Nanoelectrochemistry reveals how presynaptic neurons regulate vesicle release to sustain synaptic plasticity under repetitive stimuli

Synaptic plasticity is the ability of synapses to modulate synaptic strength in response to dynamic changes within, as well as environmental changes. Although there is a considerable body of knowledge on protein expression and receptor migration in different categories of synaptic plasticity, the contribution and impact of presynaptic vesicle release and neurotransmitter levels towards plasticity remain largely unclear. Herein, nanoelectrochemistry using carbon fiber nanoelectrodes with excellent spatio-temporal resolution was applied for real-time monitoring of presynaptic vesicle release of dopamine inside single synapses of dopaminergic neurons, and exocytotic variations in quantity and kinetics under repetitive electrical stimuli. We found that the presynaptic terminal tends to maintain synaptic strength by rapidly recruiting vesicles, changing the dynamics of exocytosis, and maintaining sufficient neurotransmitter release in following stimuli. Except for small clear synaptic vesicles, dense core vesicles are involved in exocytosis to sustain the neurotransmitter level in later periods of repetitive stimuli. These data indicate that vesicles use a potential regulatory mechanism to establish short-term plasticity, and provide new directions for exploring the synaptic mechanisms in connection and plasticity.

Interaction between Calcium Chelators and the Activity of P2X7 Receptors in Mouse Motor Synapses

The ability of P2X7 receptors to potentiate rhythmically evoked acetylcholine (ACh) release through Ca²⁺ entry via P2X7 receptors and via L-type voltage-dependent Ca²⁺ channels (VDCCs) was compared by loading Ca²⁺ chelators into motor nerve terminals. Neuromuscular preparations of the diaphragms of wild-type (WT) mice and pannexin-1 knockout (Panx1^−/−) mice, in which ACh release is potentiated by the disinhibition of the L-type VDCCs upon the activation of P2X7 receptors, were used. Miniature end-plate potentials (MEPPs) and evoked end-plate potentials (EPPs) were recorded when the motor terminals were loaded with slow or fast Ca²⁺ chelators (EGTA-AM or BAPTA-AM, respectively, 50 μM). In WT and Panx1^−/− mice, EGTA-AM did not change either spontaneous or evoked ACh release, while BAPTA-AM inhibited synaptic transmission by suppressing the quantal content of EPPs throughout the course of the short rhythmic train (50 Hz, 1 s). In the motor synapses of either WT or Panx1^−/− mice in the presence of BAPTA-AM, the activation of P2X7 receptors by BzATP (30 μM) returned the EPP quantal content to the control level. In the neuromuscular junctions (NMJs) of Panx1^−/− mice, EGTA-AM completely prevented the BzATP-induced increase in EPP quantal content. After Panx1^−/− NMJs were treated with BAPTA-AM, BzATP lost its ability to enhance the EPP quantal content to above the control level. Nitrendipine (1 μM), an inhibitor of L-type VDCCs, was unable to prevent this BzATP-induced enhancement of EPP quantal content to the control level. We propose that the activation of P2X7 receptors may provide additional Ca²⁺ entry into motor nerve terminals, which, independent of the modulation of L-type VDCC activity, can partially reduce the buffering capacity of Ca²⁺ chelators, thereby providing sufficient Ca²⁺ signals for ACh secretion at the control level. However, the activity of both Ca²⁺ chelators was sufficient to eliminate Ca²⁺ entry via L-type VDCCs activated by P2X7 receptors and increase the EPP quantal content in the NMJs of Panx1^−/− mice to above the control level.