IL-1␤ and TNF are potential targets in the management of neuropathic pain after injury. However, the importance of the IL-1 and TNF systems for peripheral nerve regeneration and the mechanisms by which these cytokines mediate effects are to be fully elucidated. Here, we demonstrate that mRNA and protein levels of IL-1␤ and TNF are rapidly upregulated in the injured mouse sciatic nerve. Mice lacking both IL-1␤ and TNF, or both IL-1 type 1 receptor (IL-1R1) and TNF type 1 receptor (TNFR1), showed reduced nociceptive sensitivity (mechanical allodynia) compared with wild-type littermates after injury. Microinjecting recombinant IL-1␤ or TNF at the site of sciatic nerve injury in IL-1␤-and TNF-knock-out mice restored mechanical pain thresholds back to levels observed in injured wild-type mice. Importantly, recovery of sciatic nerve function was impaired in IL-1␤-, TNF-, and IL-1␤/TNF-knock-out mice. Notably, the infiltration of neutrophils was almost completely prevented in the sciatic nerve distal stump of mice lacking both IL-1R1 and TNFR1. Systemic treatment of mice with an anti-Ly6G antibody to deplete neutrophils, cells that play an essential role in the genesis of neuropathic pain, did not affect recovery of neurological function and peripheral axon regeneration. Together, these results suggest that targeting specific IL-1␤/ TNF-dependent responses, such as neutrophil infiltration, is a better therapeutic strategy for treatment of neuropathic pain after peripheral nerve injury than complete blockage of cytokine production.

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The innate immune system plays a crucial role in protecting the host against infectious microorganisms. An inappropriate control of this system may have profound consequences, because of the maintained production of specific proinflammatory molecules. Glucocorticoids are the most efficient endogenous molecules that provide negative feedback on proinflammatory signaling and gene expression. Here we show that activation of this system is not detrimental for the brain but a profound neurodegeneration takes place in animals treated with the glucocorticoid receptor inhibitor Mifepristone (RU486). This drug increased the inflammatory reaction induced by a single intracerebral bolus of lipopolysaccharide (LPS). Inhibition of tumor necrosis factor alpha (TNF-alpha) totally abolished the neurotoxic effect of the endotoxin, and chronic infusion of the cytokine mimicked the treatment combining RU486 and LPS. The neuronal damage caused by TNF-alpha is dependent on both nitric oxide and caspase pathways. In controlling the cerebral innate immunity and microglial TNF-alpha production, glucocorticoids play a major role in protecting the brain against bacterial cell wall components.

There are exciting new developments regarding the molecular mechanisms involved in the influence of circulating proinflammatory molecules within cells of the blood-brain barrier (BBB) during systemic immune challenges. These molecules, when present in the circulation, have the ability to trigger a series of events in cascade, leading to either the mitogen-activated protein (MAP) kinases/nuclear factor kappa B (NF-kappaB) or the janus kinase (JAK)/signal transducer and activator of transcription (STAT) transduction pathways in vascular-associated cells of the central nervous system (CNS). The brain blood vessels exhibit both constitutive and induced expression of receptors for different proinflammatory ligands that have the ability to stimulate these signaling molecules. Depending on the challenges and the cytokines involved, the transduction signal(s) solicited in cells of the BBB may orient the neuronal activity in a very specific manner in activating the transcription and production of soluble factors, such as prostaglandins (PGs). It is interesting to note that cytokines as well as systemic localized inflammation stimulate the cells of the BBB in a nonselective manner (i.e., within both large blood vessels and small capillaries across the brain). This nonselectivity raises several questions with regard to the localized neuronal activation induced by different experimental models of inflammation and cytokines. It is possible that the selectivity of the neuronal response is a consequence of the fine interaction between nonparenchymal synthesis of soluble mediators and expression of specific receptors for these ligands within parenchymal elements of different brain nuclei. This review will present the recent developments on this concept and the mechanisms that take place in cells of the BBB, which lead to the neuronal circuits involved in restoring the body's homeostasis during systemic immunogenic challenges. The induction of fever, the hypothalamic-pituitary adrenal (HPA) axis, and other autonomic functions are among the physiological outcomes necessary for the protection of the mammalian organism in the presence of foreign material.

