The mechanism by which substance use contributes to HIV neuropathogenesis is not fully understood. HIV invades the central nervous system (CNS) within 1–2 weeks after the initial infection [37] through the transmigration of infected mature CD14+CD16+ monocytes across the blood-brain barrier (BBB) [38,39,40,41,42]. These cells can differentiate into long-lived macrophages and establish a viral reservoir within the CNS, infecting and activating myeloid cells, including microglia and macrophages, even with effective cART [43,44,45,46,47,48,49]. The infected and activated myeloid cells produce inflammatory cytokines and chemokines [50], leading to the recruitment and activation of additional myeloid populations and immune cells and damage to the BBB, resulting in chronic neuroinflammation [51]. Additionally, neurotoxic viral proteins such as Tat or gp120 that exacerbate the neurotoxic environment may also be released [52]. Studies also demonstrate that a small population of astrocytes harbor HIV DNA [53,54], but whether they are also viral reservoirs remains debated due to their low infectivity and replication rates [48,55,56,57]. A recent study demonstrated that although these reservoirs are low in number, HIV-infected astrocytes allow HIV to egress into the periphery, where they can repopulate the virus [58]. T-cells may also become infected and produce viruses in the CSF, which can circulate through the body and serve as a possible latent reservoir [59]. While neurons are poor viral reservoirs due to their lack of CD4 receptors, they are vulnerable to damage from chemokines, neurotoxins, and viral proteins such as Tat and gp120 [60]. Neurodegeneration is a hallmark feature of HAND, and both Tat and gp120 contribute to neurotoxicity through various mechanisms.

Tat, trans-activator of transcription, is the first viral protein to be transcribed and translated from the integrated HIV-1 provirus and is responsible for the recruitment of positive host transcription factor P-TEFb and recognition of the 5′TAR element in HIV-1 RNA, drastically increasing the rate of viral transcription [61,62,63]. Despite cART therapy, low levels of Tat expression can result in chronic glial activation, cytokine expression, and reductions in neuronal and synaptic density [64]. Exposure of primary microglial cells to Tat results in mitochondrial dysfunction and initiates mitophagy, activating microglia and neuroinflammation [65]. Injection of Tat into the caudate putamen of rats significantly increased levels of malondialdehyde (MDA), indicating oxidative damage and induction of neuronal apoptosis [66]. It has also been shown that Tat exposure can induce autophagy, resulting in a Tat-mediated downregulation of tight junction proteins and consequently increasing vascular permeability in an in vitro model of the BBB [67]. Tat also induces a presynaptic loss in rat hippocampal neurons associated with excessive Ca²⁺ influx via NMDAR [68].

Gp120, HIV envelope glycoprotein, progresses the pathogenesis of HAND by binding to co-receptors CCR5 and CXCR4 and allowing the virus to enter host cells [69]. Gp120 evokes synaptic and behavioral deficits in vivo that mirror significant features of HAND [70]. In vitro studies have also demonstrated gp120′s role in neurotoxicity. Exposure of primary rat cortical neurons to gp120 in vitro impairs mitochondrial function by decreasing the respiratory capacity of mitochondria and disrupting mitochondrial distribution [71]. Alterations in tight junction expression, morphological changes in brain microvascular endothelial cells, and increased stress fiber formation increase BBB permeability with gp120 exposure [72]. In human monocytes, gp120 induces the production of the cytokines TNF-a and IL-10, two proteins implicated in the immunopathology of HIV-1 [73]. An increase in NMDAR-mediated excitatory postsynaptic currents measured through whole-cell patch clamp recordings on hippocampal rat brain slices may involve a molecular mechanism for gp120-induced neuronal injury [74].

Despite the success of cART, the CNS continues to serve as a viral reservoir for toxins and cytokines. Tat and gp120 proteins are expressed in transgenic rodents to investigate the neurocognitive deficits observed in HAND [70,75,76]. In non-infectious HIV-1 transgenic rats, 7 of the 9 HIV proteins (env, Tat, rev, vif, vpr, vpu, and nef) are constitutively and systemically expressed and resemble HIV-1 seropositive individuals on cART [77]. Transgenic mice with gp120 expression constitutively express gp120 in astrocytes under the control of the promoter of glial fibrillary acidic protein (GFAP) [70]. Tat and gp120 expression in transgenic rodents have been used to study the combined effects of HIV-1 and psychostimulant drug exposure. Expressing these viral proteins in transgenic rodents can help elucidate mechanisms behind the altered dopaminergic systems observed in these comorbid conditions [78,79,80]. However, since these models only express some HIV-1 viral proteins, results from these models may miss the interactive and additive effects among these proteins.

