Identification of the neuropeptide transmitter proctolin in Drosophila larvae: characterization of muscle fiber-specific neuromuscular endings

Abstract

The cellular localization of the peptide neurotransmitter proctolin was determined for larvae of the fruitfly Drosophila melanogaster. Proctolin was recovered from the CNS, hindgut, and segmental bodywall using reverse-phase HPLC, and characterized by bioassay, immunoassay, and enzymatic analysis. A small, stereotyped population of proctolin-immunoreactive neurons was found in the larval CNS. Several of the identified neurons may be efferents. In the periphery, proctolin-immunoreactive neuromuscular endings were identified on both visceral and skeletal muscle fibers. On the hindgut, the neuropeptide is associated with endings on intrinsic circular muscle fibers. We propose that the hindgut muscle fibers are innervated by central neurons homologous to previously described proctolinergic efferents of grasshoppers. The segmental bodywall innervation consists of a pattern of segment-specific junctions on several singly identifiable muscle fibers. While it is generally accepted that Drosophila muscle fibers are innervated by glutamatergic motoneurons, our data indicate that a specialized subset of muscle fibers are also innervated by peptidergic efferents.

Identification and characterization of a G protein-coupled receptor for the neuropeptide proctolin in Drosophila melanogaster

“…Specifically, marked CG6986 receptor expression in the hindgut is consistent with the potent myostimulatory effects of proctolin on that tissue in cockroaches (1). A similar physiological action of proctolin was confirmed for the case of the Drosophila hindgut, which is also innervated by proctolin-immunoreactive neurons originating in the abdominal ganglia (9). Expression of the CG6986 receptor in Drosophila Malpighian tubules is consistent with reports that, in the locust, proctolin induces tubule secretion and stimulates tubule writhing (41).…”

“…We did not observe staining in the heart proper, but did observe minor staining in the anterior aorta and in some nerve terminals apposed to cardiac tissue. There are conflicting reports on proctolin action on heart rate in Drosophila (9,44). Given the mechanism of cardiac pacemaking and its modulation (45), any direct action of proctolin on heart muscle is likely to be excitatory, whereas indirect action onto neurons that innervate the heart could have a net excitatory or inhibitory action (46).…”

“…In insects, proctolin is a major peptide cotransmitter and it modulates contractions of both somatic and visceral muscles (7,8). Proctolin-like immunosignals are found within a small number of neurons in the larval CNS of Drosophila and these include efferents to bodywall muscles, heart, and viscera (9).…”

Synaptic neuropeptide release induced by octopamine without Ca 2+ entry into the nerve terminal

“…Octopamine is also in NMJ boutons at muscles 12 and 13 and can have complex effects at adjacent muscle 6 and 7 NMJs: octopamine inhibits phasic transmission and facilitates tonic release induced by K + , with the latter effect requiring cAMP-dependent protein kinase (PKA) (6)(7)(8). Drosophila motoneurons also contain neuropeptides, which are released in response to nerve stimulation, depolarization, and developmental cues (9)(10)(11)(12)(13). However, the impact of octopamine and cAMP on synaptic neuropeptide release is unknown, even though such regulation could be relevant to the effects of octopamine on central peptidergic neurons involved in controlling metabolism, development, and circadian behavior (14,15).…”

Plasticity and Second Messengers During Synapse Development

“…CCAP (D. Park and L.C. Griffith, unpublished), Proctolin (Anderson et al, 1988), and Insulin-like peptides (Gorczyca et al, 1993), or they can be released into the larval hemolymph which bathes the musculature (Hewes et al, 1998). Only a few of the peptides that have been extensively characterized as to their effects on NMJ function in Drosophila are considered here.…”