Factors Associated With Risk of Recurrent Transient Global Amnesia.

Importance: Transient global amnesia (TGA) is usually considered a benign event with a low recurrence rate. However, recurrence rates vary considerably among studies and there are no known risk factors for TGA. Objective: To examine risk factors for the recurrence of TGA. Design, Setting, and Participants: This retrospective cohort study involved medical record review of patients with isolated or recurrent TGA presenting to the Mayo Clinic in Rochester, Minnesota, between August 1, 1992, and February 28, 2018. A total of 1491 cases were reviewed and 1044 met diagnostic inclusion criteria for TGA, with the remainder excluded owing to indeterminate or alternate diagnoses or limited information available in the medical record. Exposures: Single vs recurrent episodes of TGA. Main Outcomes and Measures: Demographics, precipitating factors, migraine history, imaging and electrodiagnostic findings, and family history of TGA were collected. The main outcome measure was TGA recurrence. Results: Of 1044 included patients, 575 (55.1%) were male, and the mean (SD) age at inclusion was 75.0 (11.5) years. A total of 901 patients (86.3%) had a single episode of TGA and 143 (13.7%) had recurrent episodes of TGA. The 2 groups were similar in age at inclusion, sex, identifiable triggers, and duration of anterograde amnesia. The number of recurrences ranged from 1 to 9, with 137 individuals (95.8%) having 3 or fewer recurrences. The mean (SD) age at first episode of TGA was 65.2 (10.0) years for individuals with a single episode vs 58.8 (10.3) years for those with recurrent episodes (P < .001). There was a personal history of migraine in 180 individuals (20.0%) with a single episode of TGA and 52 individuals (36.4%) with recurrent episodes of TGA (P < .001), and a family history of migraine in 167 individuals (18.5%) with a single episode of TGA and 44 individuals (30.8%) with recurrent episodes of TGA (P = .001). There were no electroencephalographic findings associated with increased risk of TGA recurrence. Acute and subacute temporal lobe abnormalities on results of magnetic resonance imaging were seen rarely and did not require intervention. A family history of TGA was identified in 12 individuals (1.3%) with a single episode of TGA and 4 individuals (2.8%) with recurrent episodes of TGA. Conclusions and Relevance: This study suggests that, in this large cohort of patients with TGA, recurrent TGA was associated with earlier age at the time of first TGA episode and higher prevalence of both personal and family history of migraine compared with isolated cases. These results can be used to counsel patients about risks of recurrence and may have implications for the understanding of TGA pathophysiology.

Epinephrine and glucose modulate training-related CREB phosphorylation in old rats: relationships to age-related memory impairments.

Epinephrine enhances memory in young adult rats, in part, by increasing blood glucose levels needed to modulate memory. In old rats, epinephrine is deficient at raising blood glucose levels and thus is only moderately effective at enhancing memory. In contrast, systemic glucose injections improve memory in old rats, with resulting memory performance equal to that of young rats. The diminished response of glucose to training in old rats may blunt downstream neurochemical and molecular mechanisms needed to upregulate memory processes. In the first experiment, young adult and old rats were trained on an inhibitory avoidance task with immediate post-training injections of aCSF or glucose into the dorsal hippocampus. Old rats had significant memory impairments compared to young rats 7 days after training. Intrahippocampal injections of glucose reversed age-related deficits, improving memory scores in old rats to values seen in young rats. A second experiment examined age-related changes in activation of the transcription factor CREB, which is widely implicated in memory formation and may act downstream of hormonal and metabolic signals. Activation was assessed in response to training with systemic injections of epinephrine and glucose at doses known to enhance memory. Young adult and old rats were trained on inhibitory avoidance with immediate post-training systemic injections of saline, epinephrine, or glucose. After training, old rats had significant impairments in CREB phosphorylation in area CA1 and the dentate gyrus region of the hippocampus, and in the basolateral and lateral amygdala. Epinephrine and glucose attenuated age-related deficits in CREB phosphorylation, but were more effective in the amygdala and hippocampus, respectively. Together, these results support the view that age-related changes in blood glucose responses to epinephrine contribute to memory impairments, which may be related to alterations in regional patterns of CREB phosphorylation.

Glucose attenuates impairments in memory and CREB activation produced by an α4ß2 but not an α7 nicotinic receptor antagonist.

Glucose improves memory for a variety of tasks when administered to rats and mice near the time of training. Prior work indicates glucose may enhance memory by increasing the synthesis and release of the neurotransmitter acetylcholine in the brain. To investigate if specific acetylcholine receptor subtypes may mediate some of the memory-enhancing actions of glucose, we examined the effects of subtype-specific nicotinic acetylcholine receptor antagonists on memory in Fischer-344 rats and also examined the ability of glucose to reverse drug-induced impairments. Pre-training peripheral injections of methyllycaconitine (MLA) or dihydro-beta-erythroidine (DHßE), which are specific α7 and α4ß2 nicotinic receptor antagonists, respectively, dose-dependently impaired retention latencies in an inhibitory avoidance task when tested 7-days but not 1 h after training. Immediate post-training glucose injections attenuated the impairments, but were more effective in attenuating the DHßE-induced impairments. Likewise, peripheral or direct intrahippocampal injections of MLA or DHßE dose-dependently impaired spatial working memory scores on a spontaneous alternation task. Concurrent administration of glucose reversed DHßE- but not MLA-induced impairments. CREB phosphorylation downstream of cholinergic signaling was assessed 30 min after spontaneous alternation testing and intrahippocampal drug infusions. Both MLA and DHßE impaired hippocampal CREB phosphorylation; glucose reversed DHßE- but not MLA-induced deficits. The effectiveness of glucose in reversing DHßE- but not MLA-induced impairments in behavioral performance and CREB phosphorylation suggests that activation of α7 receptors may play an important role in memory enhancement by glucose.

