Estrogen-Dependent Functional Spine Dynamics in Neocortical Pyramidal Neurons of the Mouse

Area of Science:

• Neuroscience
• Cell Biology
• Endocrinology

Background:

• Surgical ovariectomy (OVX) in rodents decreases spine density in hippocampal CA1 pyramidal cells.
• 17β-estradiol (E2) treatment can reverse OVX-induced spine loss in the hippocampus.
• The impact of estrogen replacement therapy on neocortical structure and function post-OVX is not fully understood.

Purpose of the Study:

• To investigate the effects of OVX and subsequent E2 treatment on spine density and synaptic function in mouse neocortical neurons.
• To determine the specific estrogen receptors involved in mediating these effects in the neocortex versus the hippocampus.
• To assess whether E2 treatment fully restores synaptic connectivity after OVX.

Main Methods:

• Ovariectomy (OVX) and 17β-estradiol (E2) treatment in Thy1M-EGFP mice.
• Confocal microscopy to quantify spine density in neocortical pyramidal neurons.
• Electrophysiology to measure miniature excitatory postsynaptic current (mEPSC) frequency.
• Pharmacological manipulation using estrogen receptor agonists/antagonists (ERα/ERβ, GPER).
• Time-lapse in vivo two-photon imaging to track spine dynamics (gain and loss).

Main Results:

• OVX reduced spine density in neocortical layer 5 pyramidal neurons, which was reversed by E2 treatment.
• OVX decreased mEPSC frequency in the somatosensory cortex, indicating altered functional synapses.
• GPER activation, but not ERα/ERβ, rescued OVX-associated spine loss in the neocortex, contrasting with hippocampal effects.
• E2 treatment and GPER activation in vitro rescued reduced mEPSC frequency in neocortical slices.
• Time-lapse imaging revealed E2 reversed OVX-induced increases in spine loss and decreases in spine gain.
• Newly formed spines after E2 rescue did not preferentially reoccupy original sites.

Conclusions:

• Estrogen replacement therapy reverses OVX-induced spine loss and normalizes spine dynamics in the mouse neocortex.
• The G-protein-coupled estrogen receptor (GPER) mediates some of E2's effects on neocortical structure and function post-OVX.
• Despite restoring spine density and dynamics, estrogen replacement does not fully re-establish the original synaptic connectivity, suggesting persistent functional consequences of OVX.