Effects of sex and chronic neonatal nicotine treatment on Na2+/K+/Cl− co-transporter 1, K+/Cl− co-transporter 2, brain-derived neurotrophic factor, NMDA receptor subunit 2A and NMDA receptor subunit 2B mRNA expression in the postnatal rat hippocampus

Abstract

Chronic exposure to nicotine during the first postnatal week in rats, a developmental period that corresponds to the third trimester of human gestation, results in sexually dimorphic long-term functional defects in the adult hippocampus. One potential cause could be the sex-specific differences in the maturation of GABAA receptor-mediated responses from excitatory to inhibitory, which depends on the expression of the Na2+/K+/Cl−-co-transporter NKCC1 and the K+/Cl− co-transporter KCC2. In the rat hippocampus, this switch occurs during the first and second postnatal week in females and males, respectively, and is regulated by nicotinic receptor activation. Excitatory GABAergic signaling can increase BDNF expression, which might exacerbate sex differences by impacting synaptogenesis. We hypothesized that chronic neonatal nicotine (CNN) exposure differentially regulates the expression of these co-transporters and BDNF in males and females. We use quantitative isotopic in situ hybridization to examine the expression of mRNAs for NKCC1, KCC2, BDNF, and NMDA receptor subunits NR2A and NR2B in the postnatal day (P) 5 and 8 rat hippocampus in both sexes that were either control-treated or with 6 mg/kg/day nicotine in milk formula (CNN) via gastric intubation starting at P1. In line with prolonged GABAergic excitation, we found that at P5 males had significantly higher mRNA expression of NKCC1 and BDNF than females. CNN treatment resulted in a significant increase in KCC2 and BDNF mRNA expression in male but not female hippocampus (p<0.05). Males also had higher expression of NR2A and lower expression of NR2B at P5 compared to females (p<0.05). At P8, there were neither sex nor treatment effects on mRNA expression, indicating the end of a critical period for sensitivity to nicotine. These results suggest that differential maturation of GABAAR-mediated responses result in sex-specific sensitivity to nicotine during early postnatal development, potentially explaining the differential long-term effects of CNN on hippocampal function.

Sex differences and estrogen regulation of BDNF gene expression, but not propeptide content, in the developing hippocampus

“…Damborsky and Winzer-Serhan (2012) reported neonatal male rats have higher hippocampal Bdnf gene expression in dentate, CA1 and CA3 subregions at PN5, but not PN8. This is largely consistent with our observations with the exception of CA3, where we did not detect sex differences.…”

“…In contrast to long-standing interest in the role of BDNF in hippocampal plasticity and neurogenesis in adults, particularly with regard to effects of estrogen, far fewer studies have examined BDNF expression in the developing hippocampus. Damborsky and Winzer-Serhan (2012) reported that neonatal male rats have higher hippocampal BDNF gene expression in dentate, CA1, and CA3 subregions at PN5, but not PN8. This is largely consistent with our observations with the exception of CA3, where we did not detect sex differences.…”

Postnatal alterations in GABAB receptor tone produce sensorimotor gating deficits and protein level differences in adulthood

“…As previously outlined, immature neurons have a high intracellular concentration of Cl − and GABA induces a depolarizing effect via GABA A Rs. A transition occurs in early development that alters Cl − co-transporters, resulting in greater extracellular Cl − concentrations and enables the activation of GABA A Rs to induce a hyperpolarizing effect (Payne et al, 1996;Rivera et al, 1999;Ben-Ari, 2002;Galanopoulou, 2008;Damborsky and Winzer-Serhan, 2012). The expression of these co-transporters occur at different times in males and females, which results in an earlier hyperpolarizing effect of GABA A Rs in females as compared to males (Nunez and McCarthy, 2007;Galanopoulou, 2008;Damborsky and Winzer-Serhan, 2012).…”

Sex‐dependent effects of nicotine on the developing brain

“…Nicotine exposure during the early neonatal period of hippocampal maturation induces increased expression of the KCC2 cotransporter in male pups, a change that may influence the timing of the switch in GABA transmission from excitatory to inhibitory (Figure 3; Damborsky and Winzer-Serhan 2012). The long-term consequence of neonatal nicotine exposure is enhanced excitation in the adult hippocampal CA1, which is greater in males than females (Damborsky et al 2012).…”