Retinoic acid prevents synaptic deficiencies induced by alcohol exposure during gastrulation in zebrafish embryos

In this study, we examined the effects of alcohol exposure during gastrulation on zebrafish embryos, specifically focusing on excitatory synaptic activity associated with neurons (Mauthner cells) that are born during gastrulation. Furthermore, we determined whether co-treatment of alcohol and retinoic acid (RA) could prevent the effects of alcohol exposure during gastrulation. We exposed zebrafish embryos to ethanol (150mM), RA (1nM), or a combination of RA (1nM) plus ethanol (150mM) for 5.5h from 5.25h post fertilization (hpf) to 10.75 hpf (gastrulation). Ethanol treatment resulted in altered hatching rates, survivability and body lengths. Immunohistochemical analysis of Mauthner cells (M-cells) suggested that ethanol treatment resulted in smaller M-cell bodies and thinner axons, while electrophysiological recordings of AMPA miniature excitatory postsynaptic currents (mEPSCs) associated with M-cells showed that ethanol treated animals had a significantly reduced mEPSC frequency. Other mEPSC parameters such as amplitude, rise times and decay kinetics were not altered by exposure to alcohol. Locomotor studies showed that ethanol treatment resulted in altered C-bend escape responses. For instance, the C-bends of alcohol-treated fish were larger than control embryos. Thus, ethanol treatment during gastrulation altered a range of features in embryonic zebrafish. Importantly, co-treatment with RA prevented all of the effects of ethanol including survivability, body length, M-cell morphology, AMPA mEPSC frequency and escape response movements. Together these findings show that ethanol exposure during the brief period of gastrulation has a significant effect on neuronal morphology and activity, and that this can be prevented with RA co-treatment.

Three native somatostatin isoforms differentially affect membrane voltage-sensitive ion currents in goldfish somatotrophs

Message encoding for three isoforms of somatostatin (SS) peptides, SS-14, goldfish brain (gb)SS-28 and [Pro²]SS-14, are expressed in goldfish hypothalamus and pituitary tissues. All three native goldfish SSs are active in reducing basal and stimulated growth hormone (GH) responses in cultured goldfish pituitary cells, although with different potencies and efficacies. In the present study, we examined the effects of these three endogenous SSs on electrophysiological properties of goldfish somatotrophs and their physiological relevance. Voltage-sensitive K+ , Ca²+ and Na+ channels in identified goldfish somatotrophs in primary culture were isolated using whole-cell, amphotericin B-perforated patch-clamp techniques. None of the three SSs affected Na+ currents but all three SSs increased maximal K+ current magnitude, with SS-14 being the most effective. [Pro²]SS14 did not affect Ba²+ currents through voltage-sensitive Ca²+ channels but SS14 decreased the magnitude of early and late Ba²+ currents, whereas gbSS-28 reduced that of the late Ba²+ current. Under current-clamp conditions, SS14 and gbSS28 attenuated evoked action potential magnitudes by 34% and 18%, respectively, although [Pro²]SS14 had no effects. However, all three SSs decreased basal intracellular Ca²+ levels ([Ca²+ ](i)) and suppressed basal GH release. These data suggest that, although the ability of SS-14 and gbSS-28 to decrease basal [Ca²+](i) and GH release can be explained, at least in part, by their attenuating effects on cell excitability and current flow through voltage-sensitive Ca²+ channels, [Pro²]SS14-induced reduction in GH responses and [Ca²+](i) cannot be explained by changes in Ca²+ channel properties.

Protein kinase A modulates A-type potassium currents of larval zebrafish (Danio rerio) white muscle fibres

AIMS

Potassium (K(+)) channels are involved in regulating cell excitability and action potential shape. To our knowledge, very little is known about the modulation of A-type K(+) currents in skeletal muscle fibres. Therefore, we sought to determine whether K(+) currents of zebrafish white skeletal muscle were modulated by protein kinase A (PKA).

METHODS

Pharmacology and whole-cell patch clamp were used to examine A-type K(+) currents and action potentials associated with zebrafish white skeletal muscle fibres.

RESULTS

Activation of PKA by a combination of forskolin + 3-isobutyl-1-methylxanthine (Fsk + IBMX) decreased the peak current density by approximately 60% and altered the inactivation kinetics of A-type K(+) currents. The specific PKA inhibitor H-89 partially blocked the Fsk + IBMX-induced reduction in peak current density, but had no effect on the change in decay kinetics. Fsk + IBMX treatment did not shift the activation curve, but it significantly reduced the slope factor of activation. Activation of PKA by Fsk + IBMX resulted in a negative shift in the V(50) of inactivation. H-89 prevented all Fsk + IBMX-induced changes in the steady-state properties of K(+) currents. Application of Fsk + IBMX increased action potential amplitude, but had no significant effect on action potential threshold, half width or recovery rate, when fibres were depolarized with single pulses, paired pulses or with high-frequency stimuli.

