Kazuo Yamada

Maternal stress induces synaptic loss and developmental disabilities of offspring.

Mild prenatal stress affects the serotonergic system in the hippocampus of rat offspring. Pregnant rats were daily exposed to mild stress treatments (consisting of crowding and saline injection) during days 15 to 21 of pregnancy. Their offspring were assessed by a series of biochemical, histological and behavioral tests. On 35 days after birth, 5‐hydroxytryptamine (5‐HT) level was decreased by 17% (P < 0.05), whereas 5‐hydroxyindolacetic acid (5‐HIAA) level was increased by 18% (P < 0.05) in the offspring of prenatally stressed rats. The metabolic rate (5‐HIAA/5‐HT) was increased by 49% (P < 0.01). Synaptic density in the hippocampus of prenatally stressed offspring was also decreased by 32% (P < 0.0001) on postnatal day 35. There was no significant group difference in the spatial learning acquisition test of the Morris water maze ; however, in the reversal task, prenatally stressed 5‐week old rats spent more time than control animals searching for the platform of the pool. Escape latency in the cued test showed no significant difference. Together with data in our previous studies, that have shown 5‐HT to facilitate synapse formation and maintenance in the central nervous system, synaptic loss is suggested to occur in relation to changes of 5‐HT system in the hippocampus of prenatally stressed offspring. This may be associated with reported changes in behavior and learning ability in prenatally stressed offspring.

Long-term object location memory in rats: effects of sample phase and delay length in spontaneous place recognition test.

This study investigates the effects of sample phase and delay length on discrimination performance in the spontaneous place recognition (SPR) test in rats. Rats were allowed to explore an arena where two identical objects were presented for 5-20 min (sample phase). After a delay interval, rats were placed again in the same arena but one of the two objects was moved to a novel place (test phase). Results showed that when the sample phase was as long as 20 min, rats preferentially explored the moved object during the test phase even after a 6-24h delay was interposed. Further sequential and cumulative analyses of the test phase revealed that the preference for the object in a novel place was evident in the first and 2nd min of the test phase in rats with a longer sample phase duration. Correlation analysis showed that locomotor activity and object exploration in the sample phase were not decisive factors in spatial memory performance. The present results demonstrate the importance of the sample phase exposure time and the test phase length.

Expression of the mouse macrophage cystine transporter in Xenopus laevis oocytes

System xc- mediates transport of cystine and glutamate across the mammalian plasma membrane in a Na(+)-independent manner. This transport activity can be induced in mouse peritoneal macrophages during culture by diethylmaleate, a sulfhydryl-reactive agent. We injected mRNA from such macrophages into Xenopus oocytes and demonstrated the expression of System xc-, i.e., a Na(+)-independent, glutamate-inhibitable cysteine transport system. The expressed cystine transport activity depended on the assay temperature, in that cystine uptake measured at 37 degrees C was severalfold higher than that measured at 20 degrees C. Injection of size-fractionated mRNA indicated that the System xc- transporter of the mouse macrophage is encoded by mRNA of 1.5 to 2.9 kb.

MK-801-induced and scopolamine-induced hyperactivity in rats neonatally treated chronically with MK-801.

This study investigated the effects of chronic neonatal blockade of N-methyl-D-aspartate (NMDA) receptors on NMDA and muscarinic acetylcholine receptor-mediated neurotransmission in adulthood. Rats neonatally treated chronically with MK-801/saline were tested for 40 min, at the age of 14–16 weeks, for locomotor activity in an open field immediately after acute administration of MK-801 (0.2 mg/kg) or scopolamine (0.4–2.0 mg/kg). Rats neonatally treated with MK-801 showed significantly higher locomotor activity than those treated with saline. Acute MK-801 administration caused hyperlocomotion regardless of neonatal treatment, but the effect was more potent in rats neonatally treated with MK-801. In contrast, acute scopolamine administration did not cause hyperlocomotion in rats neonatally treated with saline, but significantly increased locomotion in those neonatally treated with MK-801. The results suggest that chronic neonatal NMDA receptor blockade causes changes in glutamatergic and cholinergic transmission in adulthood long after the cessation of treatment.

Hippocampal AP5 treatment impairs both spatial working and reference memory in radial maze performance in rats.

The possible involvement of hippocampal N-methyl-d-aspartate (NMDA) receptors in spatial reference and working memory was investigated. Rats were first trained in a four-baited/four-unbaited version of the eight-arm radial maze task in which only predetermined four arms for each rat were baited with a food pellet. After rats reached the learning criterion, their performance was tested under the treatment of a NMDA antagonist, AP5 (d,l-2-amino-5-phosphonopentanoic acid, 20-40nmol), or vehicle into the dorsal hippocampus through the bilaterally implanted guide cannulae. AP5 produced dose-dependent increments on both reference and working memory errors, but did not have any effect on the running speed. Additionally, there were significant correlations between the number of trials to criterion in acquisition and the number of reference and working memory errors induced by AP5 treatment. The results suggest that hippocampal NMDA receptors are involved in both spatial reference and working memory.

Nicotine improves AF64A-induced spatial memory deficits in Morris water maze in rats.

