Effects of serotonin (5-HT)6 receptor ligands on responding for cocaine reward and seeking in rats

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

Prefrontal cortex nicotinic receptor inhibition by methyllycaconitine impaired cocaine-associated memory acquisition and retrieval

Cocaine administration has been shown to induce plastic changes in the medial prefrontal cortex (mPFC), which could represent a mechanism by which cocaine facilitates the association between cocaine rewarding effects with contextual cues. Nicotinic acetylcholine receptors (nAChRs) in the mPFC have critical roles in cognitive function including attention and memory and are key players in plasticity processes. However, whether nAChRs in the mPFC are required for the acquisition and maintenance of cocaine-associated memories is still unknown. To assess this question, we used the conditioning place preference (CPP) model to study the effect of intra-mPFC infusion of methyllycaconitine, a selective antagonist of α7 nAChRs, on the acquisition, consolidation and expression of cocaine-associated memory in adult rats. Our findings reveal that mPFC α7 nAChRs activation is necessary for the acquisition and retrieval, but not consolidation, of cocaine induced CPP. Moreover, cocaine-induced sensitization during CPP conditioning sessions was abolished by methyllycaconitine infusion in the mPFC. Together, these results identify mPFC α7 nAChRs as critical players involved in both acquiring and retrieving cocaine-associated memories. Considering that drug seeking often depends on the association between drug-paired cues and the rewarding effects of the drug, α7 nAChRs in the mPFC could be considered as potential targets for the prevention or treatment of cocaine use disorder.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin Type 6 and 7 Receptors as a Novel Therapeutic Target for the Treatment of Schizophrenia

Schizophrenia is a serious disease of the central nervous system that affects a person's ability to think, feel and behave clearly. Even though the pathophysiological hypothesis of the disease is not clearly understood, dysfunction of dopamine, glutamate, serotonin and other neurotransmitters is widely believed to be involved. Serotonin within the synaptic vesicles functions as neurotransmitter and neurohormone in regulation of emotion, learning, memory, hormone release, cognition and motor function. Dysfunction of normal brain activity of serotonin is associated with schizophrenia. The role of serotonin 6 and 7 receptors in schizophrenia, interaction with neurotransmitters and the effect of drugs on those receptors in schizophrenia are the goal of this review. The aim of this review was to provide information for researchers and other scholars to identify the possible intervention points in the management of schizophrenia. The serotonin 6 and 7 receptors are associated with schizophrenia via modulating cyclic adenosine monophosphate, regulation of Fyn kinase and induction of structural plasticity. The above modulatory effects affect cholinergic, dopaminergic, glutamatergic, adrenergic and GABAergic systems. Recently, diverse numbers of selective agonist and antagonist ligands were developed for both receptors. SGS-518, ABT-354, Lu AE58054, SB-742,457, S-518, AVN-211, AVN-322, SYN-114 and SYN-120 are serotonin 6 receptor antagonists and aripiprazole-controlled release serotonin 7 receptor agonists under clinical trial for schizophrenia. Thus, research on novel drugs that act on serotonin 6 and 7 receptors likely facilitates the intervention into schizophrenia patients seeking better quality of life in the future.

Striatal 5-HT6 Receptors Regulate Cocaine Reinforcement in a Pathway-Selective Manner

The nucleus accumbens (NAc) in the ventral striatum integrates many neurochemical inputs including dopamine and serotonin projections from midbrain nuclei to modulate drug reward. Although D1 and D2 dopamine receptors are differentially expressed in the direct and indirect pathway medium spiny neurons (dMSNs and iMSNs, respectively), 5-HT6 receptors are expressed in both pathways, more strongly than anywhere else in the brain, and are an intriguing target for neuropsychiatric disorders. In the present study, we used viral vectors utilizing dynorphin or enkephalin promoters to drive expression of 5-HT6 receptors or green fluorescent protein (GFP) selectively in the dMSNs or iMSNs of the NAc shell. Rats were then trained to self-administer cocaine. Increased 5-HT6 receptor expression in dMSNs did not change any parameter of cocaine self-administration measured. However, increasing 5-HT6 receptors in iMSNs reduced the amount of cocaine self-administered under fixed-ratio schedules, especially at low doses, increased the time to the first response and the length of the inter-infusion interval, but did not alter motivation as measured by progressive ratio 'break point' analysis. Modeling of cocaine pharmacokinetics in NAc showed that increased 5-HT6 receptors in iMSNs reduced the rat's preferred tissue cocaine concentration at each dose. Finally, increased 5-HT6 receptors in iMSNs facilitated conditioned place preference for a low dose of cocaine. We conclude that 5-HT6 receptors in iMSNs of NAcSh increase the sensitivity to the reinforcing properties of cocaine, particularly at low doses, suggesting that these receptors may be a therapeutic target for the treatment of cocaine addiction.

