Cigarette smoke exposure and hypoxic effects on the expression of nicotinic acetylcholine receptors and apoptosis in the developing brain.

Abstract

Introduction: Cigarette smoke exposure (CSE) during pregnancy and postpartum poses a significant threat to the infant’s health, such as respiratory and ear infections, cognitive deficits, as well as an increased risk for sudden infant death syndrome (SIDS). Nicotine, the major neurotoxic agent from cigarette smoke induces its action by binding to nicotinic acetylcholine receptors (nAChRs) with one downstream consequence being increased cell death (apoptosis). Further, recent studies on the structural abnormalities in the SIDS hippocampus interested us to study the proinflammatory cytokine, IL-1b. The objectives of the thesis were to determine the alterations in expression of nAChRs and apoptotic markers (active caspase-3 (Casp-3) and TUNEL) in the hippocampus and brainstem following: a) CSE in mice, b) postnatal nicotine exposure alone or in combination with c) intermittent hypercapnic hypoxia (IHH) (acute vs repeated) in piglets, and d) in the human infant hippocampus and determine any changes according to the diagnosis of SIDS, and the presence of the risk factors of CSE and prone sleeping. Methods: nAChR subunits and IL-1b expression were determined using single immunohistochemistry (IHC), apoptotic makers (Casp-3 and TUNEL) were determined using double IHC labelling on formalin fixed paraffin embedded tissue. Results: Pre- into post-natal CSE led to alterations in nAChR subunits with simultaneous changes in apoptotic markers in mice, postnatal nicotine affected the nAChRs more severely in the brainstem than hippocampus with predominant changes being for the a3 and b1 subunits, which were also more sensitive to pre- into post-natal CSE. Repeated IHH induced much more alterations in nAChRs (predominantly the a2b2 complex) than acute IHH in both the hippocampus and brainstem medulla. SIDS infants exhibited higher TUNEL in the subiculum which was associated with CSE, higher IL-1b in the CA1 region which was associated with prone sleeping position, and lower a7 expression in the dentate gyrus and CA4. Conclusion: Alterations in the expression of nAChR subunits due to CSE, nicotine and hypoxia, are evident in the developing hippocampus and brainstem, and were associated with increased apoptosis in a region specific manner. These results provide an indication of the mechanism(s) via which CSE to an infant affects the nAChRs in the brain and the need to continue with the health campaign advising against CSE.