High frequency repetitive stimulation of an excitatory input in the hippocampus results in a post-tetanic potentiation (PTP) of short duration (about 3 min) that can be followed by a long-term synaptic potentiation (LTP) of the same excitatory input (Schwartzkroin and Wester, 1975; Andersen
et al., 1977). It has been reported that this tetanus-induced LTP cannot be
elicited in a Ca²⁺-free medium and is therefore a Ca²⁺-dependent process
(Dunwiddie et al., 1978; Dunwiddie and Lynch, 1979; Wigstrӧm et al., 1979).
Whether the induction of LTP is directly dependent upon Ca²⁺, or whether, Ca²⁺ is required because synaptic transmission is needed to initiate
certain postsynaptic process(es) (a postsynaptic depolarization, for
instance) leading to LTP, is unknown. Recent studies from this laboratory
showed that both short-term potentiation (STP; with a duration resembling
PTP) and LTP can be associatively induced if activation of a test input
co-occured with either a tetanic stimulation of separate excitatory inputs
or a sufficient depolarization of the postsynaptic neurone (Sastry et al.,
1985). In this study, experiments were performed to investigate (1) whether
associative STP could be induced when activation of the test input preceded
or followed the onset of the conditioning train and (2) whether LTP could be
induced in the absence of Ca²⁺ in the extracellular medium if sufficient depolarizations of the presynaptic terminals and postsynaptic neurones were provided.
All experiments were performed using the transversely sectioned hippocampal slice preparation. Test stimuli were delivered via an electrode located in the stratum radiatum while the conditioning tetani (100 Hz, 10 pulses per train) were delivered via another electrode located in the recorded from the apical dendritic area of CA₁ neurones. After the initial control stimulation period, 5 conditioning tetani were given at a frequency of 0.2 Hz. The test stimuli either preceded (-) or followed ( + ) the onset of each conditioning train by 0 to 100 ms. When the test stimulus followed the onset of each conditioning train, there was significant STP of the test EPSP up to a conditioning-test interval of +80 ms. When the test stimulus preceded the onset of each conditioning train, there was significant STP of the test EPSP up to a conditioning-test interval of -50 ms. Conditioning tetani that were given without co-activation of the test input resulted in a subsequent depression of the test EPSP. It is suggested that either the test or the conditioning input can initiate some postsynaptic process(es) which can in turn affect the activated presynaptic terminals to increase transmitter release or alter the subsynaptic dendritic properties.
For studying the possibility of the induction of LTP in the absence of
Ca²⁺ in the extracellular medium, population EPSPs were recorded from
apical dendritic area of CA₁ neurones in response to stratum radiatum
stimulation. After the control stimulation period, slices were exposed
either to Ca²⁺-containing or Ca²⁺-free (with Mn²⁺ and Mg²⁺ replacing
Ca²⁺) medium, with the concentration of KC1 at 10 to 80 mM. Long-term
potentiation of the population EPSPs was observed following the exposure to
high K⁺ in Ca²⁺-free media. Following a brief period of potentiation
initially, population EPSPs often exhibited a tendency toward depression
after exposure to high K⁺ in Ca²⁺-containing media. LTP induced by high
K⁺ in Ca²⁺-free medium could also be observed when a fixed number of axons were being activated, indicating that a recruitment of presynaptic fibres cannot entirely account for the potentiation. LTP of the depolarizing commands were paired with activation of the stratum radiatum
while the slices were exposed to Ca²⁺ -free medium (normal concentration of
KC1). These results suggest that extracellular Ca²⁺, synaptic transmission and thus subsynaptic receptor activation are not necessary for the induction of LTP as long as sufficient depolarizations of the presynaptic terminals and postsynaptic neurones are provided. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/26497 |
| Date | January 1987 |
| Creators | May, Patrick B. Y. |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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