Potassium-stimulated GABA Release Research Articles

Serotonin-2C receptor agonists decrease potassium-stimulated GABA release in the nucleus accumbens.

The serotonin 5-HT2C receptor has shown promise in vivo as a pharmacotherapeutic target for alcoholism. For example, recently, a novel 4-phenyl-2-N,N-dimethylaminotetralin (PAT) drug candidate, that demonstrates 5-HT2C receptor agonist activity together with 5-HT2A/2B receptor inverse agonist activity, was shown to reduce operant responding for ethanol after peripheral administration to rats. Previous studies have shown that the 5-HT2C receptor is found throughout the mesoaccumbens pathway and that 5-HT2C receptor agonism causes activation of ventral tegmental area (VTA) GABA neurons. It is unknown what effect 5-HT2C receptor modulation has on GABA release in the nucleus accumbens core (NAcc). To this end, microdialysis coupled to capillary electrophoresis with laser-induced fluorescence was used to quantify extracellular neurotransmitter concentrations in the NAcc under basal and after potassium stimulation conditions, in response to PAT analogs and other 5-HT2C receptor modulators administered by reverse dialysis to rats. 5-HT2C receptor agonists specifically attenuated stimulated GABA release in the NAcc while 5-HT2C antagonists or inverse agonists had no effect. Agents with activity at 5-HT2A receptors had no effect on GABA release. Thus, in contrast to results reported for the VTA, current results suggest 5-HT2C receptor agonists decrease stimulated GABA release in the NAcc, and provide a possible mechanism of action for 5HT2C -mediated negative modulation of ethanol self-administration.

BDNF Modulates Sensory Neuron Synaptic Activity by a Facilitation of GABA Transmission in the Dorsal Horn

Topical application of brain-derived neurotrophic factor (BDNF) to the adult rat isolated dorsal horn with dorsal root attached preparation inhibited the electrically evoked release of substance P (SP) from sensory neurons. This effect of BDNF was dose dependent (EC 50 250 pM) and reversed by the tyrosine kinase inhibitor, K-252a. BDNF-induced inhibition of SP release was blocked by the GABA B receptor antagonist CGP 55485 but not by naloxone. Acute application of BDNF significantly increased potassium-stimulated release of GABA in the dorsal horn isolated in vitro and this effect was blocked by K-252a. Intrathecal injection of BDNF into the rat lumbar spinal cord induced a short-lasting increase in hindpaw threshold to noxious thermal stimulation that was blocked by CGP 55485 and was associated with activation of ERK in dorsal horn. These data suggest that exogenous BDNF can indirectly modulate primary sensory neuron synaptic efficacy via facilitation of the release of GABA from dorsal horn interneurons.

Melatonin, vitamin E, and estrogen reduce damage induced by kainic acid in the hippocampus: potassium‐stimulated GABA release

Melatonin, vitamin E and estrogen have been shown to exert neuroprotective effects against kainic acid (KA)-induced damage in the hippocampus. The aim of the present study was to examine the changes in potassium-evoked gamma-aminobutyric acid (GABA) release in the hippocampus of KA-treated rats and to test the possible protective effects of melatonin, vitamin E or estrogen. Following the treatment of mice with KA, a marked reduction in potassium-evoked [3H]GABA release was observed. Melatonin or estrogen prevented the reduction in potassium-evoked GABA release due to kainate administration. Vitamin E also exhibited some protective effect, but it was less than that provided by melatonin or estrogen. Melatonin, estrogen and, to a lesser extent, vitamin E reduce the physiological toxicity of KA. Since KA is believed to cause neuronal alterations via oxidative processes, it is assumed that the free radical scavenging and oxidative properties of melatonin, estrogen and vitamin E account for the protective effects of these agents.

