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. Author manuscript; available in PMC: 2021 Dec 1.
Published in final edited form as: Eur J Neurosci. 2020 Sep 21:10.1111/ejn.14980. doi: 10.1111/ejn.14980

Cocaine self-administration abolishes endocannabinoid-mediated long-term depression of glutamatergic synapses in the ventral tegmental area

Ruixiang Wang a, Kathryn A Hausknecht a, Amy M Gancarz-Kausch a,b, Saida Oubraim a, Roh-Yu Shen a, Samir Haj-Dahmane a,*
PMCID: PMC7775329  NIHMSID: NIHMS1630928  PMID: 32959420

Abstract

Drugs of abuse, including cocaine, alter the mechanisms underpinning synaptic plasticity, including long-term potentiation of glutamatergic synapses in the mesolimbic system. These effects are thought to underlie addictive behaviors. In the ventral tegmental area (VTA), glutamatergic synapses also exhibit long-term depression (LTD), a type of plasticity that weakens synaptic strength. This form of synaptic plasticity is induced by low-frequency stimulation and mediated by endocannabinoid (eCB) signaling, which also modulates addictive behaviors. However, it remains unknown whether eCB-LTD in the VTA could be altered by cocaine use. Therefore, the goal of the present study was to examine the impact of cocaine self-administration on eCB-LTD of glutamatergic synapses onto VTA dopaminergic (DA) neurons. To that end, male rats underwent cocaine (0.75 mg/kg/infusion) or saline self-administration under the fixed ratio 1 schedule for 6 – 9 days. One day after the last self-administration session, the magnitude of eCB-LTD was examined using ex vivo whole-cell recordings of putative VTA DA neurons from naïve rats and rats with saline or cocaine self-administration. The results revealed that cocaine self-administration abolished eCB-LTD. The cocaine-induced blockade of eCB-LTD in the VTA was mediated by an impaired function of presynaptic CB1 receptors. Collectively, these findings indicate that cocaine exposure blunts eCB-mediated synaptic plasticity in midbrain DA neurons. This effect could be one of the cellular mechanisms that mediate, at least in part, addictive behaviors.

Keywords: psychostimulant, addiction, dopamine, CB1 receptors, synaptic plasticity

Introduction

Addictive drugs, including cocaine, alter neural plasticity mechanisms and evoke long-lasting changes in synaptic strength in the mesolimbic dopaminergic (DA) pathway, which could mediate addictive behaviors, such as compulsive use and relapse (Nestler, 2005; Kauer & Malenka, 2007; Nestler, 2013). Indeed, extensive studies have established that single or repeated exposure to cocaine causes a persistent increase in strength of glutamatergic synapses onto DA neurons in the ventral tegmental area (VTA), the origin of the mesolimbic pathway (Ungless et al., 2001; Borgland et al., 2004). Moreover, potentiation of VTA glutamatergic synapses induced by cocaine self-administration lasts weeks after cessation of cocaine intake (Chen et al., 2008). The long-term potentiation (LTP) of glutamatergic synapses is thought to mediate, at least in part, the cocaine-induced persistent activation of VTA DA neurons and addictive behaviors (Wolf, 2003; Shen et al., 2007; Lüscher & Malenka, 2011).

In addition to undergoing LTP, results from previous studies have established that glutamatergic synapses onto VTA DA neurons also exhibit long-term depression (LTD). Indeed, low-frequency presynaptic stimulation (LFS) combined with postsynaptic depolarization induces LTD (Gutlerner et al., 2002; Haj-Dahmane & Shen, 2010). Evidence from other studies has suggested that this form of synaptic plasticity can be altered by psychostimulants. Acute, in vitro application of amphetamine abolishes the LTD of glutamatergic synapses in the VTA (Jones et al., 2000). In contrast, a single in vivo cocaine exposure has been shown to enhance LFS-LTD (Ungless et al., 2001), but have no effect on spike timing-dependent LTD (Luu & Malenka, 2008). These conflicting results are probably due to the different mechanisms of LTD induction.

