Chronic lithium feeding reduces upregulated brain arachidonic acid metabolism in HIV-1 transgenic rat

Abstract

HIV-1 transgenic (Tg) rats, a model for human HIV-1 associated neurocognitive disorder (HAND), show upregulated markers of brain arachidonic acid (AA) metabolism with neuroinflammation after 7 months of age. Since lithium decreases AA metabolism in a rat lipopolysaccharide model of neuroinflammation, and may be useful in HAND, we hypothesized that lithium would dampen upregulated brain AA metabolism in HIV-1 Tg rats. Regional brain AA incorporation coefficients k* and rates J ( in ), markers of AA signaling and metabolism, were measured in 81 brain regions using quantitative autoradiography, after intravenous [1-(14) C]AA infusion in unanesthetized 10-month-old HIV-1 Tg and age-matched wildtype rats that had been fed a control or LiCl diet for 6 weeks. k* and J ( in ) for AA were significantly higher in HIV-1 Tg than wildtype rats fed the control diet. Lithium feeding reduced plasma unesterified AA concentration in both groups and J ( in ) in wildtype rats, and blocked increments in k* (19 of 54 regions) and J ( in ) (77 of 81 regions) in HIV-1 Tg rats. These in vivo neuroimaging data indicate that lithium treatment dampened upregulated brain AA metabolism in HIV-1 Tg rats. Lithium may improve cognitive dysfunction and be neuroprotective in HIV-1 patients with HAND through a comparable effect.

Figure 1

Coronal brain autoradiographs showing effects of LiCl feeding for 6 weeks on regional AA incorporation coefficients k* in wildtype and HIV-1 Tg rats. Values of k* (ml/s/g brain) x 10⁻⁴ are given on a color scale. ACg, anterior cingulate cortex; CPu, caudate putamen; Ctx, cortex; Hipp, hippocampus; Mot, motor cortex; Som, somatosensory cortex.

Figure 2

J_in for AA (Eq. (2)) in each of three brain regions, in wildtype and HIV-1 Tg rats fed a control or LiCl diet for six weeks. Means ± SD. Statistics were calculated by a 2-way ANOVA, (n = 9, except for HIV-1 Tg rats under LiCl diet n = 6).

Figure 3

Proposed sites of action of lithium on brain arachidonic acid (AA) cascade (3a) and the cascade as it is upregulated in brain HIV-1 infection (3b). Figure 3a. As illustrated, AA is released from phospholipid following activation of cPLA₂ by a receptor-mediated mechanism. Unesterified AA is a substrate for COX-2 and other oxidative enzymes, and forms PGE₂ and other bioactive products that have cellular actions. Remaining AA is recycled into phospholipid via acyl-CoA synthetase 4 (Acsl4) and an acyltransferase. Based on animal studies and as illustrated, lithium can downregulate the AA cascade by interfering with: NMDA receptor mediated release of Ca²⁺ to activate cPLA₂, transcription of cPLA₂ by the transcription factor activator protein (AP)-2, expression levels of cytosolic cPLA₂ (mRNA, protein and activity), turnover of AA within the deacylation-reacylation cycle, the unesterified brain AA concentration, protein and activity levels of COX-2, and activity of secretory sPLA₂ (not shown). Adapted from Rao et al. (Rao et al. 2008). Figure 3b. Suggested brain targets related to AA cascade in HIV-1 brain infection. Following entry of circulating macrophages containing the HIV-1 virus into brain via blood brain barrier, the macrophages or released virus infect resident microglia (productive infection) or astrocytes (nonproductive infection), via CD4, CCR5 and other receptors. Released glycoprotein-120 (gp-120) also can activate microglia. Activated microglia release cytokines that bind to astrocytic IL-1β and TNF-α receptors that are coupled to activation of cPLA₂ and sPLA₂, to initiate the AA cascade by releasing AA from membrane phospholipid (see 3A). Inducible nitric oxide synthetase (iNOS) in activated microglia releases nitric oxide (NO). NO stimulates release of glutamate by presynaptic glutamatergic nerve terminals, and excess glutamate binds to postsynaptic NMDA receptors (NMDAR) to release extracellular Ca²⁺ into the neuron, which in turn activates Ca²⁺-dependent cPLA₂ to release AA and increase its conversion to PGE₂ within the AA cascade. Excess glutamate also accumulates in the synaptic cleft because of reduced expression of the astrocytic glutamate reuptake receptor (EEAT2). Lithium’s actions, identified by interrupted arrows, are suggested to dampen (−) upregulated AA cascade parameters designated by ↑ in astrocytes and post-synaptic neuronal elements, and to block the NMDAR on post-synaptic membrane (Anthony and Bell 2008; Basselin et al. 2006; Basselin et al. 2010; Basselin et al. 2007; Ramadan et al. 2012; Ramadan et al. 2010; Rao et al. 2011; Luschen et al. 2000; Dinarello 2002; Lane et al. 1996; Marcoli et al. 2006; D’Aversa et al. 2005; Six and Dennis 2000).

