Decreased GABAA receptor binding in the medullary serotonergic system in the sudden infant death syndrome

Abstract

γ-Aminobutyric acid (GABA) neurons in the medulla oblongata help regulate homeostasis, in part through interactions with the medullary serotonergic (5-HT) system. Previously, we reported abnormalities in multiple 5-HT markers in the medullary 5-HT system of infants dying from sudden infant death syndrome (SIDS), suggesting that 5-HT dysfunction is involved in its pathogenesis. Here, we tested the hypothesis that markers of GABAA receptors are decreased in the medullary 5-HT system in SIDS cases compared with controls. Using tissue receptor autoradiography with the radioligand H-GABA, we found 25% to 52% reductions in GABAA receptor binding density in 7 of 10 key nuclei sampled of the medullary 5-HT system in the SIDS cases (postconceptional age [PCA] = 51.7 ± 8.3, n = 28) versus age-adjusted controls (PCA = 55.3 ± 13.5, n = 8) (p ≤ 0.04). By Western blotting, there was 46.2% reduction in GABAAα3 subunit levels in the gigantocellularis (component of the medullary 5-HT system) of SIDS cases (PCA = 53.9 ± 8.4, n = 24) versus controls (PCA = 55.3 ± 8.3, n = 8) (56.8% standard in SIDS cases vs 99.35% in controls; p = 0.026). These data suggest that medullary GABAA receptors are abnormal in SIDS infants and that SIDS is a complex disorder of a homeostatic network in the medulla that involves deficits of the GABAergic and 5-HT systems.

Figure 1

γ-Aminobutyric acid_A (GABA_A) receptor binding in a SIDS case and control. (A, B) Illustrative autoradiograms displaying ³H-GABA binding to GABA_A receptors in tissue sections at mid- (level of hypoglossal nucleus) and rostral (level of the prepositus nucleus) sections of the medulla from a SIDS infant at 44 postconceptional weeks (B), and a control infant with an acute cause of death at 40 postconceptional weeks (A). The density of GABA_A receptor binding in the medullary 5-HT system is lower in the SIDS case, except for the arcuate nucleus in which the binding levels are similar. Abbreviations: ARC, arcuate nucleus; HG, hypoglossal nucleus; NTS, nucleus of the solitary tract; GC, gigantocellularis; PGCL, paragigantocellularis; PIO, principal inferior olive; RO, raphé obscurus.

Figure 2

γ-Aminobutyric acid_A (GABA_A) receptor binding in the medulla and cerebellum. (A, B) Binding in the medullary regions is lower in the 44 postconceptional weeks SIDS case (B) vs. the 44 postconceptional weeks control (A), except in the arcuate nucleus in which the binding levels are similar. Fragments of cerebellar tissue (dotted lines) serve as an internal control of receptor binding levels. There is no difference between the cases in binding levels in the cerebellar tissue. ARC, arcuate nucleus; HG, hypoglossal nucleus; NTS, nucleus of the solitary tract; PIO, principal inferior olive; RO, raphé obscurus.

Figure 3

Western blot analysis of levels of γ-Aminobutyric acid_Aa3 (GABA_Aa3) receptor subunit in the gigantocellularis. A representative Western blot of the GABA_Aα3 receptor subunit demonstrates differences among SIDS cases, an acute control (AC), and a control with chronic illness (CI). Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is used as a loading control and shows little or no difference in abundance levels among the SIDS cases and controls.