The mammalian suprachiasmatic nuclei (SCN) are located in the ventral part of the hypothalamus and orchestrate circadian rhythms in physiology and behaviour. The ~20.000 neurones of the murine SCN express key molecular clock components including the Cryptochrome (Cry1/2) and Period (Per1/2/3) genes and their protein products CRY1/2 and PER1/2/3. Using different mouse models, this work demonstrates that with disrupted expression of CRY in the after-hours (Afh/Afh) mouse, cells of the ventral part of the SCN (vSCN) have a propensity to desynchronise. They receive increased GABAergic inputs and are less excitable during the projected night but not during the day compared to congenic wildtype (+/+). The linkage between CRY protein expression and the reduced excitability at night is supported by recordings from SCN cells of Cry2 deficient mice (Cry2-/-), which exhibit similar electrophysiological behaviour. Luminometrical recordings of single cell Per2 expression confirms the involvement of GABAergic signalling in both, maintaining a coherent rhythm in synchronised SCN cells from +/+ controls and the propensity of Afh/Afh SCN cells to desynchronise. A mechanism by which neuronal excitability is regulated in mammals, is the modulation of activity of small-conductance Ca2+-activated K+ (SK) channels. Western blot analysis demonstrates the expression of SK2 and SK3 channel protein in SCN neurones. Functionally, we show with whole cell electrophysiology, calcium imaging and luminometry how SK channels regulate the levels of intracellular calcium ([Ca2+]i) from day to night. In the more hyperpolarised SCN network of the Afh/Afh genotype at night, SK channel activity is altered and contributes to the lower single cell excitability. Vasoactive intestinal polypeptide (VIP) and its cognate receptor, VPAC2, are synthesised by SCN neurones and this intercellular signal facilitates coordination of suprachiasmatic neuronal activity. How the loss of VPAC2 receptor signalling affects the electrophysiology of SCN neurones and their response to excitatory inputs is unclear. Here we made patch clamp recordings of SCN neurones in brain slices prepared from animals that do not express VPAC2 receptors (Vipr2-/- mice) as well as non-transgenic animals (Vipr2+/+ mice). While Vipr2+/+ SCN neurones exhibit coordinated day-night variation in their electrical state, Vipr2-/- neurones do not and instead manifest a range of states during both day and night. We find that Vipr2+/+ neurones vary the membrane threshold potential at which they start to fire actions potentials from day to night, while Vipr2-/- neurones lack this variation. This is due to Vipr2-/- neurones lacking a voltage-gated sodium current. Subsequently we determine that this aberrant temporal control of neuronal state and excitability alters appropriate neuronal responses to a neurochemical mimic of the light-input pathway to the SCN. Conclusively, these results highlight the critical role intercellular signalling plays in the activity of individual neuronal state and their response to neural input as well as ensemble activity and function of the suprachiasmatic neural network.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:727802 |
| Date | January 2015 |
| Creators | Wegner, Sven |
| Contributors | Piggins, Hugh |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/synaptic-communication-in-the-mammalian-master-circadian-clock(f8f5aa2d-8742-4ab1-b578-0d29c28cf3be).html |
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