Hypoxic-ischemic brain injury (HIBI) after cardiac arrest remains a challenging condition and an unresolved issue in clinical practice. The depletion of energy during ischemia and the induction of oxidative stress during reperfusion activates a number of molecular pathways that lead to cell death and finally to severe neurological damage. A reliable indicator of this damage is the insufficient recovery of normal EEG activity.
However, the brain is endowed with remarkable self-protection mechanisms, relying on the activation of a large array of molecular pathways, involving the so-called neurovascular unit (NVU). The reinstatement of the integrity of NVU, which includes the basal lamina, astrocytes, neurons, peri-capillary microglia, endothelial cells (ECs) and pericytes, is emerging as a crucial factor for successful recovery of ischemia-injured brain.
This proposal is grounded by the hypothesis that hypoxia resistant neurons in the post-ischemic brain can be gently electrically stimulated to trigger neuroprotection mechanisms in the ischemia-lesioned regions, steering durable changes towards normal activity patterns and functions. Using a rodent model of post-cardiac arrest recovery and ischemic brain damage, we will characterize neuroprotective mechanisms activated by ischemia-induced brain injury (HIBI), focusing the attention on both neuronal and microvasculature response. As a means to enhance intrinsic neuroprotective mechanisms, a non-invasive paradigm of broad neuronal activation by the cervical branch of the vagus nerve stimulation will be exploited. Biochemical changes activated by HIBI per se, and potentiated by VNS, are part of an enduring neuroprotective response, thus providing a significant window for pharmacological interventions.
The very nature of brain ischemia, a lack of blood flow, restricts the ability to deliver pharmacological agents to the site of need, thereby limiting the application of pharmacological interventions in the acute phase of ischemic episode. As a consequence, a list of efforts worldwide is currently looking for innovative pharmacological approaches.
Well established data suggests that stimulation of network-level activity both directly (transcranial) and indirectly (via peripheral nerves) can have beneficial effects in seemingly complex neurological or psychiatric conditions. For example, the fact that vagus nerve stimulation (VNS) displays anti-epileptic properties, decreasing abnormal CNS activity, but is also effective against generalised major depression, suggesting an increase in network activation, are reconciled by a model in which external neuronal stimulation results in a network normalisation effect, driving the stimulated tissue away from excessively high or abnormally-low spiking frequencies, acting on the unbalanced ratio between excitatory and inhibitory signals. Similarities could be drawn with the use of heart pacemakers against cardiac arrhythmias. With such a normalisation model in mind, it is plausible that VNS stimulation could lead to improvements in the brain's network functionality (and EEG pattern) of comatose patients, increasing their currently bleak chances of survival, perhaps achieving a revolutionary new technology for the cardiology and neurology intensive care units (ICUs).
The non-invasive VNS stimultation approach we propose to apply in this study is expected to engage ipoxia-resilient neurons after electrical stimulation, thus acting as the driving force for neighboring neuron activation. Neuronal activation should then spread to large areas, leading to network activation. As part of the characterization of our animal model of post-cardiac arrest recovery and ischemic brain damage we will gain an insight on the consistency of ipoxia-resilient neurons, answering the questions: i) how many are they? ii) are they spread all over the brain parenchyma or are they confined to limited areas? (Task 1).
Our current understanding of the cascade of injury mechanisms in the ischemic brain is still limited. These can directly alter the neuronal activity patterns measurable in the EEG (e.g. through lower activity of ion pumps, increased Ca+2 influx, elevated extracellular glutamate PMID:16314180) or indirectly through abnormal metabolism (e.g. decreased mitochondrial function monitored as changes in cytochrome-c-oxidase oxidation and Cerebral Metabolic Rate of Oxygen PMID:23710974). However, t is now well established that HIBI triggers a complex and long lasting response. This response involves all components of the so-called neurovascular unit (NVU), which includes the CNS vasculature and its structural and functional connections to the neural tissue. In addition to endothelial cells, pericytes, and astrocyte end feets, the NVU includes microglia and neurons, which both contact blood vessels by fine cytoplasmic processes (PMID: 24629161; PMID: 27920202). All these cellular components reside in the same microenvironment and are therefore sensible to changes in the milieu. Experiments of Task 2 and 3 will provide a thorough characterization of molecular pathways of the NVU response to HIBI, providing support to VNS protocols and new hints for interventions aimed at improving brain repair mechanisms. Indeed, biochemical changes activated by HIBI per se, and potentiated by VNS, as part of an enduring neuroprotective response, provide a significant window for pharmacological interventions.
Neuronal local and network activation will naturally reflect to the NVU integrity, which is attracting increasing interest in relation to brain damage interventions and a deeper knoweldge of its functional properties holds therapeutic potentials, with particular reference to the recruitment of pericytes to regenerative processes (PMID: 22371274; PMID: 31743876). Notably, although molecular mechanisms underlying the ability of VNS to influence microvasculature are still poorly characterized, recent evidence shows that VNS stimulation promotes angiogenesis (PMID: 30016789; PMID: 27653860). Experiments of Task 3 are conceived to deeply investigate this issue and will provide a valuable increase of our knowledge in a farefront field with implications for the design and testing of new therapeutic strategies.