Cellular and Molecular Interactions in Astroglia, Microglia, and Neurons
Prior reviews have outlined how opiate drugs likely exacerbate neuroHIV pathology in neurons and glia (Hauser et al. 2005; Dutta and Roy 2012; Hauser et al. 2012; Reddy et al. 2012; Hauser and Knapp 2014; Liu et al. 2016a; Murphy et al. 2019) including in the enteric nervous system (Galligan 2015; Meng et al. 2015). Opioid-HIV pathophysiological interactions are complex and differ depending on the timing and duration of co-exposure, the pharmacology of the opioid drug involved, the cell types and brain regions targeted, host and viral genetics, and are highly contextual (Hauser and Knapp 2014, 2018). A summary of the cellular and molecular interactions in various CNS cell types is also reviewed in detail in Table 2.
Table 2 Cellular and molecular interactions (in vitro)
Major effects HIV pathogena ARV Opioids Outcome Model system (in vitro) Citation(s)
Mixed-Glia
HIV expression HIV No • Dynorphin
• U50,488
(KOR agonists) • ↑ HIV-1 expression,
• Dynorphin (KOR agonist) ↑ TNF-α, IL-6 mRNA and protein Human fetal neural cells, HIV-infected promonocyte (U1) line (Chao et al. 1995)
HIVSF162 No • U50,488
• U69,593
• Dynorphin1–17;
(KOR agonists)
• Morphine • KOR agonists ± TNF-α differentially ↓ HIV p24 Human, primary mixed neurons and glia (Chao et al. 1998a)
Chemokines Tat1–86 No Morphine • ↑ CCL5, CCL2
• ↑ [Ca2+]i (Beclin1 dependent)
• ↓ Autophagy Mouse, primary mixed glia (Lapierre et al. 2018)
HIVSF162 (R5) No Morphine • ↑ HIV-1 Tat-induced LTR expression
• ↑ CCR5 expression (inhibited by bivalent ligand in astrocytes)
• ↑ IL-6
• ↑ CCL5 Human, primary mixed glia (El-Hage et al. 2013)
Glial restricted precursors: survival & MOR, DOR, KOR expression Tat1–72 No Morphine (acting via DOR and/or KOR) • ↑ Caspase-3 activation & ↑ cell death by Tat or morphine via DOR, KOR
• No opioid-Tat interactions Mouse, primary glial precursors (Buch et al. 2007)
MOR expression in NPCs;
NPC survival and developmental fate Tat1–72 No Morphine • MOR expressed by subsets of NPCs
• ↑ Astrocyte and immature glial death Mouse, primary mixed glia (Khurdayan et al. 2004)
MOR and CCR5 interactions Tat1–86 (from HIVIIIB) No Morphine • ↓ Neuronal survival via CCR5 activation in glia (rescued by BDNF treatment) Mouse, primary neurons and glia (Kim et al. 2018)
HIV infectivity MOR-CCR5 dimerization HIVSF162 (R5) No Morphine
CCR5-MOR bivalent ligand 1b • MOR-CCR5 bivalent ligand blocks HIV infection in astroglia, but not microglia, with morphine
• MOR-CCR5 bivalent ligand blocks the fusion of HIV gp160 and CCR5-CD4-expressing HEK cells Human, primary astrocytes and microglia; HEK-293T cells (Yuan et al. 2013; Arnatt et al. 2016)
HIV expression and maturational fate of neurons and astroglia HIVBaL (R5) No Morphine • ↑ HIV p24 and ↑ Tat mRNA levels with morphine after 21 days
• ↓ Proliferation of neural progenitors; ↑ astroglial and ↑ neuronal differentiation Human, neural progenitors (Balinang et al. 2017)
Astrocytes
HIV expression HIVSF162 (R5) No Morphine • ↑ HIV p24
