Calcium and transmitter release

Effects of combined arsenic and lead exposure on the brain monoaminergic system and behavioral functions in rats: Reversal effect of MiADMSA

In this study, we evaluated the therapeutic efficacy of monoisoamyldimercaptosuccinic acid (MiADMSA) against individual and combined effects of arsenic (As) and lead (Pb) on the monoaminergic system and behavioral functions in rats. Pregnant rats were exposed to sodium metaarsenite (50 ppm) and lead acetate (0.2%) individually and in combination (As = 25 ppm + Pb = 0.1%) via drinking water from gestation day (GD) 6 to postnatal day (PND) 21. MiADMSA (50 mg/kg body weight) was given orally through gavage for 3 consecutive days to pups from PND 18 to PND 20. The results showed increases in synaptosomal epinephrine, dopamine, and norepinephrine levels with individual metal exposures and decreases with combined exposure to As and Pb in the cortex, cerebellum, and hippocampus in PND 21, PND 28, and 3 months age-group rats. We found decreased activity of mitochondrial monoamine oxidase in the selected brain regions following individual and combined exposures to Pb and As. In addition, rats treated with Pb and As alone or in combination showed significant deficits in open-field behavior, grip strength, locomotor activity, and exploratory behavior at PND 28 and 3 months of age. However, MiADMSA administration showed reversal effects against the As- and/or Pb-induced impairments in the monoaminergic system as well as in behavioral functions of rats. Our data demonstrated that the mixture of Pb and As induced synergistic toxicity to developing brain leading to impairments in neurobehavioral functions and also suggest therapeutic efficacy of MiADMSA against Pb- and/or As-induced developmental neurotoxicity.

The tumor microenvironment: the making of a paradigm

What has been will be again, what has been done will be done again; there is nothing new under the sun

(Ecclesiastes 1:9)

Stephen Paget was the conceptual father of the role played by the Tumor Microenvironment (TME) in tumor progression. The focus of this essay is the developmental phase of the post Paget TME research. Attempts will be made to highlight some of the pioneering work of scientists from the late sixties through the eighties of last century who laid the foundations for the contemporary scientific achievements of TME research but whose ground breaking studies are rarely cited. This review should serve as a small tribute to their great work.

How to build an inner hair cell: challenges for regeneration

During their development inner hair cells (IHCs), the primary sensory receptors in the mammalian cochlea, undergo a meticulously orchestrated series of changes in the expression of ion channels and in their presynaptic function. This review considers what we currently know about these changes in IHCs of mice and rats, which start hearing 10-12 days after birth. Just after terminal mitosis the IHCs are electrically quiescent and functionally isolated, expressing only small and slow outward K(+) currents in their basolateral membranes. By the first postnatal week the cells have acquired inward Ca(2+) and Na(+) currents that enable them to fire spontaneous action potentials at a time when the cochlea can not yet be stimulated by sound. These action potentials may be essential for normal development and survival of the IHCs themselves and of the afferent nerve fibres that synapse with them. At the onset of hearing the transition to a functionally mature sensory receptor comes about by the expression of a large and fast BK current, I(K,f), a KCNQ4 current, I(K,n), and by changes in the exocytotic machinery. Some implications of this complex developmental programme for the ideal of hair-cell regeneration in the mature mammalian cochlea are discussed.

Zinc and Calcium Reduce Lead Induced Perturbations in the Aminergic System of Developing Brain

Since alterations in monoamines and monoamine oxidase (MAO) have been postulated to play a role in toxic effects of lead (Pb) on the central nervous system, we have examined the protective effects of calcium (Ca2+) and zinc (Zn2+) supplementation on Pb-induced perturbations in the levels of monoamines and the activity of MAO. Swiss albino mice were lactationally exposed to low (0.2%) and high (1%) levels of Pb-acetate via drinking water of the mother. Pb-exposure commenced on postnatal day (PND) 1, continued up to PND 21 and stopped at weaning. Ca2+ or Zn2+ (0.02% in 0.2% Pb-water or 0.1% in 1% Pb-water) was supplemented separately to the mother up to PND 21. The levels of monoamines (epinephrine, norepinephrine, dopamine and serotonin) and the activity of MAO in the brain regions such as hippocampus, cortex, cerebellum and medulla of young (1 month old) and adult (3 month old) mice were determined in the synaptosomal fractions. The synaptosomal monoamines though increased with low level (0.2%) Pb-exposure, significantly decreased with high level (1%) Pb-exposure in all the brain regions in both the age groups. In general, the young mice seem to be more vulnerable to Pb-neurotoxicity. Ca2+ or Zn2+ supplementation significantly reversed the Pb-induced perturbations both in the levels of monoamines and in the activity of MAO. However, the recovery in monoamine levels and MAO activity was more pronounced with Ca2+ supplementation as compared to Zn2+. These results provide evidence that dietary Ca2+ and/or Zn2+ provide protection against Pb-induced neurotoxic effects.

