The low degree, or even absence, of tolerance or dependence
observed after chronic treatment with the dual inhibitors could
be explained by a more specific stimulation of the opioid binding
sites by the tonically released endogenous opioids. A limited
opioid receptor occupation by the endogenous peptides is
supported by in vivo binding studies, which demonstrated that
the increase in tonically released endogenous enkephalins is too
low to saturate opioid receptors. Moreover, chronic morphine
induces a hypersensitivity of noradrenaline-containing
neurons in the locus coeruleus, considered as one of the main
causes of the withdrawal syndrome. It is interesting to observe
that in slices of rat pons, kelatorphan was able to potentiate
strongly the firing of the locus coeruleus that had been induced
by exogenous Met-enkephalin but had no intrinsic effect, indicating
that there is little or no tonic endogenous opioid action
in this brain region . This is probably one of the major reasons
why the withdrawal syndrome is significantly milder after
chronic treatment by dual inhibitors as compared with exogenous
opioids. Indeed, it has been clearly demonstrated that the
locus coeruleus is the most critical structure implicated in the
development of dependence .
On the other hand, an important distinction between opiate
drugs and native peptides is that these two classes of ligand
differ greatly in bioavailability, metabolism and modes of opioid
receptor stimulation (review in [114,115]). This is likely to
account for the observed difference in their potency to induce
tolerance or dependence when they are administered at equieffective
analgesic doses. The poor propensity of a given agonist
to induce tolerance could be linked to its ability to
promote opioid receptor internalization and recycling to the
cell surface in a fully active state, thereby resensitizing cells to
agonist [116,117]. Endogenous opioid peptides are typically
released in a phasic or pulsatile manner. Opiate drugs, in contrast,
persist in the extracellular milieu for a prolonged period
of time, and activate opioid receptors in an abnormally prolonged
manner. Opiate drugs that induce rapid desensitization
and endocytosis of receptors may more closely mimic the
phasic actions and physiological adaptations observed with
endogenous opioid peptides. In contrast, opiate drugs such as
morphine, which are unable to induce endocytosis of receptors,
persistently stimulate them, forcing other cellular mechanisms
to compensate at downstream sites for this prolonged
activation. Thus, morphine could have a strongest capability
propensity to cause widespread changes in neural plasticity
associated with drug addiction [118] than the natural
morphine-like peptides. Side effects following chronic treatment
with opiates are probably due to multiple cellular events
involving several components of the cyclic AMP signal
transduction cascade, such as CREB
虽然由于急用我已经自己翻完了,但是看到这么负责认真的答案,真的很感谢。话不多说,100分追加奉上:)以后再有我实在不想翻的段落还找你哈,一定会高分奉上的:)
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