Uncultured lobster muscle, cultured neurons and brain slices: the neurophysiology of zinc

Essential role of vitamin C and zinc in child immunity and health

With the progressive elimination of dietary protein-energy deficits, deficiencies of micronutrients are emerging as the limiting factors in ensuring children's optimal health. Data from several countries in Asia and Latin America indicate that deficiencies of vitamin C and zinc continue to be at alarming levels. This article reviews the roles of vitamin C and zinc in supporting children's growth and development, with a particular focus on the complementary roles they play in supporting immune functions and combating infections. The contemporary relevance of vitamin C and zinc deficiency in the Asian and Latin American regions, both undergoing a rapid nutritional transition, are also discussed. Overall, there is increasing evidence that deficiency of vitamin C and zinc adversely affects the physical and mental growth of children and can impair their immune defences. Nutrition should be the main vehicle for providing these essential nutrients; however, supplementation can represent a valid support method, especially in developing regions.

Zn2+ modulation of ATP-responses at recombinant P2X2 receptors and its dependence on extracellular pH

1. Using recombinant P2X2 receptors expressed in Xenopus oocytes, the modulatory effects of zinc (Zn2+) on ATP-responses were studied under voltage-clamp conditions and at different levels of extracellular pH. 2. Zn2+ (0.3-300 microM) added to the bathing medium potentiated ATP-activated membrane currents, increasing ATP-responses by up to 20 fold. This potentiating effect was reversed on washout. Zn2+-potentiation was reduced in an exponential manner (decaying 1/e in 42 s) as the interval was lengthened between adding Zn2+ then ATP to the superfusate. 3. The potentiating effect of Zn2+ was progressively diminished by acidic shifts in extracellular pH (pHe) which, of itself, also potentiated ATP-responses at P2X2 receptors. The maximal potentiating effects of Zn2+ and H+ were not additive. 4. Neither Zn2+ nor H+ potentiation of ATP-responses was abolished by diethylpyrocarbonate (DEPC, 0.3-3 mM), which irreversibly denatures histidyl residues. Nine histidyl residues are present in the extracellular loop of P2X2 receptors. 5. Zn2+ also enhanced the blocking activity of the P2 receptor antagonist suramin at P2X2 receptors. Therefore, Zn2+ also mimics H+ in increasing suramin-activity at P2X2 receptors. 6. In summary, Zn2+ and H+ potentiate agonist and antagonist activity at P2X2 receptors but their effects are not wholly alike for receptor agonism. There, the potentiating effects of Zn2+ are time-dependent and gradually convert to inhibition while those of H+ are time-independent, persistent and more potent, suggesting that either these modulators interact in a different way with a single allosteric site or with different allosteric sites.

Inhibition of GABAA ligand-gated Cl- channels by zinc in adult rat brain: a regional study

Zinc (Zn2+) was shown to invariably inhibit muscimol-stimulated 36Cl- uptake by synaptoneurosomes in the cerebral cortex, hippocampus and cerebellum. The Zn2+ sensitivity of the GABAA receptor-gated 36Cl- uptake in the cerebral cortex was comparable to that in the hippocampus, whereas the uptake in the cerebellum was less sensitive to Zn2+. Although diazepam-potentiation of muscimol-stimulated 36Cl- uptake was unaltered by 100 microM Zn2+ in the cerebral cortex and hippocampus, diazepam caused no enhancement in the presence of Zn2+ in the cerebellum. Zn2+ inhibited [3H]diazepam binding significantly at 1 mM in the cerebral cortex and cerebellum, whereas Ni2+ increased the binding in a concentration-dependent manner in both regions. Although lower concentrations of Zn2+ did not affect [3H]Ro 15-4513 binding to diazepam-sensitive sites, higher concentrations of ZN2+ increased the binding in both regions. Unlike the diazepam-sensitive sites, the diazepam-insensitive [3H]Ro 15-4513 binding was not affected by Zn2+ or Ni2+ at any of the tested concentrations. These results suggest that the GABAA ligand-gated Cl- flux and its diazepam-potentiation are heterogeneously modulated in various brain regions. It is also suggested that cerebellar diazepam-insensitive [3H]Ro 15-4513 binding sites are insensitive to Zn2+ and Ni2+.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Zinc modulates A-type potassium currents and neuronal excitability in snail neurons