Pharmacological Evidence for 5-HT6 Receptor Modulation of 5-HT Neuron Firing in Vivo

5-Hydroxytryptamine (5-HT) neurons in the midbrain dorsal raphe nucleus (DRN) are implicated in the drug treatment and pathophysiology of a wide variety of neuropsychiatric disorders. Accumulating evidence suggests that 5-HT6 receptors may be located and functional in the DRN; therefore, 5-HT6 receptor ligands may have potential as novel modulators of 5-HT neurotransmission. The current study investigated the effect of intravenous (i.v.) administration of the selective 5-HT6 receptor agonist, WAY-181187, and antagonist, SB-399885, on the firing of 5-HT neurons in the DRN in vivo. Extracellular recordings were made in the DRN of anesthetized rats, and single 5-HT neurons were identified on the basis of electrophysiological properties combined with juxtacellular labeling and postmortem immunohistochemical analysis. WAY-181187 (1-4 mg/kg i.v.) caused a dose-dependent increase in 5-HT neuron firing rate. In comparison, SB-399885 (0.125-1 mg/kg i.v.) caused a dose-dependent decrease in 5-HT neuron firing rate, an effect reversed by WAY-181187 (3 mg/kg i.v.). These effects of WAY-181187 and SB-399885 were observed in two separate sets of experiments. In summary, the current data show the modulation of 5-HT neuronal firing by the 5-HT6 ligands WAY-181187 and SB-399885 and are consistent with the presence of 5-HT6 receptor-mediated positive feedback control of 5-HT neurons.

Therapeutic Potential of 5-HT6 Receptor Agonists

Given its predominant expression in the central nervous system (CNS), 5-hydroxytryptamine (5-HT: serotonin) subtype 6 receptor (5-HT6R) has been considered as a valuable target for the development of CNS drugs with limited side effects. After 2 decades of intense research, numerous selective ligands have been developed to target this receptor; this holds potential interest for the treatment of neuropathological disorders. In fact, some agents (mainly antagonists) are currently undergoing clinical trial. More recently, a series of potent and selective agonists have been developed, and preclinical studies have been conducted that suggest the therapeutic interest of 5-HT6R agonists. This review details the medicinal chemistry of these agonists, highlights their activities, and discusses their potential for treating cognitive issues associated with Alzheimer's disease (AD), depression, or obesity. Surprisingly, some studies have shown that both 5-HT6R agonists and antagonists exert similar procognitive activities. This article summarizes the hypotheses that could explain this paradox.

Psychostimulants: Basic and Clinical Pharmacology

Substance use disorder, and particularly psychostimulant use disorder, has considerable socioeconomic burden globally. The psychostimulants include several chemical classes, being derivatives of benzoylecgonine, phenethylamine, phenylpropanolamine, or aminoaryloxazoline. Psychostimulant drugs activate the brain reward pathways of the mesoaccumbal system, and continued use leads to persistent neuroplastic and dysfunctional changes of a variety of structures involved in learning and memory, habit-forming learning, salience attribution, and inhibitory control. There are a variety of neurochemical and neurobehavioral changes in psychostimulant addiction, for example, dopaminergic, glutamatergic, serotonergic (5-HT-ergic), and γ-amino butyric acid (GABA) changes have all noted. In this chapter, we will review pharmacological changes associated with psychostimulant use and abuse in humans and animals, and on the basis of the best characterized and most widely abused psychostimulants (amphetamines, cocaine) discuss why use transitions into abuse and review basic science and clinical strategies that might assist in treating psychostimulant abuse.

Working memory deficits and alterations of ERK and CREB phosphorylation following withdrawal from cocaine self-administration

BACKGROUND

The mechanisms underlying memory functions during withdrawal from the chronic drug use are poorly understood.

METHODS

We assessed learning and spatial working memory using the delayed alternation assay (T-maze) in rats, previously subjected to cocaine self-administration. The T-maze training was conducted 1-5 weeks after cocaine cessation; working memory efficacy was assessed at 5-8 weeks of drug withdrawal. After behavioral training and testing, the rats were sacrificed and the levels of p-CREB/CREB and p-ERK2/ERK2 in several brain areas were measured. The same molecular assessment was performed in rats with cocaine injections, but forced to drug abstinence in home cages.

