Open-field behaviors and water-maze learning in the F substrain of Ihara epileptic rats.

PURPOSE: Genetically epileptic model rats, Ihara epileptic rat (IER/F substrain), have neuropathologic abnormalities and develop generalized convulsive seizures when they reach the age of approximately 5 months. Because the neuromorphologic abnormalities are centered in the hippocampus, we expected to observe spatial cognitive deficits. The present study aimed to evaluate emotionality and learning ability of the F substrain of IER. METHODS: To determine whether deficits are caused by inborn neuropathologic abnormalities or by repeated generalized convulsions, we tested nine 6- to 12-week-old IER/F rats that had not yet experienced seizures (experiment 1) and nine 7- to 9-month-old IER/F rats that had repeatedly experienced seizures (experiment 2) with identical tasks: an open-field test and the Morris water-maze place and cue tasks. RESULTS: Both groups of IER/Fs showed behaviors that were different from those of control rats in the open-field test, and extensive learning impairments were seen in both the place task, which requires spatial cognition, and the cue task, which does not require spatial cognition but requires simple association learning. Their impaired performance of the cue task indicates that their deficiency was not limited to spatial cognition. CONCLUSIONS: Because young IER/F rats without seizure experiences also showed severe learning impairments, genetically programmed microdysgenesis in the hippocampus was suspected as a cause of the severe learning deficits of IER/Fs.

Effects of prenatal maternal stress by repeated cold environment on behavioral and emotional development in the rat offspring.

It has been reported that many types of stresses, which caused physiological and psychological alterations in dams as prenatal maternal stress, affected behavioral and emotional traits of their offspring. However, effects of environmental temperature changes, which induce various stress responses in both animals and humans, have not been assessed as prenatal maternal stress. Repeated cold stress (RCS) is a type of chronic cold stress in which environmental temperature changes rapidly and frequently several times within a day. In the present study, to investigate effects of chronic maternal stress by the RCS on behavioral and emotional development of the rat offspring (prenatal RCS rats), the RCS stress was loaded to pregnant rats between day 9 and 19 after fertilization. The prenatal RCS rats showed similar locomotor activity in an open field to control rats that were borne by non-stressed pregnant rats. On the other hand, the prenatal RCS rats showed significantly higher startle responses than the control rats in a light enhanced startle paradigm. However, treatment of diazepam decreased the startle responses in the prenatal RCS rats to the same degree as those in the control rats. The results indicated that prenatal RCS affected emotional development of the rat offspring, but not locomotor activity. Comparison of the present results with the previous studies suggests that there might be unknown common mechanisms among different prenatal maternal stresses that induce similar behavioral developmental alteration.

Effect of lesions in the lateral nucleus of the amygdala on fear conditioning using auditory and visual conditioned stimuli in rats.

The lateral nucleus of the amygdala (LA) is believed to be the site of auditory conditioned stimulus (CS) relay in classical fear conditioning. The present study attempts to determine whether the LA is specifically involved in fear conditioning using an auditory CS. Seven rats with lesions in the LA (Tone-Lateral group) and eight sham-operated rats in the control group were trained using an auditory CS (overtone based on an 800 Hz fundamental tone, 70 dB, 3.7 s) paired with foot shock (1.0 mA, 0.5 s). Five rats with lesions in the LA (Light-Lateral group) and eight unoperated rats in the control group were trained using a visual CS (25 W light, 3.7 s). The behavioral index of fear conditioning was a potentiation of the startle reflex in the presence of CS. All rats in the control group and Light-Lateral group showed this potentiation, whereas those in the Tone-Lateral group did not. These results suggest that the LA is an input site of auditory CS information into the amygdala, and that it is not a site of visual CS information input in fear conditioning. Thus, each modality of CS may have a specific subnucleus of the amygdala that mediates fear conditioning.