Effects of serotonin and serotonergic agonists and antagonists on the production of tumor necrosis factor α and interleukin-6

Abstract

Serotonin (5-HT) is a neurotransmitter and immune modulator. The effect of 5-HT on the production of cytokines by human macrophages and lymphocytes is poorly recognized. In the present article we examine the role of 5-HT in modulating the production of two pro-inflammatory cytokines, i.e. interleukin-6 (IL-6) and tumor necrosis factor-α (TNFα), as well as the role of 5-HT_1A and 5-HT₂ receptors in this process. The specific aims were to examine the effects of 5-HT, p-chlorophenylalanine (PCPA), a 5-HT depleting agent, flesinoxan, a 5-HT_1A agonist, m-chlorophenylpiperazine (mCPP), a 5-HT_2B/2C agonist, and ritanserin, a 5-HT_2A/2C antagonist, on the production of the above cytokines. We found that: (1) 5-HT, 15 μg/ml, significantly decreased IL-6 and TNFα production; (2) pCPA, 5 μM, significantly suppressed the production of IL-6 and TNFα; and (3) mCPP, 2.7 μg/ml, significantly increased the production of IL-6 and TNFα. It is concluded that intracellular 5-HT is necessary for optimal synthesis of IL-6 and TNFα; 5-HT in physiological concentrations may increase IL-6 and TNFα production by stimulating 5-HT₂ receptors; and extracellular 5-HT concentrations above the baseline physiological levels may suppress the production of the above cytokines.

Introduction

It has been postulated that disorders in central serotonin neurotransmission are implicated in the pathogenesis of major depression. A frequently reported abnormality in the metabolism of 5-HT in major depression is a lowered availability of serum/plasma tryptophan, the precursor of 5-HT (Maes and Meltzer, 1995). Moreover, it has been shown that tryptophan depletion may cause mood disturbances in normal volunteers and depressed patients, as well as a reduction of 5-HT release in the frontal forebrain (Maes and Meltzer, 1995). It is thought that antidepressants, such as selective 5-HT reuptake inhibitors (SSRIs), 5-HT noradrenaline reuptake inhibitors (SNRIs) and some tricyclic antidepressants (TCAs) exert their antidepressant activity through modulation of the serotonergic system (Maes and Meltzer, 1995). Nevertheless, a clear relationship between changes in serotonergic functions and clinical improvement during treatment with these drugs has not been demonstrated nor have precise mechanisms been described by which these drugs exert their therapeutic effect.

The cytokine hypothesis of affective disorders suggests the importance of serotonin-immune interactions in the working mechanisms of antidepressant drugs (Maes, 2001). There is now evidence that major depression is associated with an activation of the inflammatory response system as well as an activation of some aspects of cell-mediated immunity (Maes, 2001). The evidence includes, amongst other things, increased numbers of peripheral blood leukocytes, such as monocytes, neutrophils and activated T cells, increased serum concentrations of neopterin and prostaglandin E2 (PGE2) and positive acute phase reactants. Most importantly, major depression is accompanied by increased serum concentrations and increased production of monocytic and Th1-like lymphocytic cytokines, such as interleukin-1β (IL-1β), IL-2, IL-6, tumor necrosis factor-alpha (TNFα) and interferon-gamma (IFNγ) (Maes, 2001, Mikova et al., 2001). Immunotherapy with cytokines, such as interferons and IL-2, induces full-blown major depression in a large number of subjects; administration of LPS or IL-1 to rodents induces a symptom complex typical for major depression (Dantzer et al., 1998). If increased production of proinflammatory cytokines were involved in the etiology of depression, one would expect antidepressive treatments to have negative immunoregulatory effects. Recently, we reported that antidepressants with a serotonergic mode of action have inhibitory effects on IFNγ and TNFα production and a stimulatory effect on IL-10, a negative immunoregulator with anti-inflammatory properties (Kubera et al., 2000a, Kubera et al., 2001). Co-incubation of whole human blood with fluoxetine, venlafaxine, imipramine and l-5-HTP, the immediate precursor of 5-HT, significantly decreased the IFNγ/IL-10 production ratio. The exact mechanisms by which antidepressant drugs affect cytokine production have remained elusive. It may be hypothesized that part of these effects may be explained by the serotonergic activities of antidepressants, such as depletion of intracellular 5-HT stores, increase in extra-cellular 5-HT, and stimulation of 5-HT_1A or 5-HT₂ receptors.

Serotonin is synthesized and released in the circulation by enterochromaffin cells from gastric and intestinal mucosa (Grimaldi and Fillion, 2000). It is rapidly taken up by platelets, were it can be found at high concentrations, and to a lesser extent by lymphocytes and monocytes (Mössner and Lesch, 1998). In inflammatory conditions the activated platelets release 5-HT and this leads to an increase in its local concentration at the inflamed region. 5-HT is released from platelets and lymphocytes/monocytes following stimulation by endotoxic lipopolysaccharide (LPS, released by Gram-negative bacteria), platelet activating factors and interferon (IFN)γ (Finocchiaro et al., 1988). 5-HT interacts with blood cells and its modulatory effect on immune cells including T, B and NK cells, and monocytes/macrophages has been well documented (Hellstrand and Hermodsson, 1993, Young and Matthews, 1995).

The effect of 5-HT on the production of cytokines by human immunocytes has been the subject of only a few studies. Arzt et al. (1991) showed that 5-HT inhibits the production of TNFα by macrophages, whereas Hellstrand et al. (1993) found that 5-HT promotes IFNγ production by Natural Keller cells. In peripheral blood leukocytes, 5-HT induces the production of IL-16, a chemotactic factor (Laberge et al., 1996). Recently Cloez-Tayarani et al. (2003) observed that 5-HT inhibited the production of TNFα. Previously we found that (1) intracellular 5-HT is necessary for IFNγ and IL-10 production by whole blood cells; (2) exogenous 5-HT decreases IFNγ production; and (3) the inhibitory effect of 5-HT on IFNγ production may be ascribed to 5-HT₂, but not 5-HT_1A, mechanisms (Kubera et al., 2000a).

The aim of the present paper was to elucidate, whether 5-HT and 5-HT receptor subtypes affect IL-6 and TNFα production. Therefore, we studied the effects of 5-HT, pCPA, a 5-HT depleting agent, flesinoxan, a 5-HT_1A agonist, m-chlorophenylpiperazine (mCPP), a 5-HT_2B/2C agonist, and ritanserin, a 5-HT_2A/2C antagonist, on the production of IL-6 and TNFα. We included three study groups, i.e. depressed subjects and younger versus elderly normal controls in order to check whether the above serotonergic substances have different effects in depressed patients versus normal controls and in elderly versus younger volunteers.