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Stimulation of endogenous opioid release displaces mu receptor binding in rat hippocampus
J J Wagner1, R M Caudle, J F Neumaier
1Department of Pharmacology, University of Washington School of Medicine, Seattle 98195.
Abstract:
Physiological release of endogenous opioids in the rat hippocampus was detected by an in vitro radioligand displacement assay using [3H][D-Ala2,N-methyl-Phe4,glyol5]enkephalin ([3H]DAGO), a mu selective opioid agonist. In this assay, radioligand binding to opioid receptors in the in vitro hippocampal slice was reduced by competition with endogenous opioids released following tissue depolarization. Veratridine-induced opioid release caused displacement of [3H]DAGO that could be blocked by either tetrodotoxin addition or calcium removal from the incubation buffer. Maximal displacement of [3H]DAGO also required the presence of peptidase inhibitors in the incubation buffer. None of the buffer composition changes directly affected [3H]DAGO binding to rat brain membranes. Calcium-dependent displacement of [3H]DAGO binding from mu receptor sites elicited by focal electrical stimulation depended on the intensity and frequency of stimulation and positioning of the electrode in the slice. Maximal displacement of [3H]DAGO binding was observed following high intensity (150-300 microA), high frequency (10-50 Hz) stimulation of the perforant path, a major afferent fiber system to the hippocampus previously shown to contain proenkephalin-derived opioids. Low frequency stimulation (0.1-1 Hz) was ineffective. Stimulation of the mossy fibers (containing both dynorphins and enkephalins) also significantly reduced mu receptor binding, but to a lesser extent. Electrical stimulation of the hippocampal slice at sites not containing opioid peptides did not cause mu receptor displacement. These results demonstrate that under physiological conditions, the release of endogenous opioids from the major opioid containing pathways can be detected in a single hippocampal slice following high frequency stimulation.
Insights
Researchers detected endogenous opioid release in rat hippocampus using a radioligand assay. High-frequency electrical stimulation of specific pathways triggered opioid release, demonstrating physiological detection of these signaling molecules.
Area of Science:
- Neuroscience
- Neuropharmacology
Background:
- Endogenous opioids play crucial roles in brain function.
- Detecting their physiological release is vital for understanding neural circuits.
Purpose of the Study:
- To develop and validate an in vitro method for detecting physiological release of endogenous opioids in the rat hippocampus.
- To characterize the conditions required for opioid release and detection.
Main Methods:
- Utilized an in vitro radioligand displacement assay with [3H][D-Ala2,N-methyl-Phe4,glyol5]enkephalin ([3H]DAGO), a mu-selective opioid agonist.
- Employed tissue depolarization (veratridine) and focal electrical stimulation to induce opioid release.
- Investigated the roles of calcium, tetrodotoxin, and peptidase inhibitors in the release and detection process.
Main Results:
- Veratridine-induced opioid release displaced [3H]DAGO, a process blocked by tetrodotoxin or calcium removal.
- Maximal displacement required peptidase inhibitors and was calcium-dependent.
- High-frequency electrical stimulation (10-50 Hz) of the perforant path and mossy fibers caused significant [3H]DAGO displacement, indicating endogenous opioid release.
- Low-frequency stimulation was ineffective, and stimulation at non-opioid sites did not cause displacement.
Conclusions:
- The study successfully demonstrates an in vitro method to detect physiological release of endogenous opioids from major hippocampal pathways.
- High-frequency stimulation is critical for triggering detectable opioid release.
- This method provides a valuable tool for studying opioid signaling in the hippocampus under physiological conditions.