|Home | About | Journals | Submit | Contact Us | Français|
Dopaminergic and glutamatergic inputs to the nucleus accumbens shell have a central role in reward processing. Non-contingent cocaine administration generates a number of long-term AMPA receptor-dependent changes in synaptic efficacy. However, the synaptic consequences of cocaine self-administration and the potential role of dopamine in these processes remain unclear. Here, we examined the influence of D1 dopamine receptor (D1DR) activation on excitatory synaptic plasticity in the accumbens shell of adult rats following cocaine self-administration. Our results indicated that during the first 2 days following cocaine exposure both pre- and post-synaptic mechanisms contribute to a net decrease in AMPA receptor-mediated signaling. This is reflected by decreased frequency of miniature EPSCs (mEPSCs) attributable to enhanced cannabinoid receptor activity, decreased mEPSC amplitude, and increased paired-pulse ratio of evoked EPSCs. In contrast, the only changes observed in the shell 3–4 weeks following cocaine self-administration were increased mEPSCs amplitudes and AMPA/NMDA ratios. We further found that although these cocaine-induced neuroadaptations during early and late abstinence have different synaptic expression mechanisms, they were normalized by stimulation of D1DRs. Thus, pre-exposure to the D1DR agonist, SKF38393, during the initial period of abstinence increased excitatory synaptic strength, but reduced excitatory signaling after weeks of abstinence. Taken together, these results indicate that the direction of changes in excitatory transmission induced by cocaine self-administration switches over the first few weeks of abstinence. Moreover, D1DRs gate the stability of these cocaine-induced changes at glutamatergic synapses in the accumbens shell by utilizing multiple temporally distinct mechanisms, which has implications for the treatment of cocaine craving and addiction.
Repeated exposure to cocaine results in changes in glutamatergic AMPA receptor-mediated neurotransmission in the nucleus accumbens shell that may contribute to cocaine craving and relapse (Kalivas and O'Brien, 2008; Schmidt and Pierce, 2010; Wolf, 2010). The changes in AMPA-mediated synaptic transmission are sensitive to the duration of abstinence from cocaine administration. Thus, during the first few days of abstinence following repeated non-contingent cocaine administration the ratio of AMPA to NMDA receptor-mediated currents as well as the intrinsic excitability of the accumbens shell medium spiny neurons (MSNs) are decreased (Kourrich et al, 2007; Kourrich and Thomas, 2009, but see Dobi et al, 2011; Kim et al, 2011). At longer abstinence periods (>10 days) the AMPA/NMDA ratio in the shell increases (Kourrich et al, 2007) as does the amplitude and frequency of AMPA-mediated miniature EPSCs (mEPSCs) (Kourrich et al, 2007; Thomas et al, 2001; Dobi et al, 2011). Consistent with these results, the cocaine-induced synaptic changes at excitatory synapses in the shell are accompanied by changes in the expression of specific AMPA receptor subunits. Surface expression of GluA1, GluA2, and GluA2/A3 subunits of the AMPA receptor at early abstinence in the nucleus accumbens decreases (Schumann and Yaka, 2009) or remains unchanged (Boudreau and Wolf, 2005; Boudreau et al, 2009; Ghasemzadeh et al, 2009), but increases following more prolonged abstinence from experimenter-administered cocaine (Boudreau and Wolf, 2005; Boudreau et al, 2007, 2009; Ghasemzadeh et al, 2009; Schumann and Yaka, 2009; Ferrario et al, 2010).
Although ample evidence suggests that neuroadaptations in the nucleus accumbens are sensitive to the mode of cocaine intake (Crespo et al, 2002; Mu et al, 2010; McCutcheon et al, 2011; Kalivas and McFarland, 2003; Bowers et al, 2010), a systematic analysis of excitatory synaptic function following self-administered cocaine is lacking. In the nucleus accumbens shell, one study reported reduced excitation during early abstinence from cocaine self-administration using extracellular field potential recordings (Schramm-Sapyta et al, 2006). More evidence is available with regard to specific contribution of calcium-permeable, GluA2-lacking AMPA receptors after abstinence from self-administration, although the results vary (Conrad et al, 2008; Mameli et al, 2009; McCutcheon et al, 2011). In biochemical studies, cocaine self-administration decreases the surface expression of GluA1 subunits in the nucleus accumbens 1 day after the cessation of cocaine treatment but increases surface GluA1 and GluA1-pSer845 after extended forced abstinence (Conrad et al, 2008; Ferrario et al, 2011). Overall, it remains unclear to what extent the synaptic plasticity observed following experimenter-administered cocaine is preserved when cocaine intake is voluntary.
