Transmitter release from normal and degenerating locust motor nerve terminals

The comparative analysis of the myenteric plexus in pigeon and hen

Using the thiocholine method and histological techniques the myenteric plexuses of pigeon and hen were studied. Investigations revealed the presence of a nervous network in the wall of the small intestine of both animals. It consists of many nerve fibres crossing each other and creating meshes in a variety of shapes. The density of the network was different according to the species and to the parts of intestine. The myenteric plexus from the pigeon's duodenum is thicker (3.7-fold) than in the remaining part of the small intestine; in the hen this is approximately 2.2-fold thicker. The meshes of the duodenum in both species are smaller than in the jejunum and ileum. The results of histological investigations showed different localization of myenteric plexuses; in pigeon in the space between the circular and longitudinal muscle layers, and in hen within the circular muscle.

OsK2, a new selective inhibitor of Kv1.2 potassium channels purified from the venom of the scorpion Orthochirus scrobiculosus

A novel inhibitor of voltage-gated K(+) channels has been purified to homogeneity from the venom of the black scorpion Orthochirus scrobiculosus. This toxin, named OsK2, has been characterized as a 28-residue peptide, containing six conserved cysteine residues and was shown to be a potent and selective blocker of Kv1.2 channels (K(d) = 97 nM). OsK2 is the second member of the 13th subfamily of short-chain K(+) channel-blocking peptides known thus far and is therefore called alpha-KTx 13.2.

Cloning and structure of delta-latroinsectotoxin, a novel insect-specific member of the latrotoxin family: functional expression requires C-terminal truncation

The venom of the black widow spider (BWSV) (Latrodectus mactans tredecimguttatus) contains several potent, high molecular mass (>110 kDa) neurotoxins that cause neurotransmitter release in a phylum-specific manner. The molecular mechanism of action of these proteins is poorly understood because their structures are largely unknown, and they have not been functionally expressed. This study reports on the primary structure of delta-latroinsectotoxin (delta-LIT), a novel insect-specific toxin from BWSV, that contains 1214 amino acids. delta-LIT comprises four structural domains: a signal peptide followed by an N-terminal domain that exhibits the highest degree of identity with other latrotoxins, a central region composed of 15 ankyrin-like repeats, and a C-terminal domain. The domain organization of delta-LIT is similar to that of other latrotoxins, suggesting that these toxins are a family of related proteins. The predicted molecular mass and apparent mobility of the protein (approximately 130 kDa) encoded in the delta-LIT gene differs from that of native delta-LIT purified from BWSV (approximately 100 kDa), suggesting that the toxin is produced by proteolytic processing of a precursor. MALDI-MS of purified native delta-LIT revealed a molecular ion with m/z+ of 110916 +/- 100, indicating that the native delta-LIT is 991 amino acids in length. When the full-length delta-LIT cDNA was expressed in bacteria the protein product was inactive, but expression of a C-terminally truncated protein containing 991 residues produced a protein that caused massive neurotransmitter release at the locust neuromuscular junction at nanomolar concentrations. Channels formed in locust muscle membrane and artificial lipid bilayers by the native delta-LIT have a high Ca2+ permeability, whereas those formed by truncated, recombinant protein do not.

Stationary and non-stationary occurrences of miniature end plate potentials are well described as stationary and non-stationary Poisson processes in the mollusc Navanax inermis

Protractor muscles in the gastropod mollusc Navanax inermis exhibit typical spontaneous miniature end plate potentials with mean amplitude 1.71 +/- 1.19 (standard deviation) mV. The evoked end plate potential is quantized, with a quantum equal to the miniature end plate potential amplitude. When their rate is stationary, occurrence of miniature end plate potentials is a random, Poisson process. When non-stationary, spontaneous miniature end plate potential occurrence is a non-stationary Poisson process, a Poisson process with the mean frequency changing with time. This extends the random Poisson model for miniature end plate potentials to the frequently observed non-stationary occurrence. Reported deviations from a Poisson process can sometimes be accounted for by the non-stationary Poisson process and more complex models, such as clustered release, are not always needed.

Sub-miniature junction potentials in excitatory neuromuscular synapses of the locust

By recording miniature excitatory junction potentials (mejps) intracellularly at two points from a multiterminally innervated muscle fibre it is possible to select mejps whose amplitudes are not substantially affected by electrotonic decay. Many amplitude histograms of such selected mejps from untreated locust jumping muscle show a bimodal distribution with a high proportion of small-amplitude mejps (sub-mejps). Most amplitudes of excitatory junction potentials (ejps) resulting from the release of a single transmitter quantum correspond to the large-mode mejps. Tetanic nerve stimulation, in high [Mg2+]o without Ca2+, greatly reduces the proportion of sub-mejps. It is concluded that there are two modes of spontaneous transmitter release from the motor nerve endings.

Properties of miniature excitatory junctional currents at the locust nerve-muscle junction

1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na(+) (i.e. in normal medium, Ca(2+)-free medium and Cl(-)-free medium). M.e.j.c.s were also examined in isotonic Ca(2+) where the inward post-synaptic current was carried mainly by Ca(2+).2. In normal medium, mean m.e.j.c. amplitude = 2.34+/-0.05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) was tau(m.e.j.c.) = 2.62+/-0.11 msec (V(m) = -80 mV, T = 22 degrees C). Decay-time was not markedly changed in Ca(2+)-free or Cl(-)-free medium. tau(m.e.j.c.) approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. tau(m.e.j.c.) decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182.+/-24.8 mV change in V(m).5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca(2+)-free and Cl(-)-free solutions.6. The kinetics of junctional channels are altered in isotonic Ca(2+). M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j.c. = 0.74+/-0.03 nA. The decay time becomes markedly briefer, tau(m.e.j.c.) = 1.01+/-0.08 msec, indicating a reduction in mean channel life-time (V(m) = -80 mV, T = 22 degrees C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca(2+), thus producing a prolongation in mean tau(m.e.j.c.). This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of tau(m.e.j.c.) of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca(2+). The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

Collateral sprouting of insect motorneurons

The metathoracic extensor tibiae muscle of the cricket, Teleogryllus oceanicus, is innervated by two excitatory axons: a fast axon, which initiates large twitches to single stimuli, and a slow axon, which evokes minute twitches to single stimuli, but which, through facilitation and summation, evokes readily measurable tension to repetitive stimulation. The fast axon and the slow axon leave the metathoracic ganglia in different nerve roots, the fast axon through nerve 5 and the slow axon through nerve 3. The fast axon innervates muscle fibers in the middle of the extensor tibiae, and the slow axon innervates muscle fibers at the proximal and distal ends of the muscle. A central region of muscle fibers is innervated by only the fast axon. This region is flanked on either side by dually innervated fibers, fibers that receive both the fast and the slow axons. Fibers with only slow axon innervation are restricted to a wedge-shaped patch in the proximal extensor tibiae and a larger region in the most distal portion of the muscle. Sectioning nerve 5 containing the fast axon, or nerve 3 containing the slow axon, partially denervates the extensor tibiae. Functional transmission by the fast axon fails 7-10 days after nerve section. The innervation field of the intact motorneuron expands in a partially denervated muscle. The linear expansion rate of the slow axon field is about 20-40 micrometer per day. The enlarged slow field does not regress when axons regenerate to the muscle through nerve 5. The progressive expansion of the slow innervation field suggests that the expansion is due to collateral sprouting of slow axon terminals.