Inducible Expression of Tryptophan Hydroxylase without Serotonin Synthesis in Hypothalamic Dopaminergic Neurons Sampsa Vanhatalo* , † and Seppo Soinila* , ‡ *Department of Anatomy, Institute of Biomedicine; †Unit of Child Neurology, Hospital for the Children and Adolescent; ‡Department of Neurology, University Central Hospital, University of Helsinki, Helsinki, Finland Received August 24, 1998; accepted January 19, 1999 In the present study we have further studied the previous findings that rat hypothalamic dopaminergic neuronal cell groups may express tryptophan hydroxy- lase (TpH), the serotonin synthesizing enzyme, with- out a detectable serotonin synthesis. Chemical and mechanical neuronal injuries, namely colchicine treat- ment and axonal transection, respectively, were per- formed, and distributions of neurons exhibiting immu- noreactivity for TpH and/or tyrosine hydroxylase (TH), the dopamine synthesizing enzyme, were analyzed throughout the hypothalamic periventricular and ar- cuate nuclei. After colchicine treatment there was a statistically significant 87% (P  0,01) increase in the number of TpH expressing neurons, while TH expres- sion remained essentially similar. Axonal transection resulted also in a statistically significant 131% (P F 0,01) increase in the number of TpH expressing neurons, while TH expression was not significantly altered. All TpH expression coexisted with TH expres- sion, and the induction of TpH expression by neuronal injuries occurred evenly throughout the rostrocaudal length of the territory studied. A possible serotonin synthesis by TpH was examined by giving drugs that increase brain serotonin synthesis, but no immunohis- tochemically detectable serotonin synthesis could be found in any of the TpH expressing neurons. Finally the possibility was studied that the relative shortage of the cofactor tetrahydrobiopterin would limit seroto- nin synthesis. However, an administration of tetrahy- drobiopterin did not result in detectable serotonin synthesis in these neurons. Taken together these re- sults suggest that dopaminergic neurons in the hypo- thalamic periventricular and arcuate nuclei are able to express TpH, this expression is induced after neuro- nal injury, and this induction occurs similarly through- out the territories studied. TpH expression occurs independently of TH expression, and the newly ex- pressed TpH appears not to synthesize serotonin, re- gardless of pharmacological pretreatments. Thus, our findings (i) support the idea that neurons may possess inducible expression of nonfunctional transmitter- synthesizing enzymes, in this case TpH, and (ii) sug- gest that expression of an enzyme synthesizing a cer- tain transmitter may not necessarily imply the corresponding transmitter phenotype.  1999 Academic Press Key Words: serotonin; hypothalamus; superfluous ex- pression; rat. INTRODUCTION One common approach to elucidate the transmitter phenotype of a given neuron is to reveal the transmitter- synthesizing enzyme(s) present in the neuron. Expres- sion of the transmitter-specific enzyme is thus usually considered to reflect synthesis and usage of the corre- sponding transmitter. However, an increasing number of studies have reported synthesis of nonfunctional enzymes in distinct neuronal groups (21, 27, 43, 48, 60). Among these, our recent studies have shown that the hypothalamic A11 and A12 dopaminergic neurons (5) may express tryptophan hydroxylase (TpH), the seroto- nin-synthesizing enzyme, which does not synthesize serotonin in these neurons (60, 62, 63). This observa- tion is an example of the recently introduced concept of superfluous expression, i.e., expression of certain pro- teins, which do not perform any of their known func- tion(s) in the given cell (8). Such an expression is considered to result from balancing of the energy costs between expression and suppression of the correspond- ing gene (8), which then results in a minimal expres- sion of a number of gene products. However, this concept also undermines the idea that neuronal trans- mitter phenotype of a particular neuron could be char- acterized by visualizing the enzymes synthesizing trans- mitters. Dopaminergic neurons in the hypothalamic periven- tricular (PeVN) and arcuate (ARC) nuclei constitute the dopaminergic A11 and A12 neuronal groups (5, 12), and they are usually visualized by their immunoreactiv- ity for tyrosine hydroxylase (TH-IR) (5), the dopamine- synthesizing enzyme. Large proportion of these neu- rons send projections to the median eminence and the pituitary gland (19, 33, 60), and thereby play a central role in neuroendocrinological regulation by mediating a Experimental Neurology 157, 305–316 (1999) Article ID exnr.1999.7053, available online at http://www.idealibrary.com on 305 0014-4886/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.