General Principles of Neuronal Co-transmission: Insights From Multiple Model Systems

The Phosphoprotein Synapsin Ia Regulates the Kinetics of Dense-Core Vesicle Release

Common fusion machinery mediates the Ca²⁺-dependent exocytosis of synaptic vesicles (SVs) and dense-core vesicles (DCVs). Previously, Synapsin Ia (Syn Ia) was found to localize to SVs, essential for mobilizing SVs to the plasma membrane through phosphorylation. However, whether (or how) the phosphoprotein Syn Ia plays a role in regulating DCV exocytosis remains unknown. To answer these questions, we measured the dynamics of DCV exocytosis by using single-vesicle amperometry in PC12 cells (derived from the pheochromocytoma of rats of unknown sex) overexpressing wild-type or phosphodeficient Syn Ia. We found that overexpression of phosphodeficient Syn Ia decreased the DCV secretion rate, specifically via residues previously shown to undergo calmodulin-dependent kinase (CaMK)-mediated phosphorylation (S9, S566, and S603). Moreover, the fusion pore kinetics during DCV exocytosis were found to be differentially regulated by Syn Ia and two phosphodeficient Syn Ia mutants (Syn Ia-S62A and Syn Ia-S9,566,603A). Kinetic analysis suggested that Syn Ia may regulate the closure and dilation of DCV fusion pores via these sites, implying the potential interactions of Syn Ia with certain DCV proteins involved in the regulation of fusion pore dynamics. Furthermore, we predicted the interaction of Syn Ia with several DCV proteins, including Synaptophysin (Syp) and soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. By immunoprecipitation, we found that Syn Ia interacted with Syp via phosphorylation. Moreover, a proximity ligation assay (PLA) confirmed their phosphorylation-dependent, in situ interaction on DCVs. Together, these findings reveal a phosphorylation-mediated regulation of DCV exocytosis by Syn Ia.SIGNIFICANCE STATEMENT Although they exhibit distinct exocytosis dynamics upon stimulation, synaptic vesicles (SVs) and dense-core vesicles (DCVs) may undergo co-release in neurons and neuroendocrine cells through an undefined molecular mechanism. Synapsin Ia (Syn Ia) is known to recruit SVs to the plasma membrane via phosphorylation. Here, we examined whether Syn Ia also affects the dynamics of DCV exocytosis. We showed that Syn Ia regulates the DCV secretion rate and fusion pore kinetics during DCV exocytosis. Moreover, Syn Ia-mediated regulation of DCV exocytosis depends on phosphorylation. We further found that Syn Ia interacts with Synaptophysin (Syp) on DCVs in a phosphorylation-dependent manner. Thus, these results suggest that Syn Ia may regulate the release of DCVs via phosphorylation.

Dual modulatory effects on feedback from a proprioceptor in the crustacean stomatogastric nervous system

Neuromodulatory actions that change the properties of proprioceptors or the muscle movements to which they respond necessarily affect the feedback provided to the central network. Here we further characterize the responses of the gastropyloric receptor 1 (GPR1) and gastropyloric receptor 2 (GPR2) neurons in the stomatogastric nervous system of the crab Cancer borealis to movements and contractions of muscles, and we report how neuromodulation modifies those responses. We observed that the GPR1 response to contractions of the gastric mill 4 muscle (gm4) was absent, or nearly so, when the neuron was quiescent but robust when it was spontaneously active. We also found that the effects of four neuromodulatory substances (GABA, serotonin, proctolin, and TNRNFLRFamide) on the GPR1 response to muscle stretch were similar to those previously reported for GPR2. Finally, we showed that an excitatory action on gm4 due to proctolin combined with an inhibitory action on GPR2 due to GABA can allow for larger muscle contractions without increased proprioceptive feedback.NEW & NOTEWORTHY We report that the combination of GABA and the peptide proctolin increases contraction of a stomatogastric muscle while decreasing the corresponding response of the proprioceptor that reports on it. These results suggest a general mechanism by which muscle movements can be modified while sensory feedback is conserved, one that may be particularly well suited for providing flexibility to central pattern generator networks.

