Altered gamma oscillations during pregnancy through loss of δ subunit-containing GABA(A) receptors on parvalbumin interneurons

Front Neural Circuits. 2013 Sep 17;7:144. doi: 10.3389/fncir.2013.00144. eCollection 2013.

Abstract

Gamma (γ) oscillations (30-120 Hz), an emergent property of neuronal networks, correlate with memory, cognition and encoding. In the hippocampal CA3 region, locally generated γ oscillations emerge through feedback between inhibitory parvalbumin-positive basket cells (PV+BCs) and the principal (pyramidal) cells. PV+BCs express δ-subunit-containing GABA(A)Rs (δ-GABA(A)Rs) and NMDA receptors (NMDA-Rs) that balance the frequency of γ oscillations. Neuroactive steroids (NS), such as the progesterone-derived (3α,5α)-3-hydroxy-pregnan-20-one (allopregnanolone; ALLO), modulate the expression of δ-GABA(A)Rs and the tonic conductance they mediate. Pregnancy produces large increases in ALLO and brain-region-specific homeostatic changes in δ-GABA(A)Rs expression. Here we show that in CA3, where most PV+ interneurons (INs) express δ-GABA(A)Rs, expression of δ-GABA(A)Rs on INs diminishes during pregnancy, but reverts to control levels within 48 h postpartum. These anatomical findings were corroborated by a pregnancy-related increase in the frequency of kainate-induced CA3 γ oscillations in vitro that could be countered by the NMDA-R antagonists D-AP5 and PPDA. Mimicking the typical hormonal conditions during pregnancy by supplementing 100 nM ALLO lowered the γ frequencies to levels found in virgin or postpartum mice. Our findings show that states of altered NS levels (e.g., pregnancy) may provoke perturbations in γ oscillatory activity through direct effects on the GABAergic system, and underscore the importance of δ-GABA(A)Rs homeostatic plasticity in maintaining constant network output despite large hormonal changes. Inaccurate coupling of NS levels to δ-GABA(A)R expression may facilitate abnormal neurological and psychiatric conditions such as epilepsy, post-partum depression, and post-partum psychosis, thus providing insights into potential new treatments.

Keywords: CA3 interneurons; GABAA receptors; delta subunit; gamma oscillations; neurosteroids; parvalbumin; pregnancy; tonic inhibition.

MeSH terms

  • Animals
  • Brain Waves / physiology*
  • CA3 Region, Hippocampal / metabolism*
  • Female
  • Interneurons / metabolism*
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Nerve Net / metabolism*
  • Parvalbumins / metabolism*
  • Pregnancy
  • Receptors, GABA-A / genetics
  • Receptors, GABA-A / metabolism*

Substances

  • Parvalbumins
  • Receptors, GABA-A