Endocrine Abstracts

Gonadotropin-releasing hormone (GnRH) is a hypothalamic neuropeptide that acts via GnRHR on the pituitary gonadotrope. It is secreted in pulses and acts via GnRHR to activate ERK and Nuclear Factor of Activated T-cells (NFAT), mediating GnRH effects on gonadotropin expression. We monitor their activation by high content imaging (fluorescence staining for ppERK and nuclear translocation of an NFAT1c-EFP reporter) in fixed LbT2 gonadotroph cells. Single cell measures reveal high cell-cell heterogeneity, and information theoretical approaches can be used to explore its influence on information transfer. Here we use Mutual information (MI) between GnRH concentration and measured responses (I(response; GnRH)) to measure (in Bits) information transfer via GnRHR. One bit of information can resolve two different signal values. However, the MI values were always <1Bit despite 3Bit input. Joint sensing of ERK and NFAT increased MI values, but the increase was modest, suggesting that, by ignoring response dynamics, information transfer is underestimated. Therefore, using live cell measurements and MI calculations taking response trajectory into account, NFAT-EFP translocation was tracked in response to a single pulse of GnRH. The I(NFAT-NF; GnRH) was 0.4Bit at 30min and increased to 0.54 Bit by consideration of trajectories. We also tracked NFAT-EFP translocation responses in cells receiving two pulses of GnRH and found that the information gained from the second pulse was little. Similar experiments were performed using Fluo-4 measurements of [Ca²⁺]_i. The I(Ca²⁺; GnRH) was 0.8Bit, 24sec after stimulation, and increased to 1Bit by sensing trajectories. Thus, LbT2 cells are unreliable sensors of GnRH concentration because a considerable amount of information is lost through signalling. Although joint sensing, trajectory sensing and sensing repeated pulses increased information transfer, this was typically <20% suggesting that most information is lost early in the GnRH signalling cascade, prior to Ca²⁺ mobilisation.