Dorsal Raphe Nucleus 5-HT Neurons: A New Target for Rewarding Regulation and Drug Addiction

It has been proposed that reward was divided into two types, including the pleasure brought by the enjoyment of emotion and motivation made by motivating individuals to obtain the reward. According to the classification of reward attributes, it can be divided into natural reward and drug reward. Natural rewards usually refer to water, food, sex and social interaction, while drug rewards usually induced by drugs such as morphine and cocaine. The mechanisms of rewarding effect are still complex and controversial. Traditional views suggest that the limbic dopamine system is the key to inducing the reward and leading to the addictive behaviors. Take morphine for example. Morphine interacts with m-Opioid Receptors (MORs) expressed in the Ventral Tegmental Area (VTA) γ-Aminobutyric acid (GABAergic) neurons and relieves the local inhibitory tone leading to reduced inhibition of the VTA dopaminergic neurons [1]. This in turn releases Dopamine (DA) in the Nucleus Accumbens (NAc) [2], which is considered a reward center for reward [3]. 

However, the long-term use of morphine can lead to drug abuse and addiction. Morphine addiction is characterized by negative physical and emotional feelings when the drug is terminated and causes drug-craving in mental functions and drug-seeking in behavioral changes [4]. Conversely, knockout of the dopamine transporter genetically does not inhibit the rewarding properties in the model of self-administration and Conditioned Place Preference (CPP) [5,6]. In addition, morphine addiction is still observed in dopamine-deficient mice [7], indicating that additional mechanisms of morphine addiction existed outside the dopaminergic system.

5-Hydroxytryptamine (5-HT), is an important inhibitory neurotransmitter in the brain, which is synthesized and secreted by Dorsal Raphe Nucleus (DRN) 5-HT neurons. DRN 5-HT neurons can form synaptic connections with almost whole brain nucleus and project to the forebrain and limbic areas like Medial Prefrontal Cortex (mPFC), NAc, amygdala, lateral habenula nucleus and VTA to form complex neural circuits [8]. Clinical studies have shown that 5-HT reuptake inhibition like fluoxetine and ciprotidran can relieve anhedonia in depression patients, suggesting that 5-HT is closely associated with rewarding effect. Genetically deleted both dopamine and 5-HT transporter in mice inhibited the cocaine CPP [9], indicating that 5-HT might play a role in reward regulation beside dopamine. Recently, several researches have reported that motivational, reinforcement and reward waiting behaviors could be induced by activating of DRN 5-HT neurons optogenetically in mice [10-12]. Our group showed that optogenetically activated 5-HT neurons in the DRN to mediate real-time CPP, suggesting that rewarding effect was led by activation of 5-HT neurons [13]. Other work in our group has shown that long-term fluoxetine use can induce significant conditional place preference (not published). These results suggest that 5-HT plays a more important role in reward regulation. 

However, there is some controversy about the reward regulation of DRN 5-HT neurons because of 5-HT can show reward and punishment through different neural projections. This means DR 5-HT neurons can encode reward and aversion [14].

It is generally believed that the addictive drugs mediated the dysfunction of mesolimbic dopaminergic system are the reasons of addiction. In neuroanatomy, DRN 5-HT neurons interactions with the mesolimbic dopaminergic system and regulate the reward effect of addictive drugs such as morphine [15]. A recently study reported that the inhibitory pathway of GABAergic neurons in the cephalic region of the VTA project to the DRN 5-HT neurons played an important role in morphine addiction, and optogenetically activated this inhibitory pathway to block the morphine rewarding effect and rescued the mental dependence [15]. By activating this GABAergic inhibitory pathway projected to the DRN, the inhibitory neurotransmitter GABA was released locally in the DRN, and then inhibited the activity of 5-HT neurons. Our study also found that morphine could increase DRN 5-HT neurons firing rate in vivo, and inhibited 5-HT neurons after morphine administration by optogenetic blocked morphine-induced CPP [13]. 

5-HT receptor is closely related to reward regulation and drug addiction. 5-HT receptors can be divided into 7 families according to their function, structure and signal transduction characteristics, including 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7. Except for 5-HT3 receptor conjugated with cationic channel type receptor, the other receptors were G protein conjugated receptors [16]. Many researched reported that 5-HT2 receptor family might be a key factor in drug addiction. The selective 5-HT2C receptor agonist Ro60-0175 did not affect methamphetamine-induced impulsive behavior at low doses. However, a higher dose of Ro60-0175 and M100907, a selective antagonist of 5-HT2A, can reduce the impulsive behavior of methamphetamine and cocaine [17]. Injection of 5-HT2A receptor antagonist M100907 into mPFC attenuated cue-induced cocaine seeking behavior, but the same dose of M100907 did not affect cocaine self-administration. These results further confirmed the role of 5-HT2A receptor of mPFC in cue-induced cocaine seeking behavior [18]. Systematic injection of 5-HT2C receptor agonist RO60-0175 reduced self-administration of cocaine [19]. The mPFC administered 5-HT2C receptor agonist MK212 in a dose-dependent manner to reduce cocaine ignition and drug-seeking behavior induced by conditioned cues, which was reversed by selective 5-HT2C receptor antagonist SB242084 [20]. Activation of 5-HT2C receptor in VTA leads to a decrease in dopamine level in NAc. Therefore, 5-HT2A /2C receptor may play its role in reducing drug igniting and cue-induced drug seeking behavior. In summery DRN 5-HT neurons will be a new target for the treatment of drug addiction.

