The Role Of Dopamine/DARPP-32 Signal In Excitotoxicity And Seizures On Rat Forebrain

Midbrain dopaminergic neurons play a critical role in multiple brain functions. Abnormal signaling through dopaminergic pathways has been implicated in several major neurological and psychiatric disorders, including epilepsy, Parkinsonism, schizophrenia, and drug abuse. One well-studied molecular target for the actions of dopamine is DARPP-32(dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa).The Dopamine/DARPP-32/PP-1 cascade regulates the state of phosphorylation and activity of all of the physiological effectors in dopaminoceptive neurons examined to date. These include a variety of neurotransmitter receptors, voltage-gated ion channels, transcription factors, and an electrogenic ion pump.Epilepsy is one pathological condition characterized by localized bursts of electrical overactivity (seizures) in the cerebral hemispheres. Outbursts of electrical activity, commonly observed in cortical and subcortical areas, canresult in extensive neuronal cell death induced by glutamate excitotoxicity in different brain regions. The striatum is one of the major structures associated with seizure activity and dopamine may be important in this process, specifically in determining the seizure threshold and propagation. In addition, the application of dopamine D1 receptor antagonists has been proposed to explain individual susceptibility to epilepsy. So dopamine has close relationships with seizures and neuronal damage from excitotoxicity. However, the pathological and physiological mechanism of dopamine's involvement in epileptiform activity remains unclear. It is necessary to explore the effects of dopamine and DARPP-32 signaling in epilepsy.In the present study, a variety of methods including immunohistochemistry, double immunofluorescence, immune-electron-microscopic staining and Western blot have been utilized to investigate the the role of Dopamine/DARPP-32 signal in excitotoxicity and seizures on rat forebrain.The results are as follows.1. To study the distribution of DARPP-32-positive neurons in rat forebrain, the DARPP-32-immunoreactive neurons were observed in cortex, hippocampus and striatum; and were present throught the cytoplasm of the labeled neuronal somata and dendrites. So DARPP-32 is an effective marker for dopaminoceptive neurons.2. To examine the co-localization of NMD A or AMPA receptor subunits in DARPP-32-containing neurons in rat forebrain, there were different expression of NMDA or AMPA receptor subunits in DARPP-32-containing neurons. A majority (86-98%) of DARPP-32-containing neurons expressed NMD A receptor subunits and AMPA receptor subunit GluR2/3, while less ofthem (14-90%) expressed AMPA receptor subunits GluRl and GluR4 subunits in aforementioned regions. This study has provided morphological basis for functional interaction between DARPP-32 and various NMDA or AMPA receptors in the dopaminoceptive cells of the forebrain regions, and suggested its implications in modulating the neuronal property and excitotoxicity of mammalian forebrain neurons of normal animals and neurological disorders.3. We respectively analysed the relation between doses of PTZ and behavioral changes or Fos activated neurons in different groups which were systemically administered vehicle, Levodopa+sulpirid or Levodopa+sulpirid+SCH23900 respectively before being induced generalized statues epilepticus(SE) by increasing doses of PTZ eventually. Dopamine through D1 receptor exert facilitatory effect on seizures, whereas the D1 receptor antagonist SCH 23390 could inhibit seizure activity. Double-immunostaining was perfomed in rat forebrain. There were different expression of Fos in DARPP-32-containing neurons, and DARPP-32 may play a important role.4. To study the temporal changes of phosphorylated DARPP-32 in rat forebrain after onset of SE, the number of phosphorylated DARPP-32-positive cells increased significantly at 1h after the onset of SE, and decreased at 24h . The decrease in the number became ceasing at 72h. The increase of phosphorylated DARPP-32 may be partly derived from the release of endogenetic dopamine. Comparing the number of phosphorylated DARPP-32-positive cells from PTZ with SLP and SSLP groups one another, there were no statitically significant differences between the groups. The...