Age-dependent microRNA control of synaptic plasticity in 22q11 deletion syndrome and schizophrenia.

The 22q11 deletion syndrome (22q11DS) is characterized by multiple physical and psychiatric abnormalities and is caused by the hemizygous deletion of a 1.5-3 Mb region of chromosome 22. It constitutes one of the strongest known genetic risks for schizophrenia; schizophrenia arises in as many as 30% of patients with 22q11DS during adolescence or early adulthood. A mouse model of 22q11DS displays an age-dependent increase in hippocampal long-term potentiation (LTP), a form of synaptic plasticity underlying learning and memory. The sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA2), which is responsible for loading Ca(2+) into the endoplasmic reticulum (ER), is elevated in this mouse model. The resulting increase in ER Ca(2+) load leads to enhanced neurotransmitter release and increased LTP. However, the mechanism by which the 22q11 microdeletion leads to SERCA2 overexpression and LTP increase has not been determined. Screening of multiple mutant mouse lines revealed that haploinsufficiency of Dgcr8, a microRNA (miRNA) biogenesis gene in the 22q11DS disease-critical region, causes age-dependent, synaptic SERCA2 overexpression and increased LTP. We found that miR-25 and miR-185, regulators of SERCA2, are depleted in mouse models of 22q11DS. Restoration of these miRNAs to presynaptic neurons rescues LTP in Dgcr8(+/-) mice. Finally, we show that SERCA2 is elevated in the brains of patients with schizophrenia, providing a link between mouse model findings and the human disease. We conclude that miRNA-dependent SERCA2 dysregulation is a pathogenic event in 22q11DS and schizophrenia.

Phosphatase and tensin homologue (PTEN) regulates synaptic plasticity independently of its effect on neuronal morphology and migration.

The tumour suppressor PTEN is the central negative regulator of the phosphatidylinositol 3-kinase (PI3K) signalling pathway, which mediates diverse processes in various tissues. In the nervous system, the PI3K pathway modulates proliferation, migration, cellular size, synaptic transmission and plasticity. In humans, neurological abnormalities such as autism, seizures and ataxia are associated with inherited PTEN mutations. In rodents, Pten loss during early development is associated with extensive deficits in neuronal migration and substantial hypertrophy of neurons and synaptic densities; however, whether its effect on synaptic transmission and plasticity is direct or mediated by structural abnormalities remains unknown. Here we analysed neuronal and synaptic structures and function in Pten-conditional knockout mice in which the gene was deleted from excitatory neurons postnatally. Using two-photon imaging, Golgi staining, immunohistochemistry, electron microscopy, and electrophysiological tools, we determined that Pten loss does not affect hippocampus development, neuronal or synaptic structures, or basal excitatory synaptic transmission. However, it does cause deficits in both major forms of synaptic plasticity, long-term potentiation and long-term depression, of excitatory synaptic transmission. These deficits coincided with impaired spatial memory, as measured in water maze tasks. Deletion of Pdk1, which encodes a positive downstream regulator of the PI3K pathway, rescued Pten-mediated deficits in synaptic plasticity but not in spatial memory. These results suggest that PTEN independently modulates functional and structural properties of hippocampal neurons and is directly involved in mechanisms of synaptic plasticity.

Dysregulation of presynaptic calcium and synaptic plasticity in a mouse model of 22q11 deletion syndrome.

The 22q11 deletion syndrome (22q11DS) is characterized by cognitive decline and increased risk of psychiatric disorders, mainly schizophrenia. The molecular mechanisms of neuronal dysfunction in cognitive symptoms of 22q11DS are poorly understood. Here, we report that a mouse model of 22q11DS, the Df(16)1/+ mouse, exhibits substantially enhanced short- and long-term synaptic plasticity at hippocampal CA3-CA1 synapses, which coincides with deficits in hippocampus-dependent spatial memory. These changes are evident in mature but not young animals. Electrophysiological, two-photon imaging and glutamate uncaging, and electron microscopic assays in acute brain slices showed that enhanced neurotransmitter release but not altered postsynaptic function or structure caused these changes. Enhanced neurotransmitter release in Df(16)1/+ mice coincided with altered calcium kinetics in CA3 presynaptic terminals and upregulated sarco(endo)plasmic reticulum calcium-ATPase type 2 (SERCA2). SERCA inhibitors rescued synaptic phenotypes of Df(16)1/+ mice. Thus, presynaptic SERCA2 upregulation may be a pathogenic event contributing to the cognitive symptoms of 22q11DS.

