Zinc modulates GABAB binding in rat brain

Zinc reduces antiseizure activity of neurosteroids by selective blockade of extrasynaptic GABA-A receptor-mediated tonic inhibition in the hippocampus

Zinc is an abundant trace metal in the hippocampus nerve terminals. Previous studies demonstrate the ability of zinc to selectively block neurosteroid-sensitive, extrasynaptic GABA-A receptors in the hippocampus (Carver et al, 2016). Here we report that zinc prevents the seizure protective effects of the synthetic neurosteroid ganaxolone (GX) in an experimental model of epilepsy. GABA-gated and tonic currents were recorded from dissociated dentate gyrus granule cells (DGGCs), CA1 pyramidal cells (CA1PCs), and hippocampal slices from adult mice. Antiseizure effects of GX and the reversal of these effects by zinc were evaluated in fully-kindled mice expressing generalized (stage 5) seizures. In electrophysiological studies, zinc blocked the GABA-evoked and GX-potentiated GABA-gated chloride currents in DGGCs and CA1PCs in a concentration-dependent fashion similar to the competitive GABA-A receptor antagonists bicuculline and gabazine. Zinc completely blocked GX potentiation of extrasynaptic tonic currents, but not synaptic phasic currents. In hippocampus kindling studies, systemic administration of GX produced a dose-dependent suppression of behavioral and electrographic seizures in fully-kindled mice with complete seizure protection at the 10 mg/kg dose. However, the antiseizure effects of GX were significantly prevented by intrahippocampal administration of zinc (ED₅₀, 150 μM). The zinc antagonistic response was reversible as animals responded normally to GX administration 24 h post-zinc blockade. These results demonstrate that zinc reduces the antiseizure effects of GX by selectively blocking extrasynaptic δGABA-A receptors in the hippocampus. These pharmacodynamic interactions have clinical implications in neurosteroid therapy for brain conditions associated with zinc fluctuations.

The relationship between micronutrient status and sleep patterns: a systematic review

OBJECTIVE

To review articles on the relationship of dietary and circulating micronutrients with sleep patterns, and to identify issues surrounding implications for future research and public health practice.

DESIGN

A systematic review was conducted. PubMed, Embase and Scopus were searched through January 2016.

SETTING

Both experimental and observational studies were included. However, studies that focused on secondary sleep impairment due to comorbidities were excluded.

SUBJECTS

Individuals in different age groups, from infants to older adults.

RESULTS

A total of twenty-six articles were selected. In the articles reviewed, researchers generally supported a potential role of micronutrients, particularly Fe and Mg, in the development of sleep stages among infants and in reversing age-related alterations in sleep architecture in older adults. Micronutrient status has also been linked to sleep duration, with sleep duration positively associated with Fe, Zn and Mg levels, and negatively associated with Cu, K and vitamin B12 levels. The mechanisms underlying these relationships include the impact of micronutrients on excitatory/inhibitory neurotransmitters and the expression of circadian genes.

CONCLUSIONS

Although the number of studies on the relationship between micronutrient status and sleep remains low, evidence has emerged that suggests a link between dietary/circulating micronutrients and sleep. Future research is needed to investigate the dose-dependent as well as the longitudinal relationships between micronutrient levels and human sleep across populations, test the interactions among micronutrients on sleep outcomes, and ultimately examine the clinical relevance of micronutrients on sleep health.

Associations between Blood Zinc Concentrations and Sleep Quality in Childhood: A Cohort Study

