Evoked dopamine overflow is augmented in the striatum of calcitriol treated rats

Abstract

Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These actions include reducing the severity of some central nervous system lesions, possibly by upregulating trophic factors such as glial cell line-derived neurotrophic factor (GDNF). GDNF has substantial effects on the nigrostriatal dopamine (DA) system of young adult, aged and lesioned animals. Thus, the administration of calcitriol may lead to significant effects on nigrostriatal DA neuron functioning. The present experiments were designed to examine the ability of calcitriol to alter striatal DA release, and striatal and nigral tissue levels of DA. Male Fischer-344 rats were administered vehicle or calcitriol (0.3, 1.0, or 3.0 μg/kg, s.c.) once daily for eight consecutive days. Three weeks later in vivo microdialysis experiments were conducted to measure basal and stimulus evoked overflow of DA from the striatum. Basal levels of extracellular DA were not significantly affected by the calcitriol treatments. However, the 1.0 and 3.0 μg/kg doses of calcitriol led to increases in both potassium and amphetamine evoked overflow of striatal DA. Although post-mortem tissue levels of striatal DA were not altered by the calcitriol injections, nigral tissue levels of DA and its main metabolites were increased by both the 1.0 and 3.0 μg/kg doses of calcitriol. In a separate group of animals GDNF levels were augmented in the striatum and substantia nigra after eight consecutive daily injections of calcitriol. These results suggest that systemically administered calcitriol can upregulate dopaminergic release processes in the striatum and DA levels in the substantia nigra. Increases in the levels of endogenous GDNF following calcitriol treatment may in part be responsible for these changes. The ability of calcitriol to lead to augmented DA release in the striatum suggests that calcitriol may be beneficial in disease processes involving dopaminergic dysfunction.

Introduction

Calcitriol (1,25-dihydroxyvitamin D₃), the active metabolite of vitamin D₃, is currently used clinically to treat several conditions such as hypocalcemia and hypoparathyroidism. While its classical role in bone metabolism and calcium homeostasis is well known, calcitriol has also been shown to modulate several other physiological processes including cell growth, apoptosis and immune responses (Dusso et al., 2005). Calcitriol has also been shown to have numerous effects in the nervous system (Fernandes de Abreu et al., 2009, Garcion et al., 2002). Although its ability to cross the blood–brain barrier is limited, calcitriol is taken up by the brain in proportion to free circulating levels perfusing the blood–brain barrier (Gascon-Barre and Huet, 1983); and vitamin D receptors are extensively distributed throughout the brain (Eyles et al., 2005, Prufer et al., 1999, Stumpf and O’Brien, 1987). Furthermore, the enzyme responsible for the final step in calcitriol synthesis has been localized to neuronal and glial cells (Eyles et al., 2005), and it has been demonstrated that activated microglial cells can produce calcitriol (Neveu et al., 1994b). Taken together, these results suggest that calcitriol may have important physiological functions in the central nervous system.

One of the apparent functions of calcitriol in the nervous system is the modulation of trophic factors. Several studies have indicated that calcitriol regulates the expression of trophic factors in glial and neuroblastoma cell lines (Neveu et al., 1994a, Veenstra et al., 1997a, Veenstra et al., 1997b). One of the trophic factors that is regulated by calcitriol is glial cell line-derived neurotrophic factor (GDNF). Calcitriol has been shown to increase the expression of GDNF in vitro in C6 glioma cells (Naveilhan et al., 1996), and increase the release of GDNF from human glioblastoma cells (Verity et al., 1999). In vivo, calcitriol administration has been shown to increase the expression and protein levels of GDNF in the brain (Sanchez et al., 2002, Sanchez et al., 2009, Wang et al., 2000).

In young adult animals a single intracerebral administration of GDNF has significant and long-lasting effects on brain dopamine (DA) systems. These effects include changes in tissue levels of DA in the nigrostriatal system (Cass and Peters, 2010, Hudson et al., 1995, Martin et al., 1996), augmentation of stimulus-evoked overflow of DA from the striatum (Hebert et al., 1996, Xu and Dluzen, 2000), and behavioral changes that suggest activation of dopaminergic systems (Hudson et al., 1995, Kobayashi et al., 1998, Martin et al., 1996). Similar changes have been reported in aged animals following acute or chronic administration of GDNF (Emerich et al., 1996, Grondin et al., 2003, Hebert and Gerhardt, 1997, Lapchak et al., 1997). Because of its striking effects on nigrostriatal DA neurons in normal and aged animals, GDNF has also been extensively evaluated as a potential therapeutic agent in several animal models of Parkinson’s disease (Bjorklund et al., 2000, Cass et al., 1998, Gash et al., 1998, Grondin et al., 2002, Kordower et al., 2000).

Previous studies have documented that calcitriol can increase brain GDNF levels, and that GDNF leads to increases in dopaminergic activity in the brain. Thus, it is possible that calcitriol could lead to alterations in dopaminergic processes in the brains of normal animals. Understanding how calcitriol affects DA dynamics in intact animals may provide insight into its potential as a possible therapeutic agent for conditions associated with dopaminergic dysfunction. The purpose of the present study was to determine if calcitriol treatment can alter striatal DA release, and striatal and nigral tissue levels of DA, in normal animals. In vivo microdialysis was used to evaluate potassium- and amphetamine-evoked overflow of DA, and to monitor basal extracellular levels of DA and its primary metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) from the striatum of rats treated with various doses of calcitriol. Postmortem tissue levels of DA and metabolites in the striatum and substantia nigra were determined at the conclusion of each experiment, and, in a separate group of animals the ability of calcitriol to upregulate GDNF levels was evaluated.