We have previously shown that LFS-LTD of glutamatergic synapses onto VTA DA neurons is mediated by endocannabinoid (eCB) signaling (Haj-Dahmane & Shen, 2010). Importantly, results from neurochemical studies have demonstrated that non-contingent acute and chronic exposure to cocaine alters the levels of eCBs in the brain (González et al., 2002; Patel et al., 2003). Similarly, voluntary cocaine self-administration has been shown to affect the levels of 2-arachidonoylglycerol (2-AG) (Bystrowska et al., 2014), an eCB species utilized in VTA DA neurons (Parsons & Hurd, 2015). Furthermore, extensive behavioral studies have demonstrated that pharmacological and genetic manipulations of eCB signaling alter the motivational and reinforcing effects of drugs of abuse, including cocaine (Parsons & Hurd, 2015). Although these studies have established an important role of eCB signaling in controlling addictive behaviors, the impact of cocaine self-administration on eCB-mediated synaptic plasticity in the VTA remains unknown. Therefore, in the present study, we investigated whether cocaine self-administration could alter the mechanisms of eCB-mediated LTD of glutamatergic synapses onto VTA DA neurons.

Materials and Methods

Animals and cocaine self-administration

Male Sprague Dawley rats (Envigo, Indianapolis, IN, USA) were used in the present study. Before the cocaine self-administration experiment, 5-week-old rats were trained for lever press in an operant chamber to obtain a “SuperSac” solution (3% glucose and 0.125% saccharin, w/v) (Walker & Koob, 2008) under the fixed ratio (FR) 1 reinforcement schedule (0.02 ml/reward; maximum rewards/session: 100). The training was carried out in daily 1-h sessions until a criterion was reached – 100 responses/session for 3 consecutive sessions (see details in Wang et al., 2019). All the rats acquired the lever press behavior in 7 – 9 days. The jugular vein catheterization surgery was performed after the training, as described previously (Wang et al., 2019). Briefly, rats were anesthetized with ketamine (60 mg/kg)/xylazine (5 mg/kg, i.p.), following buprenorphine administration (0.025 mg/kg, s.c) for preoperative pain relief. A heparin-coated catheter (Instech Laboratories, Plymouth Meeting, PA, USA) was implanted into the right external jugular vein and connected to a back harness with an injection port (Instech). Five to seven days after the surgery, rats started to self-administer cocaine (0.75 mg/kg/infusion) or saline by pressing an active lever under the FR 1 schedule in daily 2-hour sessions. There was also an inactive lever present in the operant chamber, pressing which had no programmed consequences. The maximum number of infusions/session was set at 20. In the cocaine group, rats were required to reach a criterion of 20 infusions/session for 3 consecutive sessions. A failure to attain the criterion would lead to exclusion from data analysis and ex vivo recordings. All the cocaine rats reached the criterion in 6 – 9 days, which concluded the experiment. The saline self-administration also ended in 6 – 9 days correspondingly. Only the first 6 sessions in both groups were included in data analysis. All the procedures were approved by the Institutional Animal Care and Use Committee of University at Buffalo.

Ex vivo electrophysiology

The whole-cell patch clamp recordings were performed one day after the last self-administration session, only in rats with patent cannulae at the end of the self-administration experiment. The method of the recordings was described in detail previously (Wang et al., 2006). In brief, the rats’ brains were dissected following rapid decapitation under deep isoflurane anesthesia and placed in cold modified artificial cerebrospinal fluid (ACSF) containing the following (in mm): 110 choline-Cl, 2.5 KCl, 0.5 CaCl2, 7 MgSO4, 1.25 NaH2PO4, 26.2 NaHCO3, 11.6 sodium L-ascorbate, 3.1 sodium pyruvate, and 25 glucose, saturated with 95% O2/5% CO2. Horizontal midbrain slices (250 μm) containing the VTA were cut using a vibratome (VT1200S; Leica Biosystems, Wetzlar, Hesse, Germany) and incubated for 45 – 60 min at 35°C in standard ACSF containing the following (in mm): 119 NaCl, 2.5 KCl, 2.5 CaCl2, 1.3 MgSO4, 1 NaH2PO4, 26.2 NaHCO3, and 11 glucose, saturated with 95% O2/5% CO2. Subsequently, slices were allowed to recover at room temperature for a minimum of one hour before they were transferred to the recording chamber, continuously perfused with standard ACSF, bubbled with 95% O2/5% CO2, and maintained at 30 ± 1°C.