1. Zinc-induced actions were studied on the A-current and neuronal activity in identified and unidentified nerve cells of the snail, Helix pomatia, L., under voltage and current clamp conditions. 2. Extracellularly applied Zn2+ attenuated the peak amplitude of the A-current in a potential- and dose-dependent way (Ki = 1.8 mM at -30 mV, nH = 0.6). 3. Attenuation of the A-currents was initiated as Zn2+ shifted the potential dependence of both activation and inactivation of the currents toward more positive potential values. 4. Zinc concomitantly prolonged the time to peak and the decay time constant of the A-currents (Kd = 1.7 mM, nH = 1.4) as well. 5. Zn2+ decreased the resting membrane potential and the spike amplitude and increased the action potential duration and the input resistance of the cells in current clamp experiments. 6. A complex action of zinc increased the neuronal excitability, indicating spontaneous and synaptically evoked spike discharges. 7. Common and specific zinc binding sites are supposed on vertebrate and invertebrate A-type potassium channel proteins, where binding Zn2+ can modulate the gating properties and kinetics of the fast outward potassium currents.

Modifications of A-current kinetics in mammalian central neurones induced by extracellular zinc

1. Whole-cell voltage clamp recordings were used to study the action of the transition ion zinc on the A-current kinetics in granule cells from rat cerebellar slices. 2. The effects of zinc have been tested in the concentration range from 1 microM to 1 mM, and fully characterized on all kinetic parameters at 100 and 300 microM. All the effects observed were rapid, concentration dependent and fully reversible. 3. Steady-state inactivation curves are strongly shifted towards depolarized potentials, with activation curves much less so. These shifts lead to an increase of the peak current amplitude around physiological resting membrane potentials and to a decrease at hyperpolarized potentials. 4. The forward 'on' rate constants are slowed by Zn2+ at a concentration of 100-300 microM by a factor from 1.5 to 4. The backward 'off' rate constants are unaffected by Zn2+. 5. The development of IA inactivation, as measured from the current decay, is not affected by Zn2+ up to 1 mM. Removal of inactivation is, on the contrary, significantly slowed. 6. The results are neither compatible with the theory of the surface charge screening effect nor with a mechanism involving channel block. It seems more likely that Zn2+ interferes with the channel gating by binding to a specific domain of the channel protein. 7. After treatment with Hg2+, which is irreversible, Zn2+ still maintains its effects, which suggest that the two divalents act at different sites. 8. In view of the widespread distribution of zinc throughout the brain, its actions on the A-current could play an important role in physiological function.

Triton channels are sensitive to divalent cations and protons

Addition of Triton X-100 to planar bilayers composed of dioleoyl phosphatidyl choline, diphytanoyl phosphatidyl choline or mono-oleoyl glycerol induces single channel-like events when electrical conductivity across the bilayer is measured. Addition of divalent cations or protons causes channels to disappear; single channel conductance of remaining channels is not significantly altered; addition of EDTA or alkali (respectively) reverses the effect. It is concluded that sensitivity to divalent cations and protons need not be dependent on specific channel proteins or pore-forming toxins, but may be a feature of any aqueous pore across a lipid milieu.

An engineered cysteine in the external mouth of a K+ channel allows inactivation to be modulated by metal binding

Substitution of a cysteine in the extracellular mouth of the pore of the Shaker-delta K+ channel permits allosteric inhibition of the channel by Zn2+ or Cd2+ ions at micromolar concentrations. Cd2+ binds weakly to the open state but drives the channel into the slow (C-type) inactivated state, which has a Kd for Cd2+ of approximately 0.2 microM. There is a 45,000-fold increase in affinity when the channel changes from open to inactivated. These results indicate that C-type inactivation involves a structural change in the external mouth of the pore. This structural change is reflected in the T449C mutant as state-dependent metal affinity, which may result either from a change in proximity of the introduced cysteine residues of the four subunits or from a change of the exposure of this residue on the surface of the protein.

