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Activation of enkephalin receptors reduces calcium conductance in neuroblastoma cells
Abstract:
Enkephalin actions on the voltage-gated Ca channel were studied under voltage clamp in the neuroblastoma X glioma hybrid cell line, NG 108-15. We found that in sodium-free external solutions containing Ba2+ (20 mM), a depolarizing step pulse from a holding potential of -50 mV evoked both a rapidly decaying inward current and a sustained inward current similar to those described in other preparations. External application of 1 microM [D-Thr2,Leu5]enkephalin-Thr (DTLET), an agonist at the delta-opioid receptors specifically inhibited the sustained inward current. Naloxone, an antagonist at this receptor, blocked the effect of DTLET. Furthermore, this effect of DTLET was not observed if the cells were dialyzed with a low Ca2+ internal buffer solution [( Ca2+]i less than 10(-9) M). We conclude that in neuroblastoma cells: (1) there is a functional coupling between delta-receptors and voltage-gated sustained Ca channels, and (2) the coupling is mediated by intracellular free Ca ions.
Insights
Enkephalin, acting on delta-opioid receptors, inhibits sustained calcium currents in neuroblastoma cells. This effect is dependent on intracellular calcium levels, suggesting a crucial role for calcium ions in the signaling pathway.
Area of Science:
- Neuroscience
- Cell Biology
- Pharmacology
Background:
- Voltage-gated calcium channels are crucial for neuronal function.
- Opioid receptors, including delta-opioid receptors, modulate various cellular processes.
- Neuroblastoma X glioma hybrid cells (NG 108-15) are a valuable model for studying neuronal signaling.
Purpose of the Study:
- To investigate the effect of enkephalin on voltage-gated calcium channels in NG 108-15 cells.
- To elucidate the role of delta-opioid receptors in this modulation.
- To determine the involvement of intracellular calcium in the observed effects.
Main Methods:
- Electrophysiology: Voltage clamp technique was employed to record ionic currents.
- Pharmacological agents: [D-Thr2,Leu5]enkephalin-Thr (DTLET), a delta-opioid receptor agonist, and naloxone, an antagonist, were used.
- Intracellular manipulation: Cells were dialyzed with low calcium buffer to assess intracellular calcium dependence.
Main Results:
- A sustained inward current, characteristic of voltage-gated calcium channel activity, was observed in NG 108-15 cells.
- DTLET significantly inhibited this sustained current.
- Naloxone blocked the inhibitory effect of DTLET.
- The inhibitory effect of DTLET was abolished when intracellular free calcium concentration was reduced below 10(-9) M.
Conclusions:
- There is a functional coupling between delta-opioid receptors and voltage-gated sustained calcium channels in neuroblastoma cells.
- Intracellular free calcium ions mediate this functional coupling.
- Enkephalin's action on calcium channels is dependent on the intracellular calcium milieu.