Putative DA neurons adjacent and medial to the medial lemniscus where DA neurons are concentrated were identified by the presence of hyperpolarization-activated cation currents (Ih) (Ungless & Grace, 2012). Using the post hoc immunohistochemistry approach, we have previously shown that the vast majority (> 87%) of the recorded neurons from the VTA exhibiting Ih also express tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis (Hausknecht et al., 2015). Only neurons that exhibited Ih were included in the present study. Recordings were obtained with patch electrodes (3 – 5 MΩ) filled with a solution containing the following (in mm): 120 cesium methanesulfonate, 10 Na2-phosphocreatine, 10 KCl, 10 Hepes, 1 MgCl2, 1 EGTA, 2 Na2-ATP, and 0.25 Na-GTP (pH: 7.3, adjusted with CsOH; osmolality: 280 – 290 mOsm/kg). Picrotoxin (100 μm) was added into ACSF to block GABAA receptors. Stimulating electrodes were placed 50 – 100 μm rostral to the recorded neurons. Excitatory postsynaptic currents (EPSCs) were evoked with single square-pulses (duration: 100 – 200 μs) delivered at 0.1 Hz while neurons were voltage-clamped at −70 mV.

The LTD was induced by pairing postsynaptic depolarization to −30 mV with 2 Hz afferent stimulation for 6 min in the voltage-clamp mode. We have previously shown that this stimulation protocol reliably induces a robust LTD (Haj-Dahmane & Shen, 2010). In data processing, EPSC amplitudes were normalized to the baselines (recorded for 10 min before LTD induction). The magnitude of LTD was measured by comparing the EPSC amplitude 30 min post-LTD induction with the baseline. Since it has been suggested that drug-free operant self-administration per se may modulate synaptic plasticity (Fernández-Lamo et al., 2018), in the LTD experiment we included an age-matched “naïve” group to distinguish cocaine-induced effects from possible effects of the operant self-administration procedure.

In the experiment assessing the impact of cocaine self-administration on the function of presynaptic CB1 receptors (CB1Rs), we examined the effects of WIN 55,212–2 (10 μm), a CB1R agonist, on the amplitude of evoked EPSCs. WIN 55,212–2 was dissolved in dimethyl sulfoxide (0.001%) and applied for 30 min after 10 min of stable baseline EPSC recordings. The EPSC amplitude post-WIN 55,212–2 application was compared with the baseline.

Chemicals and drugs

Cocaine hydrochloride (Sigma-Aldrich, St. Louis, MO, USA), dissolved in 0.9% sterile saline, was made once a week for self-administration. Ketamine, xylazine, and buprenorphine were supplied by Patterson Veterinary Supply (Greeley, CO, USA). Picrotoxin was purchased from R&D Systems (Minneapolis, MN, USA). WIN 55,212–2 and AM 251 were obtained from Tocris Bioscience (Bristol, UK). All the other chemicals were carried by Fisher Scientific (Waltham, MA, USA).

Statistical analysis

Independent-samples t-tests, paired-samples t-tests, and two-way repeated measures analysis of variance (ANOVA) were used in statistical analysis. Planned comparisons were employed to detect pairwise differences if ANOVA produced main or interaction effects. The level of significance was set at α = 0.05. The statistical analyses were conducted using SAS 9.4 (SAS Institute Inc., Cary, NC, USA). Data are expressed as mean ± SEM.

Results

Cocaine showed a strong reinforcing effect in the self-administration experiment

Rats self-administered more cocaine than saline (in numbers of infusions) across the six FR1 sessions (two-way repeated measures ANOVA: saline vs. cocaine and sessions 1 – 6; group main effect, F1,13 = 80.01, p < 0.001; saline: 5.61 ± 1.88 infusions/session, average across 6 sessions, n = 6 rats; cocaine: 15.96 ± 1.21 infusions/session, n = 9 rats; Fig. 1A & B). In addition, the proportions of active responses (lever presses) were higher in the cocaine than the saline group (two-way repeated measures ANOVA; group × session interaction effect, F5,65 = 2.78, p < 0.05; saline: 28.94 ± 7.04% vs. cocaine: 74.36 ± 3.03%, p < 0.001, planned comparison; Fig. 1C & D).

Fig. 1.

Fig. 1.

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Cocaine showed a strong reinforcing effect in the self-administration experiment. (A) & (B) Rats self-administered more cocaine (0.75 mg/kg/infusion) than saline (in numbers of infusions) under the fixed ratio 1 reinforcement schedule. (C) & (D) The proportions of active responses (lever presses) were higher in the cocaine than the saline group. The left panels depict data from each session; the right panels present averages across the 6 sessions. Data are expressed as mean ± SEM. ***p < 0.001, saline vs. cocaine.

