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C Fernando Valenzuela

Publications and source records attributed to C Fernando Valenzuela.

At least 19 recordsLinked to original sources
PubMed2006

Expression and function of SNAP-25 as a universal SNARE component in GABAergic neurons.

Intracellular vesicular trafficking and membrane fusion are important processes for nervous system development and for the function of neural circuits. Synaptosomal-associated protein 25 kDa (SNAP-25) is a component of neural soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complexes that mediate the exocytotic release of neurotransmitters at chemical synapses. Previous results from mouse mutant models and pharmacological/neurotoxin blockades have demonstrated a critical role for SNAP-25-containing SNARE complexes in action potential (AP)-dependent release at cholinergic and glutamatergic synapses and for calcium-triggered catecholamine release from chromaffin cells. To examine whether SNAP-25 participates in the evoked release of other neurotransmitters, we investigated the expression and function of SNAP-25 in GABAergic terminals. Patch-clamp recordings in fetal Snap25-null mutant cortex demonstrated that ablation of SNAP-25 eliminated evoked GABA(A) receptor-mediated postsynaptic responses while leaving a low level of spontaneous AP-independent events intact, supporting the involvement of SNAP-25 in the regulated synaptic transmission of early developing GABAergic neurons. In hippocampal cell cultures of wild-type mice, punctate staining of SNAP-25 colocalized with both GABAergic and glutamatergic synaptic markers, whereas stimulus-evoked vesicular recycling was abolished at terminals of both transmitter phenotypes in Snap25-/- neurons. Moreover, immunohistochemistry and fluorescence in situ hybridization revealed coexpression of SNAP-25, VGAT (vesicular GABA transporter), and GAD65/67 (glutamic acid decarboxylase 65/67) in interneurons within several regions of the adult brain. Our results thus provide evidence that SNAP-25 is critical for evoked GABA release during development and is expressed in the presynaptic terminals of mature GABAergic neurons, consistent with its function as a component of a fundamental core SNARE complex required for stimulus-driven neurotransmission.

Animals↗
PubMed2006

Alcohol potently modulates climbing fiber-->Purkinje neuron synapses: role of metabotropic glutamate receptors.

Consumption of alcoholic beverages produces alterations in motor coordination and equilibrium that are responsible for millions of accidental deaths. Studies indicate that ethanol produces these alterations by affecting the cerebellum, a brain region involved in the control of motor systems. Purkinje neurons of the cerebellar cortex have been shown to be particularly important targets of ethanol. However, its mechanism of action at these neurons is poorly understood. We hypothesized that ethanol could modulate Purkinje neuron function by altering the excitatory input provided by the climbing fiber from the inferior olive, which evokes a powerful all-or-none response denoted as the complex spike. To test this hypothesis, we performed whole-cell patch-clamp electrophysiological and Ca2+ imaging experiments in acute slices from rat cerebella. We found that ethanol potently inhibits the late phase of the complex spike and that this effect is the result of inhibition of type-1 metabotropic glutamate receptor-dependent responses at the postsynaptic level. Moreover, ethanol inhibited climbing fiber long-term depression, a form of synaptic plasticity that also depends on activation of these metabotropic receptors. Our findings identify the climbing fiber-->Purkinje neuron synapse as an important target of ethanol in the cerebellar cortex and indicate that ethanol significantly affects cerebellar circuits even at concentrations as low as 10 mm (legal blood alcohol level in the United States is below 0.08 g/dl = 17 mm).

Alanine↗
PubMed2006

Connexin-36 gap junctions mediate electrical coupling between ventral tegmental area GABA neurons.

