Characterization of action potential‐evoked calcium transients in mouse postganglionic sympathetic axon bundles

Abstract

1. Action potential‐evoked Ca2+ transients in postganglionic sympathetic axon bundles in mouse vas deferens have been characterized using confocal microscopy and Ca2+ imaging. Axonal Ca2+ transients were tetrodotoxin sensitive. The amplitude depended on both the frequency of stimulation and the number of stimuli in a train. Removal of extracellular Ca2+ abolished the Ca2+ transient. Cd2+ (100 μm) inhibited the Ca2+ transient by 78 ± 10 %. The N‐type Ca2+ channel blocker ω‐conotoxin GVIA (0.1 μm) reduced the amplitude by −35 ± 4 %, whereas nifedipine (10 μm; L‐type) and ω‐conotoxin MVIIC (0.1 μm; P/Q type) were ineffective. Caffeine (10 mm), ryanodine (10 μm), cyclopiazonic acid (30 μm) or CCCP (10 μm) had no detectable effects. Blockade of large and small conductance Ca2+‐dependent K+ channels with iberiotoxin (0.1 μm) and apamin (1 μm), respectively, or Ca2+‐dependent Cl− channels by niflumic acid (100 μm) did not alter Ca2+ transients. In contrast, the non‐specific K+ channel blockers tetraethylammonium (10 mm) and 4‐aminopyridine (10 mm) markedly increased the amplitude of the Ca2+ transient. Blockade of delayed rectifiers and A‐like K+ channels, by tityustoxin‐K (α) (0.1 μm) and pandinustoxin‐K (α) (10 nm), respectively, also increased the Ca2+ transient amplitude. Thus, Ca2+ transients are evoked by Na+‐dependent action potentials in axons. These transients originate mainly from Ca2+ entry through voltage‐dependent Ca2+ channels (80 % Cd2+ sensitive of which 40 % was attributable to N‐type). Twenty per cent of the Ca2+ transient was not due to Ca2+ entry through voltage‐gated Ca2+ channels. Intracellular stores and mitochondria were not involved in the generation of the transient. Ca2+ transients are modulated by A‐like K+ channels and delayed rectifiers (possibly KV1.2) but not by Ca2+‐activated ion channels.

Bretylium or 6‐OHDA‐resistant, action potential‐evoked Ca2+ transients in varicosities of the mouse vas deferens

“…To determine whether uptake-1 sites were functional in all regions, mice were pre-treated with 6-OHDA. Bretylium or 6-OHDA pre-treatment did not a ect the nerve action potential-evoked Ca 2+ transients previously characterized in preterminal axon bundles (Jackson et al, 2001). These data suggest that uptake-1 sites are not present in su cient numbers to block action potential propagation in the parent axons.…”

The Role of Mitochondria in Cytoplasmic Ca2+ Cycling

“…Since CCCP itself was shown to produce depolarization of the DC m , the question arises as to whether observed cytoplasmic Ca 2+ changes were attributable to mitochondria. Action potential-evoked Ca 2+ transients in mouse postganglionic axon bundles were not affected by CCCP (Jackson et al 2001). Also, in synaptic terminals of retinal bipolar neurons mitochondrial Ca 2+ uptake was evident only sometimes and with large Ca 2+ loads (Zenisek & Matthews, 2000).…”

Kv1.3 channels in postganglionic sympathetic neurons: expression, function, and modulation

“…These findings are consistent with previous reports demonstrating that inhibition of Kv1 family channels enhances neurotransmitter release. The work of Jackson et al (22) and Uhrenholt and Nedergaard (45) indicates that inhibition of Kv1 channels enhances NE release at sympathetic neurovascular junctions. Our findings that Kv1.3 channels are present in processes innervating arteries and that MgTX enhances NE release, suggest that these channels play a role in modulating sympathetic neurovascular transmission.…”

“…These channels affect a range of neuronal functions (6, 8, 11, 16, 26, 34 -36), including spike frequency adaptation (35,36) and the regulation of cellular excitability in response to synaptic input (11). Postganglionic sympathetic neurons express Kv1 channels (5,33,43), and inhibition of these channels has been reported to modulate neurotransmitter release from these neurons (21,22,45). However, the mechanisms involved in Kv1 channel modulation of neurotransmitter release in sympathetic neurons are largely unexplored.…”