• ↑ CCL2 Human, primary astrocytes (Rodriguez et al. 2017)
Toll-like receptor (TLR) expression/function • Tat1–72
• gp120 No Morphine • ↑ TLR2 with Tat, Tat + morphine, gp120
• ↓ TLR9 with Tat, morphine, gp120 Mouse, primary astrocytes (El-Hage et al. 2011a)
Chemokines Tat1–72 No Morphine • ↑ CCL5, CCL2
• ↑ IL-6
• ↑ [Ca2+]i Mouse, primary astrocytes (El-Hage et al. 2005)
Tat1–72 No Morphine • ↑ CCL2
• ↑ CCL5
• ↑ Microglial migration Mouse, primary astrocytes (El-Hage et al. 2006a)
Tat1–72 No Morphine • ↑ CCL2, ↑ IL-6, ↑ TNF-α
• ↑ [Ca2+]i
• ↑ NF-κB trafficking and transcription
• No interaction / acceleration with morphine Mouse, primary astrocytes (El-Hage et al. 2008b)
Tat No • U50,488 (KOR agonist)
• Nor-BNI (KOR antagonist) • U50,488 ↓ CCL2
• U50,488 ↓ NF-κB Human, primary astrocytes (Sheng et al. 2003)
N/A No Morphine • ↑ CCR5, CCR3, CXCR2
• ↓ IL-8, CCL4 Human, astrocytoma U87 cell line, primary astrocytes (Mahajan et al. 2002)
• Tat1–86
• gp120IIIB No Morphine Regional differences in cytokine and ROS production —differed for each insult Mouse, primary astrocytes (Fitting et al. 2010a)
Oxidative stress / damage Tat1–72 No • DPDPE
• SNC-80
(DOR agonists) DOR agonists ↓ Tat-induced oxidative stress Human derived brain cell line (SK-N-SH) (Wallace et al. 2006)
Inflammation, maturation /plasticity • Tat86
• Tat101 No Morphine ↓ β-catenin signaling and variably decreases TrkB, BDNF, and NLRP1 mRNA in fetal astrocytes b Human, U87MG and fetal astrocytes (Chen et al. 2020)
Microglia
HIV replication HIVSF162 (R5) No • Endomorphin-1
• Endomorphin-2
(MOR agonists) • ↑ HIV p24 with endomorphin-1, but not endomorphin-2
• Endomorphin-1 acts via MOR, but not DOR / KOR Human, primary microglia (Peterson et al. 1999)
HIVSF162 (R5) No Morphine ↑ HIV p24 Human, primary microglia (El-Hage et al. 2014)
HIVSF162 (R5) No • U50,488; U69,593 (KOR agonists)
• Dynorphin Al-13 ↓ HIV p24 Human, primary microglia (Chao et al. 1996b)
• HIVJR-FL (R5)
• gp120 No β-endorphin • ↑ HIV expression
• ↑ HIV p24 (14-day post infection)
• gp120 ↑ IL-1, TNF, IL-6 Human, fetal microglia (Sundar et al. 1995)
HIVSF162 No • 8-CAC, U50,488 (KOR agonists)
• Cocaine • KOR agonist ↓ p24; blocked by KOR antagonists
• KOR agonist negates cocaine-induced ↑ HIV Human, fetal brain microglia (Gekker et al. 2004)
HIVSF162 No OPRL1 antisense
Nociceptin / orphanin FQ (OPRL1 agonist) • OPRL1 antisense (and sense) ↓ p24
• Nociceptin, no effect on p24 Human, fetal brain microglia and mixed neurons/glia (Chao et al. 1998b)
HIV expression • HIVSF162
• Tat ZDV U50,488 (KOR agonist) • ↓ p24 on day 14 with U50,488
• ↓ Neurotoxicity (U50,488)
• ↓ Quinolinate by microglia Human, fetal microglia and neural cells (Chao et al. 2000)
Chemokines and Cytokines Tat1–72 No Morphine • ↑ CCR5
• ↑ CD11b, ↑ CD40
• ↑ TNF-α, ↑ IL-6, ↑ IP-10
• ↑ iNOS Mouse, BV-2 and primary microglia (Bokhari et al. 2009)
MOR signaling Tat1–72 No Morphine • ↑ MOR (intracellular)
• ↑ MOR mRNA Mouse, N9 and primary microglia (Turchan-Cholewo et al. 2008)
Oxidative Stress Tat1–72 No Morphine • ↑ ROS [O2− (DHE), ↑ HO2•, H2O2 (DCF)]
