Abstract
| Introduction | Material
and Methods:
Animals; Drugs; Hypothalamic Incubation; Gn-RH determination;
NOS activity; | Results: Effect
of the administration of testosterone on hypothalamic NOS
activity;
Effect of the administration of testosterone on hypothalamic
Gn-RH release; | Statistical Analysis
| Discussion
| Acknowledgments | References
Roxana
Reynoso (Biochemist),
Claudia Mohn (MD, PhD),
Valeria Retory, (MD, PhD)*.,
Berta Szwarcfarb, (Biochemist),
Silvia Carbone (Biochemist),
Dora Rondina (Biochemist) and
Jaime A. Moguilevsky (MD, PhD)*
Departamento
de Fisiología, Facultad de Medicina, Universidad de
Buenos Aires. Buenos Aires, ARGENTINA.
* Established Investigator. National Research Council. Buenos
Aires ARGENTINA
Key words:
Nitric
Oxide, Sexual Maturation, GnRH, gonadotrophins
Abstract.
OBJECTIVES:
To determine the effect of testosterone administration to
prepubertal (15 days old) and peripubertal rats (30 days old)
on hypothalamic nitric oxide synthetase (NOS), and GnRH release.
METHODS:
Hypothalamic samples containing the anterior preoptic and
medial basal areas (APO-MBH) were incubated for 30 minutes
in 500 l of Earle´s medium with glucose (1 mg/ml) and
bacitracin (20 mM). GnRH was determined by RIA in the medium
and NOS activity was determined in APO-MBH after 10 min of
incubation by the conversion of 14 C arginine to 14 C citrulline
.
RESULTS:
Treatment with testosterone propionate, signicantly decreased
NOS hypothalamic activity in prepubertal male rats. (Control:
58.41± 0.85; Testosterone: 25.61± 1.40, p<0.001)
and had no effect in peripubertal male rats (Control 49.28
± 1.50; Testosterone 51.48 ± 5.2 pmoles NO/10
min/hypothalamus)
On the other hand, in prepubertal rats the treatment decreased
Gn-RH release (Control: 3.62 ± 0.23; Testosterone:
1.38 ± 0.11 (pg/ ml medium, p<0.001) and has no
effect on Gn-RH release in 30 days old rats (Control : 3.65
± 0.33.;Testosterone: 4.15 ± 0.36 pg/ ml. medium,).
CONCLUSION:
These results clearly demonstrated that testosterone has an
inhibitory effect on hypothalamic NOS activity in prepubertal
rats while did not affect the concentration of this neurotransmitter
system in peripubertal rats. These pattern is similar to that
observed with GnRH hypothalamic release since testosterone
has an inhibitoty effect in prepubertal rats and did not modify
the GnRH release in peripubertal rats. Taking into account
the well known stimulatory effect of NO on GnRH and the decrease
in the sensitivity of GnRH-gonadotrophin axis to the inhibitory
feedback effect of testosterone during sexual maturation and
the onset of puberty, it is proposed that the changes here
described are connected with maturational modifications in
the sexual hormones on-GnRH axis connected with the onset
of puberty
Introduction
The
involvement of nitric oxide (NO), as an intracellular and
extracellular gaseous neurotransmitter synthetized from L-arginine
by nitric oxide synthetase (NOS), (1) in the hypothalamic
control of gonadotrophin secretion has been demonstrated [2,3,4,5,6,7]
. NO as a diffusible signaling gas has the ability to control
and synchronize the activity of the neighboring cells. GnRH
neurons are under the regulatory effect of NO. At the median
eminence NO activates GnRH terminals which, at the same time
increases the release of the hypothalamic hormone (3). NO
secretion in the median eminence was also involved in the
modulation of GnRH release during the estrous cycle and in
the genesis of pulsatile GnRH secretion [3,4,6].
There are clear evidences that GnRH is the primary messenger
involved in the activation of pituitary gonadotrophins during
sexual maturation and in the onset of puberty (8) . GnRH hypothalamic
neurons are under the regulatory activity of different neutrotransmitter
and neuromodulator systems which change its effects during
sexual maturation and at the time of the onset of puberty
[9,10,11]. The decrease in the inhibitory effect of testosterone
on gonadotrophin secretion appears to be one of the neuroendocrine
mechanisms involved in the increases of these pituitary hormones
during the onset of puberty.[8]. Apparently these changes
are related with changes in sensitivity of neurotransmitters-GnRH
neurons to sexual hormones during sexual maturation. [10].
On these bases the possibility arises that the sexual hormones
could modify NOS hypothalamic activity during sexual maturation
as one of the neuroendocrine mechanisms implicated in the
changes in the sensitivity of hypothalamic-pituitary axis
to the negative feedback effect of testosterone that take
place during the onset of puberty.
The aim of the present paper was to determine the effect of
testosterone on hypothalamic NOS activity in prepubertal and
in peribubertal male. On the other hand the effect of testosterone
on hypothalamic GnRH release was also determined at both ages.
Material
and Methods.
Animals
Drugs
Hypothalamic Incubation
Gn-RH determination
NOS activity
Results
Effect of the administration of testosterone
on hypothalamic NOS activity
Effect of the administration of testosterone
on hypothalamic Gn-RH release
Statistical
Analysis
Discussion
Acknowledgments
This work was supported by Grants from Agencia Nacional de
Promoción Científica y Técnica (BID 1201.
PICT 6240). The fellowship Carrillo-Oñativia, (Ministerio
de Salud de la Nación) and the University of Buenos
Aires.
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