Dopamine inhibits responses of astroglia-enriched cultures to lipopolysaccharide via a β-adrenoreceptor-mediated mechanism

Abstract

We here investigated the effect of the catecholaminergic neurotransmitter dopamine (DA), on the release of two major inflammatory effectors, TNF-α and nitric oxide, in rat astroglia-enriched cultures stimulated with the bacterial endotoxin lipopolysaccharide (LPS). Upon LPS challenge, we observed a dramatic increase in the culture medium of the TNF-α protein, an effect thereafter followed by an increase of nitric oxide synthase type 2 (NOS2) mRNA and, at later times, of nitrite accumulation, an index of nitric oxide (NO) production. DA substantially inhibited the release of TNF-α and NO evoked by LPS, an effect not mimicked by selective agonists nor prevented by selective antagonists of the DA receptors. The inhibitory effects of DA were mimicked by noradrenalin and isoproterenol and fully reverted by propranolol, a selective antagonist of the β-adrenergic receptors. In addition, selective antagonists of β-adrenergic receptor type 1 (metoprolol) and type 2 (ICI-118,551) counteracted the inhibitory effects of DA on LPS-induced TNF-α and NO release. Accordingly, agents capable of elevating intracellular cyclic 3′,5′-adenosine monophosphate (cAMP), such as forskolin and dibutyryl-cAMP, mimicked DA inhibitory effects on LPS-evoked accumulation of TNF-α and nitrite. These data, consistent with a role of DA as local modulator of glial inflammatory responses, uncover the existence of an interaction between DA and heterologous β-adrenergic receptors in astroglial cells.

Introduction

Diverse forms of acute and chronic neurodegeneration, including brain trauma, stroke, Alzheimer's disease, and Parkinson's disease (PD), are accompanied by inflammation in the brain Raine, 1994, McGeer and McGeer, 1995, Allan and Rothwell, 2001, Hirsch et al., 2003. Astrocytes, together with microglia, are the main effectors cells of innate immune responses in the central nervous system (CNS) and can be activated in neurodegenerative diseases to produce an array of inflammatory mediators Eddleston and Mucke, 1993, Dong and Benveniste, 2001. The resulting inflammation may modulate neurodegenerative processes in either a beneficial or a detrimental fashion (Wyss-Coray and Mucke, 2002).

Lipopolysaccharide (LPS), a bacterial endotoxin which is responsible for multiple pathophysiological changes associated with Gram-negative infections, is widely used as a proinflammatory stimulus in a variety of experimental settings (Mukaida et al., 1996). Both in vivo Buttini et al., 1997, Arimoto and Bing, 2003 and in vitro Chung and Benveniste, 1990, Galea et al., 1992, Gayle et al., 2002 studies have demonstrated that LPS challenge induces upregulation in the expression of several inflammatory mediators in glial cells, including nitric oxide synthase (NOS) and tumor necrosis factor-α (TNF-α) which may be involved in the propagation of a variety of neurodegenerative processes Dawson and Dawson, 1998, Venters et al., 2000.

Dopamine (DA), a prominent neurotransmitter in the brain, is known to have an immunomodulatory role outside the CNS, where several peripheral cell types of the immune system, including lymphocytes and macrophages, express DA receptors Santambrogio et al., 1993, Ricci et al., 1994. DA suppresses the production of TNF-α and augments the release of IL-6 in adrenal zona glomerulosa cells (Ritchie et al., 1996) while in mice, DA receptor agonists and antagonists modulate LPS-induced TNF-α levels in plasma and in peritoneal macrophages (Hasko et al., 1996). More recently, it has been reported that DA attenuates LPS-evoked IL-12 p40 production in macrophages (Hasko et al., 2002) as well as nitric oxide synthase (NOS) expression and NO release in C6 glioma cells (Mazzio et al., 2002). While this suggests that DA may exert an immunomodulatory function, to date, there is no information available regarding the modulatory effect of DA on inflammatory responses of astroglial cells.

We here show that, in rat primary cultures enriched in astrocytes, DA inhibits the release of both TNF-α and NO, an effect not affected by selective antagonists of DA receptors nor mimicked by selective DA receptor agonists, but, rather, fully reversed by selective antagonists of β-adrenergic receptors. These data implicate the existence of a functional cross talk between DA and adrenergic receptors in astroglial cells which can modulate inflammatory responses in brain parenchyma.