Astrocytic control of synaptic NMDA receptors.

Astrocytes express a wide range of G-protein coupled receptors that trigger release of intracellular Ca2+, including P2Y, bradykinin and protease activated receptors (PARs). By using the highly sensitive sniffer-patch technique, we demonstrate that the activation of P2Y receptors, bradykinin receptors and protease activated receptors all stimulate glutamate release from cultured or acutely dissociated astrocytes. Of these receptors, we have utilized PAR1 as a model system because of favourable pharmacological and molecular tools, its prominent expression in astrocytes and its high relevance to neuropathological processes. Astrocytic PAR1-mediated glutamate release in vitro is Ca2+ dependent and activates NMDA receptors on adjacent neurones in culture. Activation of astrocytic PAR1 in hippocampal slices induces an APV-sensitive inward current in CA1 neurones and causes APV-sensitive neuronal depolarization in CA1 neurones, consistent with release of glutamate from astrocytes. PAR1 activation enhances the NMDA receptor-mediated component of synaptic miniature EPSCs, evoked EPSCs and evoked EPSPs in a Mg2+-dependent manner, which may reflect spine head depolarization and consequent reduction of NMDA receptor Mg2+ block during subsequent synaptic currents. The release of glutamate from astrocytes following PAR1 activation may also lead to glutamate occupancy of some perisynaptic NMDA receptors, which pass current following relief of tonic Mg2+ block during synaptic depolarization. These results suggest that astrocytic G-protein coupled receptors that increase intracellular Ca2+ can tune synaptic NMDA receptor responses.

Exaggerated 17-hydroxyprogesterone response to intravenous infusions of recombinant human LH in women with polycystic ovary syndrome.

Studies using pharmacological gonadotropin stimulation suggest that ovarian steroidogenesis is abnormal in the polycystic ovary syndrome (PCOS). We assessed ovarian steroid secretion in response to near-physiological gonadotropin stimuli in 12 ovulatory controls and 7 women with PCOS. A gonadotropin-releasing hormone-receptor antagonist (ganirelix, 2 mg sc) was given to block endogenous LH secretion, followed by dexamethasone (0.75 mg orally) to suppress adrenal androgen secretion. After ganirelix injection (12 h), intravenous infusions of recombinant human LH (0, 10, 30, 100, and 300 IU; each over 8 min) were administered at 4-h intervals in a pseudorandomized (highest dose last) manner. Plasma LH, 17-hydroxyprogesterone (17-OHP), androstenedione, and testosterone were measured concurrently. LH dose-steroid response relationships (mean sex-steroid concentration vs. mean LH concentration over 4 h postinfusion) were examined for each subject. Linear regression of 17-OHP on LH yielded a higher (mean +/- SE) slope in PCOS (0.028 +/- 0.010 vs. 0.005 +/- 0.005, P < 0.05), whereas extrapolated 17-OHP at zero LH was similar. The slopes of other regressions did not differ from zero in either PCOS or controls. We conclude that near-physiological LH stimulation drives heightened 17-OHP secretion in patients with PCOS, suggesting abnormalities of early steps of ovarian steroidogenesis. With the exception of 17-OHP response in PCOS, no acute LH dose-ovarian steroid responses were observed in controls or PCOS. Defining the precise mechanistic basis of heightened precursor responsiveness to LH in PCOS will require further clinical investigation.

Obese patients with polycystic ovary syndrome: evidence that metformin does not restore sensitivity of the gonadotropin-releasing hormone pulse generator to inhibition by ovarian steroids.

Women with polycystic ovary syndrome (PCOS) have reduced GnRH sensitivity to suppression by ovarian steroids, which can be ameliorated by androgen blockade. We studied nine PCOS women and nine controls to determine whether metformin could change feedback inhibition by estradiol (E(2)) and progesterone (P). LH was measured every 10 min, and FSH, E(2), P, and testosterone (T) were measured every 2 h. Frequently sampled iv glucose tolerance test was performed at the end of each admission. After the first admission, metformin (500 mg, three times a day) was started. The second admission occurred on d 8-11 of the next menstrual cycle in controls and on d 28 in PCOS patients. Patients subsequently took E(2) and P for 1 wk until the third admission. At baseline, PCOS women had higher T, free T, androstenedione, and estrone. After 4 wk of metformin, controls had a slight reduction in total T, but free T was unchanged. However, PCOS patients had reduced insulin, T, and E(2), and increased LH mean/amplitude and FSH. After ovarian steroids, controls had a greater reduction in LH pulse frequency than PCOS (61 vs. 25%). These results suggest that the beneficial effects of metformin on ovulatory function in obese PCOS women are probably not mediated by enhanced hypothalamic sensitivity.

Hypothalamic regulation of cyclic ovulation: evidence that the increase in gonadotropin-releasing hormone pulse frequency during the follicular phase reflects the gradual loss of the restraining effects of progesterone.

The luteal-follicular transition is characterized by decreasing plasma levels of E(2), progesterone (P), and inhibin A, with concomitant increases in FSH and LH levels. LH (and by inference GnRH) pulse frequency increases from 1 pulse every 3-4 h during the luteal phase to approximately 1 pulse/h at the midcycle LH surge. To examine the regulation of GnRH pulse frequency, we gave 10 normally cycling women transdermal E(2) and oral P to produce midluteal levels [364 +/- 65.0 pmol/liter (99 +/- 18 pg/ml) and 29.7 +/- 6.8 nmol/liter (9.3 +/- 2.1 ng/ml), respectively] for 10 d after the LH surge (d 0). P was then discontinued, and E(2) was given alone for 3 additional wk. Pulsatile LH secretion and follicular size were assessed on d 10, 17, 24, and 31. Results are presented as the mean +/- SEM. LH pulse frequency was 3.1 +/- 0.5 pulses/12 h after 10 d of E(2) and P, and remained low on d 17 when P had fallen below 1.6 nmol/liter (<0.5 ng/ml). In the continued presence of midluteal levels of E(2) [ approximately 360 pmol/liter (100 pg/ml)], LH pulse frequency increased on d 24 and 31 to 5.5 +/- 0.9 and 5.8 +/- 0.5 pulses/12 h, respectively, whereas pulse amplitude remained unchanged. FSH increased 2-fold, but follicular size did not change. These results are consistent with E(2) potentiating the effects of low concentrations of P on the GnRH pulse generator for at least 7 d, after which pulse frequency increases despite maintenance of E(2) levels. This supports the hypothesis that the increasing GnRH pulse frequency throughout the follicular phase reflects the gradual loss of the inhibitory actions of low concentrations of P.