The accessibility of embryonic and adult neurons within invertebrate nervous systems has made them excellent subjects for neurobiological study. The ability to readily identify individual neurons, together with their great capacity for regeneration, has been especially beneficial to investigations of synapse formation and the specificity of neuronal connectivity. Many invertebrate neurons survive for long periods following isolation into primary cell culture. In addition, they readily extend new neuritic arbors and form electrical and chemical connections at sites of contact. Thus, cell culture approaches have allowed neuroscientists greater access to, and resolution of, events underlying neurite outgrowth and synaptogenesis. Studies of identified neuromuscular synapses of Helisoma have determined a number of signaling mechanisms involved in transsynaptic communication at sites of neuron-target contact. At these sites, both anterograde and retrograde signals regulate the transformation of growth cones into functional presynaptic terminals. We have found that specific muscle targets induce both global and local changes in neurotransmitter secretion and intracellular calcium handling. Here we review recent studies of cultured Helisoma synapses and discuss the mechanisms thought to govern chemical synapse formation in these identified neurons and those of other invertebrate species.

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Identified buccal motoneuron 19 (B19), isolated from the nervous system of Helisoma and plated into cell culture, is selective in synapse formation and requires contact with appropriate muscle targets before acquiring secretory competence. Action potential-regulated transmitter release was elevated specifically at neuritic arbors in contact with appropriate muscle fibers. In contrast, contact between B19 and inappropriate muscle targets failed to induce changes in presynaptic secretory properties. Spontaneous transmitter release rate was elevated globally across neuritic arbors of B19 in appropriate muscle cocultures, including arbors without direct muscle contact. In addition, dual contacts formed between B19 and two different muscle targets resulted in significant elevations in excitation-secretion coupling only at neuritic sites of appropriate muscle contact and not at sites of contact with mismatched targets. Image analyses of presynaptic neuronal architecture revealed that appropriate, but not novel, muscle targets elicited increased arborization of neurites at sites of neuron-muscle contact. In contrast to results with neuron B19, secretory properties of buccal neuron 5 (B5), a neuron capable of forming inappropriate chemical synapses with a number of novel targets in culture, were not enhanced by inappropriate muscle contact. We conclude that muscle targets vary in their ability to induce presynaptic modifications (e.g., excitation-secretion coupling, spontaneous transmitter release, and neuritic arborization) in identified motoneurons of Helisoma. These results imply that separate molecular pathways exist which control each of these events during synaptic differentiation. The differential potency of muscle targets for induction of presynaptic changes indicates the existence of a cellular mechanism of target recognition. We hypothesize that such a mechanism underlies the ability of neuron B19 to discriminate between potential postsynaptic partners in cell culture.

1. The role of activity-dependent mechanisms in target-mediated induction of secretory properties was investigated at regenerating neuromuscular synapses of the American pond snail, Helisoma trivolvis, in cell culture. 2. Identified motoneurons were isolated into cell culture conditions that promoted neurite outgrowth. Buccal neurons 19 (B19) were cultured alone for 2 days, at which time dissociated muscle fibers were manipulated into contact with newly formed neurites. 3. Immediately before the plating of muscle fibers, the sodium channel blocker, tetrodotoxin (TTX), or the acetylcholine receptor antagonist, d-tubocurarine chloride (curare), was added to the culture dish. After 48 h of exposure, the inhibitors were removed by repeated dilution of the culture medium and electrophysiological analyses were performed. 4. Cholinoceptive assay cells were manipulated into contact with the presynaptic neurons to assess secretory properties along neuronal processes. Assay cells were used to control for variations in postsynaptic sensitivity that could result from long-term exposure to activity inhibitors. 5. These analyses demonstrated that inhibition of TTX-sensitive presynaptic activity and inhibition of curare-sensitive postsynaptic activation both blocked the induction of excitation-secretion coupling typically induced in these motoneurons by appropriate target contact. Neuron B5, which rapidly acquires functional synaptic properties in vitro, was unaffected in its secretory function by 48 h of activity inhibition. 6. Acquisition of secretory competence was not suppressed due to a reduction in the viability or long-term changes in excitability of the activity-inhibited neurons, as indicated by analyses of electrophysiological properties. 7. Although target-contact and activity both participated in the induction of secretory modifications in neuron B19, target-mediated changes did not involve retrograde effects on presynaptic neuronal excitability. 8. We hypothesize that contact-mediated mechanisms govern the initiation of presynaptic modifications in B19, however, our data indicate that the acquisition of functional excitation-secretion coupling also involves activity-dependent mechanisms. Although the mechanistic role of activity remains undefined, our results suggest that the activation of the target muscle plays a critical role in a retrograde signaling pathway underlying maturation of a functional secretory apparatus in target-contacted neuronal processes.

Regeneration of motor systems involves reestablishment of central control networks, reinnervation of muscle targets by motoneurons, and reconnection of neuromodulatory circuits. Still, how these processes are integrated as motor function is restored during regeneration remains ill defined. Here, we examined the mechanisms underlying motoneuronal regeneration of neuromuscular synapses related to feeding movements in the pulmonate snail Helisoma trivolvis. Neurons B19 and B110, although activated during different phases of the feeding pattern, innervate similar sets of muscles. However, the percentage of muscle fibers innervated, the efficacy of excitatory junction potentials, and the strength of muscle contractions were different for each cell’s specific connections. After peripheral nerve crush, a sequence of transient electrical and chemical connections formed centrally within the buccal ganglia. Neuromuscular synapse regeneration involved a three-phase process: the emergence of spontaneous synaptic transmission (P1), the acquisition of evoked potentials of weak efficacy (P2), and the establishment of functional reinnervation (P3). Differential synaptic efficacy at muscle contacts was recapitulated in cell culture. Differences in motoneuronal presynaptic properties (i.e., quantal content) were the basis of disparate neuromuscular synapse function, suggesting a role for retrograde target influences. We propose a homeostatic model of molluscan motor system regeneration. This model has three restoration events: (1) transient central synaptogenesis during axonal outgrowth, (2) intermotoneuronal inhibitory synaptogenesis during initial neuromuscular synapse formation, and (3) target-dependent regulation of neuromuscular junction formation.