Cocaine self-administration abolished eCB-LTD of glutamatergic synapses in the VTA

To examine the impact of cocaine self-administration on eCB-LTD, we first confirmed that glutamatergic synapses onto VTA DA neurons in naïve rats exhibited eCB-LTD. Consistent with our earlier finding (Haj-Dahmane & Shen, 2010), pairing low frequency (2Hz) afferent stimulation with postsynaptic depolarization (−30 mV) reliably induced LTD in naïve rats (n = 8 cells; p < 0.05, paired t-test; Fig. 2A). As reported previously, the LFS-LTD was mediated by a persistent decrease in probability of glutamate release as indicated by a significant increase in paired-pulse ratio (PPR: the ratio of EPSC 2 amplitude to EPSC 1 amplitude; inter-stimulus interval: 50 ms) 30 min after LTD induction (PPR control = 0.97 ± 0.01; PPR LTD = 1.55 ± 0.06; n = 6 cells; p < 0.001, paired t-test; Fig. 2B & C). Importantly, this form of LTD was readily blocked by the CB1R antagonist AM 251 (3 μm, applied 1 h before recordings; LTD control = 45.49 ± 3.7 %; LTD AM 251 = 4.54 ± 3.2 %; n = 8 cells for each group; p < 0.01, control vs. AM 251, independent-samples t-test; Fig. 2A) indicating that the LTD was mediated by eCB signaling.

Fig. 2.

Fig. 2.

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Cocaine self-administration abolished eCB-LTD of glutamatergic synapses onto putative VTA DA neurons. (A) A summary graph of the time course and magnitude of LFS-induced LTD in naïve rats in the absence (●) and presence of AM 251 (3 μm; ○). Note that blockade of CB1Rs abolished the LTD, indicating that the LTD was mediated by eCB signaling. (B) Representative pairs of EPSC traces taken before and during LTD, as indicated by the numbers in graph A (1: in black). (C) A summary graph of the paired-pulse ratios (PPRs) of EPSCs determined before and during LTD. The LTD was associated with a significant increase in PPR, indicating a decrease in glutamate release. (D) A summary graph of the time course and magnitude of the LTD recorded in saline- (●) and cocaine-treated rats (○). Note that cocaine self-administration blocked the LTD. (E) Representative EPSC traces taken at the periods indicated by the numbers in graph D (1: in black). (F) A summary graph of the magnitudes of the LTD recorded in saline and cocaine groups. Data are expressed as mean ± SEM. **: p < 0.01, saline vs. cocaine; ***: p < 0.001, baseline vs. LTD. Scale bars: 50 pA, 10 ms.

We next assessed the eCB-LTD of VTA glutamatergic synapses in rats undergoing saline or cocaine self-administration. Remarkably, we found that cocaine but not saline self-administration abolished eCB-LTD (two-way repeated measures ANOVA; group × time interaction effect, F2,33 = 11.31, p < 0.001; saline group: p < 0.001, 30 min after LTD induction vs. baseline, planned comparison; cocaine group: non-significant; Fig. 2D). Indeed, as illustrated in Fig. 2F, the magnitudes of eCB-LTD measured 30 min after LTD induction in saline and cocaine groups were 37.9 ± 3.39 % and 0.13 ± 1.65 %, respectively (saline group, n = 10 cells; cocaine group, n = 18 cells; saline vs. cocaine, p < 0.01). No differences were observed between naïve rats and rats with saline self-administration (planned comparison). These results indicated that cocaine self-administration impaired eCB-mediated plasticity of glutamatergic synapses impinging onto VTA DA neurons.

Cocaine self-administration altered the function of presynaptic CB1Rs

In principle, cocaine self-administration could block eCB-LTD either by inhibiting the synthesis and/or release of eCBs or by reducing the function of CB1Rs. To distinguish between these possibilities, we examined the effect of cocaine self-administration on the depression of EPSCs induced by the CB1R agonist Win 55,212–2. We found that cocaine self-administration significantly reduced the ability of Win 55,212–2 (10 μm) to inhibit the amplitude of EPSCs (two-way repeated measures ANOVA; group × time interaction effect, F1,14 = 13.47, p < 0.01; n = 8 cells for each group; saline group: p < 0.001, post-WIN 55,212–2 application vs. baseline, planned comparison; cocaine group: p < 0.01; Fig. 3A). The magnitudes of WIN 55,212–2-induced depression of EPSCs were 44.77 ± 2.63 % and 11.23 ± 1.22 % in saline and cocaine groups, respectively (saline vs. cocaine, p < 0.01; Fig. 3B). Because at the concentration we used (i.e., 10 μm) the WIN 55,212–2-induced depression of EPSCs is signaled by CB1Rs (Haj-Dahmane & Shen, 2010; Hausknecht et al., 2017), the present results indicated that cocaine self-administration impaired the function of CB1Rs in the VTA.