Little evidence is available regarding the relationship between zinc and sleep in school children. The present study aimed to examine the cross-sectional and longitudinal associations between blood zinc concentrations and sleep quality throughout childhood. A total of 1295 children from the Jintan Child Cohort in China were included in this study. Venous blood sample of zinc and subjective sleep data were collected when the children were at preschool age (3-5 years old) and early adolescence (11-15 years old). Odds ratios (ORs) reflect the odds of the sleep quality/subdomain being at a greater impairment level associated with 1 unit increase in log zinc concentration. Cross-sectional analyses showed negative correlation of blood zinc concentrations with insufficient sleep duration (OR = 0.432, p = 0.002), sleep disturbances (OR = 0.454, p = 0.009) and poor sleep quality (OR = 0.559, p = 0.049) in adolescence, but no association at preschool age (p > 0.05). Longitudinal analyses indicated that blood zinc concentrations at preschool age predict poor sleep efficiency (OR = 0.186, p = 0.000) and poor sleep quality (OR = 0.358, p = 0.020) in adolescence. Our findings suggest that sufficient zinc concentration is associated with good sleep quality, dependent on the developmental stage in childhood. Future interventional research is warranted to examine the short and long-term effect of zinc status on sleep heath.

Inhibitory zinc-enriched terminals in mouse spinal cord

The ultrastructural localization of zinc transporter-3, glutamate decarboxylase and zinc ions in zinc-enriched terminals in the mouse spinal cord was studied by zinc transporter-3 and glutamate decarboxylase immunohistochemistry and zinc selenium autometallography, respectively. The distribution of zinc selenium autometallographic silver grains, and zinc transporter-3 and glutamate decarboxylase immunohistochemical puncta in both ventral and dorsal horns as seen in the light microscope corresponded to their presence in the synaptic vesicles of zinc-enriched terminals at ultrastructural levels. The densest populations of zinc-enriched terminals were seen in dorsal horn laminae I, III and IV, whereas the deeper laminae V and VI contained fewer terminals. At ultrastructural levels, zinc-enriched terminals primarily formed symmetrical synapses on perikarya and dendrites. Only relatively few asymmetrical synapses were observed on zinc-enriched terminals. In general, the biggest zinc-enriched terminals contacted neuronal somata and large dendritic elements, while medium-sized and small terminals made contacts on small dendrites. The ventral horn was primarily populated by big and medium-sized zinc-enriched terminals, whereas the dorsal horn was dominated by medium-sized and small zinc-enriched terminals. The presence of boutons with flat synaptic vesicles with zinc ions and symmetric synaptic contacts suggests the presence of inhibitory zinc-enriched terminals in the mammalian spinal cord, and this was confirmed by the finding that zinc ions and glutamate decarboxylase are co-localized in these terminals. The pattern of zinc-enriched boutons in both dorsal and ventral horns is compatible with evidence suggesting that zinc may be involved in both sensory transmission and motor control.

Zinc-enriched GABAergic terminals in mouse spinal cord

Electrophysiological experiments have shown that zinc ions modulate glutamate and GABA receptors in brain slices. All the zinc-enriched neuronal pathways in the brain analyzed up until now have been found to be glutaminergic. Many years ago, zinc-enriched terminals with flat vesicles and symmetric synapses were found to be present in rat spinal cord by Henrik Daa Schrøder, and recently these findings have been supported by immunohistochemical and electron microscopical data in lamprey, mouse and rat. In the present study we expanded these observations by revealing a colocalization of zinc ions, zinc transporter-3 (ZnT3) and glutamic acid decarboxylase (GAD) in synaptic vesicles of zinc-enriched terminals throughout the mouse spinal cord. Confocal analysis of ZnT3 and GAD immunofluorescence was used at light microscopical levels, and a combination of zinc selenium autometallography and GAD immunocytochemistry at electron microscopic levels. Zinc-enriched/GABAergic terminals were observed in all laminae of the spinal gray matter, but most densely populated were laminae I and III in the dorsal horn. In the lateral and ventral funiculi of the white matter, rows of inhibitory zinc-enriched boutons were seen radiating from the gray matter. Ultrastructurally, colocalization of zinc ions and GAD immunoreactivity was seen in a pool of presynaptic terminals in the above locations. Some zinc-enriched terminals were not GAD-positive and some GAD-positive terminals were void of zinc ions. The majority of the zinc-enriched, not GABAergic terminals could be classified as excitatory based on their morphology, i.e. round clear vesicles and symmetric synapses. We conclude that a majority of the spinal cord zinc-enriched terminals are GABAergic. The zinc-enriched terminals with excitatory morphology are most likely glutaminergic, a few have an inhibitory morphology but are not GABAergic. These are most likely glycinergic.

