Neuropharmacological Properties of the Histaminergic System in the Zebrafish

Biogenic amine neurotransmitters are important modulators of the central nervous system. Studies on the roles of the histaminergic system have revealed that it regulates many key brain functions. It has also been implicated to be involved in many brain disorders. Modulation of histamine signalling may thus provide an option for the treatment of these conditions. Due to its relatively recent discovery, many unanswered questions about the histaminergic system remain. In this thesis, I studied the neuropharmacology of the histaminergic system using the zebrafish as a model of the vertebrate brain. In the first study, the effect of acute ethanol on the histaminergic and dopaminergic systems was analysed. The mRNAs of enzymes synthesizing dopamine and histamine were upregulated by a short ethanol exposure in a dose-dependent manner. The morphology of the histaminergic and dopaminergic systems was unaltered by ethanol treatment, suggesting that the changes observed were localized to cells that produce these transmitters under normal conditions. No significant changes were seen in histamine levels following ethanol treatment. Ethanol treatment resulted in a rapid, dose-dependent increase in locomotion of larvae. The behavioural and transcriptional changes occurred within a similar time frame, suggesting that histamine and dopamine may play a role in the locomotor effects of ethanol. In the second study, pharmacological inhibition of the vesicular monoamine transporter 2 (VMAT2) was shown to result in an almost total loss of histamine immunoreactivity in neuronal fibres and a decreased number of histamine neurons. Additionally, levels of histamine and other amine neurotransmitters were decreased. Depletion of histamine did not upregulate its synthesis, although enzymes synthesizing the other aminergic transmitters were upregulated. Behaviourally, VMAT2 inhibition resulted in a decreased histamine-dependent dark-induced flash response of zebrafish larvae, a brief spike in locomotor activity during the first second of sudden onset of darkness, which has been described as histamine dependent. In the third study, we characterized the expression of the zebrafish histamine receptor hrh3 and generated a hrh3 knockout strain using the CRISPR-Cas9 method of targeted genome editing. Double fluorescent in situ hybridization revealed that the majority of hrh3 expressing neurons in the telencephalon were glutamatergic, suggesting that hrh3 regulates glutamatergic transmission in the zebrafish. We saw that hrh3 knockout larval zebrafish adapted faster to sudden onset of darkness, indicating that hrh3 can regulate adaptation to sudden changes in the environment. Although basic behavioural parameters of hrh3 knockout larvae were unaltered, adult hrh3 knockout fish showed decreased locomotor activity, suggesting that hrh3 signalling may have different roles at different ages. No changes were seen in the morphology and transcriptional profile of other aminergic networks. However, zebrafish larvae had decreased levels of dopamine and serotonin, supporting a role for hrh3 in the regulation of these systems. In total, these results provide some new answers to open questions in the field of histamine neuropharmacology, specifically concerning the effects of alcohol on the histaminergic system, the mechanisms of vesicular storage of histamine and interactions between histamine and other aminergic systems. This information will be valuable when planning further studies to fully elucidate the pharmacological potential of this system.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2017

Author dc:creator

• Puttonen, Henri

Statement dc:rights

• Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
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• Publikationen är skyddad av upphovsrätten. Den får läsas och skrivas ut för personligt bruk. Användning i kommersiellt syfte är förbjuden.

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/228582

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F228582