CHANGES IN SPECIFIC ACTIVITY OF DOPAMINE METABOLITES AS EVIDENCE OF A MULTIPLE COMPARTMENTATION OF DOPAMINE IN STRIATAL NEURONS

— The functional status of dopaminergic nerve terminals has been studied with a method that allows the simultaneous determination of the specific activities of dopamine (DM), tyrosine (Tyr), 3‐methoxytyramine (3‐MT) and 3,4‐dihydroxyphenylacetic acid (DOPAC), after administration of [3H]tyrosine ([3H]Tyr). Combined fluorimetric, mass fragmentographic and radiometric techniques have been used. [3H]Tyrosine was given intraventricularly to unanaesthetized rats and the animals were killed by exposure for 4 s to high energy microwave radiations. The specific activities of 3‐MT and DOPAC measured 5 and 20 min after administration of [3H]Tyr, i. e. at time intervals in which the specific activity of DM is rising, are much higher than those their physiological precursor, suggesting that they are generated by more than one DM compartment. This hypothesis seems to be supported by the finding that in animals killed by decapitation instead of microwave radiations the post mortem accumulation of 3‐MT occurs to a much smaller extent for the radioactive fraction than for the endogenous one, indicating that 3‐MT formed after death may be mainly derived from DM coming from a compartment where this monoamine has been poorly labeled by the radioactive precursor.