Age-related impairments in memory and in CREB and pCREB expression in hippocampus and amygdala following inhibitory avoidance training.

This experiment examined whether age-related changes in CREB and pCREB contribute to the rapid forgetting seen in aged animals. Young (3-month-old) and aged (24-month-old) Fischer-344 rats received inhibitory avoidance training with a low (0.2 mA, 0.4 s) or moderate (0.5 mA, 0.5 s) foot shock; memory was measured 7 days later. Other rats were euthanized 30 min after training, and CREB and pCREB expression levels were examined in the hippocampus, amygdala, and piriform cortex using immunohistochemistry. CREB levels decreased with age in the hippocampus and amygdala. After training with either shock level, young rats exhibited good memory and increases in pCREB levels in the hippocampus and amygdala. Aged rats exhibited good memory for the moderate but not the low shock but did not show increases in pCREB levels after either shock intensity. These results suggest that decreases in total CREB and in pCREB activation in the hippocampus and amygdala may contribute to rapid forgetting in aged rats. After moderate foot shock, the stable memory in old rats together with absence of CREB activation suggests either that CREB was phosphorylated in a spatiotemporal pattern other than analyzed here or that the stronger training conditions engaged alternate mechanisms that promote long-lasting memory.

Expression of Neurog1 instead of Atoh1 can partially rescue organ of Corti cell survival.

In the mammalian inner ear neurosensory cell fate depends on three closely related transcription factors, Atoh1 for hair cells and Neurog1 and Neurod1 for neurons. We have previously shown that neuronal cell fate can be altered towards hair cell fate by eliminating Neurod1 mediated repression of Atoh1 expression in neurons. To test whether a similar plasticity is present in hair cell fate commitment, we have generated a knockin (KI) mouse line (Atoh1(KINeurog1)) in which Atoh1 is replaced by Neurog1. Expression of Neurog1 under Atoh1 promoter control alters the cellular gene expression pattern, differentiation and survival of hair cell precursors in both heterozygous (Atoh1(+/KINeurog1)) and homozygous (Atoh1(KINeurog1/KINeurog1)) KI mice. Homozygous KI mice develop patches of organ of Corti precursor cells that express Neurog1, Neurod1, several prosensory genes and neurotrophins. In addition, these patches of cells receive afferent and efferent processes. Some cells among these patches form multiple microvilli but no stereocilia. Importantly, Neurog1 expressing mutants differ from Atoh1 null mutants, as they have intermittent formation of organ of Corti-like patches, opposed to a complete 'flat epithelium' in the absence of Atoh1. In heterozygous KI mice co-expression of Atoh1 and Neurog1 results in change in fate and patterning of some hair cells and supporting cells in addition to the abnormal hair cell polarity in the later stages of development. This differs from haploinsufficiency of Atoh1 (Pax2cre; Atoh1(f/+)), indicating the effect of Neurog1 expression in developing hair cells. Our data suggest that Atoh1(KINeurog1) can provide some degree of functional support for survival of organ of Corti cells. In contrast to the previously demonstrated fate plasticity of neurons to differentiate as hair cells, hair cell precursors can be maintained for a limited time by Neurog1 but do not transdifferentiate as neurons.

Age-related memory impairments due to reduced blood glucose responses to epinephrine.

Increases in blood glucose levels are an important component of the mechanisms by which epinephrine enhances memory formation. The present experiments addressed the hypothesis that a dysfunction in the blood glucose response to circulating epinephrine contributes to age-related memory impairments. Doses of epinephrine and glucagon that significantly increased blood glucose levels in young adult rats were far less effective at doing so in 2-year-old rats. In young rats, epinephrine and glucose were about equally effective in enhancing memory and in prolonging post-training release of acetylcholine in the hippocampus. However, glucose was more effective than epinephrine in enhancing both memory and acetylcholine release in aged rats. These results suggest that an uncoupling between circulating epinephrine and glucose levels in old rats may lead to an age-related reduction in the provision of glucose to the brain during training. This in turn may contribute to age-related changes in memory and neural plasticity.

Diversity of Ca2+-activated K+ channel transcripts in inner ear hair cells.

