Mechanism of the calcium-dependent stimulation of transmitter release by 4-aminopyridine in synaptosomes

The mechanism of the Ca2+-dependent stimulation of neurotransmitter release by 4-aminopyridine in synaptosomes was studied. The stimulation of gamma-[3H]aminobutyric acid and [3H]acetylcholine release by 4-aminopyridine was not significantly affected either by tetrodotoxin or by the absence of Na+ in the medium, whereas the toxin notably inhibited the release of both transmitters induced by veratridine. On the other hand, the release of labeled gamma-aminobutyric acid induced by 4-aminopyridine was inhibited by both La3+ and ruthenium red, two blockers of Ca2+ transport in synaptosomes. In other experiments, 4-aminopyridine had only a slight stimulatory effect, if any, on the influx of 45Ca2+ into synaptosomes, under both resting and K+-depolarizing conditions. Ruthenium red inhibited the stimulation by K+ of the 45Ca2+ uptake, and 4-aminopyridine did not antagonize this inhibition. We conclude that the transmitter-releasing action of 4-aminopyridine in synaptosomes does not involve an excitatory effect on the membrane which may result in the opening of voltage-sensitive Na+ channels. 4-Aminopyridine does not seem to act either by enhancing Ca2+ entry into the synaptosomes. It is proposed that 4-aminopyridine facilitates the coupling between Ca2+ binding and transmitter secretion at the presynaptic membrane.

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)

Effect of 4-aminopyridine on transmitter release in synaptosomes

The effect of 4-aminopyridine (4-AP) on the release of labeled transmitters in mouse brain synaptosomes was studied in a superfusion system. 4-AP at microM concentrations notably stimulated the spontaneous release of labeled GABA and glutamate, and of acetylcholine (ACh) derived from tritiated choline. No effects on the release of labeled alpha-aminoisobutyric acid were observed. The stimulation of GABA and ACh release was dependent on the presence of Ca2+ in the superfusion media, whereas the effect on glutamate release was more variable and no clear Ca2+-dependence was observed. In contrast to these results, 4-AP did not have any effect on the release of the above transmitters stimulated by K+-depolarization in the presence of Ca2+. These results are discussed in terms of the possible participation of Ca2+ in the action of 4-AP on spontaneous transmitter release in isolated nerve endings.

Selective stimulation of neurotransmitter release from chick retina by kainic and glutamic acids

The excitatory action of kainic and glutamic acids in chick whole retina was demonstrated as an immediate stimulation of the release of labeled gamma-aminobutyric acid (GABA) and glycine in a superfusion system. This stimulatory effect was 3-10 times greater than that produced by a depolarizing K+ concentration; in addition, it was independent of Ca2+ in the medium, but notably inhibited when Na+ was omitted from the medium. Under identical experimental conditions, neither kainic nor glutamic acid had any effect on the release of labeled dopamine or alpha-aminoisobutyric acid, thus indicating that their effect is not unspecific or due to cell damage. Similar although less marked stimulation of labeled GABA and glycine release by kainic acid was obtained in subcellular retinal fractions, particularly in fraction P1, which contained photoreceptor terminals and outer segments. This stimulation was also Ca2+ independent and greatly reduced when Na+ was omitted from the medium. It is suggested that the stimulation of GABA release by kainic and glutamic acids is probably due to a Na+-dependent, carrier-mediated mechanism that responds to the entry of Na+ produced by the interaction of glutamic and kainic acids with retinal membranes. In cortical or striatal slices from mouse brain, these acids had a negligible stimulatory effect on GABA and dopamine release.

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

A method for detecting changes in the internal concentration of Ca2+ in synaptosomes from mouse brain is described. Synaptosomes were fused with phosphatidylcholine-phosphatidylserine unilamellar liposomes previously loaded with the metalochromic, Ca2+ -sensitive dye arsenazo III, in order to introduce the dye into the synaptosomes. The fusion was promoted by La3+. Changes in the differential absorption between 660 and 690 nm, which indicate changes in Ca2+ concentration, were followed in a double beam spectrophotometer. It was found that both in the dye-loaded liposomes and in the fused synaptosomes, the addition of the Ca2+ ionophore A23187 to the medium containing Ca2+ produced a notable change in the differential absorbance 660-690 nm. When the depolarizing alkaloid veratridine was added, there was no response in the liposomes, whereas a change in the differential absorbance 660-690 nm was detected in the fused synaptosomes containing arsenazo but only when Na+ was present in the medium. These fused synaptosomes were able to release labeled gamma-aminobutyric acid as a response to veratridine, in a Na+ -dependent manner, similarly to control non-fused synaptosomes. These results demonstrate the feasibility of fusion to introduce Ca2+ -sensitive dyes into isolated nerve endings from the mammalian brain and therefore to detect changes in their internal Ca2+ concentration.

