lnterleukin- 6 (lL -6 ) is a pleotrophic, pro-inflammatory cytokine and was cloned and purified by several different groups and known by 1 0 different names, each o f which reflected a different characteristic o f the protein. It was first identified as an interferon, IENP2, when attempts were made to clone intèrferon-(3 cD N A from human fibroblasts
(W eissenbach et al. 1980). 1 chose this particular cytokine to investigate because it is upregulated follow ing nerve injury (see 1.7.4) and unlike other cytokines, e.g. T N F a
and 1L-1(3, few studies existed regarding the role o f lL- 6 in chronic pain.
1.7.1 Immunological functions of IL-6
1 will now give a brief overview o f the, mainly immunological, functions o f EL- 6 that are not related to the nervous system. EL- 6 plays an important role in hem atopoiesis, the formation and development o f blood cells from a pluripotent stem cells to red (erythocytes and thrombocytes) and white (e.g macrophages, neutrophils, mast cells) blood cells. Stem cells are dormant and reside in the Gq phase o f the cell cycle. EL- 6
stimulates the proliferation o f hematopoietic stem cells, in conjunction with EL-3, by reducing the Gq residence time o f hematopoietic stem cells (Ikebuchi et al. 1987). lL- 6 has been shown to be essential in B-cell differentiation for activated B -cells to becom e im m unoglobulin-secreting plasma cells (Kishimoto 1989). The proliferation o f
Snick 1990) and IL-6 increases the survival o f som e m alignancies e.g. prostate cancer, yet have anti-tumour effects in other cancers e.g. leukemia (see (Stein and Sutherland
1998).
IL-6 stimulates the proliferation o f peripheral and thymic T-cells (Uyttenhove et al. 1988) and synergises with IL-1 to control the initial steps in T -cell activation leading to
the formation o f cytolytic T-cells (Renauld et al. 1989). The acute phase response is a com plex reaction that occurs in response to infection, trauma or injury resulting in fever, increased vascular permeability, leukocytosis, changes in plasma steroid concentrations and an increase in the synthesis o f acute phase proteins e.g. hem opexin, Oj-
macroglobulin and C-reactive protein (only in humans). IL-6 induces acute phase
proteins in vivo (Geiger et al. 1988) and increases in core temperature (Rothwell et al. 1991). Administration o f lipopolysaccharide (LPS; the active fragment o f endotoxin from gram-negative bacteria) induces fever and EL-6 production in vivo (Shalaby et al. 1989). LPS has been shown to increase EL-6 release from medial basal hypothalami (M BH) (Spangelo et al. 1990). Such local EL-6 production in the medial hypothalamus activates the hypothalamic-pituitary-adrenocortical (HPA) axis, which regulates several central effects including fever and sleep (for review, see Schobitz et al. 1994). The essential role o f E^-6 in fever is perhaps m ost clearly demonstrated by EL-6 deficient m ice in which LPS failed to induce fever (Chai et al. 1996).
EL-6 has been reported to have a role in bone m etabolism with a significant correlation found between bone mineral density and polymorphic variations o f the IL-6 gene in
humans (Murray et al. 1997). It has been suggested that the dysregulation o f IL-6 actions in bone cells m ay lead to osteoporosis and Paget’s disease (Stein and Sutherland 1998). Elevated levels o f IL-6 have also been found in the patients with other diseases including rhematoid arthritis (Houssiau et al. 1988) and H IV (Laurenzi et al. 1990).
1.7.2 Neurological functions of IL-6
Other than the role o f IL-6 in nociception (1.7.6), there is evidence that IL-6 has a role in neuronal survival, neuroprotection and neuronal differentiation. In vitro IL-6 has been shown to enhance the survival o f catecholaminergic neurons in the m escencephalon and cholinergic neurones in the septum and basal forebrain in postnatal rats (Hama et al.
