Abstract
Hypothalamic neurons expressing histamine and orexin/hypocretin (hcrt) are necessary for normal regulation of wakefulness. In Parkinson's disease, the loss of dopaminergic neurons is associated with elevated histamine levels and disrupted sleep/wake cycles, but the mechanism is not understood. To characterize the role of dopamine in the development of histamine neurons, we inhibited the translation of the two non-allelic forms of tyrosine hydroxylase (th1 and th2) in zebrafish larvae. We found that dopamine levels were reduced in both th1 and th2 knockdown, but the serotonin level and number of serotonin neurons remained unchanged. Further, we demonstrated that th2 knockdown increased histamine neuron number and histamine levels, whereas increased dopaminergic signaling using the dopamine precursor l-DOPA (l-3,4-dihydroxyphenylalanine) or dopamine receptor agonists reduced the number of histaminergic neurons. Increases in the number of histaminergic neurons were paralleled by matching increases in the numbers of hcrt neurons, supporting observations that histamine regulates hcrt neuron development. Finally, we show that histaminergic neurons surround th2-expressing neurons in the hypothalamus, and we suggest that dopamine regulates the terminal differentiation of histamine neurons via paracrine actions or direct synaptic neurotransmission. These results reveal a role for dopaminergic signaling in the regulation of neurotransmitter identity and a potential mechanism contributing to sleep disturbances in Parkinson's disease.
Highlights
In zebrafish (Danio rerio), two non-allelic forms of th are expressed in the brain in a largely complementary manner [13,14,15]
Using differential knockdown of the zebrafish th genes th1 and th2, we showed that TH2 acts as the mammalian tyrosine hydroxylase in dopamine synthesis and is involved in the regulation of histaminergic and hcrt neuron numbers
The conclusion is contradictory to our current findings and several other reports, which confirm the fact that zebrafish th2 has tyrosine hydroxylase activity in dopamine synthesis [39, 40]
Summary
Hypothalamic Histaminergic and Serotonergic Neurons Are Distinct from th Dopaminergic Neurons—Tg(f.TH:egfp) transgenic fish express GFP under the control of the fugu th promoter [16]. In the 5-dpf zebrafish brain, GFP expression was seen in olfactory bulbs, the preoptic region, the paraventricular organ, and caudal periventricular hypothalamic zones (Fig. 1A). To further characterize the identity of GFP-positive cells in the hypothalamus, we performed in situ hybridization (ISH) with th riboprobes, followed by immunohistochemistry (IHC) using TH1, TH2, histamine, or 5-HT antibodies on 5-dpf.
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