Disruption of sphingomyelin synthase 2 gene alleviates cognitive impairment in a mouse model of Alzheimer's disease

The membrane raft accommodates the key enzymes synthesizing amyloid β (Aβ). One of the two characteristic components of the membrane raft, cholesterol, is well known to promote the key enzymes that produce amyloid-β (Aβ) and exacerbate Alzheimer's disease (AD) pathogenesis. Given that the raft is a physicochemical platform for the sound functioning of embedded bioactive proteins, the other major lipid component sphingomyelin may also be involved in AD. Here we knocked out the sphingomyelin synthase 2 gene (SMS2) in 3xTg AD model mice by hybridization, yielding SMS2KO mice (4S mice). The novel object recognition test in 9/10-month-old 4S mice showed that cognitive impairment in 3xTg mice was alleviated by SMS2KO, though performance in the Morris water maze (MWM) was not improved. The tail suspension test detected a depressive trait in 4S mice, which may have hindered the manifestation of performance in the wet, stressful environment of MWM. In the hippocampal CA1, hyperexcitability in 3xTg was also found alleviated by SMS2KO. In the hippocampal dentate gyrus of 4S mice, the number of neurons positive with intracellular Aβ or its precursor proteins, the hallmark of young 3xTg mice, is reduced to one-third, suggesting an SMS2KO-led suppression of syntheses of those peptides in the dentate gyrus. Although we previously reported that large-conductance calcium-activated potassium (BK) channels are suppressed in 3xTg mice and their recovery relates to cognitive amelioration, no changes occurred by hybridization. Sphingomyelin in the membrane raft may serve as a novel target for AD drugs.

Remodeling of projections from ventral hippocampus to prefrontal cortex in Alzheimer's mice

Emotional dysregulation often accompanies cognitive deficits in Alzheimer's disease (AD). The hippocampus, most notably damaged by AD pathology, is classified into the cognition-bound posterior and emotion-bound anterior hippocampi. Since the anterior hippocampus or its rodent counterpart, the ventral hippocampus (VH), sends dense afferents to the prefrontal cortex (PFC) and the basolateral amygdala (BLA), the two structures implicated in fear responses, we investigated whether these afferents are modified in 3xTg AD model mice. An anterograde dextrin tracer injected into VH revealed that axons in PFC were more ramified in 3xTg than wild-type (WT) mice, with the synaptic density reduced. The VH projections to BLA were not affected. Intracellular accumulation of amyloid β (Aβ) or Aβ-like immunoreactivity was found in PFC and BLA neurons alike. Behaviorally, in the 2-way active avoidance test, the frequency of chamber change was higher, with the test performance better, in 3xTg than WT mice, suggesting a distorted contextual fear in the 3xTg group. Given the essential involvement of parts of PFC in contextual fear responses and that of BLA in fear responses in general, the observed remodeling of VH-to-PFC afferents and the accumulation of intracellular Aβ in BLA and PFC pyramidal cells might exercise critical influences on enhanced avoidance behavior in 3xTg mice.

Stressful learning paradigm precludes manifestation of cognitive ability in sphingomyelin synthase-2 knockout mice

Sphingomyelin synthases (SMSs) are enzymes converting ceramide to sphingomyelin. The behavioral phenotype attributed to their disruption has not been well described. We examined learning ability and hippocampal synaptic plasticity in mice deficient in SMS2 (SMS2 KO). In context-dependent fear learning and novel object recognition test, no difference in learning ability was detected in SMS2 KO and wild-type (WT) mice. By contrast, achievement of the Morris water maze (MWM) test was deteriorated in SMS2 KO mice. In the hippocampal CA1, while the basic synaptic transmission was normal, both short- and long-term synaptic plasticity was moderately suppressed. We interpret that the MWM test taking place in wet environment may represent learning paradigm under more stressful condition than those performed in dry conditions, and that the learning ability of SMS2 KO mice failed to manifest itself fully in stressful situations. In agreement, forced swimming induced depression-like behavior more easily in SMS2 KO mice. Mass spectrometry suggested a slightly altered species distribution of ceramide in the hippocampus of SMS2 KO mice. These findings support the proposal that altered synthesis of ceramide, which is the substrate of SMS2 and therefore expected to be modified in SMS2 KO mice, is associated with depression-like tendency in animal models and depressive disorder in humans.

