The role of progesterone in memory bias during spatial navigation in females

Progesterone and allopregnanolone facilitate excitatory synaptic transmission in the infralimbic cortex via activation of membrane progesterone receptors

Estrogens and progesterone can have rapid effects on neuronal function and can modify the use of spatial navigation strategies dependent upon the prefrontal cortex, striatum, and hippocampus. Here, we assessed the effects of 17β-estradiol (E2), progesterone, and its metabolite allopregnanolone, on evoked excitatory postsynaptic potentials in the infralimbic region of the female rat prefrontal cortex. Field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of layer I were first characterized by recording responses at multiple depths between the cortical surface and the underlying white matter. Current source density analysis showed that the short-latency negative component was generated by activation of synaptic currents within layer I, and that putative polysynaptic responses were generated in layers III to V. The amplitude of evoked field EPSPs in layer I was not significantly affected by 20 min application of 17β-estradiol (10 nM), but both 100 nM progesterone and 1 µM allopregnanolone caused lasting increases in field EPSP amplitude. The effects of progesterone were not blocked by the nuclear progesterone receptor antagonist RU486 (1 µM). Both progesterone and allopregnanolone are known to activate membrane progesterone receptors, and we found that the membrane progesterone receptor agonist Org OD 02-0 facilitated EPSPs, and also occluded further increases induced by either progesterone or allopregnanolone. These results provide evidence that both progesterone and allopregnanolone facilitate synaptic responses in layer I of the infralimbic cortex by activating membrane progesterone receptors.

Enhancement of response learning in male rats with intrastriatal infusions of a BDNF - TrkB agonist, 7,8-dihydroxyflavone

Enhancement of learning and memory by cognitive and physical exercise may be mediated by brain-derived neurotrophic factor (BDNF) acting at tropomyosin receptor kinase B (TrkB). Upregulation of BDNF and systemic administration of a TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), enhance learning of several hippocampus-sensitive tasks in rodents. Although BDNF and 7,8-DHF enhance functions of other brain areas too, these effects have mainly targeted non-cognitive functions. One goal of the present study was to determine whether 7,8-DHF would act beyond the hippocampus to enhance cognitive functions sensitive to manipulations of the striatum. Here, we examined the effects of intrastriatal infusions of 7,8-DHF on learning a striatum-sensitive response maze and on phosphorylation of TrkB receptors in 3-month-old male Sprague Dawley rats. Most prior studies of BDNF and 7,8-DHF effects on learning and memory have administered the drugs for days to months before assessing effects on cognition. A second goal of the present study was to determine whether a single drug treatment near the time of training would effectively enhance learning. Moreover, 7,8-DHF is often tested for its ability to reverse impairments in learning and memory rather than to enhance these functions in the absence of impairments. Thus, a third goal of this experiment was to evaluate the efficacy of 7,8-DHF in enhancing learning in unimpaired rats. In untrained rats, intrastriatal infusions of 7,8-DHF resulted in phosphorylation of TrkB receptors, suggesting that 7,8-DHF acted as a TrkB agonist and BDNF mimic. The findings that a single, intra-striatal infusion of 7,8-DHF 20 min before training enhanced response learning in rats suggest that, in addition to its trophic effects, BDNF modulates learning and memory through receptor mediated cell signaling events.

High estradiol reduces adult neurogenesis but strengthens functional connectivity within the hippocampus during spatial pattern separation in adult female rats

Adult neurogenesis in the dentate gyrus plays an important role for pattern separation, the process of separating similar inputs and forming distinct neural representations. Estradiol modulates neurogenesis and hippocampus function, but to date no examination of estradiol's effects on pattern separation have been conducted. Here, we examined estrogenic regulation of adult neurogenesis and functional connectivity in the hippocampus after the spatial pattern separation task in female rats. Ovariectomized Sprague-Dawley rats received daily injections of vehicle, 0.32 μg (Low) or 5 μg (High) of estradiol benzoate until the end of experiment. A single bromodeoxyuridine (BrdU) was injected one day after initiation of hormone or vehicle treatment and rats were tested in the delayed nonmatching to position spatial pattern separation task in the 8-arm radial maze for 12 days beginning two weeks after BrdU injection. Rats were perfused 90 min after the final trial and brain sections were immunohistochemically stained for BrdU/neuronal nuclei (NeuN) (new neurons), Ki67 (cell proliferation), and the immediate early gene, zif268 (activation). Results showed that high, but not low, estradiol reduced the density of BrdU/NeuN-ir cells and had significant inter-regional correlations of zif268-ir cell density in the hippocampus following pattern separation. Estradiol treatment did not influence pattern separation performance or strategy use. These results show that higher doses of estradiol can reduce neurogenesis but at the same time increases correlations of activity of neurons within the hippocampus during spatial pattern separation.

Combined effects of the contraceptive hormones, ethinyl estradiol and levonorgestrel, on the use of place and response memory in gonadally-intact female rats

During maze navigation rats can rely on hippocampus-mediated place memory or striatum-mediated response memory. Ovarian hormones bias whether females use place or response memory to reach a reward. Here, we investigated the impact of the contraceptive hormones, ethinyl estradiol (EE) and levonorgestrel (LNG), on memory bias. A total of 63 gonadally-intact female rats were treated with either 10 μg/kg of EE alone, 20 μg/kg of LNG alone, both 10 μg/kg of EE and 20 μg/kg of LNG together, or a sesame oil injection with 5% ethanol as a vehicle control. Rats in the control condition were tested during the diestrus phase of the estrous cycle in order to control for the low circulating levels of gonadotropin and ovarian hormones that occur with oral contraceptive administration. Rats treated with LNG alone had a bias towards the use of place memory compared to diestrus phase control rats. This bias was not observed if LNG was administered in combination with EE. Rats treated with EE or EE+LNG did not have a statistically significant difference in memory bias compared to rats in the control group. These data show that synthetic hormones contained in oral contraceptives administered to females influence which cognitive strategy is predominantly used during navigation.