Biochemical changes, early brain growth suppression and impaired detour learning in nicotine-treated chicks

Fetal growth suppression associated with chronic maternal intake of cigarette smoke is frequently observed in humans and studies using animal models suggest that in utero nicotine exposure is an important component of this growth suppression. The developing fetal central nervous system (CNS) is sensitive to the growth inhibitory effect of nicotine and morphological as well as functional CNS deficits may result from fetal nicotine exposure. The studies presented here show that nicotine exposure during early embryonic development ultimately inhibits the ability of 7-11 day old chicks to learn a detour task. The brain growth suppression caused by nicotine is paralleled by a failure of the early embryo brain to express the normal developmental increase in ornithine decarboxylase (ODC) activity. This biochemical change may be germane to the mechanism of nicotine-induced growth inhibition and/or nicotine-induced behavioral changes because the appropriate expression of ODC activity is essential to normal growth and differentiation in the fetal CNS. In the chick embryo, nicotine exposure alters several important signaling pathways that regulate ODC expression. For example, nicotine exposure lowers embryonic brain glucose levels and causes significant decreases in whole brain cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels and in cyclic AMP binding proteins (protein kinase-A regulatory activity). Also, in cultured chick cells, nicotine inhibits the ability of a potent mitogen (insulin) to induce ODC activity, but, paradoxically, in ovo nicotine exposure increased insulin binding and stimulated insulin receptor autophosphorylation in brain membranes.