• ↑ Protein carbonyls Mouse, N9 and primary microglia (Turchan-Cholewo et al. 2009)
Glutamate release Tat1–72 No Morphine ↑ Glutamate release via ↑ xc− cystine-glutamate antiporter expression/function Mouse, primary microglia (Gupta et al. 2010)
Neurons
HIV expression HIV No Morphine ↑ HIV expression Human derived, SH-SY5Y neuroblastoma cells (Squinto et al. 1990)
Homeostasis and Injury Tat1–86 No Morphine • ↑ [Ca2+]i,
• ↑ [Na+]i
• ↓ ΔΨm (mitochondrial) instability
• ↑ Dendritic degeneration Mouse, primary neurons (Fitting et al. 2014a)
Mitochondrial inner membrane potential and ROS • Tat1–86, Tat1–72
• gp120 No Morphine ↑ ΔΨm instability and oxidative stress ↑ with Tat + morphine, ↑ neuroprotection with allopregnanolone Human, primary neurons ; mouse, striatal medium spiny neurons; mouse, striatal medium spiny neurons, SH-SY5Y neuroblastoma cells (Turchan-Cholewo et al. 2006; Paris et al. 2020)
Neuronal survival Tat1–86 No Morphine • ↓ Neuronal survival from Tat + morphine and ↓ glial CX3CL1 rescued by CX3CL
• CX3CL1 (fractalkine) regulates microglial motility Mouse, primary neurons and mixed glia (Suzuki et al. 2011)
Tat1–86 No Morphine • ↓ Proliferation
• ↑ ERK1/2 activation
• ↑ p53 and p21
• ↓ Cyclin D1 and Akt levels Human, neuronal precursors (Malik et al. 2014)
Tat1–72, Tat1–86 No Morphine • ↓ Neuronal survival
• ↑ Neuronal survival with ibudilast (AV411) (inhibiting glial NF-κB blocks Tat ± morphine neurotoxicity) Mouse, primary neurons and mixed glia (Gurwell et al. 2001; El-Hage et al. 2014)
White matter/oligodendroglial pathology
Changes in OL survival and morphology Tat1–86 No Morphine (25 mg pellet, 7 days); morphine (in vitro) • ↑ Degeneration of OLs
• ↑ TUNEL reactivity
• ↑ Caspase-3 activation Mouse, Tat tg; primary OLs (Hauser et al. 2009)
Blood-brain barrier and the neurovascular unit
BBB model integrity and function Tat1–86 No Morphine • ↑ TNF-α
• ↑ IL-8
• ↓TEER
• ↑ JAM-2 expression
• ↑ Monocyte transmigration with CCL5 Human, using primary BMVEC and primary astrocytes (Mahajan et al. 2008)
ARV accumulation Tat1–86 DTG
FTC
TFV Morphine • ↓ Intracellular ARV concentrations Human, primary astrocytes (Patel et al. 2019)
HIV-1 strain differences
Neuronal Survival Tat1–86 (clades B & C) No Morphine • ↓ Neuronal survival via MOR on mixed glia
• ↑ ROS in astrocytes
• ↑ Iba1 and 3-NT microglia with morphine Mouse, primary neurons and mixed glia (Zou et al. 2011)
• gp120IIIB
• gp120MN (X4)
• gp120ADA (R5) No Morphine ↓ Neuronal survival in presence of glia with gp120MN and transiently with gp120IIIB (X4), not R5-tropic gp120, in combination with morphine Mouse, primary neurons and mixed glia (Podhaizer et al. 2012)
Proliferation and maturational fate of neural progenitors and oligodendroglia • HIVSF162 (R5)
• HIVIIIB (X4) No Morphine • ↓ Proliferation of immature neural and OL progenitors with Tat + morphine
• ↓ NPC DNA synthesis with R5-tropic HIV + morphine
• ↑ NPC DNA synthesis with X4-tropic HIV + morphine Mouse, Tat tg; Mouse, Human, primary neural progenitors (Hahn et al. 2012)