Efficacy of exogenous oral zinc in treatment of patients with carbonic anhydrase VI deficiency

BACKGROUND

We previously described a disorder in 18 patients with decreased parotid saliva gustin/carbonic anhydrase (CA) VI secretion associated with loss of taste (hypogeusia) and smell (hyposmia) and distorted taste (dysgeusia) and smell (dysosmia). Because gustin/CAVI is a zinc-dependent enzyme we instituted a study of treatment with exogenous zinc to attempt to stimulate synthesis/secretion of gustin/CAVI and thereby attempt to correct the symptoms of this disorder.

METHODS

Fourteen of the 18 patients with this disorder completed the study. They were treated with 100 mg of exogenous zinc daily for 4 to 6 months, in an open clinical trial. Both before and after treatment, measurements were obtained of parotid saliva gustin/CAVI, parotid saliva, serum and urine zinc, taste and smell function, and, in some patients, examination of circumvallate taste buds by electron microscopy.

RESULTS

Treatment success was predicated upon significant increases in parotid saliva gustin/CAVI. This occurred in 10 of the 14 patients who were labeled responders; they also exhibited improvement in taste and smell acuity, a diminution in dysgeusia and dysosmia and increased zinc concentrations in parotid saliva, serum, and urine. Taste bud morphology returned to normal in each responder in whom it was measured. No increase in gustin/CAVI occurred in 4 patients who were labeled nonresponders; they exhibited no improvement in taste or smell acuity and no increases in parotid saliva zinc. However, serum and urine zinc increased to levels similar to those measured in the 10 responders. Two of 4 nonresponders reported diminution in dysgeusia and dysosmia. Taste bud morphology did not change from the abnormal state in the 1 nonresponder in whom it was measured.

CONCLUSIONS

Zinc treatment is effective in patients in whom this trace metal increases synthesis/secretion of gustin/CAVI and ineffective in those in whom it does not. Increased gustin/CAVI in this disorder is probably associated with zinc stimulation of the gene responsible for the synthesis/secretion of gustin/CAVI. Among nonresponders, zinc was ineffective for several possible reasons, including resistance to zinc and possible sialylation of gustin/CAVI, which may render it functionally ineffective. Results suggest the hypothesis that gustin/CAVI is a trophic factor that promotes growth and development of taste buds through its action on taste bud stem cells.

Distribution of zinc in the substantia nigra of rats treated with 6-hydroxydopamine

To study the relationship between tissue accumulation of Zinc (Zn) and neurodegeneration in the nigrostriatal dopaminergic pathway, 65Zn distribution in this pathway was examined after unilateral injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra of rats. When 65ZnCl2 was intravenously injected 4 days after treatment with 6-OHDA, 65Zn was concentrated in the ipsilateral substantia nigra 6 days after 65Zn injection. On the other hand, 19 d after treatment with 6-OHDA, 65Zn distribution in the ipsilateral substantia nigra was decreased to the level of the contralateral one. When NH4(99)TcO4, which cannot go through the blood-brain barrier, was injected into rats 4 d after treatment with 6-OHDA, 99Tc was concentrated in the ipsilateral substantia nigra 30 min after 99Tc injection, but no longer detectable 6 d after injection. These results suggest that Zn is necessary for a repair process called replacement gliosis after the death of neurons and that excess Zn does not accumulate in the lesion after completion of the gliosis.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

GABAA receptor-mediated IPSCs in rat thalamic sensory nuclei: patterns of discharge and tonic modulation by GABAB autoreceptors

