Developmental hippocampal neuroplasticity in a model of nicotine replacement therapy during pregnancy and breastfeeding

Abstract

Rationale: The influence of developmental nicotine exposure on the brain represents an important health topic in light of the popularity of nicotine replacement therapy (NRT) as a smoking cessation method during pregnancy.

Objectives: In this study, we used a model of NRT during pregnancy and breastfeeding to explore the consequences of chronic developmental nicotine exposure on cerebral neuroplasticity in the offspring. We focused on two dynamic lifelong phenomena in the dentate gyrus (DG) of the hippocampus that are highly sensitive to the environment: granule cell neurogenesis and long-term potentiation (LTP).

Methods: Pregnant rats were implanted with osmotic mini-pumps delivering either nicotine or saline solutions. Plasma nicotine and metabolite levels were measured in dams and offspring. Corticosterone levels, DG neurogenesis (cell proliferation, survival and differentiation) and glutamatergic electrophysiological activity were measured in pups.

Results: Juvenile (P15) and adolescent (P41) offspring exposed to nicotine throughout prenatal and postnatal development displayed no significant alteration in DG neurogenesis compared to control offspring. However, NRT-like nicotine exposure significantly increased LTP in the DG of juvenile offspring as measured in vitro from hippocampal slices, suggesting that the mechanisms underlying nicotine-induced LTP enhancement previously described in adult rats are already functional in pups.

Conclusions: These results indicate that synaptic plasticity is disrupted in offspring breastfed by dams passively exposed to nicotine in an NRT-like fashion.

Figure 1. Offspring Weight.

Between postnatal day 2 (P2) and weaning (P21), the average weight of male pups exposed to maternal nicotine did not differ significantly from that of saline-exposed controls (ps = 0.16–0.94).

Figure 2. Plasma nicotine levels in nicotine-exposed dams and their breastfed pups.

Dam nicotine levels increased non-significantly between pregnancy and post-parturition (p = 0.14). E15: embryonic day 15; P7: postnatal day 7 days post-parturition.

Figure 3. Plasma corticosterone in pups.

Basal plasma corticosterone measured in juvenile pups exposed through breastfeeding to nicotine or saline (controls) from early embryogenesis. Corticosterone levels did not differ between groups (p = 0.67).

Figure 4. Cell proliferation.

DG proliferation assessed from Ki67-immunostained brain sections from juvenile (P15) and adolescent (P41) male offspring exposed to saline (controls) or nicotine throughout prenatal and postnatal development. Representative micrographs of Ki67-IR cells (arrows) in the subgranular zone or adjacent granule cell layer (gcl) in control P15 (A) and P41 (B) offspring. No significant difference in numbers of Ki67-IR cells was found in the dorsal hippocampus of P15 (C; p = 0.70) or P41 (D; p = 0.17) offspring. Scale bar = 25 µm.

Figure 5. Cell survival.

Developmental nicotine exposure did not affect survival of DG cells (p = 0.22). Survival of newborn cells was estimated from BrdU-immunostained brain sections in adolescent (P41) offspring exposed to saline (controls) or nicotine from early embryogenesis until weaning and injected with BrdU at P15.

Figure 6. Neuronal differentiation.

(A) Proportion of BrdU-IR cells differentiating into neurons in adolescent (P41) offspring exposed to saline (controls) or nicotine from early embryogenesis until weaning and injected with BrdU at P15 did not differ between groups (p = 0.48). (B) Orthogonal confocal image of a BrdU/NeuN-labeled DG cell (red: BrdU; green: NeuN). Scale bar = 10 µm.

Figure 7. Basal synaptic transmission.

Basal excitatory synaptic transmission was not affected by nicotine exposure. Upper panel: averaged traces of evoked field excitatory postsynaptic potential (fEPSP) recorded from the DG of P15 rat pups. The small depolarization ahead of fEPSP is the fiber volley, which represents the amount of activated presynaptic fibers. Basal excitatory synaptic transmission is therefore related to the ratio of fiber volley vs fEPSP at different stimulating currents (see the four averaged traces). We compared this ratio between control and nicotine-treated pups and found no differences between these two groups (lower panel: comparing slope of regression lines fitting fEPSPs and fiber volley amplitudes between two groups, p = 0.21).

Figure 8. Long-term potentiation.

Long-term potentiation was enhanced by nicotine exposure. Upper panel: Scattered plots of fEPSP against time revealed changes in fEPSP after long-term potentiation (LTP) induction by theta-burst stimulation. Note that LTP triggered in pups exposed to nicotine was stronger than that in control pups. Lower panel: Average traces of fEPSP obtained at time a (before LTP induction) and b (60 min after LTP induction) in representative recordings obtained from saline- and nicotine-exposed pups (% potentiation at 60 minutes after TBS: p = 0.037).