GABA function • HIVBaL (R5)
• gp120 (ADA, MN, and IIIB)
• Tat1–86 No Morphine • Tat or morphine ↓ KCC2 levels via CCR5
• ↑ KCC2 prevents Tat and R5 HIV, gp120, but not X4, gp120 neurotoxicity ± morphine Human, primary neurons, hNPCs (Barbour et al. 2020)
Astroglial CCL5 and neuroprotection • gp120IIIB (X4)
• gp120BaL (R5) No • Morphine (10 μM)
• DAMGO • Morphine ↑ astroglial CCL5 blocking gp120BaL neurotoxicity
• Morphine (or CXCL12) does not block gp120IIIB neurotoxicity Rat, mixed neurons and glia; isolated neurons, astrocytes and microglia (Avdoshina et al. 2010)
aassumed Clade B, unless noted otherwise, b statistical findings for some results are unclear
ARV, antiretroviral(s); BMVEC, brain vascular endothelial cells; [Ca2+]i intracellular calcium concentration; 8-CAC, 8-carboxamidocyclazocine; DAMGO, D-Ala2, N-MePhe4, Gly-ol]-enkephalin; DCF, dihydro-dichlorofluorescein; DOR, δ-opioid receptor; DHE, dihydroethidium; DTG, dolutegravir; DPDPE, [D-Pen2,D-Pen5]enkephalin; FTC, emtricitabine; GABA, γ-aminobutyric acid; Iba1, ionized calcium-binding adapter molecule 1; JAM-1, junctional adhesion molecule-1; KCC2, K+-Cl− cotransporter 2; KOR, κ-opioid receptor; LTR, long terminal repeat; ΔΨm, mitochondrial inner membrane potential; MOR, μ-opioid receptor; [Na+]i, intracellular sodium concentration; nor-BNI, nor-binaltorphimine; NPCs, neural progenitor cells; OLs, oligodendroglia; ROS, reactive oxygen species; TEER, transendothelial electrical resistance; TFV, tenofovir; TUNEL, terminal deoxynucleotidyl transferase dUTP nick end labeling; ZDV, zidovudine
For practicality, the table is limited to key studies in the CNS with emphasis on neuropathological or neuroimmune rather than psychosocial outcomes. With deference toward the excellent studies we excluded: (1) on opioid and HIV effects on PBMCs, or on isolated lymphocytes and monocytes, not directly related to the central nervous system or BBB; (2) on HIV or opioid and ARV interactions in the peripheral nervous system; and (3) studies not directly examining opioid-HIV interactions (irrespective of whether a positive or negative interaction was found)

Opioid and HIV Interactive Pathology in Astroglia
Although the extent to which astroglia display productive infection is debated (Russell et al. 2017; Ko et al. 2019), there is nevertheless considerable evidence of proviral integration in the CNS of PWH (Gorry et al. 2003; Churchill et al. 2009), infectious animal models (Eugenin et al. 2011), and/or cultured human fetal astrocytes (Tornatore et al. 1994; Liu et al. 2004; Do et al. 2014; Narasipura et al. 2014; Li et al. 2015; Nath 2015; Li et al. 2020). Integrated HIV sequences have been identified in astrocytes in HIV-infected CNS tissue by laser capture microdissection (Churchill et al. 2006). Astroglia appear to infect via non-classical, CD4-independent mechanisms, that can have the appearance of virologic synapses, adding to the debate (Liu et al. 2004; Do et al. 2014; Li et al. 2015; Nath 2015; Al-Harthi et al. 2019; Li et al. 2020).
