Subcellular and regional distribution of 125I-labeled alpha-bungarotoxin binding in rat brain and its relationship to acetylcholinesterase and choline acetyltransferase.

1. The subcellular distribution of binding sites for 125I-labeled alpha-bungarotoxin was studied in rat cerebral cortex. Primary fractions showing higher specific activity than homogenate were P2 (crude mitochondria and nerve endings) and P3-P2 was subfractionated on a Ficoll gradient with the P2B (nerve ending) subfraction exhibiting the greatest recovery (65%) and enrichment of toxin binding. Toxin binding showed a distribution similar to that of acetylcholinesterase, choline acetyltransferase, and sodium and potassium ion-activated ATPase. 2. P2B and P3 were subfractionated on five-step discontinuous sucrose gradients. The highest specific activity of toxin binding and acetylcholinesterase was associated with fractions of relatively low buoyant density, while choline acetyltransferase activity was associated with fractions of higher density. 3. Toxin binding, acetylcholinesterase, and choline acetyltransferase activities were relatively high in olfactory lobes, cerebral cortex, thalamic region, caudate nucleus, and brain stem; intermediate in hippocampus; low in cerebellum. 4. The relationship of toxin binding to the putative acetylcholine receptor in brain is discussed.