A procedure for detecting changes in the internal Ca2+ concentration in isolated nerve endings using the metallochromic dye arsenazo III

Inhibitory effect of capsaicin on the ascending pathway of the guinea-pig ileum and antagonism of this effect by ruthenium red

A segment of guinea-pig ileum, which was continuous with a strip of longitudinal muscle-myenteric plexus (LM-MP) at the anal end, was used to examine the effect of capsaicin on ascending excitatory pathways. Electrical field stimulation of the LM-MP caused an ascending contraction of the segment. After initially causing contraction capsaicin (3 microM) inhibited the ascending contraction. This inhibitory effect of capsaicin exhibited rapid desensitization and was abolished after extrinsic (mesenteric) denervation. Desensitization to calcitonin gene-related peptide (CGRP) prevented the capsaicin-induced inhibition without affecting the ascending contraction. Neither naloxone nor alpha- and beta-adrenoceptor antagonists affected the capsaicin-induced inhibition. CGRP (25 nM) also inhibited the ascending contraction, mimicking the inhibition induced by capsaicin. Ruthenium red (0.1-3 microM) antagonized the capsaicin-induced inhibition in a concentration-related manner, but did not affect the CGRP-induced inhibition. These findings suggest that the inhibitory effect of capsaicin on the ascending pathways might be mediated via the release of CGRP from extrinsic nerve terminals, and that the site of the antagonism of the action of capsaicin by ruthenium red is prejunctional.

Binding of lanthanum ions and ruthenium red to synaptosomes and its effects on neurotransmitter release

A technique for studying the binding of La3+ to synaptosomes in a double-beam spectrophotometer, using murexide as indicator, is described. The binding of La3+ was very rapid and Scatchard plots revealed two components, with KD values of 0.6 and 27 microM in a Na+-free medium (sucrose medium) and 2.3 and 63 microM in an ionic medium containing 135 mM Na+. The binding of the cationic dye ruthenium red (RuR) showed only one site, with a KD of 3.7 microM. La3+ binding was partially inhibited by RuR and vice versa, and La3+ was also capable of partially displacing RuR previously bound to the synaptosomes, particularly in the sucrose medium. The release of labeled gamma-aminobutyric acid (GABA) stimulated by K+ depolarization was inhibited by La3+ concentrations at or above 1 microM, in the ionic medium, whereas in the sucrose medium 2.5 microM or higher La3+ concentrations notably stimulated the spontaneous release of both GABA and glutamic acid. It is concluded that La3+ and RuR share at least one type of binding site, which is probably the high-affinity La3+ site. Since both La3+ and RuR at low concentrations have been shown to block the depolarization-induced Ca2+ entry in synaptosomes, this site might be related to the voltage-dependent Ca2+ entry involved in neurotransmitter release.

Effects of drugs on neurotransmitter release: experiments in vivo and in vitro

Calcium ions play a fundamental role in the release of transmitters in the nervous system. Therefore, drugs capable of modifying Ca2+ transport are useful tools for studying the mechanisms of such release in vivo and in vitro. In this article the action of some of these drugs on motor behavior, as well as on Ca2+ uptake and neurotransmitter release in synaptosomes, is reviewed. Ruthenium red (RuR) inhibits Ca2+ uptake and transmitter release in synaptosomes, and produces flaccid paralysis when injected intraperitoneally (IP) and convulsions after intracranial administration. Drugs which stimulate the Ca2+-dependent transmitter release in synaptosomes, such as 4-aminopyridine, antagonize the paralysis produced by RuR. Lanthanum ions also inhibit Ca2+ uptake and neurotransmitter release in synaptosomes, but no paralysis was observed after La2+ IP injection. However, this cation blocks the binding of RuR to the presynaptic membrane, and prevents the RuR-induced paralysis. Veratridine and the Ca2+ chelator EGTA were used to demonstrate in synaptosomes that besides the Ca2+-dependent mechanism of release of the central inhibitory transmitter gamma-aminobutyric acid (GABA), there seems to be a strictly Na+-dependent process which is not shared by other transmitters such as acetylcholine or dopamine.

Stimulation of [3H]gamma-aminobutyric acid release by calcium chelators in synaptosomes

The effect of EGTA on the release of labeled gamma-aminobutyric acid (GABA), glutamate, acetylcholine, and dopamine was studied in superfused synaptosomes from mouse brain. In the absence of both Ca2+ and Mg2+, EGTA and also EDTA at 50 microM or higher concentrations induced a 2.5-5-fold stimulation of [3H]GABA release, similar to that produced by potassium depolarization, whereas only a slight effect, or no effect at all, was observed on the release of the other transmitters studied. The GABA-releasing action of EGTA was practically abolished in the presence of Mg2+. In contrast, the effect of EDTA was also observed when the medium contained Mg2+. Studies on the ionic dependence showed that the stimulation of GABA release by EGTA was abolished in a Na+-free medium. Li+ did not substitute Na+ for the EGTA effect, which was also independent of chloride. This Na+ dependence does not seem to involve voltage-sensitive channels, since tetrodotoxin did not affect the GABA-releasing action of EGTA, whereas in parallel superfusion chambers it blocked over 80% the stimulation of GABA release by veratridine. In contrast, two calcium channel blockers in synaptosomes, La3+ and the cationic dye ruthenium red, greatly inhibited the GABA-releasing effect of EGTA. L-2,4-Diaminobutyric acid, an inhibitor of the Na+-dependent GABA carrier, did not affect the releasing action of EGTA, whereas in a parallel experiment this drug inhibited by more than 90% the exchange of labeled GABA with unlabeled GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in free-calcium levels and pH in synaptosomes during transmitter release

The free cytoplasmic Ca2+ concentration [( Ca2+]i) in rat brain synaptosomes estimated by the indicator quin 2 is 104 +/- 8 nM (S.D.) in artificial cerebrospinal fluid (1.2 mM Ca2+), but decreases at lower Ca2+ concentrations in the medium. The presence of quin 2 in the synaptosomes does not affect either the spontaneous release of transmitter (gamma-aminobutyric acid) or the release induced by K+ depolarisation. In quin 2-loaded synaptosomes, depolarisation by K+ causes an abrupt increase in [Ca]i (less than 2-fold) that is approximately proportional to the extent of depolarisation, whereas depolarisation by veratrine alkaloids produces a slow rise in [Ca]i. The increase in [Ca]i produced by K+ depolarisation does not occur in the absence of Ca2+ in the medium. The data are consistent with a direct correlation between [Cai] and transmitter release in functional synaptosomes. The pH in synaptosomes estimated by the indicator quene 1 is 7.04 +/- 0.07 and is stable in media containing 5 mM bicarbonate. The pH in synaptosomes was decreased by protoveratrine but not by K+ depolarisation.