Synaptic transmission is impaired at neuronal autonomic synapses in agrin-null mice

Neuronal synapse formation is a multistep process regulated by several pre- and postsynaptic adhesion and signaling proteins. Recently, we found that agrin acts as one such synaptogenic factor at neuronal synapses in the PNS by demonstrating that structural synapse formation is impaired in the superior cervical ganglia (SCG) of z+ agrin-deficient mice and in SCG cultures derived from those animals. Here, we tested whether synaptic function is defective in agrin-null (AGD-/-) ganglia and began to define agrin's mechanism of action. Our electrophysiological recordings of compound action potentials showed that presynaptic stimulation evoked action potentials in approximately 40% of AGD-/- ganglionic neurons compared to 90% of wild-type neurons; moreover, transmission could not be potentiated as in wild-type or z+ agrin-deficient ganglia. Intracellular recordings also showed that nerve-evoked excitatory postsynaptic potentials in AGD-/- neurons were only 1/3 the size of those in wild-type neurons and mostly subthreshold. Consistent with these defects in transmission, we found an approximately 40-50% decrease in synapse number in AGD-/- ganglia and cultures, and decreased levels of differentiation at the residual synapses in culture. Furthermore, surface levels of acetylcholine receptors (AChRs) were equivalent in cultured AGD-/- and wild-type neurons, and depolarization reduced the synaptic localization of AChRs in AGD-/- but not wild-type neurons. These findings provide the first direct demonstration that agrin is required for proper structural and functional development of an interneuronal synapse in vivo. Moreover, they suggest a novel role for agrin, in stabilizing the postsynaptic density of nAChR at nascent neuronal synapses.

A null mutation for the alpha3 nicotinic acetylcholine (ACh) receptor gene abolishes fast synaptic activity in sympathetic ganglia and reveals that ACh output from developing preganglionic terminals is regulated in an activity-dependent retrograde manner

In vertebrates, synaptic activity exerts an important influence on the formation of neural circuits, yet our understanding of its role in directing presynaptic and postsynaptic differentiation during synaptogenesis is incomplete. This study investigates how activity influences synaptic differentiation as synapses mature during early postnatal life. Specifically, we ask what happens to presynaptic terminals when synapses develop without functional postsynaptic receptors and without fast synaptic transmission. To address this issue, we investigated cholinergic nicotinic synapses in sympathetic ganglia of mice with a null mutation for the alpha3 nicotinic ACh receptor gene. Disrupting the alpha3 gene completely eliminates fast excitatory synaptic potentials on postganglionic sympathetic neurons, establishing a crucial role for alpha3-containing postsynaptic receptors in synaptic transmission. Interestingly, the preganglionic nerve terminals form morphologically normal synapses with sympathetic neurons, and these synapses persist without activity in postnatal animals. Surprisingly, when stimulating the preganglionic nerve at physiological rates, we discovered a significant decrease in ACh output from the presynaptic terminals in these alpha3(-/-) sympathetic ganglia. We show that this decrease in ACh output from the presynaptic terminals results, in part, from a lack of functional high-affinity choline transporters. We conclude the following: (1) fast synaptic transmission in mammalian SCG requires alpha3 expression; (2) in the absence of activity, the preganglionic nerve forms synapses that appear morphologically normal and persist for several weeks; and (3) to sustain transmitter release, developing presynaptic terminals require an activity-dependent retrograde signal.

Agrin plays an organizing role in the formation of sympathetic synapses

Agrin is a nerve-derived factor that directs neuromuscular synapse formation, however its role in regulating interneuronal synaptogenesis is less clear. Here, we examine agrin's role in synapse formation between cholinergic preganglionic axons and sympathetic neurons in the superior cervical ganglion (SCG) using agrin-deficient mice. In dissociated cultures of SCG neurons, we found a significant decrease in the number of synapses with aggregates of presynaptic synaptophysin and postsynaptic neuronal acetylcholine receptor among agrin-deficient neurons as compared to wild-type neurons. Moreover, the levels of pre- and postsynaptic markers at the residual synapses in agrin-deficient SCG cultures were also reduced, and these defects were rescued by adding recombinant neural agrin to the cultures. Similarly, we observed a decreased matching of pre- and postsynaptic markers in SCG of agrin-deficient embryos, reflecting a decrease in the number of differentiated synapses in vivo. Finally, in electrophysiological experiments, we found that paired-pulse depression was more pronounced and posttetanic potentiation was significantly greater in agrin-deficient ganglia, indicating that synaptic transmission is also defective. Together, these findings indicate that neural agrin plays an organizing role in the formation and/or differentiation of interneuronal, cholinergic synapses.