withdrawal days (WD). The left graph compares the effect of naspm in MSN recorded on WD1 (n=6 cells/4 rats) versus WD7–8 (n=5 cells/4 rats) (data are mean ± SEM). For sample traces, scale bars indicate 40 ms × 100 pA. The bar graph on the right summarizes data from WD1 and WD78, as well as from WD2–4 (n=5 cells/4 rats) and WD>40 (n=8 cells/5 rats) (the latter data are taken from Figure 2). Data in the right graph are expressed as % reduction in EPSC₋₇₀ after naspm (average of min 9–12 after naspm; this period is indicated by gray shading in the left graph) (*p<0.05). (C) Timeline of experimental procedures for results shown in panels D and E (see legend to Figure 1 for details). (D) Training phase: Mean ± SEM number of methamphetamine infusions, active hole and inactive hole nose-pokes over the ten, 6-h daily self-administration training sessions (n=12 rats). (E) Seeking test: Each rat was tested twice, on WD7 and WD30. Data are mean ± SEM nose-pokes in the previously active hole and in the inactive hole during seeking tests.