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BRAIN TRYPTOPHAN METABOLISM ON THE 5‐HYDROXYTRYPTAMINE AND KYNURENINE PATHWAYS IN A STRAIN OF RATS WITH A DEFICIENCY IN PLATELET 5‐HT

1 Brain 5‐hydroxytryptamine (5‐HT) metabolism has been compared in albino (Sprague‐Dawley; SD) and in Fawn‐Hooded (FH) rats, which have an inherited platelet 5‐HT deficiency. 2 It was confirmed that blood 5‐HT levels in the FH rats were about one quarter of those in the SD rats. 3 Brain 5‐HT and 5‐HIAA were however higher in FH rats on a per gram basis; there was no difference between the strains on a per brain basis, because of the smaller brain weights of the FH rats. 4 Brain and plasma tryptophan were not significantly different in the two strains. Plasma kynur‐enine was higher in the FH rats, and brain kynurenine was also higher either on a per gram or on a per brain basis. 5 The reserpine‐releasable brain 5‐HT was the same proportion of total brain 5‐HT in the two strains. 6 Experiments with pargyline suggested that the turnover of 5‐HT was somewhat higher in the FH rats on a per gram basis, but not significantly so on a per brain basis. 7 It is concluded that although brain tryptophan metabolism may be somewhat accelerated along both the 5‐HT and kynurenine pathways in the FH rats there is no gross deficiency in the binding of 5‐HT in their brains analogous to that found in their platelets.