Irrespective of whether they become infected, MOR-expressing, HIV or HIV protein-exposed astrocytes release greater amounts of inflammatory cytokines and dysfunction sufficient to harm bystander neurons upon treatment with opiates (El-Hage et al. 2005, 2008b; Zou et al. 2011; El-Hage et al. 2014). MOR-expressing subsets of glia, especially microglia and astroglia, are prominent in driving the interactive opioid and HIV neuropathogenesis (Hauser et al. 2007, 2012; Hauser and Knapp 2014; Liu et al. 2016a; Chilunda et al. 2019; Murphy et al. 2019). When MOR is deleted from glia (astrocytes and microglia), morphine no longer increases the death of Tat-exposed striatal medium spiny neurons (MSNs) (Zou et al. 2011). Conversely, if MOR is deleted from MSNs, morphine exacerbates the neurotoxic effects of Tat in MSNs (Zou et al. 2011). The proinflammatory effects of Tat alone or in combination with morphine on glia are mediated through a Beclin-1-dependent autophagy pathway (Rodriguez et al. 2017; Lapierre et al. 2018). Drugs with selective glial anti-inflammatory activity (i.e., ibudilast or AV411) attenuated the deleterious effects of HIV and opiate exposure, including HIV-1 replication, cytokine release, and neurotoxicity in vitro (El-Hage et al. 2014). Thus, the observed neuronal death is largely mediated by MOR-expressing glia (Zou et al. 2011), including astroglia (El-Hage et al. 2005, 2008b) and microglia (Turchan-Cholewo et al. 2008; Bokhari et al. 2009; Turchan-Cholewo et al. 2009; Gupta et al. 2010).
The direct contributions of astrocytes to opioid and HIV interactions have been discussed previously (Dutta and Roy 2012; Hauser et al. 2012; Reddy et al. 2012; Hauser and Knapp 2014). Subsets of astroglia can express MOR, DOR and KOR (Stiene-Martin and Hauser 1991; Eriksson et al. 1992; Ruzicka et al. 1995; Gurwell et al. 1996; Hauser et al. 1996; Peterson et al. 1998; Stiene-Martin et al. 1998, 2001), as well as endogenous opioid peptides (Vilijn et al. 1988; Shinoda et al. 1989; Spruce et al. 1990; Hauser et al. 1990; Low et al. 1992). It appears that the ‘early’ events triggering the release of proinflammatory cytokines (i.e., TNF-α and IL-1β) from astroglia can be mediated by HIV Tat exposure alone (El-Hage et al. 2005, 2006a, b, 2008a). Opioids enhance HIV-1-induced inflammation later during the inflammatory cascade by exacerbating the sustained release of CCL5 from astrocytes, which subsequently triggers the release of CCL2 thereby enhancing the recruitment and activation of macrophages/microglia (El-Hage et al. 2008a) (Fig. 1). This is caused by the morphine-dependent exacerbation of Tat-induced increases in intracellular calcium concentration ([Ca2+]i) in astroglia (El-Hage et al. 2005), which accelerates the trafficking of NF-κB p65 (RelA) subunits to the nucleus and sustained CCL2, CCL5, and IL-6 transcription in astrocytes (El-Hage et al. 2008b).
