Cell death in the lateral geniculate nucleus, and its possible relationship with nicotinic receptors and sudden infant death syndrome (SIDS)

Abstract

The role of the lateral geniculate nucleus (LGN) in vision has been extensively studied, yet its extraretinal capacities are still being investigated, including its role in arousal from sleep. The β2 nicotinic acetylcholine receptor (nAChR) subunit is involved in the laminal organisation of the LGN with magnocellular (MC) and parvocellular (PC) neurons. Sudden infant death syndrome (SIDS) occurs during a sleep period and, neuropathologically, is associated with increased neuronal cell death and altered nAChRs. A recent qualitative pilot study from our group implicates the possibility of increased neuronal death/apoptosis in the SIDS LGN. The present study used quantitative analysis to report the baseline expression of apoptotic and nAChR subunits α7 and β2 in the PC and MC layers of the LGN, to determine correlations amongst these markers within layers and across layers, and to evaluate changes in the expression of these markers in the LGN of SIDS infants, along with associations with SIDS risk factors, such as age, sex, cigarette smoke exposure, bed-sharing, and presence of an upper respiratory tract infection (URTI). Tissue was immunohistochemically stained for cell death markers of active caspase-3 (Casp-3) and TUNEL, and for the α7 and β2 nAChR subunits. Amongst 43 cases of sudden and unexpected deaths in infancy (SUDI), classifications included explained deaths (eSUDI, n = 9), SIDS I (n = 5) and SIDS II (n = 29). Results indicated a strong correlation of the apoptotic markers and β2 nAChR subunit between the LGN layers, but not across the markers within the layers. Amongst the diagnostic groups, compared to eSUDI, the SIDS II cases had decreased Casp-3 expression while β2 nAChR expression was increased in both PC and MC layers. Amongst the SIDS risk factors, URTI and bed-sharing were associated with changes in neuronal death but not in the α7 and β2 markers. In conclusion, our findings do not support a role for the α7 and β2 nAChRs in apoptotic regulation of the LGN layers during infancy. However, for SIDS victims, an inverse correlation between the changes for markers of apoptosis and the β2 nAChR subunit expression suggests altered LGN function.

Keywords: Acetylcholine; Apoptosis; Cholinergic; LGN; SUDI; Sleep.

Fig. 1

Microscopic morphology of the LGN layers and cells obtained by Cresyl Violet staining. (A) Micrograph illustrating the location of the LGN in coronal section at level 43 of Atlas of the Human Brain by Mai et al., 1997 and its relation to the hippocampus dentate gyrus (DG), commencement of the Cornu ammonis 3 (CA3), and fimbria. (B) The six laminae are indicated. 1-2 represent the magnocellular layer (MC) and 3-6 represent the parvocellular layer (PC), in between each layer are koniocellular neurons. (C, D & E) are magnifications of the (C) Magnocellular cells, (D) Koniocellular cells, and (E) Parvocellular cells and the boxed C & E in panel B indicate the approximate locations from where images for analysis were obtained. Scale bar represents 2.0cm for A, 1.5cm for B, and 200μm for C-E

Fig. 2

Immunostaining for the markers quantified. (A-B) TUNEL (thick arrows, positive is brown nucleus) and Casp-3 (thin arrows, positive is blue cytoplasm), (C-D) α7 nAChR subunit, and (E-F) β2 nAChR subunit. Solid black arrows represent positive neurons and white arrows represent negative neurons. Scale bar represents 100μm for all images