Effects of veratridine and high potassium on μ-opioid receptor internalization in the rat spinal cord: Stimulation of opioid release versus inhibition of internalization

Abstract

Veratridine and high K + evoked opioid release in rat spinal cord slices, measured by μ-opioid receptor (MOR) internalization. Veratridine elicited up to 75% MOR internalization with an atypical concentration-response: its effect appeared abruptly from 5 μM to 10 μM, and declined thereafter to disappear at 100 μM. Veratridine at 100 μM also abolished MOR internalization induced by an exogenous MOR agonist, endomorphin-2, showing that its inhibitory phase was caused by direct inhibition of MOR internalization. In contrast, veratridine elicited neurokinin 1 receptor internalization (used to measure substance P release) following a monophasic dose-response curve. KCl evoked up to 80% MOR internalization following a monophasic, steep dose-response curve with an EC 50 of 19 mM. K + -evoked MOR internalization disappeared in the presence of lidocaine or in the absence of peptidase inhibitors, indicating that it involved neuronal firing and peptide release. Veratridine and high K + produced substantially more MOR internalization when applied for 2 min than for 20 min, because of a direct inhibition of MOR internalization with the longer incubation times. These results show that short incubations with 20 μM veratridine or 30 mM KCl are optimal stimuli to evoke MOR internalization in the spinal cord.

Opioid receptor internalization contributes to dermorphin-mediated antinociception

“…Although the present manuscript is the first to show DERM-A594 internalization in a whole animal, opioid-induced MOPr internalization has been shown previously in intact animals using immunohistochemistry (Trafton and Basbaum, 2000; He et al, 2002; Trafton and Basbaum, 2004; Chen et al, 2007; Chen et al, 2008; Lao et al, 2008). The advantage of assessing MOPr internalization with a fluorescently labeled opioid agonist in vivo is that internalization is restricted to those receptors bound by agonist at the plasma membrane.…”

Acute inflammation induces segmental, bilateral, supraspinally mediated opioid release in the rat spinal cord, as measured by μ-opioid receptor internalization

“…In each neuron, endosomes were counted by changing the focus to scan the soma (not the dendrites) in the z-axis. Whereas high concentrations of exogenous opioids elicit the appearance of numerous endosomes and the almost complete disappearance of MOR immunoreactivity from the cell surface (Marvizon et al, 1999a; Song and Marvizon, 2003b; Chen et al, 2007), endogenously released opioids produce a more subtle pattern (Song and Marvizon, 2005; Chen et al, 2008a; Chen et al, 2008b; Lao et al, 2008). Endosomes are observed, but a large part of the MOR immunoreactivity remains at the cell surface forming clusters (Supplementary Fig.…”