Stroke is an important cause of morbidity and mortality in industrialized countries and few therapies exist thus far. It is generally acknowledged that many agents, proven neuroprotective in experimental models, fail in clinical practice, possibly because they cannot respond to the complex multifaceted nature of the ischemic cascade after stroke [
1‐
3]. Hypothermia is a well established and robust neuroprotective treatment and has been the focus of research as it may act on several pathways simultaneously [
4,
5]. The most common form of stroke is a transient focal cerebral ischemia attacking the Middle Cerebral Artery (MCA) or one of its collaterals [
1]. In the endothelin-1 (Et-1) rat model, a potent vasoconstrictor is infused in the vicinity of the MCA, thereby occluding this vessel (>75%) during 30 minutes before gradual reperfusion takes place [
6]. The core and the penumbra of the insult are represented by the ipsilateral striatum and cortex, respectively. In this and in other MCA occlusion (MCAO) models, mild hypothermia (33°C) has proven to reduce the infarct volume by half, 24 hours after the administration of Et-1, as the result of an almost complete recovery of the penumbra [
7,
8]. We also showed that in the Et-1 model, reduction in infarct size by hypothermic treatment coincides with decreased levels of apoptosis and oxidative stress in the penumbra [
9]. However, the influence of hypothermia on neuroinflammation and neurological outcome has not yet been investigated in this model. Shortly after stroke, cytokines are released from a variety of cells in the central nervous system. The most important ones produced are interleukin-1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α), the former described as a pro-inflammatory cytokine and the latter as a more pleiotropic one with neurotoxic as well as neuroprotective properties [
10‐
12]. Transforming Growth Factor beta (TGF-β) on the other hand is an anti-inflammatory cytokine that is also upregulated after stroke and serves to protect the brain against ischemic injury [
13]. The primary source of IL-1β is activated microglia or macrophages, followed by astrocytes. Some production exists from neurons, endothelial cells, oligodendroglia and circulating immune cells [
14]. TNF-α and TGF-β, in general, follow IL-1β's production profile [
15,
16]. Consequently, an essential role in the upregulation of IL-1β, TNF-α and TGF-β can be assigned to activated microglia and macrophages [
17,
18]. Higher concentrations of these cytokines further activate microglia and astrocytes and a full-blown inflammatory response develops, usually leading to an aggravation of the damage after stroke [
16,
19]. It is generally accepted that hypothermia is able to reduce the inflammatory response after stroke [
4,
20]. However, there is no evidence whether this reduction occurs in the core of the insult and/or in the penumbra. In addition, short-term effects of treatment may be different from long-term effects. Indeed, recent literature indicates that cerebral ischemia induces a biphasic response including a quick, merely detrimental, inflammatory response and a slow response, which could be beneficial. Moreover, the switch depends on the severity of the insult. For example, at an early stage, reactive microglia have shown neurotoxic properties, while sustained activation appeared beneficial to neuronal repair [
16,
21]. To address these essential questions, we investigated the effects of a 2 hours mild hypothermic treatment on microgliosis, astrogliosis and cytokine production as a function of time up to 1 week after an Et-1-induced insult and compared the results with neurological outcome and infarct size at the same time points. This study is the first to show that hypothermia has a modulatory effect on the neuroinflammatory response after an Et-1-induced stroke.