Many of cocaine's characteristic behavioral and neuronal effects result from increases in extracellular dopamine and the subsequent stimulation of D1 dopamine receptors (D1DRs) (Pierce and Kumaresan, 2006). Notably, stimulation of D1DRs in the nucleus accumbens shell facilitates reinstatement of cocaine seeking (Schmidt et al, 2006). In terms of synaptic strength, acute application of D1DR agonists suppresses AMPA receptor-mediated currents via a pre-synaptic mechanism (Pennartz et al, 1992; Nicola et al, 1996; Harvey and Lacey, 1996). Agonist-induced activation of D1DRs, however, has also been shown to increase the surface expression of AMPA receptors in a PKA-dependent manner (Chao et al, 2002; Mangiavacchi and Wolf, 2004), an effect that was potentiated following cocaine self-administration (Anderson et al, 2008). D1DR activation may elicit lasting synaptic effects that persist beyond the duration of the agonist application (Schilström et al, 2006). Such lasting effects are of particular relevance with regard to cocaine-induced relapse. Even after prolonged periods of abstinence in human cocaine users, a single exposure to cocaine may elicit an extended period of relapse to cocaine taking, an effect that is presumably driven by enduring cocaine-triggered neuroadaptations in the relevant brain circuits. In this study, we used a conventional regimen of cocaine self-administration to examine AMPA-mediated neuronal transmission and to determine the lasting influence of D1DR stimulation on cocaine-induced synaptic plasticity in the nucleus accumbens shell of adult rats following brief and extended abstinence.
Male Sprague–Dawley rats (Rattus norvegicus) weighing 300–350g were obtained from Taconic Laboratories (Germantown, NY). Animals were individually housed, with food and water available ad libitum in the home cage. A 12–12-h light–dark cycle was used with the lights on at 0700 hours. All behavioral training was performed during the light cycle. All experimental protocols were approved by the Institutional Animal Care and Use Committee of the University of Pennsylvania.
Before surgery, the rats were anesthetized with injections of 80mg/kg ketamine (i.p.) and 12mg/kg xylazine (i.p.). An indwelling catheter (Strategic Applications) was placed into the right jugular vein and sutured in place. The catheter was routed subcutaneously to a mesh platform placed and sutured between the shoulder blades. Catheters were flushed daily with 0.3ml of the antibiotic Timentin (0.93mg/ml) dissolved in heparinized saline. The catheters were sealed with plastic obturators when not in use. After a 7-day recovery period from surgery, the rats were placed in operant chambers (Med Associates) and allowed to lever-press for intravenous cocaine infusions (0.25mg cocaine per 56μl saline over a 5s infusion) during daily 2-h self-administration sessions. Each cocaine infusion was followed by a 20-s timeout period during which responses had no scheduled consequences. The rats were initially trained using a fixed ratio 1 (FR1) schedule of reinforcement. When stable responding was achieved under the FR1 schedule, they were switched to an FR5 schedule. Rats self-administered cocaine for 14 days and were paired with yoked saline controls. The yoked animals received an infusion of saline every time its pair received cocaine. There were no consequences to lever pressing by the yoked saline animals.
A subset of rats was implanted with bilateral guide cannulae (14mm 24-gauge tubing; Small Parts) 2mm dorsal to the nucleus accumbens shell for microinjection experiments. The stereotaxic coordinates, relative to bregma, were as follows: 1.0mm anterior, ±1.0mm lateral, 5.0mm ventral. Cannulae were cemented in place by affixing dental acrylic to three stainless steel screws fastened to the skull. In order to prevent occlusion of the guide cannulae, an obturator (14mm, 33-gauge wire) was inserted into each one.
