Role of Na⁺/H⁺ exchangers, excitatory amino acid receptors and voltage-operated Ca²⁺ channels in human immunodeficiency virus type 1 gp 120-mediated increases in intracellular Ca²⁺ in human neurons and astrocytes

Human immunodeficiency virus type 1 (HIV-1) dementia is the commonest form of dementia in North American people less than 60 years of age. HIV-1 envelope glycoprotein gp120 has been implicated in the neurotoxicity observed in, and the pathogenesis of, HIV-1 dementia. Recombinant gp120 (gp120) was pressure-applied on to cultured human fetal neurons and astrocytes and, by using single-cell calcium imaging, we determined the mechanisms responsible for gp120-induced increases in the levels of intracellular calcium ([Ca²⁺](i)). Significant dose-related increases in [Ca²⁺](i) were observed in neurons and astrocytes. In neurons, 5 pM gp120 increased [Ca²⁺](i) by 290 ± 13 nM and increases of 2210 ± 211 nM were found at 209 nM, the highest concentration of gp120 tested. The apparent EC₅₀ value for gp120 of 223 ± 40 pM in neurons was not significantly different from that in astrocytes. Immunoelution of gp120 with polyclonal anti-gp120 and Ca²⁺- free conditions blocked increases in [Ca²⁺](i) by gp120. Increases in [Ca²⁺](i) were significantly (P < 0.005) attenuated by the Na⁺/H⁺ exchange blocker 5-(N-methyl-N-isobutyl)-amiloride in neurons and astrocytes. The L-type calcium channel blockers nimodipine, diltiazem and CdCl₂ + NiCl₂ significantly (P < 0.005) reduced increases in [Ca²⁺](i) in neurons, but not astrocytes. Increases in [Ca²⁺](i) by gp120 were not significantly affected by blockers of N-, P- and Q-type calcium channels. The N-methyl-D- aspartate receptor antagonists (±)-2-amino-5-phosphonopentanoic acid (AP5), memantine and dizocilpine significantly (P < 0.01) lowered gp120-induced increases in [Ca²⁺](i) in neurons. AP5 and memantine, but not dizocilpine, significantly (P < 0.01) reduced increases in [Ca²⁺]i by gp120 in astrocytes. Gp120 appears to activate astrocyte Na⁺/H⁺ exchangers to release glutamate and potassium and, subsequent to this, increases in [Ca²⁺](i) in neurons and astrocytes result from activation of excitatory amino acid receptors on astrocytes and neurons, and voltage-operated calcium channels on neurons. Drugs that block gp120-induced changes in [Ca²⁺](i) in neurons and astrocytes may help in the treatment of HIV-1 dementia.