Giant GABAB-mediated synaptic potentials induced by zinc in the rat hippocampus: paradoxical effects of zinc on the GABAB receptor

The interaction of zinc with pre- and postsynaptic GABAB receptors was studied in adult rat hippocampal slices using intracellular recording in CA1 and CA3 pyramidal neurons. Zinc (50-300 microM) antagonized baclofen responses with a variable potency, whereas CGP-35348 (100 microM) or barium (300 microM) produced a more substantial and consistent inhibition. Zinc also induced giant GABAA-mediated depolarizing potentials (GDP) in these neurons. After blocking GABAA and excitatory synaptic transmission, monosynaptic hyperpolarizing inhibitory postsynaptic potentials (IPSP) mediated by GABAB receptors (IPSPB) were inhibited by CGP-35348 or barium; however, zinc increased the latency and prolonged the duration of the IPSPB and also induced the appearance of spontaneous giant GABAB-mediated hyperpolarizing potentials (GHP). In some cells, IPSPBs in zinc exhibited a multiphasic appearance. The early component was partially inhibited by 300 microM zinc and was followed by a late GHP. CGP-35348 at 100 microM inhibited the early monosynaptic IPSPB but not the GHP; however, at 300 microM both components were blocked. Paired-pulse inhibition of the IPSPB was used to assess the effect of zinc on presynaptic GABAB receptors. Neither the zinc-chelating agent CP94 (400 microM) nor zinc affected this phenomenon. CGP-35348, barium and polyvalent cations, such as cadmium, copper, cobalt, manganese, iron and aluminum, failed to induce giant potentials in hippocampal neurons. It is concluded that zinc is apparently unique in synchronizing the release of GABA to produce GDPs and GHPs.

Properties of GABA-mediated synaptic potentials induced by zinc in adult rat hippocampal pyramidal neurones

1. Intracellular recording techniques were used to study the actions of the transition ion, zinc, on CA1 and CA3 pyramidal neurones in adult rat hippocampal slices. 2. Zinc (300 microM) hyperpolarized pyramidal neurones, increased the membrane excitability and also induced periodic, spontaneous giant depolarizing potentials associated with a conductance increase mechanism. 3. The occurrence of spontaneous giant depolarizations was dependent on the zinc concentration (10 microM-1 mM) with an apparent dissociation constant of 98 microM. The frequency of zinc-induced depolarizations was unaffected by the membrane potential from -50 to -100 mV. 4. Stimulation of the Schaffer collaterals or mossy fibre pathways evoked an excitatory and inhibitory synaptic potential complex. In the presence of zinc, nerve fibre stimulation evoked, in an all-or-none fashion, a giant depolarizing potential with an increased membrane conductance. Both spontaneous and evoked depolarizations were inhibited by 1 microM tetrodotoxin. 5. Evoked giant depolarizations were labile with too frequent stimulation resulting in a failure of generation. A minimum time of 140 s was required between stimuli to ensure successive giant depolarizations. 6. Spontaneous and evoked zinc-induced depolarizing potentials were inhibited by bicuculline (10 microM) or picrotoxin (40 microM) and enhanced by pentobarbitone (100 microM) or flurazepam (10 microM), suggesting that these potentials are mediated by activation of gamma-aminobutyric acidA (GABAA) receptors. 7. Ionophoretic application of GABA produced biphasic responses at -60 mV membrane potential. The reversal potentials for the depolarizing and hyperpolarizing GABA responses were -56 +/- 5 and -66 +/- 8 mV respectively. The giant depolarizations induced by zinc reversed at -57 +/- 4 mV. This suggests a dendritic location for the generation of these potentials. 8. Excitatory amino acid antagonists, 2-amino-5-phosphonovalerate (APV, 40 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) did not affect the amplitude but slightly reduced the frequency of the giant depolarizations. 9. It is concluded that zinc induces a synchronized release of GABA, quite independent of intact excitatory synaptic transmission, which acts on GABAA receptors producing large depolarizing synaptic potentials. This increased level of GABA release may be of physiological and pathological importance since zinc is a naturally occurring metal ion endogenous to the central nervous system.

Neurons of origin of zinc-containing pathways and the distribution of zinc-containing boutons in the hippocampal region of the rat