Antagonism of the ruthenium red-induced paralysis in mice by 4-aminopyridine, guanidine and lanthanum

4-Aminopyridine and guanidine were administered intraperitoneally to mice during the complete flaccid paralysis induced by treatment with ruthenium red (RuR). At 1-8 min after 4-aminopyridine or guanidine the animals had recovered completely from the paralysis, whereas the control mice injected only with RuR remained paralytic for at least 60 min. Intraperitoneal injections of LaCl3 had no apparent effects on animal motility and did not reverse the paralysis produced by RuR. However, when La3 + was administered 30 min prior to RuR the occurrence of flaccid paralysis was totally prevented. The results obtained are discussed in terms of the possible antagonist effects of the compounds used on acetylcholine release at neuromuscular junctions.

Mathematical models of synaptic plasticity: I. Posttetanic potentiation

A mathematical model of post-tetanic potentiation is proposed. The model uses differential equations and is based upon physiological postulates of the electrical, metabolic, and neuroendocrine activities that are related to synaptic connectivity. These activities may modify some important parameters in synaptic function. In the proposed model these parameters are restricted to the presynapse in view of the physiological evidence indicating that posttetanic potentiation is probably due to presynaptic mechanisms. The model takes into consideration the size of the transmitter pool available for release, the mobilization of transmitter from and to this pool, and the fraction of transmitter released. Based upon the above postulates, we have simulated different phases of the phenomenon of posttetanic potentiation, and we have presented the results of several preparations in which this event has been studied. This work represents a successful attempt to reproduce the dynamics of posttetanic potentiation based upon physiological results with a mathematical model.

Mathematical model of synaptic plasticity: III. Heterosynaptic changes

A mathematical model, using differential equations, of heterosynaptic plasticity is proposed. The model is based on physiological studies of invertebrates in which nonspecific conditioning, such as sensitization and heterosynaptic inhibition, starts to be elucidated and behavioral studies of classical and instrumental conditioning, which we postulate to have the same mechanisms as those found in nonspecific conditioning. The model permits us to simulate the following heterosynaptic changes: sensitization, heterosynaptic inhibition, classical and instrumental conditioning--including short- and long-term memory--extinction and recuperation--spontaneously and by stimulation.

Mathematical model of synaptic plasticity: II. Habituation

A mathematical model of the phenomenon of habituation as a homosynaptic depression of the amount of transmitter release is proposed. The model is based on the physiological studies of habituation in invertebrates and in the spinal cord of vertebrates, where a single synapse has been isolated and some of the physiological mechanisms of this process have been elucidated. The model simulates the following properties of habituation: (1) reduced amount of transmitter release attributed to a repetitive stimulus through changes in the membrane permeability to Ca2+ ions; (2) spontaneous recovery by rest; (3) the amplitude and frequency dependence of habituation; (4) modulation of habituation: sensitization, through an increase in membrane Ca2+ permeability, and presynaptic inhibition, through a reduced depolarization of the physiological stimulus; (5) long-term habituation attributed to repetitive trials of habituation and spontaneous recovery.

Glutamate decarboxylase activity in chick brain and retina. Inhibition of the immature enzyme by Triton-X-100

We have studied the effect of Triton-X-100 on glutamate decarboxylase (GAD) activity in brain and retina from chick embryos of 12 and 16 days' incubation and from chicks 4--6 weeks old. GAD activity was measured in five different homogenization media. Triton-X-100 inhibited the enzyme by about 60% in both brain and retina of 12-day embryos and by about 50% in 16-day embryos, independently of the homogenization medium. In chicks only about 20% inhibition by the detergent was observed in brain whereas no effect was found in retina. These results indicate that the evaluation of the experimental conditions of enzyme assays at different ages is essential for developmental studies of GAD activity in nervous tissue.

