Spatial learning and long-term potentiation of mutant mice lacking D-amino-acid oxidase

We evaluated the role of D-amino-acid oxidase on spatial learning and long-term potentiation (LTP) in the hippocampus, since this enzyme metabolizes D-amino-acids, some of which enhance the N-methyl-D-aspartate receptor functions. The Morris water maze learning and the LTP in the CA1 area of the hippocampal slice were observed in wild-type mice and mutant mice lacking D-amino-acid oxidase. The mutant mice showed significantly shorter platform search times in the water maze and significantly larger hippocampal LTPs than the wild-type mice. These results suggest that the abundant D-amino-acids in the mutant mouse brain facilitate hippocampal LTP and spatial learning.

Impaired spatial learning after hypergravity exposure in rats

Most astronauts experience spatial disorientation after exposure to weightlessness, indicating that constant gravity is utilized as a stable external reference during spatial cognition. We attempted to elucidate the role of constant gravity in spatial learning using a radial arm maze test on rats housed in a hypergravity environment (HG) produced by a centrifuge device. Male Wistar rats were kept in 2G linear acceleration for 2 weeks before the spatial learning task, which lasted for 10 days. The control rats were placed close to the centrifuge device but not exposed to hypergravity. Spatial learning was evaluated by the accuracy and the re-entry rate, which were the rate of correct arm entries and the rate of entries into the arms that they had already visited, respectively. Locomotor activity was measured by number of entries per minute. The number of baits the animal took per minute was also measured. The results showed that accuracy was significantly inferior and the re-entry rate was significantly higher in the HG rats than in the controls, suggesting that animals use a constant gravity as a stable external reference in spatial learning. However, these differences disappeared at 5 days later, indicating that the HG rats learned the spatial task more rapidly than the controls. Locomotor activity was higher in the HG rats and there was no difference in number of baits per minute between the HG and control animals. In conclusion, if one sensory cue necessary for spatial cognition is disturbed by gravity change, animals can subsidize with other sensory cues such as proprioceptive and motor efference copy signals through increased locomotor activities.

How sleep affects the developmental learning of bird song

Sleep affects learning and development in humans and other animals, but the role of sleep in developmental learning has never been examined. Here we show the effects of night-sleep on song development in the zebra finch by recording and analysing the entire song ontogeny. During periods of rapid learning we observed a pronounced deterioration in song structure after night-sleep. The song regained structure after intense morning singing. Daily improvement in similarity to the tutored song occurred during the late phase of this morning recovery; little further improvement occurred thereafter. Furthermore, birds that showed stronger post-sleep deterioration during development achieved a better final imitation. The effect diminished with age. Our experiments showed that these oscillations were not a result of sleep inertia or lack of practice, indicating the possible involvement of an active process, perhaps neural song-replay during sleep. We suggest that these oscillations correspond to competing demands of plasticity and consolidation during learning, creating repeated opportunities to reshape previously learned motor skills.

TMS induced plasticity in human cortex

Repetitive transcranial magnetic stimulation (RTMS) is a novel, non-invasive and painless technique to stimulate the human brain with the intent to alter excitability or function of the stimulated cortex or its connections. This review focuses on RTMS induced changes in excitability, the potential mechanisms underlying these effects, and the usefulness of this knowledge to gain insight into mechanisms of other processes such as learning.

[Effects of different light-dark cycle on learning and memory in mice]

OBJECTIVE

To study the effect of different light-dark cycles on learning and memory in mice.

METHOD

Seventy-two ICR mice were raised under different light-dark cycles including LD 5h/5h, LD 12h/12h and LD 22h/22h for 6 weeks. The locomotor activity was recorded continuously. Morris water-maze task was used as the judging criteria for spatial learning and memory.

RESULT

The locomotor activity rhythm was consistent with the light-dark cycle. The period of light-dark cycle shorter than 24 h such as 10 h could effect on the ability of learning and memory in mice.

CONCLUSION

The short period of light-dark cycle can improve the ability of learning and memory in mice.