Communication between neurons in the mammalian brain is primarily through chemical synapses; however, evidence is accumulating in support of electrical synaptic transmission between some neuronal types in the mature nervous system. The authors have recently demonstrated that the gap junction (GJ) blocker quinidine suppresses stimulus-induced and dopamine-evoked coupling of gamma amino butyric acid (GABA) neurons in the ventral tegmental area (VTA) of mature rats (Stobbs et al., 2004). The aim of this study was to evaluate the role of connexin-36 (Cx36) GJs in mediating electrical coupling between VTA GABA neurons in P50-80 rats in vivo and P25-50 rats in vitro. Single stimulation of the internal capsule (IC) evoked VTA GABA neuron spike couplets in mature rats when activated antidromically, and multiple poststimulus spike discharges (PSDs) when activated with brief high-frequency stimulation of the IC (ICPSDs). The Cx36 GJ blocker mefloquine (30 mg/kg) suppressed VTA GABA neuron ICPSDs in mature freely behaving rats. VTA GABA neurons recorded via whole-cell patch clamp in the midbrain slice preparation of P25-50 rats showed robust expression of Cx36 transcripts when tested with single-cell quantitative reverse transcription polymerase chain reaction. In P50-80 rats, Cx36 protein immunoreactivity was evident in the VTA and surrounding structures. Dye-coupling between VTA neurons was observed following Neurobiotin labeling of VTA GABA neurons, as well as with the fluorochrome Alexa Fluor 488 using real-time video fluorescent microscopy. Thus, mature VTA GABA neurons appear to be connected by electrical synapses via Cx36 GJs, whose coupling is enhanced by corticotegmental input and by dopamine.

Animals↗
PubMed2006

Alcohol increases efficacy of immature synapses in a neurosteroid-dependent manner.

Fetal ethanol exposure persistently affects hippocampal circuits leading to learning and memory disabilities. Although the mechanisms responsible for these effects are not fully understood, several studies implicate neurosteroids as mediators of the actions of ethanol. A neurosteroid that appears to be critical for the fetal actions of ethanol is pregnenolone sulfate (PREGS). We found that chronic prenatal ethanol exposure increases PREGS levels in the fetal brain and that an endogenous PREGS-like neurosteroid strengthens excitatory transmission in the neonatal hippocampus. Therefore, we hypothesized that ethanol could affect synaptic transmission in the developing hippocampus in a PREGS-dependent manner. We used patch-clamp electrophysiological techniques and found that 50 mm ethanol strengthens AMPA receptor-mediated transmission in the CA1 region by reducing the failure rate of low-efficacy synapses. This effect was age-dependent and was occluded by application of exogenous PREGS. An anti-PREGS antibody scavenger and blockade of PREGS synthesis prevented the effect of ethanol. These data indicate that the deleterious effects of ethanol on hippocampal development are mediated in part by alterations in neurosteroid production, which results in premature stabilization of excitatory synapses.

Age Factors↗
PubMed2005

Developmentally regulated actions of alcohol on hippocampal glutamatergic transmission.

Ethanol exposure during fetal development is a leading cause of learning disabilities. Studies suggest that it alters learning and memory by permanently damaging the hippocampus. It is generally assumed that this is mediated, in part, via alterations in glutamatergic transmission. Although NMDA receptors are presumed to be the most sensitive targets of ethanol in immature neurons, this issue has not been explored in the developing hippocampus. We performed whole-cell patch-clamp recordings in hippocampal slices from neonatal rats. Unexpectedly, we found that acute ethanol (10-50 mM) exposure depresses inward currents elicited by local application of exogenous AMPA, but not NMDA, in CA3 pyramidal neurons. These findings revealed a direct effect of ethanol on postsynaptic AMPA receptors. Ethanol significantly decreased the amplitude of both AMPA and NMDA receptor-mediated EPSCs evoked by electrical stimulation. This effect was associated with an increase in the paired-pulse ratio and a decrease in the frequency of miniature EPSCs driven by depolarization of axonal terminals. These findings demonstrate that ethanol also acts at the presynaptic level. Omega-conotoxin-GVIA occluded the effect of ethanol on NMDA EPSCs, indicating that ethanol decreases glutamate release via inhibition of N-type voltage-gated Ca2+ channels. In more mature rats, ethanol did not affect the probability of glutamate release or postsynaptic AMPA receptor-mediated currents, but it did inhibit NMDA-mediated currents. We conclude that the mechanism by which ethanol inhibits glutamatergic transmission is age dependent and challenge the view that postsynaptic NMDA receptors are the primary targets of ethanol early in development.

