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BZ withdrawal can lead to adaptive modification of glutamatergic synaptic transmissionin hippocampal CA1 neurons. Two subtypes of glutamate receptors, AMPAR andNMDAR, showed different patterns of modification upon BZ withdrawal. The possiblemechanisms and intrinsic relationship of these two receptors during BZ withdrawal werestudied in this dissertation.AMPAR-mediated mEPSC amplitudes were increased 30% in CA1 neurons with a 2-foldincrease in single-channel conductance. The potentiated AMPAR-mediated synaptictransmission was also manifested by an increased slope of the input-output curve.CaMKII’s contribution to the potentiation was validated by pre-incubation of slices withthe selective inhibitor, KN-93, or intracellular inclusion of AIP, both of which preventedthe increases of AMPAR current amplitude and conductance. Increased NAS inhibitionwas consistent with synaptic incorporation of homomeric GluR1 AMPARs. In 1-daywithdrawn rats, only GluR1 levels were increased in immunoblots of the PSD-enrichedsubcellular fraction from CA1 minislices consistent with increased mEPSC amplitude,but not conductance. In 2-day withdrawn rats, total, but not relative phospho-Thr286CaMKII levels increased in the PSD-enriched subfraction in parallel with increasedGluR1 and phospho-Ser831 GluR1 expression levels implying that CaMKII mediatesAMPAR phosphorylation and increased channel conductance in FZP-withdrawn CA1neurons. Whole-cell and field (f)EPSP recordings revealed that LTP expression, induced by low-intensity theta burst stimulation, was impaired in CA1 neurons from FZPwithdrawnrats although no memory deficits were detectable using a novelty preferenceparadigm. The findings suggest that synaptic insertion and subsequent CaMKII-mediatedphosphorylation of homomeric GluR1 AMPARs might contribute to BZ withdrawalinducedpotentiation of AMPARs analogous to mechanisms underlying activitydependentplasticity.NMDAR function was depressed during BZ withdrawal. The contribution of the NR2Bsubunit to NMDAR downregulation was indicated both by the effect of ifenprodil onNMDAR currents and by the decreased expression level of both NR1 and NR2B, but notNR2A subunits in PSD-enriched fractions from 2-day FZP-withdrawn rats. Thedepression of NMDAR currents was secondary to AMPAR potentiation, since AMPAincubation could accelerate the reduction in NMDAR currents in 1-day FZP withdrawnrat, and could counteract the hyperactivity of AMPAR when total charge transfer wasmeasured. Calcium-related homeostatic regulation through calcium-permeable AMPARsand L-type voltage-gated calcium channels was proposed to explain the possibleCaMKII/calmodulin-mediated mechanisms underlying the enhancement of AMPA anddepression of NMDAR currents, and their relationship to the appearance of withdrawalanxiety. |
