Voltage-dependent kappa-opioid modulation of action potential waveform-elicited calcium currents in neurohypophysial terminals
Department of Physiology
Medical Subject Headings
*Action Potentials; Analgesics, Non-Narcotic; Analgesics, Opioid; Animals; Calcium; Calcium Channels; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; GTP-Binding Proteins; Male; Patch-Clamp Techniques; Pituitary Gland, Posterior; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; *Synapses; Synaptic Transmission
Life Sciences | Medicine and Health Sciences | Neuroscience and Neurobiology
Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca(2+) channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca(2+) channel kinetics, and the associated Ca(2+) influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the kappa- and micro-opioid inhibition of Ca(2+) currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the kappa- and micro-receptors in neurohypophysial terminals. The kappa-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, micro-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of kappa-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca(2+) channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca(2+)-dependent neuropeptide release in other CNS terminals, as well.
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Citation: J Cell Physiol. 2010 Oct;225(1):223-32. Link to article on publisher's site