Characteristics of d-lysergic acid diethylamide binding to subcellular fractions derived from rat brain

Abstract

Synaptosomes of rat cerebral cortical gray matter contain low concentrations of high affinity ( K D 1 = 9 × 10 −9 M) and medium affinity ( K D 2 = 1.2 × 10 −6 M) binding sites for LSD ( D-lysergic acid diethylamide). Cerebral cortical synaptic membranes contain 2.2-times as many high affinity binding sites per milligram of protein as synaptosomes, but only about 0.18-times as many medium affinity sites per milligram of protein. These binding sites were not detected in synaptosomes from other regions of the brain, or in myelin or mitochondria from either the cortex or the rest of the brain. All these fractions, however, do exhibit low affinity binding ( K D 3 > 10 −4 M). Binding was determined by equilibrium dialysis employing 3H-LSD of high specific activity. The high affinity binding component is stable for at least 90 min at 37°. Incubation with proteolytic enzymes reduces the binding. Of the LSD bound to cortex both in vivo and in vitro, 20–25 per cent is irreversibly bound. There is a pH dependency of the binding which suggests that at 5 × 10 −9 M and at 2 × 10 −5 M, LSD binds preferentially in the protonated form to a fraction of the binding sites. LSD binding to synaptosomes is inhibited by ten hallucinogenic compounds of widely varying structures, but not by their nonhallucinogenic congeners. All of these hallucinogenic compounds may produce their effects by binding to the same sites which bind LSD. Chlorpromazine also inhibits LSD binding. Prostaglandin E 1 and cyclic AMP substantially increase LSD binding.