Fabian Fernandez

Pharmacotherapy for cognitive impairment in a mouse model of Down syndrome

Ts65Dn mice, a model for Down syndrome, have excessive inhibition in the dentate gyrus, a condition that could compromise synaptic plasticity and mnemonic processing. We show that chronic systemic treatment of these mice with GABAA antagonists at non-epileptic doses causes a persistent post-drug recovery of cognition and long-term potentiation. These results suggest that over-inhibition contributes to intellectual disabilities associated with Down syndrome and that GABAA antagonists may be useful therapeutic agents for this disorder.

Hippocampal-dependent learning requires a functional circadian system.

Decades of studies have shown that eliminating circadian rhythms of mammals does not compromise their health or longevity in the laboratory in any obvious way. These observations have raised questions about the functional significance of the mammalian circadian system, but have been difficult to address for lack of an appropriate animal model. Surgical ablation of the suprachiasmatic nucleus (SCN) and clock gene knockouts eliminate rhythms, but also damage adjacent brain regions or cause developmental effects that may impair cognitive or other physiological functions. We developed a method that avoids these problems and eliminates rhythms by noninvasive means in Siberian hamsters (Phodopus sungorus). The present study evaluated cognitive function in arrhythmic animals by using a hippocampal-dependent learning task. Control hamsters exhibited normal circadian modulation of performance in a delayed novel-object recognition task. By contrast, arrhythmic animals could not discriminate a novel object from a familiar one only 20 or 60 min after training. Memory performance was not related to prior sleep history as sleep manipulations had no effect on performance. The GABA antagonist pentylenetetrazol restored learning without restoring circadian rhythms. We conclude that the circadian system is involved in memory function in a manner that is independent of sleep. Circadian influence on learning may be exerted via cyclic GABA output from the SCN to target sites involved in learning. Arrhythmic hamsters may have failed to perform this task because of chronic inhibitory signaling from the SCN that interfered with the plastic mechanisms that encode learning in the hippocampus.

Nociceptin/Orphanin FQ Increases Anxiety-Related Behavior and Circulating Levels of Corticosterone During Neophobic Tests of Anxiety

Intracranial administration of nociceptin/orphanin FQ (N/OFQ) increases circulating concentrations of adrenocorticotrophic hormone and corticosterone in unstressed rats, and elevates the responsiveness of these hormones during mild stress. Furthermore, N/OFQ and its cognate receptor are both abundant in a variety of limbic nuclei, and stress exposure decreases neuronal N/OFQ content in forebrain neurons. In light of these and other findings, we examined the potential involvement of N/OFQ in regulation of anxiety-related behaviors in rats. In the open field, elevated plus maze, and dark-light neophobic tests, intracerebroventricular N/OFQ (1.0 pmole–1.0 nmole) increased the expression of anxiety-related behaviors. Specifically, N/OFQ increased the latency to enter, decreased the number of entries into, and decreased the time spent in the exposed or brightly lit environments of all three tests. N/OFQ also enhanced thigmotactic responses in the open field test. The effects of diazepam and of the benzodiazepine inverse agonist FG 7142 were also assessed in independent groups of rats. In all three tests, the behavioral effects of N/OFQ resembled the anxiogenic actions of FG 7142, and contrasted with the anxiolytic actions of diazepam. N/OFQ administration also increased circulating concentrations of corticosterone during anxiety testing, in comparison with the concentrations in vehicle-treated controls. We conclude that N/OFQ administration is anxiogenic, and elevates responsiveness of the hypothalamic pituitary-adrenal axis during neophobic tests of anxiety. This supports the possibility that N/OFQ neurotransmission participates in processing of emotionally-salient and stressful stimuli, and suggests that normal functioning of the N/OFQ system may be important in physiological and psychological well-being.

Episodic-like memory in Ts65Dn, a mouse model of Down syndrome

Ts65Dn mice, like individuals with Down syndrome (DS), demonstrate a functional dissociation between explicit and implicit forms of memory, showing selective impairment in explicit or declarative learning tasks. Here, we explored Ts65Dn explicit memory deficits further by evaluating the ability of these mice to assimilate the temporal and spatial contexts under which previously novel objects had been encountered. We found that Ts65Dn mice could in fact form contextual representations of objects over the course of a few hours, contrary to their inability to discriminate object novelty over a more prolonged period of 24h. These results suggest that Ts65Dn mice might have particular difficulties in declarative tasks requiring long-term memory, presenting an especially important putative therapeutic target for pre-clinical and clinical DS research.

Spatial Memory and Long-Term Object Recognition Are Impaired by Circadian Arrhythmia and Restored by the GABAAAntagonist Pentylenetetrazole