Neurophysiological and neurochemical studies on the action of the anticonvulsant gamma-hydroxy, gamma-ethyl, gamma-phenyl-butyramide

The effect of gamma-hydroxy, gamma-ethyl, gamma-phenyl-butyramide (HEPB) on afterdischarges produced by hippocampal stimulation in cats was studied. HEPB notably diminished the duration of afterdischarges and in some cats blocked their propagation into the substantia nigra and the amygdala. HEPB treatment also antagonized the enhancement of afterdischarge duration produced by subconvulsive doses of bicuculline, whereas treatment with diphenylhydantoin strongly potentiated this effect of bicuculline. The intracisternal injection of HEPB or gamma-aminobutyric acid (GABA) in mice resulted in a potentiation of strychnine-induced convulsions. On the other hand, neurochemical experiments in mouse brain cortex slices and in synaptosomes demonstrated that HEPB did not affect the high affinity uptake of [3H] GABA, its spontaneous or Ca2+ dependent release stimulated by depolarizing K+ concentrations, and its Na+ independent binding to synaptic plasma membranes.

Convulsions or flaccid paralysis induced by ruthenium red depending on route of administration

Ruthenium red was administered to mice and cats intracranially or intraperitoneally. In mice, intracisternal administration produced status epilepticus and tonic convulsions. In contrast, intraperitoneal administration induced total flaccid paralysis lasting several hours. These effects of Ruthenium red were partially blocked by the simultaneous administration of CaCl2. EDTA, at doses much greater than those of Ruthenium red, produced effects similar to those of the dye, which were also blocked by CaCl2 administration. In cats, intraventricular or intrahippocampal administration of Ruthenium red through a permanently implanted cannula produced after a few minutes subclinical paroxysmal activity in all brain regions recorded. After several hours the animals developed typical grand mal seizures. Intraperitoneal injection of Ruthenium red to cats did not affect the EEG but markedly depressed muscular activity. Administration of carbachol to the latter animals produced myoclonic responses. These results are discussed in relation to the inhibitory effect of Ruthenium red on Ca2+ transport and binding to membranes, and to the role of this cation on neurotransmitter release.

Properties of soluble and particulate cysteine sulfinate decarboxylase of the adult and the developing rat brain

Some properties of cysteine sulfinate decarboxylase (CSD) activity were studied in the pellet and supernatant of a 18,000 X g centrifugation of isotonic sucrose rat brain homogenates. About 50% of the enzyme activity was found associated to the particulate fraction and 16% of the activity remained particle bound after hypo-osmotic shock of the 18,000 X g pellet. The activity of the 18,000 X g supernatant showed a lower dependence on exogenous pyridoxal phosphate (PLP) than the activity in the particulate fraction. The CSD activity of these fractions also differed in optimal pH and in apparent kinetic constants. The enzyme associated to particles showed the highest Vmax and the lowest Km. The activity and kinetic characteristics of CSD were studied during brain postnatal development. In the newborn brain only a small amount of the enzyme activity was found associated to the 18,000 X g pellet. CSD in brain homogenates of immature rats was less dependent on free PLP and showed a higher Km and a lower Vmax as compared with the enzyme in the adult brain. Between birth and adulthood the enzyme activity increased more than 10-fold in the particulate fraction and 2-fold in the soluble fraction. It is concluded that the differences observed in CSD activity between newborn and adult brain are due to an increase of the particulate form of the enzyme during postnatal development.

GABA metabolism and cerebral protein synthesis

Glutamate decarboxylase (GAD) activity and [14C)leucine incorporation into brain protein in vivo were measured in mice injected with L-glutamic acid-gamma-hydrazide and 60 min later with pyridoxal-5'-phosphate; this combined treatment produces a decrease in GAD activity and an increase in GABA levels. Under such conditions, protein synthesis was inhibited to the same extent as GAD activity. A parellelism between the decrease in GAD activity and that in protein synthesis was also observed in brain cortex slices treated with GAD inhibitors. This treatment did not affect leucine incorporation into protein in liver slices. The results support the previously suggested hypothesis that a certain pool of GABA may have a role in the regulation of protein synthesis in brain, and further suggest that the synthesis of GABA may be more important in this respect than its total concentration.

