Differential ultrastructure of synaptic terminals on ventral longitudinal abdominal muscles in Drosophila larvae

Abstract

The innervation of ventral longitudinal abdominal muscles (muscles 6, 7, 12, and 13) of third-instar Drosophila larvae was investigated with Nomarski, confocal, and electron microscopy to define the ultrastructural features of synapse-bearing terminals. As shown by previous workers, muscles 6 and 7 receive in most abdominal segments "Type I" endings, which are restricted in distribution and possess relatively prominent periodic terminal enlargements ("boutons"); whereas muscles 12 and 13 have in addition "Type II" terminals, which are more widely distributed and have smaller "boutons". Serial sectioning of the Type I innervation of muscles 6 and 7 showed that two axons with distinctive endings contribute to it. One axon (termed Axon 1) has somewhat larger boutons, containing numerous synapses and presynaptic dense bodies (putative active zones for transmitter release). This axon also has more numerous intraterminal mitochondria, and a profuse subsynaptic reticulum around or under the synaptic boutons. The second axon (Axon 2) provides somewhat smaller boutons, with fewer synapses and dense bodies per bouton, fewer intraterminal mitochondria, and less-developed subsynaptic reticulum. Both axons contain clear synaptic vesicles, with occasional large dense vesicles. Approximately 800 synapses are provided by Axon 1 to muscles 6 and 7, and approximately 250 synapses are provided by Axon 2. In muscles 12 and 13, endings with predominantly clear synaptic vesicles, generally similar to the Type I endings of muscles 6 and 7, were found, along with another type of ending containing predominantly dense-cored vesicles, with small clusters of clear synaptic vesicles. This second type of ending was found most frequently in muscle 12, and probably corresponds to a subset of the "Type II" endings seen in the light microscope. Type I endings are thought to generate the 'fast' and 'slow' junctional potentials seen in electrophysiological recordings, whereas the physiological actions of Type II endings are presently not known.

The atypical cadherin flamingo regulates synaptogenesis and helps prevent axonal and synaptic degeneration in Drosophila

“…One type of ectopic synapse is the formation of type I boutons. For example, muscle 12 normally receives synaptic inputs from four types of motoneurons forming type Ib, Is, type II, and type III synaptic boutons (Jia et al, 1993;Atwood et al, 1993). In fmi mutants, muscle 12 often received ectopic synapses from axons coming from the transverse nerve or the intersegmental nerve even if its normal synaptic inputs were present.…”

Ultrastructure of neuromuscular junctions in Drosophilal: Comparison of wild type and mutants with increased excitability

“…Four synaptic bouton types can be demonstrated at the Drosophila larval body wall muscles at the light microscopical level (Johansen et al, 1989a;Budnik and Gorczyca, 1992;Gorczyca et al, 1993;Atwood et al, 1993;this report) and are summarized in Table 1. All bouton types have been reported to contain glutamate, the main excitatory transmitter at these neuromuscular junctions (Jan and Jan, 1976b;Johansen et al, 1989a,b).…”

“…Muscles 6, 7, 12, and 13 are all innervated by motor axon terminals containing this type of bouton (Johansen et al, 1989a,b). Atwood et al (1993) have divided the large boutons into Type Ib and Type Is based on a quantitative analysis of muscles 6 and 7. Muscle 13 contains large Type Ib and small Type Is boutons that are clearly separable on the basis of size.…”

Actions of MDMA at glutamatergic neuromuscular junctions

“…The innervation and synaptic properties of the Is and Ib motor nerve terminals that innervate muscle 6 of the third instar Drosophila larva have been thoroughly studied (Atwood et al, 1993;Kurdyak et al, 1994;Li et al, 2002). The Ib and Is terminals have different morphology and physiology, though they can be recruited separately or in unison.…”

“…The Ib and Is terminals have different morphology and physiology, though they can be recruited separately or in unison. The terminals of the Is axon contain small varicosities along its length and give rise to large EPSPs in the muscle, while the Ib axon has large varicosities and produces smaller EPSPs (Atwood et al, 1993;Kurdyak et al, 1994;Stewart et al, 1994) (Fig. 4A and B).…”