BACKGROUND: The superior (SC) and inferior (IC) colliculi in the midbrain are involved in the generation of behavioural and cardiovascular responses to threatening stimuli. In our previous study, we observed that disinhibition of neurons in the SC and IC resulted in a response characterised by highly synchronised bursts of sympathetic and respiratory activity. In this study we examined whether the response also includes generation of somatomotor activity. We also determined whether the coordinated response could be generated by naturalistic stimuli following disinhibition of the colliculus.
METHODS: In urethane-anaesthetised, pancuronium paralysed and artificially ventilated rats, microinjections of picrotoxin (100 pmol/ 100 nl) into dorsal sites of the SC and IC evoked the same pattern of response observed in our previous work.
RESULTS: Coordinated bursts of activity were seen in sympathetic (splanchnic nerve and heart rate), respiratory (phrenic nerve) and somatomotor activity (sciatic or brachial nerve). In addition following cessation of the immediate drug effects bright light, abrupt sound or whisker stimulation evoked a synchronised pattern of response in outflows characterised by short bursts activity. Simultaneous stimulation of more than one type of sensory input evoked an enhanced response. Injections into sites located 0.5 mm away from these ‘hot spots’ elicited no response and there was no effect of naturalistic stimuli.
CONCLUSIONS: The results are consistent with the view that neurons in the SC and IC can generate highly co-ordinated behavioural responses, accompanied by cardiovascular and respiratory changes that are appropriate for a defensive behavioural response triggered by a sudden threatening stimulus. Activation of multimodal sensory inputs evokes an alerting response only following removal of inhibitory input from a discrete region of the colliculus.
DISCLOSURE: N/A, Supported by NHMRC 1028183 and by Capes.