1989; Hama et al. 1991). In the spinal cord o f embryonic (E15) rats, IL-6 enhanced the survival o f acetylcholinesterase (AChE)-positive neurones and increased the number o f
neuron-like cells in a dose-dependent manner (Kushima and Hatanaka 1992). More recently, the addition o f both IL-6 and the soluble IL-6 receptor (see section 1.7.5) promoted the survival o f newborn rat dorsal root ganglion (Thier et al. 1999). The first study to report neuroprotective effects o f IL-6 in vivo demonstrated that IL-6 attenuated the N M D A -induced neurotoxic effects on rat striatal cholinergic neurones but not on G ABA ergic neurones (Toulmond et al. 1992). Since then, IL-6 has been shown to protect certain neuronal subpopulations against other neuronal insults such as glutamate- induced cell death in hippocampal neurones (Yamada and Hatanaka 1994), 1-methy 1-4- phenyl pyridinium (MPP^) neurotoxicity in fetal dopaminergic neurones (experimental model o f Parkinson’s disease)(Akaneya et al. 1995) and axotom y in developing spinal motoneurons (Ikeda et al. 1996).
IL-6 was reported to induce neurite outgrowth in PC 12 (pheochromocytoma) cells (Satoh et al. 1988) how ever others demonstrate that IL-6 w ill only trigger such neuronal differentiation in PC 12 cells in the presence o f its soluble receptor (Marz et al. 1997). These discrepancies have been suggested to be a result o f differences in the number o f IL-6 receptors expressed on the PC 12 subclones (Gadient and Otten 1997). This neuronal differentiation effect o f IL-6 has also been observed in cultured hippocampal neurones from the fetal rat brain (Sarder et al. 1996).
1.7.3 IL-6 in the brain
Elevated levels o f IL-6 occur in many CNS diseases, for instance IL-6 levels are
increased in the cerebro-spinal fluid (CSF) o f patients with bacterial meningitis (Waage
et al. 1989), A lzheim er’s disease and Parkinson’s disease (Blum -Degen et al. 1995). Furthermore, significantly higher levels o f IL-6 protein has been found in the
dopaminergic, striatal regions o f parkinsonian patients than those in control subjects (M ogi et al. 1994). Similarly, IL-6 protein was more frequently detectable in the temporal cortex o f patients with A lzheim er’s disease than control brains (W ood et al.
1993). Traumatic brain injury also results in increased IL-6 levels (Kossmann et al. 1996). Although there is an upregulation o f IL-6 levels, the role o f IL-6 in the pathologic process o f diseases such as A lzheim er’s and Parkinson’s remains to be defined.
The majority o f animal studies have examined the levels o f EL-6 in the brain in terms o f m RNA. IL-6 m R N A is evident in a number o f brain regions including the striatum,
neocortex, cerebellum, brain stem and hippocampus, with a tendency for higher levels o f EL-6 m R N A in forebrain structures compared to more caudal regions (Gadient and
Otten 1995). M any neuronal types throughout the brain express IL-6 m RNA including neurones o f the dorsomedial (periventricular), ventromedial, medial preoptic nucleus o f the hypothalamus and habenular nucleus, pyramidal and granular neurones o f the hippocampus, cerebellar granular neurones, pyramidal neurones o f the cerebral cortex (for review, see Schobitz et al. 1994) and cerebellar Purkinje neurones (Gadient and
Otten 1994).
EL-6 receptor (EL-6R) m R N A is usually coexpressed with EL-6 mRNA. The coexpression o f EL-6 m R N A and EL-6R m R N A also occurs at low levels within the
white matter o f fibre tracts in the forebrain e.g. the corpus callosum, lateral olfactory tract (Yan et al. 1992). This suggests that oligodendrocytes, responsible for the myelination o f axons o f the CNS neurones that com prise these tracts, express EL-6 and EL-6R. The most abundant cell type in the CNS, astrocytes, which provide neuronal support also express EL-6 and EL-6R mRNA. Incidentally, EL-6 has been shown to stimulate astrocyte
proliferation in vivo using m ice which displayed a cerebral overexpression o f IL-6 (Campbell et al. 1993). This wide-ranging expression o f IL-6 m R N A and IL-6R m R N A shows that several different cell types have the potential to express IL-6 or respond to
IL-6 released or perhaps synthesize IL-6 and respond to IL-6. W hether this means that IL-6 is involved in cell-to-cell signaling remains to be established, as is the functional
purpose o f such potential pathways.
