Experimental data on developmental elimination of retrosplenial cortex GABAergic interneurons in a mouse model of ethanol exposure during the last trimester of human pregnancy.

It has been previously shown that 40% of murine cortical interneurons are eliminated via apoptosis during the first two weeks of postnatal development [1], [2], [3]. Here, we report data on the effect of ethanol exposure on this process in a mouse model of binge-like alcohol exposure during last trimester of human pregnancy (equivalent to the first postnatal week in mice). We used transgenic mice that express the Venus fluorescent protein in GABAergic interneurons under the control of the vesicular GABA transporter promoter (VGAT-Venus mice) [4]. Mice were exposed to air (controls) or ethanol for 4 hr/day on postnatal days 4 to 9 using vapor inhalation chambers [5]. This exposure paradigm produces peak blood ethanol concentrations between 300 and 400 mg/dl. Transcardial perfusions were performed under anesthesia at postnatal days 5, 7, 10 and 30. Cryostat-prepared floating sections were stained with the fluorescent DNA dye, 4'6-diamidino-2-phenylindole (DAPI). We then quantified the density of Venus-positive GABAergic interneurons in layers I, II-IV and V of the retrosplenial cortex, which is part of the limbic memory system [6], and is sensitive to ethanol-induced apoptosis during the first postnatal week in mice [7], [8], [9], [10], [11]. The data show that density of interneurons decreases in the retrosplenial cortex layers during the first week of life and that ethanol exposure does not significantly alter this process. These data may be of interest to investigators who are studying the effect of ethanol and other teratogenic agents on developing interneurons in the cerebral cortex.

Enhancement of parvalbumin interneuron-mediated neurotransmission in the retrosplenial cortex of adolescent mice following third trimester-equivalent ethanol exposure.

Prenatal ethanol exposure causes a variety of cognitive deficits that have a persistent impact on quality of life, some of which may be explained by ethanol-induced alterations in interneuron function. Studies from several laboratories, including our own, have demonstrated that a single binge-like ethanol exposure during the equivalent to the third trimester of human pregnancy leads to acute apoptosis and long-term loss of interneurons in the rodent retrosplenial cortex (RSC). The RSC is interconnected with the hippocampus, thalamus, and other neocortical regions and plays distinct roles in visuospatial processing and storage, as well as retrieval of hippocampal-dependent episodic memories. Here we used slice electrophysiology to characterize the acute effects of ethanol on GABAergic neurotransmission in the RSC of neonatal mice, as well as the long-term effects of neonatal ethanol exposure on parvalbumin-interneuron mediated neurotransmission in adolescent mice. Mice were exposed to ethanol using vapor inhalation chambers. In postnatal day (P) 7 mouse pups, ethanol unexpectedly failed to potentiate GABAA receptor-mediated synaptic transmission. Binge-like ethanol exposure of P7 mice expressing channel rhodopsin in parvalbumin-positive interneurons enhanced the peak amplitudes, asynchronous activity and total charge, while decreasing the rise-times of optically-evoked GABAA receptor-mediated inhibitory postsynaptic currents in adolescent animals. These effects could partially explain the learning and memory deficits that have been documented in adolescent and young adult mice exposed to ethanol during the third trimester-equivalent developmental period.

The mouse-equivalent of the human BDNF VAL66MET polymorphism increases dorsal hippocampal volume and does not interact with developmental ethanol exposure.

A relatively common polymorphism in the human brain-derived neurotrophic factor (BDNF) gene (Val66Met, which corresponds to Val68Met in mice) has been shown to modulate cognitive function and vulnerability to mental health disorders. This substitution impairs trafficking and activity-dependent release of BDNF. A number of studies with both humans and transgenic mice suggest that carriers of the Met allele have deficits in the structure and/or function of the hippocampal formation. Using a relatively new transgenic mouse model of this polymorphism, we recently demonstrated that it modulates the effects of developmental ethanol exposure in the hippocampus. Here, we further characterized the effect of this polymorphism on hippocampal morphology and its interaction with ethanol vapor exposure during the 2nd and 3rd trimester equivalents of human pregnancy. We found that BDNFmet/met mice have slightly larger hippocampal volumes than BDNFval/val mice. Ethanol vapor exposure during the 2nd and 3rd trimester equivalents of human pregnancy increased hippocampal volume in a single hippocampal subregion, the CA1 stratum radiatum. Ethanol exposure did not interact with BDNF genotype to affect volume in any hippocampal subregion. These results suggest that the Val66Met polymorphism does not reduce hippocampal size (i.e., it rather increases it slightly) or increase susceptibility to prenatal ethanol exposure-induced structural hippocampal damage during adulthood.

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.