Facilitative effect of high frequency subthreshold repetitive transcranial magnetic stimulation on complex sequential motor learning in humans

We investigated the effect of repetitive transcranial magnetic stimulation (rTMS) applied to the motor cortex, on the motor learning of sequential finger movements. Fifteen healthy subjects were trained to perform seven sequential finger movements of the left hand. Ten Hertz or sham rTMS with a resting motor threshold of 80% was applied to each subject during the task period. Stimulation with 10Hz rTMS produced a better learning performance in terms of target score and execution time than sham stimulation. We conclude that high-frequency rTMS may modulate the excitability of the motor cortex and facilitate the sequential motor learning process in normal subjects. These findings may provide a basis for the development of therapeutic applications of rTMS in patients with impaired motor skill.

Sleep deprivation impairs hippocampus-mediated contextual learning but not amygdala-mediated cued learning in rats

Prolonged sleep deprivation results in cognitive deficits. In rats, for example, sleep deprivation impairs spatial learning and hippocampal long-term potentiation. We tested the effects of sleep deprivation on learning in a Pavlovian fear conditioning paradigm, choosing a sleep deprivation paradigm in which REM sleep was completely prevented and non-REM sleep was strongly decreased. During conditioning, rats were given footshocks, either alone or paired with a tone, and tested 24 h later for freezing responses to the conditioning context, and to the tone in a novel environment. Whereas control animals had robust contextual learning in both background and foreground contextual conditioning paradigms, 72 h of sleep deprivation before conditioning dramatically impaired both types of contextual learning (by more than 50%) without affecting cued learning. Increasing the number of footshocks did not overcome the sleep deprivation-induced deficit. The results provide behavioural evidence that REM/non-REM sleep deprivation has neuroanatomically selective actions, differentially interfering with the neural systems underlying contextual learning (i.e. the hippocampus) and cued learning (i.e. the amygdala), and support the involvement of the hippocampus in both foreground and background contextual conditioning.

[Studies on the injury effects of hippocampus induced by high power microwave radiation in rat]

OBJECTIVE

To study the changes of morphology and function in rat hippocampus induced by high power microwave (HPM) radiation.

METHODS

Fifty male Wistar rats were radiated by HPM. Then their learning and memory abilities were tested with Y maze and were sacrificed 6 h, 1 d, 3 d and 7 d after radiation. The hippocampus was taken out to study the basic pathologic changes, apoptosis and the expressions of neuron-specific enolase (NSE) and glial fibrillary acidic protein (GFAP) by means of HE staining, Nissel body staining, in situ terminal end labeling and immunohistochemistry.

RESULTS

The learning and memory abilities of rats reduced significantly after HPM radiation. HPM also resulted in rarefaction, edema and hemangiectasia of hippocampus, nervous cells degeneration and necrosis, decrease or disappearance of Nissel bodies. The injuries were more serious in field CA4 and dentate gyrus, which showed dose-effect relationship, and were progressively aggravated within 7 days. The apoptosis cells were significantly increased. NSE was increased in neurons. The NSE positive areas were also seen in the interstitial matrix and blood vessels. GFAP was increased in astrocytes, which became shorter and thicker.

CONCLUSION

HPM can damage the abilities of learning and memory and results in morphologic changes in hippocampus. The major pathologic changes are degeneration, apoptosis and necrosis of neurons and edema in interstitium. NSE and GFAP play an important role in the pathologic process.

Facilitation of visuo-motor learning by transcranial direct current stimulation of the motor and extrastriate visual areas in humans

Performance of visuo-motor tasks requires the transfer of visual data to motor performance and depends highly on visual perception and cognitive processing, mainly during the learning phase. The primary aim of this study was to determine if the human middle temporal (MT)+/V5, an extrastriate visual area that is known to mediate motion processing, and the primary motor cortex are involved in learning of visuo-motor coordination tasks. To pursue this, we increased or decreased MT+/V5, primary contralateral motor (M1) and primary visual cortex excitability by 10 min of anodal or cathodal transcranial direct current stimulation in healthy human subjects during the learning phase of a visually guided tracking task. The percentage of correct tracking movements increased significantly in the early learning phase during anodal stimulation, but only when the left V5 or M1 was stimulated. Cathodal stimulation had no significant effect. Also, stimulation of the primary visual cortex was not effective for this kind of task. Our data suggest that the areas V5 and M1 are involved in the early phase of learning of visuo-motor coordination.

