Involvement of presynaptic voltage-dependent Kv3 channel in endothelin-1-induced inhibition of noradrenaline release from rat gastric sympathetic nerves

[Ca2+ ] changes in sympathetic varicosities and Schwann cells in rat mesenteric arteries-Relation to noradrenaline release and contraction

AIM

This study aimed to assess intracellular Ca²⁺ dynamics in nerve cells and Schwann cells in isolated rat resistance arteries and determine how these dynamics modify noradrenaline release from the nerves and consequent force development.

METHODS

Ca²⁺ in nerves was assessed with confocal imaging, noradrenaline release with amperometry and artery tone with wire myography. Ca²⁺ in axons was assessed after loading with Oregon Green 488 BAPTA-1 dextran. In other experiments, arteries were incubated with Calcium Green-1-AM which loads both axons and Schwann cells.

RESULTS

Schwann cells but not axons responded with a Ca²⁺ increase to ATP. Electrical field stimulation of nerves caused a frequency-dependent increase in varicose [Ca²⁺ ] ([Ca²⁺ ]_v ). ω-conotoxin-GVIA (100 nmol/L) reduced the [Ca²⁺ ]_v transient to 2 and 16 Hz by 60% and 27%, respectively; in contrast ω-conotoxin GVIA inhibited more than 80% of the noradrenaline release and force development at 2 and 16 Hz. The K_V channel blocker, 4-aminopyridine (10 µmol/L), increased [Ca²⁺ ]_v , noradrenaline release and force development both in the absence and presence of ω-conotoxin-GVIA. Yohimbine (1 µmol/L) increased both [Ca²⁺ ]_v and noradrenaline release but reduced force development. Acetylcholine (10 µmol/L) caused atropine-sensitive inhibition of [Ca²⁺ ]_v , noradrenaline release and force. In the presence of ω-conotoxin-GVIA, acetylcholine caused a further inhibition of all parameters.

CONCLUSION

Modification of [Ca²⁺ ] in arterial sympathetic axons and Schwann cells was assessed separately. K_V 3.1 channels may be important regulators of [Ca²⁺ ]_v , noradrenaline release and force development. Presynaptic adrenoceptor and muscarinic receptor activation modify transmitter release through modification of [Ca²⁺ ]_v .

KV1 and KV3 Potassium Channels Identified at Presynaptic Terminals of the Corticostriatal Synapses in Rat

In the last years it has been increasingly clear that K_V-channel activity modulates neurotransmitter release. The subcellular localization and composition of potassium channels are crucial to understanding its influence on neurotransmitter release. To investigate the role of K_V in corticostriatal synapses modulation, we combined extracellular recording of population-spike and pharmacological blockage with specific and nonspecific blockers to identify several families of K_V channels. We induced paired-pulse facilitation (PPF) and studied the changes in paired-pulse ratio (PPR) before and after the addition of specific K_V blockers to determine whether particular K_V subtypes were located pre- or postsynaptically. Initially, the presence of K_V channels was tested by exposing brain slices to tetraethylammonium or 4-aminopyridine; in both cases we observed a decrease in PPR that was dose dependent. Further experiments with tityustoxin, margatoxin, hongotoxin, agitoxin, dendrotoxin, and BDS-I toxins all rendered a reduction in PPR. In contrast heteropodatoxin and phrixotoxin had no effect. Our results reveal that corticostriatal presynaptic K_V channels have a complex stoichiometry, including heterologous combinations K_V1.1, K_V1.2, K_V1.3, and K_V1.6 isoforms, as well as K_V3.4, but not K_V4 channels. The variety of K_V channels offers a wide spectrum of possibilities to regulate neurotransmitter release, providing fine-tuning mechanisms to modulate synaptic strength.