Mice with targeted genetic reduction of GABA(A) receptor alpha1 subunits display performance differences in Morris water maze tasks.

Recent research has begun to demonstrate that specific subunits of GABA(A) receptors may be involved in the normal expression of specific behaviors. The present research used mice with GABA(A) receptors whose alpha1 subunits contained mutations of serine 270 to histidine and leucine 277 to alanine in the TM2 region. The purpose was an attempt to examine the possible role that this particular subunit may have in learning the spatial and nonspatial version of the Morris water maze task. Mutant animals, compared to controls, displayed elevated levels of pool circling in both the spatial task and the nonspatial task. These results suggested that normal performance of the spatial and nonspatial water maze tasks may be dependent upon a natural alpha1 subunit array.

Acute mild footshock alters ethanol drinking and plasma corticosterone levels in C57BL/6J male mice, but not DBA/2J or A/J male mice.

Stress is an often-reported cause for alcohol consumption in humans. Acute intermittent footshock is a frequently used paradigm to produce stress in laboratory animals including mice. The effect produced by intermittent footshock stress on ethanol self-administration has been inconsistent: both increases and decreases in ethanol consumption have been reported. The current set of studies further investigates, in three commonly studied mouse strains, the effect of footshock stress on ethanol self-administration. Furthermore, the effect of footshock on plasma corticosterone levels was determined to investigate potential biochemical correlates. Adult male C57BL/6J, DBA/2J, and A/J mice were allowed to self-administer 10% (wt/vol) ethanol for 12 days in a standard 23-h two-bottle paradigm before receiving either 15 min of mild inescapable footshock or no footshock. Shock intensity was equal to the mean intensity at which each strain vocalized as previously determined. Following footshock, animals had the opportunity to self-administer ethanol for an additional 23 h. Separate animals were subjected to either footshock or no shock prior to collection of plasma for corticosterone. Mild footshock stress altered ethanol self-administration and increased plasma corticosterone levels in C57BL/6J mice. Footshock stress did not alter ethanol self-administration or plasma corticosterone levels in DBA/2J or A/J mice. These data demonstrate that mild footshock stress is a suboptimal method of modeling the stress-induced increases in ethanol consumption often reported by humans.

Acute ethanol administration selectively impairs spatial memory in C57BL/6J mice.

It has been shown in rats that acute ethanol administration, via a single intraperitoneal injection, selectively impairs the memory of certain spatial tasks. It is unknown whether these same results can be produced in the C57BL/6J mouse strain. Male C57BL/6J mice were trained in a spatial task in the Morris water maze. After training, an ethanol test was administered in which each mouse was given an injection of one of four randomly assigned doses: ethanol, at a dose of 1.25, 1.75, or 2.25 g/kg, or a saline control dose that remained constant at 1.75 g/kg. Thirty minutes after injection, the mice were given the spatial task. Next, the same mice were given training for a nonspatial task in the Morris water maze. After training, another ethanol test was administered. Again, the mice were randomly assigned one of the aforementioned doses. Thirty minutes after injections, the mice were given the nonspatial task. Results from Study 1, by using latency, showed that acute ethanol administration selectively impaired spatial memory (P<.05) at 1.75 and 2.25 g/kg doses, yet it failed to significantly impair nonspatial memory except at the 2.25 g/kg dose. Results from Study 2, by using path lengths, showed similar effects, in that acute ethanol administration selectively impaired spatial memory (P<.05) at the 2.25 g/kg dose, yet it failed to impair nonspatial memory at any dose. These findings demonstrate that acute ethanol administration selectively impairs spatial memory in C57BL/6J mice.

Effect of acute ethanol and acute allopregnanolone on spatial memory in adolescent and adult rats.

