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Abstract
Opioid peptides are found throughout the central nervous system, and have profound
effects on neuroendocrine function. In man, exogenous opiates and opioids elevate
circulating prolactin, GH and TSH, and suppress the release of the gonadotrophins
and pro-opiocortin-related peptides. However, unlike in other species, there is substantial
evidence for a physiological role of endogenous opioids only in the case of the gonadotrophins
and ACTH/LPH. Most evidence suggests that LH and FSH are modulated via the hypothalamus
or amygdala, where concentrations of opioids and opioid receptors are very high. Endogenous
opioids appear to be principally concerned with the frequency-modulated release of
GnRH, and this may be important clinically in patients presenting with amenorrhoea.
ACTH/LPH are under tonic inhibition by endogenous opioids acting at hypothalamic and/or
pituitary levels, and changes in this inhibition may be responsible for the release
of these peptides in response to certain forms of stress. It has been reported that
the opiate antagonist, naloxone, is clinically useful in paradoxically inhibiting
the release of ACTH in patients with Nelson's syndrome, but this requires adequate
confirmation. Vasopressin is under biphasic opiate control, but the principal effect
is probably opiate-mediated inhibition of vasopressin release. The endogenous ligand
for this response is likely to be dynorphin. Suppression of vasopressin release by
opiates may become a useful therapy in the treatment of the 'Syndrome of inappropriate
ADH'.