CONCLUSION

PKA modulates the A-type K(+) current in zebrafish skeletal muscle and affects action potential properties. Our results provide new insights into the role of A-type K(+) channels in muscle physiology.

Limits to the development of fast neuromuscular transmission in zebrafish

Zebrafish embryos have small and slow miniature end-plate currents (mEPCs), whereas only a few days later larval mEPCs are an order of magnitude larger and faster, being among the fastest of all neuromuscular synapses. To identify the bases for these changes we compared, in embryos and larvae, the properties and distributions of acetylcholine (ACh) receptors (AChRs) and acetylcholinesterase (AChE) as well as the ultrastructure of the developing neuromuscular junctions (NMJs). To mimic synaptic release, patches of muscle membrane were exposed briefly (for 1 ms) to a saturating concentration (10 mM) of ACh. The AChR deactivation kinetics were twice as slow in embryos compared with larvae. In both embryos and larvae, AChRs demonstrated open channel block by millimolar ACh, and this was detected during mEPCs, indicating that a high concentration of ACh is released at immature and mature NMJs. AChR and AChE distributions were compared using the selective fluorescently conjugated labels alpha-bungarotoxin and fasciculin 2, respectively. In larvae, punctate AChR clusters were detected whereas junctional AChE staining was less intense than that found at adult NMJs. Transmission electron microscopy revealed immature nerve endings in embryos that were closely juxtaposed to the surrounding muscle cells, whereas mature larval NMJs had a wider synaptic cleft with a conspicuous basal lamina over a limited region of synaptic contact. Our results indicate that ACh is released at high concentrations at immature NMJs, but its clearance is prolonged and the AChRs are dispersed, resulting in a slow mEPC time course until a mature cleft appears with densely packed faster AChRs and abundant AChE.

NMDA receptor regulation by Src kinase signalling in excitatory synaptic transmission and plasticity

Regulation of postsynaptic glutamate receptors is one of the main mechanisms for altering synaptic efficacy in the central nervous system. Recent studies have given insight into the upregulation of the NMDA receptor by Src family tyrosine kinases, which bind to scaffolding proteins in the NMDA receptor complex. Src acts as a common step in signalling cascades that link G-protein-coupled receptors with protein kinase C via the intermediary cell-adhesion kinase beta. This signalling to NMDA receptors is required for long-term potentiation in the CA1 region of the hippocampus.

CAKbeta/Pyk2 kinase is a signaling link for induction of long-term potentiation in CA1 hippocampus

Long-term potentiation (LTP) is an activity-dependent enhancement of synaptic efficacy, considered a model of learning and memory. The biochemical cascade producing LTP requires activation of Src, which upregulates the function of NMDA receptors (NMDARs), but how Src becomes activated is unknown. Here, we show that the focal adhesion kinase CAKbeta/Pyk2 upregulated NMDAR function by activating Src in CA1 hippocampal neurons. Induction of LTP was prevented by blocking CAKbeta/Pyk2, and administering CAKbeta/Pyk2 intracellularly mimicked and occluded LTP. Tyrosine phosphorylation of CAKbeta/Pyk2 and its association with Src was increased by stimulation that produced LTP. Finally, CAKbeta/Pyk2-stimulated enhancement of synaptic AMPA responses was prevented by blocking NMDARS, chelating intracellular Ca(2+), or blocking Src. Thus, activating CAKbeta/Pyk2 is required for inducing LTP and may depend upon downstream activation of Src to upregulate NMDA receptors.

Development of spontaneous glycinergic currents in the Mauthner neuron of the zebrafish embryo

