On the mechanism by which extracellular sodium depletion causes 5-hydroxytryptamine release from rat brain synaptosomes

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Induction of 5-hydroxytryptamine release by tramadol, fenfluramine and reserpine

Tramadol is a centrally acting analgesic with several modes of action. Enhancement of 5-hydroxytryptamine release contributes to its actions. We investigated in which way tramadol induces 5-hydroxytryptamine release. Rat brain frontal cortex slices were preincubated with [3H]5-hydroxytryptamine, then superfused using conditions which impaired either carrier mediated release or exocytosis. Tramadol (10 and 100 microM), fenfluramine (1 microM) and reserpine (10 microM) enhanced the basal release of [3H]5-hydroxytryptamine. In the presence of a high concentration of 6-nitroquipazine effects of tramadol were reduced and those of fenfluramine abolished. Effects by reserpine were enhanced, indicating that [3H]5-hydroxytryptamine depletion was counteracted by reuptake. When NaCl was replaced by LiCl, tramadol did not affect [3H]5-hydroxytryptamine release, fenfluramine induced a small and reserpine a marked facilitation. Omission of CaCl2 did not alter fenfluramine and reserpine effects while those by tramadol were reduced. It is concluded that tramadol induces both carrier mediated 5-hydroxytryptamine release as well as exocytosis.

The characteristics of arginine transport by rat cerebellar and cortical synaptosomes

The uptake of L-[3H]arginine into synaptosomes prepared from rat cerebellum and cortex occurred by a high-affinity carrier-mediated process. The uptake of arginine appeared to be potentiated by removal of extracellular Na+, inhibited by high levels of extracellular K+, but not by depolarization with veratridine or 4-amino pyridine. The effect of Na+ removal or K+ elevation did not seem to be due to changes in intracellular Ca2+ or pH. In both brain regions, uptake was significantly inhibited by L-arginine, L-lysine, L-ornithine, and L-homoarginine, but not by D-arginine nor L-citrulline. Uptake was also inhibited by NG-monomethyl-L-arginine acetate, but not by NG-nitro-L-arginine methyl ester nor NG-nitro-L-arginine except in the cortex at a concentration of 1 mM. The results indicate that the carrier system operating in synaptosomes showed many of the characteristics of the ubiquitous y+ system seen in many other tissues, although its apparent sensitivity to variations in extracellular Na+ was unusual.

The uptake of gamma-aminobutyric acid and glutamate by synaptosomes from the visual cortex of albino and pigmented rabbits

The synaptosomal uptake of glutamate and gamma aminobutyric acid [GABA] in the visual cortex of albino and pigmented rabbits was compared. GABA uptake was similar in both pigmented and albino rabbits, but glutamate uptake was greater in the pigmented rabbit. The kinetics of glutamate uptake in albino and pigmented rabbits suggested that the number of functioning glutamate synapses may be lower in the albino. The significance of this with respect to the differences in visual processing in the two types of rabbit is discussed.

On the significance of perfusion rate in the study of glutamate release from superfused synaptosomes

The effect of perfusion rate on the apparent release of [(3)H]glutamate from prelabelled and superfused rat cortical synaptosomes was examined. The proportion of tissue [(3)H]glutamate released in response to a 4 ml depolarizing pulse of 15 mM K+ increased almost linearly with perfusion rates from 1 ml min(-1) to 10 ml min(-1). Release did not increase markedly between 10 ml min(-1) and 20 ml min(-1). The basal efflux of [(3)H]glutamate also increased with perfusion rate. The increase in both basal efflux and (K+)-induced release is interpreted as being due to a greater amount of released transmitter avoiding recapture by uptake processes as perfusion rate increases. This is supported by the observation that increasing the potential number of uptake sites in the tissue decreases both the basal and (K+)-evoked release of the transmitter. The significance of this with respect to optimal perfusion rates for studies on the regulation of glutamate release is discussed.

Postnatal development of the calcium-dependency of glutamate release from rat cortical synaptosomes: comparison with 5-hydroxytryptamine release

The ontogeny of the Ca(2+)-dependency of the depolarisation-induced release of preloaded [3H]glutamate and [3H]5-hydroxytryptamine [5-HT] from rat cortical synaptosomes was examined. 5-HT release was found to be exclusively Ca(2+)-dependent at all ages studied. In contrast, glutamate release only showed a significant Ca(2+)-dependent component from postnatal day 10 [PND 10] onwards. This correlated with the ontogeny of the glutamate accumulating activity of synaptic vesicles, a finding consistent with vesicles being the site of Ca(2+)-dependent release. The effectiveness of K(+)-depolarisation in inducing the Ca(2+)-dependent release of both transmitters increased during the early neonatal period, reaching near adult levels at PND20 for 5-HT and PND30 for glutamate.

On the role of nitric oxide as a cellular messenger in brain

The characteristics of the high-affinity uptake of [3H]-L-arginine into cerebellar and cortical synaptosomes were investigated. Uptake into cerebellar synaptosomes was often greater than seen in cortical synaptosomes under similar experimental conditions, and this was reflected by a higher Vmax in synaptosomes from this brain region. Uptake into synaptosomes prepared from both brain regions was markedly enhanced by removing extracellular Na+, adn inhibited by high concentrations of extracellular K+. Depolarisation with 4-aminopyridine or veratridine has no effect on uptake. Uptake was also unaffected by hyperpolarisation. The profile of inhibition of arginine uptake by related amino acids was similar to that seen for the y+ carrier, but the other characteristic alluded to above suggest that the carrier is distinct from the classical y+ system. The possible relationship between the carrier and the metabolism of arginine through the nitric oxide [NO] pathway, and the role of NO in the central nervous system is discussed.

Effects of short-term hypoxia on [3H]glutamate release from preloaded hippocampal and cortical synaptosomes

The effect of short-term hypoxia on the release of [3H]glutamate from preloaded hippocampal and cortical synaptosomes was studied in a rapid superfusion system. The technique minimised the loss of released glutamate by reuptake. The results indicated that the effects of short term hypoxia were qualitatively similar to those reported in previous studies using more long-term hypoxia, but were significantly smaller. The non-Ca(2+)-dependent efflux of glutamate from cortical synaptosomes was increased by hypoxia as was the Ca(2+)-dependent release from hippocampal tissue. Possible mechanisms for these findings were discussed. The small amplitude of these changes in comparison to the effects seen in slowly perfused tissue in vitro and in vivo indicated that the contribution made by changes in neuronal efflux to the overall increase in extracellular glutamate seen in hypoxia is relatively minor.

Changes in synaptosomal glutamate release during postnatal development in the rat hippocampus and cortex

The effectiveness of K+ depolarisation in inducing the release of [3H]L-glutamate from preloaded hippocampal and cortical synaptosomes was examined in rats aged from postnatal day 4 (PND 4) to adult. In the lower age groups studied (PND 4-PND 15), the response to depolarisation was always smaller than that seen in the adult. From PND 15, the sensitivity of the release process increased steadily to a maximum level in the adult. The relatively small amounts of glutamate released in response to K(+)-depolarisation in the younger age groups may be a factor which contributes to the relative insensitivity of neonatal brain to ischaemic damage. Discrete variations in the sensitivity to K+ depolarisation observed in animals aged from PND 4 to PND 15 may be involved in plastic changes in neural activity which are known to occur during this important development period.