Tryptophan hydroxylase (TPH) neurons are a specialized population of neuromodulatory neurons that synthesize serotonin (5-hydroxytryptamine, 5-HT), a critical neurotransmitter involved in mood regulation, sleep-wake cycles, pain modulation, and cognitive function[1][2]. In the context of neurodegenerative diseases, TPH-expressing neurons in the raphe nuclei undergo degeneration and dysfunction, contributing to non-motor symptoms that often precede motor manifestations by years[3].
Tryptophan hydroxylase (EC 1.14.16.4) is a tetrahydrobiopterin-dependent monooxygenase that catalyzes the rate-limiting step in serotonin biosynthesis:
| Isoform | Gene | Location | Function |
|---|---|---|---|
| TPH1 | TPH1 | Peripheral (enterochromaffin cells, mast cells) | Peripheral 5-HT synthesis |
| TPH2 | TPH2 | Central nervous system (raphe nuclei) | CNS serotonin production |
The CNS isoform TPH2 is expressed exclusively in serotonergic neurons of the raphe nuclei, while TPH1 is found in peripheral tissues[4].
The dorsal raphe nucleus (DRN) and median raphe nucleus (MRN) contain the majority of TPH-expressing neurons:
TPH neurons receive input from:
They project to:
Serotonergic dysfunction in AD is increasingly recognized as a major contributor to neuropsychiatric symptoms[5]:
Mechanistic links:
TPH neuron dysfunction in PD contributes to non-motor symptoms[6]:
Mechanistic links:
Serotonergic alterations contribute to psychiatric symptoms[7]:
| Drug Class | Examples | Mechanism | Indication |
|---|---|---|---|
| SSRIs | Fluoxetine, Sertraline | 5-HT reuptake inhibition | Depression in ND |
| SNRIs | Venlafaxine, Duloxetine | 5-HT + NE reuptake | Depression, pain |
| 5-HT1A agonists | Buspirone | Autoreceptor modulation | Anxiety |
| Tricyclics | Amitriptyline | Multiple receptor actions | Depression, pain |
Walther DJ, Bader M. [A unique central tryptophan hydroxylase isoform](https://doi.org/10.1016/S0006-2952(03). 2003. ↩︎
Jacobsen JP, et al. The selective serotonin reuptake inhibitor, sertraline, causes cognitive dysfunction but not antidepressant-like behavioral effects in BDNF/hET mice. 2012. ↩︎
Politis M, et al. Serotonergic dysfunction in Parkinson's disease. 2010. ↩︎
Gutknecht L, et al. [Spatio-temporal expression of tryptophan hydroxylase isoforms in human and rat brain](https://doi.org/10.1016/S0169-328X(01). 2001. ↩︎
Chen CP, et al. Serotonergic deficits in Alzheimer's disease. 2015. ↩︎
Schapira AHV, et al. Non-motor features of Parkinson disease. 2017. ↩︎
Jellinger KA. Neuropathology of Huntington's disease. 2017. ↩︎