Diminished potassium-stimulated GABA release in vivo in genetically epilepsy-prone rats

This experiment was conducted to assess the physiological relevance of observed changes in transmitter amino acid content in severe seizure genetically epilepsy-prone rats (GEPR-9s) by use of microdialysis. Adult male GEPR-9s and non-epileptic control rats were implanted with guide cannulae, and 6 mm (loop) dialysis probes were inserted unilaterally into rostral caudate and perfused with artificial cerebrospinal fluid. Each subject was perfused in the awake state with 100 or 150 mM K + for 80 min in separate counterbalanced sessions, and 20-min fractions collected. High K + perfusion increased extracellular fluid GABA and glutamate (GLU) in a concentration-dependent manner in both GEPR-9s and non-epileptic control rats. However, in the presence of 150 mM K + GABA release was decreased in GEPR-9s relative to controls throughout the stimulation interval. In contrast, the increase in extracellylar fluid GLU after high K + was not different in the two groups. These results suggest an important role for mechanisms underlying GABA release in the seizure susceptibility observed in GEPR-9s.

Short-term dopaminergic regulation of GABA release in dopamine deafferented caudate-putamen is not directly associated with glutamic acid decarboxylase gene expression

In vivo microdialysis and in situ hybridization were combined to study dopaminergic regulation of γ-amino butyric acid (GABA) neurons in rat caudate-putamen (CPu). Potassium-stimulated GABA release in CPu was elevated following a dopamine deafferentation. Local perfusion with exogenous dopamine (50 μM) for 3h via the microdialysis probe attenuated the potassium-stimulated increase in extracellular GABA in CPu. Expression of glutamic acid decarboxylase (GAD) mRNA was also increased in the dopamine deafferented CPu. However, local perfusion with dopamine had no significant attenuating effect on the increased GAD mRNA expression. These findings indicate that dopaminergic regulation of GABA neurons in the dopamine deafferented CPu includes both a short-term effect at the level of GABA release independent of changes in GAD mRNA expression and a long-term modulation at the level of GAD gene expression.

Release of taurine and GABA from cerebellar slices from developing and adult mice

The properties of the release of exogenous radioactive taurine and GABA from cerebellar slices from developing and adult mice were investigated using a superfusion system. Potassium stimulation (50 mM K +) caused, approximately, a 1.4-fold enhancement in the release of preloaded taurine from slices from adult mice, while the response to potassium in 7-day-old mice was about 6-fold. The potassium stimulation caused, approximately, a 3-fold increase in the release of preloaded GABA from cerebellar slices from 7-day-old mice, whereas the enhancement was about 10-fold in the adult. The actual molar amount of taurine released from the immature cerebellum was strikingly large, about 16 times larger than the amount of GABA released upon the same stimulus. Spontaneous taurine efflux was potentiated by taurine and GABA, the responses being more pronounced in the 7-day-old cerebellum, suggesting that the immature cerebellum is more prone to stimulation by homo- and heteroexchange than the mature cerebellum. Potassium-stimulated taurine release was inhibited by GABAergic substances in the adult but not in the developing cerebellum. Potassium-stimulated GABA release from the adult cerebellum was greatly increased by GABA and also moderately by muscimol and bicuculline, the effect of the latter being antagonized by taurine and hypotaurine. Taurine was thus able to modulate GABA release through bicuculline-sensitive receptors, but this modulation was not evident in cerebellar slices from 7-day-old mice. An exposure of the slices to sodium-free media greatly enhanced taurine and GABA release in both age groups. The stimulated release of GABA generally exhibited a similar calcium dependency in the adult and 7-day-old cerebella but in 7-day-old mice the stimulated release of taurine was not strictly calciumdependent.

Modulation of GABAergic neurotransmission in the brain by dipeptides

The effects of endogenous and synthetic peptides containing GABA or its analogues on the GABA/benzodiazepine/chloride ionophore complex. GABAB receptor, Cl fluxes, GABA release and GABA uptake were studied using synaptic membranes, crude synaptoneurosomal preparations and slices prepared from the rat and mouse brain. The sodium-independent binding of GABA was strongly inhibited by GABA-histidine, followed by gamma-glutamyl-homotaurine, GABA-glycine and gamma-glutamyl-GABA. The binding of diazepam was slightly enhanced by the same peptides. The peptides alone had no effect on the chloride fluxes, but GABA-histidine, gamma-glutamyl-GABA and GABA-glycine enhanced while gamma-glutamyl-homotaurine and GABA-taurine inhibited GABA-stimulated chloride uptake. GABA-histidine was the most effective displacer of baclofen binding, but gamma-glutamylhomotaurine was entirely ineffective. The uptake of GABA was markedly inhibited in synaptosomal preparations by GABA-histidine, while all other peptides were less effective. gamma-Glutamyl-taurine attenuated but gamma-glutamyl-homotaurine and GABA-glycine enhanced the potassium-stimulated release of GABA. The present actions of GABA-histidine in vitro may be of significance for GABAergic neurotransmission in vivo.

