Transmitter release from presynaptic terminals of electric organ: inhibition by the calcium channel antagonist omega Conus toxin

Cholinergic synaptosomes from electroplax of the ray Ommata discopyge release both ATP and ACh when depolarized with high K+ concentration in the presence of Ca2+. Others have shown that the ATP and ACh are released in the molar ratio found in isolated synaptic vesicles. Thus, it is assumed that the release of ATP reflects exocytosis of synaptic vesicles, and that transmitter release can be indirectly monitored by assaying ATP release. We present further evidence for this assumption and examine the effects of presynaptic neurotoxins on this ATP release. As expected for transmitter release, we find that depolarization-evoked ATP release is supported by Sr2+ and Ba2+ and is inhibited by the Ca channel antagonists Co2+ and Mn2+. Likewise, the presynaptic toxins omega-CmTX and omega-CgTX, omega peptides from the venom of the marine snails Conus magus and Conus geographus, respectively, inhibit 80% of the depolarization-evoked ATP release. Half-maximal inhibition of ATP release occurs with approximately 0.5 microM of either toxin. The toxins' effects are reversible, and when toxin is washed away, the time dependence of recovery of release is approximately first order and half complete within 40 min with omega-CmTX and 15 min with omega-CgTX. The Ca2+ ionophore A23187 induces Ca2+-dependent ATP release from resting synaptosomes. As would be expected of a Ca channel antagonist, omega-CmTX does not affect this ionophore-induced release. Leptinotarsin-d (LPTd), a putative Ca channel agonist from the Colorado potato beetle, evokes Ca2+-dependent ATP release from resting synaptosomes. omega-CmTX does not block LPTd-evoked release of ATP, which suggests that omega-CmTX and LPTd act at different sites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Post-synaptic potentiation: interaction between quanta of acetylcholine at the skeletal neuromuscular synapse.

1. Post-synaptic responses to acetylcholine (ACh) released from nerve terminals and from iontophoretic micropipettes were investigated in skeletal muscle fibres of the snake. Each fibre has a compact end-plate consisting of fifty to seventy synaptic boutons. The fibres were voltage clamped, and synaptic currents were recorded from visually identified end-plates. 2. When acetylcholinesterase (AChE) is inhibited, a potentiating interaction is observed between two or more quanta that are released close to each other from a synaptic bouton and act upon partially overlapping postsynaptic areas. The potentiation is expressed as a prolongation of the synaptic current. This potentiation also occurs under normal conditions of release when about 300 quanta are distributed over the entire end-plate, so thet the presynaptic release sites are separated by an average of 2 mum. An analogous potentiating interaction is observed when micropipettes, closely apposed to the subsynaptic membrane, substitute for quantal release sites. ACh from one pipette potentiates the response to ACh from another pipette less than 2 mum away. 3. In contrast, with AChE fully active no post-synaptic potentiation is seen when the normal complement of quanta is released over the entire end-plate. The time course of the synaptic currents in response to a single quantum or to 300 quanta is similar. It is concluded that functionally the quanta act independently of each other, because AChE isolates each quantum from its neighbours by limiting the lifetime of ACh and its lateral diffusion in the synaptic cleft. The estimated area over which a quantum normally acts is less than 2mum2. 4. Post-synaptic receptors are not saturated by the ACh in a quantum, since the peak of the quantal response adds linearly to the response produced by an appropriate background concentration of ACh from a pipette. This conclusion is supported by the observation that upon inhibition of AChE the peak amplitude of the quantal current response increases by about 20% with no change in its time to peak. 5. It is suggested that post-synaptic potentiation between quanta may play a role in signalling at synapses in which non-linear dose-response characteristics have been observed and where transmitter is not as repidly inactivated as the neuromuscular synapse.     

Synaptic excitation and inhibition resulting from direct action of acetylcholine on two types of chemoreceptors on individual amphibian parasympathetic neurones.

