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.

Presynaptic target of Ca2+ action on neuropeptide and acetylcholine release in Aplysia californica

“…Our observation of apparently docked DCVs in active zones contrasts with the common assertion that neuronal DCVs are rarely if ever docked at the membrane, and are excluded from active zones (De Camilli & Jahn, 1990; Verhage et al 1991; Golding, 1994; Karhunen et al 2000; but see Sharman et al 2000). The fact that single APs, short trains, or brief depolarizations often fail to release detectable amounts of peptide (Whim & Lloyd, 1989, 1994; Seward et al 1995; Vilim et al 1996; Leenders et al 1999) has often been attributed to an absence of docked vesicles ready for release.…”

The Stomatogastric Nervous System as a Model for Studying Sensorimotor Interactions in Real-Time Closed-Loop Conditions

“…To extrapolate a transfer function from these experiments we made the following assumptions about the muscle behavior and AGR activation: (1) as gm1 muscles are slow, non-twitch muscles and do not generate intrinsic action potentials (Jorge-Rivera and Marder, 1997), they can be approximated as a low-pass filter for the motoneuronal input (as also suggested in other systems: Partridge, 1966; Beer and Chiel, 2003). This ignores the fact that muscle dynamics can vary greatly depending on activity history and hormonal influences (Jorge-Rivera et al, 1998; Sharman et al, 2000; Birmingham and Tauck, 2003). Since we were not trying to create a realistic model of the muscle, but rather the proprioceptive response we tuned the muscle model together with the AGR model to generate the output which fit our data best (see also Discussion).…”

GABA Enhances Transmission at an Excitatory Glutamatergic Synapse

“…Studies of the distribution of GABA immunoreactivity in a variety of crustacean species, including lobsters, show that none of the STG motor neuron somata stain for GABA (Cournil et al, 1990;Mulloney and Hall, 1990;Swensen et al, 2000). However, GABA is found in inputs to the stomatogastric ganglion (Cournil et al, 1990;Blitz and Nusbaum, 1999), some of which project through the STG into the motor nerves (Swensen et al, 2000), and may be responsible for recent observations of inhibitory synapses onto stomach muscles (Sharman et al, 2000). Particularly relevant for this work, Sharman et al (2000) found ultrastructural evidence of inhibitory synapses onto the axons of excitatory motor neurons in crabs.…”

“…Until recently, it was thought that the muscles of the crustacean stomach receive only excitatory innervation, although a few stomach muscles show a picrotoxin-sensitive increase in Cl Ϫ conductance in response to GABA (Albert et al, 1986). New anatomical evidence has suggested that there may be GABAergic innervation to some of the crustacean stomach muscles (Sharman et al, 2000;Swensen et al, 2000). This prompted us to reinvestigate the possible role of GABA at some of the neuromuscular junctions of the lobster Homarus americanus.…”