By Author
  By Title
  By Keywords

March 2016, Volume 66, Issue 3

Original Article

Paroxetine: An update of response on intestinal motility

Ayesha Afzal  ( Department of Pharmacology, Wah Medical College, Wah Cantt, Pakistan. )
Khalida Ajmal  ( Department of Pharmacology, Wah Medical College, Wah Cantt, Pakistan. )
Sabeen Shakir  ( Department of Pharmacology, Rawal Medical College, Rawalpindi, Pakistan. )
Bushra Tayabba Khan  ( Department of Pharmacology, Army Medical College, Rawalpindi, Pakistan. )
Iffat Ara  ( Department of Pharmacology, Wah Medical College, Wah Cantt, Pakistan. )


Objectives: To find out the possible effects of paroxetine on gastrointestinal smooth muscles in vitro as they can cause severe nausea and vomiting at the start of therapy which later settles down.
Method: Power lab (USA) was used for recording the contractions of ileal smooth muscle of rabbits in response to acetylcholine, serotonin and paroxetine.
Results: The percent responses with acetylcholine, serotonin and paroxetine were 100, 158.7 and 6.45 percent respectively indicating that acetylcholine and serotonin causes an increase in contractility of isolated ileal smooth muscle in comparison to paroxetine which has a depressant effect on motility.
Conclusion: Inability of paroxetine to enhance the serotonergic transmission in vitro causes a decrease in its qualitative response.
Keywords: Serotonin, Paroxetine, Acetylcholine, Gastrointestinal tract, Diarrhea, Power lab. (JPMA 66: 240; 2016)


Developments of antidepressants in the last five decades were based on monoaminergic hypothesis, which postulates that depression is because of deficiencies or fluctuation in the levels of serotonin, nor-epinephrine and dopamine. Several preclinical studies have suggested, that by targeting the specific serotonin receptors with selective agonist or antagonist not only improves the efficacy but also reduces time required for the therapeutic effect of antidepressants to appear.1 To explore the underlying mechanism of excessive nausea and vomiting produced by selective serotonin reuptake inhibitors, we observed the effects of paroxetine on ileal smooth muscles of rabbits in vitro. About 95% of serotonin is released in the gastrointestinal tract in response to release of acetylcholine.2,3 So acetylcholine and serotonin-mediated intestinal activity was taken as the control.4


This experimental study was carried out in Multidisciplinary Lab Army Medical College Rawalpindi, from March 2013 to July 2013 after obtaining approval from the institutional ethical committee.
Chemicals Acetylcholine Chloride, Serotonin Carnitine Sulfate and Paroxetine Hydrochloride were purchased from local market. All the solutions and dilutions (10-9 to 10-6 M) were prepared fresh at the time of experiments.5

Preparation of Tissue

Eighteen healthy rabbits weighing between 2.5-3.0 Kg were randomly divided into three groups (n=6). One overnight fasting rabbit was sacrificed and the small intestine was removed. The ileum was cut into 2 inches pieces6 and transferred to an organ bath of 50ml capacity containing tyrode\\\'s solution (in mM: NaCl, 136.8mM; KCl, 2.7mM; MgCl2, 0.5mM; CaCl2, 1.3mM; NaH2PO4, 0.14mM; NaHCO3, 12.0mM, Dextrose, 5.5mM) and aerated continuously with 95% oxygen and 5% carbondioxide.5 One end of the ileal strip was attached to the bottom of the oxygen tube in tissue bath and the other end was connected to a research grade force Displacement transducer.2 After equilibration the isotonic ileal smooth muscle activity was recorded through the Displacement Transducer on Power lab.7
Group 1 Cumulative concentration response curve of acetylcholine (n=6)
Using varying concentrations (10-9-10-6M) the cumulative dose-response curves of acetylcholine was constructed. To prevent tissue sensitization new tissue was used each time (n=6). This group served as a control for the study, and serotonin and paroxetine mediated contractions were compared with acetylcholine induced contractions.2
Group 2 Cumulative concentration-response curve of serotonin (n=6)
Serotonin mediated isotonic contractions were recorded using concentrations 10-9 to 10-6 M in the same manner as used for acetylcholine.2
Group 3 Cumulative concentration-response curve of paroxetine (n=6)
By using varying concentrations of paroxetine (10-9-10-6 M) the ileal smooth muscle activity was recorded in a similar manner as for group 1 and 2.

Statistical Analysis

The results were expressed as means ± standard deviation. The arithmetic means of amplitudes of contractions and SDs were calculated using Kruskal-Wallis test (One-Way Anova).


Paroxetine exerts a depressive effect on contraction of ileal smooth muscles and a significant decrease of paroxetine-induced contractions was observed at 10-7 M and 10-6 M concentrations (Figure).

To evaluate the decrease in magnitude of paroxetine-induced ileal contractility the response was compared with that of acetylcholine and serotonin on isolated ileal smooth muscle. Maximum constrictor response of serotonin was 58.7% more than the maximal acetylcholine response.
Paroxetine caused a significant decrease in ileal smooth muscle contractions compared to the control group (100% to 6.45%) and amplitude of contractions was found statistically significant.


