Science︱ Does Serotonin help overcome cocaine addiction?

Written by Zheng Yuanjia, edited by Zheng Yuanjia, Wang Sizhen Drug addiction (drug addiction), commonly known as "drug addiction", refers to the characteristics of specific, compensatory and adaptive changes in the body due to the long-term interaction of drugs with the body.
Once the drug is stopped, it will Causes the body's severe discomfort and/or urgent psychological needs, manifested as a strong desire for drugs and compulsive drug-seeking behavior
.

 Among people who chronic cocaine (cocaine) use, about 20% will lose control and become addicted [1].
The typical feature of addiction is an increase in dopamine (DA) levels [2], which triggers various forms of Synaptic plasticity is also the basis of adaptive behavior [3-5]
.

Using optogenetic technology to self-stimulate dopamine neurons (hereinafter referred to as oDASS), and selectively release dopamine in ventral tegmental area (VTA) neurons, which can induce neuronal adaptation similar to that produced by addictive drugs, and produce compulsive and The bimodal distribution of non-forced individuals [6]
.

 In addition, cocaine can also inhibit the serotonin serotonin (5-HT) transporter (SERT), causing the 5-HT level in the striatum to change instantaneously, and it is pharmacologically reduced in the forebrain 5-HT can promote compulsive cocaine seeking behavior [7]
.

These indicate that the different effects of the 5-HT system may be related to drug addiction [8, 9]
.

However, the related circuits and underlying cellular mechanisms remain unclear
.

 On September 10, 2021, the Christian Lüscher research group of the University of Geneva School of Clinical Medicine in Switzerland published an article titled "Synaptic mechanism underlying serotonin modulation of transition to cocaine addiction" on Science, revealing the neural circuit model of addiction Serotonin 5-hydroxytryptamine (5-HT) regulates the transition from behavior to compulsion by inhibiting key synapses
.

In order to explore the role of 5-HT regulation, first, the authors used SERT Met172 knock-in (SertKI) mice to carry transporters (the transporters neither bind cocaine nor change the background 5-HT levels [10-12], Using a 5-HT sensor, it was observed that cocaine (1.
5 mg/kg intraperitoneal injection) did not induce changes in 5-HT levels in the dorsal striatum (DS) of SertKI mice (Figure 1 AC)
.

Figure 1A-C 5- HT sensor detects calcium transients caused by cocaine (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Then, the mice were given the following cocaine self-administration training
.

1) Acquisition period (acquisition period) 12-15 days: When the mouse presses an activation lever, it can trigger the intravenous input of cocaine (0.
5 mg/kg).
At this time, there is a reminder light.
When the pause lever is pressed, there is no drug or light; 2) Punishment 4 days: a progressive punishment procedure, that is, a 0.
2 mA foot shock punishment is given while cocaine is injected (Figure 1D)
.

The results showed that there was no difference in the number of lever presses between the SertKI group and the wild-type (WT) group during the acquisition phase (Figure 1E), but during the punishment phase, some mice reduced the trigger for cocaine self-administration (Figure 1F), and some did not.
No reduction
.

Figure 1D-F Cocaine self-administration (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) By analyzing behavioral parameters, rats with different behaviors are divided into two categories: renouncersand and Perseverers, there are penalties for abandonment but no longer taking cocaine, and perseverers are punished and still taking cocaine, that is, addiction tendency (Figure 1 G and H)
.

56% of SertKI mice are persistent, while only 12% of WT mice (Figure 1I)
.

The percentage of adherence was not correlated with baseline cocaine SA (Figure 1J)
.

Figure 1G-J Different types of cocaine SA (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Next, the author used oDASS to make dopamine neurons self-stimulate to induce neuronal adaptability.
This results in a compulsive tendency (instead of self-administration of cocaine as described above), and the use of citalopram to increase 5-HT levels (Figure 2 AC)
.

ChR2 is expressed in the DA neurons of the mouse ventral tegmental area (VTA), so that pressing the activation lever can induce a short laser stimulation.
Repeat the aforementioned behavioral observations and found that regardless of whether citalopram is used or not, the acquired stage mice can be very good.
Good control of oDASS (Figure 2D), but there are also differences in the punishment phase (Figure 2E)
.

Figure 2A-E Optogenetic dopamine self-stimulation (Citalopram treatment) (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Using the above classification method, the abandonment category and the behavioral parameter are also classified according to the above classification method.
Stick to two clusters of class (figure 2 F and G)
.

Only 15% of rats treated with citalopram were persistent, while in the control group, 60% were persistent (Figure 2H)
.

Similar to the previous results, there was no significant correlation between the adherence rate and the oDASS baseline rate (Figure 2I)
.

Figure 2F-I Different types of compulsive dopamine self-stimulation (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Some studies have shown that from the perspective of oDASS, the orbitofrontal cortex (OFC) to the back The afferent synaptic potentials of the lateral striatum (DS) can drive "persistence" behavior [13], so the author wanted to understand whether cocaine self-administration is also a similar mechanism
.

In order to selectively stimulate the OFC-DS projection, the author expressed Chrimson (a red-shifted opsin) in OFC neurons (Figure 3A), and illuminates the DS brain slices 24-48 hours after the animal’s last punishment Terminals to induce excitatory postsynaptic currents (EPSCs)
.

