Graduate School of Frontier Biosciences, Osaka University

Japanese

Synaptic Plasticity Group

  Name Email TEL
Associate Prof. TOMINAGA-YOSHINO, Keiko +81-6-6879-4662
TEL +81-6-6879-4661
FAX +81-6-6879-4661
Postal Mail Address Graduate School of Frontier Biosciences
1-3 Yamadaoka, Suita, Osaka 565-0871 Japan

One of the most typical functions of the brain is memory. Some 20 years ago, memory was a theme in the field of psychology. But it is now in the field of biology. Brain functions are realized by the activity of neuronal networks composed of a huge number of neurons. The efficiency of information transfer within the networks is changeable. Even the networks themselves can change by experience. Information transfer between neurons is performed at synapse, the site of neurons' contact, by releasing information molecule (=neurotransmitter) from pre-synaptic cell and capturing it by post-synaptic cell. The amount of released neurotransmitter or the efficacy of capture can change. Moreover, synapses are found to be newly formed upon activity or abandoned upon inactivity. These changes are called "synaptic plasticity". Thus, the fundamental process of brain memory can be analyzed biologically in the questions such as: how does the release of neurotransmitter increase; how do the terminals of neurons bud out. Memory is classified into a short-term one, quickly formed but short-living, and a long-term one, slowly formed but long-living. Biologically, the former corresponds to a short-term plasticity, the process occurring in existing synapse with no need of protein synthesis, and the latter to a long-term plasticity, the process accompanied by new synapse formation (=synaptogenesis) and protein synthesis. The former was profoundly analyzed after the discovery of excellent model systems for analysis such as "sensitization of gill withdrawal response in Aplysia (=a sea slug)" or "post-tetanic potentiation in hippocampus (=a part of cerebral cortex)". The year 2000's Nobel Prize for Medicine was awarded to Dr. Eric Kandel for his study on Aplysia.  I (Ogura) was once engaged in the study of post-tetanic potentiation and made a small contribution to it by showing the critical role of postsynaptic calcium elevation. The cellular bases of short-term plasticity have been largely revealed by the analyses of these phenomena.  In contrast, the the cellular bases of long-term plasticity remain still nearly unexplored. It is mainly because of the absence of a good model system for analysis. Once it is established, the biologists of the world can concentrate their efforts into that and the mechanism should be revealed, as was the case in the short-term plasticity. We are challenging this issue.

21_img1.jpg




Fig.1 Scanning electron micrograph of a cultured hippocampal neuron growing on a glial cell







 

21_img2.jpg




Fig.2 Hippocampal slice cultured for 14 days

1. Activity-induced synapse formation

Brain slices prepared from newborn rat pups can be maintained for months, if oxygen and nutrition are properly supplied. This "cultured brain slice" is amenable to the long-term microscopic observation of ongoing processes, to drug application, or to gene manipulation. It is a good candidate for the wanted model system. We recently found that repetitive stimulation leads to synaptic reinforcement accompanied by synaptogenesis. Single, however strong, stimulation was not enough. Three repetitions, with a proper interval, were needed. We are now intriguing what different cellular events follow after the 1st and 3rd stimulations, what processes occur in the interval periods, etc.

21_img3.jpg



Fig.3 Growing tip of a neuron in a cultured hippocampal slice after stimulation








21_img4.jpg


Fig.4 Electron micrograph of a cultured slice with synapses bifurcating after stimulation

 

2. Inactivity-induced synapse elimination and neuronal death

Neurons isolated from newborn rat's cerebellum cannot be maintained long, unless stimulant (=potassium chloride or glutamic acid) was added to the culture medium. This activity-dependent survival or inactivity-induced apoptosis (=cell death) had been thought that an elevation of intracellular Ca2+ level above a certain limit due to an enhanced Ca2+ inflow guarantees the survival (by blocking apoptotic signal cascades). But direct measurements of Ca2+ done by us did not show the difference in cytoplasmic Ca2+. Enhanced Ca2+ inflow was counterbalanced by enhanced Ca2+ pumping. What differed between surviving and dying neurons were local Ca2+ turnover and exo/endocytotic behavior. Probably some neurotrophic (=survival promoting) substance is released and incorporated. We further hypothesize that the death of a whole cell may share a common mechanism with the death of an individual synapse.

3. Physiology of glial cells

The brain is composed not only of nerve cells but also of glial cells. Glial cells have long been regarded as physiologically dull cells, such like a mere structural support or a housekeeper. Recently, however, they are found to exert important functions such as the synthesis of neurotrophic substances. They transmit signals between them quite similarly to neurons (although slow). We like to cast new light to glial cell functions.

Back