View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by OAR@UM Review Journal of Pharmacy And Pharmacology Understanding the molecular pharmacology of the serotonergic system: using fluoxetine as a model Lino Sghendo and Janet Mifsud Department of Clinical Pharmacology and Therapeutics, University of Malta, Msida, Malta Keywords Abstract molecular and clinical pharmacology; molecular and receptor pharmacology; Objectives Serotonin is a monoamine neurotransmitter that is widely distributed pharmacokinetics and drug disposition in the body and plays an important role in a variety of psychological and other body functions such as mood, sexual desire and function, appetite, sleep, memory and Correspondence learning, temperature regulation and social behaviour. This review will assess the Janet Mifsud, Department of Clinical use of fluoxetine, one of the most commonly used selective serotonin reuptake Pharmacology and Therapeutics, University of Malta, Msida, MDS2040, Malta. inhibitors, as a model for understanding the molecular pharmacology of the sero- E-mail: [email protected] toninergic system. Key findings Seven serotonin receptor families have been discovered to date. All Received June 21, 2011 serotonin receptors, except 5-HT3, are G-protein coupled, seven transmembrane Accepted September 22, 2011 receptors that activate an intracellular second messenger cascade. The 5-HT3 recep- tor is a ligand-gated ion channel. Furthermore, 5-HT1A receptors are known as doi: 10.1111/j.2042-7158.2011.01384.x autoreceptors since their stimulation inhibits the release serotonin in nerve termi- nals.A transporter protein found in the plasma membrane of serotonergic neurones is responsible for the reuptake of this neurotransmitter. Selective serotonin reuptake inhibitors, such as fluoxetine, act primarily at the serotonin transporter protein and have limited, if any, reaction with other neurotransmitter systems. Selective seroto- nin reuptake inhibitors appear to bind with the serotonin transporter with different rates of occupancy, duration and potency. Summary The following review focuses on the interaction of serotonin with this membrane transporter in the body and assesses the use of fluoxetine as a reference drug in the understanding of this interaction. Introduction The selective serotonin reuptake inhibitor (SSRI) fluoxetine is significant.[4] Fluoxetine is a chiral molecule that is commer- one of the most widely used drugs in its class, and it is used in cially available as a racemic mixture, i.e. a 50 : 50 mixture of the treatment of depression, obsessive-compulsive disorder, its enantiomers (R)-fluoxetine and (S)-fluoxetine. bulimia and panic disorder.[1] It is considered to be the refer- Fluoxetine, similar to other SSRIs, has been shown to exert ence drug for the treatment of depression. The other com- its antidepressant effect by interacting with a membrane mercially available SSRIs include citalopram, escitalopram, protein. The following review focuses on the interaction of fluvoxamine, paroxetine and sertraline. Despite the fact that serotonin with this membrane protein in the body and how SSRIs have very different molecular structures, they all have fluoxetine, as the reference drug in this class, interferes with similar mechanisms of action, albeit with slightly different this interaction in order to carry out its pharmacological pharmacokinetic properties, which may lead to small differ- effects. The developments with respect to the molecular ences in some pharmacokinetic parameters such as half-life, understanding of these interactions are key to the develop- and differences in clinical and adverse and drug interac- ment of more targeted drug molecules acting on this impor- tions.[2] Fluoxetine was the first drug of its class that was tant system. Table 1 gives an overview of the mode of action, approved by the regulatory authorities in Europe and the adverse effects and pharmacokinetic parameters of fluoxetine USA. It is well absorbed from the gastrointestinal tract and its and other SSRIs. Newer SSRIs have fewer adverse effects than bioavailability is about 72%. Peak plasma concentrations are fluoxetine, i.e. fewer activating side effects (e.g. insomnia, agi- observed after 6 to 8 h.