Microwave effects on the nervous system

Studies have evaluated the electroencephalography (EEG) of humans and laboratory animals during and after Radiofrequency (RF) exposures. Effects of RF exposure on the blood-brain barrier (BBB) have been generally accepted for exposures that are thermalizing. Low level exposures that report alterations of the BBB remain controversial. Exposure to high levels of RF energy can damage the structure and function of the nervous system. Much research has focused on the neurochemistry of the brain and the reported effects of RF exposure. Research with isolated brain tissue has provided new results that do not seem to rely on thermal mechanisms. Studies of individuals who are reported to be sensitive to electric and magnetic fields are discussed. In this review of the literature, it is difficult to draw conclusions concerning hazards to human health. The many exposure parameters such as frequency, orientation, modulation, power density, and duration of exposure make direct comparison of many experiments difficult. At high exposure power densities, thermal effects are prevalent and can lead to adverse consequences. At lower levels of exposure biological effects may still occur but thermal mechanisms are not ruled out. It is concluded that the diverse methods and experimental designs as well as lack of replication of many seemingly important studies prevents formation of definite conclusions concerning hazardous nervous system health effects from RF exposure. The only firm conclusion that may be drawn is the potential for hazardous thermal consequences of high power RF exposure.

[Effects of 2450 MHz microwave on long-term potentiation of hippocampus and lipofuscin contents in rat brain]

OBJECTIVE

To study the mechanism of the effects of microwave on learning and memory.

METHOD

Long-term potentiation (LTP) of hippocampus induced potential and lipofuscin content in rat brain were studied. After irradiated by a 2450 MHz microwave, rats hippocampus induced potential in vivo was recorded and lipofuscin content in the brain was measured by fluorospectrophotometry.

RESULT

Continuous microwave with 10-25 mW/cm2 intensity could inhibit the amplitude of the population spike (PS) of weak and strong stimuli induced LTP with an intensity-effect relationship. At 25 mW/cm2, lipofuscin content was significantly higher than control and 10 mW/cm2 group (P<0.05).

CONCLUSION

Continuous microwave with 10-25 mW/cm2 intensity impairs learning and memory by restraining hippocampus LTP and brain lipofuscin content.

How the Shape of Pre- and Postsynaptic Signals Can Influence STDP: A Biophysical Model

Spike-timing-dependent plasticity (STDP) is described by long-term potentiation (LTP), when a presynaptic event precedes a postsynaptic event, and by long-term depression (LTD), when the temporal order is reversed. In this article, we present a biophysical model of STDP based on a differential Hebbian learning rule (ISO learning). This rule correlates presynaptically the NMDA channel conductance with the derivative of the membrane potential at the synapse as the postsynaptic signal. The model is able to reproduce the generic STDP weight change characteristic. We find that (1) The actual shape of the weight change curve strongly depends on the NMDA channel characteristics and on the shape of the membrane potential at the synapse. (2) The typical antisymmetrical STDP curve (LTD and LTP) can become similar to a standard Hebbian characteristic (LTP only) without having to change the learning rule. This occurs if the membrane depolarization has a shallow onset and is long lasting. (3) It is known that the membrane potential varies along the dendrite as a result of the active or passive backpropagation of somatic spikes or because of local dendritic processes. As a consequence, our model predicts that learning properties will be different at different locations on the dendritic tree. In conclusion, such site-specific synaptic plasticity would provide a neuron with powerful learning capabilities.

