Luciana A. Rava

Decreased Kainate Receptor Binding in the Arcuate Nucleus of the Sudden Infant Death Syndrome

The human arcuate nucleus is postulated to be homologous to ventral medullary surface cells in animals that participate in ventilatory and blood pressure responses to hypercarbia and asphyxia. Recently, we reported a significant decrease in muscarinic cholinergic receptor binding in the arcuate nucleus in victims of the sudden infant death syndrome compared with control patients that died of acute causes. To test the specificity of the deficit to muscarinic cholinergic binding, we examined kainate binding in the arcuate nucleus in the same database. We assessed 3H-kainate binding to kainate receptors with tissue receptor autoradiography in 17 brainstem nuclei. Analysis of covariance was used to examine differences in binding by diagnosis, adjusted for postconceptional age (the covariate). Cases were classified as SIDS, 47; acute control, 15; and chronic group with oxygenation disorder, 17. (Acute controls are infants who died suddenly and unexpectedly and in whom a complete autopsy established a cause of death). The arcuate nucleus was the only region in which there was a significant difference in the age-adjusted mean kainate binding between the SIDS group (37 ± 2 fmol/mg tissue) and both the acute controls (77 ± 4 fmol/mg tissue) (p < 0.0001) and the chronic group (69 ± 4 fmol/mg tissue) (p < 0.0001). There was a positive correlation between the density of muscarinic cholinergic and kainate binding in the SIDS cases only (R = 0.460; p = 0.003). The neurotransmitter deficit in the arcuate nucleus in SIDS victims involves more than one receptor type relevant to carbon dioxide and blood pressure responses at the ventral medullary surface.

Anatomic distribution of the growth-associated protein GAP-43 in the developing human brainstem.

GAP-43 is a membrane phosphoprotein whose expression is high in neurons undergoing development or remodeling or axonal connections. This study used a monospecific antibody to GAP-43 to investigate the sequences of fiber tract elongation and synaptic development in the human brainstem. Immunocytochemistry was performed in 14 fetal and infant brainstems; two child and adult cases were also examined for comparison. At midgestation, GAP-43 immunostaining was moderately intense across nuclei and fiber tracts, except for the corticospinal tract, where levels were higher, and cranial nerve nucleus VII, superior olive, inferior colliculus, inferior olivary hilum, inferior cerebellar peduncle, medial lemniscus, and medial longitudinal fasciculus, where staining was nearly absent. By the end of the neonatal period, the relative distribution of GAP-43 immunostaining appeared well-established and similar, although not identical, to that in the child and adult brainstem. Immunostaining was absent or negligible in almost all the cranial nerve somato- and branchiomotor nuclei, auditory-relay nuclei, and vestibular nuclei, while remaining intense in visceral-related nuclei, reticular formation, cochlear nucleus, and periaqueductal gray. Staining was also virtually absent in all fiber tracts at birth, except for the corticospinal tract and central tegmental tract. Persistence of GAP-43 staining in the corticospinal tract past the fetal period suggests that this tract remains in a plastic state beyond initial axonal elongation. Intense immunostaining in visceral-related nuclei into adulthood suggests that these regions may continue to undergo synaptic reorganization. This study provides baseline information relevant to understanding developmental brainstem disorders in early human life.

Tritiated-naloxone binding to brainstem opioid receptors in the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is defined as the sudden death of an infant under 1 year of age that remains unexplained after a thorough case investigation, including a complete autopsy. We hypothesized that SIDS is associated with altered 3H - naloxone binding to opioid receptors in brainstem nuclei related to respiratory and autonomic control. We analyzed 3H - naloxone binding in 21 regions in SIDS and control brainstems using quantitative tissue receptor autoradiography. Three groups were analyzed: SIDS (n = 45); acute controls (n = 14); and a chronic group with oxygenation disorders (n = 15). Opioid binding was heavily concentrated in the caudal nucleus of the solitary tract, nucleus parabrachialis medialis, spinal trigeminal nucleus, inferior olive, and interpeduncular nucleus in all cases analyzed (n = 74). The arcuate nucleus on the ventral medullary surface contained negligible binding in all cases (n = 74), and therefore binding was not measurable at this site. We found no significant differences among the three groups in the age-adjusted mean 3H - naloxone binding in 21 brainstem sites analyzed. The only differences we have found to date between SIDS and acute controls are decreases in 3H - quinuclidinyl benzilate binding to muscarinic cholinergic receptors and in 3H - kainate binding to kainate receptors in the arcuate nucleus in alternate sections of this same data set. The present study suggests that there is not a defect in opioid receptor binding in cardiorespiratory nuclei in SIDS brainstems.

Developmental changes in heterogeneous patterns of neurotransmitter receptor binding in the human interpeduncular nucleus

The interpeduncular nucleus (IPN) exhibits many complex features, including multiple subnuclei, widespread projections with the forebrain and brainstem, and neurotransmitter heterogeneity. Despite the putative importance of this nucleus, very little is known about its neurochemical development in the human. The human IPN is cytoarchitectonically simple, unlike the rat IPN, which displays considerable heterogeneity. In the following study, we hypothesized that the developing human IPN is neurochemically heterogeneous despite its cytological simplicity. The chemoarchitecture in this study was defined by neurotransmitter receptor binding patterns by using quantitative tissue autoradiography for the muscarinic, nicotinic, serotoninergic, opioid, and kainate receptors. We examined neurotransmitter receptor binding in the developing human IPN in a total of 15 cases. The midbrains of five midgestational fetuses (19–26 gestational weeks) and six infants (38–74 postconceptional weeks) were examined. The midbrain of one child (4 years) and three adults (20–68 years) were analyzed as indices of maturity. At all ages examined, high muscarinic binding was localized to the lateral subdivision of the IPN, high serotoninergic binding was localized to the dorsal IPN, and high opioid receptor binding was localized to the medial IPN. The developmental profile was unique for each radioligand. We report a heterogenous distribution of neurotransmitter receptor binding in the developing human IPN, which supports a complex role for it in human brain function. J. Comp. Neurol. 390:322–332, 1998.