Subcellular fractionation of rat cerebellum: an electron microscopic and biochemical investigation. III. Isolation of large fragments of the cerebellar glomeruli

By a combination of differential and sucrose density gradient (both discontinuous and linear) centrifugation, large fragments of the cerebellar glomeruli were isolated in high purity from hand homogenised tissue. The final preparation contained only about 1% of the tissue protein, but over 90% of its volume was accounted for by the glomerulus particles. The ultrastructure of the glomerulus particles was well preserved. The enzyme profile was characteristic: the glomerulus particles were enriched in glutamate decarboxylase (GAD) activity (relative specific activity (RSA), 2.54), but the RSA of choline acetyltransferase (ChAc) was only 1.05. These findings are consistent with the view that GAD activity is very high in the inhibitory Golgi terminals, which occupy only a small fraction of the total volume of the particles, and acetylcholine may be a transmitter only in a relatively small fraction of the mossy fibre terminals. The glomerulus particles also contained a high concentration of succinate dehydrogenase (SDH) activity (RSA, 1.91), whereas the RSA of glutamate dehydrogenase (GDH) was only 1.15. The great asset of this preparation for future investigations is that it is composed almost exclusively from pre- and postsynaptic neuronal structures. Fractions containing neuropil fragments of non-glomerular origin were also obtained, but the profile of the estimated enzymes did not indicate unique characteristics.

Seizure susceptibility in the developing mouse and its relationship to glutamate decarboxylase and pyridoxal phosphate in brain

The relationship between the susceptibility to convulsions, the content of pyridoxal 5'-phosphate and the activity of pyridoxal kinase (EC 2.7.1.35) and glutamate decarboxylase (EC 4.1.1.15) in brain, was studied in the developing mouse. Seizures were induced by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH), a drug previously reported to reduce the levels of pyridoxal 5'-phosphate and as a consequence to inhibit the activity of glutamate decarboxylase in brain of adult mice. It was found that the seizure pattern, as well as the time of appearance of convulsions, differed between 2- and 5-day old mice and 10-day old or older mice, indicating a progressive increase in seizure susceptibility during development. In brain, pyridoxal kinase activity and pyridoxal 5'-phosphate levels were decreased by the administration of PLPGH at all ages studied, whereas glutamate decarboxylase activity was inhibited less than 25% in 2- and 5-day old mice, and about 50% thereafter. Parallelly, the activation of glutamate decarboxylase by pyridoxal 5'-phosphate added in vitro to control homogenates was less in 2- and 5-day old mice than in older animals. It is concluded that the increase in the susceptibility to seizures induced by PLPGH during development is probably related to the increase observed in the sensitivity of glutamate decarboxylase in vivo to a decrease of pyridoxal 5'-phosphate levels. The correlation between pyridoxal 5'-phosphate, glutamate decarboxylase, and seizure susceptibility seems to be established at about 10 days of age.

Differences in some properties of newborn and adult brain glutamate decarboxylase

Some properties of glutamate decarboxylase (EC 4.1.1.15) activity in brain of newborn and adult mouse were studied comparatively. It was found that glutamate decarboxylase of the newborn brain was strongly inactivated by homogenization in hypotonic medium, centrifugation of isotonic sucrose homogenates, preincubation at 37 degrees C or the addition of Triton-X-100, whereas the adult brain enzyme was practically unaffected by any of these conditions. It was also found that the newborn glutamate decarboxylase was less activated by pyridoxal 5'-phosphate and less inhibited by pyridoxal 5'-phosphate oxime-O-acetic acid, than the adult enzyme. These differences do not exist for brain dihydroxyphenylalanine decarboxylase (EC 4.1.1.26) and are not due to the release of inhibitors from the newborn brain. On the basis of the results obtained it is postulated that two forms of glutamate decarboxylase exist in brain: a newborn form, which is unstable and has high affinity for pyridoxal 5'-phosphate, and an adult form, which is much more stable and has low affinity for pyridoxal 5'-phosphate. The possible implications of these findings in the establishment of the gamma-aminobutyric acid dependent synaptic inhibitory mechanisms during development are discussed.