Disinhibition of the midbrain colliculi unmasks coordinated autonomic, respiratory and somatomotor responses to auditory and visual stimuli.

Müller-Ribeiro FC, Dampney RA, McMullan S, Fontes MA, Goodchild AK Am J Physiol Regul Integr Comp Physiol. 2014 Aug 6. Previous work has shown that if you cause blockade of GABA-A receptors in the colliculi of awake rats, the rats will show defensive/escape responses to stimuli that normally wouldn't bother them. This suggested that the colliculi are involved in activation of the sympathetic nervous system, but that the neurons are usually under tonic GABAergic inhibition. Based on that idea, this paper studies the effect of GABA-A blockade in the superior and inferior colliculi and its effect on splanchnic, sciatic, and phrenic nerve activity in anesthetized rats. Normal, anesthetized rats should have no increases in nerve activity or MAP after the rat is exposed to clapping, flashes of light, and paw-pinches. That's exactly what they saw in their preparation. They then blocked GABA-A receptors with picrotoxin (which rarely caused changes in baseline nerve activity), and gave the rats the same stimuli. After blockade, they saw that all three of those stimuli could cause synchronized increases in nerve activity among all three of the nerves. This is strong proof that the colliculi are relay points for autonomic activity that are normally kept under GABAergic inhibition. They followed this up by trying to figure out where this GABA might be coming from - to do this they removed the forebrain and saw the exact same effects. Because they saw the complete response in decerebrate rats, they can now say that GABA is coming from either the midbrain or the brainstem, tonicically inhibiting the colliculi's activation of nerve activity, and also that whatever signal is driving that GABAergic tone is ALSO located in the midbrain or brainstem. -DH