The Effect of the Protein Synthesis Inhibitors Emetine, Anisomycin and Puromycin, the Adenylyl Cyclase Activator, Forskolin, and the Protein Kinase A Inhibitor Rp-cAMPs, on the Late Phase of Long-Term Potentiation

The aim of this project was to elucidate how different drugs (protein synthesis inhibitors, enzyme inhibitors and enzyme activators) affect the late phase of LTP (long-term potentiation). The hippocampus from 2-3 week old rats was cut into 400 μm thick slices, which were then incubated in an oxygenated artificial cerebrospinal fluid, containing essential ions and glucose. A stable baseline of fEPSP (field excitatory post synaptic potential) in the synaptic layer of the hippocampus region CA1 was first obtained. LTP was induced by three trains of high frequency stimulation (tetanization, consisting of: 100 Hz, 100 impulses, 10 s interval x 3). To compare synapses subjected to LTP with synapses in a control pathway, two stimulating electrodes were used, and LTP was only induced by one of them. The recording electrode was connected to a computer, which graphically showed the outcome of the experiment. After a stabilized LTP (~ 3 h after induction) the maintenance (L-LTP) was studied in the presence of different drugs. Alternatively, the drug was applied before induction of LTP. Another ambition was to find out how the NMDA component (the part of the synaptic response mediated by NMDA receptors) was affected by LTP induction. The results showed that anisomycin, emetine and puromycin (protein synthesis inhibitors) decreased basal synaptic transmission by inducing a slowly developing depression of fEPSPs but had no specific blocking effect on L-LTP. However, L-LTP was impeded by a combined application of emetine and puromycin. Rp-cAMPs (protein kinase A inhibitor) caused smaller levels of LTP but had no specific effect on L-LTP; forskolin (adenylyl cyclase activator) induced potentiation followed by depression of fEPSPs; this effect interacted with paired pulse facilitation, indicative of a presynaptic mechanism, but did not interact with any part of LTP. The NMDA component seemed to contribute to the maintenance of LTP displaying a potentiation of about 50% of the AMPA component (the part of the synaptic response mediated by AMPA receptors) 1 h after induction of LTP and about 61% of the AMPA component 4 h after induction of LTP. But after statistical analysis it was concluded that there was no statistical significance for the asymmetrical changes of the NMDA component and the AMPA components between 1 h and 4 h after LTP induction, and no significance for the NMDA component at all. That makes the NMDA sniffing experiments impossible to use as a base for any conclusions. The lack of clear effect on L-LTP obtained with drugs related to control of protein synthesis might be due to a high level of constituent protein synthesis in slices from young animals.