The influence of reproductive status on the stimulatory action of N-methyl-D,L-aspartate on growth hormone secretion, in vitro in rainbow trout, Oncorhynchus mykiss |
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| Authors: | AC Holloway JF Leatherland |
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| Institution: | (1) Department of Zoology, University of Guelph, Guelph, ON, N1G 2W1, Canada |
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| Abstract: | The glutamate agonist, N-methyl-D,L-aspartate (NMA) stimulates the secretion of growth hormone (GH) from pituitary fragments
in vitro and increases plasma GH levels in vivo in rainbow trout, Oncorhynchus mykiss (Flett et al. 1994; Holloway and Leatherland
1997a,b); however gonadal steroid hormones appear to modulate this response in experimental situations. This study examines
whether steroid hormones also modulate the GH-regulatory actions of NMA during the normal reproductive cycle of rainbow trout
by examining the relationship between the stage of sexual maturation and the pituitary release of GH in vitro in response
to an NMA (10-8 M) challenge. NMA had no effect on mean GH release from the pituitary glands of fish that were immature (GSI <1.0), from
males during early development (GSI 1.0-3.0), or from sexually mature males (with free running milt) and females (ovulated).
However, NMA significantly increased GH release from pituitary glands taken from females during the early stages of gonadal
growth (GSI 1.0-9.0) and from males and females sampled during the later stages of gonadal growth (males GSI 3.01-6.0; females
GSI 9.01-15.0). The GH-stimulatory action of NMA in males and females progressed to a maximum effect during the late stages
of gonadal growth, and disappeared in ovulated females and free running males. Moreover, in female fish, the maximal GH release
in response to the NMA challenge is positively correlated with plasma 17β-estradiol levels; no such correlation was evident
for plasma testosterone levels in males. Changes in the GH response to NMA during maturation while gonadal steroid levels
fluctuate provides further evidence to suggest that the effects of NMA on GH secretion are intimately linked to endogenous
gonadal steroid hormone levels.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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| Keywords: | rainbow trout growth hormone N-methyl-D L-aspartate gonadal steroid hormones estradiol testosterone |
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