Asphyxia is a prominent perinatal cause of death in human infants. Some babies may be particularly vulnerable to degrees of birth hypoxia that would not normally be considered dangerous. Sudden infant death syndrome (SIDS) cases may occur because the infant was at heightened risk from levels of apnoea-induced hypoxaemia commonly encountered in this age-group. However, the mechanism underlying the apparently increased susceptibility to hypoxia is unclear. Alterations or delays in maturation of systems that monitor and respond to low blood oxygen levels may render the infant less able to cope with hypoxic events. In higher centres of the brain, this may be manifested as developmental differences in the characteristics of one or more transmitter systems when vulnerable and neurologically normal infants are compared.
Glutamate, GABA and adenosine neurotransmission systems are active and widespread throughout the CNS. During a hypoxia-ischaemia insult, an imbalance between glutamate-mediated excitation and GABA- or adenosine-mediated inhibition could impair the infant's arousal responses and/or other relevant higher functions.
The aim of this project was to assess possible differences in the above neurotransmitter receptor functions in babies that had suffered a fatal asphyxiation episode of unknown origin, and in children dying from SIDS, as compared with neurologically intact controls. Development changes in the glutamate-NMDA, GABAA and adenosine A1 and A2 receptor systems from 22 gestation weeks to adult age were also examined.
Human cerebral cortex tissue (primary motor, superior frontal, mid temporal and occipital) was obtained at autopsy from newborn cases between 22 and 42 weeks' gestation, postnatal infants 18 days to 273 days old and adults cases 24 to 78 years of age. Ligand binding assays were carried out on homogenates obtained from rapidly thawed tissue pieces; data were processed with the EBDA and LIGAND programs and ANOVAs were performed on the resulting data. Protocols developed for use in adult tissue had to be optimized for foetal tissue. Optimization of the binding assay involved changes in the centrifugation conditions, washing requirements and the amount of synaptic plasma membrane added to each experiment. [3H]MK801, "central-type" benzodiazepines ([3H]flunitrazepam and [3H]diazepam), and [3H]cyclopentyladenosine displaced with R-PIA ([3H]CPA/R-PIA) and [3H]CGS21680 were used to assess glutamate-NMDA, GABAA and adenosine A1 and A2a receptor sites, respectively.
In the four cortical region studied, [3H]MK801 bound to a single class of glutamate- NMDA receptor sites in all assays. Perinatal infants had a lower affinity for the receptor in the anterior regions of the cortex. The density of [3H]MK801 binding sites showed marked increases in all cortical regions during human brain development. The extent to which glutamate could enhance [3H]MK801 binding became significantly lower in prefrontal and motor cortex as the third trimester of gestation progressed, so that at term, little activation was apparent. In occipital and temporal cortex, this parameter was low throughout the third trimester. The glutamate-mediated enhancement of [3H]MK801 binding was significantly different in the adult as compared with the infant, suggesting that a different form of the glutamate-NMDA receptor was present in the immature brain.
The affinity of [3H]flunitrazepam for the GABAA receptor binding site and the number of receptor sites present in the human cortex did not differ during brain development. [3H]diazepam KD showed marked regional and developmental variations: infant tissues showed a distinctly lower affinity than adults for this ligand. The [3H]diazepam receptor density continued to increase during gestation and posmatal development, but declined by the time full maturation of the brain had occurred. Data on the GABA activation of [3H]diazepam binding indicated that significant differences in the form of the receptor occurred during gestational, during postnatal development, and in adult subjects.
Adenosine A2a receptor sites in human adult caudate showed similar characteristics to those in rat striatum. However, higher KD values were found in human neonatal caudate than in adult cases, indicating that the receptor type may be different in neonates. While the affinity for the adenosine A1 receptor site did not vary during gestational development, postnatal cases had a lower affinity (higher KD values), suggesting that the form of the receptor is changing during the first year of life. The amount of adenosine A1 sites in the frontal cortex also increased during brain development.
Due to small case numbers the studies on SIDS cases are only preliminary. Overall, only minor differences in the three neurotransmitter systems were found. Adenosine A1 receptors did not differ from those in controls, while the glutamate-NMDA system showed that SIDS infants had a lower response to glutamate. In the temporal cortex of SIDS infants there was an overall higher number of GABAA receptor sites when compared with controls. These receptor systems need to be further investigated before they can be ruled out as playing no part in the mechanism of SIDS.
The glutamate-NMDA, GABAA and adenosine A1 receptors were examined in newborn infants dying between 22 weeks' gestation and term, either from unexpected acute cerebral hypoxia, or from other non-cerebral conditions incompatible with life. The lack of correlation between either glutamate-NMDA or GABAA receptor density and the degree of hypoxia, as assessed pathologically, suggested that receptor differences preceded the hypoxic episode. The receptor affinity was found to be invariant in the three receptor systems when asphyxia cases were compared with controls. For the glutamate- NMDA receptor, term control cases were shown to have more receptor sites that preterm controls in the frontal cortex, whereas vulnerable cases did not exhibit this difference, suggesting that perinatal asphyxia cases may fail to accumulate optimal densities of the glutamate-NMDA receptor by term. The GABAA and adenosine A1 receptors showed no significant variation in receptor density between perinatal asphyxia cases and controls. Glutamate enhancement of [3H]MK801 and GABA enhancement of [3H]diazepam did not vary in three of the four cortical areas across case-groups. In hypoxia cases, the temporal region, showed a significant difference in the Group X Dose-response Interaction in both receptors. GABA-mediated transmission acts to counterbalance the excitatory effects of glutamate, so these variations in the functionalities of the receptor systems could affect the excitation/inhibition balance in the brain. In consequence, the infant would be places at a heightened risk should asphyxia occur.