Neuropeptides as Primary Mediators of Brain Circuit Connectivity

Across sleep and wakefulness, brain function requires inter-neuronal interactions lasting beyond seconds. Yet, most studies of neural circuit connectivity focus on millisecond-scale interactions mediated by the classic fast transmitters, GABA and glutamate. In contrast, neural circuit roles of the largest transmitter family in the brain–the slow-acting peptide transmitters–remain relatively overlooked, or described as “modulatory.” Neuropeptides may efficiently implement sustained neural circuit connectivity, since they are not rapidly removed from the extracellular space, and their prolonged action does not require continuous presynaptic firing. From this perspective, we review actions of evolutionarily-conserved neuropeptides made by brain-wide-projecting hypothalamic neurons, focusing on lateral hypothalamus (LH) neuropeptides essential for stable consciousness: the orexins/hypocretins. Action potential-dependent orexin release inside and outside the hypothalamus evokes slow postsynaptic excitation. This excitation does not arise from modulation of classic neurotransmission, but involves direct action of orexins on their specific G-protein coupled receptors (GPCRs) coupled to ion channels. While millisecond-scale, GABA/glutamate connectivity within the LH may not be strong, re-assessing LH microcircuits from the peptidergic viewpoint is consistent with slow local microcircuits. The sustained actions of neuropeptides on neuronal membrane potential may enable core brain functions, such as temporal integration and the creation of lasting permissive signals that act as “eligibility traces” for context-dependent information routing and plasticity. The slowness of neuropeptides has unique advantages for efficient neuronal processing and feedback control of consciousness.

Leucokinin and Associated Neuropeptides Regulate Multiple Aspects of Physiology and Behavior in Drosophila

Leucokinins (LKs) constitute a family of neuropeptides identified in numerous insects and many other invertebrates. LKs act on G-protein-coupled receptors that display only distant relations to other known receptors. In adult Drosophila, 26 neurons/neurosecretory cells of three main types express LK. The four brain interneurons are of two types, and these are implicated in several important functions in the fly's behavior and physiology, including feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. The 22 neurosecretory cells (abdominal LK neurons, ABLKs) of the abdominal neuromeres co-express LK and a diuretic hormone (DH44), and together, these regulate water and ion homeostasis and associated stress as well as food intake. In Drosophila larvae, LK neurons modulate locomotion, escape responses and aspects of ecdysis behavior. A set of lateral neurosecretory cells, ALKs (anterior LK neurons), in the brain express LK in larvae, but inconsistently so in adults. These ALKs co-express three other neuropeptides and regulate water and ion homeostasis, feeding, and drinking, but the specific role of LK is not yet known. This review summarizes Drosophila data on embryonic lineages of LK neurons, functional roles of individual LK neuron types, interactions with other peptidergic systems, and orchestrating functions of LK.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Intrinsic sources of tachykinin-related peptide in the thoracic ganglion mass of the crab, Cancer borealis

Neuropeptides comprise the largest class of neural and neuroendocrine signaling molecules. Vertebrate tachykinins (TKs) and the structurally-related invertebrate tachykinin-related peptides (TRPs) together form the largest neuropeptide superfamily, with a number of conserved neural and neuroendocrine functions across species. Arthropods, including crustaceans, have provided many insights into neuropeptide signaling and function. Crustacean tachykinin-related peptide occurs in endocrine organs and cells and in two of the major crustacean CNS components, the supraoesophageal ganglion ("brain") and the stomatogastric nervous system. However, little is known about TRP sources in the remaining major CNS component, the thoracic ganglion mass (TGM). To gain further insight into the function of this peptide, we aimed to identify intrinsic TRP sources in the TGM of the Jonah crab, Cancer borealis. We first adapted a clearing protocol to improve TRP immunoreactivity specifically in the TGM, which is a dense, fused mass of multiple ganglia in short-bodied crustaceans such as Cancer species of crabs. We verified that the clearing protocol avoided distortion of cell body morphology yet increased visibility of TRP immunoreactivity. Using confocal microscopy, we found TRP-immunoreactive (TRP-IR) axon tracts running the length of the TGM, TRP-IR neuropil in all ganglia, and approximately 110 TRP-IR somata distributed throughout the TGM, within and between ganglia. These somata likely represent both neural and neuroendocrine sources of TRP. Thus, there are many potential intrinsic sources of TRP in the TGM that are positioned to regulate behaviors such as food intake, locomotion, respiration, and reproduction.

