Cardiovascular Elements of Innate Alerting Responses: Role of the Superior Colliculus

Rapid behavioural and physiological responses to alerting stimuli are vital to animal survival, yet the neural mechanisms underlying these responses remain elusive. Of particular interest is the superior colliculus (SC), a prominent midbrain structure known to process visual information and drive innate behavioural responses to a variety of ecologically salient stimuli. This project aimed to improve understanding of the involvement of the SC in both behavioural and cardiovascular responses to overhead looming stimuli, which mimic a rapidly approaching object and evoke defensive reactions primarily mediated by SC circuits. First experiments utilised fifteen male Long-Evans rats exposed to a forward loom, reverse loom, or no loom (n=5 per group). Animals were euthanised and brain tissue from the SC underwent histological processing to examine the cell activation marker c-Fos. Behavioural categorisation of rats exposed to the forward loom differed significantly from those of rats exposed to the reverse loom, with almost all forward loom rats favouring an escape response over freezing. Within this group, the greatest c-Fos expression occurred within the medial, as opposed to lateral, SC, which is associated with the overhead visual field. Follow-up optogenetic experiments sought to assess the contribution made by medial SC neurons in mediating cardiovascular and behavioural response to overhead loom. A total of eight male Long-Evans rats were instrumented with optic fibres targeting inhibitory opsins to the rostral medial SC and telemetry pressure transmitters to record arterial BP. Animals were presented with four forward looms, the third of which was accompanied by delivery of optogenetic inhibition via implanted optic fibres. Following the overhead loom, some animals were found to rapidly increase systolic BP (sBP), accompanied by a gradual decrease in HR, although this was not seen in every case. Inhibition of the SC was expected to diminish both behavioural and physiological responses to overhead looms, but findings indicate that laser stimulation during the loom produced an increase in sBP compared to baseline and did not reduce freezing duration compared to a loom alone. Whilst optogenetic inhibition of the rostral medial SC did not act as expected, the results nevertheless demonstrate extensive involvement of the SC in both physiological and behavioural responses to the overhead loom.