Developmental Hippocampal Neuroplasticity in a Model of Nicotine Replacement Therapy during Pregnancy and Breastfeeding

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

Cellular plasticity in limbic brain regions of suicides

Considerable evidence implicates adult hippocampal neurogenesis in the aetiology of depression and in antidepressant efficacy, and recent postmortem studies support the hypothesis that depressed suicides display reduced neuronal proliferation in the dentate gyrus. However, at the time these studies began relatively little was known about the contributions of structural plasticity and neurogenesis in other human brain regions to depression and suicide. Therefore, the work contained herein was completed with the goal of exploring whether cellular plasticity in limbic regions implicated in affective regulation and learning, namely the amygdala and subventricular-olfactory bulb system, was altered in depressed suicides. These studies reveal that deficits in cellular plasticity extend well beyond the hippocampus among individuals who die by suicide. Subventricular zone-derived neuroblasts of depressed suicides appear to display a migratory deficit resulting in the ectopic differentiation of doublecortin-immunoreactive cells along the rostral migratory stream. In the basolateral amygdala, expression of doublecortin and polysialylated neural cell adhesion molecule—two proteins associated with hyperplasticity—is increased only among depressed non-suicides, suggesting that depressed suicides display a deficit in certain forms of compensatory neuroplasticity. Among depressed suicides, the relationship between doublecortin and glial cell line- derived neurotrophic factor (GDNF) signaling in the basolateral amygdala is altered as well, which appears to reflect modified isoform-specific regulation of GDNF receptors by microRNAs. Taken together, this work supports the hypothesis that depression and, to an even greater extent, suicide is associated with wide-ranging deficits in cellular plasticity. Although the affected neurons seem to differ in terms of age and location, these hyperplastic limbic system cells seem to share a common set of impairments associated with an altered capacity for structural remodeling. This work further suggests that changes in neuronal responses to neurotrophin signaling may contribute to the problem, and implicate dysregulation at the level of protein, mRNA, and microRNA expression in these deficits.De nombreuses études impliquent la neurogenèse hippocampique adulte dans l'étiologie de la dépression et l'efficacité des antidépresseurs. De récentes études post mortem soutiennent l'hypothèse que la prolifération neuronale au sein du gyrus denté soit réduite chez les suicidés ayant souffert de dépression. Nous ne savons toutefois presque rien au sujet de la possible implication, dans la dépression et le suicide, de la neurogenèse et, plus largement, de la plasticité cellulaire au sein d’autres régions du cerveau humain. Par conséquent, le travail présenté dans cette thèse fut réalisé dans le but d'examiner si la plasticité cellulaire dans l'amygdale et dans le système ventriculaire-olfactif, deux régions limbiques impliquées dans la régulation affective et l'apprentissage, est modifiée dans la dépression et le suicide. Mes recherches ont révélé que des déficits de plasticité cellulaire s’étendent bien au-delà de l'hippocampe chez les personnes qui se suicident. Ainsi, les neuroblastes dérivés de la zone sous-ventriculaire chez les suicidés dépressifs semblent présenter un déficit migratoire menant à une différenciation ectopique de cellules doublecortine-immunoréactives le long du courant migratoire rostral. Dans l'amygdale basolatérale, l’expression de doublecortine et de la molécule d'adhérence cellulaire neurale polysialylée—deux protéines associées à l’hyperplasticité—est augmentée seulement au sein d’échantillons cérébraux de dépressifs ne s’étant pas suicidés (morts accidentelles) par comparaison à des témoins appariés. Ces données suggèrent que les dépressifs suicidés présentent un déficit au niveau de certaines mécanismes de neuroplasticité compensatoire. Parmi les dépressifs suicidés, la relation entre la doublecortine et la signalisation du facteur neurotrophique dérivé des cellules gliales (GDNF) dans l'amygdale basolatérale est elle aussi altérée. Plus spécifiquement, nous avons trouvé que la régulation par microARN de certains isoformes du récepteur principal du GDNF semble s’effectuer différemment chez les suicidés. Pris dans leur ensemble, ces résultats appuient l'hypothèse que la dépression et, dans une plus grande mesure, le suicide, sont associés à d’importants déficits de plasticité cellulaire. Bien que les neurones limbiques affectés diffèrent en termes de maturité et d'emplacement, ils semblent partager un ensemble de déficiences affectant leur capacité à participer à la réorganisation structurelle. En outre, le travail présenté dans cette thèse suggère qu’un changement dans les réponses neuronales au GDNF peut contribuer au déficit de plasticité cérébrale observé dans la dépression et le suicide

Increased doublecortin (DCX) expression and incidence of DCX-immunoreactive multipolar cells in the subventricular zone-olfactory bulb system of suicides

Postmortem studies have confirmed the occurrence of adult hippocampal neurogenesis in humans and implicated this process in antidepressant response, yet neurogenesis in other regions remains to be examined in the context of depression. Here we assess the extent of subventricular zone-olfactory bulb (SVZ-OB) neurogenesis in adult humans having died by suicide. Protein expression of proliferative and neurogenic markers Sox2, proliferating cell nuclear antigen, and doublecortin (DCX) were examined in postmortem SVZ and OB samples from depressed suicides and matched sudden-death controls. In the SVZ, DCX-immunoreactive (IR) cells displayed phenotypes typical of progenitors, whereas in the olfactory tract (OT), they were multipolar with variable size and morphologies suggestive of differentiating cells. DCX expression was significantly increased in the OB of suicides, whereas SVZ DCX expression was higher among unmedicated, but not antidepressant-treated, suicides. Although very few DCX-IR cells were present in the control OT, they were considerably more common in suicides and correlated with OB DCX levels. Suicides also displayed higher DCX-IR process volumes. These results support the notion that OB neurogenesis is minimal in adult humans. They further indicate that the differentiation and migration of SVZ-derived neuroblasts may be altered in unmedicated suicides, leading to an accumulation of ectopically-differentiating cells in the OT. Normal SVZ DCX expression among suicides receiving antidepressants suggests a potentially novel mode of action of antidepressant medication. Given the modest group sizes and rarity of DCX-IR cells assessed here, a larger-scale characterization will be required before firm conclusions can be made regarding the identity of these cells

Plasma corticosterone in pups.

<p>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).</p

Cell proliferation.

<p>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 (<b>A</b>) and P41 (<b>B</b>) offspring. No significant difference in numbers of Ki67-IR cells was found in the dorsal hippocampus of P15 (<b>C</b>; p = 0.70) or P41 (<b>D</b>; p = 0.17) offspring. Scale bar = 25 µm.</p

Long-term potentiation.

<p>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 <i>a</i> (before LTP induction) and <i>b</i> (60 min after LTP induction) in representative recordings obtained from saline- and nicotine-exposed pups (% potentiation at 60 minutes after TBS: p = 0.037).</p

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

<p>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.</p

Neuronal differentiation.

<p>(<b>A</b>) 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>B</b>) Orthogonal confocal image of a BrdU/NeuN-labeled DG cell (red: BrdU; green: NeuN). Scale bar = 10 µm.</p