Environmental Enrichment Attenuates Traumatic Brain Injury: Induced Neuronal Hyperexcitability in Supragranular Layers of Sensory Cortex.

We have previously demonstrated that traumatic brain injury (TBI) induces significant long-term neuronal hyperexcitability in supragranular layers of sensory cortex, coupled with persistent sensory deficits. Hence, we aimed to investigate whether brain plasticity induced by environmental enrichment (EE) could attenuate abnormal neuronal and sensory function post-TBI. TBI (n = 22) and sham control (n = 21) animals were randomly assigned housing in either single or enriched conditions for 7-9 weeks. Then, in terminal experiments, extracellular recordings were obtained from barrel cortex neurons in response to whisker motion, including those mimicking motion in awake animals undertaking different tasks. Long-term EE exposure (6 weeks) attenuated TBI-induced hyperexcitability in layers 2-3, such that neuronal activity in TBI animals exposed to EE was restored to control levels. Little to no EE-induced changes in population neuronal responses occurred in input layer 4 and output layer 5. However, single-cell responses demonstrated EE-induced hypoexcitation in L4 post-TBI. EE was also able to fully ameliorate sensory hypersensitivity post-TBI, although it was not found to improve motor function. Long-term enrichment post-TBI induces changes at both the population and single-cell level in the sensory cortex, where EE may act to restore the excitation/inhibition balance in supragranular cortical layers.

Cortical hypoexcitation defines neuronal responses in the immediate aftermath of traumatic brain injury.

Traumatic brain injury (TBI) from a blow to the head is often associated with complex patterns of brain abnormalities that accompany deficits in cognitive and motor function. Previously we reported that a long-term consequence of TBI, induced with a closed-head injury method modelling human car and sporting accidents, is neuronal hyper-excitation in the rat sensory barrel cortex that receives tactile input from the face whiskers. Hyper-excitation occurred only in supra-granular layers and was stronger to complex than simple stimuli. We now examine changes in the immediate aftermath of TBI induced with same injury method. At 24 hours post-trauma significant sensorimotor deficits were observed and characterisation of the cortical population neuronal responses at that time revealed a depth-dependent suppression of neuronal responses, with reduced responses from supragranular layers through to input layer IV, but not in infragranular layers. In addition, increased spontaneous firing rate was recorded in cortical layers IV and V. We postulate that this early post-injury suppression of cortical processing of sensory input accounts for immediate post-trauma sensory morbidity and sets into train events that resolve into long-term cortical hyper-excitability in upper sensory cortex layers that may account for long-term sensory hyper-sensitivity in humans with TBI.