Abstract
Corticotropin-releasing hormone (CRH) neurons located within the paraventricular nucleus (PVN) of the hypothalamus control the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis controls the neuroendocrine stress response as well as exhibits spontaneous activity patterns. CRH neurons are activated by real or perceived threats, resulting in a surge of corticosteroid (CORT) secretion. Under basal (unstressed) conditions CORT is known to be secreted with a prominent circadian (~ 24-hour) and ultradian (~ hourly) rhythm; with the peak of circadian secretion before the transition into the active phase of the organism. However, to date, the spontaneous activity patterns of the PVN CRH neuron population, which may control CORT rhythms, are unknown. Therefore, the overall goal of this PhD thesis was to gain insight into the diurnal and ultradian activity patterns of the PVN CRH neurons and how these activity patterns may be influenced by acute stress exposure.
This was investigated by using in vivo GCaMP6s fibre photometry to optically measure the activity patterns of the PVN CRH neuron population in awake behaving adult male mice while simultaneously monitoring home-cage activity. A knock-in mouse line, where Cre-recombinase is expressed by the endogenous CRH gene (CRH-ires-Cre mouse line), was used in combination with an adeno-associated-virus construct, that carried a Cre-dependant fluorescent calcium indicator (GCaMP6s), to unilaterally transduce PVN CRH neurons followed by implantation of an optical fibre above the PVN. This optical fibre was connected to the photometry system during experimental recording.
Fibre photometry recordings of PVN CRH neuron spontaneous activity were carried out over 24-h under unstressed basal conditions. These recordings revealed an ultradian activity pattern that occurred approximately every 54-min. These ultradian activity bursts were characterised by ~20-min long elevations in CRH population activity interspersed with fast transient events. Surprisingly, these ultradian activity bursts were highly correlated with the animal’s total activity in their home-cage, including locomotion and exploratory head movements. The frequency of ultradian activity bursts (~ every 54-min) was also remarkedly similar to the previously reported frequency of ultradian CORT secretion (~ 1 pulse/h), leading to the speculation that ultradian PVN CRH population activity may drive ultradian CORT secretion.
Interestingly, the observed diurnal changes in PVN CRH population activity were subtle, and the time course did not match circadian CORT secretion. Specifically, ultradian PVN CRH population activity was not elevated before the transition into the active phase of the animal; instead, CRH activity bursts lasted ~ 4-min longer during the night.
Exposure to an acute stress (20-min novel environment) did not significantly disrupt the ongoing ultradian rhythms of the PVN CRH neuron population. Finally, PVN CRH neuron population responses to acute stress (5-min white noise) did not differ between ZT0 and ZT12. This result may suggest that the CRH population response to a stressor follows a set ‘template’ regardless of the timing of such stressor across a 24-h day.
Taken together, these data are the first to reveal ultradian and diurnal rhythms in PVN CRH neuron population activity across the 24-h day with a strong link to the animal’s home-cage activity. Furthermore, acute stress did not affect ongoing ultradian rhythms significantly within the PVN CRH population, and the neural responses to acute stress were not found to depend on the time of day. Overall this research expands our in vivo knowledge about basal and stress-evoked neuronal activity patterns within the PVN CRH population across the 24-h day.