Cognitive Improvement by Photic Stimulation in a Mouse Model of Alzheimer's Disease

We previously reported that activity of the large conductance calcium-activated potassium (big-K, BK) channel is suppressed by intracellular Aβ in cortical pyramidal cells, and that this suppression was reversed by expression of the scaffold protein Homer1a in 3xTg Alzheimer's disease model mice. Homer1a is known to be expressed by physiological photic stimulation (PS) as well. The possibility thus arises that PS also reverses Aβ-induced suppression of BK channels, and thereby improves cognition in 3xTg mice. This possibility was tested here. Chronic application of 6-hour-long PS (frequency, 2 Hz; duty cycle, about 1/10; luminance, 300 lx) daily for 4 weeks improved contextual and tone-dependent fear memory in 3xTg mice and, to a lesser extent, Morris water maze performance as well. Hippocampal long-term potentiation was also enhanced after PS. BK channel activity in cingulate cortex pyramidal cells and lateral amygdalar principal cells, suppressed in 3xTg mice, were facilitated. In parallel, neuronal excitability, elevated in 3xTg mice, was recovered to the control level. Gene expression of BK channel, as well as that of the scaffold protein Homer1a, was found decreased in 3xTg mice and reversed by PS. It is known that Homer1a is an activity-dependently inducible immediate early gene product. Consistently, our previous findings showed that Homer1a induced by electrical stimulation facilitated BK channels. By using Homer1a knockouts, we showed that the present PS-induced BK channel facilitation is mediated by Homer1a expression. We thus propose that PS might be potentially useful as a non-invasive therapeutic measure against Alzheimer's disease.

Impaired cliff avoidance reaction in dopamine transporter knockout mice

RATIONALE

Impulsivity is a key feature of disorders that include attention-deficit/hyperactivity disorder (ADHD). The cliff avoidance reaction (CAR) assesses maladaptive impulsive rodent behavior. Dopamine transporter knockout (DAT-KO) mice display features of ADHD and are candidates in which to test other impulsive phenotypes.

OBJECTIVES

Impulsivity of DAT-KO mice was assessed in the CAR paradigm. For comparison, attentional deficits were also assessed in prepulse inhibition (PPI) in which DAT-KO mice have been shown to exhibit impaired sensorimotor gating.

RESULTS

DAT-KO mice exhibited a profound CAR impairment compared to wild-type (WT) mice. As expected, DAT-KO mice showed PPI deficits compared to WT mice. Furthermore, the DAT-KO mice with the most impaired CAR exhibited the most severe PPI deficits. Treatment with methylphenidate or nisoxetine ameliorated CAR impairments in DAT-KO mice.

CONCLUSION

These results suggest that DAT-KO mice exhibit impulsive CAR behavior that correlates with their PPI deficits. Blockade of monoamine transporters, especially the norepinephrine transporter (NET) in the prefrontal cortex (PFC), may contribute to pharmacological improvement of impulsivity in these mice.

Serotonergic involvement in the amelioration of behavioral abnormalities in dopamine transporter knockout mice by nicotine

