Stress and nutritional influences on GnRH and somatostatin neuronal systems in the ewe
The effect of stressful stimulation and protein malnutrition on the gonadotrophic and somatotrophic axis of sheep is discussed with special references to the relationship between these stimuli and the GnRH and somatostatin neuronal systems in the hypothalamus. Generally, long-term stimulation and chronic underfeeding reduce gonadal functions in the sheep. There is evidence for the GnRH-dependent pathway for the mechanism of these phenomena in female sheep. GnRH neurons respond to long-term stress in diminishing of neuropeptide release from the nerve terminals due to the depression of its axonal transport. Chronic restriction of dietary proteins in lambs reduces the plasma LH concentrations but does not impair the development of GnRH neurons nor the synthesis and processing of GnRH. It is suggested that malnutrition delays the first ovulation probably due to the neural mechanism responsible for the preovulatory GnRH/LH output. Stress has rather unclear effect on growth hormone (GH) secretion in the sheep. Prolonged, but not short stressful stimulation provokes the rapid release of somatostatin, which is sustained during long-term stimulation. These results suggest that effect of stress on somatotrophic axis depends on the period of stressful stimulation. Chronic malnutrition enhances secretion of GH by an increase in amplitude of GH pulses and reduces the secretory activity of somatostatin neurons. It is postulated that nutrients can influence GH secretion in the sheep by mechanism dependent on the hypothalamic somatostatin.
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