Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part II. Age-associated alterations in serotonin receptor binding profiles within medullary nuclei supporting cardiorespiratory homeostasis

Abstract

The failure of chemoreflexes, arousal, and/or autoresuscitation to asphyxia may underlie some sudden infant death syndrome (SIDS) cases. In Part I, we showed that some SIDS infants had altered 5-hydroxytryptamine (5-HT)2A/C receptor binding in medullary nuclei supporting chemoreflexes, arousal, and autoresuscitation. Here, using the same dataset, we tested the hypotheses that the prevalence of low 5-HT1A and/or 5-HT2A/C receptor binding (defined as levels below the 95% confidence interval of controls-a new approach), and the percentages of nuclei affected are greater in SIDS versus controls, and that the distribution of low binding varied with age of death. The prevalence and percentage of nuclei with low 5-HT1A and 5-HT2A/C binding in SIDS were twice that of controls. The percentage of nuclei with low 5-HT2A/C binding was greater in older SIDS infants. In >80% of older SIDS infants, low 5-HT2A/C binding characterized the hypoglossal nucleus, vagal dorsal nucleus, nucleus of solitary tract, and nuclei of the olivocerebellar subnetwork (important for blood pressure regulation). Together, our findings from SIDS infants and from animal models of serotonergic dysfunction suggest that some SIDS cases represent a serotonopathy. We present new hypotheses, yet to be tested, about how defects within serotonergic subnetworks may lead to SIDS.

Keywords: Autoradiography; Blood pressure recovery; Gasping; Hypoxia; Inferior olive; Nucleus of the solitary tract; Raphe.

Figure 1.

The diagram illustrates the different laboratory databases utilized in this study and the numbers of SIDS and control cases within each database. The individual databases are comprised of infants with 5-HT_1A or 5-HT_2A/C binding data from receptor ligand autoradiography. The combined database is comprised of infants that have both 5-HT_1A and 5-HT_2A/C binding data. Created with Biorender.com.

Figure 2.

Representative autoradiograms of ¹²⁵I-DOI binding to 5-HT_2A/C receptors and ³H-DPAT binding to 5-HT_1A receptors in a 53 postconceptional week SIDS infant. Mid and rostral levels of the medulla are shown and measured nuclei are labeled. A representation of a radioactivity standard is shown with fentamol/mg (fmol/mg) given from high binding (red) to low binding (dark blue). Binding to 5-HT_2A/C receptors is heavily concentrated in the reticular formation, that is IRZ, PGCL, and GC, in the rostral medulla compared to low binding in the reticular formation of the mid-medulla. This finding is relevant to the chemoarchitecture of gasping because the 5-HT_2A receptor is essential for gasping. HG, hypoglossal nucleus; NTS, nucleus of the solitary tract; DMX, dorsal motor nucleus of vagus; PIO, principal inferior olive; MAO, medial accessory olive; DAO, dorsal accessory olive; RO, raphe obscurus; GC, nucleus gigantocellularis; IRZ, intermediate reticular zone; PGCL, nucleus paragigantocellularis lateralis. The figure is reproduced from Haynes et al (47) with modifications.

Figure 3.

Examples of binding data plotted versus postconceptional age for ^I25I-DOI binding to 5-HT_2A/C receptors and ³H-DPAT binding to 5-HT_1A receptors in the raphe obscures (ROb/RMg). Controls are shown as gray circles, SIDS infants who statistically have normal binding are shown as blue circles, and SIDS infants who have low binding are shown as red circles. Because the definition of low binding adjusted for laboratory dataset (see Statistical analysis of low receptor binding section), the plots show binding adjusted by dataset for ease of visualization.

Figure 4.

Graphs of percentages of SIDS infants with low-binding values for 5-HT_1A and 5-HT_2A/C by nuclei in SIDS (top) compared to controls (bottom). Nuclei are separated by “source nuclei” (nuclei containing 5-HT cell bodies) and “target nuclei” (nuclei containing 5-HT terminals without 5-HT cell bodies). The PIO was not measured for 5-HT_1A. RO, raphe obscurus; GC, nucleus gigantocellularis; PGCL, nucleus paragigantocellularis lateralis; IRZ, intermediate reticular zone; HG, hypoglossal nucleus; DMX, dorsal motor nucleus of vagus; NTS, nucleus of the solitary tract; MAO, medial accessory olive; DAO, dorsal accessory olive; PIO, principal inferior olive. Created with Biorender.com.

Figure 5.

Graphs of percentages of nuclei defined as low binding in SIDS (left) compared to controls (right) for 5-HT_1A and 5-HT_2A/C. The infants are separated into bins based on postconceptional (PC) age; postnatal (PN) age ranges are given for reference. The numbers in each group are indicated above the bar. ROb/RMg, raphe obscurus; GC, nucleus gigantocellularis; PGCL, nucleus paragigantocellularis lateralis; IRZ, intermediate reticular zone; HG, hypoglossal nucleus; DMX, dorsal motor nucleus of vagus; NTS, nucleus of the solitary tract; MAO, medial accessory olive; DAO, dorsal accessory olive; PIO, principal inferior olive. Created with Biorender.com.