1. The patterns of discharge of spontaneous GABAA-mediated inhibitory postsynaptic currents (sIPSCs), originating from the nucleus reticularis thalami (NRT), and their modulation by GABAB autoreceptors, were studied in rat thalamocortical (TC) neurones using whole-cell voltage-clamp recordings in brain slices. 2. sIPSCs were recorded in all ventro-basal (VB) and dorsal lateral geniculate (LGN) neurones. In VB neurones, in the presence of tetraethylammonium (TEA, 5 mM), these sIPSCs can occur in bursts at frequencies of either 0.1 or 1-2 Hz. In the presence of tetrodotoxin (TTX), these bursting activities are replaced by the continuous discharge of miniature IPSCs (mIPSCs), recorded in the absence of TEA, at a frequency of 4 Hz. The kinetic properties of mIPSCs were similar in VB and LGN TC neurones. 3. In VB TC neurones the GABAB receptor agonist (+/-)-baclofen, at a concentration of 0.05 microM, decreased the mIPSC frequency by 22% without affecting their amplitude distribution. Increasing the (+/-)-baclofen concentration to 1 and 10 microM caused similar reductions (41 and 47%, respectively) in the mIPSCs frequency: these values were significantly different from the one observed with 0.05 microM (+/-)-baclofen. In LGN TC neurones, where mIPSCs originate from both NRT and local interneurone terminals, 1 microM (+/-)-baclofen produced a 66% reduction in the mIPSC frequency. 4. The GABAB receptor antagonist CGP55845A (50 nM) not only blocked the baclofen-mediated decrease in mIPSC frequency, but also produced a 52% increase in the mIPSC frequency compared with control in three out of seven neurones. Application of CGP55845A (50-500 nM) alone produced a 77% increase in the mIPSC frequency in three out of nine VB neurones, and in the LGN, CGP55845A (100 nM) produced a 53% increase in four out of nine neurones. CGP55845A (100 nM) also reversibly increased the amplitude of evoked GABAA IPSCs by 74 and 57% in three out of three VB and three out of five LGN neurones, respectively. 5. Application of GABA (1.5-5 microM) decreased the mIPSC frequency in VB TC neurones by a similar extent (48%) as 1-10 microM (+/-)-baclofen. 6. In the presence of 100 microM Cd2+, (+/-)-baclofen still decreased the mIPSC frequency by about 40%, indicating that the effect of presynaptic GABAB receptor activation on spontaneous GABA release did not occur through a reduction of voltage-dependent Ca2+ currents. 7. Cd2+ (100 microM) decreased the amplitude of both mIPSCs and isoguvacine-induced current by 30 and 19%, respectively, indicating an effect of this divalent cation on postsynaptic GABAA receptors. 8. We conclude that GABAB autoreceptors are present on the GABAergic terminals within the thalamic sensory nuclei and that these receptors can be tonically activated by the ambient GABA.

Regional zinc staining in postmortem hippocampus from schizophrenic patients

Schizophrenia-associated deficits in learning and memory have been associated with a decrease in the volume of the hippocampus, but the specific nature of the neuronal deficit remains unknown. Many critical afferent pathways in the hippocampus contain ionic zinc. Alterations of these pathways could be manifest as a decrease in ionic zinc levels within hippocampal afferent pathways. This possibility was examined in postmortem hippocampal tissue taken from schizophrenic patients, patients with other psychiatric disorders and matched, non-mentally ill subjects using a modified Timm's silver staining method. The three groups exhibited similar patterns of zinc staining within the hippocampal formation as well as similar levels of zinc within the mossy fiber projection system. A greater prevalence of zinc staining within the inner molecular layer of the dentate gyrus was observed in female as compared to male donors and in older as compared to younger donors. The results of the present study demonstrate that loss of ionic zinc within the hippocampus does not appear to be part of the pathology of schizophrenia.

Biological half-lives of zinc and manganese in rat brain

The brains of rats injected intravenously with 65ZnCl2 or 54MnCl2 were subjected to high-resolution autoradiography. The distribution of 65Zn and 54Mn in each brain region gradually decreased from 6 days to 42 days for 65Zn and from 15 days to 60 days for 54Mn after the injection. The biological half-lives of Zn in each region studied were in the range of 16-43 days; the longest was observed in the amygdaloid nuclei. The regions where the long biological half-life was observed were consistent with the ones with the high density of Zn-containing neuron terminals reported previously. The biological half-lives of Mn in each region were 51-74 days; the longest were those in the hypothalamic nuclei and thalamus.