Fig. 1 Opioids exacerbate HIV-1-induced CNS inflammation, in part, by augmenting CCL5-dependent increases in CCL2—key sites of opioid-HIV convergent interactions in glial inflammatory signaling cascades. Subpopulations of striatal glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes in wildtype mice normally express CCR2 immunoreactivity (a-b; arrows), CCL2 (c; arrow), or μ-opioid receptor (MOR) (d; arrows) immunoreactivity (scale bars a-b = 25 μm; c-d = 15 μm). CCR2 deletion (−/−) significantly reduces HIV-1 Tat ± morphine-induced increases in GFAP+ astroglia (e) and F4/80+ macrophages/microglia (f) compared to wild type (+/+) mice at sites near (300 ± 100 μm) the site of Tat injection (*p < 0.05 vs. wild type mice) (see, El-Hage et al. 2006a). In wild-type mice, Tat ± morphine administration markedly increases the proportion of CCL2 immunoreactive astrocytes (g) or macrophages/microglia (h) [*p < 0.05 vs. other groups in wild-type or CCL5(−/−) mice; bp < 0.05 vs. vehicle- or Tat plus morphine-treated wild-type mice; #p < 0.05 vs. equivalent treatment in wild-type mice], while in CCL5 null mice, significant increases in CCL2 immunoreactivity were only seen in macrophages/microglia co-exposed to Tat and morphine (§p < 0.05 vs. vehicle injected CCL5 knockout mice) (see, El-Hage et al. 2008a). CCL5 expression in striatal GFAP-immunoreactive astrocytes (arrows) increases following Tat injections (i, j) compared to wild-type control mice (not shown) (El-Hage et al. 2008a). Opioids exacerbate HIV-1-induced CNS inflammation, in part, by increasing CCL5 and augmenting CCR5-dependent increases in CCL2 production by astrocytes resulting in the activation and recruitment of microglia/macrophages and spiraling inflammation (k). Additional factors likely mediate the proinflammatory cascade, but these are less well substantiated (?). Moreover, autocrine and reciprocal paracrine (astroglial ↔ macrophage/microglial) intercellular, feedback amplification mechanisms from macrophages/microglia are likely to be operative (dashed red arrow) (also see, Kang and Hebert 2011) and occur elsewhere within the cascade (not shown); effects of HIV-1 Tat/HIV, red arrows; sites of opioid convergence, blue arrows; pro-BDNF:mature BDNF (mBDNF) ratio (Kim et al. 2018). (a-f) Modified and reprinted with permission from El-Hage et al. (2006a). (g-k) Modified and reprinted with permission from El-Hage et al. (2008a)

Opioid and HIV Interactive Pathology in Microglia
Unlike in astrocytes, opiate and HIV interactions in microglia tend to be self-limiting (Turchan-Cholewo et al. 2009). Opiates initially trigger large increases in the production of proinflammatory cytokines (Hauser, unpublished), reactive oxygen (ROS) and nitrogen (RNS) species (Turchan-Cholewo et al. 2009), and the release of glutamate (Gupta et al. 2010) and ATP (Sorrell and Hauser 2014) extracellularly in Tat-exposed microglia. The release of glutamate is mediated by the catalytic subunit of the cystine-glutamate antiporter xc− (xCT) (Gupta et al. 2010). Interestingly, following acute increases in the release of cytokines (e.g., TNF-α; unpublished), morphine no longer increases Tat-induced cytokine levels at 24 h; instead, their levels are reduced by opiate-dependent proteasome inhibition. The proteasome inhibitor, MG115, mimics the effects of morphine in decreasing proteasome activity at 24 h and blocks TNFα, IL-6, and CCL2 release from microglia, but does not increase ROS or RNS production (Turchan-Cholewo et al. 2009). The ubiquitin proteasome system (UPS) is typically viewed as contributing to opiate tolerance and physical dependence by modulating MOR downregulation (Massaly et al. 2014; Caputi et al. 2019), rather than MOR activity constraining the UPS. Thus, while HIV-exposed, MOR-expressing microglia show a burst of ROS and proinflammatory cytokine production in response to morphine, the cytokine release collapses within 24 h seemingly because sustained opiate exposure inhibits the UPS thereby preventing degradation of the IκB subunit and nuclear translocation of NF-κB (Turchan-Cholewo et al. 2009).
While neither astroglia nor microglia alone mimic the full inflammatory profile seen with opiates and HIV in the CNS; in combination, the neuroimmune signature more accurately mimics that seen in neuroHIV. Accordingly, we have proposed that opioids promote positive feedback through separate actions in astroglia and microglia in neuroHIV—resulting in spiraling inflammation and cytotoxicity (Hauser et al. 2005, 2007).