Hormonal axes in Drosophila: regulation of hormone release and multiplicity of actions

Hormones regulate development, as well as many vital processes in the daily life of an animal. Many of these hormones are peptides that act at a higher hierarchical level in the animal with roles as organizers that globally orchestrate metabolism, physiology and behavior. Peptide hormones can act on multiple peripheral targets and simultaneously convey basal states, such as metabolic status and sleep-awake or arousal across many central neuronal circuits. Thereby, they coordinate responses to changing internal and external environments. The activity of neurosecretory cells is controlled either by (1) cell autonomous sensors, or (2) by other neurons that relay signals from sensors in peripheral tissues and (3) by feedback from target cells. Thus, a hormonal signaling axis commonly comprises several components. In mammals and other vertebrates, several hormonal axes are known, such as the hypothalamic-pituitary-gonad axis or the hypothalamic-pituitary-thyroid axis that regulate reproduction and metabolism, respectively. It has been proposed that the basic organization of such hormonal axes is evolutionarily old and that cellular homologs of the hypothalamic-pituitary system can be found for instance in insects. To obtain an appreciation of the similarities between insect and vertebrate neurosecretory axes, we review the organization of neurosecretory cell systems in Drosophila. Our review outlines the major peptidergic hormonal pathways known in Drosophila and presents a set of schemes of hormonal axes and orchestrating peptidergic systems. The detailed organization of the larval and adult Drosophila neurosecretory systems displays only very basic similarities to those in other arthropods and vertebrates.

Different microcircuit responses to comparable input from one versus both copies of an identified projection neuron

Neuronal inputs to microcircuits are often present as multiple copies of apparently equivalent neurons. Thus far, however, little is known regarding the relative influence on microcircuit output of activating all or only some copies of such an input. We examine this issue in the crab (Cancer borealis) stomatogastric ganglion, where the gastric mill (chewing) microcircuit is activated by modulatory commissural neuron 1 (MCN1), a bilaterally paired modulatory projection neuron. Both MCN1s contain the same co-transmitters, influence the same gastric mill microcircuit neurons, can drive the biphasic gastric mill rhythm, and are co-activated by all identified MCN1-activating pathways. Here, we determine whether the gastric mill microcircuit response is equivalent when stimulating one or both MCN1s under conditions where the pair are matched to collectively fire at the same overall rate and pattern as single MCN1 stimulation. The dual MCN1 stimulations elicited more consistently coordinated rhythms, and these rhythms exhibited longer phases and cycle periods. These different outcomes from single and dual MCN1 stimulation may have resulted from the relatively modest, and equivalent, firing rate of the gastric mill neuron LG (lateral gastric) during each matched set of stimulations. The LG neuron-mediated, ionotropic inhibition of the MCN1 axon terminals is the trigger for the transition from the retraction to protraction phase. This LG neuron influence on MCN1 was more effective during the dual stimulations, where each MCN1 firing rate was half that occurring during the matched single stimulations. Thus, equivalent individual- and co-activation of a class of modulatory projection neurons does not necessarily drive equivalent microcircuit output.

Geoffrey Burnstock - An accidental pharmacologist

Geoffrey Burnstock, the founder of the field of purinergic signaling research passed away in Melbourne, Australia on June 3rd, 2020, at the age of 91. With his death, the world of biomedical research lost one of its most passionate, creative and unconventional thought leaders. He was an inspiration to the many researchers he interacted with for more than 50 years and a frequent irritation to those in the administrative establishment. Geoff never considered himself a pharmacologist having being trained as a zoologist and becoming an autonomic neurophysiologist based on his evolving interests in systems and disease-related research. By the end of his life he had: published some 1550 papers; been cited more than 125,000 times; had an h-index of 156 and had supervised over 100 Ph.D. students. His indelible legacy, based on a holistic, data-based, multidisciplinary, unconventional "outside the box" approach to research was reflected in two of the seminal findings in late 20th century biomedical research: the purinergic neurotransmitter hypothesis and the concept of co-neurotransmission, both of which were initially received by his peers with considerable skepticism that at times verged on disdain. Nonetheless, while raising hackles and threatening the status quo, Geoff persevered and prevailed, becoming a mentor for several generations of biomedical researchers. In this review we provide a joint perspective on Geoff Burnstock's legacy in research.

