Ca2+ -independent transmission at the central synapse formed between dorsal root ganglion and dorsal horn neurons

GABA and Glycine Synaptic Release on Axotomized Motoneuron Cell Bodies Promotes Motor Axon Regeneration

Motor axon regeneration after traumatic nerve injuries is a slow process that adversely influences patient outcomes because muscle reinnervation delays result in irreversible muscle atrophy and suboptimal axon regeneration. This advocates for investigating methods to accelerate motor axon growth. Electrical nerve stimulation and exercise both enhance motor axon regeneration in rodents and patients, but these interventions cannot always be easily implemented. A roadblock to uncover novel therapeutic approaches based on the effects of activity is the lack of understanding of the synaptic drives responsible for activity-mediated facilitation of axon regeneration. We hypothesized that the relevant excitatory inputs facilitating axon regrowth originate in GABA/glycine synapses, which become depolarizing after downregulation of the potassium chloride cotransporter 2 in motoneurons following axotomy. To test this, we injected tetanus toxin (TeTx) into the tibialis anterior (TA) muscle of mice to block the release of GABA/glycine specifically onto TA motoneurons. Thereafter, we axotomized all sciatic motoneurons by nerve crush and analyzed the time courses of muscle reinnervation in TeTx-treated (TA) and untreated (lateral gastrocnemius [LG]) motoneurons. Muscle reinnervation was slower in TA motoneurons with blocked GABA/glycine synapses, as measured by recovery of M responses and anatomical reinnervation of neuromuscular junctions. Post hoc immunohistochemistry confirmed the removal of the vesicle-associated membrane proteins 1 and 2 by TeTx activity, specifically from inhibitory synapses. These proteins are necessary for the exocytotic release of neurotransmitters. Therefore, we conclude that GABA/glycine neurotransmission on regenerating motoneurons facilitates axon growth and muscle reinnervation, and we discuss possible interventions to modulate these inputs on regenerating motoneurons.

Action Potential Firing Patterns Regulate Dopamine Release via Voltage-Sensitive Dopamine D2 Autoreceptors in Mouse Striatum In Vivo

Dopamine (DA) in the striatum is vital for motor and cognitive behaviors. Midbrain dopaminergic neurons generate both tonic and phasic action potential (AP) firing patterns in behavior mice. Besides AP numbers, whether and how different AP firing patterns per se modulate DA release remain largely unknown. Here by using in vivo and ex vivo models, it is shown that the AP frequency per se modulates DA release through the D2 receptor (D2R), which contributes up to 50% of total DA release. D2R has a voltage-sensing site at D131 and can be deactivated in a frequency-dependent manner by membrane depolarization. This voltage-dependent D2R inhibition of DA release is mediated via the facilitation of voltage-gated Ca2+ channels (VGCCs). Collectively, this work establishes a novel mechanism that APs per se modulate DA overflow by disinhibiting the voltage-sensitive autoreceptor D2R and thus the facilitation of VGCCs, providing a pivotal pathway and insight into mammalian DA-dependent functions in vivo.

Non-ionotropic voltage-gated calcium channel signaling

Voltage-gated calcium channels (VGCCs) are the major conduits for calcium ions (Ca2+) within excitable cells. Recent studies have highlighted the non-ionotropic functionality of VGCCs, revealing their capacity to activate intracellular pathways independently of ion flow. This non-ionotropic signaling mode plays a pivotal role in excitation-coupling processes, including gene transcription through excitation-transcription (ET), synaptic transmission via excitation-secretion (ES), and cardiac contraction through excitation-contraction (EC). However, it is noteworthy that these excitation-coupling processes require extracellular calcium (Ca2+) and Ca2+ occupancy of the channel ion pore. Analogous to the "non-canonical" characterization of the non-ionotropic signaling exhibited by the N-methyl-D-aspartate receptor (NMDA), which requires extracellular Ca2+ without the influx of ions, VGCC activation requires depolarization-triggered conformational change(s) concomitant with Ca2+ binding to the open channel. Here, we discuss the contributions of VGCCs to ES, ET, and EC coupling as Ca2+ binding macromolecules that transduces external stimuli to intracellular input prior to elevating intracellular Ca2+. We emphasize the recognition of calcium ion occupancy within the open ion-pore and its contribution to the excitation coupling processes that precede the influx of calcium. The non-ionotropic activation of VGCCs, triggered by the upstroke of an action potential, provides a conceptual framework to elucidate the mechanistic aspects underlying the microseconds nature of synaptic transmission, cardiac contractility, and the rapid induction of first-wave genes.

GABA and glycine synaptic release on axotomized motoneuron cell bodies promotes motor axon regeneration

Motor axon regeneration after traumatic nerve injuries is a slow process that adversely influences patient outcomes because muscle reinnervation delays result in irreversible muscle atrophy and suboptimal axon regeneration. This advocates for investigating methods to accelerate motor axon growth. Electrical nerve stimulation and exercise both enhance motor axon regeneration in rodents and patients, but these interventions cannot always be easily implemented. A roadblock to uncover novel therapeutic approaches based on the effects of activity is the lack of understanding of the synaptic drives responsible for activity-mediated facilitation of axon regeneration. We hypothesized that the relevant excitatory inputs facilitating axon regrowth originate in GABA/glycine synapses which become depolarizing after downregulation of the potassium chloride cotransporter 2 in motoneurons following axotomy. To test this, we injected tetanus toxin (TeTx) in the tibialis anterior (TA) muscle of mice to block the release of GABA/glycine specifically on TA motoneurons. Thereafter, we axotomized all sciatic motoneurons by nerve crush and analyzed the time-courses of muscle reinnervation in TeTx- treated (TA) and untreated (lateral gastrocnemius, LG) motoneurons. Muscle reinnervation was slower in TA motoneurons with blocked GABA/glycine synapses, as measured by recovery of M- responses and anatomical reinnervation of neuromuscular junctions. Post-hoc immunohistochemistry confirmed the removal of the vesicular associated membrane proteins 1 and 2 by TeTx activity, specifically from inhibitory synapses. These proteins are necessary for exocytotic release of neurotransmitters. Therefore, we conclude that GABA/glycine neurotransmission on regenerating motoneurons facilitates axon growth and muscle reinnervation and discuss possible interventions to modulate these inputs on regenerating motoneurons.

Stronger stimulus triggers synaptic transmission faster through earlier started action potential

Synaptic transmission plays an important and time-sensitive role in the nervous system. Although the amplitude of neurotransmission is positively related to the intensity of external stimulus, whether stronger stimulus could trigger synaptic transmission faster remains unsolved. Our present work in the primary sensory system shows that besides the known effect of larger amplitude, stronger stimulus triggers the synaptic transmission faster, which is regulated by the earlier started action potential (AP), independent of the AP's amplitude. More importantly, this model is further extended from the sensory system to the hippocampus, implying broad applicability in the nervous system. Together, we found that stronger stimulus induces AP faster, which suggests to trigger the neurotransmission faster, implying that the occurrence time of neurotransmission, as well as the amplitude, plays an important role in the timely and effective response of nervous system.

Ca2+ -independent transmission at the central synapse formed between dorsal root ganglion and dorsal horn neurons

A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca2+ -dependent secretion (CDS). The discovery and mechanistic investigations of Ca2+ -independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here, we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (i) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (ii) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca2+ ; (iii) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (iv) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.