Amyloid fibrils in Gerstmann-Sträussler-Scheinker (GSS) disease are composed of a fragment of the prion protein (PrP), the N and C termini of which correspond to ragged residues 81-90 and 144-153. A synthetic peptide spanning the sequence 82-146 (PrP 82-146) polymerizes into protease-resistant fibrils with the tinctorial properties of amyloid. We investigated the biological activity of PrP 82-146 and of two nonamyloidogenic variants of PrP 82-146 with scrambled amino acid sequence 106-126 or 127-146. Cortical neurons prepared from rat and mouse embryos were chronically exposed to the PrP 82-146 peptides (10-50 microM). PrP 82-146 and the partially scrambled peptides induced neuronal death with a similar dose-response pattern, indicating that neurotoxicity was independent of amyloid fibril formation. Neurotoxicity was significantly reduced by coadministration of an anti-oligomer antibody, suggesting that PrP 82-146 oligomers are primarily responsible for triggering cell death. Neurons from PrP knock-out (Prnp0/0) mice were significantly less sensitive to PrP 82-146 toxicity than neurons expressing PrP. The gliotrophic effect of PrP 82-146 was determined by [methyl-3H]-thymidine incorporation in cultured astrocytes. Treatment with PrP 82-146 stimulated [methyl-3H]-thymidine uptake 3.5-fold. This activity was significantly less when the 106-126 or 127-146 regions were disrupted, indicating that PrP 82-146 amyloid activates the gliotrophic response. Prnp0/0 astrocytes were insensitive to the proliferative stimulus of PrP 82-146. These results underline the role of cerebral accumulation of abnormally folded PrP fragments and indicate that cellular PrP governs the pathogenic process.