Systemic injection of the endotoxin lipopolysaccharide (LPS) upregulates the gene encoding CD14 early in the circumventricular organs (CVOs) and later in the brain parenchyma. The present study tested the hypothesis that the parenchymal production of the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) by microglial cells is responsible for triggering CD14 transcription in an autocrine/paracrine loop-like manner. In a first set of experiments, Sprague Dawley rats were killed 1, 3, 6, and 12 hr after an intracerebroventricular administration of recombinant rat TNF-alpha or vehicle solution. Second, anti-rat TNF-alpha-neutralizing antibody or vehicle solution was administrated into the lateral ventricle 10 hr before an intraperitoneal injection of LPS. Central administration of the cytokine caused a strong induction of IkappaBalpha, TNF-alpha, and CD14 mRNA in parenchymal microglial cells. The hybridization signal for these transcripts was localized to the edge of the ventricles and the site of infusion. The time-related expression of each mRNA suggested that TNF-alpha has the ability to trigger its own production followed by the transcription of the LPS receptor; the signal for IkappaBalpha, TNF-alpha, and CD14 peaked at 1, 3, and 6 hr, respectively. The genes encoding TNF-alpha and mCD14 were also induced in the CVOs and within microglial cells across the brain parenchyma in response to intraperitoneal LPS administration. This induction in parenchymal cells of the brain was prevented in animals that received the anti-TNF-antisera intracerebroventricularly 10 hr before the systemic treatment with the endotoxin. The present data provide the evidence that microglial-derived TNF-alpha is responsible for the production of the LPS receptor CD14 during endotoxemia. This autocrine/paracrine stimulatory loop may be of great importance in controlling the inflammatory events that take place in the CNS during innate immune response as well as under pathological conditions.

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Tumor necrosis factor (TNF)-alpha is usually referred to as a proinflammatory cytokine that plays a central role in initiating the cascade of other cytokines and factors for an appropriate immune response to infection. Like systemic phagocytes, recent studies have reported that specific cellular populations of the CNS have the ability to express and release the proinflammatory cytokine in response to peripheral administration of the bacterial endotoxin lipopolysaccharide (LPS). Whether such phenomenon represents a general mechanism of systemic immunogenic stimuli and how the severity of the challenge may influence TNF-alpha transcription in the brain has yet to be defined. Adult male rats were sacrificed 1, 3, 6, 12, 24 and 48 hours (h) after intraperitoneal (IP) injection of LPS (25-250 microg/100 g) or intramuscular (IM) injection of turpentine. Brains and pituitary glands were removed, cut, and TNF-alpha mRNA assayed by in situ hybridization using a full-length rat cRNA probe. The results show no positive signal under basal conditions or following sterile inflammation into the left hind limb. Systemic LPS caused a profound increase in the expression of the gene encoding TNF-alpha in the leptomeninges, choroid plexus (chp) and all sensorial circumventricular organs (CVOs). Interestingly, a migratory-like pattern of TNF-alpha-positive cells became apparent around the sensorial CVOs at 3 h, while a ubiquitous-like positive signal was found throughout the brain 6 h after the injection with the highest dose of LPS. The IP LPS injection also stimulated TNF-alpha transcription in the anterior pituitary lobe; the signal was maximal 1 h after the injection and returned gradually to basal levels at 12 h, whereas the mRNA encoding the cytokine was detected later in the neurohypophysis, i.e. 3 and 6 h post challenge. Dual-labeling procedure provided the evidence of an LPS-dependent induction of TNF-alpha in different phagocytic cellular populations of the brain, including parenchymal microglial cells during severe endotoxemia. The fact that these myeloid-derived cells have the ability to express the LPS receptor CD14 in the brain may well explain the transcriptional activation of the cytokine in response to the bacterial endotoxin, but not to systemic localized inflammation.

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