The effects of heavy particle irradiation on exploration and response to environmental change

Free radicals produced by exposure to heavy particles have been found to produce motor and cognitive behavioral toxicity effects in rats similar to those found during aging. The present research was designed to investigate the effects of exposure to 56Fe particles on the ability of male Sprague-Dawley rats to detect novel arrangements in a given environment. Using a test of spatial memory previously demonstrated to be sensitive to aging, open field activity and reaction to spatial and non-spatial changes were measured in a group that received a dose of 1.5 Gy (n=10) of 56Fe heavy particle radiation or in non-radiated controls (n=10). Animals irradiated with 1.5 Gy of 56Fe particles exhibited some age-like effects in rats tested, even though they were, for the most part, subtle. Animals took longer to enter, visited less and spent significantly less time in the middle and the center portions of the open field, independently of total frequency and duration of activity of both groups. Likewise, irradiated subjects spend significantly more time exploring novel objects placed in the open field than did controls. However, irradiated subjects did not vary from controls in their exploration patterns when objects in the open field were spatially rearranged. Thus, irradiation with a dose of 1.5 Gy of 56Fe high-energy particle radiation elicited age-like effects in general open field exploratory behavior, but did not elicit age-like effects during the spatial and non-spatial rearrangement tasks.

The effects of proton exposure on neurochemistry and behavior

Future space missions will involve long-term travel beyond the magnetic field of the Earth, where astronauts will be exposed to radiation hazards such as those that arise from galactic cosmic rays. Galactic cosmic rays are composed of protons, alpha particles, and particles of high energy and charge (HZE particles). Research by our group has shown that exposure to HZE particles, primarily 600 MeV/n and 1 GeV/n 56Fe, can produce significant alterations in brain neurochemistry and behavior. However, given that protons can make up a significant portion of the radiation spectrum, it is important to study their effects on neural functioning and on related performance. Therefore, these studies examined the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by the Morris water maze. Male Sprague-Dawley rats received a dose of 0, 1.5, 3.0 or 4.0 Gy of 250 MeV protons at Loma Linda University and were tested in the different behavioral tests at various times following exposure. Results showed that there was no effect of proton irradiation at any dose on any of the endpoints measured. Therefore, there is a contrast between the insignificant effects of high dose proton exposure and the dramatic effectiveness of low dose (<0.1 Gy) exposures to 56Fe particles on both neurochemical and behavioral endpoints.

Radiation-induced cognitive dysfunction: The protective effect of ethyol in young rats

PURPOSE

To evaluate the protective learning and memory effect of Ethyol in irradiated young rats.

METHODS AND MATERIALS

One hundred twenty-eight 45-day-old Wistar rats received whole brain fractionated radiation (30 Gy), whereas 48 rats received sham irradiation. Four irradiated subgroups were defined: saline, 37.5 mg/kg, 75 mg/kg, and 150 mg/kg Ethyol. Sequential behavioral studies including one-way and two-way avoidance tasks were undertaken before and after radiation.

RESULTS

Before radiation, the performances of all groups were similar. For the one-way avoidance task, at 1, 3, and 6 months postradiation, saline-irradiated rats had a lower percentage of avoidance than sham- or Ethyol- (75 or 150 mg/kg) irradiated rats (p <or= 0.001). Nevertheless, lower percentage of avoidance (p <or= 0.001) was only seen at 1 month postradiation in saline-irradiated rats, after comparison with their preradiation values. For the two-way avoidance task, performed only after radiation, saline-irradiated rats had a lower percentage of avoidance than sham- or Ethyol- (150 or 75 mg/kg) irradiated rats at 1, 2.5, 4.5, and 7.5 months (p = 0.000). Rats treated with Ethyol did not differ from sham-irradiated rats in both avoidance tasks, except for the 37.5 mg/kg-Ethyol group which showed lower performances 6 months postradiation (p = 0.005, one-way avoidance).

CONCLUSIONS

Treatment with 75 or 150 mg/kg of Ethyol prevents radiation-induced learning and transitory memory dysfunction in young rats.

Suppression of adult neurogenesis impairs olfactory learning and memory in an adult insect

Although adult neurogenesis has now been demonstrated in many different species, the functional role of newborn neurons still remains unclear. In the house cricket, a cluster of neuroblasts, located in the main associative center of the insect brain, keeps producing new interneurons throughout the animal's life. Here we address the functional significance of adult neurogenesis by specific suppression of neuroblast proliferation using gamma irradiation of the insect's head and by examining the impact on the insect's learning ability. Forty gray irradiation performed on the first day of adult life massively suppressed neuroblasts and their progeny without inducing any noticeable side effect. We developed a new operant conditioning paradigm especially designed for crickets: the "escape paradigm." Using olfactory cues, visual cues, or both, crickets had to choose between two holes, one allowing them to escape and the other leading to a trap. Crickets lacking adult neurogenesis exhibited delayed learning when olfactory cues alone were used. Furthermore, retention 24 hr after conditioning was strongly impaired in irradiated crickets. By contrast, when visual cues instead of olfactory ones were provided, performance of irradiated insects was only slightly affected; when both olfactory and visual cues were present, their performance was not different from that of controls. From these results, it can be postulated that newborn neurons participate in the processing of olfactory information required for complex operant conditioning.