RESULTS

After 5 weeks of cocaine withdrawal from self-administration, a significant impairment of working memory under increased working memory load (inter-trial delay extended to 30s), with no changes at baseline conditions (inter-trial delay 10s), was noticed. Neither acquisition phase nor working memory performance measured 6-8 weeks after the last drug intake differed between cocaine or saline pretreated rats. Upon T-maze training and 8-week withdrawal, cocaine-pretreated rats had higher levels of p-CREB/CREB in prefrontal cortex and dorsal striatum and lower in hippocampus compared to saline rats. Increased levels of p-ERK2/ERK2 were observed in dorsal striatum, hippocampus and decreased in nucleus accumbens. In cocaine-pretreated caged rats no changes in p-CREB/CREB levels were observed, while ERK2 levels either decreased (frontal cortex) or increased (nucleus accumbens).

CONCLUSION

Our results suggest that cocaine self-administration results in cognitive impairments and alterations in ERK/CREB signaling pathway long after discontinuation of drug use.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

The effect of active and passive intravenous cocaine administration on the extracellular signal-regulated kinase (ERK) activity in the rat brain

According to a current hypothesis of learning processes, recent papers pointed out to an important role of the extracellular signal-regulated kinase (ERK), in drug addiction. We employed the Western blotting techniques to examine the ERK activity immediately after cocaine iv self-administration and in different drug-free withdrawal periods in rats. To distinguish motivational vs. pharmacological effects of the psychostimulant intake, a "yoked" procedure was used. Animals were decapitated after 14 daily cocaine self-administration sessions or on the 1st, 3rd or 10th extinction days. At each time point the activity of the ERK was assessed in several brain structures, including the prefrontal cortex, hippocampus, dorsal striatum and nucleus accumbens. Passive, repeated iv cocaine administration resulted in a 45% increase in ERK phosphorylation in the hippocampus while cocaine self-administration did not change brain ERK activity. On the 1st day of extinction, the activity of the ERK in the prefrontal cortex was decreased in rats with a history of cocaine chronic intake: by 66% for "active" cocaine group and by 35% for "yoked" cocaine group. On the 3rd day the reduction in the ERK activity (25-34%) was observed in the hippocampus for both cocaine-treated groups, and also in the nucleus accumbens for "yoked" cocaine group (40%). On the 10th day of extinction there was no significant alteration in ERK activity in any group of rats. Our findings suggest that cortical ERK is involved in cocaine seeking behavior in rats. They also indicate the time and regional adaptations in this enzyme activity after cocaine withdrawal.

Cocaine-induced glutamate receptor trafficking is abrogated by extinction training in the rat hippocampus

BACKGROUND

It has been demonstrated that long-term exposure to cocaine leads to plastic changes in the brain that contribute to the manifestation of addictive behaviors. While attention has mostly focused on the meso-cortico-limbic pathway, the hippocampus seems to play a role in the craving induced by cues in drug addicts, in particular in cue- and drug-induced reinstatement of cocaine seeking. Since glutamate appears to be critical for context-induced drug seeking behaviors, the major aim of our work was to investigate the expression of hippocampal AMPA and NMDA glutamate receptors following repeated cocaine exposure and during extinction training.

METHODS

We thus employed the yoked control operant paradigm and exposed the animals to contingent or non-contingent cocaine exposure for 2 weeks and sacrificed the animals after the last self-administration (SA) session and following 1 or 10 days of extinction. Protein levels of glutamate receptors were analyzed by Western blotting.

RESULTS

We found increased levels of the main subunits of both NMDA and AMPA receptors in the post-synaptic density (PSD) fraction, but not in the whole homogenate, of the hippocampus of animals repeatedly exposed to cocaine indicating increased trafficking toward the membrane of these receptors. Also, we found that extinction abolished such effect, suggesting that the trafficking was tightly linked to the presence of the psychostimulant.

CONCLUSIONS

These data reveal a novel, previously unappreciated role of glutamate receptors in the action of cocaine and cocaine-extinction behavior in rat hippocampus.

The effects of cocaine on different redox forms of cysteine and homocysteine, and on labile, reduced sulfur in the rat plasma following active versus passive drug injections

The aim of the present studies was to evaluate cocaine-induced changes in the concentrations of different redox forms of cysteine (Cys) and homocysteine (Hcy), and products of anaerobic Cys metabolism, i.e., labile, reduced sulfur (LS) in the rat plasma. The above-mentioned parameters were determined after i.p. acute and subchronic cocaine treatment as well as following i.v. cocaine self-administration using the yoked procedure. Additionally, Cys, Hcy, and LS levels were measured during the 10-day extinction training in rats that underwent i.v. cocaine administration. Acute i.p. cocaine treatment increased the total and protein-bound Hcy contents, decreased LS, and did not change the concentrations of Cys fractions in the rat plasma. In turn, subchronic i.p. cocaine administration significantly increased free Hcy and lowered the total and protein-bound Cys concentrations while LS level was unchanged. Cocaine self-administration enhanced the total and protein-bound Hcy levels, decreased LS content, and did not affect the Cys fractions. On the other hand, yoked cocaine infusions did not alter the concentration of Hcy fractions while decreased the total and protein-bound Cys and LS content. This extinction training resulted in the lack of changes in the examined parameters in rats with a history of cocaine self-administration while in the yoked cocaine group an increase in the plasma free Cys fraction and LS was seen. Our results demonstrate for the first time that cocaine does evoke significant changes in homeostasis of thiol amino acids Cys and Hcy, and in some products of anaerobic Cys metabolism, which are dependent on the way of cocaine administration.