The DRN 5-HT neurons is believed sensitive to reward and drug addiction, our group presents evidence that morphine regulates DRN 5-HT neurons to induce a rewarding effect and that the DRN 5-HT system participates in chronic morphine-induced CPP [13]. However, further studies are needed to establish the exact role of the DRN 5-HT system on other addictive drug effects that leads to addiction and withdrawal symptoms. The mechanisms including molecular, receptors, synaptic, neurons and even neural circuits should be investigated in the future, and provided more directions to clinical treatment of drug addiction.

1. Wise RA, Rompre PP (1989) Brain dopamine and reward. Annu Rev Psychol 40: 191-225.
2. Fields HL, Margolis EB (2015) Understanding opioid reward. Trends Neurosci 38: 217-225.
3. Russo SJ, Nestler EJ (2013) The brain reward circuitry in mood disorders. Nat Rev Neurosci 14: 609-625.
4. Vargas-Perez H, Ting-A-Kee RA, Heinmiller A, Sturgess JE, van der Kooy D (2007) A test of the opponent-process theory of motivation using lesions that selectively block morphine reward. Eur J Neurosci 25: 3713-3718.
5. Rocha BA, Fumagalli F, Gainetdinov RR, Jones SR, Ator R, et al. (1998) Cocaine self-administration in dopamine-transporter knockout mice. Nat Neurosci 1: 132-137.
6. Sora I, Wichems C, Takahashi N, Li XF, Zeng Z, et al. (1998) Cocaine reward models: Conditioned place preference can be established in dopamine- and in serotonin-transporter knockout mice. Proc Natl Acad Sci USA 95: 7699-7704.
7. Hnasko TS, Sotak BN, Palmiter RD (2005) Morphine reward in dopamine-deficient mice. Nature 438: 854-857.
8. Luo M, Zhou J, Liu Z (2015) Reward processing by the dorsal raphe nucleus: 5-HT and beyond. Learn Mem 22: 452-460.
9. Sora I, Hall FS, Andrews AM, Itokawa M, Li XF, et al. (2001) Molecular mechanisms of cocaine reward: combined dopamine and serotonin transporter knockouts eliminate cocaine place preference. Proc Natl Acad Sci USA 98: 5300-5305.
10. Liu Z, Zhou J, Li Y, Hu F, Lu Y, et al. (2014) Dorsal raphe neurons signal reward through 5-HT and glutamate. Neuron 81: 1360-1374.
11. Li Y, Zhong W, Wang D, Feng Q, Liu Z, et al. (2016) Serotonin neurons in the dorsal raphe nucleus encode reward signals. Nature Communications 7: 10503.
12. Miyazaki K, Miyazaki KW, Sivori G, Yamanaka A, Tanaka KF, et al. (2020) Serotonergic projections to the orbitofrontal and medial prefrontal cortices differentially modulate waiting for future rewards. Sci Adv 6: eabc7246.
13. Fu B, Yao JQ, Lu C, Wang B, Li Z, et al. (2022) Dorsal Raphe Nucleus Serotoninergic Neurons Mediate Morphine Rewarding Effect and Conditioned Place Preference. Neuroscience 480: 108-116.
14. Nagai Y, Takayama K, Nishitani N, Andoh C, Koda M, et al. (2020) The Role of Dorsal Raphe Serotonin Neurons in the Balance between Reward and Aversion. Int J Mol Sci 21: 2160.
15. Li Y, Li CY, Xi W, Jin S, Wu ZH, et al. (2019) Rostral and Caudal Ventral Tegmental Area GABAergic Inputs to Different Dorsal Raphe Neurons Participate in Opioid Dependence. Neuron 101: 748-761.
16. Slifirski G, Król M, Turlo J (2021) 5-HT Receptors and the Development of New Antidepressants. Int J Mol Sci 22: 9015.
17. Fletcher PJ, Rizos Z, Noble K, Higgins GA (2011) Impulsive action induced by amphetamine, cocaine and MK801 is reduced by 5-HT(2C) receptor stimulation and 5-HT(2A) receptor blockade. Neuropharmacology 61: 468-477.
18. Katsidoni V, Apazoglou K, Panagis G (2011) Role of serotonin 5-HT2A and 5-HT2C receptors on brain stimulation reward and the reward-facilitating effect of cocaine. Psychopharmacology (Berl) 213: 337-354.
19. Fletcher PJ, Rizos Z, Sinyard J, Tampakeras M, Higgins GA (2008) The 5-HT2C receptor agonist Ro60-0175 reduces cocaine self-administration and reinstatement induced by the stressor yohimbine, and contextual cues. Neuropsychopharmacology 33: 1402-1412.
20. Pentkowski NS, Duke FD, Weber SM, Pockros LA, Teer AP, et al. (2010) Stimulation of medial prefrontal cortex serotonin 2C (5-HT(2C)) receptors attenuates cocaine-seeking behavior. Neuropsychopharmacology 35: 2037-2048.