Circadian Structural Plasticity Drives Remodeling of E Cell Output

Behavioral outputs arise as a result of highly regulated yet flexible communication among neurons. The Drosophila circadian network includes 150 neurons that dictate the temporal organization of locomotor activity; under light-dark (LD) conditions, flies display a robust bimodal pattern. The pigment-dispersing factor (PDF)-positive small ventral lateral neurons (sLNv) have been linked to the generation of the morning activity peak (the "M cells"), whereas the Cryptochrome (CRY)-positive dorsal lateral neurons (LNds) and the PDF-negative sLNv are necessary for the evening activity peak (the "E cells") [1, 2]. While each group directly controls locomotor output pathways [3], an interplay between them along with a third dorsal cluster (the DN1ps) is necessary for the correct timing of each peak and for adjusting behavior to changes in the environment [4-7]. M cells set the phase of roughly half of the circadian neurons (including the E cells) through PDF [5, 8-10]. Here, we show the existence of synaptic input provided by the evening oscillator onto the M cells. Both structural and functional approaches revealed that E-to-M cell connectivity changes across the day, with higher excitatory input taking place before the day-to-night transition. We identified two different neurotransmitters, acetylcholine and glutamate, released by E cells that are relevant for robust circadian output. Indeed, we show that acetylcholine is responsible for the excitatory input from E cells to M cells, which show preferential responsiveness to acetylcholine during the evening. Our findings provide evidence of an excitatory feedback between circadian clusters and unveil an important plastic remodeling of the E cells' synaptic connections.

Modulatory Roles of ATP and Adenosine in Cholinergic Neuromuscular Transmission

A review of the data on the modulatory action of adenosine 5’-triphosphate (ATP), the main co-transmitter with acetylcholine, and adenosine, the final ATP metabolite in the synaptic cleft, on neuromuscular transmission is presented. The effects of these endogenous modulators on pre- and post-synaptic processes are discussed. The contribution of purines to the processes of quantal and non-quantal secretion of acetylcholine into the synaptic cleft, as well as the influence of the postsynaptic effects of ATP and adenosine on the functioning of cholinergic receptors, are evaluated. As usual, the P2-receptor-mediated influence is minimal under physiological conditions, but it becomes very important in some pathophysiological situations such as hypothermia, stress, or ischemia. There are some data demonstrating the same in neuromuscular transmission. It is suggested that the role of endogenous purines is primarily to provide a safety factor for the efficiency of cholinergic neuromuscular transmission.

Unique synaptic topography of crest-type synapses in the interpeduncular nucleus

Neurons in the central nervous system display a great diversity of synaptic architecture. While much of our knowledge on the excitatory synapse morphology derives from the prototypical asymmetric synapses, little has been studied about the atypical crest-type synapse that exists in the restricted brain regions. Here, we used focused ion beam scanning electron microscopy (FIB/SEM) to image a neuropil volume of interpeduncular nucleus (IPN) and manually reconstructed several dendrites to obtain an insight about the topography and quantitative features of crest synapses. Three-dimensional reconstruction showed numerous U-shaped structures protruding from the IPN dendrites. On either faces of the U-shaped structure, a pair of crest synapses are aligned in parallel such that there exists a positive correlation between the postsynaptic density (PSD) area of synapses that participate in pair formation. Interestingly, mitochondria are excluded from the site of crest synapses. Several presynaptic axons run through the hollow, cylindrical space of the U-shape grooves such that the plasma membrane of the axon and the dendrite are organized in a tight opposition without any intervening glial membrane. Unlike the peculiar dendritic morphology, IPN neurons possess typical somatic morphology with an oval, centrally located nucleus. In conclusion, our data reveals a hitherto unknown unique topographical feature of crest synapses in the IPN.

