Crab stomach pyloric muscles display not only excitatory but inhibitory and neuromodulatory nerve terminals

Abstract

Movements of the foregut in crustaceans are produced by striated muscles that are innervated by motor neurons in the stomatogastric ganglion (STG). Firing of the STG motor neurons generates excitatory junctional potentials (EJPs) in the stomach muscles. We now provide evidence for the existence of separate inhibitory and neuromodulatory innervations of some pyloric muscles in the foregut of several crabs, Callinectes sapidus, Cancer magister, and Cancer borealis. Electron microscopic examination of several pyloric muscles revealed three distinct types of nerve terminals. Excitatory terminals were readily identified by the spherical shape of their small, clear synaptic vesicles. These terminals also housed a few large dense core vesicles. Inhibitory nerve terminals were recognized by the elliptical shape of their small, clear synaptic vesicles, and contacted the muscles at well-defined synapses equipped with dense bar active zones. Bath application of GABA reduced the amplitudes of EJPs in a pyloric muscle of C. borealis, consistent with the presence of GABAergic inhibitory innervation. Neuromodulatory terminals were characterized by their predominant population of large dense and dense core vesicles. These terminals formed synapses with presynaptic dense bars on the muscle, as well as on the excitatory and inhibitory nerve terminals. The presence of the inhibitory and neuromodulatory terminals creates a functional context for previously described reports of neuromodulatory actions on stomach muscles and suggests that the transfer function from STG motor patterns to pyloric movement may be orchestrated by a complex innervation from sources outside of the STG itself.