Levels o f IL-6 in the CNS are low or undetectable under normal physiological conditions, which suggests that although many different cells are capable o f IL-6 synthesis, relatively little constitutive biosynthesis actually occurs. However, follow ing CNS disease (as mentioned previously), inflammation and injury there is increased synthesis o f IL-6. Sources o f IL-6 in these conditions include astrocytes, microglia, endothelial cells of the vascular system, T cells, macrophages, endothelial cells o f the blood brain barrier (B B B ), neurones and oligodendrocytes (for review see (Gruol and N elson 1997). In terms o f motoneuron injury for instance, the transection o f the facial nerve causes a rapid upregulation o f IL-6 m R N A in the facial nucleus (Kiefer et al.
1993). With such a wide distribution o f IL-6 and IL-6R m R N A the functional roles o f
IL-6 in the central nervous system will perhaps increase in the future. At present, as previously discussed, the literature indicates that IL-6 has roles in fever, regulation o f the HP A axis, neuronal differentiation, survival and protection.
1.7.4 IL-6 expression in the spinal cord and periphery
In humans, IL-6-like immunoreactivity (DR.) has been found in juvenile and foetal dorsal
root ganglion (DRG) (Nordlind et al. 2000) and nerve explants have been shown to secrete IL-6 from Schwann cells (Rutkowski et al. 1999). Peripheral nerve-like structures also exhibit IL-6-like IR in normal human skin and to a greater extent in inflamed skin (Nordlind et al. 1996).
IL-6 m R N A is expressed in sympathetic and sensory ganglia o f adult rats (Gadient and Otten 1996). M any animal studies have examined the effect o f nerve injury on IL-6
expression. Complete rat sciatic nerve transection resulted in a large increase in IL-6 m R N A in the sciatic nerve within hours o f this insult (Zhong and Heumann 1995; Bourde et al. 1996); this increase was transient returning to control levels within 24
hours. In m ice, a persistent upregulation o f IL-6 m RNA in the sciatic nerve is seen follow ing nerve injury and is present 21 days post-injury (Reichert et al. 1996).
F ollow ing sciatic nerve crush, plasma levels o f IL-6 increase (W ells et al. 1992) and IL- 6 m R N A is induced in Schwann cells at the site o f injury (Bolin et al. 1995; Grothe et al. 2000). These changes are also transient returning to control levels within 24 hours.
Comparing sciatic crush injury and axotomy showed the same increase (35-fold) and tim ecourse o f IL-6 m R N A expression (Ito et al. 1998). In the Bennett (CCI), Seltzer (PSL) and G azelius (photochemically induced ischaem ic lesion) m odels o f neuropathic pain, an increase in IL-6 positive cells in the sciatic nerve has been reported (Cui et al. 2000). In addition, this study also reports a highly significant increase in the number o f IL-6-positive cells in allodynic Bennett/Seltzer rats compared to non-allodynic
Bennett/Seltzer rats. CCI induced a significant increase in IL-6 m R N A in the sciatic nerve after 7 days (Okamoto et al. 2001) and in the ipsilateral D R G (Murphy et al. 1999b). A xotom y resulted in upregulation o f IL-6 m R N A peaking at 2-4 days after injury in rat D R G neurones (Murphy et al. 1995). N erve crush and spinal nerve root transection also induce, to a lesser extent, IL-6 m R N A in D R G neurones (Murphy et al. 1999a). A bilateral elevation o f IL-6 protein is seen in the infraorbital nerve and
brainstem 3-10 days follow ing constriction o f this nerve (Anderson and Rao 2001). All