Dopamine dysfunction has also been associated with HIV infection, and research has revealed selective damage to brain regions to which dopamine cells project, including the striatum [29,81]. While cART diminishes damage to these regions, PLWH on cART still shows striatal dysfunction, increased microglial activation, and inflammation, leading to neuronal damage [82,83,84,85,86]. Dopamine signaling has immunomodulatory effects by regulating the activation of myeloid and T-cells, the production of cytokines, transmigration, and phagocytosis [87,88,89,90], which could influence the development of HIV infection in the CNS and neurocognitive function. CD14+CD16+ monocytes, key mediators of HIV neuropathogenesis, express mRNA for all five dopamine receptors [88]. Dopamine and D1-like receptor agonists increased CD14+CD16+ cell motility, adhesion, and transmigration across the BBB in an in vitro model, suggesting that elevated extracellular dopamine in the CNS of PLWH with SUD contributes to HIV neuropathogenesis by increasing the accumulation of monocytes in dopamine-rich regions [87,88]. Elevated dopamine increases the susceptibility of macrophages to HIV [29] and spurs the production of inflammatory cytokines [91]. These effects of dopamine on immune function may promote the spread of viral infection, viral reservoirs, neuroinflammation, and neurotoxicity. Substance use is associated with increased HIV neuropathogenesis and neurocognitive decline in PLWH in the cART era [92,93,94,95], suggesting increased extracellular dopamine from substance use may play a role in the neurotoxic effects of HIV even when viral titers are very low.

Emerging evidence from pre-clinical studies suggests an HIV-mediated potentiation of drug reward [17,75,79,96,97]. These effects are theorized to be mediated by viral proteins, specifically Tat, which continues to be produced under viral suppression. Recent studies suggest that Tat may affect dopamine homeostasis by inhibiting dopamine transporter function allosterically, potentially elevating extracellular dopamine [98,99,100]. Psychostimulants, such as cocaine and meth, have complex interactions with Tat. Intravenous cocaine self-administration increased striatal DAT binding and showed an increased sensitivity to cocaine’s reinforcing effects in transgenic rats with Tat expression through a leftward shift in the dose-response curve [97]. Tat induces conformational changes in DAT that increase the affinity of cocaine for the transporter protein [101]. The similar and distinct effects of chronic cocaine administration and HIV-1 infection appear to enhance neurotoxicity, as evidenced by the overexcitation of mPFC pyramidal neurons in transgenic rats with Tat expression [102]. HIV-Tat and cocaine combined exposure in human and rat primary hippocampal neurons caused a significant depolarization of the mitochondrial membrane potential, indicative of mitochondrial damage. In contrast, Tat alone or cocaine alone only caused a slight effect on mitochondrial membrane potential [103].

Expression of Tat in transgenic mice augments methamphetamine-induced sensitization, as shown by increased locomotor activity and decreased expression of dopamine receptors demonstrated with RT-PCR and immunohistochemistry [79]. Like the mechanism of meth, Tat alters DA homeostasis by inhibiting DAT, and the combination of Tat and meth decreases DAT function more than either condition alone [104]. Increasing doses of meth resulted in impaired working and spatial memory in Tat transgenic mice compared to non-Tat or non-meth-treated mice, suggesting cooperative effects between Tat and meth on neurocognition [105]. Tat also increases microglial activation, indicating neuroinflammation [79]. Combined exposure to Tat and meth increased cellular ROS production, leading to neuronal injury [106]. In sum, these studies find that HIV infection enhances the effects of psychostimulants on dopaminergic pathways in the brain, including the reward pathway. This raises the possibility that PLWH have a greater risk of developing SUDs than those without HIV. Future studies examining compounds that can specifically block Tat binding site(s) in DAT and new forms of cART that do not affect normal DAT function, may provide an early intervention for mitigating the negative effects of HAND in PLWH with SUD.