Hippocampal NMDA receptors are involved in rats' spontaneous object recognition only under high memory load condition.
The possible involvement of hippocampal N-methyl-D-aspartate (NMDA) receptors in spontaneous object recognition was investigated in rats under different memory load conditions. We first estimated rats' object memory span using 3-5 objects in "Different Objects Task (DOT)" in order to confirm the highest memory load condition in object recognition memory. Rats were allowed to explore a field in which 3 (3-DOT), 4 (4-DOT), or 5 (5-DOT) different objects were presented. After a delay period, they were placed again in the same field in which one of the sample objects was replaced by another object, and their object exploration behavior was analyzed. Rats could differentiate the novel object from the familiar ones in 3-DOT and 4-DOT but not in 5-DOT, suggesting that rats' object memory span was about 4. Then, we examined the effects of hippocampal AP5 infusion on performance in both 2-DOT (2 different objects were used) and 4-DOT. The drug treatment before the sample phase impaired performance only in 4-DOT. These results suggest that hippocampal NMDA receptors play a critical role in spontaneous object recognition only when the memory load is high.
Institute of Psychology and Behavioral Neuroscience, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan.
Manami
M
Sugita
Institute of Psychology and Behavioral Neuroscience, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan. Electronic address: kayamada@human.tsukuba.ac.jp.
Kazuo
K
Yamada
Institute of Psychology and Behavioral Neuroscience, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan.
Natsumi
N
Iguchi
Institute of Psychology and Behavioral Neuroscience, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan.
Yukio
Y
Ichitani
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D000704:Analysis of Variance; D000818:Animals; D004192:Discrimination, Psychological / Q000187:drug effects; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005106:Exploratory Behavior / Q000187:drug effects / Q000502:physiology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D021641:Recognition, Psychology / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors; D014796:Visual Perception / Q000187:drug effects / Q000502:physiology
D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D015763:2-Amino-5-phosphonovalerate
Hippocampus; Memory load; NMDA receptors; Rats; Spontaneous object recognition