Dopamine transporter knockout (DAT KO) mice exhibit elevated extracellular dopamine levels in brain regions that include the striatum and the nucleus accumbens, but not the prefrontal cortex. DAT KO mice model some aspects of psychiatric disorders, including schizophrenia. Smoking is more common in patients with schizophrenia, suggesting that nicotine might ameliorate aspects of the behavioral abnormalities and/or treatment side effects seen in these individuals. We report nicotine-induced normalization of effects on locomotion and prepulse inhibition of acoustic startle (PPI) in DAT KO mice that require intact serotonin 5-HT1A systems. First, we observed that the marked hyperactivity displayed by DAT KO mice was reduced by administration of nicotine. This nicotine effect was blocked by pretreatment with the non-specific nicotinic acetylcholine (nACh) receptor antagonist mecamylamine, or the 5-HT1A antagonist WAY100635. Secondly, we examined the effects of nicotine on PPI in DAT KO mice. Treatment with nicotine significantly ameliorated the PPI deficits observed in DAT KO mice. The ameliorating action of nicotine on PPI deficits in DAT KO mice was blocked by mecamylamine, the α₇ nACh receptor antagonist methyllycaconitine or WAY100635, while the α₄β₂ nACh receptor antagonist dihydro-β-erythroidinehydrobromide (DHβE) produced only a non-significant trend toward attenuation of nicotine effects. Finally, we observed that administration of the 5-HT1A receptor agonist 8-OH-DPAT also ameliorated the deficit in PPI observed in DAT KO mice. This amelioration was antagonized by pretreatment with WAY100635. These data support the idea that nicotine might ameliorate some of the cognitive dysfunctions found in schizophrenia in a 5-HT1A-dependent fashion. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

[Trend in the study of cocaine addiction]

Psychostimulant drugs including cocaine increase extracellular levels of monoamines by blocking the neuronal plasma membrane transporters. Increased extracellular dopamine levels in mesocorticolimbic dopamine systems have been postulated to mediate the rewarding effects of cocaine. Studies in genetically modified mice models, particularly knockout mice have contributed a great deal to our understanding of the mechanisms underlying psychostimulant actions. Phenotypic analysis of genetically modified mice models has been instrumental in identifying the role of specific molecular targets of cocaine. In this article, we summarize studies that have reported the effects of cocaine using genetically modified mice especially gene knockouts of the monoamine transporters and receptors.

[Brain deformation caused by hyperplasia molar teeth (macrodonts) in the Japanese field vole (Microtus montebelli)]

Protrusion of hyperplastic upper molar teeth (macrodonts) into the cranial cavity was observed in the Japanese field vole. Protrusions of the upper molars occurred on both sides, but were large on the left than on the right. The teeth invaded the cranial cavity and brain parenchyma. Large concavities on the ventral side of the brain were interposed between the lateral olfactory tract and the optic tract. Invation by one macrodont reached the basal nuclei, hypothalamic and thalamic nuclei. The brain was deformed bilaterally, and nearly the entire olfactory bulb, globus pallidus, internal capsule, and lateral preoptic nucleus, and a portion of the hypothalamus, thalamus and anterior commissure were absent on the left side.

Retinal projections to the subcortical nuclei in the Japanese field vole (Microtus montebelli)

Retinal projections in the Japanese field vole (Microtus montebelli) were determined by anterograde transport of horseradish peroxidase (HRP). Injection of HRP into the unilateral vitreous body demonstrated that the terminal labeling of the optic projections was seen bilaterally in the suprachiasmatic nucleus (SCH), the ventral (GLv) and dorsal (GLd) lateral geniculate nuclei, the intergeniculate leaflet (IGL), the medial pretectal nucleus (NTOM) of the pretectum (PT) and the superficial layer of the superior colliculus (CS), with contralateral predominance, and only contralaterally labeled terminals were found in the lateroposterior thalamic nucleus (LP), the lateral pretectal nucleus (NTOL) of the PT, the dorsal (DTN) and medial (MTN) terminal nuclei of the accessory optic system (AOS). The distribution area of the retinofugal terminals was divided into a three laminar arrangement in the GLd, i.e., layers 1 and 3 and layer 2, received the retinal input from contralateral and ipsilateral eye, respectively, as in arboreal squirrels. The contralateral CS received retinal fibers in the superficial layer, while ipsilateral optic fibers projected sparsely to the stratum opticum of the colliculi. Retinal connections to the DTN and MTN of the AOS were clearly discerned but no lateral terminal nucleus with retinal afferents was found. In addition, the AOS had no inferior fasciculus. These findings indicate that the vole has a contradictory features of a well- and a less-developed sense of vision. Namely, the image forming visual system such as the retino-GLd was as well-developed as in a squirrel, on the other hand, the non-image forming visual system such as the retino-AOS was less-developed as in an insectivore's brain.