Abstract-Repetitive transcranial magnetic stimulation (rTMS) has become a popular method of modulating neural plasticity in humans. Clinically, rTMS is delivered at high intensities to modulate neuronal excitability. While the high-intensity magnetic field can be targeted to stimulate specific cortical regions, areas adjacent to the targeted area receive stimulation at a lower intensity and may contribute to the overall plasticity induced by rTMS. We have previously shown that low-intensity rTMS induces molecular and structural plasticity in vivo, but the effects on membrane properties and neural excitability have not been investigated. Here we investigated the acute effect of low-intensity repetitive magnetic stimulation (LI-rMS) on neuronal excitability and potential changes on the passive and active electrophysiological properties of layer 5 pyramidal neurons in vitro. Whole-cell current clamp recordings were made at baseline prior to subthreshold LI-rMS (600 pulses of iTBS, n = 9 cells from 7 animals) or sham (n = 10 cells from 9 animals), immediately after stimulation, as well as 10 and 20 min post-stimulation. Our results show that LI-rMS does not alter passive membrane properties (resting membrane potential and input resistance) but hyperpolarises action potential threshold and increases evoked spike-firing frequency. Increases in spike firing frequency were present throughout the 20 min poststimulation whereas action potential (AP) threshold hyperpolarization was present immediately after stimulation and at 20 min post-stimulation. These results provide evidence that LI-rMS alters neuronal excitability of excitatory neurons. We suggest that regions outside the targeted region of high-intensity rTMS are susceptible to neuromodulation and may contribute to rTMS-induced plasticity. Ó

Low intensity repetitive magnetic stimulation of neural tissue modulates neuronal excitability and has promising therapeutic potential in the treatment of neurological disorders. However, the underpinning cellular and biochemical mechanisms remain poorly understood. This study investigates the behavioural effects of low intensity repetitive magnetic stimulation (LI-rMS) at a cellular and biochemical level. We delivered LI-rMS (10 mT) at 1 Hz and 10 Hz to B50 rat neuroblastoma cells in vitro for 10 minutes and measured levels of selected metabolites immediately after stimulation. LI-rMS at both frequencies depleted selected tricarboxylic acid (TCA) cycle metabolites without affecting the main energy supplies. Furthermore, LI-rMS effects were frequency-specific with 1 Hz stimulation having stronger effects than 10 Hz. The observed depletion of metabolites suggested that higher spontaneous activity may have led to an increase in GABA release. Although the absence of organised neural circuits and other cellular contributors (e.g., excitatory neurons and glia) in the B50 cell line limits the degree to which our results can be extrapolated to the human brain, the changes we describe provide novel insights into how LI-rMS modulates neural tissue.

The neuromuscular junction (NMJ) is the site of communication between motor nerve axons and muscle fibres. It is composed of four specialised cell types: motor neurons, Schwann cells, muscle fibres and the recently discovered kranocytes. The function of the NMJ is to transmit signals from the motor neuron to the skeletal muscle fibre quickly and reliably, to ensure precise control of skeletal muscle contraction and therefore voluntary movement. The reliability of transmission is aided by specialised architecture (multiple active zones, junctional folds) that promotes high levels of transmitter release, large and reliable postsynaptic responses to transmitter binding and rapid termination of signalling events. In the last century, the structure and function of the NMJ has been extensively studied, which has been instrumental in uncovering many of the fundamental processes of chemical synaptic transmission. Key Concepts: The somatic neuromuscular junction is the site of communication between motor neurons and skeletal muscle fibres. Specialisations of the neuromuscular junction mean that activity in and release of transmitter from motor neurons produces contraction of skeletal muscle fibres rapidly and reliably. The neuromuscular junction comprises four cell types: the motor neuron, terminal Schwann cell, skeletal muscle fibre and kranocyte, with the motor neuron and muscle fibre separated by a gap called the synaptic cleft. The motor nerve terminal contains synaptic vesicles, filled with neurotransmitter, which release their transmitter into the synaptic cleft at multiple specialised sites called active zones, in response to action potential firing. Released transmitter acts at receptors on the muscle membrane, which occur in high‐density clusters at the peaks of muscle membrane infoldings called junctional folds. Junctional folds are unique to the neuromuscular junction, increasing the reliability of transmission by localisation of acetylcholine receptors to the crests of the folds and enhancing the effect of depolarisation by localisation of sodium channels in the troughs. Schwann cells are essential for the development and maintenance of the neuromuscular junction and play important roles in the remodelling and regeneration of damaged neuromuscular junctions. Acetylcholinesterase in the synaptic cleft hydrolyses acetylcholine and limits the temporal and spatial effects of released of acetylcholine, ensuring precision of muscle control. Transmitter binding causes two types of electrical signals in skeletal muscle, miniature endplate potentials caused by the spontaneous release of a single vesicle of acetylcholine and larger endplate potentials. Endplate potentials are caused by activity‐dependent release of multiple transmitter‐filled vesicles and trigger action potential firing in, and thus contraction of, the muscle fibre. The neuromuscular junction is an accessible and relatively easy to study synapse that has led to tremendous progress in our understanding of synapses and in particular neurotransmitter release and continues to be a useful experimental model and educational tool.

The neuromuscular junction (NMJ) provides a connection between the somatic nervous and muscular systems. The present work documents, for the first time, the mature form of the NMJ in an Australian marsupial, the western gray kangaroo Macropus fuliginosus. Marsupials give birth to highly altricial young and the majority of motor system development thus occurs postnatally. We demonstrate that NMJ formation in the forelimbs of kangaroos precedes that in hindlimbs by several weeks, consistent with evidence of precocial forelimb development from other indices of motor system maturation in marsupials. Forelimb and hindlimb NMJs reach a similar level of development by the end of the first postnatal month, an outcome that results from accelerated hindlimb development in the early postnatal period together with a more unexpected stagnation or even regression of forelimb NMJ maturity over the same period. This study suggests that the pattern of NMJ maturation may be adjusted in forelimb as opposed to hindlimb. Elucidation of the underlying cellular control mechanisms may inform current understanding of NMJ formation, and as such the broader processes involved in synaptic formation.

No abstract