Animals↗
PubMed2005

Alcohol is a potent stimulant of immature neuronal networks: implications for fetal alcohol spectrum disorder.

Ethanol consumption during development affects the maturation of hippocampal circuits by mechanisms that are not fully understood. Ethanol acts as a depressant in the mature CNS and it has been assumed that this also applies to immature neurons. We investigated whether ethanol targets the neuronal network activity that is involved in the refinement of developing hippocampal synapses. This activity appears during the growth spurt period in the form of giant depolarizing potentials (GDPs). GDPs are generated by the excitatory actions of GABA and glutamate via a positive feedback circuit involving pyramidal neurons and interneurons. We found that ethanol potently increases GDP frequency in the CA3 hippocampal region of slices from neonatal rats. It also increased the frequency of GDP-driven Ca2+ transients in pyramidal neurons and increased the frequency of GABA(A) receptor-mediated spontaneous postsynaptic currents in CA3 pyramidal cells and interneurons. The ethanol-induced potentiation of GABAergic activity is probably the result of increased quantal GABA release at interneuronal synapses but not enhanced neuronal excitability. These findings demonstrate that ethanol is a potent stimulant of developing neuronal circuits, which might contribute to the abnormal hippocampal development associated with fetal alcohol syndrome and alcohol-related neurodevelopmental disorders.

Alcohol-Induced Disorders, Nervous System↗
PubMed2005

Differential expression of NMDA receptor subunits and splice variants among the CA1, CA3 and dentate gyrus of the adult rat.

N-Methyl-d-aspartate (NMDA)-type glutamate receptors in the hippocampus are important mediators of both memory formation and excitotoxicity. It is thought that glutamatergic neurons of the CA1, CA3 and dentate gyrus regions of the hippocampus contribute differentially to memory formation and are differentially sensitive to excitotoxicity. The subunit and/or splice variant composition of the NMDA receptor controls many aspects of receptor function such as ligand affinity, calcium permeability and channel kinetics, as well as interactions with intracellular anchoring and regulatory proteins. Thus, one possible explanation of the differences in NMDA receptor-dependent processes, such as synaptic plasticity and excitotoxicity, among the hippocampal sub-regions is that they differ in subunit and/or splice variant expression. Here we report that the NMDA receptor subunits NR1 and NR2B, along with the four splice variant cassettes of the NR1 subunit are differentially expressed in the CA1, CA3 and dentate gyrus of the hippocampus. Expression of the AMPA receptor subunits GluR1 and GluR2 also differ. These differences may contribute to functional differences, such as with excitotoxicity and synaptic plasticity, that exist between the sub-regions of the hippocampus.

Actinin↗
PubMed2005

Neurosteroid-induced plasticity of immature synapses via retrograde modulation of presynaptic NMDA receptors.

Neurosteroids are produced de novo in neuronal and glial cells, which begin to express steroidogenic enzymes early in development. Studies suggest that neurosteroids may play important roles in neuronal circuit maturation via autocrine and/or paracrine actions. However, the mechanism of action of these agents is not fully understood. We report here that the excitatory neurosteroid pregnenolone sulfate induces a long-lasting strengthening of AMPA receptor-mediated synaptic transmission in rat hippocampal neurons during a restricted developmental period. Using the acute hippocampal slice preparation and patch-clamp electrophysiological techniques, we found that pregnenolone sulfate increases the frequency of AMPA-mediated miniature excitatory postsynaptic currents in CA1 pyramidal neurons. This effect could not be observed in slices from rats older than postnatal day 5. The mechanism of action of pregnenolone sulfate involved a short-term increase in the probability of glutamate release, and this effect is likely mediated by presynaptic NMDA receptors containing the NR2D subunit, which is transiently expressed in the hippocampus. The increase in glutamate release triggered a long-term enhancement of AMPA receptor function that requires activation of postsynaptic NMDA receptors containing NR2B subunits. Importantly, synaptic strengthening could also be triggered by postsynaptic neuron depolarization, and an anti-pregnenolone sulfate antibody scavenger blocked this effect. This finding indicates that a pregnenolone sulfate-like neurosteroid is a previously unrecognized retrograde messenger that is released in an activity-dependent manner during development.