New light on cortical neuropeptides and synaptic network plasticity

Neuropeptides, members of a large and evolutionarily ancient family of proteinaceous cell-cell signaling molecules, are widely recognized as extremely potent regulators of brain function and behavior. At the cellular level, neuropeptides are known to act mainly via modulation of ion channel and synapse function, but functional impacts emerging at the level of complex cortical synaptic networks have resisted mechanistic analysis. New findings from single-cell RNA-seq transcriptomics now illuminate intricate patterns of cortical neuropeptide signaling gene expression and new tools now offer powerful molecular access to cortical neuropeptide signaling. Here we highlight some of these new findings and tools, focusing especially on prospects for experimental and theoretical exploration of peptidergic and synaptic networks interactions underlying cortical function and plasticity.

Role of the Serotonin Receptor 7 in Brain Plasticity: From Development to Disease

Our knowledge on the plastic functions of the serotonin (5-HT) receptor subtype 7 (5-HT7R) in the brain physiology and pathology have advanced considerably in recent years. A wealth of data show that 5-HT7R is a key player in the establishment and remodeling of neuronal cytoarchitecture during development and in the mature brain, and its dysfunction is linked to neuropsychiatric and neurodevelopmental diseases. The involvement of this receptor in synaptic plasticity is further demonstrated by data showing that its activation allows the rescue of long-term potentiation (LTP) and long-term depression (LTD) deficits in various animal models of neurodevelopmental diseases. In addition, it is becoming clear that the 5-HT7R is involved in inflammatory intestinal diseases, modulates the function of immune cells, and is likely to play a role in the gut-brain axis. In this review, we will mainly focus on recent findings on this receptor’s role in the structural and synaptic plasticity of the mammalian brain, although we will also illustrate novel aspects highlighted in gastrointestinal (GI) tract and immune system.

Presynaptic MAST kinase controls opposing postsynaptic responses to convey stimulus valence in Caenorhabditis elegans

Animals need to quickly extract the valence information of sensory stimulus and assess whether the stimulus is attractive or aversive. Deciphering the molecular and circuit mechanisms that determine the stimulus valence is fundamental to understand how the nervous system generates the animal behaviors. Here we report that the AFD thermosensory neurons of C. elegans evoke in its postsynaptic AIY interneurons opposing neuronal responses that correlate with the valence of thermal stimuli. The C. elegans homologs of MAST kinase, Stomatin, and Diacylglycerol kinase function in AFD and regulate the opposing AIY responses. Our results further suggest that the alteration between excitatory and inhibitory AIY responses is mediated by controlling the balance of two opposing signals released from the AFD neurons.

Presynaptic plasticity is known to modulate the strength of synaptic transmission. However, it remains unknown whether regulation in presynaptic neurons can evoke excitatory and inhibitory postsynaptic responses. We report here that the Caenorhabditis elegans homologs of MAST kinase, Stomatin, and Diacylglycerol kinase act in a thermosensory neuron to elicit in its postsynaptic neuron an excitatory or inhibitory response that correlates with the valence of thermal stimuli. By monitoring neural activity of the valence-coding interneuron in freely behaving animals, we show that the alteration between excitatory and inhibitory responses of the interneuron is mediated by controlling the balance of two opposing signals released from the presynaptic neuron. These alternative transmissions further generate opposing behavioral outputs necessary for the navigation on thermal gradients. Our findings suggest that valence-encoding interneuronal activity is determined by a presynaptic mechanism whereby MAST kinase, Stomatin, and Diacylglycerol kinase influence presynaptic outputs.

Molecular and Cellular Mechanisms Underlying Somatostatin-Based Signaling in Two Model Neural Networks, the Retina and the Hippocampus

Neural inhibition plays a key role in determining the specific computational tasks of different brain circuitries. This functional "braking" activity is provided by inhibitory interneurons that use different neurochemicals for signaling. One of these substances, somatostatin, is found in several neural networks, raising questions about the significance of its widespread occurrence and usage. Here, we address this issue by analyzing the somatostatinergic system in two regions of the central nervous system: the retina and the hippocampus. By comparing the available information on these structures, we identify common motifs in the action of somatostatin that may explain its involvement in such diverse circuitries. The emerging concept is that somatostatin-based signaling, through conserved molecular and cellular mechanisms, allows neural networks to operate correctly.