We used whole cell and outside-out patch-clamp techniques with reticulospinal Mauthner neurons of zebrafish embryos to investigate the developmental changes in the properties of glycinergic synaptic currents in vivo from the onset of synaptogenesis. Miniature inhibitory postsynaptic currents (mIPSCs) were isolated and recorded in the presence of TTX (1 microM), kynurenic acid (1 mM), and bicuculline (10 microM) and were found to be sensitive to strychnine (1 microM). The mIPSCs were first observed in 26-29 h postfertilization (hpf) embryos at a very low frequency of approximately 0.04 Hz, which increased to approximately 0.5 Hz by 30-40 hpf, and was approximately 10 Hz in newly hatched (>50 hpf) larvae, indicating an accelerated increase in synaptic activity. At all embryonic stages, the amplitudes of the mIPSCs were variable but their means were similar ( approximately 100 pA), suggesting rapid formation of the postsynaptic matrix. The 20-80% rise times of mIPSCs in embryos were longer (0.6-1.2 ms) than in larvae (approximately 0.3 ms), likely due to slower diffusion of glycine at the younger, immature synapses. The mIPSCs decayed with biexponential (tau(off1) and tau(off2)) time courses with a half-width in 26-29 hpf embryos that was longer and more variable than in older embryos and larvae. In 26- to 29-hpf embryos, tau(off1) was approximately 15 ms and tau(off2) was approximately 60 ms, representing events of intermediate duration; but occasionally long mIPSCs were observed in some cells where tau(off1) was approximately 40 ms and tau(off2) was approximately 160 ms. In 30-40 hpf embryos, the events were faster, with tau(off1) approximately 9 ms and tau(off2) approximately 40 ms, and in larvae, events declined somewhat further to tau(off1) approximately 4 ms and tau(off2) approximately 30 ms. Point-per-point amplitude histograms of the decay of synaptic events at all stages resulted in the detection of similar single channel conductances estimated as approximately 45 pS, indicating the presence of heteromeric glycine receptors (GlyRs) from the onset of synaptogenesis. Fast-flow (1 ms) application of a saturating concentration of glycine (3-10 mM) to outside-out patches obtained at 26-29 hpf revealed GlyR currents that decayed biexponentially with time constants resembling the values found for intermediate and long mIPSCs; by 30-40 hpf, the GlyR currents resembled fast mIPSCs. These observations indicate that channel kinetics limited the mIPSC duration. Our data suggest that glycinergic mIPSCs result from the activation of a mixture of fast and slow GlyR subtypes, the properties and proportion of which determine the decay of the synaptic events in the embryos.

Regulation of neuregulin signaling by PSD-95 interacting with ErbB4 at CNS synapses

Neuregulins (NRGs) and their receptors, the ErbB protein tyrosine kinases, are essential for neuronal development, but their functions in the adult CNS are unknown. We report that ErbB4 is enriched in the postsynaptic density (PSD) and associates with PSD-95. Heterologous expression of PSD-95 enhanced NRG activation of ErbB4 and MAP kinase. Conversely, inhibiting expression of PSD-95 in neurons attenuated NRG-mediated activation of MAP kinase. PSD-95 formed a ternary complex with two molecules of ErbB4, suggesting that PSD-95 facilitates ErbB4 dimerization. Finally, NRG suppressed induction of long-term potentiation in the hippocampal CA1 region without affecting basal synaptic transmission. Thus, NRG signaling may be synaptic and regulated by PSD-95. A role of NRG signaling in the adult CNS may be modulation of synaptic plasticity.

Properties of miniature glutamatergic EPSCs in neurons of the locomotor regions of the developing zebrafish

As a first step in understanding the development of synaptic activation in the locomotor network of the zebrafish, we examined the properties of spontaneous, glutamatergic miniature excitatory postsynaptic currents (mEPSCs). Whole cell patch-clamp recordings were obtained from visually identified hindbrain reticulospinal neurons and spinal motoneurons of curarized zebrafish 1-5 days postfertilization (larvae hatch after the 2nd day of embryogenesis). In the presence of tetrodotoxin (TTX) and blockers of inhibitory receptors (strychnine and picrotoxin), we detected fast glutamatergic mEPSCs that were blocked by the AMPA/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). At positive voltages or in the absence of Mg(2+), a second, slower component of the mEPSCs was revealed that the N-methyl-D-aspartate (NMDA) receptor-selective antagonist DL-2-amino-5-phosphonovalerate (AP-5) abolished. In the presence of both CNQX and AP-5, all mEPSCs were eliminated. The NMDA component of reticulospinal mEPSCs had a large single-channel conductance estimated to be 48 pS. Larval AMPA/kainate and NMDA components of the mEPSCs decayed with biexponential time courses that changed little during development. At all stages examined, approximately one-half of synapses had only NMDA responses (lacking AMPA/kainate receptors), whereas the remainder of the synapses were composed of a mixture of AMPA/kainate and NMDA receptors. There was an overall increase in the frequency and amplitude of mEPSCs with an NMDA component in reticulospinal (but not motoneurons) during development. These results indicate that glutamate is a prominent excitatory transmitter in the locomotor regions of the developing zebrafish and that it activates either NMDA receptors alone at functionally silent synapses or together with AMPA/kainate receptors.