Spatial learning and memory deficits induced by exposure to iron-56-particle radiation

It has previously been shown that exposing rats to particles of high energy and charge (HZE) disrupts the functioning of the dopaminergic system and behaviors mediated by this system, such as motor performance and an amphetamine-induced conditioned taste aversion; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, spatial learning and memory were assessed in the Morris water maze 1 month after whole-body irradiation with 1.5 Gy of 1 GeV/nucleon high-energy (56)Fe particles, to test the cognitive behavioral consequences of radiation exposure. Irradiated rats demonstrated cognitive impairment compared to the control group as seen in their increased latencies to find the hidden platform, particularly on the reversal day when the platform was moved to the opposite quadrant. Also, the irradiated group used nonspatial strategies during the probe trials (swim with no platform), i.e. less time spent in the platform quadrant, fewer crossings of and less time spent in the previous platform location, and longer latencies to the previous platform location. These findings are similar to those seen in aged rats, suggesting that an increased release of reactive oxygen species may be responsible for the induction of radiation- and age-related cognitive deficits. If these decrements in behavior also occur in humans, they may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere.

50 Hz magnetic field exposure influence on human performance and psychophysiological parameters: two double-blind experimental studies

Two double-blind studies were performed to examine magnetic field (MF) exposure effects and to determine the impact of temporal variation (continuous vs. intermittent exposure) of 100 mu T(rms) 50 Hz MF diurnal exposure on psychological and psychophysiological parameters in healthy humans. Three cephalic exposure sessions of 30-min, i.e., sham, continuous, and intermittent (15 s ON/OFF cycles) MF conditions, were involved. Each subject participated in all sessions, which were spaced at 1-wk intervals. In each session, mood ratings and performance measures were obtained before, during, or after exposure and several electrophysiological data (event-related brain potentials [ERP]) were recorded after each exposure session. These criteria were chosen to evaluate sensory functions as well as automatic and voluntary attentional processes. In experiment 1, 21 healthy male volunteers (20 to 27 years of age) were studied. Ten subjects were exposed at 13:30 h, and 11 subjects were exposed at 16:30 h. Statistically significant changes in the amplitude of ERP were observed after MF exposure in the dichotic listening task, indexing selective attention processes. Eighteen of the 21 original male volunteers took part in experiment 2, undertaken to better understand the results related to information processing involved in selective attention and control for ultradian rhythmicity. Exposure time for all the subjects was at 13:30 h. The analysis of the data again revealed significant amplitude changes of the ERP recorded in the dichotic listening task. Moreover, they demonstrated ERP latency and reaction time slowing in the oddball paradigm, a visual discrimination task after real MF exposure. These results also indicate that a low level 50 Hz MF may have a slight influence on event-related potentials and reaction time under specific circumstances of sustained attention.

Effects of prenatal low-dose beta radiation from tritiated water on learning and memory in rats and their possible mechanisms