1. Synaptic transmission was studied in visually identified parasympathetic ganglion cells that modulate the heart beat of the mudpuppy Necturus maculosus).2. The brief pulse of acetylcholine (ACh) released from terminals of the vagus nerve after each impulse can produce two distinct post-synaptic responses in individual principal cells of the ganglion: (i) within a milli-second of release, ACh generates a rapid and strong excitatory post-synaptic potential (e.p.s.p.) that normally initiates a post-synaptic impulse; (ii) this excitation is usually followed by a slow hyperpolarizing inhibitory post-synaptic potential (i.p.s.p.) that lasts for several seconds. The magnitude and time course of the i.p.s.p. depends on the frequency and number of vagal stimuli. When the hydrolysis of ACh is inhibited by prostigmine, a train of nerve stimuli may be followed by an i.p.s.p. lasting half a minute or longer.3. The rapid e.p.s.p. and slow i.p.s.p. result from the direct action of ACh on two different types of chemoreceptors in the post-synaptic membrane of the principal cell. The e.p.s.p. can be preferentially blocked by the nicotinic antagonist dihydro-beta-erythroidine (5 x 10(-7)M), while the i.p.s.p. is selectively blocked by the muscarinic antagonist atropine (5 x 10(-9)M).4. Potentials resembling nerve-evoked e.p.s.p.s and i.p.s.p.s can be produced by iontophoretic release of ACh from micropipettes onto the post-synaptic membrane. Application of the muscarinic agonist bethanechol generates exclusively inhibitory responses.5. The reversal potential for the i.p.s.p. is about -105 mV, which is approximately the equilibrium potential for potassium (E(K)). When the external K(+) concentration is altered, the reversal potential for inhibition is shifted to the new value of E(K) as expected from the Nernst equation. Changes in the external Na(+) and Cl(-) concentrations have no appreciable effect on the reversal potential. Thus, the i.p.s.p. is the result of a conductance increase for K(+).6. The conductance change producing the i.p.s.p. is voltage sensitive. When the membrane potential is shifted from -40 to -60 mV, the i.p.s.p becomes larger and longer. Beyond -60 mV the inhibitory response decreases in proportion to the driving force on K(+) without any further change in time course.7. The inhibitory response produced by an iontophoretically applied pulse of bethanechol has a delayed onset of about 150 msec at 24 degrees C. The early portion of this response, including the delay, is proportional to t(3), where t is time. The proportionality factor (the apparent rate constant) decreases elevenfold when the temperature is lowered by 10 degrees C. This suggests that a multi-step process is involved in the activation of the conductance increase that leads to the inhibitory response. Inhibitory responses with similar kinetics were produced in heart muscles of the mudpuppy upon application of ACh.     

Functional profiling of neurons through cellular neuropharmacology

We describe a functional profiling strategy to identify and characterize subtypes of neurons present in a peripheral ganglion, which should be extendable to neurons in the CNS. In this study, dissociated dorsal-root ganglion neurons from mice were exposed to various pharmacological agents (challenge compounds), while at the same time the individual responses of >100 neurons were simultaneously monitored by calcium imaging. Each challenge compound elicited responses in only a subset of dorsal-root ganglion neurons. Two general types of challenge compounds were used: agonists of receptors (ionotropic and metabotropic) that alter cytoplasmic calcium concentration (receptor-agonist challenges) and compounds that affect voltage-gated ion channels (membrane-potential challenges). Notably, among the latter are K-channel antagonists, which elicited unexpectedly diverse types of calcium responses in different cells (i.e., phenotypes). We used various challenge compounds to identify several putative neuronal subtypes on the basis of their shared and/or divergent functional, phenotypic profiles. Our results indicate that multiple receptor-agonist and membrane-potential challenges may be applied to a neuronal population to identify, characterize, and discriminate among neuronal subtypes. This experimental approach can uncover constellations of plasma membrane macromolecules that are functionally coupled to confer a specific phenotypic profile on each neuronal subtype. This experimental platform has the potential to bridge a gap between systems and molecular neuroscience with a cellular-focused neuropharmacology, ultimately leading to the identification and functional characterization of all neuronal subtypes at a given locus in the nervous system.     

Acetylcholine receptors at neuromuscular synapses: phylogenetic differences detected by snake alpha-neurotoxins.

Phylogenetic differences in acetylcholine receptors from skeletal neuromuscular synapses of various species of snakes and lizards have been investigated, using the snake venom alpha-neurotoxins alpha-atratoxin (cobrotoxin) and alpha-bungarotoxin. The acetylcholine receptors of the phylogenetically primitive lizards, like those from all other vertebrates previously tested, are blocked by these alpha-neurotoxins. In contrast, receptors from snakes and advanced lizards are insensitive to one or both of the toxins. It is suggested that toxin-resistant acetylcholine receptors appeared early in the evolution of Squamata and preceded the appearance of alpha-neurotoxins.     