Increase in efficiency and reduction in Ca2+ dependence of exocytosis during development of mouse inner hair cells

Developmental changes in the coupling between Ca2+ entry and exocytosis were studied in mouse inner hair cells (IHCs) which, together with the afferent endings, form the primary synapse of the mammalian auditory system. Ca2+ currents (ICa) and changes in membrane capacitance (DeltaCm) were recorded using whole-cell voltage clamp from cells maintained at body temperature, using physiological (1.3 mM) extracellular Ca2+. The magnitudes of both ICa and DeltaCm increased with maturation from embryonic stages until postnatal day 6 (P6). Subsequently, ICa gradually declined to a steady level of about -100 pA from P13 while the Ca2+-induced DeltaCm remained relatively constant, indicating a developmental increase in the Ca2+ efficiency of exocytosis. Although the size of ICa changed during development, its activation properties did not, suggesting the presence of a homogeneous population of Ca2+ channels in IHCs throughout development. The Ca2+ dependence of exocytosis changed with maturation from a fourth power relation in immature cells to an approximately linear relation in mature cells. This change applies to the release of both a readily releasable pool (RRP) and a slower secondary pool of vesicles, implying a common release mechanism for these two kinetically distinct pools that becomes modified during development. The increased Ca2+ efficiency and linear Ca2+ dependence of mature IHC exocytosis, especially over the physiological range of intracellular Ca2+, could improve the high-fidelity transmission of both brief and long-lasting stimulation. These properties make the mature cell ideally suited for fine intensity discrimination over a wide dynamic range.

Direct measurements of presynaptic calcium and calcium-activated potassium currents regulating neurotransmitter release at cultured Xenopus nerve-muscle synapses

The understanding of neurotransmitter release at vertebrate synapses has been hampered by the paucity of preparations in which presynaptic ionic currents and postsynaptic responses can be monitored directly. We used cultured embryonic Xenopus neuromuscular junctions and simultaneous pre- and postsynaptic patch-clamp current-recording procedures to identify the major presynaptic conductances underlying the initiation of neurotransmitter release. Step depolarizations and action potential waveforms elicited Na and K currents along with Ca and Ca-activated K (KCa) currents. The onset of KCa current preceded the peak of the action potential. The predominantly omega-CgTX GVIA-sensitive Ca current occurred primarily during the falling phase, but there was also significant Ca2+ entry during the rising phase of the action potential. The postsynaptic current began a mean of 0.7 msec after the time of maximum rate of rise of the Ca current. omega-CgTX also blocked KCa currents and transmitter release during an action potential, suggesting that Ca and KCa channels are colocalized at presynaptic active zones. In double-ramp voltage-clamp experiments, KCa channel activation is enhanced during the second ramp. The 1 msec time constant of decay of enhancement with increasing interpulse interval may reflect the time course of either the deactivation of KCa channels or the diffusion/removal of Ca2+ from sites of neurotransmitter release after an action potential.

Definition of the readily releasable pool of vesicles at hippocampal synapses

A readily releasable pool of quanta, tentatively identified with docked synaptic vesicles, has been defined by analysis of the neurotransmitter release caused by application of hypertonic solutions. The goal of this work is to determine the relationship of this functionally defined readily releasable pool to the one drawn upon by action potential-evoked release. We find that hypertonic solutions do not act through changes in intracellular calcium. Since the release produced by action potentials and hypertonic solutions varies in parallel as the pool size is changed, we conclude that the same pool is shared by both mechanisms. This conclusion, taken together with other observations in the literature, means that the synaptic release probability depends on the size of the readily releasable pool.

