Serotonin reverses the inhibitory effect of a brain soluble fraction on synaptosomal membrane Na+, K(+)-ATPase activity.

Normal operation of the Na+ pump (Na+, K(+)-ATPase) is essential for the maintenance of neurotransmission. Filtration through Sephadex G-50 of a brain soluble fraction allowed the separation of peaks I and II fractions, respectively stimulating and inhibiting synaptosomal membrane Na+, K(+)-ATPase activity. Peaks I and II were isolated from rat cerebral cortex and their effect together with serotonin (5-HT) was studied on ATPase activity by estimating K(+)-p-nitrophenylphosphatase activity in brain cortex synaptosomal membranes. It was observed that 10(-5) or 10(-4) M 5-HT failed to modify either control membrane enzyme activity or peak I activatory effect; on the other hand, such 5-HT concentrations significantly suppressed peak II inhibitory effect. This ability of 5-HT to reverse the inhibitory effect of endogenous factors on Na+, K(+)-ATPase activity could well be a new 5-HT modulatory action within the brain.