Regulation of kainate receptors by cAMP-dependent protein kinase and phosphatases

Modulation of synaptic efficacy in field CA1 of the rat hippocampus by forskolin

Activation of cAMP-dependent protein kinase (kinase A) has recently been shown to enhance responses evoked by stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in cultured hippocampal pyramidal neurons. Here we report results of experiments designed to determine if activation of the cAMP cascade potentiates synaptic strength in field CA1 of rat hippocampal slices. We find that bath application of the direct adenylate cyclase activator forskolin (50 microM) enhances the field excitatory postsynaptic potential (EPSP) slope and population spike amplitude evoked by stimulation of Schaffer/commissural afferents. This effect is potentiated by the phosphodiesterase inhibitor and adenosine receptor antagonist 3-isobutyl-1-methylxanthine (IBMX). The enhancement produced by forskolin is suppressed in the presence of adenylate cyclase inhibitors and is not mimicked by the inactive forskolin analogue 1,9-dideoxyforskolin, indicating that, indeed, activation of adenylate cyclase mediates the effects of forskolin in field CA1. Our observations support the idea that changes in intracellular cAMP levels can modulate synaptic efficacy of excitatory glutamatergic synapses in the mammalian hippocampus.

Induction of stable long-term potentiation in the presence of the protein kinase C antagonist staurosporine

We have studied the effects of staurosporine, an antagonist of the catalytic subunit of protein kinase C, on the mechanisms of long-term potentiation (LTP) in rat hippocampal slices maintained in vitro. Application of staurosporine did not affect pre-established LTP, but resulted in a decaying potentiation when high frequency stimulation was delivered in its presence. However, coactivation of two inputs to the same group of CA1 neurons during high frequency stimulation transformed the decaying potentiation into stable LTP. Staurosporine also reduced the NMDA receptor-mediated component of synaptic responses to burst stimulation. It is concluded that the PKC antagonist interferes with LTP induction, but not expression mechanisms.

Calmodulin-dependent protein kinase II. Multifunctional roles in neuronal differentiation and synaptic plasticity

One of the most important mechanisms for regulating neuronal functions is through second messenger cascades that control protein kinases and the subsequent phosphorylation of substrate proteins. Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is the most abundant protein kinase in mammalian brain tissues, and the alpha-subunit of this kinase is the major protein and enzymatic molecule of synaptic junctions in many brain regions. CaM-kinase II regulates itself through a complex autophosphorylation mechanism whereby it becomes calcium-independent following its initial activation. This property has implicated CaM-kinase II as a potential molecular switch at central nervous system (CNS) synapses. Recent studies have suggested that CaM-kinase II is involved in many diverse phenomena such as epilepsy, sensory deprivation, ischemia, synapse formation, synaptic transmission, long-term potentiation, learning, and memory. During brain development, the expression of CaM-kinase II at both protein and mRNA levels coincides with the active periods of synapse formation and, therefore, factors regulating the genes encoding kinase subunits may play a role in the cell-to-cell recognition events that underlie neuronal differentiation and the establishment of mature synaptic functions. Recent findings have demonstrated that the mRNA encoding the alpha-subunit of CaM-kinase II is localized in neuronal dendrites. Current speculation suggests that the localized translation of dendritic mRNAs encoding specific synaptic proteins may be responsible for producing synapse-specific changes associated with the processing, storage, and retrieval of information in neural networks.