The release of catecholamines by an endogenous factor that inhibits neuronal Na+,K(+)-ATPase.

It is known that the inhibition of Na+,K(+)-ATPase induces neurotransmitter release in several experimental models. In this laboratory it was previously observed that a brain soluble fraction separated by Sephadex G-50 (peak II) is able to inhibit Na+,K(+)-ATPase but not other membrane-bound enzymes. The object of the present study was to test the effect of brain peak II fraction on neurotransmitter content of the pineal nerves synaptic vesicles. Uninjected rats and rats injected 30 min before with 5-hydroxydopamine (30 mg per kg,i.p.) were used. 5-hydroxydopamine produces a false neurotransmitter whose presence in the synaptic vesicles is visualized after fixation with glutaraldehyde-osmium as an electron dense material with the electron microscope which fills totally or partially the vesicles. In uninjected rats the osmiophilia and the chromaffin reaction of the electron dense core were studied. The pineal glands were incubated in Tyrode solution without calcium in the presence and absence of peak II at room temperature and processed for electron microscopy. When the glands from rats pretreated with 5-hydroxydopamine were incubated with peak II a significant decrease in the number of vesicles totally stained was observed. This indicates a reduction in false neurotransmitter content, specially in the matrix of the synaptic vesicles. In the presence of an aged peak II, which does not inhibit Na+,K(+)-ATPase, this effect on the synaptic vesicles was not observed. When the glands from uninjected rats were incubated with peak II no changes in the osmiophilia and the chromaffin reaction of the synaptic vesicles were found. The osmiophilia and the chromaffin reaction of the cores marks the monoamines storage site (catechol and indoleamines in the pineal nerves). These results are coherent with the idea of a relationship between the inhibition of Na+,K(+)-ATPase activity and the release of a pool of neurotransmitter stored in the nerve endings.