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.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

GABAB binding sites in early adult and aging rat brain

The effect of aging on GABAB binding was investigated in rat brain. Receptor autoradiography was used to investigate both GABAB and GABAA binding at 2 months, 3 months, 13 months, and 23 months. GABAB binding decreases significantly between 2 months and 23 months of age, as does GABAA binding, with was investigated in rat brain. Receptor autoradiography was used to investigate both GABAB and GABAA binding at 2 months, 3 months, 13 months, and 23 months. GABAB binding decreases significantly between 2 months and 23 months of age, as does GABAA binding, with the greatest decrease between 2 and 3 months. The decrease in GABAB binding appears to be due to a decrease in binding site affinity rather than a decrease in receptor density. The noncompetitive GABAB antagonist zinc, the competitive GABAB antagonist CGP 35348, and the guanyl nucleotide analogue GTP-gamma-S all inhibit GABAB binding identically in 2 month and 23 month brain. These data indicate subtle age-related changes in the GABAB binding in early adult life but little change with senescence.

Postnatal ontogeny of GABAB binding in rat brain

The postnatal development of GABAB binding sites in rat brain was studied by quantitative receptor autoradiography using [3H]GABA under selective conditions. Binding levels peak at regionally specific times during the first three weeks of life and then decline to adult levels. GABAB binding peaked in the globus pallidus, vestibular and spinal trigeminal nuclei, and the CA3 region of the hippocampus at postnatal day 3; in the striatum, nucleus accumbens, inferior olive, septum, dentate gyrus and CA1 region of the hippocampus at postnatal day 7; in the neocortex and thalamus at postnatal day 14; and in the medial geniculate at postnatal day 21. Following these regionally specific peaks, binding decreased to postnatal day 28 levels. Further significant decreases in binding were observed in all regions examined between postnatal day 28 and adulthood. Comparisons of binding site pharmacology reveal equipotent displacement of GABAB binding by several competitive agonists and antagonists in postnatal day 7 and adult rat brain, indicating that immature and adult binding sites have similar pharmacological properties with regard to these compounds. The GABAB receptor antagonist CGP 54626A, however, inhibited binding more potently in the postnatal day 7 thalamus and neocortex than in these areas in the adult brain. The guanyl nucleotide analogue guanosine 5'-O-(3-thiotriphasphate) inhibited GABAB binding extensively in both postnatal day 7 and adult brain. The non-competitive antagonist zinc also inhibited GABAB binding at both ages and was more potent in postnatal day 7 brain than in adult brain. Saturation analyses reveal two binding sites with similar affinities in both immature and adult rat brain, indicating that postnatal modulation of GABAB binding reflects changes in binding site density rather than modulation of binding site affinity. While immature GABAB binding sites share most pharmacological characteristics with adult binding sites and appear to be coupled to G-proteins at an early age, their interactions with zinc and CGP 54626A suggest that GABAB binding sites in immature brain may have a distinct pharmacological profile. Our data suggest significant regional and pharmacological changes in GABAB binding during development. The implications of these findings are discussed with regards to a possible role of GABAB receptors in the development of the central nervous system.

Brain uptake of trace metals, zinc and manganese, in rats

The brains of rats injected intravenously with 65ZnCl2 or 54MnCl2 were subjected to high resolution autoradiography. 65Zn and 54Mn were largely concentrated in choroid plexus 1 h after injection and then gradually decreased, with increases in other brain regions, suggesting that both metals were taken up gradually into brain mainly via cerebrospinal fluid in the choroid plexus. By 3 days after injection, relatively high level of 65Zn was seen in the dentate gyrus and CA3 region of the hippocampus and in the cerebral cortex. The level of 54Mn was also high in the former, while relatively low in the latter.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

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.