Age Factors↗
PubMed2005

Neurosteroid paradoxical enhancement of paired-pulse inhibition through paired-pulse facilitation of inhibitory circuits in dentate granule cells.

Neurosteroids are produced in the brain independently of peripheral endocrine glands to act locally in the nervous system. They exert potent promnesic effects and play significant roles in mental health-related disorders. In part, neurosteroids act by affecting ligand-gated ion channels and metabotropic receptors through rapid non-genomic processes. We have previously demonstrated that neurosteroids also affect synaptic transmission presynaptically in the CA1 region of the hippocampus. Here we describe the effects of the most abundant neurosteroid in the rodent brain, pregnenolone sulfate (PregS), on signal processing in the dentate subfield of the hippocampus. We show that PregS acts presynaptically at low concentrations (300 nM) to enhance paired-pulse facilitation (PPF) in perforant pathway terminals on dentate granule cells. Similar effects were found with two steroid sulfatase inhibitors demonstrating a potential contribution of endogenous steroids to dentate synaptic plasticity. This enhanced presynaptic facilitation paradoxically increases paired-pulse inhibition (PPI) at short interpulse intervals. Based on these data, a model of dentate gyrus circuit interactions is proposed for the presynaptic action of PregS on the filtering dynamics of the dentate subfield at frequencies similar to those of the endogenous signals from the entorhinal cortex. These modeling studies are consistent with experimental measurements demonstrating positive modulation by PregS at low frequencies and negative modulation at high frequencies. These studies show an important role for the presynaptic action of neurosteroids in modulating input signals to the hippocampus.

Animals↗
PubMed2005

Synchronized network activity in developing rat hippocampus involves regional hyperpolarization-activated cyclic nucleotide-gated (HCN) channel function.

The principal form of synchronized network activity in neonatal hippocampus consists of low frequency 'giant depolarizing potentials' (GDPs). Whereas contribution of both GABA and glutamate to their generation has been demonstrated, full understanding of the mechanisms underlying these synchronized activity bursts remains incomplete. A contribution of the h-current, conducted by HCN channels, to GDPs has been a topic of substantial interest. Here we focus on HCN1, the prevalent HCN channel isoform in neonatal hippocampus, and demonstrate an HCN1 spatiotemporal expression pattern in both CA3 principal cells and interneurons that correlates with the developmental profile of GDPs. Abrogation of HCN physiological function in CA3, via the selective I(h)-blocker ZD7288, disrupts GDP generation. Furthermore, ZD7288 specifically abolishes spontaneous bursting of the CA3 pyramidal cells at frequencies typical of GDPs without major influence on interneuronal firing. These findings support a pivotal role for HCN channels expressed by CA3 neurons, and particularly CA3 pyramidal cells, in GDP-related network synchronization.

Animals↗
PubMed2004

Alcohol enhances GABAergic transmission to cerebellar granule cells via an increase in Golgi cell excitability.

