Sex Does Matter: Sex Differences in Molecular Regulation of Synapses
Material below summarizes the article Sex Differences in Molecular Signaling at Inhibitory Synapses in the Hippocampus, on August 12, 2015, in JNeurosci and authored by Nino Tabatadze, Guangzhe Huang, Renee M. May, Anant Jain, and Catherine S. Woolley.
Many brain disorders vary between the sexes, in their incidence, symptoms, and/or responses to treatment. In most cases, no one knows the sources of these differences: Are they due to intrinsic biological differences between male and female brains? Are they due to cultural differences in how boys and girls are raised – the different experiences of men and women in society?
In this study, we describe intrinsic biological sex differences in molecular regulation of synaptic function in experimental animals. The differences were present in animals that had their gonads removed, so they didn’t depend on differences in circulating sex hormones. The differences were present in brain slices in a dish, so they didn’t depend on direct experience, or even on the rest of the body. This is why we refer to the sex differences we found as being “intrinsic” biological differences.
A little background will be helpful to understand our findings. The billions of neurons in the brain are wired into circuits through trillions of tiny junctions called synapses. At most synapses, neurons communicate when packets of neurotransmitter are released by one neuron to activate receptors located on the next neuron in a chain. The type of neurotransmitter receptor present at a synapse determines whether that synapse is excitatory, tending to activate the next cell in line, or inhibitory, tending to silence a downstream partner. The amount of neurotransmitter released at synapses is fine-tuned by a variety of molecules, one group of which is called endocannabinoids (so-named because they are endogenous molecules that activate the same receptors as tetrahydrocannabinol, or THC, the active ingredient in marijuana). When endocannabinoids are released at synapses, they decrease neurotransmitter release. You can think of it like controlling water from a faucet: endocannabinoids turn the water down.
Endocannabinoids are known to influence many diverse aspects of physiology and behavior including learning and memory, motivational state, appetite, responses to stress, and pain; they are also involved in neurological disorders such as epilepsy. Understanding what controls the synthesis, release, and breakdown of endocannabinoids has broad implications both for normal and pathological brain function.
We discovered that the regulation of a key endocannabinoid in the brain, anandamide, differs between male and female rats. We found this by studying how estradiol applied to brain slices regulates neurotransmitter release at inhibitory synapses in the hippocampus, a brain region involved in learning and memory, responses to stress, and in epilepsy (among many other things). We use acute estradiol application to slices as a model for how estrogens synthesized in the brain, independently of the gonads, could affect neurophysiology. Our previous work had shown that estradiol suppresses neurotransmitter release through endocannabinoid actions at inhibitory synapses in the hippocampus of females, but not of males. This was surprising because estradiol acutely regulates excitatory synapses in the hippocampus both sexes.
The goals of our new study were to define the intracellular signaling that links estradiol to suppression of neurotransmitter release and to identify where in this pathway males and females differ.
Using a series of electrophysiological, biochemical, anatomical, and molecular techniques, we demonstrated that estradiol suppresses inhibitory synapses through membrane-associated estrogen receptor α (ERα), which interacts with metabotropic glutamate receptor 1 (mGluR1); when estradiol stimulates this interaction (in females), it results in activation of phospholipase C, the production of inositol triphosphate (IP3), and activation of the IP3 receptor; activation of the IP3 receptor increases intracellular calcium and ultimately leads to anandamide mobilization to inhibit neurotransmitter release. Interestingly, males had all the components of this pathway, but estradiol did not stimulate the interaction between ERα and mGluR1 in males.
This and other results in the paper point to regulation of the ERα-mGluR1 interaction as being the likely site in the pathway where males and females differ. In the course of completing these experiments, we also discovered that inhibitory synapses in females are clamped down by tonic (constant) anandamide release, whereas in males, this tonic release of anandamide was absent.
Thinking about the water faucet analogy above, the two main findings in the paper, that females show estradiol regulation of anandamide release and tonic anandamide release, both of which males appear to lack, indicate that females have extra controls on the tap of inhibitory neurotransmitter that males don't have.
We don’t know yet whether our results will translate to humans, but we think it is important for the scientific community to be aware of sex differences in regulation of endocannabinoids. One reason is that there are multiple clinical trials using drugs that modulate endocannabinoids, including anandamide. Knowing that the brains of males and females may respond differently to these drugs should help in the design and interpretation of clinical trials so that the results are not obscured by sex differences and the outcomes are meaningful to both men and women.
Finally, it is very important not to over-interpret the results of studies on sex differences in the brain. We found a sex difference in molecular regulation of synapses in the hippocampus. It can be tempting to jump to a conclusion that sex differences at the molecular level necessarily translate to sex differences at the behavioral level. But that may not be the case. For example, some sex differences may serve to compensate for other differences, making males and females more similar at a behavioral level.
So while it is true that the findings in our study are important for understanding how drugs that act on the endocannabinoid system may have different effects in males and females, understanding how the differences we found do or do not translate to behavioral differences will require new studies designed specifically to investigate that question.
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