| Summary: | 碩士 === 國立成功大學 === 藥理學研究所 === 86 === There are two main research subjects in the present dissertation
:
The first subject focused on the effect of action of
topiramate (TPM) on the hippocampal neurons. Topiramate is a
structurally novel anticonvulsant, which was recently approved
for adjunctive therapy in the partial and secondarily
generalized seizures. The present study was aimed at elucidating
the mechanisms underlying the anticonvulsant efficacy of
topiramate using intra- and extracellular recording techniques
in the in vitro rat hippocampal slices.
When stimuli were delivered every 20 seconds, topiramate had
no measurable effect on both field excitatory postsynaptic
potentials (fEPSPs) and population spikes (PSs). However,
increasing the stimulation frequency from 0.05 to 0.2 Hz,
topiramate significantly decreased the slope of fEPSP and the
amplitude of PS in a concentration-dependent manner. The
amplitude of presynaptic fiber volley was also reduced.
Topiramate did not affect the magnitude of paired-pulse
inhibition and monosynaptically evoked inhibitory postsynaptic
potentials (IPSPs).
Sustained repetitive firing was elicited by injection of long
duration (500 msec) depolarizing current pulses (500-800 pA).
Superfusion with topiramate significantly reduced the number of
action potentials evoked by a given current pulse. After
blockade of GABA receptors by bicuculline, burst firing which
consisted of a train of several spikes riding on a large
depolarizing wave termed paroxysmal depolarizing shift (PDS) was
recorded. Application of topiramate reduced the duration of PDS
and later spikes with less effect on the initial action
potential.
These results suggest that frequency-dependent inhibition of
neuronal activity due to blockade of Na+ channels may account
for the anticonvulsant efficacy of topiramate. But topiramate
does not affect GABAergic or glutaminergic synaptic transmission
.
The second subject was aimed at investigating the role for
mitogen-activated protein kinases (MAP kinases) in the late
phase of long-term potentiation (LTP) in area CA1 of the
hippocampus. LTP is an activity-dependent enhancement of
synaptic transmission that is thought to function as a cellular
mechanism for learning and memory. Similar to the stages of
memory storage, LTP has both an early phase (E-LTP) which does
not depend on new protein synthesis, and a late phase (L-LTP)
which requires both translation and transcription. The L-LTP is
also critically depending on cAMP and activation of
cAMP-depend protein kinase.
MAP kinases are highly expressed in post-mitotic neurons that
act as regulators of cell proliferation and differentiation by
translocating into the nucleus. It has been shown that the
activation of MAP kinases by PKA is important for the
invertebrate species- Aplysia for learning and memory. Therefore
we want to study if the activation of MAP kinases by PKA also
play an important role in the mammal learning and memory by
using extracellular recording technique and p42/p44 MAP kinase a
ssay system in the in vitro rat hippocampal slices.
When forskolin and IBMX were added to the bath, fEPSP was
enhanced over 6 hours in the hippocampal CA1 area. To determine
the role for the MAP kinases in L-LTP, we perfused the compound
PD98059, an inhibitor of MEK, to block activation of the MAP
kinase cascade during the activation of PKA. We found PD98059
did disrupt L-LTP but not E-LTP or normal synaptic transmission.
We also found paired-pulse facilitation (PPF) ratio was
decreased when forskolin and IBMX were added, but returned to
the baseline after 2 hours later.
We next using the p42/p44 MAP kinase assay system to confirm
if the PKA could activate MAP kinases in the hippocampal CA1
area. After adding forskolin for 5 minutes, MAP kinases activity
was transiently increased, which returned to control level with
20 minutes.
These results suggest MAP kinases could be activated by the
activation of PKA, and through to regulate L-LTP in the
hippocampus CA1 area.
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