[3] Food does not appear to affect the tation,tremor and anxiety) and fewer gastrointestinal adverse systemic bioavailability of fluoxetine, although it may delay events (e.g. nausea, vomiting, diarrhoea, weight loss and its absorption by 1 to 2 h, which is probably not clinically anorexia). These side effects had lead to non-adherence issues © 2011 The Authors. JPP © 2011 Royal Pharmaceutical Society 2012 Journal of Pharmacy and Pharmacology, 64, pp. 317–325 317 Molecular pharmacology of serotonin 318 Table 1 Comparison of properties of SSRIs[2,5] Fluoxetine Citalopram Escitalopram Paroxetine Sertraline Recommended dose (for major 5–20 mg/day 20–40 mg/day 10–20 mg/day 20 mg/day 50 mg/day depressive disorder) Drug interaction potential High Relatively low Relatively low Moderate to high Relatively low Most common side effects Nausea Nausea Nausea Nausea Nausea Headache Dry mouth Insomnia Drowsiness Headache Insomnia Drowsiness Diarrhoea Headache Insomnia Nervousness Insomnia Headache Dry mouth Diarrhoea Anxiety Increased sweating Dizziness Dry mouth Drowsiness Diarrhoea Weakness Sexual dysfunction (ejaculation Anorexia Fatigue failure, decreased libido) Royal Pharmaceutical Society 2012 Diarrhoea Sexual dysfunction Drowsiness Increased sweating Dizziness Diarrhoea Fatigue Insomnia Half-life 96–386 h 35 h 27–32 h 20 h (highly variable) 26 h Time to steady state 30–60 days 7 days 7 days 10–14 days 7–14 days Protein binding 94% 80% 56% 95% 98% Metabolism By 2D6, 2C9/10, 2C19 None; no active None; no active 2D6; no active None; no active Active metabolites (potential for metabolites metabolites metabolites metabolites Journal of Pharmacy and Pharmacology drug–drug interactions) Linear kinetics No Yes Yes No Yes Secondary binding Least SSRI; Most SSRI Most SSRI Muscarinic cholinergic receptors Dopamine reuptake (more potent properties of SSRIs Norepinephrine reuptake, dopamine (most potent blocker of muscarinic dopamine uptake inhibitor than reuptake, serotonin-2C receptors, receptors among the SSRIs); other SSRIs); norepinephrine cytochrome P450 2D6, cytochrome histamine H1 receptors; nitric oxide reuptake; sigma receptors Lino Sghendo and Janet Mifsud © 2011 The Authors. JPP © 2011 P450 3A4 synthase cytochrome P450 2D6 , 64 , pp. 317–325 Lino Sghendo and Janet Mifsud Molecular pharmacology of serotonin in some patients, which undermined the efficacy of fluoxet- conformational changes and move one or more molecules ine. However these side effects resolve in the majority of per ‘cycle’, unlike channels, which stay open or closed, thus patients and become significantly less frequent with contin- allowing floods of molecules to move across bilayer mem- ued treatment over a 6-month period.[2] branes. The serotonin transporter resembles other biogenic amine transporters structurally and functionally, such as Serotonin transporter norepinephrine and dopamine transporters. The protein structure is composed of 12 transmembrane Serotonin (5-hydroxytryptamine, 5-HT), a monoamine neu- helices with an extracellular loop between transmembrane rotransmitter, plays an important role in many behaviours, helices 3 and 4.[10] Both polypeptide termini are located including sleep, appetite, memory, sexual behaviour, neu- within the cytoplasm and six putative phosphorylation sites, roendocrine function and mood. In the brain, the highest which are potential targets for protein kinase A and protein level of serotonin is found in the dorsal and median raphe kinase C, exist in the same compartment. The areas impor- nuclear complex.[6] Neurones in the raphe system project tant for selective serotonin affinity are localised within helices beyond the raphe nucleus, most often in the lateral direction. 1 to 3 and helices 8 to 12. A binding site for serotonin is Numerous serotonergic neurones are also found in the believed also to be the target of selective inhibitors, such as reticular region of the lower brain stem. Neurones in the fluoxetine (Figure 1). brain stem are often divided into a caudal system and a rostral system. Nerve cells in the caudal system descend to the spinal cord along many pathways and are largely involved in sensory, motor and autonomic functioning. Serotonergic cells in the Mechanism of action rostral system of the brainstem largely terminate in the dorsal Serotonin reuptake transporters are dependent on extracel- and median raphe nuclei. lular sodium (Na+) ions and extracellular chloride (Cl–) One other major serotonergic pathway in the brain is that ions. Unlike sodium ions, choride ions can be at least partly from the cerebellum, which terminates in the cerebellar substituted for by bicarbonate ions (HCO –). Intracellular cortex and the cerebellar nuclei. Many serotonergic neurones 3 potassium ions (K+) are also used in the process but can be contribute to this pathway. The presence of serotonin has also replaced by other ions, most notably hydrogen (H+) ions. been observed in the pons, medulla, thalamus, hypothala- The driving force for the energetically unfavourable trans- mus, substantia nigra and locus coeruleus.[7] Serotonin is port of serotonin is the sodium ion influx down its concen- synthesised from the amino acid precursor tryptophan, pack- tration gradient.[12] The Na+/K+ ATPase pump maintains the aged into vesicles