Extracellular Ca2+ changes during transmitter application in the leech central nervous system

Changes in extracellular Ca2+ concentration ([Ca2+]e) evoked by transmitters and transmitter agonists, respectively, and by elevation of bath K+ concentration were recorded in isolated segmental ganglia of the leech Hirudo medicinalis using Ca(2+)-selective microelectrodes. A 1-min bath application of kainate (10 microM), glutamate (1 mM), aspartate (1 mM), or carbachol (200 microM) decreased [Ca2+]e by up to 1 mM, whereas the inhibitory transmitters gamma-amino butyric acid (GABA, 100 microM) and serotonin (5-HT, 100 microM) did not change [Ca2+]e. The amplitude of the kainate-induced changes in [Ca2+]e increased with repetitive applications, and changes were blocked by 6-cyano-7-dinitroquinozaline-2,3-dione (CNQX). Elevation of bath K+ concentration from 4 to 40 mM led to a Ni(2+)-sensitive decrease in [Ca2+]e by 0.9 mM. Our results suggest that excitatory transmission in the leech central nervous system might be accompanied by substantial decreases in [Ca2+]e.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

Neurons respond to hyposmotic conditions by an increase in intracellular free calcium

The effect of hyposmotic conditions on the concentration of intracellular free calcium ([Ca2+]i) was studied in cultured cerebellar granule cells and cerebral cortical neurons after loading of the cells with the fluorescent Ca2+ chelator Fluo-3. It was found that in both types of neurons exposure to media with a decrease in osmolarity of 20 to 50% of the osmolarity in the isosmotic medium (320 mOsm) led to a dose dependent increase in [Ca2+]i with a time course showing the highest value at the earliest measured time point, i.e. 40 s after exposure to the hyposmotic media and a subsequent decline towards the basal level during the following 320 s. The response in the cortical neurons was larger than in the granule cells but both types of neurons exhibited a similar increase in [Ca2+]i after exposure to 50 mM K+ which was of the same magnitude as the increase in [Ca2+]i observed in the cortical neurons exposed for 40 s to a medium with a 50% reduction in osmolarity. In both types of neurons the blocker of voltage gated Ca2+ channels verapamil had no effect on the hyposmolarity induced increase in [Ca2+]i. On the contrary, this increase in [Ca2+]i was dependent upon external calcium and could be inhibited partly or completely by the inorganic blockers of Ca2+ channels Mg2+ and La3+. Dantrolene which prevents release of Ca2+ from internal stores had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study on the effect of verapamil and prazosin on the thermoregulatory responses induced by cold or exogenous noradrenaline

1. Thermoregulatory responses to verapamil (VER; 5 mg/kg/hr) and prazosin (PRA; 0.75 mg/kg/3 hr) were investigated in rabbits exposed to cold (4 degrees C) and compared with those observed after treatment with noradrenaline (NA; 1.2 mg/kg/2 hr) under thermoneutral conditions (21 degrees C). 2. Both drugs abolished the thermogenic response to cold. 3. In the case of NA hyperthermia, only PRA was effective, i.e. the drug inhibited the thermal, metabolic and vasoconstricting actions of this amine. 4. Possible mechanisms, responsible for the thermoregulatory activity of both drugs are being discussed.

Depression of miniature endplate potential frequency by acetylcholine and its analogues in frog

1. Acetylcholine (ACh), 7.5 x 10(-5) M, and carbachol, 5 x 10(-6) M (CCh) depressed the frequency of miniature endplate potentials (m.e.p.ps) in the frog (Rana temporaria) sartorius neuromuscular junction with active acetylcholinesterase to about 50-55% of the controls. 2. A similar depression was produced by the nicotinic agonists, nicotine, suberyldicholine and tetramethylammonium. 3. The muscarinic agonists, oxotremorine, methylfurmethide and methacholine were without effect on m.e.p.p. frequency. The muscarinic antagonist, atropine and the nicotinic antagonist, (+)-tubocurarine, had no effect on the depression of m.e.p.p. frequency evoked by CCh. 4. The ganglionic blockers, benzhexonium and IEM-1119, were also without effect on the CCh-evoked depression of m.e.p.p. frequency. 5. Pretreatment of muscles with anticholinesterases did not prevent the CCh-induced drop in m.e.p.p. frequency. 6. The effect of CCh was proportionally the same as in the controls in preparations where the m.e.p.p. frequency was changed by elevation of K+ and in the presence of theophylline, noradrenaline, dibutyryl adenosine 3':5'-cyclic monophosphate (db cyclic AMP) and db cyclic GMP. 7. An inhibitor of Na+,K(+)-ATPase, ouabain, 5 x 10(-5) mol l-1, prevented or reversed the depression of m.e.p.p. frequency by CCh. However, the depression was present in a nominally K(+)-free medium. Insulin and adrenaline, which are considered to be Na+,K(+)-ATPase activators, were without effect on depression of m.e.p.p. frequency. 8. The depression of m.e.p.p. frequency by 5 x 10(-6) M CCh was the same at temperatures between 5 and 30 degrees C with a Q10 near to 1.0. When threshold amounts of CCh were used (6 x 10-7 and 3 x 10-7 M), the depression was less at higher temperatures.9. The receptive structures responsible for the CCh (or ACh)-evoked depression of m.e.p.p. frequency differ pharmacologically from muscarinic, nicotinic ganglionic and neuromuscular junction ACh-receptors as well as from the synaptic cholinesterase, in contrast to previous reports (Duncan & Publicover, 1979).The low temperature-dependence points to the possibility that physical rather than biochemical processes are limiting in this presynaptic effect of cholinomimetics.