Fig. 3.

Fig. 3.

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Cocaine self-administration reduced the function of presynaptic CB1Rs in the VTA. (A) The lower panel is a summary graph of the time course and magnitude of WIN 55,212–2-induced depression of EPSC amplitudes obtained in saline- (●) and cocaine-treated rats (○). The upper panel depicts superimposed EPSC traces taken before and after WIN 55,212–2 (10 μm) application, as indicated by the numbers in the lower panel (1: in black). (B) A summary plot of WIN 55,212–2-induced depression of EPSC amplitudes obtained in saline- and cocaine-treated rats. Note that cocaine self-administration significantly reduced the magnitude of WIN-55,212–2 induced depression of EPSC amplitudes. Data are expressed as mean ± SEM. **: p < 0.01, saline vs. cocaine. Scale bars: 50 pA, 20 ms.

Discussion

In the present study, we have shown that cocaine self-administration abolishes eCB-LTD of glutamatergic synapses onto VTA DA neurons, similar to the observation after acute, in vitro amphetamine treatment (Jones et al., 2000). Taken together, psychostimulants impair the brain’s built-in mechanism to weaken excitatory synaptic strength in VTA DA neurons. The lack of this “brake” restraint makes it easier for psychostimulants to increase the glutamatergic synaptic strength and excite VTA DA neurons (Jones et al., 2000; Kauer, 2004). Cocaine exposure also abolishes eCB-LTD of glutamatergic synapses onto medium spiny neurons in the nucleus accumbens (Fourgeaud et al., 2004). As such, psychostimulant-induced blockade of eCB-mediated glutamatergic LTD is ubiquitous in the mesolimbic system, which partially underpins the addictive effects of psychostimulants.

It is worth noting that in the present study, the blockade of eCB-LTD was observed one day after the last cocaine self-administration session. It is unclear yet whether this effect persists. An earlier study shows that glutamatergic LTP is still occluded in the VTA after 21 days of abstinence following cocaine self-administration, indicating persistent enhancement in VTA glutamatergic signaling, which possibly underlies cocaine relapse (Chen et al., 2008). Likewise, it would be interesting to know whether the abolition of glutamatergic eCB-LTD in the VTA is persistent after protracted cocaine abstinence, which remains to be investigated.

The LFS-LTD in the VTA has been thoroughly characterized, which is independent of NMDA and metabotropic glutamatergic receptors (mGluRs) (Jones et al., 2000; Haj-Dahmane & Shen, 2010). Instead, it is mediated by retrograde eCB signaling and 2-AG is the most likely eCB messenger (Melis et al., 2004; Haj-Dahmane & Shen, 2010). Long-term depression mediated by mGluRs has also been observed in the VTA, which, however, is induced by high-frequency (66 Hz) stimulation (Bellone & Luscher, 2005). For LFS-induced eCB-LTD in the VTA, there are several key steps in the induction process. First, depolarization of DA neurons leads to Ca2+ influx via L-type Ca2+ channels, which facilitates 2-AG synthesis and retrograde release. Then, 2-AG binds to Gi/o-coupled CB1Rs on the presynaptic terminals and inhibits the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signaling cascade, resulting in a reduction in glutamate release and LTD (Thomas et al., 2000; Haj-Dahmane & Shen, 2010; Atwood et al., 2014). The present study has replicated previous findings, showing that LFS-LTD of VTA glutamatergic synapses is indeed mediated by eCB, as the CB1R antagonist AM 251 (3 μm) completely blocks LTD. In addition, a reduction in glutamate release is associated with the LTD, indicating what occurs is presynaptic LTD.