Obturators were removed from the guide cannulae and 33-gauge stainless steel microinjectors (Small Parts) were inserted. These microinjectors were cut to a length that extended 2mm below the ventral end of the guide cannulae and into the shell of the nucleus accumbens. Bilateral infusions were performed simultaneously over a 120-s time period in a total volume of 0.5μl per side. Following the microinjections, guide cannulae were left in place for 60s in order to allow the solution to diffuse away from the tips of the cannulae before they were removed. The animals were killed 20–30min following the microinjections and the brains processed for electrophysiological analyses. The correct placement of the guide cannulae was verified by the presence of the cannula track in the nucleus accumbens shell during electrophysiological recordings.
The rats were decapitated following isoflurane anesthesia 1–2 days (early abstinence) or 23–30 days (late abstinence) following 14 days of cocaine self-administration. The brain was removed and coronal slices (300μm) containing the nucleus accumbens were cut with a Vibratome (VT1000S, Leica Microsystems) in an ice-cold artificial cerebrospinal fluid solution (ACSF), in which NaCl was replaced by an equiosmolar concentration of sucrose. ACSF consisted of 130mM NaCl, 3mM KCl, 1.25mM NaH2PO4, 26mM NaHCO3, 10mM glucose, 1mM MgCl2, and 2mM CaCl2 (pH 7.2–7.4 when saturated with 95% O2/5% CO2). Slices were incubated in ACSF at 32–34°C for 45min and kept at 22–25°C thereafter, until transfer to the recording chamber. A subset of slices from each animal was incubated with ACSF supplemented with SKF38393 (10μM; Sigma) for 1–3h. The partial SKF38393 was chosen on the bases of its pharmacological similarity to dopamine (Ryman-Rasmussen et al, 2005) and its ability to modulate excitability of the nucleus accumbens neurons with efficiency that matches or exceeds that of the full agonist SKF81297 (Johansen et al, 1991). Slices pre-exposed to SKF38393 were washed with regular ACSF for at least 10min before initiation of the recordings. Importantly, we observed no correlation between the SKF38393 incubation and wash-out times and any of the measures reported in this study (median r2=0.03). The osmolarity of all solutions was 305–315mOsm. Slices were viewed using infrared differential interference contrast optics under an upright microscope (Eclipse FN1, Nikon Instruments) with a 40 × water-immersion objective.
The recording chamber was continuously perfused (1–2ml/min) with oxygenated ACSF heated to 32±1°C using an automatic temperature controller (Warner Instruments). Picrotoxin (100μM) was added to all solutions to block the GABAA receptor-mediated currents. For mEPSC experiments, tetrodotoxin (1μM) was present in the bath. Recording pipettes were pulled from borosilicate glass capillaries (World Precision Instruments) to a resistance of 4–7MΩ when filled with the intracellular solution. The intracellular solution contained (in mM): 100 CsCH3O3S, 50 CsCl, 3 KCl, 0.2 BAPTA, 10 HEPES, 1 MgCl2, 2.5 phosphocreatine-2Na, 2Mg-ATP, 0.25 GTP-Tris, 5 QX-314, adjusted to pH 7.2–7.3 (pH 7.2–7.3 with CsOH, osmolarity 280–290mOsm). For some mEPSC recordings, the intracellular solution contained (in mM): 145 potassium gluconate, 2 MgCl2, 2.5 KCl, 2.5 NaCl, 0.1 BAPTA, 10 HEPES, 2Mg-ATP, 0.5 GTP-Tris, and 5 QX-314 (pH 7.2–7.3 with KOH, osmolarity 280–290mOsm). No differences in amplitude, frequency, or decay kinetics were observed between K gluconate and Cs-containing solutions and these data were pooled for analyses. Alexa 568 (200μM) was also included in the intracellular solution in some recordings. MSNs in the nucleus accumbens shell were identified by their morphology and the low resting membrane potential (−70 to −85mV). eEPSC and mEPSC recordings were conducted in whole-cell voltage-clamp mode (Vh=−70 and +40mV for eEPSC and Vh=−70mV for mEPSCs). AM251 was applied via the Y-tube perfusion system (Murase et al, 1989) modified for optimal solution exchange in brain slices (Hevers and Lüddens, 2002). In experiments with AM251, mEPSCs were analyzed after a minimum of 3min of AM251 application. All recordings were conducted with a MultiClamp700B amplifier (Molecular Devices). Currents were low-pass filtered at 2kHz and digitized at 20kHz using a Digidata 1440A acquisition board and pClamp10 software (both from Molecular Devices). Access resistance (10–30MΩ) was monitored throughout the recordings by injection of 10mV hyperpolarizing pulses and data were discarded if access resistance changed by >25% over the course of data acquisition. Evoked responses were triggered by 100μs constant-current pulses generated by an A310 Accupulser (World Precision Instruments) and delivered at 0.1Hz via a bipolar tungsten stimulation electrode positioned within 100μm of the recorded cell. The amplitude of the current pulses was controlled by a stimulus isolator (ISO-Flex, AMPI) and was adjusted to elicit monosynaptic responses in the range of 100–300pA (the required stimulus intensity ranged from 15 to 80μA).