Performance on many memory tests varies across the day and is severely impaired by disruptions in circadian timing. We developed a noninvasive method to permanently eliminate circadian rhythms in Siberian hamsters (Phodopussungorus) so that we could investigate the contribution of the circadian system to learning and memory in animals that are neurologically and genetically intact. Male and female adult hamsters were rendered arrhythmic by a disruptive phase shift protocol that eliminates cycling of clock genes within the suprachiasmatic nucleus (SCN), but preserves sleep architecture. These arrhythmic animals have deficits in spatial working memory and in long-term object recognition memory. In a T-maze, rhythmic control hamsters exhibited spontaneous alternation behavior late in the day and at night, but made random arm choices early in the day. By contrast, arrhythmic animals made only random arm choices at all time points. Control animals readily discriminated novel objects from familiar ones, whereas arrhythmic hamsters could not. Since the SCN is primarily a GABAergic nucleus, we hypothesized that an arrhythmic SCN could interfere with memory by increasing inhibition in hippocampal circuits. To evaluate this possibility, we administered the GABAA antagonist pentylenetetrazole (PTZ; 0.3 or 1.0 mg/kg/day) to arrhythmic hamsters for 10 days, which is a regimen previously shown to produce long-term improvements in hippocampal physiology and behavior in Ts65Dn (Down syndrome) mice. PTZ restored long-term object recognition and spatial working memory for at least 30 days after drug treatment without restoring circadian rhythms. PTZ did not augment memory in control (entrained) animals, but did increase their activity during the memory tests. Our findings support the hypothesis that circadian arrhythmia impairs declarative memory by increasing the relative influence of GABAergic inhibition in the hippocampus.

Object recognition memory is conserved in Ts1Cje, a mouse model of Down syndrome.

Ts1Cje and Ts65Dn are genetic mouse models of Down syndrome (DS). Like individuals with DS, these mice exhibit various hallmarks of hippocampal pathology, and deficits in hippocampal-based, declarative learning and memory tasks. Both spatial navigation and novel object recognition, two prototypical domains of declarative memory function, have been strongly characterized in the Ts65Dn DS model. Indeed, Ts65Dn mice show navigation problems in the Morris water maze, impaired alternation in a T-maze, and deficient working and reference memory in the radial arm maze task. They, likewise, show an inability to detect object novelty over time. In contrast to the Ts65Dn model, hippocampal-dependent cognition has been less well characterized in Ts1Cje. Although Ts1Cje mice have been found to exhibit spatial difficulties in the Morris water maze and reduced spontaneous alternation, their ability to process object-based information has never been examined. Here, we report that Ts1Cje mice perform normally in short-term and long-term novel object recognition tasks. The ability of Ts1Cje mice to detect object novelty, unlike Ts65Dn, may point to differences in the extent of hippocampal pathology in the two DS mouse models.

Poor Sleep as a Precursor to Cognitive Decline in Down Syndrome : A Hypothesis

We propose that sleep disruption is a lever arm that influences how cognition emerges in development and then declines in response to Alzheimer disease in people with Down syndrome. Addressing sleep disruptions might be an overlooked way to improve cognitive outcomes in this population. This article is a contribution to a Special Issue on Down Syndrome curated by the editors of the Journal of Alzheimer’s Disease & Parkinsonism.

Normal protein composition of synapses in Ts65Dn mice: a mouse model of Down syndrome

Down syndrome (DS) is the most prevalent form of intellectual disability caused by the triplication of ∼230 genes on chromosome 21. Recent data in Ts65Dn mice, the foremost mouse model of DS, strongly suggest that cognitive impairment in individuals with DS is a consequence of reduced synaptic plasticity because of chronic over‐inhibition. It remains unclear however whether changes in plasticity are tied to global molecular changes at synapses, or are due to regional changes in the functional properties of synaptic circuits. One interesting framework for evaluating the activity state of the DS brain comes from in vitro studies showing that chronic pharmacological silencing of neuronal excitability orchestrates stereotyped changes in the protein composition of synaptic junctions. In the present study, we use proteomic strategies to evaluate whether synapses from the Ts65Dn cerebrum carry signatures characteristic of inactive cortical neurons. Our data reveal that synaptic junctions do not exhibit overt alterations in protein composition. Only modest changes in the levels of synaptic proteins and in their phosphorylation are observed. This suggests that subtle changes in the functional properties of specific synaptic circuits rather than large‐scale homeostatic shifts in the expression of synaptic molecules contribute to cognitive impairment in people with DS.

A bicistronic lentiviral vector based on the 1D/2A sequence of foot-and-mouth disease virus expresses proteins stoichiometrically.

Classic IRES sequences are notorious for exerting biased expression in favor of upstream coding regions when placed into polycistronic vectors. Here, we report the development of a bicistronic lentiviral system based on the 1D/2A sequence from the foot-and-mouth disease virus that is able to maintain tightly balanced control of upstream and downstream protein expression for several days at a stoichiometry very closely approaching 1.0. Our results suggest that the 1D/2A sequence can be optimized in an FUGW lentiviral setting to coordinate expression of multiple polypeptides, presenting a potentially valuable tool to signaling network researchers and to the gene therapy community.

Circadian locomotor rhythms are normal in Ts65Dn "Down syndrome" mice and unaffected by pentylenetetrazole.

Ts65Dn mice are used extensively as a model for Down syndrome. Recent studies have reported conflicting evidence as to whether these mice express circadian rhythms. The authors therefore recorded locomotor activity patterns from these animals while they were housed under a standard light-dark cycle, constant darkness (DD), and constant light (LL). Contrary to expectations, Ts65Dn mice had more robust circadian rhythms with slightly shorter periods compared with their wild-type littermates. They also exhibited increased rhythm period and marked activity suppression when moved from DD to LL (i.e., Aschoff’s rule). Administration of the GABAA antagonist pentylenetetrazole did not influence any of these circadian parameters. Thus, locomotor activity is under strict circadian control in Ts65Dn mice, suggesting that their cognitive deficits and sleep disturbances are not due to dysfunctional circadian timing as proposed previously.