Recovery from open channel block by acetylcholine during neuromuscular transmission in zebrafish

At larval zebrafish neuromuscular junctions (NMJs), miniature end plate currents (mEPCs) recorded in vivo have an unusually fast time course. We used fast-flow application of acetylcholine (ACh) onto outside-out patches to mimic the effect of synaptic release onto small numbers of ACh receptor channels (AChRs). Positively charged ACh acted at hyperpolarized potentials and at millimolar concentrations as a fast ("flickering") open channel blocker of AChRs. Because of filtering, the open channel block resulted in reduced amplitude of single channel currents. Immediately after brief (1 msec) application (without significant desensitization) of millimolar ACh at hyperpolarized potentials, a slower, transient current appeared because of delayed reversal of the block. This rebound current depended on the ACh concentration and resembled in time course the mEPC. A simple kinetic model of the AChR that includes an open channel-blocking step accounted for our single channel results, as well as the experimentally observed slowing of the time course of mEPCs recorded at a hyperpolarized compared with a depolarized potential. Recovery from AChR block is a novel mechanism of synaptic transmission that may contribute in part at all NMJs.

In vivo recording from identifiable neurons of the locomotor network in the developing zebrafish

The zebrafish is a popular model for developmental studies due to its accessibility by cellular, molecular and genetic approaches. As a complement to these other methods, we have devised an exposed hindbrain/spinal cord preparation in the curarized zebrafish embryo and larva that permits intracellular labeling and patch clamp recording from individually identified sensory neurons, motoneurons and interneurons in vivo. Regular bursts of synaptic potentials and action potentials were observed under whole-cell current clamp in embryonic motoneurons and in some identified interneurons. Larval neurons showed prolonged depolarizations with synaptically driven bursts of action potentials. Frequent spontaneous synaptic potentials were observed and synaptic currents were effectively space clamped. It is thus feasible to study in vivo the properties of identifiable neurons of the developing locomotor network in the zebrafish, including their synaptic activity, firing patterns and interconnections.

Ionotropic and metabotropic activation of a neuronal chloride channel by serotonin and dopamine in the leech Hirudo medicinalis

1. Cl- channels on the pressure-sensitive (P) neuron in the leech are directly activated by synaptic release of serotonin (5-HT) and are indirectly stimulated by the cAMP second messenger pathway, suggesting an unusual dual regulation of the channels. We have investigated the mode of action of 5-HT and dopamine (DA) on a Cl- channel in adult P cells in culture by recording from cell-attached patches. 2. 5-HT increased Cl- channel activity only when included in the recording pipette and not when applied in the bath. 3. Pipette or, more effectively, bath application of DA led to an increase in Cl- channel activity. This effect was blocked by the potent and specific dopaminergic (DA1) receptor blocker, SCH-23390. 4. The stimulation by DA, but not by 5-HT, was also blocked by the cAMP-dependent protein kinase A (PKA) inhibitor Rp-cAMP and was mimicked by the membrane-permeant cAMP analogue dibutyryl cAMP (db-cAMP). 5. Our results show that 5-HT directly gates a Cl- channel that is also activated by DA via the cAMP pathway. This study demonstrates that a ligand-gated channel can be independently operated by another transmitter acting via a second messenger pathway.

The distribution and partial characterization of FMRFamide-related peptides in the salivary glands of the locust, Locusta migratoria

The distribution and partial characterization of FMRFamide-related peptides in the salivary glands of the locust, Locusta migratoria, were investigated by means of immunohistochemistry, radioimmunoassay and reversed-phase high performance liquid chromatography. Whole-mount preparations of glands stained positively against anti-FMRFamide antisera, and contained the equivalent of 837 +/- 80 fmol FMRFamide/gland pair, as determined by radioimmunoassay. FMRFamide-like immunoreactivity occurred in the processes of the transverse nerves of both the prothoracic and mesothoracic ganglia, but was not found in the salivary motoneurons 1 or 2 of the suboesophageal ganglion, both of which directly innervate the salivary glands via the salivary nerve 7b; nor was it found within the salivary nerve 7b itself, leading to the salivary glands. It was, however, found as a superficial nerve plexus on the surface of nerve 7 at the suboesophageal ganglion, but did not appear to extend to the salivary glands. The origin of this staining is unclear. High performance liquid chromatography of salivary gland tissue extracts, monitored by radioimmunoassay, revealed 4 peaks of immunoreactive material, 2 of which co-migrated with AFIRFamide and GQERNFLRFamide, previously isolated from the locust ventral nerve cord. These 2 synthetic peptides did not elevate basal levels of the second messengers cyclic AMP or cyclic GMP in the salivary glands.