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Arginine vasopressin and ion transporters in the modulation of brain excitability during birth and birth asphyxia seizures

Final Report Summary - PRESSBIRTH (Arginine vasopressin and ion transporters in the modulation of brain excitability during birth and birth asphyxia seizures)

Severe birth asphyxia (BA) is a life-threatening condition of the newborn, with an annual death rate of a million neonates globally. BA is typically caused by compromised umbilical/placental functions during complicated delivery, which leads to a fall in the oxygen level and increase in the carbon dioxide level in the neonate. Moderate and mild forms of BA and the ensuing hypoxic-ischemic encephalopathy (HIE) have lifelong deleterious consequences, ranging from psychiatric disorders to neurological manifestations such as cerebral palsy. The conventional view of BA/HIE is that it is mainly an energy-metabolic disorder of the neonate brain. Our research project takes another, complementary angle, examining the roles of the pH-regulatory as well as neuroendocrine mechanisms involving the neonatal hypothalamic-pituitary-adrenal (HPA) axis. The HPA-axis is strongly activated during normal delivery, and further enhanced by BA. In this ongoing research program, we investigate the acute and long-term mechanisms and consequences of the BA-induced brain pH and oxygen changes as well as the "over-activation" of the HPA-axis, focusing on the hypothalamic neurohormone arginine vasopressin (AVP) and its surrogate marker, copeptin (CPT). To this end, we have developed and optimized new rodent models and physiological recording techniques for translational work, and participated in clinical studies.
A direct effect of AVP on neuronal network functions was observed using neonatal (around P0) hippocampal slices, where physiologically-relevant (low nanomolar) concentrations of AVP suppressed the activity of neuronal networks in a manner which implies an AVP-dependent neuroprotective action even in an altricial species, such as the rat, in which BA is a minor problem. An identical effect was seen in more mature rat and also neonatal guinea pig slices (a precocial species), pointing to a high translational relevance of these findings. The identification of the cellular basis (AVP-induced activation of interneurons) and of a specific receptor class (Vla) mediating this response opens the possibility of generating novel drugs targeting the HPA-axis for neonatal neuroprotection.
Our animal model of birth asphyxia is based on 6 and 11 day-old (P6 and P11) rats exposed to a gas mixture of 4-5% or 9% O2 plus 20% CO2 for 30-45 min (“steady asphyxia”); or a gas mixture where the O2 level is altered in a stepwise manner between 5% and 9% in the constant presence of 20% CO2. Large changes in blood acid-base parameters as well as a massive release of peripheral AVP are triggered in response to the experimental asphyxia, which validates our models at P6 and P11 for translational research on preterm and full term human neonates, respectively. The data we have obtained indicate that systems-level mechanisms controlling brain pH levels as well as peripheral and central AVP signaling play key roles in the protection of the fetal brain during normal and complicated delivery.
With P6 and P11 rats, the post-asphyxia recovery period starts with Rapid or Graded Restoration of Normocapnia (RRN and GRN), in which the former induces a much faster recovery of brain pH in the alkaline direction. Notably, transient changes in the hypoxic load of the brain, rather than steady hypoxia as such, lead to an enhanced propensity to seizures. A finding with particularly important therapeutic implications is that using GRN by adding 5% CO2 to the ambient air blocks the intense behavioral and electrographic/cortical seizures seen at P11. This effect is caused by the slower recovery of brain pH during GRN vs. RRN. Because therapeutic maneuvers based on inhaled CO2-containing gas mixture are not easily done in the NICU, we have examined the efficacy of the carbonic anhydrase inhibitor, acetazolamide (AZA) and found, indeed, that AZA potently suppresses the post-asphyxia seizures. AZA is drug with a well-established safety profile, and we are currently looking for collaborators for setting up a clinical trial on this drug.
Our work has also provided a rational basis for the development of prognostic biomarkers to identify those newborns that are at high risk for poor outcome following BA. In a retrospective clinical study we showed that elevated plasma copeptin levels measured after birth asphyxia showed a strong correlation with poor neurological outcome at the age of 2 years.