PRE- AND POSTSYNAPTIC ORIGIN OF THE NOREPINEPHRINE METABOLITES FORMED DURING TRANSMITTER RELEASE ELICITED BY NERVE STIMULATION

¹ Instituto de Investigaciones Farmacológicas, Consejo Nacional de investigaciones Cientificas y Técnicas, Buenos Aires, Argentina

The norepinephrine stores in the isolated nerve-muscle preparation of the cat nictitating membrane were labeled with ³H-norepinephrine and both spontaneous and stimulation-induced release of tritiated compounds was studied. When the postganglionic nerves were stimulated at 10 shocks/sec, about 30% of the total increase in radioactivity was due to norepinephrine metabolites. Normetanephrine and the O-methylated-deaminated fraction represented more than two-thirds of the increase in metabolites. Cocaine and different concentrations of phenoxybenzamine were examined in their actions on neuronal and extraneuronal accumulation of norepinephrine in order to correlate these effects with the changes in the metabolism of the released transmitter. The percentage of normetanephrine in the overflow induced by stimulation followed closely the changes in extraneuronal accumulation of norepinephrine. These results are consistent with the view that normetanephrine formed during transmitter release by stimulation originates postsynaptically. Phenoxybenzamine or cocaine did not affect spontaneous release of normetanephrine in concentrations which modified the nerve-stimulation induced overflow of metabolite suggesting that normetanephrine in spontaneous outflow was formed predominantly by presynaptic catechol-O-methyltransferase. There was a good correlation between inhibition of neuronal accumulation of norepinephrine and the reduction in formation of the O-methylated-deaminated metabolites during nerve stimulation. Therefore, it appears that the O-methylated-deaminated metabolites are formed intraneuronally from released norepinephrine which is reincorporated into the nerve terminals. The spontaneous outflow of the O-methylated-deaminated metabolites was due to the activity of presynaptic monoamine oxidase and catechol-O-methyltransferase on norepinephrine leaking from the granules into the neuronal cytoplasm.