The effects of ethanol differ in adolescent and adult rats on a number of measures. The evidence of the effects of ethanol on spatial memory in adolescents and adults is equivocal. Whether adolescents are more or less sensitive to ethanol-induced impairment of spatial memory acquisition remains unclear; with regard to the effects of acute ethanol on spatial memory retrieval there is almost no research looking into any age difference. Thus, we examined the effects of acute ethanol on spatial memory in the Morris Watermaze in adolescents and adults. Allopregnanolone (ALLO) is a modulator of the GABA(A) receptor and has similar behavioral effects as ethanol. We sought to also determine the effects of allopreganolone on spatial memory in adolescent and adults. Male adolescent (post natal [PN]28-30) and adult (PN70-72) rats were trained in the Morris Watermaze for 6 days and acute doses of ethanol (saline, 1.5 and 2.0 g/kg) or ALLO (vehicle, 9 and 18 mg/kg) were administered on Day 7. A probe trial followed on Day 8. As expected, there were dose effects; higher doses of both ethanol and ALLO impaired spatial memory. However, in both the ethanol and ALLO conditions adolescents and adults had similar spatial memory impairments. The current results suggest that ethanol and ALLO both impair hippocampal-dependent spatial memory regardless of age in that once learning has occurred, ethanol or ALLO does not differentially impair the retrieval of spatial memory in adolescents and adults. Given the mixed results on the effect of ethanol on cognition in adolescent rats, additional research is needed to ascertain the factors critical for the reported differential results.

Investigation of ethanol-induced impairment of spatial memory in gamma2 heterozygous knockout mice.

GABA(A) receptors, the major inhibitory receptors in the mammalian central nervous system, are affected by a number of drug compounds, including ethanol. The pharmacological effects of certain drugs have been shown to be dependent upon specific GABA(A) receptor subunits. Because benzodiazepines and ethanol have similar effect signatures, it has been hypothesized that these drugs share the gamma2-containing GABA(A) receptors as a mechanism of action. To probe the involvement of the gamma2 subunit in ethanol's actions, spatial memory for the Morris water maze task was tested in gamma2 heterozygous knockout mice and wild type littermate controls following ethanol administration at the following doses: 0.0, 1.25, 1.75, and 2.25 g/kg. While baseline learning and memory were unaffected by reduction of gamma2 containing GABA(A) receptors, ethanol dose-dependently impaired spatial memory equally in gamma2 heterozygous knockouts and wild type littermate controls.

Knockin mice with ethanol-insensitive alpha1-containing gamma-aminobutyric acid type A receptors display selective alterations in behavioral responses to ethanol.

Despite the pervasiveness of alcohol (ethanol) use, it is unclear how the multiple molecular targets for ethanol contribute to its many behavioral effects. The function of GABA type A receptors (GABA(A)-Rs) is altered by ethanol, but there are multiple subtypes of these receptors, and thus far, individual subunits have not been definitively linked with specific behavioral actions. The alpha1 subunit of the GABA(A)-R is the most abundant alpha subunit in the brain, and the goal of this study was to determine the role of receptors containing this subunit in alcohol action. We designed an alpha1 subunit with serine 270 to histidine and leucine 277 to alanine mutations that was insensitive to potentiation by ethanol yet retained normal GABA sensitivity and constructed knockin mice containing this mutant subunit. Hippocampal slice recordings from these mice indicated that the mutant receptors were less sensitive to ethanol's potentiating effects. Behaviorally, we observed that mutant mice recovered more quickly from the motor-impairing effects of ethanol and etomidate, but not pentobarbital, and showed increased anxiolytic effects of ethanol. No differences were observed in ethanol-induced hypnosis, locomotor stimulation, cognitive impairment, or in ethanol preference and consumption. Overall, these studies demonstrate that the postsynaptic effects of ethanol at GABAergic synapses containing the alpha1 subunit are important for specific ethanol-induced behavioral effects.

Impairments in spatial learning and memory: ethanol, allopregnanolone, and the hippocampus.

Acute ethanol administration impairs performance in many cognitive tasks that are dependent on hippocampal function. For example, acute ethanol administration produces dose-dependent impairments in spatial learning. Ethanol also decreases the spatial specificity of hippocampal place cells. Such findings raise the possibility that ethanol affects learning and memory by altering, either directly or indirectly, neuronal activity in the hippocampus and related structures. Acute ethanol administration induces a dose- and time-dependent increase in brain concentration of the neuroactive steroid allopregnanolone. Allopregnanolone is a potent GABAA receptor agonist and produces effects similar to the effects produced by ethanol. Blockade of de novo biosynthesis of allopregnanolone alters many of ethanol's effects including ethanol-induced suppression of spontaneous activity in medial septum/diagonal band of Broca neurons and hippocampal pyramidal neurons. These findings suggest that ethanol-induced increases in allopregnanolone levels might play a central role in the effects of acute ethanol on cognitive processing and hippocampal function. The impact of ethanol on spatial cognitive processing and hippocampal function will be reviewed. In addition, the possibility that ethanol-induced changes in neuroactive steroid levels contribute to the impact of ethanol on spatial learning and hippocampal function will be explored.