Alcohol intoxication alters coordination and motor skills, and this is responsible for a significant number of traffic accident-related deaths around the world. Although the precise mechanism of action of ethanol (EtOH) is presently unknown, studies suggest that it acts, in part, by interfering with normal cerebellar functioning. An important component of cerebellar circuits is the granule cell. The excitability of these abundantly expressed neurons is controlled by the Golgi cell, a subtype of GABAergic interneuron. Granule cells receive GABAergic input in the form of phasic and tonic currents that are mediated by synaptic and extrasynaptic receptors, respectively. Using the acute cerebellar slice preparation and patch-clamp electrophysiological techniques, we found that ethanol induces a parallel increase in both the frequency of spontaneous IPSCs and the magnitude of the tonic current. EtOH (50 mm) did not produce this effect when spontaneous action potentials were blocked with tetrodotoxin. Recordings in the loose-patch cell-attached configuration demonstrated that ethanol increases the frequency of spontaneous action potentials in Golgi cells. Taken together, these findings indicate that ethanol enhances GABAergic inhibition of granule cells via a presynaptic mechanism that involves an increase in action potential-dependent GABA release from Golgi cells. This effect is likely to have an impact on the flow of information through the cerebellar cortex and may contribute to the mechanism by which acute ingestion of alcoholic beverages induces motor impairment.

Alcoholic Intoxication↗
PubMed2004

Treatment of rats with antipsychotic drugs: lack of an effect on brain N-acetyl aspartate levels.

BACKGROUND: Proton magnetic resonance spectroscopy (1H-MRS) studies of schizophrenia suggest an effect of the disease or of antipsychotic medications on brain N-acetyl aspartate (NAA), a marker of neuronal viability. We studied in the rat the effect of antipsychotic drugs on NAA in several brain regions where NAA reductions have been reported in chronically medicated patients with schizophrenia. METHODS: Three groups of nine rats each were treated with haloperidol (6 mg/kg/day), clozapine (70 mg/kg/day) and vehicle for 6 weeks and were sacrificed. Concentrations of NAA were determined by high-performance liquid chromatography (HPLC) from the following brain regions: cortex, striatum, thalamus, hippocampus and cerebellum. RESULTS: Mixed-factorial ANOVA of NAA concentrations revealed no significant effect of drug group [F(2, 24) = 0.034; p = 0.966] or a group by brain region interaction [F(8, 44) = 0.841; p = 0.572]. There was a significant main effect of region [F(4, 21) = 6.104; p = 0.002] with higher NAA in the cortex. CONCLUSIONS: These results are consistent with the only other study of the effect of typical and atypical antipsychotic drugs on NAA in the rat brain. The well-documented lower NAA in chronically treated schizophrenia patients is probably not a simple effect of antipsychotic medications.

Animals↗
PubMed2004

Fetal alcohol exposure alters neurosteroid levels in the developing rat brain.

Neurosteroids are modulators of neuronal function that may play important roles in brain maturation. We determined whether chronic prenatal ethanol exposure altered neurosteroid levels in the developing brain. Rat dams were exposed to: (i) a 5% ethanol-containing liquid diet that produces peak maternal blood alcohol levels near the legal intoxication limit (approximately 0.08 g/dL); (ii) an isocaloric liquid diet containing maltose-dextrin instead of ethanol with pair-feeding; (iii) rat chow ad libitum. Neurosteroid levels were assessed in offspring brains using radioimmunoassay or gas chromatography-mass spectrometry techniques. A prenatal ethanol exposure-induced increase in pregnenolone sulfate levels, but not dehydroepiandrosterone sulfate levels, was evident at the earliest time point studied (embryonic day 14). This effect lasted until post-natal day 5. Levels of other neurosteroids were assessed at embryonic day 20; pregnenolone levels, but not allopregnanolone levels, were elevated. Pregnenolone sulfate levels were not altered in the maternal brain. Neither pregnenolone nor pregnenolone sulfate levels were significantly altered in the fetal liver, placenta and maternal blood, indicating that the effect of ethanol is not secondary to accumulation of peripherally-produced steroids. Fetal ethanol exposure has been shown to decrease both cellular and behavioral responsiveness to neurosteroids, and our findings provide a plausible explanation for this effect.

Age Factors↗
PubMed2003

Evidence that nitric oxide regulates the acute effects of ethanol on rat N-methyl-D-aspartate receptors in vitro.