All analyses of intracellular recordings were carried out with Clampfit 10 (Molecular Devices). The time constant of decay was based on a monoexponential fit to the decay phase of an average mEPSCs trace computed from a minimum of 50 individual mEPSCs. For the cumulative probability distribution plots, 100–150 individual mEPSCs were analyzed. Mean mEPSC frequencies were analyzed from 10- to 20-s long trace segments. Paired-pulse ratios (PPRs) were calculated by averaging 5–10 responses at each stimulus interval and dividing the peak amplitude of the second eEPSC by the peak amplitude of the first eEPSC. AMPA/NMDA current ratios were computed by dividing the mean peak eEPSC at −70mV (AMPA-mediated) by the mean amplitude at +40mV, 35ms after the peak over a 2ms window (NMDA-mediated). The mean eEPSCs were based on 50 individual responses at each holding potential.
Cells from 4 to 7 animals were analyzed in each experimental condition. The data were expressed as mean±SEM of cells per group (all figures) as well as the mean±SEM of recorded cells from a single animal that were then averaged across the group (see Table 1). These analyses produced identical statistical interpretation of the results. Statistical analyses were performed with Microsoft Excel or StatView 5.0.1 for Windows. Statistical comparisons were done using two-tailed unpaired Student's t-test, two-way ANOVA (with Tukey's HSD post hoc) or Kolmogorov–Smirnov (K–S) as appropriate.
We began our study by examining the effect of cocaine self-administration on a basic measure of synaptic signaling, mEPSCs, in the nucleus accumbens shell MSNs (Figure 1a). mEPSCs are quantal post-synaptic responses to spontaneous, action potential-independent release of neurotransmitter vesicles that can be isolated by application of a voltage-gated Na+ channel blocker, tetrodotoxin (1μM). Following 1–2 days of forced cocaine abstinence, the basal mEPSC amplitude in cells from cocaine-experienced animals was 23% smaller than in cells from yoked saline controls (Figures 1b and c). To examine if mEPSCs were sensitive to D1DR stimulation, a subset of slices was incubated with D1DR agonist, SKF38393 (10μM, see Subjects and methods section). Surprisingly, pre-treatment with SKF38393 increased the mEPSC amplitudes in slices from cocaine-experienced animals to control levels while mEPSC amplitudes in slices from yoked saline rats were not affected (Figures 1b and c). The time constant of mEPSC decay was significantly slower in slices from cocaine-experienced animals under basal (ie, in the absence of SKF38393 pre-treatment) conditions (yoke τ=5.2±0.2ms; cocaine τ=6±0.3ms, t(26)=2.2, p<0.05), but was increased in the yoked saline group following exposure to SKF38393 (yoke SKF τ=6.2±0.3, t(24)=2.53, p<0.05 vs yoke no SKF; cocaine SKF τ=6±0.3ms) (Figure 1b). The differences in the decay time may reflect the differences in the relative abundance of GluA subunits subtypes (Jonas, 2000; Lu et al, 2009). The basal frequency of mEPSCs in cocaine-experienced animals was 43% smaller than in yoked saline controls. Following SKF38393 exposure, mEPSC frequency increased to control levels (Figures 1d and e). The mean frequency of mEPSCs recorded in the yoked group under the basal conditions was not different from the mean mEPSC frequency measured after SKF38393 pre-exposure, although 3 out of 12 recorded cells had very frequent events (Figure 1e, inset). mEPSC frequency is traditionally interpreted to reflect the probability of neurotransmitter release. Therefore, our data suggest that the probability of glutamate release is decreased following cocaine self-administration and that incubation with SKF38393 counteracts this synaptic adaptation. Thus, the deficits in both the amplitude and the frequency of mEPSC are reversed by D1DR stimulation during early abstinence from cocaine self-administration.