Ethylcholine mustard aziridinium ion (AF64A) is a neurotoxic derivative of choline that produces not only long-term presynaptic cholinergic deficits, but also various memory deficits in rats similar to some characteristics observed in Alzheimers disease patients. This study investigated whether nicotine (NCT) administration attenuated spatial learning deficits induced by intracerebroventricular AF64A treatment. AF64A (6 nmol/6 microl)-or saline (SAL)-treated rats were trained in Morris water maze task. NCT (0.025-0.25mg/kg) was subcutaneously injected 5 min before the training every day. The results showed that moderate dose (0.10mg/kg) of NCT attenuated AF64A-induced prolongation of escape latency. Furthermore, NCT dose-dependently recovered the AF64A-induced decrease of time spent in the target quadrant in the probe test. These results suggest that NCT improves AF64A-induced spatial memory deficits, and thus it is a potential therapeutic agent for the treatment of memory deficits in dementia.

Tickling during adolescence alters fear-related and cognitive behaviors in rats after prolonged isolation

Social interactions during adolescence are important especially for neuronal development and behavior. We recently showed that positive emotions induced by repeated tickling could modulate fear-related behaviors and sympatho-adrenal stress responses. In the present study, we examined whether tickling during early to late adolescence stage could reverse stress vulnerability induced by socially isolated rearing. Ninety-five male Fischer rats were reared under different conditions from postnatal day (PND) 21 to 53: group-housed (three rats/cage), isolated-nontickled (one rat/cage) and isolated-tickled (received tickling stimulation for 5min a day). Auditory fear conditioning was then performed on the rats at PND 54. Isolated-tickled rats exhibited significantly lower freezing compared with group-housed rats in the first retention test performed 48h after conditioning and compared with isolated-nontickled rats in the second retention test performed 96h after conditioning. Moreover, group-housed and isolated-tickled rats tended to show a significant decrease in freezing responses in the second retention test; however, isolated-nontickled rats did not. In the Morris water maze task that was trained in adulthood (PND 88), but not in adolescence (PND 56), isolated-nontickled rats showed slower decrease of escape latency compared to group-housed rats; however, tickling treatment significantly improved this deficit. These results suggest that tickling stimulation can alleviate the detrimental effects of isolated rearing during adolescence on fear responses and spatial learning.

Long-term effects of traumatic stress on subsequent contextual fear conditioning in rats

Exposure to stressful events affects subsequent sensitivity to fear. We investigated the long-term effects of a traumatic experience on subsequent contextual fear conditioning and anxiety-like behaviors in rats (Experiment 1). In addition, we tested whether the administration of the glucocorticoid synthesis inhibitor metyrapone (MET) attenuated the sensitization of fear induced by traumatic stress (Experiment 2). Male rats were subjected to a multiple stress (MS) session, which consisted of 4 foot shocks (1mA, 1s) and forced swimming for 20min, followed by exposure to a situational reminder 7days after the MS session. MET (25 or 100mg/kg, intraperitoneal) was administered 30min before MS. The contextual fear conditioning was performed 14days after MS. MS enhanced the conditioned fear response for at least 14days after the conditioning, and pretreatment with MET did not affect the enhancement of conditioned fear. These results suggest that glucocorticoid secretion triggered by MS is not involved in regulating the long-term stress-induced sensitization of fear.

Effects of repeated tickling on conditioned fear and hormonal responses in socially isolated rats.

Positive emotional states have been reported to modify human resilience to fear and anxiety, but few animal models are available to elucidate underlying mechanisms. In the current study, we examined whether 2 weeks of tickling, which is considered to evoke positive emotions, alters conditioned fear and hormonal reactions in Fischer rats. We conditioned rats to fear an auditory tone which was initially paired with a mild foot-shock (0.2mA), and retention test was conducted 48h and 96h after conditioning. During these tests, we found that prior tickling treatment significantly diminished fear-induced freezing. To examine the effects of tickling on sympatho-adrenal and hypothalamic-pituitary-adrenal responses associated with conditioned fear, we measured plasma catecholamine and corticosterone levels in the retention test 96h after conditioning. The plasma catecholamine concentration of non-tickled rats was higher than basal levels, whereas tickled rats showed significantly reduced concentrations of both plasma adrenaline and noradrenaline. No significant differences in plasma corticosterone levels were observed between tickled and non-tickled rats. These results suggest that repeated exposure to tickling can modulate fear-related behavior and sympatho-adrenal stress responses.

Sleep deprivation impairs spontaneous object-place but not novel-object recognition in rats.

Effects of sleep deprivation (SD) on one-trial recognition memory were investigated in rats using either a spontaneous novel-object or object-place recognition test. Rats were allowed to explore a field in which two identical objects were presented. After a delay period, they were placed again in the same field in which either: (1) one of the two objects was replaced by another object (novel-object recognition); or (2) one of the sample objects was moved to a different place (object-place recognition), and their exploration behavior to these objects was analyzed. Four hours SD immediately after the sample phase (early SD group) disrupted object-place recognition but not novel-object recognition, while SD 4-8h after the sample phase (delayed SD group) did not affect either paradigm. The results suggest that sleep selectively promotes the consolidation of hippocampal dependent memory, and that this effect is limited to within 4h after learning.