Abstract
Chronic gonadotropin-releasing hormone agonist (GnRHa) administration is used where suppression of hypothalamic-pituitary-gonadal axis activity is beneficial, such as steroid-dependent cancers, early onset gender dysphoria, central precocious puberty and as a reversible contraceptive in veterinary medicine. GnRH receptors, however, are expressed outside the reproductive axis, e.g. brain areas such as the hippocampus which is crucial for learning and memory processes. Previous work, using an ovine model, has demonstrated that long-term spatial memory is reduced in adult rams (45 weeks of age), following peripubertal blockade of GnRH signaling (GnRHa: goserelin acetate), and this was independent of the associated loss of gonadal steroid signaling. The current study investigated whether this effect is reversed after discontinuation of GnRHa-treatment. The results demonstrate that peripubertal GnRHa-treatment suppressed reproductive function in rams, which was restored after cessation of GnRHa-treatment at 44 weeks of age, as indicated by similar testes size (relative to body weight) in both GnRHa-Recovery and Control rams at 81 weeks of age. Rams in which GnRHa-treatment was discontinued (GnRHa-Recovery) had comparable spatial maze traverse times to Controls, during spatial orientation and learning assessments at 85 and 99 weeks of age. Former GnRHa-treatment altered how quickly the rams progressed beyond a specific point in the spatial maze at 83 and 99 weeks of age, and the direction of this effect depended on gonadal steroid exposure, i.e. GnRHa-Recovery rams progressed quicker during breeding season and slower during non-breeding season, compared to Controls. The long-term spatial memory performance of GnRHa-Recovery rams remained reduced (P<0.05, 1.5-fold slower) after discontinuation of GnRHa, compared to Controls. This result suggests that the time at which puberty normally occurs may represent a critical period of hippocampal plasticity. Perturbing normal hippocampal formation in this peripubertal period may also have long lasting effects on other brain areas and aspects of cognitive function.
Highlights
Gonadotropin-releasing hormone (GnRH) is a hypothalamic decapeptide that, following its release from axon terminals at the median eminence, stimulates the release of luteinizing hormoneD
The results demonstrate that peripubertal gonadotropin-releasing hormone agonist (GnRHa)-treatment suppressed reproductive function in rams, which was restored after cessation of GnRHa-treatment at 44 weeks of age, as indicated by similar testes size in both GnRHa-Recovery and Control rams at 81 weeks of age
GnRHa can be prescribed for treatment of central precocious puberty (CPP) (Chen and Eugster, 2015) and gender dysphoria (GD) (Hembree et al, 2009) to temporarily halt reproductive development
Summary
Gonadotropin-releasing hormone (GnRH) is a hypothalamic decapeptide that, following its release from axon terminals at the median eminence, stimulates the release of luteinizing hormoneD. As GnRH agonists (GnRHa) result in continued receptor stimulation, as opposed to ultradian cyclic changes, theiradministration initially results in an increase in LH and FSH secretion (‘flare-effect’), followed by the down-regulation of GnRH receptor expression in the pituitary gland and suppression of reproductive axis function (Garner, 1994; Chen and Eugster, 2015). GnRHa is typically prescribed when the suppression of the reproductive axis is required, such as steroid-sensitive conditions like prostate cancer, uterine fibroids and endometriosis (Garner, 1994). GnRHa can be prescribed for treatment of central precocious puberty (CPP) (Chen and Eugster, 2015) and gender dysphoria (GD) (Hembree et al, 2009) to temporarily halt reproductive development
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