Sex differences in molecular signaling at inhibitory synapses in the hippocampus

The possibility that mechanisms of synaptic modulation differ between males and females has far-reaching implications for understanding brain disorders that vary between the sexes.Wefound recently that 17b-estradiol (E2) acutely suppresses GABA ergic inhibition in the hippocampus of female rats through a sex-specific estrogen receptor (ERa), mGluR, and endocannabinoid-dependent mechanism. Here, we define the intracellular signaling that links ERa, mGluRs, and endocannabinoids in females and identify where in this pathway males and females differ. Using a combination of whole-cell patch-clamp recording and biochemical analyses in hippocampal slices from young adult rats, we show that E2 acutely suppresses inhibition in females through mGluR1 stimulation of phospholipase C, leading to inositol triphosphate (IP₃) generation, activation of the IP3 receptor (IP₃R), and postsynaptic endocannabinoid release, likely of anandamide. Analysis of sex differences in this pathway showed that E2 stimulates a much greater increase in IP₃ levels in females than males, whereas the group I mGluR agonist DHPG increases IP₃ levels equivalently in each sex. Coimmunoprecipitation showed that ERa-mGluR1 and mGluR1–IP₃R complexes exist in both sexes but are regulated by E2 only in females. Independently of E2, a fatty acid amide hydrolase inhibitor, which blocks breakdown of anandamide, suppressed>50%of inhibitory synapses in females with no effect in males, indicating tonic endocannabinoid release in females that is absent in males. Together, these studies demonstrate sex differences in both E2-dependent and E2-independent regulation of the endocannabinoid system and suggest that manipulation of endocannabinoids in vivo could affect physiological and behavioral responses differently in each sex.