Development of Transient Outward Currents Coupled With Ca2+-Induced Ca2+Release Mediates Oscillatory Membrane Potential in Ascidian Muscle Cells

A mechanism for graded motor control encoded in the channel properties of the muscle ACh receptor

The larva of the invertebrate chordate Ciona intestinalis possesses only 36 striated muscle cells and lacks body segmentation. It can swim, however, like a vertebrate tadpole, and how its simple body achieves such sophisticated motor control remains puzzling. We found that muscle contractions in Ciona larvae are variable and can be changed by sensory stimuli, so that neuromuscular transmission can convert the variable neural inputs into graded muscle activity. We characterized the molecular nature of the nicotinic acetylcholine receptor (nAChR) at neuromuscular synapses. When heterologously expressed in Xenopus oocytes, this nAChR channel exhibited two biophysical features resembling vertebrate neuronal nAChRs rather than the muscle type: inward rectification and high Ca(2+) permeability. Both of these properties were abolished by a simple mutation at the channel pore in one of the non-α subunits, called BGDE3, so as to adopt the sequence of related subunits in vertebrates, γ and ε. In vivo exchange of native BGDE3 with this mutant severely disrupted graded motor control, producing instead sporadic all-or-none-like flexions. The graded nature of excitation-contraction (E-C) coupling in this organism is based on the traits of the nAChR channel pore, which confer fine controllability on such a coarse motor architecture.

Complex life forms may arise from electrical processes

There is still not an appealing and testable model to explain how single-celled organisms, usually following fusion of male and female gametes, proceed to grow and evolve into multi-cellular, complexly differentiated systems, a particular species following virtually an invariant and unique growth pattern. An intrinsic electrical oscillator, resembling the cardiac pacemaker, may explain the process. Highly auto-correlated, it could live independently of ordinary thermodynamic processes which mandate increasing disorder, and could coordinate growth and differentiation of organ anlage.

A glycine receptor is involved in the organization of swimming movements in an invertebrate chordate

BACKGROUND

Rhythmic motor patterns for locomotion in vertebrates are generated in spinal cord neural networks known as spinal Central Pattern Generators (CPGs). A key element in pattern generation is the role of glycinergic synaptic transmission by interneurons that cross the cord midline and inhibit contralaterally-located excitatory neurons. The glycinergic inhibitory drive permits alternating and precisely timed motor output during locomotion such as walking or swimming. To understand better the evolution of this system we examined the physiology of the neural network controlling swimming in an invertebrate chordate relative of vertebrates, the ascidian larva Ciona intestinalis.

RESULTS

A reduced preparation of the larva consisting of nerve cord and motor ganglion generates alternating swimming movements. Pharmacological and genetic manipulation of glycine receptors shows that they are implicated in the control of these locomotory movements. Morphological molecular techniques and heterologous expression experiments revealed that glycine receptors are inhibitory and are present on both motoneurones and locomotory muscle while putative glycinergic interneurons were identified in the nerve cord by labeling with an anti-glycine antibody.

CONCLUSIONS

In Ciona intestinalis, glycine receptors, glycinergic transmission and putative glycinergic interneurons, have a key role in coordinating swimming movements through a simple CPG that is present in the motor ganglion and nerve cord. Thus, the strong association between glycine receptors and vertebrate locomotory networks may now be extended to include the phylum chordata. The results suggest that the basic network for 'spinal-like' locomotion is likely to have existed in the common ancestor of extant chordates some 650 M years ago.

