Taketoshi Ono

Effects of maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on parvalbumin- and calbindin-immunoreactive neurons in the limbic system and superior colliculus in rat offspring

Previous studies have reported that maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces socioemotional and cognitive disturbances in rat offspring. In the present study, the effects of maternal TCDD exposure on putative inhibitory interneurons were investigated in the medial prefrontal cortex (mPFC), basolateral amygdala (BLA), hippocampus (HP), and superior colliculus (SC) in rat offspring. Dams were given TCDD (1.0μg/kg) on gestational day 15. When offspring rats reached adulthood (14 weeks old), parvalbumin (PV)- and calbindin (Calb)-immunoreactive neurons were immunohistochemically investigated. The histological investigations indicated that the mean area of the mPFC had increased, whereas the mean area of the SC decreased in the exposed male rats. In the exposed female rats, the mean SC area increased. Furthermore, the number and area of PV-immunoreactive neurons increased in the mPFC of the female exposed rats. In contrast, the number of PV-immunoreactive neurons in the BLA, HP, and SC decreased in the male and female exposed rats. The number of Calb-immunoreactive neurons decreased in the HP of the male and female exposed rats and the SC of the female exposed rats. Because PV- and Calb-immunoreactive neurons, which are putatively GABAergic, have been implicated in various higher brain functions, the effects of TCDD on socioemotional and cognitive functions might be mediated partly through these alterations in PV- and Calb-immunoreactive neurons in these areas.

Hippocampal and Parahippocampal Neuronal Responses to Spatial and Non-Spatial Factors in Rats and Monkeys

Rat hippocampal (HF) neurons were recorded while the rat ran on a treadmill affixed to a motion stage that was translocated along a figure 8-shaped track. Comparison of HF spatial firing patterns across different experimental conditions indicated that place neuron activity encodes intra-maze multiple information including location of animals, locomotion, the reinforcement episodes, and vestibular sensation or optic flow. Place neurons were also recorded from the monkey HF during virtual navigation. Most place-differential responses disappeared or changed their spatial tuning (i.e., remapping) when the arrangements of the distal cues were altered/moved in the virtual spaces. The results suggest that the HF encodes multifold information within the maze, which are gated by the extra-maze distal cues.

Role of the hippocampal formation in sequence memory

It is suggested that episodic memory is defined as a context-dependent sequence memory, and that the hippocampal formation (HF) is essential in such memory. To investigate HF involvement in a context-dependent sequence memory, multiple single unit activities were recorded from the monkey HF during performance of real translocation and virtual navigation tasks. The results indicated that place-related neuronal activity in the HF was task-or context-dependent, and cross-correlation data suggest that the context-dependent information may be encoded by interaction among pyramidal neurons based on asymmetrical connections. Rat CA1 HF neurons were also recorded during a conditional sequence memory task. Consistent with the computational studies, 2 types of the HF neurons were found; some neurons responded to single item regardless of sequences in which the item was presented, while other neurons displayed sustained firing during serial presentation of several items. In humans, event-related potentials (ERPs) were recorded during a sound-sequence memory task. The results suggest that the ERPs around 300-700 msec latency were specifically involved in sound sequence information processing. Furthermore, equivalent dipoles for the ERPs were localized in the medial temporal lobe including the HF and parahippocampal gyrus. These results suggest that the HF is crucial in context-dependent sequence information processing, which may be the neural basis of episodic memory.