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Multiple factors influencing the in vitro release of [Met5]-enkephalin from rat hypothalamic slices
K E Nikolarakis1, O F Almeida, A Herz
1Department of Neuropharmacology, Max-Planck-Institut für Psychiatrie, Planegg-Martinsried, F.R.G.
Abstract:
This study examined several in vivo and in vitro factors which influence the release of [Met5]-enkephalin (Met-ENK) from male rat hypothalamic slices superfused in vitro. Met-ENK release was significantly stimulated by corticotropin-releasing hormone (CRH; 10(-12)-10(-8) M), an effect which was abolished in the presence of the CRH-receptor antagonist, alpha-helical CRF9-41 (10(-6) M). The amount of Met-ENK release diminished with time in experiments in which the slices were continuously exposed to CRH. The opioid receptor antagonist naloxone (10(-6) M) stimulated Met-ENK release, even in the presence of the Na+ -channel blocker tetrodotoxin (10(-6) M), a result indicating presynaptic opioid feedback inhibition of Met-ENK release. The role of gonadal steroids in the control of Met-ENK release in vitro was also examined. It was found that the basal and CRH-induced release of Met-ENK was not changed 1 week after castration. However, a significant increase in the basal release of this peptide was observed 4 weeks after gonadectomy, and the Met-ENK-releasing efficacy of CRH was found to be reduced. The Met-ENK content of hypothalami from 1-week castrates was not significantly changed from control levels, but was significantly reduced in those from 4-week castrates. These long-term effects of castration could be overcome by the subcutaneous implantation of testosterone-containing capsules at the time of castration.
Insights
Corticotropin-releasing hormone (CRH) stimulates [Met5]-enkephalin (Met-ENK) release from rat hypothalamus, while naloxone indicates opioid feedback. Castration alters Met-ENK release and content, reversible with testosterone.
Area of Science:
- Neuroendocrinology
- Opioid peptide signaling
- Hypothalamic function
Background:
- [Met5]-enkephalin (Met-ENK) is a key endogenous opioid peptide.
- Hypothalamic Met-ENK release is influenced by various neuroendocrine factors.
- Understanding Met-ENK regulation is crucial for neurobiological research.
Purpose of the Study:
- To investigate the influence of corticotropin-releasing hormone (CRH) on Met-ENK release.
- To explore the role of opioid receptor feedback in Met-ENK release.
- To examine the impact of gonadal steroids on hypothalamic Met-ENK release in vitro.
Main Methods:
- Superfusion of male rat hypothalamic slices.
- Application of CRH and its antagonist (alpha-helical CRF9-41).
- Use of opioid antagonist (naloxone) and sodium channel blocker (tetrodotoxin).
- Assessment of Met-ENK release and content following castration and testosterone replacement.
Main Results:
- CRH significantly stimulated Met-ENK release, an effect blocked by its antagonist.
- Naloxone stimulated Met-ENK release even with tetrodotoxin, suggesting presynaptic inhibition.
- Castration initially showed no effect, but after 4 weeks, basal Met-ENK release increased, CRH efficacy decreased, and peptide content reduced.
- Testosterone replacement reversed these long-term castration effects.
Conclusions:
- CRH is a potent stimulator of hypothalamic Met-ENK release.
- Presynaptic opioid feedback mechanisms regulate Met-ENK release.
- Gonadal steroids, particularly testosterone, play a significant role in modulating hypothalamic Met-ENK release and content over time.