Neuroinflammatory Processes in Drug Addiction

Innate Immune Signaling in the Brain

Pattern Recognition Receptors and Their Ligands

The innate immune system functions to recognize foreign pathogens for their elimination. These pathogenic elements are detected by pattern recognition receptors (PRRs). These receptors recognize specific molecular signatures associated with bacteria and viruses, termed pathogen-associated molecular patterns (PAMPs). PRRs are promiscuous receptors that have also been found to recognize endogenous molecules associated with cell stress or trauma, known as damage-associated molecular patterns (DAMPs). This is considered “sterile” inflammation, when innate immune activation occurs without the presence of a foreign pathogen. The release of DAMPs has been implicated in the pathologies of numerous peripheral immunological diseases. DAMP release also occurs in the brain, which is normally a sterile environment. PRRs play critical roles in addiction pathology. To date, five classes of PRRs have been identified including: Toll-like receptors (TLRs), C-type lectin receptors, nucleotide binding domain receptors (leucine-rich repeat containing or nucleotide-binding oligomerization domain [NOD]-like receptors), RIG-I-like receptors, and absent in melanoma 2 (AIM2)-like receptors . TLRs are the most studied PRRs, and have been implicated in both cocaine and alcohol addiction. To date, 10 TLRs have been identified in humans and 12 in mice. TLR ligands include a variety of molecules from bacterial endotoxin to mammalian high-mobility group box 1 (HMGB1) and heat shock proteins ( Table 20.1 ). TLRs are characterized by an N-terminal extracellular leucine-rich repeat sequence and an intracellular Toll/IL-1 receptor/resistance motif (TIR). TLR signaling operates through key adapter proteins that initiate the signaling cascade upon ligand recognition. All TLRs, except for TLR3, utilize the MyD88 adapter protein complex. TIRAP/MyD88 complex formation causes activation of the IL-1 receptor–associated kinases (IRAKs) and the TNF receptor–associated factor 6 (TRAF6) leading to IκB and MAPK activation. IκB and MAPK activation result in activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and activated protein-1 (AP-1) transcription factors, respectively. These transcription factors regulate the expression of proinflammatory cytokines that propagate and magnify the immune response. Because TLRs share common intracellular signaling with several cytokines, subsequent cytokine release leads to an amplification of immune responses. Activation of these transcription factors is involved in addiction (detailed below). TLR signaling was initially described in peripheral immune cells. Thus the precise signaling pathways for the TLRs in each brain cell type have yet to be delineated. Both microglia and astrocytes appear to show canonical TLR4 signaling in response to ethanol resulting in NF-κB activation. However, responses in neurons are poorly understood. There is debate on whether neurons are capable of activating NF-κB. Some suggest that neurons do,not while others find that NF-κB activation in glutamatergic and cultured neurons regulates plasticity, learning, and memory. Furthermore, activated NF-κB subunit colocalizes with dorsal horn spinal neurons and different neuronal cell lines exhibit NF-κB-dependent regulation of μ-opioid receptor expression.

A key feature of TLR signaling is the induction of proinflammatory DAMP release. These DAMPs can subsequently further TLR activation by binding to their respective receptors. Because the brain is sterile, TLR activation in the brain in response to drugs of abuse likely involves DAMP-mediated signaling. One such DAMP that has been found to play a role in alcohol addiction in particular is the protein high-mobility group box 1 (or HMGB1), a nuclear chromatin binding protein that can be released during cellular stress, activation, or damage. Upon its release, HMGB1 acts as an immune mediator via TLR4 or RAGE receptors. HMGB1 has been implicated in alcohol addiction pathology and might be involved in other drugs of abuse (detailed further later in this chapter). HMGB1 is also released prior to hyperexcitable states, such as seizures, and modulates glutamatergic signaling. Neuroimmune activation and neuronal signaling could be interconnected by DAMP release. Several cytokines have been found to regulate normal brain function and could be dysregulated by drugs of abuse. Thus DAMP and cytokine paracrine and autocrine signaling across glia through kinase cascades may represent brain plasticity mechanisms that could contribute to the development of addiction