We next examined the mEPSCs following longer periods of abstinence. After 3–4 weeks of forced abstinence, the basal mEPSC amplitude in cocaine-experienced animals was approximately 28% larger than in yoked saline controls (Figures 2a and b). This increase is slightly greater than that reported following 10–14 days of withdrawal from experimenter-administered cocaine in the mouse (Kourrich et al, 2007). As in early abstinence, pre-treatment of slices with SKF38393 eliminated this difference (Figures 2a and b). In contrast to our observations in early abstinence, there were no basal differences in mEPSC decay time (yoke τ=6±0.4ms; cocaine τ=5.7±0.2ms; yoke SKF τ=6.2±0.3ms; cocaine SKF τ=5.8±0.3ms, Figure 2a) or mEPSC frequency (Figures 2c and d) after this longer period of abstinence. Furthermore, pre-treatment with SKF38393 had no effect on these measures in either the yoked saline or the cocaine-experienced groups.
It can be argued that because exposure to cocaine occurred in animals rather than slices, it is the animals that represent an appropriate n value for the number of statistical samples. We have, therefore, re-analyzed our mEPSCs data by calculating an average of all recorded cells in each animal and then computing the mean of such values for all animals within an experimental group. The results of these analyses for mEPSCs and all other experiments are presented in Table 1. Note that the interpretation of the statistical analyses was the same with both methods of generating the mean values.
We attributed the early abstinence decrease in mEPSC frequency to reduced probability of glutamate release. However, the basal decrease in mEPSC amplitude in cocaine-experienced rats may have forced a fraction of small-amplitude synaptic events below the detection limit and contributed to the observed frequency decrease (Stell and Mody, 2002). To confirm the pre-synaptic origin of mEPSC frequency deficits, we examined the PPRs, a measure of synaptic efficacy that is inversely proportional to the pre-synaptic probability of release (Manabe et al, 1993). In early abstinence, the PPR in cells from cocaine-experienced animals was significantly larger than in yoked saline controls across a range of inter-stimulus intervals (average yoke PPR=1.04±0.05; average cocaine PPR=1.24±0.06), consistent with a decrease in the probability of release (Figures 3a and c). This difference was eliminated by pre-treatment with SKF38393 (average yoke PPR=1.15±0.08; average cocaine PPR=1.04±0.03; Figures 3a and c). No differences in PPR were observed among any of the groups in late abstinence (average yoke=1.14±0.07; average cocaine 1.1±0.04; average yoke SKF=1.05±0.06; average cocaine SKF=1±0.03; Figures 3b and d). Collapsing all cells per animal and across each group led to similar conclusions: cocaine experience significantly increased the PPR in early abstinence, an effect that was rescued by pre-treatment with SKF38393 (F(3,13)=4.568, p=0.021) with significant effects of cocaine vs yoke, cocaine vs cocaine SKF, and cocaine vs yoke SKF groups (Tukey's HSD). There was no effect in late abstinence (F(3,13)=1.154, p=0.364). Thus, the PPR measurements provide an important confirmation to the mEPSC frequency data both before and after SKF38393 pre-treatment and suggest that D1DR stimulation affects pre-synaptic mechanisms that increase the probability of glutamate release specifically in cocaine-experienced animals in early abstinence from cocaine self-administration.
