Zinc potentiation of neurotransmission and inhibition of background cationic conductance in rat cultured hippocampal neurones

Ca2+ store-independent augmentation of [Ca2+]i responses to G-protein coupled receptor activation in recombinantly TRPC5-expressed rat pheochromocytoma (PC12) cells

Mammalian homologues of the Drosophila canonical transient receptor potential (trp) protein (TRPC) have been implicated to function as receptor-operated Ca(2+) channels (ROCs) or store-operated Ca(2+) channels (SOCs). To determine the role of TRPC5 protein in neural cells, TRPC5 was recombinantly expressed in rat pheochromocytoma cells (PC12) and changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) and Na(+) concentration ([Na(+)](i)) were analyzed. TRPC1 and TRPC3 mRNAs were endogenously expressed in PC12 cells. In TRPC5-expressed cells (TRPC5-cells), the resting [Ca(2+)](i) and [Na(+)](i) were significantly higher than those in control cells. The [Ca(2+)](i) increases induced by bradykinin and uridine 5'-triphosphate were significantly larger in TRPC5-cells. TRPC5 expression did not change in store-operated Ca(2+) entry elicited by thapsigarigin. TRPC5-cells showed larger inward current and increase of [Na(+)](i) in response to BK than control cells. These results suggest that TRPC5 channels expressed in PC12 cells function as ROCs activated by G-protein/phospholipase C coupled receptors, but not as SOCs.

Dynamic inhibition of excitatory synaptic transmission by astrocyte-derived ATP in hippocampal cultures

Originally ascribed passive roles in the CNS, astrocytes are now known to have an active role in the regulation of synaptic transmission. Neuronal activity can evoke Ca2+ transients in astrocytes, and Ca2+ transients in astrocytes can evoke changes in neuronal activity. The excitatory neurotransmitter glutamate has been shown to mediate such bidirectional communication between astrocytes and neurons. We demonstrate here that ATP, a primary mediator of intercellular Ca2+ signaling among astrocytes, also mediates intercellular signaling between astrocytes and neurons in hippocampal cultures. Mechanical stimulation of astrocytes evoked Ca2+ waves mediated by the release of ATP and the activation of P2 receptors. Mechanically evoked Ca2+ waves led to decreased excitatory glutamatergic synaptic transmission in an ATP-dependent manner. Exogenous application of ATP does not affect postsynaptic glutamatergic responses but decreased presynaptic exocytotic events. Finally, we show that astrocytes exhibit spontaneous Ca2+ waves mediated by extracellular ATP and that inhibition of these Ca2+ responses enhanced excitatory glutamatergic transmission. We therefore conclude that ATP released from astrocytes exerts tonic and activity-dependent down-regulation of synaptic transmission via presynaptic mechanisms.

Zinc in the retina

Experimental evidence exists to suggest that zinc can have positive and negative effects on the physiology of cells depending on the "local" concentration, localisation (extracellular vs. intracellular) and/or state (bound vs. free). The retina contains particularly high amounts of zinc suggesting a pivotal role in the tissue. There is also suggestive evidence that zinc deficiency in humans may result in abnormal dark adaptation and/or age-related macular degeneration. The purpose of this article is to provide an overview of various proposed functions for zinc, particularly in the retina. Endogenous chelatable zinc in the retina is localised mainly to the photoreceptors and retinal pigment epithelial cells. Moreover, the zinc localisation in the photoreceptors varies in dark and light, suggesting a role for zinc in a light-regulated process. Some zinc is also located to other areas of the retina but clearly defined zinc-enriched neurones could not be identified as has been shown to occur in certain areas of the brain. Neurones post-synaptic to zinc-enriched neurones in the brain have been suggested to be particularly vulnerable in ischaemia. The role of zinc in retinal ischaemia has been investigated to determine how it is involved in the process. It would appear that when zinc is administered in low concentrations it generally has a positive effect on an insulted retina as in ischaemia. However, higher concentrations of zinc exacerbates the influence of the insult and also acts as a toxin. Use of zinc supplements in diet must, therefore, be taken with caution.

Zinc increases the excitability of dopaminergic neurons in rat substantia nigra

The effect of zinc ions (Zn(2+)) on the neuronal excitability of substantia nigra (SN) where the zinc level is known higher in Parkinsonian brains than that in normal brains has not yet been elucidated. We, therefore, examined the effect of Zn(2+) on the intrinsic electrical properties of dopaminergic SN neurons, using a whole-cell recording method. Zn(2+) hyperpolarized dopaminergic SN neurons at resting state. Also Zn(2+) shortened the duration of evoked spikes, developed a fast afterhyperpolarization, and increased their firing frequency. Voltage-clamp studies showed that Zn(2+) decreased 4-aminopyridine-sensitive outward currents, suggesting that a transient A-like potassium channel be one of the major targets Zn(2+) can modulate in the SN neurons.

Spontaneous single-channel activity of neuronal TRP5 channel recombinantly expressed in HEK293 cells

Mammalian homologues of the Drosophila transient receptor potential (trp) protein (TRP) form Ca(2+) permeable cation channels activated in response to stimulation of G-protein-coupled receptors. Establishing biophysical characteristics of basal TRP activity is of great importance in understanding modulatory processes, which underlie enhancement of TRP activity via receptor stimulation. We have examined spontaneous activity of the TRP5 channel recombinantly expressed in human embryonic kidney cells, using the conventional whole-cell mode of the patch-clamp technique in a low-Ca(2+) external solution. The unitary Na(+) conductance of the TRP5 channel was linear, being 47.6 pS. By contrast, the open probability of the TRP5 channel showed a voltage-dependent decrease below -50 mV. These biophysical properties are important hallmarks in distinguishing the TRP5 channel in native neuronal preparations, whose spontaneous activity may contribute to control of resting membrane potentials and generation of action potentials.