The rhodopsin-porphyropsin system in freshwater fishes—2 Turnover and interconversion of visual pigment prosthetic groups in light and darkness: Role of the pigment epithelium

The freshwater fish Scardinius erythrophthalmus L. has a mixture of rhodopsin and porphyropsin in its retina. In the light, the porphyropsin is converted into rhodopsin, which is reconverted into porphyropsin in the dark. The mechanism of this effect is investigated in the present paper. Experiments in which fish were injected with retinyl-11,12-³H₂ acetate showed that there is continual turnover of visual pigment prosthetic groups in fish kept in the dark. Therefore in darkness there exists a mechanism for exchanging the retinol-based group of rhodopsin with the 3-dehydroretinol-based prosthetic group of porphyropsin. Hitherto, the switch to porphyropsin had been puzzling, because it was supposed that, unless exposed to light, visual pigment molecules were stable entities in the living retina. Both retinol and 3dehydroretinol are stored in the pigment epithelium of Scardinius, amounting to at least three equivalents of the retinal visual pigments. It was found that the change to a porphyropsindominated retina in darkness was accompanied by a parallel change to a 3-dehydroretinoldominated pigment epithelium. In the light, the reverse process took place: while rhodopsin replaced porphyropsin in the retina, retinol replaced 3-dehydroretinol in the pigment epithelium. No changes were observed in the composition of “vitamins A” in the liver. Large injections of retinyl acetate did not prevent the conversion of rhodopsin into porphyropsin in the dark or under conditions of subdued lighting. It is concluded that the pigment epithelium determines the composition of retinal visual pigments by regulating the composition of its supplies of retinol and 3-dehyroretinol available for pigment synthesis. The possible basis of this regulatory process is discussed.     

Transduction heats in retinal rods: tests of the role of cGMP by pyroelectric calorimetry.

The sensory dark current of vertebrate retinal rods is believed to be controlled by light activation of a chain of coupled biochemical cycles that finally regulate the cationic conductance of the plasma membrane by hydrolytically reducing the level of cGMP in rod outer segment cytoplasm. The scheme has been tested by measuring heat production by live frog retinas when stimulated with sequences of light flashes of progressively increasing energy. Using pyroelectric poly(vinylidene 1,1-difluoride) detectors that simultaneously measure transretinal voltage and retinal temperature change, four heat effects assignable to known biochemical cycles in rods have been found. As the dark current shuts down after a flash causing 180-1800 rhodopsin photoisomerizations per rod, a heat burst, q1, raises the retinal temperature 1-2 microK. q1 is closely regulated in size and slightly precedes dark current shutdown. Isobutylmethylxanthine slows and enlarges q1, delaying the dark-current response. Increasing cytoplasmic Ca2+ stops the dark current without affecting q1. Although rod heat production is consistent with splitting of 1-3 microM of free cytoplasmic cGMP during transduction, the kinetics of the two processes do not match the predictions of current cGMP control models.     

Disulfide-Depleted Selenoconopeptides: Simplified Oxidative Folding of Cysteine-Rich Peptides

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μ-Conotoxins that differentially block sodium channels NaV1.1 through 1.8 identify those responsible for action potentials in sciatic nerve

Voltage-gated sodium channels (VGSCs) are important for action potentials. There are seven major isoforms of the pore-forming and gate-bearing α-subunit (Na(V)1) of VGSCs in mammalian neurons, and a given neuron can express more than one isoform. Five of the neuronal isoforms, Na(V)1.1, 1.2, 1.3, 1.6, and 1.7, are exquisitely sensitive to tetrodotoxin (TTX), and a functional differentiation of these presents a serious challenge. Here, we examined a panel of 11 μ-conopeptides for their ability to block rodent Na(V)1.1 through 1.8 expressed in Xenopus oocytes. Although none blocked Na(V)1.8, a TTX-resistant isoform, the resulting "activity matrix" revealed that the panel could readily discriminate between the members of all pair-wise combinations of the tested isoforms. To examine the identities of endogenous VGSCs, a subset of the panel was tested on A- and C-compound action potentials recorded from isolated preparations of rat sciatic nerve. The results show that the major subtypes in the corresponding A- and C-fibers were Na(V)1.6 and 1.7, respectively. Ruled out as major players in both fiber types were Na(V)1.1, 1.2, and 1.3. These results are consistent with immunohistochemical findings of others. To our awareness this is the first report describing a qualitative pharmacological survey of TTX-sensitive Na(V)1 isoforms responsible for propagating action potentials in peripheral nerve. The panel of μ-conopeptides should be useful in identifying the functional contributions of Na(V)1 isoforms in other preparations.