Prostaglandin E₂-Induced Luteinizing Hormone- Releasing Hormone Release Involves Mobilization of Intracellular Ca⁺²*

Abstract

The present in vitro experiments were performed to examine the involvement of Ca+2 in the mechanism by which prostaglandin E2 (PGE2) induces LHRH release from the median eminence (ME) of the hypothalamus. Stepwise decreases in the Ca+2 concentration of the incubation medium reduced the LHRH response to PGE2. Nevertheless, neither complete omission of Ca+2 (residual Ca+2 concentration, 3.5 microM) nor chelation of residual Ca+2 with EGTA prevented the stimulatory effect of the PG, suggesting that a significant portion of the PGE2 effect on LHRH release is independent of extracellular Ca+2. Blockade of Ca+2 influx with verapamil, a Ca+2 entry blocker, demonstrated that this component of the PGE2 effect is completely independent of inward Ca+2 movement. Depletion of intraterminal Ca+2 stores by exposure of the MEs to the Ca+2 ionophore A23187 in medium without Ca+2 containing EGTA almost completely obliterated the subsequent LHRH response to PGE2, indicating that normal intraterminal Ca+2 levels are important for the PGE2 effect to occur. Preloading the ME terminals with 45Ca+2 and subsequent stimulation with PGE2 demonstrated that even in the absence of extracellular Ca+2, PGE2 stimulates Ca+2 efflux from the terminals, and this Ca+2 movement occurs temporarily associated with LHRH release. Depolarization of ME terminals with 56 mM K+ in the presence of normal Ca+2 concentration resulted in massive efflux of 45Ca+2 and a greater LHRH response than that produced by PGE2, suggesting that the effect of PGE2 is not the consequence of a nonspecific general depolarization of ME nerve terminals. Thus, although a full LHRH response to (exogenous) PGE2 necessitates normal extraterminal Ca+2 concentrations, the results indicate that translocation of Ca+2 from intracellular stores is an event involved in the mechanism by which PGE2 releases LHRH.

Estradiol Enhances Prostaglandin E2Receptor Gene Expression in Luteinizing Hormone-Releasing Hormone (LHRH) Neurons and Facilitates the LHRH Response to PGE2by Activating a Glia-to-Neuron Signaling Pathway

“…The presence of EP1-R mRNA in LHRH neurons of immature mice in situ and in GT1-1 cells, plus the detection of less abundant amounts of all three alternatively spliced forms of EP3-R mRNA in these cells, is consistent with earlier findings demonstrating the involvement of calcium (Ojeda and Negro-Vilar, 1985) and cAMP in the intracellular mechanism underlying the stimulatory effect of PGE 2 on LHRH release. Although the sensitivity of the combined immunohisto- chemistry and in situ hybridization procedure used was insufficient to detect EP3-R mRNA in L HRH neurons in vivo, the presence of all three alternatively spliced forms of the EP3-R gene in GT1-1 cells in vitro and the appearance of borderline hybridization signals in some L HRH neurons in situ suggest that the EP3-R gene is indeed expressed in L HRH neurons of the intact hypothalamus.…”

“…Among the neurotransmitters affecting L HRH release, norepinephrine (N E) is, perhaps, one of the best characterized, because its effects on L HRH secretion have been thoroughly documented using both in vivo and in vitro approaches (for review, see Barraclough and Wise, 1982;Ramirez et al, 1984;Kalra, 1986). The stimulatory effect of NE on LHRH release requires the intermediacy of prostaglandin E 2 (PGE 2 ) (Ojeda et al, 1979(Ojeda et al, , 1982, and the intracellular mechanisms underlying the actions of PGE 2 include mobilization of calcium from intracellular stores (Ojeda and Negro-Vilar, 1985) and cAMP formation .…”

Prostaglandin E 2 release from astrocytes triggers gonadotropin-releasing hormone (GnRH) neuron firing via EP2 receptor activation

“…Our electrophysiological data show that the EP1 receptor agonists 17PT-PGE 2 and sulprostone had no effect on GnRH neuronal activity and that the depletion of intracellular calcium stores with thapsigargin did not alter the depolarizing response of GnRH neurons to PGE 2 . These results, together with earlier findings demonstrating that PGE 2 -induced GnRH release from median eminence explants requires the mobilization of intracellular calcium stores (40), suggest that the EP1 receptor-dependent stimulation of GnRH release is exerted at the level of GnRH nerve terminals rather than GnRH cell bodies. Thus, PGE 2 may stimulate both GnRH neuron firing and GnRH release by acting at the cell soma and nerve terminal levels, respectively.…”

Copper amplification of prostaglandin E2 stimulation of the release of luteinizing hormone-releasing hormone is a postreceptor event.

“…Prostaglandin E2 (PGE2) stimulates the release of luteinizing hormone-releasing hormone (LH-RH) from hypothalamic tissue incubated under in vitro conditions (1)(2)(3)(4)(5), and this stimulation has a partial requirement for the influx of extracellular calcium (1,6). Several lines of evidence strongly suggest that, as in other secretory cells, calcium influx coupled to an increase in the intracellular levels of cAMP serve as a major intracellular pathway mediating PGE2 action on the LH-RH neuron or on other neurons, the secretory products of which stimulate LH-RH release.…”

“…We (21) (6,13) and in copper-amplified PGE2 action (14). There is evidence from studies on other secretory cells that supports the view that the influx of Ca2' occurs at a step beyond the formation of cAMP (23)(24)(25)(26) and the following chain of events has been proposed (26): hormone-receptor binding -* cAMP synthesis --protein kinase activation --calcium channel activation -* increase in cytosolic calcium -* many steps -+ release.…”