As mentioned above, mechanistically, cocaine self-administration could abolish eCB-LTD by impairing 2-AG synthesis and/or release, or by blocking the function of presynaptic CB1Rs. To distinguish between these possibilities, we have assessed the effects of a CB1R agonist WIN 55,212–2 on evoked EPSCs. WIN 55,212–2 leads to a greater reduction in EPSC amplitude in the saline than the cocaine group. This observation indicates that the blockade of eCB-LTD after cocaine self-administration is mediated by an impaired function of presynaptic CB1Rs. Interestingly, a reduced CB1R function has been shown in the prefrontal cortex in mice, rats, and humans after chronic cocaine exposure (Alvaro-Bartolome & García-Sevilla, 2013). Such effects could occur in multiple brain areas. Importantly, we have observed persistent reduction in CB1R function and impaired eCB-LTD in VTA DA neurons in rats with parental ethanol exposure (PE) (Hausknecht et al., 2017). Similar to rats with repeated cocaine exposure, rats with PE have potentiated glutamatergic synapses onto VTA DA neurons and exhibit increased drug addiction risk (Hausknecht et al., 2015; Wang et al., 2019). As such, a reduced CB1R function and the subsequent blockade of eCB-LTD in VTA DA neurons could be a critical cellular mechanism underlying addictive behaviors. In addition, it has also been observed that PE leads to tonic eCB signaling in the VTA, resulting in a reduction in action potential-independent spontaneous glutamate release (Hausknecht et al., 2017). Therefore, the diminished eCB-LTD after PE might be a result of occlusion, i.e., eCB-LTD has taken place due to tonic eCB signaling and hence it cannot be further induced. So far, it is unclear whether cocaine self-administration also alters tonic eCB signaling, which subsequently affects glutamatergic eCB-LTD in the VTA. This issue remains to be investigated.

The eCB-LTD has been observed in inhibitory GABAergic synapses (eCB-LTDi) onto VTA DA neurons as well (Pan et al., 2008a; Pan et al., 2008b). This type of synaptic plasticity could also be altered by cocaine: eCB-LTDi is facilitated by acute cocaine treatment, and repeated in vivo cocaine exposure occludes further induction of eCB-LTDi (Pan et al., 2008b). The augmented eCB signaling by cocaine reduces GABA release, which disinhibits DA neurons and increases DA release from the VTA (Wang et al., 2015). As such, cocaine alters both glutamatergic and GABAergic eCB-LTD, which act in concert to elevate DA release and enable the addictive effects of cocaine. Our previous work has shown that repeated cocaine exposure leads to overexcitation of VTA DA neurons to the degree of depolarization block, which causes allostatic changes in the function of the mesolimbic DA system (Shen et al., 2007).

It has been documented that DA itself could abolish LFS-LTD of VTA glutamatergic synapses by activating D2-like DA receptors (D2Rs) (Thomas et al., 2000). One explanation is that activation of D2Rs could inhibit intracellular Ca2+ currents (Cardozo & Bean, 1995; Thomas et al., 2000) and thus block 2-AG synthesis. Cocaine increases extracellular DA in the VTA, which could, in theory, subsequently diminish glutamatergic eCB-LTD via D2R activation. It is worth noting, though, that somatodendritic DA release in the VTA (Rice & Patel, 2015) is very limited because of the small reserve pool of DA vesicles (Kita et al., 2009). As yet, the role of D2R activation in 2-AG synthesis and cocaine-induced eCB-LTD blockade in the VTA remains to be elucidated.

In conclusion, Cocaine self-administration abolishes eCB-mediated LFS-LTD of glutamatergic synapses onto VTA DA neurons by reducing presynaptic CB1R function. The cocaine-induced dysregulation of eCB signaling in VTA DA neurons impairs the mechanism to weaken the strength of glutamatergic synapses, making VTA DA neurons more excitable. This could contribute to the addictive effects of cocaine.

Acknowledgements

This work was supported by the National Institutes of Health grant awards DA045863 (SH-D) and AA019482 (R-YS).

Abbreviations

CB1R

endocannabinoid CB1 receptor

D2R

D2-like dopaminergic receptor

DA

dopaminergic

eCB

endocannabinoid

EPSC

excitatory postsynaptic current

Ih

hyperpolarization-activated cation current

LFS

low-frequency stimulation

LTD

long-term depression

LTP

long-term potentiation

mGluR

metabotropic glutamatergic receptor

PPR

paired-pulse ratio

VTA

ventral tegmental area

Footnotes

Conflict of Interest Statement

The authors declare no conflict of interest.

Data Accessibility Statement

Data will be made accessible upon request.

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