| Abstract: | Evidence is presented of the existence of a substance in brain (substance B) that reverses presynaptic inhibition of evoked acetylcholine (AcCho) release. Addition of guinea pig brain synaptosomal incubation medium or whole brain extracts to assays in which 1,1-dimethyl-4-phenylpiperazinium-evoked release of [3H]AcCho from guinea pig myenteric plexus synaptosomes reversed purinergic, muscarinic, and alpha 2-adrenergic agonist inhibition. However, the extracts did not increase basal or evoked release of AcCho. Results of chromatography on Bio-Gel P-2 of a concentrated whole brain extract suggest this factor has an Mr of 700. Substance B was resistant to boiling under acid or alkaline conditions as well as incubation with phospholipase C and various proteases; ashing, however, completely destroyed activity. This endogenous factor was also found to antagonize agonist-mediated inhibition of electrically evoked contractions of myenteric plexus-longitudinal muscle strips from the guinea pig ileum. The demonstration of substance B with its ability to reverse presynaptic inhibition of AcCho release reveals one mode of presynaptic modulation. |
