Abstract:

Stressors play a pivotal role in the occurrence of depressive illnesses. Disorders of monoamine neurotransmitters and their receptors may be the fundamental causes of depression. Additionally, abnormal expression of glutamic acid (Glu) and its receptor may be a major reason for depression. Consequently, the study of the relationship between monoamine and glutamic acid neurotransmitter in stress-induced depression has significances to reveal the profound mechanism of depression. NR2B subunits which are highly expressed in the cortex, hippocampus and olfactory bulb, are one of the key subunits of NMDA receptors. Orbital frontal cortex (orbital frontal cortex, OFC), which plays a significant role in higher brain function, such as emotional and complex behavior, is one of the major sub-regions of prefrontal. This study was to investigate the effect of orbital frontal cortex D1 dopamine receptor on Glu and its receptors, especially on NR2B subunits of N-methyl-D-aspartic acid (NMDA) receptors in depression induced by chronic unpredictable mild stress (CUMS). CUMS-induced depression model was established in Sprague-Dawley rats, and intra-orbital frontal cortex microinjections of D1 dopamine receptor agonist SKF38393 and its antagonist SCH23390 were respectively adopted by rat brain stereotaxic coordinates. The behavioral observations were conducted by measurement of sucrose preference test, open-field test and tail suspension test. The concentration of Glu and the expression of NR2B subunits in orbital frontal cortex were detected by high-performance liquid chromatography (HPLC) and Western blot (WB) respectively. In comparison to control groups, depression-like behavioral changes were observed in CUMS rats, the concentration of dopamine and its D1 receptor were decreased; conversely, the increase of Glu and NR2B subunits of its NMDA receptors were observed in orbital frontal cortex. Depression-like behavioral of CUMS rats was obviously improved after pretreatment with injection of SKF38393, the expression of Glu and NR2B subunits of its NMDA receptors were also decreased. Normal rats showed depression-like behavioral which similar to the CUMS rats after pretreatment with injection of D1 dopamine receptor antagonist SCH23390, meanwhile, in orbital frontal cortex the expression of Glu and NR2B subunits of its NMDA receptors were significantly increased. These results suggest that the lowered dopamine release may be caused by chronic unpredictable mild stress, as the result of insufficient dopamine, orbital frontal cortex releases extra amounts of glutamic acid and its NMDA receptors are over activated. All those above then may lead to depression. Antidepressant effect of dopamine may be functioned by inhibiting the expression of Glu and NR2B subunits of its NMDA receptors.

Key words: chronic unpredictable mild stress, depression, orbital frontal cortex, D1 dopamine receptor, glutamic acid