ER mediation of T's protection against synergistic neurotoxicity of HIV proteins with cocaine (Coc); cholesterol's [M] neuroprotection is incomplete. Control conditions of incubation with Locke's Buffer vehicle or the HIV proteins or Coc only produced similar neuronal death. (Top Panel) Significant interaction of the HIV proteins with Coc, as illustrated by the lines diverging from parallel, produces toxic synergism (*p < 0.0001. (Middle Panel) The ER specific antagonist, ICI-182,780 (ICI), completely blocked T-mediated neuroprotection. ICI's toxicity and T's neuroprotection did not differ from vehicle control (#p < 0.0001). (Bottom Panel) Cholesterol produced significant, concentration-dependent protection against synergistic neurotoxicity of HIV proteins w/ Coc (#p < 0.0001); however full protection was demonstrated only at 100 nM.