Pregnant adult Wistar rats were randomly divided into four groups. Three of these groups were irradiated with beta rays by a single intraperitoneal injection of tritiated water ((3)H(2)O) administered on the 13th day of gestation. The doses absorbed by their offspring were estimated to be 4.6, 9.2 and 27.3 cGy. The influence of radiation on the postnatal learning ability and memory behavior and on brain development of the offspring was investigated. The number of pyramidal cells (in areas CA1, CA2, CA3 and CA4) and neurons in the hippocampus of the offspring was also measured. In addition, the Ca(++) conductance of hippocampal pyramidal cells cultured in vitro was observed. The results showed that an exposure to 4.6 cGy could prolong avoidance response time significantly and decrease the number of hippocampal pyramidal cells in the CA1 area compared to controls. An exposure to 9.2 cGy significantly decreased the establishment of conditioned reflexes and the number of hippocampal pyramidal cells in the CA3 area. This exposure also induced the degeneration and malformation of hippocampal neurons cultured in vitro, in addition to decreasing the number of hippocampal neurons observed on each culture day. A dose of 27.3 cGy significantly decreased brain and body weights and the maximum electric conductance of Ca(++) in hippocampal pyramidal neurons. In general, dose-dependent effects were observed for most of the parameters assessed in the present study. Possible mechanisms are discussed.

[Effects of low-dose tritiated water on the developing central nerve system and their mechanisms]

OBJECTIVES

To provide experimental data for evaluating tritium risk of nuclear-energy plant and instituting safety protection standard of nuclear effects of low-dose tritiated water(HTO) on the developing central nerve system.

METHODS

Pregnant adult C57BL/6J strain mice and Wistar rats were irradiated with beta-rays from HTO by a single intraperitoneal injection on the 12.5th and 13th day of gestation. The activity of HTO was (x 10(4) Bq/g. B.W) 24.09, 48.18 and 144.54, respectively. Neurobehavior and learning and memory ability were tested. Ca2+ currents of hippocampal neurons were examined by patch-clamp. Apoptosis of brain cells was detected morphologically and biochemically by DNA electrophoresis. SDS-PAGE and Western blotting were used to detect P53 protein in the cultures.

RESULTS

After irradiation with beta-rays from HTO in utero, the offspring showed hyperactivity in their young age period and then showed depression-hypoactivity in adulthood. The exposed animals had difficulties in both learning and memory retention and the developing neurons of the central nerve system showed lower Ca2+ currents, higher number of apoptosis, a typical "ladder pattern" DNA and increasing P53 protein.

CONCLUSIONS

Lower Ca2+ currents, higher number of apoptosis of brain cells and abnormal expression of P53 protein were the substance basis of the damage of central nerve system caused by tritium.

Behavioral evaluation of microwave irradiation

Establishing safe exposure levels for microwave irradiation is important since new, more powerful emitters are developed and the potential for accidental exposure is increasing. Analysis of the behavior of exposed laboratory animals has proven to be an accurate and repeatable metric for assessing the effects of microwave irradiation. Determining the specific absorption rate (SAR) at which an animal will cease an ongoing behavior has proven useful in the development of safe exposure levels for humans. Behaviors that have been used are simple tasks, and the point at which behavior changes significantly or ceases has often been referred to as "work stoppage." The tasks have been used to evaluate the overwhelming effects of heating produced by microwave irradiation. Both whole-body exposures and partial-body exposures with hotspots have been evaluated. Recent studies have suggested that microwave effects on specific cognitive aspects of behavior such as attention, learning, memory, discrimination, and time perception may occur at SAR levels far below the SARs needed to cause work stoppage. New research studies are underway to evaluate microwave-induced cognitive effects.

Differential learning impairments produced by prenatal exposure to ionizing radiation in mice

PURPOSE

To investigate the behavioural effects of prenatal irradiation on different days of gestation on the performance of two learning tasks by adult mice.

MATERIALS AND METHODS

CD1 mice were exposed in utero to 1 Gy of 250 kV X-rays on gestational days 13, 15 or 18. Other animals were sham-exposed. Male mice were tested as adults in a radial arm maze on two learning tasks considered dependent upon either spatial memory or visual associative memory.

RESULTS

Performance of the animals on the tasks was a function of the day on which exposure occurred. Compared with sham-exposed animals, exposure on day 18 produced a highly significant deficit in performance on the spatial task, and a small improvement in the visually cued task. Exposure on day 15 produced no deficit in performance on the spatial task, but a highly significant deficit in the cued task. Exposure on day 13 produced no significant deficits on either task.

CONCLUSIONS

These differential effects on performance appear to be consistent with radiation-induced insult to different memory systems within the developing mouse brain. These and further studies will help provide better estimates of the risks of radiation at different times during gestation on cognitive function in humans.