Figure 3

Proposed sites of action of lithium on brain arachidonic acid (AA) cascade (3a) and the cascade as it is upregulated in brain HIV-1 infection (3b). Figure 3a. As illustrated, AA is released from phospholipid following activation of cPLA₂ by a receptor-mediated mechanism. Unesterified AA is a substrate for COX-2 and other oxidative enzymes, and forms PGE₂ and other bioactive products that have cellular actions. Remaining AA is recycled into phospholipid via acyl-CoA synthetase 4 (Acsl4) and an acyltransferase. Based on animal studies and as illustrated, lithium can downregulate the AA cascade by interfering with: NMDA receptor mediated release of Ca²⁺ to activate cPLA₂, transcription of cPLA₂ by the transcription factor activator protein (AP)-2, expression levels of cytosolic cPLA₂ (mRNA, protein and activity), turnover of AA within the deacylation-reacylation cycle, the unesterified brain AA concentration, protein and activity levels of COX-2, and activity of secretory sPLA₂ (not shown). Adapted from Rao et al. (Rao et al. 2008). Figure 3b. Suggested brain targets related to AA cascade in HIV-1 brain infection. Following entry of circulating macrophages containing the HIV-1 virus into brain via blood brain barrier, the macrophages or released virus infect resident microglia (productive infection) or astrocytes (nonproductive infection), via CD4, CCR5 and other receptors. Released glycoprotein-120 (gp-120) also can activate microglia. Activated microglia release cytokines that bind to astrocytic IL-1β and TNF-α receptors that are coupled to activation of cPLA₂ and sPLA₂, to initiate the AA cascade by releasing AA from membrane phospholipid (see 3A). Inducible nitric oxide synthetase (iNOS) in activated microglia releases nitric oxide (NO). NO stimulates release of glutamate by presynaptic glutamatergic nerve terminals, and excess glutamate binds to postsynaptic NMDA receptors (NMDAR) to release extracellular Ca²⁺ into the neuron, which in turn activates Ca²⁺-dependent cPLA₂ to release AA and increase its conversion to PGE₂ within the AA cascade. Excess glutamate also accumulates in the synaptic cleft because of reduced expression of the astrocytic glutamate reuptake receptor (EEAT2). Lithium’s actions, identified by interrupted arrows, are suggested to dampen (−) upregulated AA cascade parameters designated by ↑ in astrocytes and post-synaptic neuronal elements, and to block the NMDAR on post-synaptic membrane (Anthony and Bell 2008; Basselin et al. 2006; Basselin et al. 2010; Basselin et al. 2007; Ramadan et al. 2012; Ramadan et al. 2010; Rao et al. 2011; Luschen et al. 2000; Dinarello 2002; Lane et al. 1996; Marcoli et al. 2006; D’Aversa et al. 2005; Six and Dennis 2000).