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Opioid
modulation of hypothalamic catecholaminergic neurotransmission
and the pre-ovulatory LH surge in the rat
Bayram Yilmaz & Desmond P. Gilmore
Key words:
catecholamines, opioid receptors,
LH surge, HPLC-ECD
Abstract
We have investigated the inter-relationship between the opioid
and catecholaminergic systems in the control of LH secretion,
and the involvement of m- and
k-opioid subtypes in this process.
Conscious female rats were intraperitoneally injected with either
m- (diamorphine) or k-opioid
agonists (U-50488H) alone or with their respective antagonists
(naloxone and MR2266) before the critical period on pro-estrus.
Hypothalamic catecholamine and plasma LH levels were determined
by HPLC-ECD and RIA, respectively. Both m-
and k-agonists significantly decreased
concentrations of noradrenaline and its metabolite (DHPG) in
all the hypothalamic regions examined concomitant with inhibition
of the LH surge. Dopamine levels were selectively reduced only
by the m-agonist in the MPOA.
The inhibitory effects of both opioid agonists were mostly reversed
following their co-administration with naloxone and MR2266 (except
the k-antagonist on LH). These
results indicate that both the m-
and k-opioid subtypes may be involved
in the inhibition of the LH surge by altering the hypothalamic
noradrenaline content.
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