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A persistent sodium current contributes to oscillatory activity in heart interneurons of the medicinal leech
1Department of Biology, Emory University, Atlanta, GA 30322.
Insights
In leeches, normally antiphasic heart interneurons synchronize in special solutions. This synchronization involves a sodium-dependent inward current (IP) and is not blocked by calcium channel blockers.
Area of Science:
- Neuroscience
- Electrophysiology
- Leach Physiology
Background:
- Heart interneurons in the medicinal leech (Hirudo medicinalis) typically exhibit antiphasic activity due to reciprocal inhibition.
- Altered saline conditions, specifically calcium-free and cobalt-containing solutions, can induce synchronous oscillations in these neurons.
Purpose of the Study:
- To investigate the ionic mechanisms underlying synchronous oscillations and plateau potentials in leech heart interneurons.
- To characterize the inward current (IP) responsible for these oscillatory behaviors.
Main Methods:
- Electrophysiological recordings, including single-electrode voltage-clamp techniques, were used on leech heart interneurons.
- Experiments involved manipulating extracellular and intracellular ionic compositions, including the use of TEA+ (tetraethylammonium), Cs+ (cesium), Co++ (cobalt), and varying Na+ (sodium) concentrations.
Main Results:
- Cobalt-containing solutions induced synchronous oscillations in heart interneurons.
- Internal TEA+ enabled full plateau potentials during Co++ induced oscillations, which were unaffected by Cs+.
- A voltage-dependent inward current (IP), primarily carried by Na+, was identified and showed little inactivation.
- IP was enhanced in Ca++-free, Co++-containing salines and was not blocked by calcium channel blockers, suggesting it does not utilize Ca++ channels.
Conclusions:
- The synchronous oscillations and plateau potentials in leech heart interneurons under specific conditions are mediated by a Na+-dependent inward current (IP).
- This inward current (IP) is distinct from calcium currents and plays a crucial role in generating the observed oscillatory activity.
Abstract:
1. Normal activity in bilateral pairs of heart interneurons, from ganglia 3 or 4, in the medicinal leech (Hirudo medicinalis) is antiphasic due to their reciprocally inhibitory connections. However, Ca(++)-free Co(++)-containing salines lead to synchronous oscillations in these neurons. 2. Internal TEA+ allows expression of full plateaus during Co++ induced oscillations in heart interneurons; these plateaus are not blocked by Cs+. Similar plateaus are also observed with internal TEA+ alone, but under these conditions activity in heart interneurons from ganglia 3 or 4 is antiphasic. 3. Plateaus in heart interneurons induced by Co++ and internal TEA+ involve a conductance increase. 4. A voltage-dependent inward current, IP, showing little inactivation, was isolated using single-electrode voltage-clamp in heart interneurons. This current is carried at least in part by Na+; the current is reduced when external Na+ is reduced and is carried by Li++ when substituted for Na+. 5. Calcium channel blockers such as La3+ and Co++ block neither the TEA+ induced plateaus nor IP, suggesting that Na+ is not using Ca++ channels. Moreover, IP is enhanced by Ca(++)-free CO(++)-containing salines. Thus, IP is correlated with the TEA(+)- and Co(++)-induced plateau behavior.