The suprachiasmatic nucleus receives dense serotonergic projections from the raphe nuclei and this input has been implicated in the modulation of circadian rhythms. This input appears to have many functions including the transmission of non-photic information during the day and the modulation of photic information at night. However, it has emerged that this input may also be involved in the transmission of light information with activation of 5-HT2C receptors at night having a photo-mimetic effect. The studies described in this thesis aim to clarify the role of 5-HT2C receptors in the control of circadian rhythms in the rat model and compare their actions to light. The acute effects of 5-HT2C receptor agonist administration on clock gene expression were investigated in the rat SCN. Systemic administration of the 5-HT2A/2C agonist DOI to rats during early night induced c-fos, Per1 and Per2 expression in a manner similar to light. This response was time of day dependent with maximal induction occurring in the early night, and no response during the day. The role of 5-HT2C receptors in this response was confirmed with the use of the selective 5-HT2C receptor agonist RO-60 0175. The effect of 5-HT2C receptor activation on the phase of expression of various circadian rhythms including temperature, melatonin and clock gene expression in the SCN and periphery was examined. Both DOI administration and light exposure at night phase delayed rhythms of melatonin and temperature. Similarly, the selective 5-HT2C receptor agonist RO-60 0175 phase delayed rhythms of 6-sulphatoxymelatonin, a response which was antagonised by the 5-HT2C receptor antagonist SB-242084. The expression of functional and clock genes within the pineal was also phase delayed following both light and 5-HT2C receptor agonist administration. However, the phase of expression of clock genes within the SCN or liver did not shift in response to either a single nocturnal light pulse or agonist administration. To investigate the site of action of 5-HT2C receptor agonists, rat SCN explants were maintained in culture allowing exposure of agonists to denervated tissue. The acute effect of DOI administration at various circadian times on c-fos and Per1 expression was assessed. 5-HT2C receptor activation significantly increased Per1 expression when administered during early subjective night, but had no effect during either subjective day or late subjective night, similar to that observed in vivo. Finally, the suitability of immortalised rat SCN cells for investigation of the intracellular actions of 5-HT2C receptors in the circadian system was assessed. Using RT-PCR the expression of various serotonin receptors in the SCN2.2 cell line was compared with that observed in punches of adult rat SCN. The mRNA for 5-HT1B and 5-HT2A receptor was expressed in both the SCN2.2 cell line and the adult rat SCN. However, 5-HT2C receptor mRNA along with 5-HT3 receptor, 5-HT5A receptor and 5-HT7 receptor mRNA was expressed in the adult rat SCN tissue but not the SCN2.2 cells. These significant differences in serotonin receptor expression limit the usefulness of this cell line for further investigation. Together these experiments further implicate 5-HT2C receptors in the control of circadian rhythms. The role of these receptors appears limited to early night, with activation showing photo-mimetic responses. Furthermore, the location of action appears to be post-synaptic within the SCN, altering the core clock genes, which in turn phase delay various circadian rhythms. / Thesis(Ph.D.)-- School of Paediatrics and Reproductive Health, 2008