Down-regulation of quantal size at frog neuromuscular junctions: possible roles for elevated intracellular calcium and for protein kinase C

Previous work showed that quantal size can be at least doubled at the frog neuromuscular junction by pretreatment with hormones or hypertonic solutions, primarily by the release of more acetylcholine (ACh) per quantum. Once increased, quantal size slowly declined over hours. Quantal size was measured from miniature end-plate potentials (MEPPs) or currents (MEPCs). In the present experiments, preparations in which quantal size had been increased were exposed to 17-25 mM [K+], quantal size decreased within minutes. Release of comparable numbers of quanta by nerve stimulation did not decrease size. K(+)-solutions did not decrease size if Ca2+ was omitted or replaced with Sr2+. The phosphokinase C (PKC) activators phorbol 12,13-diacetate (PDA) and 1-oleoyl-2-acetyl-rac-glycerol (OAG) also decreased quantal size within minutes when applied in a hypertonic solution that increased the rate of spontaneous release. Phorbol 12,13-dideconate, which does not activate PKC, did not decrease quantal size. The size decrease triggered by K(+)-solutions or PKC activators was blocked by 100 microM 1-(5-isoquinolinyl-sulfonyl)-2-methyl-piperazine (H7), a protein kinase inhibitor. Apparently, increasing [K+] elevated intracellular [Ca2+], which activates PKC, and which leads to the down-regulation of quantal size. During the period in which size is decreasing, there appears to be large and normal subpopulations of MEPP sizes, with normal gradually replacing large. This suggests that large quanta are formed by adding additional ACh to preformed quanta shortly before they are available for release.

Insulin improves rat soleus axon function by increasing glucose transport

The effect of varying glucose and insulin concentration on neuromuscular transmission was investigated in rat soleus and extensor digitorum longus (EDL) nerve-muscle preparations using conventional microelectrode techniques. Soleus and EDL axons were similar in that both failed after approximately 70 min of glucose deprivation and often did not recover from glucose deprivation. Soleus and EDL axons differed in the following ways: 1) soleus axons required greater than 7.64 mM of glucose to give sustained function vs. greater than 2.78 mM of glucose for the EDL; 2) 30 microU/ml of insulin improved soleus axon function over a range of glucose concentrations from 4.17 to 9.72 mM vs. no effect of 1,000 microU/ml on EDL axons; 3) resting glucose transport was slow into soleus axons and rapid into EDL axons; and 4) 10 microU/ml of insulin increased glucose transport into soleus axons vs. no effect of 1,000 microU/ml of insulin on glucose transport into EDL axons. These results suggest that insulin improves soleus axon function by increasing glucose transport, whereas insulin has no effect on EDL axons.

alpha-Melanocyte-stimulating hormone (alpha-MSH) release from perifused rat hypothalamic slices

A perifusion system was developed to investigate the control of alpha-melanocyte-stimulating hormone (alpha-MSH) release from rat brain. Hypothalamic slices were perifused with Krebs-Ringer bicarbonate (KRB) medium supplemented with glucose, bacitracin and bovine serum albumin. Fractions were set apart every 3 min and alpha-MSH levels were measured by means of a specific and sensitive radioimmunoassay method. Hypothalamic tissue in normal KRB medium released alpha-MSH at a constant rate corresponding to 0.1% of the total hypothalamic content per 3 min. The basal release was not altered by Ca2+ omission in the medium or addition of the sodium channel blocker tetrodotoxin (TTX). Depolarizing agents such as potassium (50 mM) and veratridine (50 microM), which is known to increase Na+ conductance, significantly stimulated alpha-MSH release in a Ca2+-dependent manner. When Na+-channels were blocked by TTX (0.5 microM) the stimulatory effect of veratridine was completely abolished whereas the K+-evoked release was unaffected. These findings suggest that: voltage-dependent sodium channels are present on alpha-MSH hypothalamic neurons; depolarization by K+ induces a marked stimulation of alpha-MSH release; K+- and veratridine-evoke releases are calcium-dependent. Altogether, these data provide evidence for a neurotransmitter or neuromodulator role for alpha-MSH in rat hypothalamus.