Circadian gene expression of tryptophan hydroxylase 2 in the Dorsal and Median Raphe nuclei is altered by dysregulated glucocorticoid rhythms

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Stress-related psychiatric disorders have been characterized by dysregulated rhythms of the hypothalamic-pituitary-adrenal axis (HPA axis) and a dysfunctional serotonergic system. However, the molecular mechanism of this relationship is not entirely described. This thesis has explored the link between an altered HPA axis circadian rhythm and the effect on tryptophan hydroxylase 2 (tph2) mRNA expression, the rate-limiting enzyme in the biosynthesis of serotonin. Three different experimental conditions were used in this study: 1) normal control conditions, 2) administration of the long-acting synthetic glucocorticoid methylprednisolone (MPL), and 3) constant light exposure for five weeks (LL). To evaluate the effects of these models, Radioimmunoassays (RIA) were used to assess plasma corticosterone (CORT) levels, and in situ hybridization histochemistry (ISHH) was used to measure tph2 mRNA in six different regions (interfascicular part DRI, caudal part DRC, ventral part DRV, dorsal part DRD, ventrolateral part/ventrolateral periaqueductal grey part DRVL/VLPAG and medium raphe nucleus MnR) of the Raphe Complex over a 24-hour period within 5 different time points (3 am, 9 am, 3pm, 6 pm, 9 pm). Relative to controls, the MPL model suppressed CORT, while the LL model displayed a hyperactive CORT secretion. Linear Mixed Model analyses showed that tph2 mRNA expression changed in the DRD, DRV, DRVL/VLPAG and in the MnR nucleus. The MPL induced a bi-phasic rhythm in expression with the loss of the nadir at 9 am and the peak at 6 pm whereas the LL treatment triggered high-levels of expression at 3 am and 9 pm with a flat expression between 9 am and 6 pm. This altered expression of tph2 mRNA was specific to the caudal levels of each area, with exception of the DRD. Measurements from each rostro-caudal bregma level showed precise changes which might be indicating a specific neuroanatomical functionality of each region of the Raphe Complex.
Date of Award25 Jun 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorBecky L Conway-Campbell (Supervisor), Jamie Walker (Supervisor) & Stafford L Lightman (Supervisor)


  • tph2 mRNA
  • circadian rhythms
  • serotonergic system
  • glucocorticoids

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