Compared with mice that gave up cocaine self-administration and oDASS, mice that continued cocaine self-administration and oDASS had a higher AMPA/NMDA ratio (Figure 3 BE).
This indicates that the enhanced OFC-DS pathway is associated with oDASS and oDASS.
The "persistence" of cocaine self-administration A is related
.

In addition, the authors have not observed EPSCs rectification under various conditions, which also shows that the enhancement of OFC-DS pathway is caused by the increase in the number of AMPA receptors
.

Figure 3A-C OFC-DS pathway is related to the "persistence" of self-administration of oDASS and cocaine (SA: self-administration) (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Next , The author also studied whether 5-HT has an effect on synaptic transmission in the OFC-DS pathway
.

5-HT (4 μM) immersion can induce inhibition of presynaptic excitatory transmission (Figure 3F and G), and can be blocked by the 5-HT1B receptor antagonist NAS181 (20 μM), but not by 5-HT1A receptors Antagonist WAY100635 (1 μM) blocked (Figure 3F and G)
.

In addition, it was observed that the coefficient variance (1/CV2) decreased, and the paired pulse ratio (PPR) increased, indicating that presynaptic inhibition is expressed by a decrease in the probability of glutamate release (Figure 3H and I)
.

Figure 3F-I The effect of 5-HT on synaptic transmission of OFC-DS pathway (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) It has been reported that chemical methods can also reduce OFC activity Reduce the proportion of "persistence" in oDASS [6].
Based on the results observed in this study, the author further hypothesized that presynaptic inhibition may reduce the possibility of OFC-DS synaptic long-term potentiation (LTP), while LTP This in turn prevents cocaine self-administration from becoming compulsive
.

Therefore, in order to establish a causal relationship between 5-HT-induced presynaptic depression and compulsive cocaine self-administration, first, the authors specifically knocked out 5-HT1B receptors projected to DS in OFC neurons (Figure 4 A, B)
.

The 5-HT1B agonist CP39129 (2 μM) was used to confirm that functional knockout of the 5-HT1B receptor induces presynaptic inhibition (Figure 4 C, D)
.

Figure 4A&B specific knockout of 5-HT1B receptor in OFC neurons (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Then, one month later, 5-HT1B gene knockout Mice can again learn cocaine self-administration (Figure 4E)
.

The author also observed that, similar to the previous one, there is no difference in the acquisition phase (Figure 1E, Figure 4F), and the two types of persistence and abandonment can be observed again in the punishment phase (Figure 4G, H)
.

Compared with the control group, the percentage of persisters in mice that specifically knocked out 5-HT1B projections was higher (57% vs 13%) (Figure 4I)
.

In the control group and knockout mice, the adherence rate still had no correlation with baseline (Figure 4J)
.

In the 5-HT1B knockout group, the percentage of persistence was also very close to the percentage of persistence in the SertKI group and saline-treated oDASS mice
.

Fig.
4C-K Cocaine self-administration after knocking out 5-HT1B receptor (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Fig.
5 Article summary picture: 5-HT on synapse Regulation (picture quoted from: Yue L et al.
, Science.
2021;373: 1252-1256) Conclusion and discussion of the article, enlightenment and prospects So far, in summary, the synaptic mechanism is clear, and the regulation effect of 5-HT Reduce the possibility of addiction (Figure 5)
.

Specifically, in WT mice, cocaine binds to SERT to prevent 5-HT reuptake
.

Elevated extracellular 5-HT activates 5-HT1B receptors, leading to presynaptic inhibition of OFC neurons, reducing the possibility of evoking postsynaptic potentials at OFC-DS synapses, and ultimately driving the occurrence of compulsive behaviors
.

In SertKI mice, cocaine cannot bind to SERT, so extracellular 5-HT is not affected by cocaine injection [10]
.

In 5-HT1B knockout mice, although cocaine still inhibited 5-HT reuptake, the delivery of OFC-DS was not inhibited
.

The addictive tendency caused by the toxicity and susceptibility of cocaine may be determined by the 5-HT signal
.

Changes in 5-HT synthesis, synaptic release, reuptake efficiency, and extracellular levels may all be determinants
.

 Although this research mainly focuses on cocaine, the synaptic mechanism regulated by 5-HT may also be related to the mechanisms of other addictive drugs, such as amphetamines and opioids [14-16]
.

Studying the 5-HT regulation mechanism can not only prevent the transition of drugs to compulsion, but also promote the restoration of control, which helps overcome the limitations of the efficacy of 5-HT reuptake blockers in human addiction studies [17-19] , Or help to design selective agonists to supplement the use of psychedelics in the treatment of addiction [20]
.

 In summary, this study used the 5-HT transporter SERT, extracellular 5-HT, and 5-HT1B receptors to study the effects of 5-HT signaling on addictive behaviors, and also explored the effects of 5-HT on addictive behavior.
The role of key synapses and the circuit model more comprehensively reveal that in the neural circuit model of addiction, 5-HT regulates the transition of behavior to compulsion through the inhibition of key synapses
.

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