Acute exposure to ethanol has been shown to inhibit the function of N-methyl-D-aspartate (NMDA) receptors (NMDAR). However, the mechanism by which ethanol produces inhibition of NMDAR and the factors that modulate this effect are not completely understood. Nitric oxide (NO) is an important modulator of NMDAR function in the hippocampus. Therefore, we examined the effects of NO donors on the ethanol-induced inhibition of NMDAR. Primary cultures of hippocampal neurons were prepared from postnatal day 3 rats. After 7 days in culture, NMDAR currents were recorded by using whole-cell patch-clamp electrophysiological techniques. Effects of acute exposure to ethanol on these currents were assessed in the absence and presence of NO donors. We found that the NO donors 1-hydroxy-2-oxo-3-(N-ethyl-2-aminoethyl)-e-ethyl-1-triazene (NOC-12, 100 microM) and S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 500 microM) inhibit currents gated by 100 microM NMDA plus 0.5 microM glycine. The inhibitory effect of NOC-12 on NMDAR currents could not be observed when 100 microM of the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) was present. Importantly, it was found that ethanol inhibits NMDAR responses to a significantly lesser extent in the presence of these donors. Ethanol (65 mM) inhibited NMDAR responses by 42+/-2%. In the presence of NOC-12 or SNAP, ethanol inhibited NMDAR responses by 21+/-4% and 11+/-7%, respectively. The effect of NOC-12 on ethanol's actions on NMDAR currents was blocked by PTIO. Our results suggest that NO is a novel modulator of the acute effects of ethanol on NMDAR function.

Animals↗
PubMed2003

Alcohol potently inhibits the kainate receptor-dependent excitatory drive of hippocampal interneurons.

Kainate receptors (KA-Rs) are members of the glutamate-gated family of ionotropic receptors, which also includes N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. KA-Rs are important modulators of interneuron excitability in the CA1 region of the hippocampus. Activation of these receptors enhances interneuron firing, which robustly increases spontaneous inhibitory postsynaptic currents in pyramidal neurons. We report here that ethanol (EtOH) potently inhibits this KA-R-mediated effect at concentrations as low as those that can be achieved in blood after the ingestion of just 1-2 drinks (5-10 mM). Pressure application of kainate, in the presence of AMPA and NMDA receptor antagonists, evoked depolarizing responses in interneurons that triggered repetitive action potential firing. EtOH potently inhibited these responses to a degree that was sufficient to abolish action potential firing. This effect appears to be specific for KA-Rs, as EtOH did not affect action potential firing triggered by AMPA receptor-mediated depolarizing responses. Importantly, EtOH inhibited interneuron action potential firing in response to KA-R activation by synaptically released glutamate, suggesting that our findings are physiologically relevant. KA-R-dependent modulation of glutamate release onto pyramidal neurons was not affected by EtOH. Thus, EtOH increases excitability of pyramidal neurons indirectly by inhibiting the KA-R-dependent drive of gamma-aminobutyric acid (GABA)ergic interneurons. We postulate that this effect may explain, in part, some of the paradoxical excitatory actions of this widely abused substance. The excitatory actions of EtOH may be perceived as positive by some individuals, which could contribute to the development of alcoholism.

Animals↗
PubMed2003

Cocaine inhibits 5-HT3 receptor function in neurons from transgenic mice overexpressing the receptor.

Studies have shown that cocaine alters the function of recombinant 5-HT(3) receptors and that behavioral responses to cocaine are affected by 5-HT(3) receptor ligands. However, the actions of cocaine on brain 5-HT(3) receptors have not been characterized because these receptors are not abundantly expressed in most neuronal populations. We examined the effect of cocaine on 5-HT(3) receptor function in cultured hippocampal neurons from transgenic mice overexpressing the receptor. Cocaine competitively inhibited 5-HT(3) receptors with an IC(50) of approximately 4 microM, indicating that brain 5-HT(3) receptors are important targets for the actions of this commonly abused substance.

Animals↗