Relationship of Dihydropyridine Binding Sites with Calcium‐Dependent Neurotransmitter Release in Synaptosomes

In the present work, we have studied the effect of ruthenium red (RuR), La3+ and 4-aminopyridine (4-AP) on the specific binding of (+)-[3H]PN200-110 to synaptosomes, as well as the effect of nitrendipine, nifedipine, and BAY K 8644 on gamma-[3H]aminobutyric acid [( 3H]GABA) release induced by potassium depolarization and by 4-AP in synaptosomes. Scatchard plots indicated that neither RuR nor 4-AP modifies the KD and Bmax of [3H]PN200-110 specific binding, whereas La3+ decreased the Bmax by about 25%; when the effect of the drugs on the total binding of PN200-110 was studied, a similar inhibition by La3+ was found. The calcium antagonists, nitrendipine and nifedipine, did not affect at all the potassium-stimulated release of [3H]GABA nor its release induced by 4-AP. The calcium agonist BAY K 8644 failed to affect both the spontaneous and the potassium-stimulated GABA release. Our results suggest that the binding sites of dihydropyridines in presynaptic membranes are not related to the calcium channels involved in neurotransmitter release with which RuR, La3+, and 4-AP interact.

Taurine and GABA release from mouse cerebral cortex slices: effects of structural analogues and drugs.

The effects of structural analogues, excitatory amino acids and certain drugs on spontaneous and potassium-stimulated exogenous taurine and GABA release were investigated in mouse cerebral cortex slices using a superfusion system. Spontaneous efflux of both amino acids was rather slow but could be enhanced by their uptake inhibitors. Taurine efflux was facilitated by exogenous taurine, hypotaurine, beta-alanine and GABA, whereas GABA, nipecotic acid and homotaurine effectively enhanced GABA release. The stimulatory potency of the analogues closely corresponded to their ability to inhibit taurine and GABA uptake, respectively, indicating that these efflux processes could be mediated by the carriers operating outwards. Glutamate induced GABA release, whereas taurine efflux was potentiated by aspartate, glutamate, cysteate, homocysteate and kainate. The centrally acting drugs, including GABA agonists and antagonists, as well as the proposed taurine antagonist TAG (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide), had no marked effects on spontaneous taurine and GABA release. Potassium ions stimulated dose-dependently both taurine and GABA release from the slices, the responses of taurine being strikingly slow but sustained. Exogenous GABA and nipecotic acid accelerated the potassium-stimulated GABA release, whereas picrotoxin and bicuculline were ineffective. The potassium-stimulated taurine release was unaltered or suppressed by exogenous taurine and analogues, differing in this respect from GABA release. The apparent magnitude of the depolarization-induced GABA release is thus influenced by the function of membrane transport sites, but the same conclusion cannot be drawn with regard to taurine. Haloperidol and imipramine were able to affect the evoked release of both taurine and GABA.

The role of enkephalins in the production of epileptogenic activity and autonomic dysfunction: Origin of arrhythmia and sudden death in the epileptic patients?

Autonomic dysfunction, including arrhythmias, has been shown to be associated with epileptogenic activity. This study examines the potential role for enkephalins in this process. A long lasting elevation of immunoreactive methionine (met)-enkephalin content in the septum, hypothalamus, amygdala, and hippocampus of rats occurs after pentylenetetrazol-induced convulsions (Brain Research 297: 121–125, 1984). Brennan et al (Life Sciences 27:1097–1101 1980) reported a greater percent inhibition of potassium-stimulated GABA release with increasing concentrations of met-enkephalin. Snead and Bearden (Science 210:1031–1033, 1980) found that leucine-enkephalin in the central nervous system may induce epileptogenic activity. In addition, (d-alanine 2) met-enkephalin has been shown to produce a centrally mediated vasopressor response as well as attenuation of the baroreceptor reflex in conscious cats (Hypertension 3:395–407, 1981), possibly leading to autonomic imbalance. The latter may precipitate arrhythmias and sudden unexplained death in the epileptic patient. Resolution of the question of whether enkephalins elicit epileptogenic activity and autonomic dysfunction via inhibition of GABA release is important since an understanding of this mechanism should eventually allow the design of pharmacologic agents to prevent the epileptogenic activity, autonomic dysfunction and the associated sudden death.

