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

Abstract

Synaptic release of neurotransmitters is evoked by activity-dependent Ca 2+ entry into the nerve terminal. However, here it is shown that robust synaptic neuropeptide release from Drosophila motoneurons is evoked in the absence of extracellular Ca 2+ by octopamine, the arthropod homolog to norepinephrine. Genetic and pharmacology experiments demonstrate that this surprising peptidergic transmission requires cAMP-dependent protein kinase, with only a minor contribution of exchange protein activated by cAMP (epac). Octopamine-evoked neuropeptide release also requires endoplasmic reticulum Ca 2+ mobilization by the ryanodine receptor and the inositol trisphosphate receptor. Hence, rather than relying exclusively on activity-dependent Ca 2+ entry into the nerve terminal, a behaviorally important neuromodulator uses synergistic cAMP-dependent protein kinase and endoplasmic reticulum Ca 2+ signaling to induce synaptic neuropeptide release.N euromodulators induce presynaptic signaling to regulate fast neurotransmission triggered by activity-induced Ca 2+ entry into the nerve terminal. Neuromodulators also influence the very low rate of spontaneous quantal release of classical transmitters. However, because spontaneous release is functionally relevant only in specialized cases (1), neuromodulators are not believed to typically induce physiologically significant release in the absence of extracellular Ca 2+ . Studies of endocrine cells suggest that release of peptides packaged in large dense-core vesicles (LDCVs) also requires Ca 2+ entry because of the rarity of spontaneous LDCV fusion and the inefficient permeation of peptides through the LDCV-fusion pore (2, 3). However, because it is difficult to measure peptide release at intact synapses, a much more limited dataset supports the conclusion that Ca 2+ influx into the nerve terminal is absolutely required for peptidergic transmission. Thus, it remains unclear how neuromodulators control synaptic neuropeptide release.With this background in mind, we set out to study regulation of neuropeptide release at the Drosophila neuromuscular junction (NMJ) by octopamine-induced cAMP signaling. Octopamine, the homolog of norepinephrine in Drosophila, controls many behaviors by activating central G protein-coupled receptors that induce adenylyl cyclase activation and intracellular Ca 2+ release (4, 5).

Ryanodine‐ and CaMKII‐dependent release of endogenous CGRP induces an increase in acetylcholine quantal size in neuromuscular junctions of mice

“…The results of our work suggest that triggering of CGRP‐containing LDCVs exocytosis requires not only the mandatory release of calcium from the ryanodine‐sensitive Ca 2+ ‐stores but also the subsequent activation of CaMKII. Taking into account the data obtained at Drosophila NMJs (Shakiryanova et al., 2007, 2011), our results suggest that conservative signaling pathway exists in NMJs of different species, which regulates the exocytosis of LDCVs via RyRs and CaMKII. Interestingly, the increase in ACh quantal size during exogenous CGRP application could not be prevented by CaMKII blocking.…”

“…Our suggestion is supported by previous results describing that the pool of the LDCVs is localized away from the extracellular calcium influx at the active zones (Pécot‐Dechavassine & Brouard, 1997), as well as ryanodine‐sensitive Ca 2+ ‐stores (Bouchard, Pattarini, & Geiger, 2003). In addition, the RyR‐mediated LDCV mobilization, trafficking, and exocytosis have been recently described at Drosophila NMJs (Shakiryanova, Zettel, Gu, Hewes, & Levitan, 2011; Shakiryanova et al., 2007; Wong, Shakiryanova, & Levitan, 2009). …”

Polymorphism and division of labour in a socially complex ant: neuromodulation of aggression in the Australian weaver ant,Oecophylla smaragdina

“…In comparison to minor workers, defensive majors have more 5-HT-immunoreactive cells in primary visual regions, more extensive serotonergic varicosities in sensory and integrative brain regions [67], and more elaborate axonal arborization in a serotonergic neuron integral to sensory integration [27]. Although relationships between neuromodulators and circuit development have been described in other taxa [68,69], the influence of neuroanatomical differences in 5-HT circuitry on 5-HT signalling and behaviour remains to be studied in ants. In O. smaragdina, major and minor worker differences in endogenous OA titres may alter circuitry to generate subcaste-related aggression.…”

Transmission, Development, and Plasticity of Synapses

“…Many of these mechanistic differences in DCV biology have been probed at Drosophila synapses using transgenic animals expressing an artificial neuropeptide-GFP-tagged rat atrial natriuretic factor peptide (ANF-GFP) (Rao et al 2001a). This transgenic line has proved highly valuable for DCV biology, revealing mechanisms of activity-dependent recruitment of DCVs (Shakiryanova et al 2005), DCV capture at active terminals (Wong et al 2012;Bulgari et al 2014), presynaptic ER-dependent Ca 2+ release driving DCV fusion (Shakiryanova et al 2007(Shakiryanova et al , 2011, and partial depletion of DCV contents upon release (Wong et al 2015). In addition to aspects of DCV release and trafficking, studies have revealed the role of the basic helix-loop-helix transcription factor Dimmed in driving neuroendocrine cell fate, including its role in regulating the expression of specific DCV proteins (Hewes et al 2006;Hamanaka et al 2010;Park et al 2011Park et al , 2014.…”