RESUMEN
Long-term depression (LTD) of synaptic strength is induced by glutamate-triggered increases in postsynaptic [Ca2+], through either influx or release from intracellular stores. Induction of LTD has also been reported to require release of Ca2+ from presynaptic stores and activation of presynaptic Ca2+/calmodulin-dependent protein kinase II. This finding leads to the hypothesis that the intercellular messenger nitric oxide (NO) may be a means by which postsynaptic Ca2+ triggers changes expressing LTD in presynaptic terminals. We report that bath application of the oxadiazoloquinoxalone derivative ODQ (4 microM), a selective inhibitor of NO-sensitive guanylyl cyclase (NOGC), markedly attenuated (90%) the magnitude of LTD induced by low-frequency stimulation (LFS; 1 Hz/15 min) of Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Both the NO donor S-nitroso-N-acetylpenicillamine (100 microM) and the membrane-permeant cyclic guanine 3',5'-monophosphate (cGMP) analogue 8-(-4-chlorophenylthio) guanosine (8-pCPT)-cGMP (50 microM) enhanced the magnitude of LTD, which is consistent with he hypothesis that activation of NOGC plays a role in the induction of LTD. Nicotinamide (20 mM), an inhibitor of NO-activated ADP ribosyltransferase, did not impair the induction of LTD. In contrast to de novo LTD, the reversal of long-term potentiation by LFS (depotentiation) was only partially blocked (55%) by ODQ, and heterosynaptic LTD was not impaired at all, suggesting that there are both NOGC-dependent and -independent forms of LTD. Because postsynaptic intracellular infusion of ODQ (500 microM) failed to block the induction of LTD, we conclude that activation of presynaptic NOGC is a necessary step in the induction of an NOGC-dependent component of LTD.
Asunto(s)
Guanilato Ciclasa/metabolismo , Hipocampo/enzimología , Potenciación a Largo Plazo/fisiología , Plasticidad Neuronal/fisiología , Óxido Nítrico/metabolismo , Animales , GMP Cíclico/análogos & derivados , GMP Cíclico/farmacología , Electrofisiología , Inhibidores Enzimáticos/farmacología , Hipocampo/química , Hipocampo/efectos de los fármacos , Potenciación a Largo Plazo/efectos de los fármacos , Masculino , Plasticidad Neuronal/efectos de los fármacos , Niacinamida/farmacología , Técnicas de Cultivo de Órganos , Oxadiazoles/farmacología , Penicilamina/análogos & derivados , Penicilamina/farmacología , Inhibidores de Agregación Plaquetaria/farmacología , Quinoxalinas/farmacología , Ratas , Ratas Sprague-Dawley , S-Nitroso-N-Acetilpenicilamina , Sinapsis/química , Sinapsis/enzimología , Tionucleótidos/farmacologíaRESUMEN
1. Extracellular bath application of the selective Ca2+/calmodulin-dependent kinase II (CaMKII) inhibitor KN-62 to hippocampal slices in vitro blocked the induction of long-term depression (LTD) by low-frequency Schaffer collateral stimulation (1 Hz/15 min) of the same concentration as has been shown previously to prevent induction of long-term potentiation (LTP) at these synapses. 2. In contrast, postsynaptic intracellular infusion of KN-62 into single CA1 pyramidal neurons did not prevent induction of LTD, although it was quite effective in blocking LTP. 3. We conclude that there is a presynaptic CaMKII that must be activated to induce LTD, whereas postsynaptic CaMKII stimulation is needed to evoke LTP. 4. Bath application of KN-62 also blocked depotentiation by low-frequency stimuli of previously induced LTP, suggesting that induction of depotentiation and de novo LTD may require the same CaMKII-dependent mechanisms.
Asunto(s)
Proteínas Quinasas Dependientes de Calcio-Calmodulina/fisiología , Potenciación a Largo Plazo/fisiología , Plasticidad Neuronal/fisiología , Sinapsis/enzimología , 1-(5-Isoquinolinesulfonil)-2-Metilpiperazina/análogos & derivados , 1-(5-Isoquinolinesulfonil)-2-Metilpiperazina/farmacología , Animales , Proteína Quinasa Tipo 2 Dependiente de Calcio Calmodulina , Proteínas Quinasas Dependientes de Calcio-Calmodulina/antagonistas & inhibidores , Proteínas Quinasas Dependientes de Calcio-Calmodulina/metabolismo , Inhibidores Enzimáticos/farmacología , Técnicas In Vitro , Potenciación a Largo Plazo/efectos de los fármacos , Masculino , Plasticidad Neuronal/efectos de los fármacos , Células Piramidales/efectos de los fármacos , Células Piramidales/fisiología , Ratas , Ratas Sprague-Dawley , Sinapsis/efectos de los fármacos , Sinapsis/ultraestructuraRESUMEN
Sublethal ischemia or hypoxia triggers adaptive changes that protect the brain against future hypoxic/ischemic damage. Preexposure of in vitro hippocampal slices to brief periods of hypoxia increases the resistance of Schaffer collateral-CA1 synaptic potentials to further, longer periods of hypoxia that would otherwise cause an irreversible loss of synaptic transmission. Since hypoxia has been shown to cause alterations in the patterns of protein synthesis, we hypothesized that newly-expressed proteins might mediate hypoxia-induced neuroprotection. We report here that the induction of neuroprotection by hypoxic preconditioning in rat hippocampal slices is blocked by either cycloheximide, a protein synthesis inhibitor, or by Actinomycin D, an inhibitor of RNA synthesis. In contrast, pharmacological blockade of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors did not prevent the induction of neuroprotection by hypoxia. Carbon monoxide (CO), which can lock heme moieties in their oxygenated configurations, did prevent hypoxia from inducing neuroprotection. We conclude that hypoxia activates protective mechanisms via deoxygenation of a heme moiety, triggering expression of gene products which protect synaptic function from subsequent hypoxic damage.