Recent methods allow the study of neurons that contain zinc in synaptic vesicles of their boutons (Timm-stainable boutons) by the intravital precipitation (local or throughout the CNS) of the vesicular zinc with selenium compounds and its subsequent retrograde transport to the parent neurons, where the precipitate can be silver enhanced. The present study is a description of the distribution of zinc-containing neurons, their possible connections and their terminal fields within the hippocampal region of the rat. Problems inherent to the methods are addressed. Finally, based on the results and a review of literature, the possible function of zinc in the hippocampal region is considered. Neurons which contain silver-enhanced precipitates were observed in layers II, V and VI of the lateral entorhinal area and in layers V and VI of the medial entorhinal area. In the parasubiculum, labeled cells were seen in layer II/III of the parasubiculum a and in layer V. Labeled cells in the presubiculum were concentrated in layers III and V, in the hippocampal pyramidal cell layer and the dentate granule cell layer, but neurons containing precipitates were largely absent from the subiculum. Zinc-containing axonal boutons defined subpopulations within principal hippocampal neuron populations. Within layer II of the lateral entorhinal cortex and the pyramidal cell layer for regio inferior deeply situated neurons were labeled, whereas superficially placed pyramidal cells were labeled in regio superior. The neuropil staining described in the present study corresponded to that found in earlier studies. However, glial and vascular staining or unspecific background were largely absent, and the neuropil staining could unequivocally be identified light microscopically. Methodological problems are most prominently reflected in unstained mossy fibers in some animals. Based on series from animals treated with decreasing doses of sodium selenite and increased survival times, this problem can be related to small amounts of circulating reactive selenium and a competition of zinc compartments (vesicles) for the selenium. Staining will fail where the competition prevents individual compartments from reaching a threshold amount of zinc precipitate for silver amplification. A guide to evaluate histological material is provided. The distribution of zinc-containing boutons and their cells of origin indicate that zinc-containing and zinc-negative projections are not organized as parallel pathways. The mossy fibers provide an example of a pure zinc-containing pathway. Projections from regio superior to the dorsal presubiculum are likely to be zinc-negative while projections from the same area to the subiculum are zinc-containing.(ABSTRACT TRUNCATED AT 400 WORDS)

A novel modulatory binding site for zinc on the GABAA receptor complex in cultured rat neurones

1. The properties of gamma-aminobutyric acidA (GABAA) receptor-ion channel complexes and the interaction with the transition metal zinc, were studied on rat sympathetic and cerebellar neurones in dissociated culture using patch clamp recording techniques. 2. The antagonism of GABA-induced membrane currents by zinc on sympathetic neurones was subject to developmental influence. Using embryonic sympathetic neurones acutely cultured for 24-72 h, GABA responses were more depressed by zinc when compared to responses evoked on adult neurones cultured for the same period. For neurones developing in vivo, the percentage inhibition of GABA responses produced by zinc in embryonic neurones was estimated to decline by 50% after 48.2 days following birth. 3. Embryonic sympathetic neurones maintained in culture for prolonged periods (40-50 days in vitro, DIV) became less sensitive to zinc when compared to neurones cultured for shorter periods (10-20 DIV). The decrease in the zinc inhibition for neurones maintained in vitro proceeded at an apparent rate of 0.55% per day. 4. Activation of the GABA receptor by muscimol (0.2-2 microM) was also antagonized by zinc (50-100 microM). 5. Lowering the pH of the perfusing Krebs solution did not affect the inhibition of GABA responses by zinc on sympathetic neurones. 6. Modulation of the GABAA receptor by some benzodiazepines, a barbiturate, a steroid based on pregnanolone, or antagonists bicuculline and picrotoxinin, did not interfere with the antagonism exerted by zinc on sympathetic neurones. A novel binding site for zinc on the GABAA receptor is proposed. 7. Analysis of the GABA-activated current noise on sympathetic neurones revealed two kinetic components to the power spectra requiring a double Lorentzian fit. The time constant describing the fast component (tau 2, 2.1 ms) was unaffected by zinc, whereas the slow component time constant (tau 1, 21.7 ms) was slightly reduced to 17.1 ms. 8. The apparent single-channel conductance for GABA-activated ion channels was determined from the power spectra (gamma s = 22.7 pS) and also from the relationship between the mean GABA-induced inward current and the variance of the current (gamma v = 24 pS). Zinc (25-100 microM) did not affect the single-channel conductance. 9. Single GABA-activated ion channels were recorded from outside-out patches taken from the soma of large cerebellar neurones. Single GABA channels were capable of activation to multiple current amplitudes which were assessed into the following conductance levels: 8, 18, 23, 29 and 34 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAA receptors are differentially sensitive to zinc: dependence on subunit composition

GABAA receptors with different subunit composition, were expressed in kidney cells and studied by whole cell recording. Expressed GABAA receptors were differentially sensitive to inhibition by zinc; receptors which lacked the gamma subunit were inhibited by zinc. Embryonic neurons also exhibited zinc-sensitive GABA responses, in contrast to adult neurones. This developmentally-sensitive aspect of GABAA receptor pharmacology may be partly dependent on expression of the gamma subunit.