The current study was carried out to observe the effects of paroxetine on ileal smooth muscle of rabbit in vitro and to find out the possible reason that may be responsible for causing severe nausea and vomiting at the start of therapy. Acetylcholine and serotonin gradually increases the ileal smooth muscle contractility, whereas paroxetine in contrast to acetylcholine and serotonin decreases the smooth muscle contractility. Acetylcholine mediated ileal contractions was taken as a standard for comparison in our experimental study.8
By acting via muscarinic receptors (M3) acetylcholine causes an inositol triphosphate (IP3) mediated release of intracellular calcium,9 the release of diacylglycerol (which activates protein kinase C), causing contraction of smooth muscles.10
Serotonin produced 158.7 percent of acetylcholine mediated response on ileal smooth muscles of rabbit.11 Serotonin by acting directly through 5-HT4 (G- protein coupled receptors) located on both cholinergic interneurons and motor neurons2 on enterocytes and indirectly via 5-HT3 receptors on mucosal nerves and vagal afferents effects the intestinal motility.12 The 5-HT4 receptors stimulation by serotonin leads to an increase in the acetylcholine release which in turn increases the intestinal activity.13
Paroxetine causes a dose dependant decrease in the contractile activity of isolated ileal smooth muscle, in turn causing an increase in the gut transient time because of its influence on vagal and adrenergic inputs.14 In addition serotonergic receptors (5-HT1A and 5-HT3) they are also known to influence vagal afferents pathway and alter the reflex accommodation pathways,15 hence causing decrease in amplitude of contractions.16


Paroxetine decreases the intestinal motility due to unmasking of its anticholinergic activity. In addition it also causes down regulation of serotonin transporter in the gastrointestinal tract in vitro.


We are grateful to National University of Sciences and Technology (NUST) Islamabad for providing financial support for this research study.
Conflict of interest: Authors show no conflict of interest.
Funding: From National University of Sciences and Technology Islamabad (NUST).
Ethical Approval: From institutional ethical Committee.
Declaration: Part of the study has already been published in doi: Int J Basic Clin Pharmacol. 2015; 4(2): 265-268.


1. Ferres A, Pillar F and Vidal R. RNA mediated serotonin transporter suppression rapidly increasing serotonergic neurotransmission and hippocampal neurogenesis. Transl Psychiatry 2013; 3: e211.
2. Afzal A, Khan BT, Sharif M. Fluoxetine causes a decrease in intestinal motility. Int J Basic Clin Pharmacol 2015; 4: 265-8
3. Coates DM, Johnson CA, Meerveld VG, Mawe MG. Effects of serotonin transporter inhibition on gastrointestinal motility and colonic sensitivity in the mouse. Neurogastroenterol Motiity 2006 18: 464-71.
4. Jabeen Q, Aziz N, Afzal Z, Gilani HA. The spasmogenic and spasmolytic activities of lavandula Stoechas are mediated through muscarinic receptor stimulation and calcium channel blockade. Int J Pharmacol 2007; 3: 61-7.
5. Noor A, Najmi MH, Bakhtiar S. Effect of bradykinin induced contraction on isolated smooth muscle of guniea pig . Indian J Pharmacol 2011; 43: 449-55.
6. Gregory VC, Lucki I. The role of serotonin receptor subtypes in treating depression: A review of animal studies. Psychology 2010; 213: 265-87.
7. Tanko Y, Alladey O, Ahmad KM, Muhammad A, Musa KY. The effect of methanol leaves extract of Ficus Glumosa on gastrointestinal motility and castor oil induced diarrhoe in laboratory animals. J Nat Prod Plant Resour 2012; 2: 360-7.
8. Chetty N, Irving RH, Coupar MI. Actiation of 5-HT3 receptors in the rat and mouse intestinal tract: a comparative study. Br J Pharmacol 2006; 148: 1012-21.
9. James BR. Acetylcholine. Wormbook. ed . [online] [Cited 14 July 2014]. Available from: URL: www_acetylcholine/acetylcholine.pdf.
10. Tuladhar RB, Costall B, Naylor JR. Modulation of 5-HT4 receptor function in the rat isolated ileum by fluoxetine: the involvement of endogenous 5-Hydroxytryptamine. Br J Pharmacol 2002; 136: 150-6.
11. Camelleri M. Serotonergic modulation of visceral sensation: Lower gut. Gut 2002; 51 suppl 1: 181-6.
12. Mujezinovic I, Cupic V, Samajlovic A, Muminovic M. Identification of serotonergic (5-H1A-Type) receptor in broiler small intestine by application of its serotonin and antagonist. Vetnary Glasnick 2011; 65: 51-9.
13. Pithadia BA, Jain MS. 5-Hydroxytryptamine Receptor Subtypes and their modulation with Therpeutic Potentials. J Clin Med Res 2009; 1: 72-80.
14. Spiller R. Serotonergic modulating drugs for functional gastrointestinal diseases. Br J Pharmacol 2002; 54: 11-20.
15. Wagstaff AJ, Cheer SM, Matheson AJ, Ormvod, Goa KL. Paroxetine: an update of its use in psychiatric disorders in adults. Drugs 2002; 62: 655-703.
16. Chial JH, Camilleri M, Burton D, Thomforde G, Olden WK, Stephens D. Selective effects of serotonergic psychoactive agents on gastrointestinal function in health. Am J Physiol Gastrointest Liver Physiol 2002; 284: G130-7.

Journal of the Pakistan Medical Association has agreed to receive and publish manuscripts in accordance with the principles of the following committees: