Long‐term in vivo modulation of synaptic efficacy at the neuromuscular junction of Rana pipiens frogs

Abstract

Prolonged changes in motor neurone activity can result in long-term changes in synaptic transmission. We investigated whether mechanisms commonly thought to be involved in determining synaptic efficacy of vertebrate motor neurones are involved in these long-term changes. The nerve supplying the cutaneus pectoris muscle was chronically stimulated via skin surface electrodes in freely moving frogs for 5-7 days. Chronic stimulation induced a 50% reduction in evoked endplate potential (EPP) amplitude at stimulated neuromuscular junctions (NMJs). These changes appear to be presynaptic since miniature EPP (mEPP) amplitude was unchanged while mEPP frequency was decreased by 46% and paired-pulse facilitation was increased by 26%. High frequency facilitation (40 Hz, 2 s) was also increased by 89%. Moreover, stimulated NMJs presented a 92% decrease in synaptic depression (40 Hz, 2 s). An increase in mitochondrial metabolism was observed as indicated by a more pronounced labelling of active mitochondria (Mitotracker) in stimulated nerve terminals, which could account for their greater resistance to synaptic depression. NMJ length visualized by α-bungarotoxin staining of nAChRs was not affected. Presynaptic calcium signals measured with Calcium Green-1 were larger in stimulated NMJs at low frequency (0.2 Hz) and not different from control NMJs at higher frequency (40 Hz, 2 s and 30 s). These results suggest that some mechanisms downstream of calcium entry are responsible for the determination of synaptic output, such as a down-regulation of some calcium-binding proteins, which could explain the observed results. The possibility of a change in frequenin expression, a calcium-binding protein that is more prominently expressed in phasic synapses, was, however, refuted by our results.

Physiological Activity Depresses Synaptic Function through an Effect on Vesicle Priming

“…A similar presynaptic effect has been reported recently in vivo with chronic stimulation at the neuromuscular junction (Belair et al, 2005), indicating that these mechanisms are likely to be important in many forms of synaptic plasticity. It will be of great interest to determine under what conditions synaptic vesicle priming assumes the primary role in homeostatic modulation and to determine the molecular pathways involved in the induction and expression of this form of synaptic plasticity.…”

Mitochondrial Regulation of Neuronal Plasticity

“…Inhibition of protein kinase C blocked potentiation and mitochondrial calcium release. Chronic stimulation of frog motor neurons resulted in a large decrease in synaptic depression which was associated with increased mitochondrial energy metabolism and a larger presynaptic calcium signal, suggesting roles for mitochondria in synaptic calcium handling and efficacy [29]. Recordings from crayfish neuromuscular junctions showed that posttetanic potentiation (PTP), a process mediated by presynaptic Ca 2+ influx, is blocked when mitochondrial Ca 2+ uptake is inhibited [30] suggesting that PTP results from the slow release of Ca 2+ from mitochondria.…”

ER vesicles and mitochondria move and communicate at synapses

“…Electron microscopy shows heterogeneous Ca 2+ sequestration within ER and mitochondria in the dendrites of hippocampal neurons (Pivovarova et al, 2002) and pharmacological exclusion of ER or mitochondria from Ca 2+ buffering modifies synaptic potentials (Markram et al, 1995;Billups and Forsythe, 2002;Medler and Gleason, 2002;Belair et al, 2005). It remains unclear, however, whether and how the neuronal activity is dependent on the spatiotemporal distribution of ER and mitochondria and which role the interactions between the organelles might play.…”

“…ER and mitochondria are involved in the modulation of synaptic transmission (Markram et al, 1995;Billups and Forsythe, 2002;Medler and Gleason, 2002;Belair et al, 2005). The disturbances in [Ca 2+ ] i handling by these organelles can cause necrosis and apoptosis (White et al, 2005).…”