Cannabinoid receptor system can both regulate glutamate release in the striatum and is sensitive to modulation by D1DRs (Patel et al, 2003; Andre et al, 2010). Activation of CB1 receptors is known to decrease neurotransmitter release (reviewed in Kano et al, 2009). Reduced release probability after cocaine self-administration in early abstinence may therefore suggest stronger basal CB1 receptor activation that could be sensitive to SKF38393 pre-treatment. In this scenario, CB1 receptor blockade should increase the mEPSC frequency before, but not after the SKF38393 pre-treatment. Indeed, we found that in early abstinence, application of a CB1 receptor antagonist, AM251 (3μM) increased the mEPSC frequency by 9.3±5.3% of baseline values in cells from yoked saline controls, but led to a 48±8.9% increase in cells from cocaine-experienced rats (Figure 4a). After slices had been incubated with SKF38393, this difference was eliminated (Figure 4a). At 3–4 weeks of abstinence from cocaine, AM251 increased the mEPSC frequency to a similar extent in yoke and cocaine slices both before and after incubation with SKF38393 (Figure 4b). These data are consistent with our prediction that D1DR stimulation in cocaine-experienced animals affects CB1 receptor signaling and that this may contribute to the reduction of glutamate release in early abstinence from cocaine self-administration.
Multiple studies have found that exposure to cocaine modulates the ratio of AMPA to NMDA receptor-mediated currents, an index of the relative excitatory synaptic strength (reviewed in Bowers et al, 2010). We examined the sensitivity of this widely used measure to SKF38393 pre-treatment. At 1–2 days of forced abstinence from cocaine self-administration, the AMPA/NMDA ratio did not differ between yoked saline and cocaine groups (yoke: 4.53±0.71; cocaine: 4.69±0.62; Figures 5a and b). SKF38393 pre-treatment did not affect the AMPA/NMDA ratio although a trend toward smaller values was observed in slices from cocaine-experienced animals (yoke SKF: 4.37±0.48; cocaine SKF: 3.75±0.38, p=0.19; Figures 5a and b). At 3–4 weeks of abstinence from cocaine self-administration, AMPA/NMDA ratio was 52% larger in the cocaine-experienced group (yoke: 4.18±0.49; cocaine: 6.34±0.69; Figures 5c and d) similar to a previous report (Kourrich et al, 2007). SKF38393 exposure reduced the AMPA/NMDA ratio in cells from cocaine-experienced animals to control levels, but had a negligible effect on cells from yoked saline pairs (yoke SKF: 4.71±0.7; cocaine SKF: 4.34±0.39; Figures 5c and d). We conclude that AMPA/NMDA ratio, a broad measure of excitatory synaptic strength, is sensitive to D1DR stimulation in nucleus accumbens shell MSNs of cocaine-experienced, but not yoked saline control animals and that this unique sensitivity develops only after an extended period of abstinence from cocaine self-administration.
Our data suggest that D1 receptor stimulation triggers lasting synaptic changes in the slices from cocaine-experienced animals. We sought to investigate whether treatment with SKF38393 in vivo is similarly capable of modulating the strength of excitatory synapses onto the accumbens shell MSNs. To accomplish this, the animals were implanted with guide cannulae targeting the accumbens shell bilaterally. At 24h after the last cocaine self-administration session (early abstinence), each animal received a microinfusion of saline into one brain hemisphere and of SKF38393 into the other hemisphere. Following this treatment, the slices were prepared according to the standard protocol and recordings performed within 5h of microinjections. We found a robust reduction of both mEPSC frequency and amplitude in cells from saline-treated hemispheres of cocaine-experienced animals (Figure 6). In cells from SKF38393-treated hemispheres, both the mEPSC frequency and mEPSCs amplitude deficits were rescued, similar to the rescue observed after in vitro exposure to SKF38393 (Figure 6, compare with Figure 1). The mEPSC decay times, however, were not different between treatment groups (yoke τ=4.4±0.3ms, cocaine τ=4.8±0.3ms, yoke SKF τ=4.2±0.2ms, cocaine SKF τ=4.8±0.2ms). Thus, we confirm a lasting modulation of excitatory synapses by D1DRs in an intact accumbal circuit. Stimulation of D1DRs is known to elicit a variety of cocaine-associated behaviors when administered into the nucleus accumbens (reviewed in Schmidt and Pierce, 2010) and these findings shed light onto possible underlying mechanisms.