Cell contact induces multiple types of electrical excitability from ascidian two-cell embryos that are cleavage arrested and contain all cell fate determinants

Ascidian early embryonic cells undergo cell differentiation without cell cleavage, thus enabling mixture of cell fate determinants in single cells, which will not be possible in mammalian systems. Either cell in a two-cell embryo (2C cell) has multiple fates and develops into any cell types in a tadpole. To find the condition for controlled induction of a specific cell type, cleavage-arrested cell triplets were prepared in various combinations. They were 2C cells in contact with a pair of anterior neuroectoderm cells from eight-cell embryos (2C-aa triplet), with a pair of presumptive notochordal neural cells (2C-AA triplet), with a pair of presumptive posterior epidermal cells (2C-bb triplet), and with a pair of presumptive muscle cells (2C-BB triplet). The fate of the 2C cell was electrophysiologically identified. When two-cell embryos had been fertilized 3 h later than eight-cell embryos and triplets were formed, the 2C cells became either anterior-neuronal, posterior-neuronal or muscle cells, depending on the cell type of the contacting cell pair. When two-cell embryos had been fertilized earlier than eight-cell embryos, most 2C cells became epidermal. When two- and eight-cell embryos had been simultaneously fertilized, the 2C cells became any one of three cell types described above or the epidermal cell type. Differentiation of the ascidian 2C cell into major cell types was reproducibly induced by selecting the type of contacting cell pair and the developmental time difference between the contacting cell pair and 2C cell. We discuss similarities between cleavage-arrested 2C cells and vertebrate embryonic stem cells and propose the ascidian 2C cell as a simple model for toti-potent stem cells.

GABAergic synaptic transmission modulates swimming in the ascidian larva

To examine the role of the amino acid GABA in the locomotion of basal chordates, we investigated the pharmacology of swimming and the morphology of GABA-immunopositive neurones in tadpole larvae of the ascidians Ciona intestinalis and Ciona savignyi. We verified that electrical recording from the tail reflects alternating muscle activity during swimming by correlating electrical signals with tail beats using high-speed video recording. GABA reversibly reduced swimming periods to single tail twitches, while picrotoxin increased the frequency and duration of electrical activity associated with spontaneous swimming periods. Immunocytochemistry for GABA revealed extensive labelling throughout the larval central nervous system. Two strongly labelled regions on either side of the sensory vesicle were connected by an arc of labelled fibres, from which fibre tracts extended caudally into the visceral ganglion. Fibre tracts extended ventrally from a third, more medial region in the posterior sensory vesicle. Two rows of immunoreactive cell bodies in the visceral ganglion extended neurites into the nerve cord, where varicosities were seen. Thus, presumed GABAergic neurones form a network that could release GABA during swimming that is involved in modulating the time course and frequency of periods of spontaneous swimming. GABAergic and motor neurones in the visceral ganglion could interact at the level of their cell bodies and/or through the presumed GABAergic fibres that enter the nerve cord. The larval swimming network appears to possess some of the properties of spinal networks in vertebrates, while at the same time possibly showing a type of peripheral innervation resembling that in some protostomes.

The Edinger-Westphal nucleus of the juvenile rat contains transient- and repetitive-firing neurons

Classically, the Edinger-Westphal nucleus is described as containing neurons controlling accommodation and pupillary constriction via projections to the ciliary ganglion. However, in several species including rat, some Edinger-Westphal neurons have ascending or descending CNS projections suggesting that the Edinger-Westphal nucleus might also have non-ocular functions. To further characterize the function of this nucleus we studied the electrophysiological properties of Edinger-Westphal neurons in a slice preparation from juvenile rats. The position of the Edinger-Westphal nucleus was determined using an immunohistochemical procedure directed at the peptide Urocortin, which is expressed in Edinger-Westphal neurons. Passive and active membrane responses were investigated and two different neuron types were identified. One type had a transient firing response to 400 ms depolarizing current pulses and one type had a repetitive firing response. Transient-firing neurons had an outward rectifying response inhibiting firing, possibly due to slowly inactivating I(D)-like potassium channels since low concentrations (200 microM) of the potassium channel blocker 4-aminopyridine elicited repetitive firing. In all neurons, low threshold Ca(2+) spikes were seen and these were blocked by nickel(II) chloride hexahydrate, suggesting that they are mediated via low voltage-activated Ca(2+) channels. Some biocytin-labeled neurons had axons or axonal collaterals projecting laterally or dorsally, suggesting possible non-ocular targets. In conclusion, the rat Edinger-Westphal nucleus contains two separate types of neurons with distinct electrophysiological properties.