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Modulation of long-term potentiation in the hippocampus Xie, Zheng

Abstract

Brief trains of high-frequency stimulation of monosynaptic excitatory pathways in the hippocampus can cause a long-term synaptic potentiation (LTP) that can last for hours in vitro or weeks in vivo. This activity-mediated LTP is thought to be involved in certain forms of learning and memory. Although mechanisms underlying the induction and maintenance of LTP process have been extensively investigated, factors that modulate LTP remain unresolved. Suppression of ƴ-aminobutyric acid (GABA) -ergic inhibition by GABA antagonists can facilitate LTP of the excitatory postsynaptic potential (EPSP) in the hippocampus. It is, however, unclear whether long-term changes in GABAergic inhibition occur after a tetanic stimulation, and if so, how such a change affects LTP of the EPSP. In this project, experiments were conducted to determine if the CA1 neuronal inhibitory postsynaptic potentials (IPSPs) were affected following a high frequency stimulation. Somatostatin co-exists with GABA in some interneurons in the CA1 area. Since the peptide might be co released with GABA, experiments were carried out to determine if somatostatin modifies GABAergic IPSPs. Whether the peptide could modulate the induction of LTP of the EPSP was also tested. Previous studies from this laboratory showed that substances collected from the hippocampus or the neocortex during a tetanic stimulation could induce LTP if applied on hippocampal slices. It is generally believed that the induction of LTP requires the activation of postsynaptic NMDA receptors and that the maintenance of LTP is at least in part due to a presynaptic mechanism. It is possible that a release of substances from the postsynaptic cells or the nearby glia during a tetanic stimulation of the hippocampal afferents could result in a retrograde interaction of these substances with the presynaptic terminals leading to changes that sustain LTP. In the present study, these substances were further characterized, and the mechanisms of their release as well as LTP-inducing action elucidated. Reports in the literature suggest that a deficiency in α-tocopherol (vitamin E) leads to enhancement in lipid peroxide content in the hippocampus as well as impairment of spatial learning. Oxygen free radicals have also been shown to accelerate the decay of established LTP. Studies were, therefore, initiated to examine if the vitamin causes LTP and if its deficiency leads to impairment of LTP-induction. EPSPs and IPSPs were recorded from CA1 neurons in guinea pig hippocampal slices in response to stratum radiatum stimulation. In control CA1 neurons, tetanic stimulation of the stratum radiatum caused LTP of the EPSP and the fast IPSP without changing the slow IPSP. If BAPTA (a Ca²+ chelator) or K-252b (a PKC inhibitor) was injected into the CA1 neurons, LTP of the EPSP did not occur. However, in these drug-injected neurons LTP of the fast IPSP was enhanced, and LTP of the slow IPSP occurred after tetanus. With the potentiation of the lPSPs in the drug-injected neurons, the shape of the EPSP was distorted in most neurons. These findings indicate that LTP occurs not only at excitatory synapses but also at inhibitory synapses. The tetanus-induced increases in intracellular free Ca²+ and PKC activity potentiate the EPSP, while decreasing LTP of the IPSPs so that the distortion of the EPSP by the IPSPs is minimized for a better expression of LTP of the EPSP. Somatostatin hyperpolarized the CA1 neurons and decreased the input resistance. The peptide depressed both the fast IPSP and the slow IPSP, without changing the EPSP. These actions of somatostatin were not due to interactions of the peptide with GABA A or GABA B receptors. However, the activation of GABA B receptors by baclofen, reduced the somatostatin-induced hyperpolarization of the CA1 neurons. The suppression of the GABA A receptor mediated IPSP by somatostatin appears to be through a postsynaptic action of the agent. It is concluded that interactions between somatostatin and GABAergic responses may be related to the peptide- and GABA-receptors being coupled to the same channels or to the sharing of the same second messenger systems for effects. By modulating the IPSPs, somatostatin could logically interfere with the induction of LTP of the EPSP. However, application of somatostatin failed to facilitate or block LTP. Samples were collected from the rabbit neocortex during a tetanic (50 Hz, 5 s) stimulation (tetanized neocortical sample, TNS). Application of TNS caused LTP of the EPSP and population spike without changing the membrane potential or the input resistance of the CA1 neurons. The TNS-induced LTP required activation of afferents. TNS also induced a short-term potentiation (STP) of the fast IPSP without changing the slow IPSP. Since the TNS- and tetanus-induced LTPs occluded each other, these two LTPs may share some common mechanisms. Different fractions of TNS (50 kDa) were able to cause LTP. APV did not block the TNS-induced LTP. However, PKC inhibitors such as sphingosine and K-252b blocked the TNS-induced LTP. TNS failed to induce LTP in Ca²+-chelaated CA1 neurons. If TNS was collected from the rabbits pretreated with MK-801 (i.p.), a non-competitive NMDA antagonist, this TNS failed to induce LTP. Gel electrophoresis of the substances in TNS revealed the presence of an acidic protein with a molecular weight of about 69 kDa. It, therefore, appears that NMDA receptor activation is required for the release but not for the LTP-inducing action of substances in TNS. α-Tocopherol phosphate (referred to as α-tocopherol) induced a slowly developing LTP of the EPSP without changing the fast and slow IPSPs, and the electrical properties of the CA1 neurons. The agent failed to induce a further potentiation of the EPSP during a pre-established tetanus-induced LTP. The α-tocopherol-induced LTP was decreased by AP3 (an ACPD antagonist) but not by APV (a NMDA antagonist). Furthermore, chelation of postsynaptic free Ca²+ with BAPTA or inhibition of PKC by sphingosine and K-252b prevented the α- tocopherol-induced LTP. Sodium ascorbate (a water soluble antioxidant) failed to induce LTP. DMSO (a lipid soluble antioxidant) was able to potentiate the EPSP as long as the application of the agent continued, but the EPSP quickly returned to the pre-application level once the application of the agent was stopped. In hippocampal slices obtained from vitamin E deficient rats, both tetanic stimulation and α-tocopheroI failed to induce LTP of the EPSP in the CA1 neurons. It is possible that the structure and the function of the membrane or certain receptors on neurons are affected by the lack of c-tocopherol in the vitamin deficient rats and, therefore, an acute application of x-tocopherol may not able to correct the changes induced by the long-term vitamin E deficiency. In conclusion, various mechanisms that modulate LTP of the EPSP were examined in guinea pig hippocampal slices. It appears that the elevation of postsynaptic [Ca²+] and the activation of PKC are needed not only to cause LTP of the EPSP but also to diminish LTP of the IPSP5 so that the expression of LTP of the EPSP is not distorted. Somatostatin suppresses GABAergic IPSPs through mechanisms other than to interfere with the amino acid receptors and the peptide appears not to affect LTP of the EPSP. Tetanic stimulation of the neocortical surface causes a release of LTP-inducing substances whose release but not action, depends on NMDA receptor activation. While vitamin E appears to induce LTP, animals with the vitamin deficiency appear to ha a diminished ability to induce LTP.

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