Santschi L, Reyes-Harde M, Stanton P K
Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
J Neurophysiol. 1999 Sep;82(3):1577-89. doi: 10.1152/jn.1999.82.3.1577.
Although it is widely agreed that cyclic AMP is necessary for the full expression of long-term potentiation of synaptic strength, it is unclear whether cyclic AMP or cyclic AMP-dependent protein kinase (PKA) play roles in the induction of long-term depression (LTD). We show here that two PKA inhibitors, H-89 (10 microM) and KT5720 (1 microM), are unable to block induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Rather, H-89 enhanced the magnitude of LTD induced by submaximal low-frequency stimulation. Raising [cGMP] with zaprinast (20 microM), a selective type V phosphodiesterase inhibitor, reversibly depressed synaptic potentials. However, coapplication of H-89 plus zaprinast converted this to a robust LTD that depended critically on activation of cyclic GMP-dependent protein kinase (PKG). Chemically induced LTD is activity-independent because it could be induced without stimulation and in tetrodotoxin (0.5 microM). Additionally, chemical LTD did not require activation of N-methyl-D-aspartate or GABA receptors and could be reversed by LTP. Stimulus-induced LTD occluded chemical LTD, suggesting a common expression mechanism. In contrast to bath application, postsynaptic infusion of H-89 into CA1 pyramidal neurons did not enhance LTD, suggesting a presynaptic site of action. Further evidence for a presynaptic locus was supplied by experiments where H-89 applied postsynaptically along with bath application of zaprinast was unable to produce chemical LTD. Thus simultaneous presynaptic generation of cyclic GMP and inhibition of PKA is sufficient to induce LTD of synaptic transmission at Schaffer collateral-CA1 synapses.
尽管人们普遍认为环磷酸腺苷（cAMP）对于突触强度长期增强的充分表达是必需的，但尚不清楚环磷酸腺苷或环磷酸腺苷依赖性蛋白激酶（PKA）是否在长时程抑制（LTD）的诱导中发挥作用。我们在此表明，两种PKA抑制剂H-89（10微摩尔）和KT5720（1微摩尔）无法阻断体外海马切片中Schaffer侧支-CA1突触处LTD的诱导。相反，H-89增强了次最大低频刺激诱导的LTD的幅度。用扎普司特（20微摩尔）（一种选择性V型磷酸二酯酶抑制剂）提高[cGMP]可使突触电位可逆性降低。然而，H-89与扎普司特共同应用可将其转化为强烈的LTD，这关键取决于环鸟苷酸依赖性蛋白激酶（PKG）的激活。化学诱导的LTD不依赖于活动，因为它可以在无刺激的情况下以及在河豚毒素（0.5微摩尔）存在时诱导产生。此外，化学LTD不需要N-甲基-D-天冬氨酸或GABA受体的激活，并且可以被长时程增强（LTP）逆转。刺激诱导的LTD可阻断化学LTD，提示存在共同的表达机制。与浴灌应用相反，将H-89突触后注入CA1锥体神经元并未增强LTD，提示其作用位点在突触前。突触后应用H-89并同时进行浴灌扎普司特的实验提供了进一步的突触前位点证据，即无法产生化学LTD。因此，突触前同时产生环鸟苷酸和抑制PKA足以诱导Schaffer侧支-CA1突触处突触传递的LTD。