In this study, we determined the persistent effects of cocaine self-administration and D1DR activation on the efficacy of AMPA receptor-mediated synaptic transmission in the nucleus accumbens shell following brief (1–2 days) and extended (3–4 weeks) abstinence. Our results indicate that incubation with the D1DR agonist, SKF38393, had no appreciable influence on most measures of excitatory synaptic strength in the shell of control rats. In contrast, in cells from cocaine-experienced animals both the early abstinence suppression as well as the late abstinence potentiation of AMPA receptor-mediated signaling were normalized by SKF38393 pre-treatment. Such reversal of cocaine-associated synaptic plasticity occurred in measures implicating pre-synaptic (mEPSC frequency, PPRs) and post-synaptic (mEPSC amplitude, AMPA/NMDA ratio) mechanisms (Table 2). Moreover, the effects of SKF38393 were observed after drug incubation ceased and also after microinjection of this drug in vivo, suggesting recruitment of signaling and effector mechanisms that persist in the absence of pharmacological stimulation. These results describe a range of synaptic adaptations triggered by cocaine self-administration and identify a unique role for D1DRs in gating the maintenance of these neuroadaptations.
During early abstinence, the basal amplitude of mEPSCs was decreased in accumbens shell MSNs from rats that self-administered cocaine. This effect may indicate a decrease in the expression of post-synaptic AMPA receptors (Conrad et al, 2008; Schumann and Yaka, 2009; Ferrario et al, 2010), which should decrease the AMPA/NMDA ratio. In fact, with non-contingent cocaine administration some studies report decreased AMPA/NMDA ratio in the accumbens shell at a similar abstinence time period (Kourrich et al, 2007; Mameli et al, 2009) although others observe no changes (Kim et al, 2011). Despite a reduction in mEPSC amplitude, we did not observe basal changes in AMPA/NMDA ratio during early abstinence. This may be explained if a decrease in NMDA receptor-mediated currents occurred in parallel with a decrease in AMPA receptor-mediated currents. We observed a small reduction in the basal amplitude of NMDA receptor eEPSCs in cocaine-experienced animals during early abstinence (at normalized stimulus intensity; yoke: 56.3±9.2pA; cocaine=45.8±8.4pA). Nevertheless, it must be noted that the amplitude of AMPA mEPSCs does not reliably predict a change in the AMPA/NMDA ratio because the populations of post-synaptic AMPA receptors activated during miniature and evoked events differ (Thomas et al, 2001; Sara et al, 2011).
In contrast to early abstinence, cocaine self-administration followed by 3–4 weeks of forced abstinence increased basal AMPA mEPSC amplitude and increased the AMPA/NMDA ratio. These results are consistent with electrophysiological studies following experimenter-administered cocaine (Thomas et al, 2001; Kourrich et al, 2007) and reports of increased surface expression of AMPA receptors following an extended period of abstinence (Boudreau and Wolf, 2005; Boudreau et al, 2007, 2009; Ghasemzadeh et al, 2009; Schumann and Yaka, 2009; Ferrario et al, 2010). The decrease in mEPSC amplitude during early abstinence as well as the increases in mEPSC amplitude and the AMPA/NMDA ratio in late abstinence were normalized following pre-treatment of slices from cocaine-experienced animals with SKF38393. Thus, cocaine-associated excitatory synaptic plasticity in the accumbens shell can be dynamically regulated by D1DR signaling. Adaptive changes in D1DR expression and signaling observed throughout abstinence (Anderson and Pierce, 2005) are likely to contribute to such dynamic regulation.
