Long-term effects of low prenatal alcohol exposure on GABAergic interneurons of the murine posterior parietal cortex.

BACKGROUND: Fetal alcohol spectrum disorders (FASDs) are characterized by a wide range of physical, cognitive, and behavioral impairments that occur throughout the lifespan. Prenatal alcohol exposure (PAE) can lead to adult impairments in cognitive control behaviors mediated by the posterior parietal cortex (PPC). The PPC plays a fundamental role in the performance of response tasks in both primates and rodents, specifically when choices between similar target and nontarget stimuli are required. Furthermore, the PPC is reciprocally connected with other cortical areas. Despite the extensive literature investigating the molecular mechanisms underlying PAE impairments in cognitive functions mediated by cortical areas, little is known regarding the long-term effects of PAE on PPC development and function. Here, we examined changes in the cellular organization of GABAergic interneurons and their function in PPC using behaviorally naïve control and PAE mice. METHODS: We used a limited access model of PAE in which C57BL/6J females were exposed to a solution of 10% (w/v) ethanol and 0.066% (w/V) saccharin for 4 h/day throughout gestation. Using high-throughput fluorescent microscopy, we quantified the levels of GABAergic interneurons in the PPC of adult PAE and control offspring. In a separate cohort, we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) using whole-cell patch clamp recordings from PPC layer 5 pyramidal neurons. RESULTS: PAE led to a significant overall reduction of parvalbumin-expressing GABAergic interneurons in PAE mice regardless of sex. Somatostatin- and calretinin-expressing GABAergic interneurons were not affected. Interestingly, PAE did not modulate sIPSC amplitude or frequency. CONCLUSIONS: These results suggest that impairments in cognitive control observed in FASD may be due to the significant reduction of parvalbumin-expressing GABAergic interneurons in the PPC. PAE animals may show compensatory changes in GABAergic function following developmental reduction of these interneurons.

Reinforcer value moderates the effects of prenatal alcohol exposure on learning and reversal.

Introduction: Fetal Alcohol Spectrum Disorders (FASD) are the leading cause of preventable developmental disability and are commonly characterized by alterations in executive function. Reversal learning tasks are reliable, cross-species methods for testing a frequently impaired aspect of executive control, behavioral flexibility. Pre-clinical studies commonly require the use of reinforcers to motivate animals to learn and perform the task. While there are several reinforcers available, the most commonly employed are solid (food pellets) and liquid (sweetened milk) rewards. Previous studies have examined the effects of different solid rewards or liquid dietary content on learning in instrumental responding and found that rodents on liquid reward with higher caloric content performed better with increased response and task acquisition rate. The influence of reinforcer type on reversal learning and how this interacts with developmental insults such as prenatal alcohol exposure (PAE) has not been explored. Methods: We tested whether reinforcer type during learning or reversal would impact an established deficit in PAE mice. Results: We found that all male and female mice on liquid reward, regardless of prenatal exposure were better motivated to learn task behaviors during pre-training. Consistent with previous findings, both male and female PAE mice and Saccharine control mice were able to learn the initial stimulus reward associations irrespective of the reinforcer type. During the initial reversal phase, male PAE mice that received pellet rewards exhibited maladaptive perseverative responding whereas male mice that received liquid rewards performed comparable to their control counterparts. Female PAE mice that received either reinforcer types did not exhibit any deficits on behavioral flexibility. Female saccharine control mice that received liquid, but not pellet, rewards showed increased perseverative responding during the early reversal phase. Discussion: These data suggest that reinforcer type can have a major impact on motivation, and therefore performance, during reversal learning. Highly motivating rewards may mask behavioral deficits seen with more moderately sought rewards and gestational exposure to the non-caloric sweetener, saccharine, can impact behavior motivated by those reinforcers in a sex-dependent manner.

Characterization of motor function in mice developmentally exposed to ethanol using the Catwalk system: Comparison with the triple horizontal bar and rotarod tests.

Studies with human subjects indicate that ethanol exposure during fetal development causes long-lasting alterations in motor coordination that are, in part, a consequence of cerebellar damage. Studies with rats exposed to ethanol during the neonatal brain growth spurt have consistently recapitulated these deficits. However, studies with mice have yielded mixed results. We hypothesized that the use of highly sensitive motor function tests, such as the Catwalk test, would reliably detect motor function deficits in mice developmentally exposed to ethanol. Venus-vesicular GABA transporter transgenic mice were ethanol exposed during postnatal days 4-9 using vapor inhalation chambers and then subjected to the Catwalk test during adolescence. Catwalk data were rigorously analyzed using an innovative multistep statistical approach. For comparison, motor coordination and strength were assessed with the triple horizontal bar and rotarod tests. Unexpectedly, we found that out of 186 parameters analyzed in the Catwalk test, only one was affected by ethanol exposure (i.e., reduced coupling between left front paw and the right hind paw). In the triple horizontal bar test, ethanol-exposed mice were able to hold to the bars for less time than controls. Surprisingly, ethanol-exposed mice performed better in the rotarod test than controls. These data indicate that neonatal ethanol exposure of mice causes mixed effects on motor function during adolescence. The Catwalk test suggests that gait is generally preserved in these mice, whereas the triple horizontal bar test revealed deficits on motor strength and the rotarod test an increase in motor coordination.