Figure 6.

The percentages of infants with low binding for 5-HT_1A and 5-HT_2A/C in individual nuclei are shown in SIDS (left) compared to controls (right) and divided by age bins (Early, Mid, and Late). Nuclei are separated by “source nuclei” (nuclei containing 5-HT cell bodies) and “target nuclei” (nuclei containing 5-HT terminals without 5-HT cell bodies). The PIO was not measured for 5-HT_1A. ROb/RMg, raphe obscurus; GC, nucleus gigantocellularis; PGCL, nucleus paragigantocellularis lateralis; IRZ, intermediate reticular zone; HG, hypoglossal nucleus; DMX, dorsal motor nucleus of vagus; NTS, nucleus of the solitary tract; MAO, medial accessory olive; DAO, dorsal accessory olive; PIO, principal inferior olive. Created with Biorender.com.

Figure 7.

Schematic diagram of vestibular nucleus (VN), nucleus of solitary tract (NTS), medial accessory olive (MAO), climbing fiber (CF), Purkinje cells (PC), fastigial nuclei (FN), and gigantocellularis (GC) circuitry that underlie dampening and recovery for blood pressure changes signaled by the NTS and VN. Change in body position (e.g. prone versus supine), or factors inducing shock are mediated by the VN and NTS. Signals are transmitted to MAO neurons and then to Purkinje cells, via climbing fibers, that project to FN. FN induces changes in autonomic motor output, body movement, arousal and upper airway tone via projections to GC and HG, among others. The circuitry is also sensitive to chemoreceptor activation, with the FN terminating prolonged apnea periods. Created with Biorender.com.

Figure 8.

Schematic representation of regional involvement of sites (nuclei) with abnormal 5-HT_1A and/or 5-HT_2A/C binding in SIDS infants compared to autopsy controls. Notably all of the involved sites are involved in protective responses to asphyxia and modulated by 5-HT in the putative subset of SIDS representing a serotonopathy (see text). The key site in SIDS infants is the mid-to-rostral medulla (rectangle), a critical (“segmental”) feature of the 5-HT-related pathology uncovered by us in SIDS over at least 2 decades of research, including in Parts I and II. The nuclei with abnormal 5-HT receptor binding in the SIDS cases are denoted in green. The RVLM is included in this regional tissue “block” (rectangle) of the hindbrain that is affected in the SIDS and includes the anatomic loci of the putative human homologue of the pre-Bötzinger complex, intermediate reticular zone (IRZ), paragigantocellularis lateralis (PGCL), and more medially, gigantocellularis (GC). The rostral medulla also contains the major 5-HT-synthesizing (source) neurons in the caudal (medullary [as opposed to the rostral (mesopontine dorsal and median raphe)]) domains of the brainstem serotoninergic system, that is caudal raphe, IRZ, PGCL, and GC. The caudal raphe includes the raphe obscurus, raphe magnus, and raphe pallidus. Of note, the source neurons and other affected (non-source) nuclei in SIDS cases (i.e. hypoglossal nucleus [HG], dorsal motor nucleus of the vagus [DMX], nucleus of the solitary tract [NTS], and inferior olivary complex [IOC]), receive 5-HT (target) projections, albeit not exclusively in the entire brainstem or forebrain (not shown). The non-HT-source (target) nuclei of the HG, NTS, DMX, and IOC all demonstrate abnormal 5-HT_1A and/or 5-HT_2A/C receptor binding in the SIDS cases, anatomic portions of which are included in the affected hindbrain segment (rectangle) of the rostral medulla. The medial accessory olive (MAO), which is a critical component of the affected olivocerebellar circuit involved in blood pressure recovery (see text), is included in the IOC. The major sites involved in chemosensory processing are highlighted with notation in blue asterisk for central and peripheral oxygen (O₂) (hypoxia) sensors and black asterisk for carbon dioxide (CO₂) (hypercapnia) sensors. Peripheral chemoreceptors are located in the carotid body, and peripheral baroreceptors in the carotid sinus and aortic arch. Important sites of chemosensory circuitry in the abnormal hindbrain segment in the SIDS cases (rectangle), support their role in the putative defective defense responses in SIDS. The retrotrapezoid nucleus, known to be essential for brainstem CO₂ chemosensitivity (98) is not shown in this diagram because the anatomic locus of its site in humans is not resolved, and thus receptor autoradiography and other studies have yet to be performed in SIDS versus controls. This schematic representation makes the key points of the results of Parts I and II in SIDS cases that brainstem: (1) serotoninergic-related pathology reported to date by us appears to be concentrated in the mid-to-rostral medulla; (2) is segmentally constricted in the medullary hindbrain; and (3) involves essential anatomic loci that mediate serotoninergic defense responses in arousal, chemosensitivity, autoresuscitation, and cardiopulmonary reflexes, many of which are operative during a sleep period. Other abbreviations include: PoO, pontis oralis; LDT, lateral dorsal tegmentum; PPN, pedunculopontine nucleus; PoC, pontis caudalis; LC, locus coeruleus; FN, fastigial nuclei. Created with Biorender.com.