Chemical anatomy of excitatory endings in the dorsal cochlear nucleus of the rat: differential synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc

In order to identify cytochemical traits relevant to understanding excitatory neurotransmission in brainstem auditory nuclei, we have analyzed in the dorsal cochlear nucleus the synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc, three molecules probably involved in different steps of excitatory glutamatergic signaling. High levels of glutamate immunolabeling were found in three classes of synaptic endings in the dorsal cochlear nucleus, as determined by quantitation of immunogold labeling. The first type included auditory nerve endings, the second were granule cell endings in the molecular layer, and the third very large endings, better described as "mossy." This finding points to a neurotransmitter role for glutamate in at least three synaptic populations in the dorsal cochlear nucleus. The same three types of endings enriched in glutamate immunoreactivity also contained histochemically detectable levels of aspartate aminotransferase activity, suggesting that this enzyme may be involved in the synaptic handling of glutamate in excitatory endings in the dorsal cochlear nucleus. There was also extrasynaptic localization of the enzyme. Zinc ions were localized exclusively in granule cell endings, as determined by a Danscher-selenite method, suggesting that this ion is involved in the operation of granule cell synapses in the dorsal cochlear nucleus.

Zinc and brain injury

Zinc is an essential catalytic or structural element of many proteins, and a signaling messenger that is released by neural activity at many central excitatory synapses. Growing evidence suggests that zinc may also be a key mediator and modulator of the neuronal death associated with transient global ischemia and sustained seizures, as well as perhaps other neurological disease states. Manipulations aimed at reducing extracellular zinc accumulation, or cellular vulnerability to toxic zinc exposure, may provide a novel therapeutic approach toward ameliorating pathological neuronal death in these settings.

Interaction of H+ and Zn2+ on recombinant and native rat neuronal GABAA receptors

1. The interaction of Zn2+ and H+ ions with GABAA receptors was examined using Xenopus laevis oocytes expressing recombinant GABAA receptors composed of subunits selected from alpha1, beta1, gamma2S and delta types, and by using cultured rat cerebellar granule neurones. 2. The potency of Zn2+ as a non-competitive antagonist of GABA-activated responses on alpha1beta1 receptors was reduced by lowering the external pH from 7.4 to 5.4, increasing the Zn2+ IC50 value from 1.2 to 58.3 microM. Zinc-induced inhibition was largely unaffected by alkaline pH up to pH 9.4. 3. For alpha1beta1delta subunits, concentration-response curves for GABA were displaced laterally by Zn2+ in accordance with a novel mixed/competitive-type inhibition. The Zn2+ IC50 at pH 7.4 was 16.3 microM. Acidification of Ringer solution resulted in a reduced antagonism by Zn2+ (IC50, 49.0 microM) without affecting the type of inhibition. At pH 9.4, Zn2+ inhibition remained unaffected. 4. The addition of the gamma2S subunit to the alpha1beta1delta construct caused a marked reduction in the potency of Zn2+ (IC50, 615 microM), comparable to that observed with alpha1beta1gamma2S receptors (IC50 639 microM). GABA concentration-response curves were depressed in a mixed/non-competitive fashion. 5. In cultured cerebellar granule neurones, Zn2+ inhibited responses to GABA in a concentration-dependent manner. Lowering external pH from 7.4 to 6.4 increased the IC50 from 139 to 253 microM. 6. The type of inhibition exhibited by Zn2+ on cerebellar granule neurones, previously grown in high K+-containing culture media, was complex, with the GABA concentration-response curves shifting laterally with reduced slopes and similar maxima. The Zn2+-induced shift in the GABA EC50 values was reduced by lowering the external pH from 7.4 to 6.4. 7. The interaction of H+ and Zn2+ ions on GABAA receptors suggests that they share either a common regulatory pathway or coincident binding sites on the receptor protein. The apparent competitive mode of block induced by Zn2+ on alpha1beta1delta receptors is shared by GABAA receptors on cerebellar granule neurones, which are known to express delta-subunit-containing receptors. This novel mechanism is masked when a gamma2 subunit is incorporated into the receptor complex, revealing further diversity in the response of native GABAA receptors to endogenous cations.

Inhibition by ATP of calcium oscillations in rat cultured hippocampal neurones

1 The effect of adenosine 5'-triphosphate (ATP) on glutamatergic synaptic transmission in hippocampus was examined by an indicator of intracellular Ca2+ oscillations. These oscillations were postsynaptic responses by glutamate released from presynaptic sites. ATP completely inhibited the oscillations in a concentration-dependent manner. 2 The ATP-induced inhibition was mediated via P2-purinoceptors since ATP exhibited the inhibitory action even in the presence of P1-purinoceptor antagonists. Also non-hydrolysable ATP analogues and uridine 5'-triphosphate (UTP) inhibited the oscillation. 3 The rank order of agonist potency of ATP analogues for inhibition of the Ca2+ oscillation was as follows: 2-methyl-thio-adenosine 5'-triphosphate > or = ATP > adenosine 5'-O-(3-thiotriphosphate)>UTP> alpha, beta-methylene-adenosine 5'-triphosphate. These inhibitory effects were insensitive to suramin. Judging from this rank order of potency, the inhibitory P2-purinoceptor could be assigned to a subclass of GTP-binding protein coupled-type receptors. 4 The site of action of ATP was thought to be presynaptic since ATP did not affect the postsynaptic Ca2+ responses by glutamate. These results suggest the existence of a presynaptic inhibitory P2-receptor that inhibits glutamate release in the hippocampus.