Estradiol increases spine density and NMDA-dependent Ca2+ transients in spines of CA1 pyramidal neurons from hippocampal slices

To investigate the physiological consequences of the increase in spine dens ity induced by estradiol in pyramidal neurons of the hippocampus, He perfor med simultaneous whole cell recordings and Ca2+ imaging in CAI neuron spine s and dendrites in hippocampal slices. Four- to eight-days in vitro slice c ultures were exposed to 17 beta-estradiol (EST) for an additional 4- to 8-d ay period, and spine density was assessed by confocal microscopy of DiI-lab eled CAI pyramidal neurons. Spine density was doubled in both apical and ba sal dendrites of the CA1 region in EST-treated slices; consistently, a redu ction in cell input resistance was observed in EST-treated CAI neurons. Dou ble immunofluorescence staining of presynaptic (synaptophysin) and postsyna ptic (alpha-subunit of CaMKII) proteins showed an increase in synaptic dens ity after EST treatment. The slops of the input/output curves of both alpha -amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) and N-methyl-D- aspartate: (NMDA) postsynaptic currents were steeper in EST-treated CAI neu rons, consistent with the observed increase in synapse density. To characte rize NMDA-dependent synaptic currents and dendritic Ca2+ transients during Schaffer collaterals stimulation, neurons were maintained at +40 mV in the presence of nimodipine, picrotoxin, and 6-cyano-7-nitroquinoxaline-2,3-dion e (CNQX). No differences in resting spine or dendritic Ca2+ levels were obs erved between control and EST-treated CAI neurons. Intracellular Ca transie nts during afferent stimulation exhibited a faster slope and reached higher levels in spines than in adjacent dendrites. Peak Ca2+ levels were larger in both spines and dendrites of EST-treated CAI neurons. Ca2+ gradients bet ween spine heads and dendrites during afferent stimulation were also larger in EST-treated neurons. Both spine and dendritic Ca2+ transients during af ferent stimulation were reversibly blocked by D,L-2-amino-5-phosphonovaleri c acid (D,L-APV. The increase in spine density and the enhanced NMDA-depend ent Ca2+ signals in spines and dendrites induced by EST may underlie a thre shold reduction for induction of NMDA-dependent synaptic plasticity in the hippocampus.