Studies have shown that women report more sleep difficulties and are more likely to be diagnosed with insomnia compared to men. Sleep disturbances are more likely to occur in women during times of hormonal fluctuations, including pregnancy and menopause, thus indicating that sex hormones play a role in the sleep-wake cycle. Mechanisms by which sleep is disrupted by estradiol (E2) are largely unknown. Rodents are proving to be a useful model to help elucidate estrogenic mechanisms that regulate sleep-wake behaviors. Two major sleep centers in the brain are in the preoptic area – the median preoptic nucleus (MnPO) and the ventrolateral preoptic nucleus (VLPO). Our current findings show that E2 action in the MnPO is necessary and sufficient to induce sleep-wake behaviors. Moreover, our previous work suggests that E2 increases extracellular adenosine in the MnPO. It has been found that subcutaneous injection of E2 increases wake and decreases sleep in ovariectomized female rats, however the mechanism by which this occurs is largely unknown. We hypothesize it is due to E2 affecting the signaling of adenosine receptors (A1R and A2AR).
During wakefulness, adenosine accumulates in the brain and is a measure of increased sleep pressure. A1R and A2AR are expressed in the MnPO and play an important role in regulating the effects of adenosine in the brain. Infusion of an A2AR agonist into the MnPO has been found to increase sleep and decrease wake in rats, while, more surprisingly, preliminary data from our lab suggests that infusion of an A1R agonist into the MnPO has been found to have the opposite effect. Interestingly, in the presence of subphysiological levels of E2, this effect of a A2AR agonist on sleep-wake states is attenuated. As such, E2 has been hypothesized to influence the inhibitory/excitatory adenosinergic balance in the MnPO. One potential target of E2 could be G protein-coupled receptor 37 (GPR37), as it has been shown to inhibit A2AR surface expression and function in the striatum.
To examine GPR37 mRNA and A2AR mRNA expression, ovariectomized (ovx) female rats were treated with E2 or oil for 2 days and sacrificed on the third day. I found that E2 increases GPR37 mRNA expression and increases GPR37 and A2AR mRNA expression in cell nuclei. Through a Lumit assay with animals treated in the same way, I found that E2 increases the interaction of GPR37 and A2AR protein in the preoptic area. I also found that knockdown of GPR37 in the MnPO significantly attenuates the effects of E2 on wakefulness, NREM sleep, and REM sleep in the dark phase but not the light phase. Furthermore, when GPR37 is knocked down, in the presence of subthreshold levels of E2, an A2AR agonist infused into the MnPO is trending to increase NREM sleep behavior. In the control group, subthreshold levels of E2 effectively blocks the pro-sleep effects of an A2AR agonist. Overall, E2 appears to influence the sleep/wake phenotype through adenosinergic signaling and expression, likely through increased GPR37 expression.