Endocrine Abstracts

Hypothyroidism refers to a thyroid hormone (TH) deficiency which mainly affects women (10/1 ratio). The higher default incidence in women remains unexplained. Diagnosis of hypothyroidism is based on a single thyroid-stimulating-hormone (TSH) measurement albeit TSH secretion displays pulsatile patterns which are under the positive control of hypothalamic thyrotropin-releasing-hormone (TRH) and negative feedback exerted by THs. Pituitary thyrotrophs form the smallest (2-4%) pituitary endocrine population and the processes which govern their TSH secretion in vivo remain largely unknown. The aim of this study is to explore whether pituitary thyrotrophs form, in vivo, a functionally-organized and sexually dimorphic cell population capable of generating TSH pulses. Circulating mouse TSH levels were measured using an ultra-sensitive in-house mTSH ELISA, uncovering various pulsatile patterns which would differentially stimulate the thyroid gland. In order to identify the origin of TSH secretion, calcium signals were monitored in vivo in mouse models, as a surrogate of the stimulus-TSH secretion coupling and cell-cell communication between thyrotrophs. Using thin needles of gradient-index (GRIN) lenses (7.6 mm long, 0.6 mm diameter) implanted at the pituitary level and a 2g head-mounted miniscope, multicellular calcium activities of the thyrotroph population were monitored in freely-moving TSHβ-crexR26fl-flGCaMP6f mice. Such signaling events were monitored during longitudinal studies (weeks to months) in which individual animals act as their own controls. We unveiled that the thyrotroph population functions in vivo as a robust generator of recurrent, slowly-evolving calcium waves which propagate between neighboring thyrotrophs and lead to the generation of ultradian TSH fluctuations. To further dissect the mechanisms underlying these sequences of calcium events, we conducted experiments on anesthetized animals, in which hypothalamic (i.e. TRH) inputs were blunted. Several injection patterns of hypothalamic TRH or TSH unveiled how TSH pulses were built-up in vivo. In male mice, TSH pulsatility resulted from a finely-tuned combination of prolonged TRH inputs together with an ultrashort intra-pituitary feedback loop exerted by TSH itself. In female mice, distinct patterns of calcium signals were observed in both basal and TRH/TSH-stimulated conditions. Moreover, 3D confocal imaging revealed sexually dimorphic, multicellular thyrotroph motifs (clusters in females vs. columns in males) which were spatially organized in fixed ‘cleared’ pituitaries. Altogether, our data support that a sexually dimorphic thyrotroph organization underlies a sexually dimorphic stimulus-TSH secretion coupling in animal models.