Summary
The widespread occurrence of opioid peptides and their receptors in brain and periphery
correlates with a variety of actions elicited by opioid agonists and antagonists on
hormone secretion. Opioid actions on pituitary and pancreatic peptides are summarized
in Table 1.
In rats opioids stimulate ACTH and corticosterone secretion while an inhibition of
ACTH and cortisol levels was observed in man. In both species, naloxone, an opiate
antagonist, stimulates the release of ACTH suggesting a tonic suppression by endogenous
opioids. In rats, a different stimulatory pathway must be assumed through which opiates
can stimulate secretion of ACTH. Both types of action are probably mediated within
the hypothalamus.
LH is decreased by opioid agonists in many adult species while opiate antagonists
elicit stimulatory effects, both apparently by modulating LHRH release. A tonic, and,
in females, a cyclic opioid control appears to participate in the regulation of gonadotropin
secretion.
Exogenous opiates potently stimulate PRL and GH secretion in many species. Opiate
antagonists did not affect PRL or GH levels indicating absence of opioid control under
basal conditions, while a decrease of both hormones by antagonists was seen after
stimulation in particular situations. In rats, opiate antagonists decreased basal
and stress-induced secretion of PRL.
Data regarding TSH are quite contradictory. Both inhibitory and stimulatory effects
have been described.
Oxytocin and vasopressin release were inhibited by opioids at the posterior pituitary
level. There is good evidence for an opioid inhibition of suckling-induced oxytocin
release. Opioids also seem to play a role in the regulation of vasopressin under some
conditions of water balance.
The pancreatic hormones insulin and glucagon are elevated by opioids apparently by
an action at the islet cells. Somatostatin, on the contrary, was inhibited. An effect
of naloxone on pancreatic hormone release was observed after meals which contain opiate
active substance. Whether opioids play a physiologic role in glucose homeostasis remains
to be elucidated.
Key-Words:
Opiate Receptor
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Opioid Peptide
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Opiate Antagonist
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ACTH
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LH
-
FSH
-
GH
-
TSH
-
Vasopressin
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Oxytocin
-
Insulin
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Glucagon
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Food Intake