Neonatal ethanol exposure triggers apoptosis in the murine retrosplenial cortex: Role of inhibition of NMDA receptor-driven action potential firing.

Exposure to ethanol during the last trimester equivalent of human pregnancy causes apoptotic neurodegeneration in the developing brain, an effect that is thought to be mediated, in part, by inhibition of NMDA receptors. However, NMDA receptors can rapidly adapt to the acute effects of ethanol and are ethanol resistant in some populations of developing neurons. Here, we characterized the effect of ethanol on NMDA and non-NMDA receptor-mediated synaptic transmission in the retrosplenial cortex (RSC), a brain region involved in the integration of different modalities of spatial information that is among the most sensitive regions to ethanol-induced neurodegeneration. A single 4-h exposure to ethanol vapor of 7-day-old transgenic mice that express the Venus fluorescent protein in interneurons triggered extensive apoptosis in the RSC. Slice electrophysiological recordings showed that bath-applied ethanol inhibits NMDA and non-NMDA receptor excitatory postsynaptic currents (EPSCs) in pyramidal neurons and interneurons; however, we found no evidence of acute tolerance development to this effect after the 4-h in-vivo ethanol vapor exposure. Acute bath application of ethanol reduced action potential firing evoked by synaptic stimulation to a greater extent in pyramidal neurons than interneurons. Submaximal inhibition of NMDA EPSCs, but not non-NMDA EPSCs, mimicked the acute effect of ethanol on synaptically-evoked action potential firing. These findings indicate that partial inhibition of NMDA receptors by ethanol has sizable effects on the excitability of glutamatergic and GABAergic neurons in the developing RSC, and suggest that positive allosteric modulators of these receptors could ameliorate ethanol intoxication-induced neurodegeneration during late stages of fetal development.

SOX2 regulation by hedgehog signaling controls adult lingual epithelium homeostasis.

Adult tongue epithelium is continuously renewed from epithelial progenitor cells, a process that requires hedgehog (HH) signaling. In mice, pharmacological inhibition of the HH pathway causes taste bud loss within a few weeks. Previously, we demonstrated that sonic hedgehog (SHH) overexpression in lingual progenitors induces ectopic taste buds with locally increased SOX2 expression, suggesting that taste bud differentiation depends on SOX2 downstream of HH. To test this, we inhibited HH signaling in mice and observed a rapid decline in Sox2 and SOX2-GFP expression in taste epithelium. Upon conditional deletion of Sox2, differentiation of both taste and non-taste epithelial cells was blocked, and progenitor cell number increased. In contrast to basally restricted proliferation in controls, dividing cells were overabundant and spread to suprabasal epithelial layers in mutants. SOX2 loss in progenitors also led non-cell-autonomously to taste cell apoptosis, dramatically shortening taste cell lifespans. Finally, in tongues with conditional Sox2 deletion and SHH overexpression, ectopic and endogenous taste buds were not detectable; instead, progenitor hyperproliferation expanded throughout the lingual epithelium. In summary, we show that SOX2 functions downstream of HH signaling to regulate lingual epithelium homeostasis.

Zinc-Dependent Transcriptional Regulation in Paracoccus denitrificans.

Zinc homeostasis is critical for bacterial survival and is mediated largely at the transcriptional level by the regulation of zinc uptake and efflux genes. Here we use RNA-seq to assess transcriptional changes as a result of zinc limitation in the denitrifying bacterium Paracoccus denitrificans. The results identify the differential expression of 147 genes, most of which were upregulated in zinc-depleted medium. Included in this set of genes are a large number of transition metal transporters, several transcription factors, and hypothetical proteins. Intriguingly, genes encoding nitric oxide reductase (norCB) and nitrite reductase (nirS) were also upregulated. A Zur consensus binding motif was identified in the promoters of the most highly upregulated genes. The zinc uptake regulator (Zur) from this organism was also characterized and shown to bind to the Zur motif in a zinc-dependent manner. This work expands our current understanding of the transcriptional response of gram-negative bacteria to zinc limitation and identifies genes involved in denitrification as part of the Zur regulon.