During the early abstinence from cocaine self-administration, we observed a decrease in mEPSC frequency accompanied by an increase in the PPR, implicating a pre-synaptic site of action. These results highlight a difference with the effects of non-contingent cocaine administration that have been reported to increase the mEPSC frequency in two recent reports (Dobi et al, 2011; Kim et al, 2011). Interestingly, Kim et al (2011) found this increase to be specific to D1-expressing MSNs, with a significant decrease in mEPSC frequencies observed in D2-expressing MSNs.
Although D1DRs are predominantly expressed post-synaptically (Levey et al, 1993; Hersch et al, 1995; Yung et al, 1995; Caille et al, 1996), up to a quarter of D1DRs in the nucleus accumbens are pre-synaptic heteroreceptors (Dumartin et al, 2007). Electrophysiological evidence indicates that acute stimulation of D1DRs reduces the frequency of mEPSCs and increases the PPR (Pennartz et al, 1992; Nicola et al, 1996). We found that following SKF38393 pre-exposure, slices from cocaine-experienced animals during early abstinence showed a prominent increase in mEPSC frequency that was accompanied by a decrease in the PPR. These observations suggest that SKF38393 increased the probability of glutamate release in cocaine-experienced animals following brief, but not longer abstinence.
A wealth of clinical evidence suggests that interaction between the dopamine and endocannabinoid systems is involved in the pathophysiology of disorders involving the limbic system (Parolaro and Rubino, 2008; Fernández-Ruiz et al, 2010; Kuepper et al, 2010; Pisani et al, 2011). Deletion of CB1 receptors reduces reinforcing properties of cocaine as does administration of CB1 receptor antagonists (Xi et al, 2006; Li et al, 2009; Ramiro-Fuentes et al, 2010). The molecular mechanisms underlying these effects are likely to involve glutamate receptors although the precise details of interaction between the dopamine, glutamate, and cannabinoid signals remain to be worked out. We identify cannabinoid receptor activation as a contributor to the pre-synaptic changes observed in slices from cocaine-experienced animals. Application of the CB1 antagonist, AM251, significantly increased mEPSC frequency in the shell of cocaine-experienced relative to saline control animals in the early abstinence. This difference was abolished following treatment with SKF38393, in line with the reported ability of D1DR agonists to decrease endogenous cannabinoid levels (Patel et al, 2003). Following extended cocaine abstinence, AM251 had similar effects both with and without the SKF38393 incubation, consistent with the lack of differences in mEPSC frequency and the PPR at this time point.
Sensitivity of AMPA receptor-mediated synaptic plasticity to cocaine history including sensitivity to duration of abstinence has been reported by a number of groups (reviewed in Wolf and Ferrario, 2010). Among the most robust findings is the enhancement of AMPA signaling following extended abstinence from non-contingent cocaine. To our knowledge, our results provide the first confirmation that this enhancement persists following extended abstinence from cocaine self-administration and that this enhancement is characterized by post-synaptic neuroadaptations. Notably, in early abstinence from cocaine self-administration we identified a broader pattern of synaptic changes attributable to both pre- and post-synaptic mechanisms.
We found that administration of a D1DR agonist only affected those measures of synaptic efficacy that were altered by cocaine experience (ie, decreased mEPSCs amplitude, frequency, PPR, at 1–2 days and increased mEPSCs amplitudes and AMPA/NMDA ratios at 3–4 weeks of abstinence). It is thus tempting to speculate that synapses modified by cocaine self-administration possess a unique sensitivity to D1DR stimulation. Moreover, D1DRs appear to gate the ability to adjust such modifications. Neuroadaptations associated with cocaine experience may underlie an impaired ability to assign motivational salience to novel rewards unrelated to the drug, similar to impaired learning after saturation of hippocampal synapses by tetanic stimulation (McNaughton et al, 1986; Moser et al, 1998). Normalization of excitatory synaptic efficacy following D1DR stimulation suggests that synapses altered by cocaine self-administration remain plastic and maintain the potential to recover normal function, which has implications for the treatment of cocaine craving and addiction (Haney and Spealman, 2008; Self and Nestler, 1995).
This work was supported by NIH Grants R01 DA22339 and K02 DA18678 to RCP.
The authors declare no conflict of interest.