Andrew N. Spencer

The physiology of a coelenterate neuromuscular synapse

Summary1.Postsynaptic potentials can be recorded intracellularly from epitheliomuscular cells overlying the inner nerve-ring in the medusaPolyorchispenicillatus (Cnidaria, Hydrozoa) (Fig. 1). These postsynaptic potentials lead to the generation of muscle action potentials which propagate through the swimming-muscle sheets. It is the swimming motor neuron network that innervates this epithelium. An alternative motor pathway is present that involves a network of small, multipolar neurons that is interpolated between swimming motor neurons and overlying epithelial cells (Fig. 2).2.That the postsynaptic potentials recorded are due to release of a chemical transmitter is supported by the following evidence: (a) PSPs have a constant delay

Multiple Shaker potassium channels in a primitive metazoan

(\bar x = 3.2{\text{ms)}}

Neuronal control of locomotion in hydrozoan medusae

following the presynaptic spike (Figs. 3, 4); (b) high Mg++ concentrations reduce the amplitude of PSPs and eventually block transmission (Fig. 10); (c) there is no electrical coupling between presynaptic neurons and postsynaptic epithelial cells.3.There is an inverse relationship between the duration of presynaptic action potentials and the amplitude of PSPs (Figs. 5, 6). The duration of presynaptic action potentials is a reflection of the degree of synchrony of spiking in the motor network so that short duration motor spikes are associated with synchronous firing. In such cases, the simultaneous release of transmitter substance at a number of neighbouring synapses will cause rapid temporal summation of PSPs in postsynaptic cells. Similarly, long duration presynaptic spikes are associated with asynchronous transmitter release and consequently with small PSPs (Figs. 8, 9).4.Changes in PSP amplitude are seen in all postsynaptic cells of a localised region (Fig. 7).5.The muscle action potential can be separated into two components, the velar and subumbrellar action potentials (Fig. 11). This biphasic nature of muscle action potentials recorded in the synaptic region results from all-or-none action potentials that are generated at the velar and subumbrellar borders of this region conducting back electrotonically. These action potentials made to conduct antidromically towards the synapses by electrical stimulation of the muscle sheets decrement as they travel through the synaptic region (Fig. 12). The nature of electrical coupling between epithelial cells in the synaptic non-muscular region and the muscle sheets proper must be different.6.Larger amplitude PSPs are associated with muscle action potentials that follow with a shorter latency, and that have the two components (velar and subumbrellar) following each other more rapidly (Figs. 5, 9).7.Action potentials in the motor network are brought into phase as they conduct around the margin. This leads to more synchronous activation of synapses and hence larger PSPs at regions distant from the initiation site of the motor spike. The resulting decrease in the latency of muscle APs at these distant sites will automatically compensate for the conduction delay of motor spikes.

The action potential and contraction in subumbrellar swimming muscle ofPolyorchis penicillatus (Hydromedusae)

The hydromedusa Polyorchis penicillatus is a good model system to study neurotransmission in coelenterates. Using a radioimmunoassay for the peptide sequence Arg-Phe-NH2 (RFamide), two peptides have now been purified from acetic acid extracts of this medusa. The structure of one of these peptides was established as pyroGlu-Leu-Leu-Gly-Gly-Arg-Phe-NH2, and was named Pol-RFamide. This peptide belongs to the same peptide family as a recently isolated neuropeptide from sea anemones (pyroGlu-Gly-Arg-Phe-NH2). Using antisera to Pol-RFamide, the peptide was found to be exclusively localized in neurones of Polyorchis, among them neurones associated with smooth-muscle fibres. This suggests that Pol-RFamide might be a transmitter or modulator at neuromuscular junctions.

Organization of Conducting Systems in “Simple” Invertebrates: Porifera, Cnidaria and Ctenophora

Voltage-gated potassium channels are critical elements in providing functional diversity in nervous systems. The diversity of voltage-gated K+ channels in modern triploblastic metazoans (such as mollusks, arthropods and vertebrates) is provided primarily by four gene subfamilies (Shaker, Shal, Shab, and Shaw), but there has been no data from the ancient diploblastic metazoans until now. Diploblasts, represented by jellyfish and other coelenterates, arose during the first major metazoan radiation and are the most structurally primitive animals to have true nervous systems. By comparing the K+ channels of diploblasts and triploblasts, we may determine the fundamental set of K+ channels present in the first nervous systems. We now report the isolation of two Shaker subfamily cDNA clones, jShak1 and jShak2, from the hydrozoan jellyfish Polyorchis penicillatus (Phylum Cnidaria). JShak1 and jShak2 express transient outward currents in Xenopus oocytes most similar to Shaker currents from Drosophila in their rates of inactivation and recovery from inactivation. The finding of multiple Shaker subfamily genes is significant in that multiple Shaker genes also exist in mammals. In Drosophila, multiple Shaker channels are also produced, but by a mechanism of alternative splicing. Thus, the Shaker K+ channel subfamily had an established functional identity prior to the first major radiation of metazoans, and multiple forms of Shaker channels have been independently selected for in a wide range of metazoans.

