A novel anti-inflammatory surface for neural electrodes

in microelectrode technology. The clinical benefits of brain implants are far-reaching and include improvements in quality of life for patients suffering from neurological impairments and diseases, such as Parkinson’s disease, epilepsy, blindness, and paralysis. However, a number of major bottlenecks currently hinder the realization of the therapeutic potential of brain implants, one of which being the ability of the implant to function reliably over the remaining lifetime of the patient. It is generally believed that many complications in long-term implant functionality are due to an adverse brain tissue response elicited by the implant. This inflammatory response, featured by the formation of the astroglial scar, can result in isolation of the implant from target neurons both electrically and mechanically, and is arguably the biggest stumbling block in realizing chronic recordings from neural electrodes. A common strategy to dampen the tissue response is local delivery of anti-inflammatory drugs directly at the implant-tissue interface. [3–5] Although such approach has been shown to successfully moderate host response during the acute phase, [6] alone it may be inadequate to have long-term functional consequences, due to limitations in the duration of drug release. Since the sustained chronic tissue response to the implant is considered to be a result of both foreign body reaction [7] and mechanical mismatch induced micromotion, [8,9] we propose to modulate the sustained tissue response by endowing the implant with an intrinsic anti-inflammatory surface. Here, we demonstrate through both in vitro cell culture studies and in vivo rodent studies, that immobilized alpha-MSH creates an inherently anti-inflammatory neural electrode surface such that it significantly attenuates glial response for at least 4 weeks post-implantation. Alpha-MSH is an endogenous tridecapeptide that is secreted by pituitary cells, astrocytes, monocytes, keratinocytes, etc., and is found in the skin, brain, and other tissues. [10,11] Among its broad, potent anti-inflammatory functions, studies have shown alpha-MSH inhibits pro-inflammatory cytokines and neurotoxic nitric oxide (NO) production by microglia stimulated with beta-amyloid and interferon-gamma, which simulated inflammation associated with Alzheimer’s disease. [12] Since microglia, the resident macrophages in the brain, are the frontline defense cells against invasive implants, the neuroimmunomodulatory peptide alpha-MSH was selected in our study to directly mitigate microglial response to foreign brain implants. Coupling of the active molecule to the neural implant surface was accomplished through silane chemistry and the use of a hetero-bifunctional crosslinker with both thiol- and amino-reactive moieties, [13] as demonstrated in Figure 1. The surface coverage of alpha-MSH peptide was 0.0212 nmol cm –2 , quantified by application of sulfo-SDTB that reacts with primary amine, specifically in this case, those presented from the lysine residue and the N-terminus of the alpha-MSH peptide. In order to first investigate whether the alpha-MSH peptide remains biologically active when covalently immobilized to the Si surface, we cultured primary rat microglial cells on the peptide modified surface and subjected the culture to lipopolysaccharide (LPS) bacterial endotoxin, which elicits an inflammatory response in vitro. As shown in Figure 2A, the production of nitric oxide (NO), an indicator of inflammation, was reduced to nearly 50% of that observed in the control culture, suggesting that the anti-inflammatory property of alphaMSH was preserved during the tethering process. Moreover, we examined the gene expression of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin 1 (IL-1) using real time RT-PCR. Both of these cytokines play a key role in mediating the inflammation process in vivo, and microglia are the known major cellular source for production of these cytokines. Modulating the level of TNFalpha and IL-1 is critical as excessive production of these factors could not only amplify the inflammation process by stimulating glial cell proliferation and activation, but also cause neuronal death. LPS is commonly used to induce microglial activation and cause production of neurotoxic pro-inflammatory cytokines. This is consistent with our finding where the mRNA expression level of TNF-alpha had an over three-fold increase in the control Si group after subjecting to LPS stimulation (Fig. 2B). In contrast, TNF-alpha expression