LC/MS/MS evaluation of cocaine and its metabolites in different brain areas, peripheral organs and plasma in cocaine self-administering rats

BACKGROUND

We employed a cocaine intravenous self-administration model based on positive reinforcement of animals' instrumental reactions (i.e., lever pressing) rewarded by a dose of the drug. We also carried out simultaneous characterization of the pharmacokinetics of cocaine and its metabolites in rats during withdrawal; in this part of the experiments, we investigated the cocaine (2 mg/kg, iv)-induced changes in the distribution, rate constant, clearance and t₁/₂ of the parent drug and its metabolites in different structures of the brain and in peripheral tissues.

METHODS

By using liquid chromatography-tandem mass spectrometry (LC/MS/MS) we measured the levels of cocaine and its major metabolites.

RESULTS

Our results demonstrate differences in the levels of cocaine after cocaine self-administration in the rat, with the highest concentration seen in the striatum and the lowest in the cerebellum. Cocaine metabolites determined in the rat brain remained at very low levels (benzoylecgonine), irrespectively of the brain area, whereas the norcocaine concentration varied from 1.56 μg/g (the nucleus accumbens) to 2.73 μg/g (the striatum).

CONCLUSION

A tandem LC/MS/MS is a valid method for evaluation of brain and peripheral levels of cocaine and its metabolites. Our results demonstrate brain area-dependent differences in the levels of cocaine after its self-administration in the rat. There were also differences in pharmacokinetic parameters among the brain areas and peripheral tissues following a bolus iv injection of cocaine to rats withdrawn from cocaine; among brain structures the slowest metabolic rate was detected for the striatum.

Evidence for a role of a dopamine/5-HT6 receptor interaction in cocaine reinforcement

The putative 5-HT6 receptor agonist ST1936 has been shown to increase extracellular dopamine (DA) in the n.accumbens (NAc) shell and in the medial prefrontal cortex (PFCX). These observations suggest that 5-HT6 receptors modulate DA transmission in mesolimbic and mesocortical terminal DA areas. To investigate the behavioral counterpart of this interaction we studied in rats 1) the ability of ST1936 to maintain i.v. self-administration in fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement; 2) the effect of 5-HT6 receptor blockade on cocaine stimulated overflow of DA in dialysates from the PFCX and from the NAc shell and on cocaine i.v. self-administration. ST1936 was i.v. self-administered at unitary doses of 0.5-1 mg/kg on an FR1 and PR schedule of reinforcement, with breaking point of about 4. Pretreatment with the 5-HT6 antagonist SB271046 reduced by about 80% responding for ST1936. SB271046 also reduced cocaine-induced increase of dialysate DA in the NAc shell but not in the PFCX and impaired i.v. cocaine self-administration. These observations indicate that ST1936 behaves as a weak reinforcer and suggest that 5-HT6 receptors play a role in cocaine reinforcement via their facilitatory interaction with DA projections to the NAc shell. This novel 5-HT/DA interaction might provide the basis for a new pharmacotherapeutic strategy of cocaine addiction.

Oxidative stress biomarkers in some rat brain structures and peripheral organs underwent cocaine

Oxidative stress (OS) generates or intensifies cocaine-evoked toxicity in the brain and peripheral organs. The aim of this study was to examine superoxide dismutase (SOD) activity and lipid peroxidation [measured by malondialdehyde (MDA) levels] in rats during maintenance of cocaine self-administration and after withdrawal by a yoked-triad procedure. Our results indicate that repeated cocaine self-administration provoked an elevation of SOD activity in the hippocampus, frontal cortex, dorsal striatum, and liver. MDA levels were reduced in the brain, increased in the liver, kidney, and heart during maintenance of self-administration, and increased in the kidney in cocaine-yoked rats. In addition, following extinction training, we found enhanced MDA levels and SOD activity in the rat hippocampus, while changes in the activity of OS biomarkers in other brain structures and peripheral tissues were reminiscent of the changes seen during cocaine self-administration. These findings highlight the association between OS biomarkers in motivational processes related to voluntary cocaine intake in rats. OS participates in memory and learning impairments that could be involved in drug toxicity and addiction mechanisms. Therefore, further studies are necessary to address protective mechanisms against cocaine-induced brain and peripheral tissue damage.