Opioid and HIV Interactive Pathology in Neurons
Besides accentuating HIV-induced neurotoxicity via glial-mediated mechanisms, morphine appears to converge with HIV Tat to dysregulate ion homeostasis and dendritic injury through potential direct actions on neurons, even though some contributions of glia cannot be excluded in this study (Fitting et al. 2014a). Combined morphine and Tat exposure accelerates the formation of Tat-induced focal dendritic varicosities/swelling via a MOR-related mechanism that was caused by focal increases in Na+ influx and [Ca2+]i, an overload of Na+/K+-ATPase, ATP depletion, and a collapse in mitochondrial inner membrane potential (Fitting et al. 2014a). Importantly, morphine’s additive effects were mediated via a MOR-related mechanism, as the exacerbating effects of morphine were absent in neurons from MOR knockout mice, thus excluding TLR4 involvement (Fitting et al. 2014a). Further, morphine exacerbated Tat-dependent focal losses in ion homeostasis by mobilizing [Ca2+]i through ryanodine-2 (RyR2)-sensitive sites (Fitting et al. 2014a) (Fig. 2). Although morphine typically acts via MOR in an inhibitory manner by activating Gi/o-proteins (Sharma et al. 1977; Moises et al. 1994; Al-Hasani and Bruchas 2011), MOR-dependent stimulation of PI3-kinase and Ca2+ mobilization (Leopoldt et al. 1998) in neurons via the Gβγ protein subunit (Mathews et al. 2008) is presumed operative here (Fig. 2).
Fig. 2 Morphine exacerbates the excitotoxic effects of HIV Tat by mobilizing Ca2+ from ryanodine (RyR)-sensitive internal stores. (a) Tat-induced increases in [Ca2+]i were not attenuated by ryanodine, whereas ryanodine and pyruvate attenuate combined Tat and morphine-induced increases in [Ca2+]i. Nimodipine (L-type Ca2+ channel blocker) and dantrolene did not show any effects. (b) Average [Ca2+]i over 10 min indicated ryanodine significantly blocked combined Tat and morphine-induced increases in [Ca2+]i, whereas no effects were noted for nimodipine, dantrolene, or pyruvate. *p < 0.05 vs. control, #p < 0.05 vs. Tat 50 nM, §p < 0.05 vs. TM, TM: Tat 50 nM + Morphine 500 nM. (c) Summary of HIV-1 Tat and morphine interactive neuronal injury in striatal medium spiny neurons. Combined Tat and morphine promotes structural and functional defects in dendrites via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), N-methyl-D-aspartic acid receptors (NMDAR), and MOR, causing influxes of Na+ and/or Ca2+, compensatory increases in Na+/K+-dependent ATPase activity, and a rapid loss in ATP mobilization with an inability to extrude excess Na+ via Na+/K+-ATPase caused by mitochondrial hyperpolarization. Dysregulation of [Ca2+]i homeostasis by combined Tat and morphine appears to be mediated downstream of [Na+]i at the level of calcium mobilization, which in turn appears to be regulated via ryanodine (RyR)-sensitive sites, and enhanced by morphine exposure likely via MOR-dependent stimulation of PI3-kinase and Ca2+ mobilization via the Gβγ protein subunit. (a-b) Modified and reprinted with permission from Fitting et al. (2014a)
Glial-derived neuronal injury is not unidirectional. Neuronal damage-associated molecular patterns (DAMPs) and dysfunction can trigger both infected and uninfected glia to become reactive, resulting in further neuronal damage and escalating pathology. Neuronal injury can reactivate HIV in latently infected microglia (Alvarez-Carbonell et al. 2019). While the events underlying the disruption of neuronal-microglial activation that trigger the emergence of latent HIV are unclear, the induction of HIV expression appears to involve the production of DAMPs by injured neurons and can be turned “on”, e.g., by methamphetamine-induced sigma-1 (σ1) receptor activation, TNF-α and IL-1β, and TLR3 activation can be turned “off” by CX3CL1/fractalkine or glucocorticoid receptor activation (Alvarez-Carbonell et al. 2017, 2019).