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Neurophysiological and neurochemical data have suggested that substance P may function as a neurotransmitter at synapses in the central nervous system, particularly in primary sensory neurons (1, 2). Mechanisms responsible for terminating the action of substance P at the synapse are however, unknown. Benuck and Marks (3) have partially puri fied the enzyme responsible for inactivation of substance P in rat brain. However, extensive research with the biogenic amines has suggested that enzymatic inactivation of a neurotrans mitter is an exception rather than the rule (4). Several putative transmitters are taken up into nerve terminals through a specific high affinity uptake system which appears to account for their inactivation process. Although presence of a high affinity uptake system is not a criterion necessary for transmitter identification, such would greatly favor the candidacy of a compound for the transmitter (4). was synthesized by the conventional method described in a previous paper (5). The synthetic substance P was labelled with tritium by a catalysed gas-exposure method, using platinum as the catalyst (6, 7). To remove the catalyst, unstable generally labelled compounds, labile tritium and tritiated impurities which arise during irradiation, the irradiated substance P was lyophilized eight times. The purity of the labelled compound was checked as follows: 1) When the compound was dansylated by the method of Gray and Hartley (8) and was submitted to thin layer chromatography of silica gel, a single spot was observed. Fur thermore, the spot coincided with the peak of radiochemical activity determined by a scan of thin layer chromatogram. 2) The biological activity on guinea pig ileum was approxi mately 80% of that of the parent compound. 3) A mixture of an aliquot of labelled and parent substance P was submitted to gel chromatography using a Sephadex G-15 column. When the column was screened for both guinea-pig ileum contracting activity and radio activity the patterns of both were similar. These results indicated that the labelled substance P was of high purity. The compound is a powder, with a specific activity of 1.31 Ci/mmole

SUBSTANCE P IN THE CENTRAL NERVOUS SYSTEM OF RABBITS : UPTAKE SYSTEM DIFFERS FROM PUTATIVE TRANSMITTERS