Population activity of the superior colliculus and associated cardiovascular outputs
Detecting and reacting to biologically significant stimuli is essential for the survival of all organisms. Orienting is the reflexive shift of the eyes and head towards salient events which precede stereotyped motor behaviours (e.g., freeze, flight, and pursuit) to produce an ecologically appropriate response based on sensory information gained from the external environment. Orienting is predominantly mediated by the superior colliculus (SC), a midbrain region responsible for sensory integration and motor command initiation. Sympathetic cardiovascular responses (increased blood pressure and heart rate) are also associated with orienting and believed to support the metabolic demands of stereotyped motor behaviours, however the extent of neural circuits that are involved remain unclear. The deeper lamina of the SC (dSC) has been proposed to drive cardiovascular control in orienting based on previous work where dSC stimulation evoked pressor responses and had formed the starting point for investigation in this thesis. The aim of this project was to further characterise the role of the dSC as a potential driver of cardiovascular activation. This was achieved via simultaneous fibre photometry-telemetry recordings of dSC neural activity and blood pressure responses during exposure to sensory stimuli to evoke orienting behaviour in conscious, head-fixed mice. It was hypothesised that a positive association would offer a naturalistic physiological context for previous work that examined the dSC through experimental electrical, chemical, and optogenetic stimulation. The genetically encoded calcium indicator (GECI), GCaMP, was injected into the dSC of wildtype C57BL/6J mice using an adeno-associated viral (AAV) delivery system and instrumented with a photometry optic fibre overlying the injection site. Head-fixation was achieved by clamping of the lightweight washer attached to the skull. The cohort (n=9) was telemetered and underwent a three-day experimental protocol consisting of exposure to auditory (white noise), visual (classic loom paradigm) and audiovisual (white noise and loom combined) stimuli. Photometry-telemetry recordings revealed significant increases in population-level dSC neural activity during the stimulus period that were accompanied by surges in systolic arterial pressure (sAP) and heart rate (HR). The correlation between the magnitudes of dSC activity and cardiovascular responses to sensory stimuli was however found to be weak, in contrast to orofacial whisking events that were synchronised to spikes in dSC activity during basal and stimulus-evoked behaviour.