Neuronal mechanisms of a hydromedusan shadow reflex

SummaryAn antiserum to the sequence Arg-Phe-amide (RFamide) was used to stain the nervous systems of various physonectid siphonophores. In the stem of Nanomia bijuga, this antiserum stained an ectodermal nerve net, which was interrupted, at regular intervals, by transverse collars of neurons. Injection of Lucifer yellow into the “giant axon” of the stem showed that this axon was dye-coupled to an ectodermal nerve net that resembled the RFamide-positive network. Ectodermal nets of neurons were also found in the pneumatophore, gastrozooids, tentacles and tentilla. At the junctions of the pneumatophore, the gastrozooids, the dactylozooids and the gonozooids with the stem, and at the junctions of tentacles and tentilla, collars or rings of neurons occurred. The stem was connected to the phyllozooids and nectophores by muscular lamellae, which were bordered by chains of neurons. At the margin of the nectophores, an immunoreactive nerve ring was found. Connected to this ring and located in the“seitliche Zapfen” (“sidely-located patche”), were two agglomerations of nerve cells. On the upper side of the bell margin, positioned at 90° relative to the “seitliche Zapfen”, a delta-shaped neuronal structure was found. This structure was connected to the nerve ring and was associated with a muscle, which ran a short distance along the exumbrellar surface.The nervous systems of Agalma elegans, Forskalia edwardsi, Forskalia leuckarti and Halistemma rubrum resembled that of Nanomia bijuga in all major respects.

Chemical and Electrical Synaptic Transmission in the Cnidaria

Summary1.The swimming control systems of 13 hydromedusan species were examined electrophysiologically and morphologically. Despite obvious differences in the structure, behavior and life style of these medusae, the basic organization of swimming system components is similar.2.Motor neurons that activate swimming muscles are located in the inner nerve-ring comprising electrically-coupled condensed networks (Figs. 2, 3). Individual neurons of these networks are of far larger diameter than other neurons of the nerve-rings (Figs. 4–6).3.Spontaneous activity persists in the swim motor neuron networks in seawater containing excess Mg++ suggesting that the network may perform a pacemaker function.4.The swimming muscle sheet includes circular, striated epitheliomuscular cells of the subumbrella and velum, and an interposed non-muscular epithelial region which overlies the inner nerve-ring (Fig. 1). Gap junctions are common throughout this tissue sheet (Figs. 13, 14, 18). Electrical(Fig. 10) and dye-coupling (Fig. 3) of cells in this sheet suggests that direct current spread between myocytes is important in transmission of excitation throughout the subumbrella.5.Recordings from epithelial cells immediately adjacent to swim motor neurons reveal graded potentials presumably of synaptic origin (Figs. 2, 10). The synaptic potentials, and muscle action potentials, are blocked in seawater containing excess Mg++. Synaptic contacts were observed between swim motor neurons and overlying epithelial cells throughout the inner nerve-rings of all medusae examined (Figs. 19–21).

Isolation of the neuropeptide <Glu-Trp-Leu-Lys-Gly-Arg-Phe-NH2 (Pol-RFamide II) from the hydromedusa Polyorchis penicillatus

Summary1.The action potentials of Polyorchis ‘swimming’ muscle cells have a characteristic square waveform.2.The depolarizing phase of the action potential is dependent on both Na+ and Ca+ + influx since i) sodium ions must be present in the bathing solution for generation of an action potential; ii) the amplitude of action potentials is increased by increasing the Ca+ + concentration of the bathing medium; iii) cobalt ions rapidly block the action potential.3.The action potential is resistant to tetrodotoxin in the bathing solution.4.The durations of action potentials are directly proportional to the size of jellyfish.5.The times to peak tension of contractions in the subumbrellar muscle sheet are directly related to the durations of the action potentials producing those contractions.6.Tetraethylammonium ions increase the duration of the plateau phase of action potentials, and increase the duration of the contractions.