Rapid Effects Of Glucocorticoids On Glutamate-or NMDA-Evoked Currents In Rat Hippocampal Neurons And Its Mechanisms

Since the 60s of last centUry people first observed that steroids had raPid effects onneurons. Scientists from all work of lines have more and more evidence about steroidsthat couldn't be explounded by the traditional, genomic mechanisms. TherefOre, the raPid,nongenondc mechbosm was put forward to explain raPid actions of steroids. Though theconcePt has 'been gradually accepted by scieniists. It is Still a secrect ahollt the concreteactions of steroids. In the present stud% we examined the raPid effects of glucocorticoids(GCs) on glutamate- or NMDA-evoked currellt (IGLu, INMDA) in primary cultUred rathippocampal neurons by Whole-cell patch clamP tecboque. The main results as follows f1 .The general electrophysiological characteristics of glUtamate- or NMDA-evokedcurrent in primary cu1tUred rat hippocamPal neurons. IGLu and INMDA displayed sighficantproperties of desensitizitation and inward rectification. NMDA or glularnate (10#mol/L)evoked an inward current in Mg'+-free external solution at a holding protential (Vh) of--60mV In most cases, the decay of IGLu was fitted by a double experimental fiJnctionwith relatively fast (Tf)and slow(T,)time constan.2. Extracellular aPplication ofcorticosterone (B, 10-9-- l0-'mol/L) had no effect onthe resting membrane conductance or voltage-gated channels; extracellular aPplication ofNMDA or GLU with B (l0D~ l0-'mol/L) simultaneously, the peaks of IGLu and INMDAwere remarkablely suppressed by B and variation of the decay of IGLu or INMDA had nostatistical meaning. PretIeat the neurons with B for 4 min, there was no significantdifference of inhibitory rates of B on IGLu or INMDA betWeen pretreatineni grouP andnon-pretreatment group. The rapid effect of B on IGLu or INMDAwas reversible andnonconcentration dependent.3 .Bovine seruxn albumin-conjugated corticosterone (B-BSA, l0-7-- 10-'moliL) hadthe similar inhibitOry effect on IGLu or INMoa to B, Whch was nonconcelltration dependent;however, BSA had no effect on IGLu or INMDA'4.When B (l0"--l0-'mol/L) was concluded in the electrode /intemal solution, Bhad no effect on extracellular NMDA-induced currellt. Howevef, the peak of currentinduced by extracellular coaPplication of B (10-'~l0-'moUL) and NMDA (l0#mol/L)was smaller than that of eXtracellular only aPpplication of NMDA (B in the eXternalsolution had the same concetration as that of B in the electrode).This result denotes: Binhibition of NMDA-evoked current was due to B directly exertion its influence onNMDA receptors located at plasmalemrna or on cy'toplasm membrane in some wayindirectly to affect NMDA receptors.5.When recording was made with the recording/control intemal solutionsupplemented with a broad spectrUm protein kinase inhibitor staurosporine (5x l0-'mol/L),the peak of INMDA was significanly inhibited, and the inhibition of lNMDA induced byextracellular coaPPlication of B and NMDA, was not only prevented, but enhanced.6. Intracellular dialysis with a more selective PKA inhibitor RP-cAMPS (l0-'mol/L),RP-cAMPS marvelously inhibited INMDA and reversed B (l0-'mol1) inhibition on INMDA'PKA inhibitor RP-cAMPS had the sinillar effects on INMDA and on inhibition of INMDAinduced by B to staurosporine.7. PKA agonist 8-Br-cAMP (l0#moI/L) had no effect on lNMDA and B (10-'moMi)suppression of NMDA-evoked current.8. A C a2+/calmo dul in- dep endent pro te in kinas e I I (C aMKII) antagoni st KN - 6 2(5x l0-'mol/L) presented in the intemal solution had no significan effect on the currentpeaks induced by extracellular NMDA (l0#mol/L), but prevented the inhibitOry effect ofB on INMDA'These results indicate that GCs have raPid, reversible idebitory effects on lNMDAIntracellular aPplication of B thIough microelectrode had no effect on INMDA, howeveT,extracellular aPplication of B or B-BSA suppressed peaks of INMDA: All these denote Bexerts its influence on NMDA receptor by cytoplasm membrane mechAnsms, which isnaxnely raPid, nongenom...