Ruthenium red interferes with the tetanus toxin inhibition of potassium-stimulated GABA release from rat cerebral cortex slices

The influence of Ruthenium Red on the release of exogenous GABA from rat cerebral cortex slices was studied in a superfusion system. Ruthenium Red inhibited the 50 mM K +-induced GABA release in a dose-dependent manner. It also attenuated the GABA release elicited by Na +-free and ouabain-containing media both with and without calcium ions. The veratrine-induced release was inhibited only in the absence of calcium. Ruthenium Red also impaired the inhibition of GABA release by tetanus toxin and decreased the mortality in mice injected with tetanus toxin. It is suggested that Ruthenium Red interferes with the fixation of tetanus toxin to the membrane binding sites involved in the incorporation of the toxin to its intracellular targets.

δ-Aminolaevulinic acid is a potent agonist for GABA autoreceptors

ACUTE attacks of the hereditary hepatic porphyrias are frequently accompanied by neuropsychiatrie symptoms. The presentation of nervous system involvement varies; in decreasing order of frequency, there may be motor neuropathy, confusion, psychiatric manifestations, hyperexcitability and epileptic-type seizures1. Psychiatric manifestations include depression, anxiety, insomnia and an organic brain syndrome2. Although the aetiology of the neural dysfunction in the acute attack is unknown, several mechanisms have been suggested at a neurochemical level. It is possible that porphyrin precursors, which are produced in excess by the liver during acute episodes, gain access to the nervous system and exert direct neurotoxic effects3–5; alternatively, a metabolic defect in haem biosynthesis in nerve cells might result in a functional haemoprotein deficiency which becomes critical at the time of the acute attack6. The porphyrin precursor, δ-aminolaevulinic acid (δ-ALA), exhibits various effects in mammalian central nervous tissue preparations. It inhibits high-affinity uptake and increases efflux of γ-aminobutyric acid (GABA)3 and L-glutamate4 in rat cortical synaptosomes; it also inhibits the (Na+ + K+) ATPase isolated from rabbit brain and red blood cells7. These effects occur only at relatively high concentrations of δ-ALA (at least 10−4M), and it is very unlikely that such concentrations are reached in the brain during acute porphyric attacks8. Recently, there has been strong evidence that release of GABA is subject to negative feedback control through presynaptic receptors on GABAergic terminals9–11. Here we report that δ-ALA inhibits potassium-stimulated release of GABA from preloaded synaptosomes, probably by acting as an agonist at these presynaptic receptors. This effect is evident at concentrations of δ-ALA (10−6M) which are well within levels recorded in the cerebrospinal fluid of porphyric patients showing neuropsychiatrie symptomatology12.

The effect of penicillin on the release of γ-aminobutyric acid from cerebral cortex slices

Penicillin is known to block the depressant action of GABA on cortical and spinal neurons. In this study we examined the effect of penicillin on the release of endogenous amino acids from rat cerebral cortex slices. The principal finding was that penicillin blocked the potassium-stimulated release of GABA, a calcium-dependent process. It did not block the veratridine-stimulated release of GABA, a calcium-independent process. It did not affect the release of other putative transmitter amino acids. Inhibition of GABA release from nerve endings may explain in part the ability of penicillin to depress synaptic inhibitory mechanisms.

Potassium-stimulated release of GABA, glycine, and taurine from the chick retina.

The effect of depolarizing potassium concentration on the release of [14C]glycine, [3H]GABA, and [3S]taurine was investigated in the whole chick retina and in a synaptosomal fraction prepared from the chick retina. In the whole retina, increasing potassium concentration above 40 mM resulted in an increased release of the three amino acids. The release of glycine was the most stimulated and that of taurine, the least. The potassium-evoked release of glycine and GABA was calcium dependent. In the synaptosomal fraction, 68.5 mM potassium significantly stimulated the efflux of GABA and glycine by a calcium-dependent mechanism. The release of taurine from this fraction was unaffected by high potassium.