t has been repeatedly shown in heroin addicts
that the short acting -opioid agonist heroin will cause a
prompt increase in serum prolactin levels, resulting directly
from an abrupt lowering of dopamine levels in the tuberoinfundibular
dopaminergic systems (85).In humans, and to a
greater extent than in rodents, prolactin release is essentially
solely under tonic inhibition by dopaminergic tone in the
tuberoinfundibular dopaminergic system.However, it was
found that during chronic methadone treatment, there is
adaptation or tolerance to this phenomenon, an attenuation,
but not a complete removal or ablation of this response
caused by dopamine lowering and resulting in elevation of
serum prolactin levels (85).Even during long-term methadone
maintenance treatment, as reported in 1978, it was
found that peak plasma levels of the -opiate agonist methadone
are related temporally to the peak plasma levels of
prolactin (85).These findings suggest that the long-acting
opioid methadone administered orally continues to have
an impact at least on the tuberoinfundibular dopaminergic
system, with a lowering of dopaminergic tone, resulting in
a modest increase of prolactin levels, although not exceeding
upper levels of normal. However, that attenuation occurs
suggests that there may be either a lowering of dopaminergic
levels and tone in the turberoinfundibular dopaminergic
system of that region or, alternatively, an attenuation of
responsivity of the -opiate-receptor system.It has been
shown that the -opiate-receptor system similarly plays a
role in modulating prolactin levels in humans (86).In normal
healthy volunteers, dynorphin A causes a prompt rise
in serum prolactin levels, resulting again presumably from
a lowering of dopaminergic tone in the tuberoinfundibular
system (86).This is a , but also a -opioid-receptor effect,
as documented by use of two different opioid antagonists
with different receptor selectivity (86).