Hair cells express a complement of ion channels, representing shared and distinct channels that confer distinct electrophysiological signatures for each cell. This diversity is generated by the use of alternative splicing in the alpha subunit, formation of heterotetrameric channels, and combinatorial association with beta subunits. These channels are thought to play a role in the tonotopic gradient observed in the mammalian cochlea. Mouse Kcnma1 transcripts, 5' and 3' ESTs, and genomic sequences were examined for the utilization of alternative splicing in the mouse transcriptome. Comparative genomic analyses investigated the conservation of KCNMA1 splice sites. Genomes of mouse, rat, human, opossum, chicken, frog and zebrafish established that the exon-intron structure and mechanism of KCNMA1 alternative splicing were highly conserved with 6-7 splice sites being utilized. The murine Kcnma1 utilized 6 out of 7 potential splice sites. RT-PCR experiments using murine gene-specific oligonucleotide primers analyzed the scope and variety of Kcnma1 and Kcnmb1-4 expression profiles in the cochlea and inner ear hair cells. In the cochlea splice variants were present representing sites 3, 4, 6, and 7, while site 1 was insertionless and site 2 utilized only exon 10. However, site 5 was not present. Detection of KCNMA1 transcripts and protein exhibited a quantitative longitudinal gradient with a reciprocal gradient found between inner and outer hair cells. Differential expression was also observed in the usage of the long form of the carboxy-terminus tail. These results suggest that a diversity of splice variants exist in rodent cochlear hair cells and this diversity is similar to that observed for non-mammalian vertebrate hair cells, such as chicken and turtle.

Differential expression of KCNQ4 in inner hair cells and sensory neurons is the basis of progressive high-frequency hearing loss.

Human KCNQ4 mutations known as DFNA2 cause non-syndromic, autosomal-dominant, progressive high-frequency hearing loss in which the cellular and molecular basis is unclear. We provide immunofluorescence data showing that Kcnq4 expression in the adult cochlea has both longitudinal (base to apex) and radial (inner to outer hair cells) gradients. The most intense labeling is in outer hair cells at the apex and in inner hair cells as well as spiral ganglion neurons at the base. Spatiotemporal expression studies show increasing intensity of KCNQ4 protein labeling from postnatal day 21 (P21) to P120 mice that is most apparent in inner hair cells of the middle turn. We have identified four alternative splice variants of Kcnq4 in mice. The alternative use of exons 9-11 produces three transcript variants (v1-v3), whereas the fourth variant (v4) skips all three exons; all variants have the same amino acid sequence at the C termini. Both reverse transcription-PCR and quantitative PCR analyses demonstrate that these variants have differential expression patterns along the length of the mouse organ of Corti and spiral ganglion neurons. Our expression data suggest that the primary defect leading to high-frequency loss in DFNA2 patients may be attributable to high levels of the dysfunctional Kcnq4_v3 variant in the spiral ganglion and inner hair cells in the basal hook region. Progressive hearing loss associated with aging may result from an increasing mutational load expansion toward the apex in inner hair cells and spiral ganglion neurons.

Differential expression of genes within the cochlea as defined by a custom mouse inner ear microarray.

Microarray analyses have contributed greatly to the rapid understanding of functional genomics through the identification of gene networks as well as gene discovery. To facilitate functional genomics of the inner ear, we have developed a mouse inner-ear-pertinent custom microarray chip (CMA-IE1). Nonredundant cDNA clones were obtained from two cDNA library resources: the RIKEN subtracted inner ear set and the NIH organ of Corti library. At least 2000 cDNAs unique to the inner ear were present on the chip. Comparisons were performed to examine the relative expression levels of these unique cDNAs within the organ of Corti, lateral wall, and spiral ganglion. Total RNA samples were obtained from the three cochlear-dissected fractions from adult CF-1 mice. The total RNA was linearly amplified, and a dendrimer-based system was utilized to enhance the hybridization signal. Differentially expressed genes were verified by comparison to known gene expression patterns in the cochlea or by correlation with genes and gene families deduced to be present in the three tissue types. Approximately 22-25% of the genes on the array had significant levels of expression. A number of differentially expressed genes were detected in each tissue fraction. These included genes with known functional roles, hypothetical genes, and various unknown or uncharacterized genes. Four of the differentially expressed genes found in the organ of Corti are linked to deafness loci. None of these are hypothetical or unknown genes.

Identification of unique transcripts from a mouse full-length, subtracted inner ear cDNA library.

A small-scale full-length library construction approach was developed to facilitate production of a mouse full-length cDNA encyclopedia representing approximately 250 enriched, normalized, and/or subtracted cDNA libraries. One library produced using this approach was a subtracted adult mouse inner ear cDNA library (sIEa). The average size of the inserts was approximately 2.5 kb, with the majority ranging from 0.5 to 7.0 kb. From this library 22,574 sequence reads were obtained from 15,958 independent clones. Sequencing and chromosomal localization established 5240 clusters, with 1302 clusters being unique and 359 representing new ESTs. Our sIEa library contributed 56.1% of the 7773 nonredundant Unigene clusters associated with the four mouse inner ear libraries in the NCBI dbEST. Based on homologous chromosomal regions between human and mouse, we identified 1018 UniGene clusters associated with the deafness locus critical regions. Of these, 59 clusters were found only in our sIEa library and represented approximately 50% of the identified critical regions.