A growing body of data is showing that the AMPA subtype of glutamate receptors play a major role in regulating short- and long-term forms of synaptic plasticity. Furthermore, it is now clear that regulation of plasticity occurs ? in large part ? by regulating the trafficking of AMPA receptor subunits, and their insertion and removal from the synapse. Notably, this trafficking is now known to be dependent , in large part, upon AMPA receptor subunit phosphorylation by 3 major signaling pathways known to be targets for mood stablilizers ? the PKC, PKA and MAPK cascades. In view of the growing body of data suggesting that severe mood disorders may be associated with impairments of cellular plasticity, we undertook the present series of studies to determine if two clinically effective, but structurally highly dissimilar antimanic agents, lithium & VPA regulate synaptic expression of AMPA receptor subunit GluR1. Administration of chronic lithium or valproate (at therapeutically relevant concentrations) reduced rat hippocampal synaptosomal levels of GluR1 after by 40% and 20%, respectively. In cultured hippocampal neurons, both lithium and VPA also significantly down-regulated the surface expression of GluR1 ~ 40% maximumally, in a dose and time-dependent manner. Surface staining with an anti-N terminal GluR1 antibody confirmed the result. Double-immunostaining of GluR1 and synaptotagmin showed that the numbers of GluR1 positive synapses of lithium and valproate-treated neurons were attenuated after chronic treatment. However, total protein levels of GluR1, and synaptotagmin remained unchanged after lithium and valproate treatment in vitro and in vivo. Phosphorylation of a specific PKA site (GluRp845) was significantly attenuated by lithium and valproate treatment by 52 and 33% respectively. Sp-cAMP treatment reversed the attenuation of phosphorylation by lithium and valproate and also brought GluR1s back to the surface, suggesting that phosphorylation of GluRp845 is involved in the mechanism of GluR1 surface attenuation. In striking contrast, drugs, which are known to induce mania, such as imipramine increase the synaptic expression of GluR1 in vivo in hippocampus. These studies suggest that regulation of glutamatergically mediated synaptic plasticity may play a role in the treatment of mood disorders, and raises the possibility that agents more directly affecting synaptic GluR1 may represent novel therapies for this devastating illness. In order to develop a new potential drug, which mimics the effect of mood stabilizers on GluR1 phosphorylation, TAT-peptides (TAT-p845 and TAT-SRC) were designed and synthesized. Tat peptide (YGRKKRRQRRR) is a leading peptide, which enables the functional peptide to pass through the blood brain barrier and cell membrane, allowing it to get into cytosol or synapses of the neurons. A previous study has successfully utilized peptide i. p. injection into animals to disrupt the interaction of PSD-95 with NMDA receptors in the brain, and to provide a neuroprotective effect on a stroke animal model. TAT-p845 was able to inhibit the phosphorylation of AMPA receptors at its PKA site and down-regulate the surface expression of GluR1 in cultured hippocampal neurons, which is the same effect produced by lithium and valproate (our unpublished data). Moreover, this TAT-p845 was able to pass the blood brain barrier and inhibit the phosphorylation of GluR1 in the hippocampus in vivo, which again demonstrated its ability to induce the same effects as lithium and valproate (our unpublished data). Behavioral studies are currently going on to determine the effects of this peptide on mood associated behavior. TAT-p845 which attenuate AMPA receptor levels at synapses, may offer exciting possibilities as a new class of medicine with the potential for treatment of: bipolar disorder. In addition to GluR1, we also investigated the effects of mood stabiillizers on GluR2 synaptic expression/trafficking. The GluR2 is known to regulate calcium permeability, rectification, and single channel conductance of AMPA receptors; notably, these receptors are dominantly influenced by inclusion of a GluR2 subunit in the receptor complex -suggesting that alterations in synaptic GluR2 trafficking would have a major effect on synaptic plasticity .Rats were treated with lithium, or valproate, or imipramine for 4 weeks and synaptosomal fractions from the hippocampus were prepared. Western blot analysis with an anti-GluR2 antibody was performed to determine the GluR2 content at synapses and in the brain. In order to understand the underlining mechanism, GluR-2 expression at the neuronal surface were determined by biotinylation assay and GluR-2 levels at the synapses was investigated by double immunostaining with anti-GluR2 and anti-synaptotagmin antibodies in cultured hippocampal neurons. We found that both lithium and valproate reduced hippocampal synaptosomal levels of GluR2 after chronic administration. Lithium significantly reduced GluR2 total protein levels in chronically treated animals, while valproate does not. In cultured hippocampal neurons, both lithium and VPA also significantly down-regulated the surface expression of GluR 2 in a time-dependent manner. Double-immunostaining of GluR2 and synaptotagmin showed that the GluR2 immunostaining at synapses of lithium and valproate-treated neurons was attenuated after 4 days of treatment. Total protein levels of GluR2 is also significantly attenuated by lithium, but not valproate, confirming the in vivo data. In striking contrast, drugs, which are known to provoke mania, such as imipramine increase the synaptic expression of GluR2 in vivo in hippocampus. These studies suggest that regulation of glutamatergically mediated synaptic plasticity may play a role in the treatment of mood disorders, and raises the possibility that agents more directly affecting synaptic GluR2 may represent novel therapies for this devastating illness.
| Catapano, Lisa A; Manji, Husseini K (2008) Kinases as drug targets in the treatment of bipolar disorder. Drug Discov Today 13:295-302 |
| Catapano, Lisa A; Manji, Husseini K (2007) G protein-coupled receptors in major psychiatric disorders. Biochim Biophys Acta 1768:976-93 |
| Du, Jing; Suzuki, Katsuji; Wei, Yanling et al. (2007) The anticonvulsants lamotrigine, riluzole, and valproate differentially regulate AMPA receptor membrane localization: relationship to clinical effects in mood disorders. Neuropsychopharmacology 32:793-802 |
| DU, Jing; Quiroz, Jorge; Yuan, Peixiong et al. (2004) Bipolar disorder: involvement of signaling cascades and AMPA receptor trafficking at synapses. Neuron Glia Biol 1:231-43 |
