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Spinal release of immunoreactive Met-enkephalin by intraventricular beta-endorphin and its analogs in anesthetized
Abstract:
We have reported previously that i.v.t. beta-endorphin increases the release of immunoreactive Met-enkephalin but not Leuenkephalin or dynorphins from the spinal cord. To determine if the effect is specific to beta-endorphin, the present investigation tested i.v.t. beta-endorphin, its analogs and other opiate agonists with different opioid receptor activities for their ability to release Met-enkephalin using an intrathecal perfusion technique. Human beta-endorphin and its analogs, human beta-endorphin-(1-30), -(1-29) and -(1-28) which have an identical amino acid sequence in the NH2-terminus showed reduced stepwise potencies in releasing Met-enkephalin. The results correlated well with their analgesic potencies. Des-Met5-camel beta-endorphin (64 micrograms i.v.t.) which does not have a complete sequence of Met-enkephalin in its NH2-terminus but still retains 20% of camel beta-endorphin analgesic potency caused the spinal release of Met-enkephalin. Morphine (mu opioid receptor agonist, 40 micrograms), D-Ala2-D-Leu5-enkephalin (delta opioid receptor agonist, 80 micrograms) and U-50488H (kappa opioid receptor agonist, 160 micrograms) injected i.v.t. were unable to cause any release of Met-enkephalin. High-performance liquid chromatography after Sephadex G-50 gel chromatography indicated that the immunoreactive Met-enkephalin in the spinal perfusate released by i.v.t. beta-endorphin had a retention time identical to authentic Met-enkephalin. Intraventricular injection of Met-enkephalin, 4 nmol (2.3 micrograms), caused little increase of Met-enkephalin immunoreactivity in the spinal perfusate, whereas 4 nmol of i.v.t. beta-endorphin caused a marked increase of Met-enkephalin in the spinal perfusate. Inhibition of peptidase by i.v.t. aprotinin and bacitracin does not prevent the spinal release of Met-enkephalin induced by i.v.t. beta-endorphin. It is concluded that the release of Met-enkephalin was specific to beta-endorphin and the results were not due to cross-immunoreactivity of beta-endorphin or its metabolites.
Insights
Intraventricular beta-endorphin specifically triggers Met-enkephalin release from the spinal cord. This effect is unique to beta-endorphin and its analogs, not other opioid agonists.
Area of Science:
- Neuroscience
- Pharmacology
- Endocrinology
Background:
- Previous research indicated that intraventricular (i.v.t.) beta-endorphin increases spinal cord immunoreactive Met-enkephalin release.
- The specificity of this effect for beta-endorphin versus other endogenous opioids was not fully established.
Purpose of the Study:
- To investigate whether the spinal release of Met-enkephalin induced by i.v.t. beta-endorphin is specific to this peptide.
- To compare the Met-enkephalin-releasing capacity of beta-endorphin and its analogs with other opioid receptor agonists.
Main Methods:
- Intrathecal perfusion technique was employed to collect spinal perfusate.
- Various beta-endorphin analogs and opioid agonists (morphine, D-Ala2-D-Leu5-enkephalin, U-50488H) were administered i.v.t.
- High-performance liquid chromatography and Sephadex G-50 gel chromatography were used to identify released Met-enkephalin.
Main Results:
- Human beta-endorphin and its truncated analogs dose-dependently released Met-enkephalin, correlating with their analgesic potencies.
- Des-Met5-camel beta-endorphin also induced Met-enkephalin release, despite lacking a complete Met-enkephalin sequence.
- Morphine, delta, and kappa opioid receptor agonists did not cause Met-enkephalin release; i.v.t. beta-endorphin was more potent than Met-enkephalin itself in inducing release.
Conclusions:
- The spinal release of Met-enkephalin is specifically triggered by beta-endorphin and its active analogs.
- The observed effect is not due to cross-reactivity with other opioid peptides or agonists acting on mu, delta, or kappa receptors.
- Peptidase inhibition did not affect beta-endorphin-induced Met-enkephalin release, suggesting direct action rather than metabolite involvement.