CHANGES IN STEADY-STATE LEVELS OF TRYPTOPHAN-HYDROXYLASE PROTEIN IN ADULT-RAT BRAIN AFTER NEONATAL 6-HYDROXYDOPAMINE LESION

A recently developed technique of immunoautoradiography on nitrocellul ose transfers of serial frozen sections was used to determine tryptoph an hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopa mine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nuc leus raphe dorsalis, containing the cell bodies of origin for these ne rve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive. transverse levels across the entire caudorostral extent o f all these regions. Similar measurements of tyrosine hydroxylase prot ein across the substantia nigra and the neostriatum verified the disap pearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin -hyperinnervated neostriatum was up by 36% above control, i.e, signifi cantly less than the number of its serotonin axon terminals or varicos ities. This was therefore indicative of a lowering of the tryptophan h ydroxylase protein content per serotonin ending. Interestingly, it tig ht correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the contro l neostriatum allowed the prediction the tryptophan hydroxylase concen tration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably a t those raphe dorsalis levels known to give rise to the serotonin hype rinnervation of neostriatum. It is hypothesized that the lower steady- state level of tryptophan hydroxylase inside the terminals and cell bo dies of hyperinnervating serotonin neurons was the result of a feedbac k inhibition of the synthesis of the enzyme by its end-product, presum ably because of the increased amount of serotonin in these terminals.