博士 / 國立陽明大學 / 生理學研究所 / 92 / Synaptic plasticity is a persistent change in the strength of synaptic efficacy that is long been considered as cellular mechanism of learning and memory in the mammalian CNS. Long-term potentiation (LTP) and long term depression (LTD) of excitatory synaptic transmission are two important forms of synaptic plasticity which is use-dependent change in synaptic efficacy, providing the best models of this cellular mechanism underlying memory storage in the CNS. Different experimental protocols typically used to induce LTP and LTD. Homosynaptic LTP can be induced by a brief high frequency stimulation; In contrast, one Hz stimulation for 15 min can reliably induce homosynaptic LTD. Associative LTP and LTD on the other hand can be induced by simultaneously pairing presynaptic stimulation with postsynaptic activity, including depolarization of postsynaptic membrane, back-propagating action potential of postsynaptic neurons, and activity produced by other synaptic inputs. The coincidence of action potentials（APs）and field excitatory postsynaptic potential（fEPSP）was found to induce changes in synaptic efficacy that was up- or down-regulated by pairing of APs and fEPSP. It is important that precise timing between pre- and postsynaptic activities may be used to encode information in neural networks. The induction of homosynaptic LTP in the rat hippocampus require N-methyl-D aspartate (NMDA) receptor activation and elevation of the postsynaptic calcium concentration, which in turn activate many second messengers required for LTP induction.
In this study, we first examined the effect of heat-shock pretreatment on homosynaptic LTP at rat hippocampal CA1 synapses. Immunohistochemical results indicated that the expression of heat shock protein 70（HSP70）in the CA1 area was elevated at 2 h, reached a maximum at 16 h, and then decayed at 48 h after heat-shock pretreatment. Thus, it is obvious that the expression of HSP70 was time-dependent. Focusing on that time point we found that high frequency stimulation (at 100 Hz) induced LTP of 191 ± 8% in control slices (n=7), which was suppressed by scopolamine to 115 ± 3 %. Heat-shock pretreatment successfully prevented such suppression (217% ± 38%, n=7). Both HSP expression and LTP responses were relatively small taken either 2 or 48 h after heat-shock treatment. These results suggest that the induction of HSPs is time-dependent and can prevent scopolamine-mediated LTP suppression.
Furthermore, we studied the role of b-adrenergic receptors in modulating associative LTP and LTD induced at rat hippocampal CA1 synapses. Two independent Schaffer collateral pathways were stimulated in hippocampal slices. The fEPSP response evoked in one pathway (the weak pathway) was small, usually 20–30% of the maximu, whereas a large response, usually 80–90% of the maximum, was evoked in the strong pathway. After recording of the baseline fEPSP evoked at 0.033 Hz, LTP of the weak pathway could be associatively induced by paired stimulation of the weak and strong pathways 100 times at 6 sec intervals, with stimulation of the weak pathway preceded 3–10 msec. However, pairing protocols with an interval between stimulation of the two pathways＞10 msec resulted in no LTP. The induced LTP was NMDA receptor dependent, because 50 mM D,L-AP5 blocked its induction. Bath application of 1 mM Iso enhanced LTP by increasing the window of the stimulation interval up to 15 msec but did not affect the magnitude of the LTP induced by pairing protocols with intervals＜10 msec. Similar results were obtained when the experiments were repeated using whole-cell recording. These results suggest that activation of b-adrenergic receptors can enhance associative LTP by increasing the width of the time window rather than the magnitude of the LTP. Enhancement of LTP by b-adrenergic receptors was blocked in slices by pretreatment with inhibitors of PKA or MAPK, suggesting that these signaling cascades are involved in this process. Associative LTD could be induced by paired stimulation of it and the strong pathway repeated 100 times at 0.167 Hz, with stimulation of the strong pathway preceding it within 100 ms. The associative LTD was NMDA receptor- and phophatase 2B-dependent because bath application of 50 mM D, L-AP5 or 10 mM cypermethrin blocked its induction. Bath application of 1 mM isoproterenol inhibited associative LTD, and this effect was blocked by timolol, suggesting the involvement ofβ-adrenergic receptors. The inhibitory effect of β-adrenergic receptors on LTD induction was blocked in slices pretreated with inhibitors of protein kinase A and mitogen-activated protein kinase, suggesting that these signal cascades are downstream effectors following activation of β-adrenergic receptors.
Finally, we investigated the expression of STDP at LPP synapses. One stimulating electrode was placed at LPP to evoke fEPSP. At the same time, another stimulating electrode was placed at mossy fiber to evoke antidromic APs. After 10 mins basal recording of fEPSP evoked at 0.033 Hz, associative LTP can be induced by pairing fEPSP with APs (fEPSP lead APs) with the delay interval within 30 ms for 100 times at 0.167 Hz. When the delay interval＞30 ms, no LTP was induced. LTP induced by the present protocol was NMDA receptor dependent, as 50 mM D, L-AP5 blocked its induction. Interestingly, pairing APs with fEPSP (APs lead fEPSP) result in LTD with the delay interval＜40ms. LTD induced here was also NMDA receptor dependent. LTP can not be induced when slices were pretreated with blockers of CaMKⅡ, PKC and MAPK, suggesting that these signal cascades were involved in the induction of associative LTP at LPP synapses. LTP and LTD induced by the present pairing protocol can be maintained for at least 4 hours suggesting present protocol can induce late phase LTP (L-LTP) and LTD (L-LTD). The L-LTP and L-LTD require protein synthesis because anisomycin blocked its expression.
| Identifer | oai:union.ndltd.org:TW/092YM005116011 |
| Date | January 2004 |
| Creators | Yi-Wen Lin, 林以文 |
| Contributors | Tsai-Hsien Chiu, Ming-Yuan Min, Hsiu-Wen Yang, 邱蔡賢, 閔明源, 楊琇雯 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 0 |
Page generated in 0.0018 seconds