Neural Circuit Repair by Low-Intensity rTMS

Electromagnetic brain stimulation is a promising treatment in neurology and psychiatry. However, clinical outcomes are variable and underlying mechanisms remain ill-defined, impeding the development of new effective stimulation protocols. There is increasing application of repetitive transcranial magnetic stimulation (rTMS) to the cerebellum to induce forebrain plasticity through its long-distance cerebello-cerebral circuits. To better understand what magnetic stimulation does within the cerebellum, we have developed tools to generate defined low-intensity (LI) magnetic fields and deliver them in vivo, in 3D organotypic culture and in primary cultures, over a range of stimulation parameters. Here we show that low-intensity rTMS (LI-rTMS) to the cerebellum induces axon growth and synapse formation providing olivocerebellar reinnervation. This repair depends on stimulation pattern, with complex biomimetic patterns being most effective, and this requires the presence of a cellular magnetoreceptor, cryptochrome. To explain these reparative changes, we found that repair-promoting LI-rTMS patterns, but not ineffective ones, increased c-fos expression in Purkinje neurons, consistent with the production of reactive oxygen species by activated cryptochrome. Rather than activating neurons via induced electric currents, we propose that weak magnetic fields act through cryptochrome, activating intracellular signals that induce climbing fibre-Purkinje cell reinnervation. This information opens new routes to optimize cerebellar magnetic stimulation and its potential role as an effective treatment for neurological diseases.

Neural circuit repair by low-intensity magnetic stimulation requires cellular magnetoreceptors and specific stimulation patterns

Although electromagnetic brain stimulation is a promising treatment in neurology and psychiatry, clinical outcomes are variable, and underlying mechanisms are ill-defined, which impedes the development of new effective stimulation protocols. Here, we show, in vivo and ex vivo, that repetitive transcranial magnetic stimulation at low-intensity (LI-rTMS) induces axon outgrowth and synaptogenesis to repair a neural circuit. This repair depends on stimulation pattern, with complex biomimetic patterns being particularly effective, and the presence of cryptochrome, a putative magnetoreceptor. Only repair-promoting LI-rTMS patterns up-regulated genes involved in neuronal repair; almost 40% of were cryptochrome targets. Our data open a new framework to understand the mechanisms underlying structural neuroplasticity induced by electromagnetic stimulation. Rather than neuronal activation by induced electric currents, we propose that weak magnetic fields act through cryptochrome to activate cellular signaling cascades. This information opens new routes to optimize electromagnetic stimulation and develop effective treatments for different neurological diseases.

What does low-intensity rTMS do to the cerebellum?

Non-invasive stimulation of the human cerebellum, such as by transcranial magnetic stimulation (TMS), is increasingly used to investigate cerebellar function and identify potential treatment for cerebellar dysfunction. However, the effects of TMS on cerebellar neurons remain poorly defined. We applied low-intensity repetitive TMS (LI-rTMS) to the mouse cerebellum in vivo and in vitro and examined the cellular and molecular sequelae. In normal C57/Bl6 mice, 4 weeks of LI-rTMS using a complex biomimetic high-frequency stimulation (BHFS) alters Purkinje cell (PC) dendritic and spine morphology; the effects persist 4 weeks after the end of stimulation. We then evaluated whether LI-rTMS could induce climbing fibre (CF) reinnervation to denervated PCs. After unilateral pedunculotomy in adult mice and 2 weeks sham or BHFS stimulation, VGLUT2 immunohistochemistry was used to quantify CF reinnervation. In contrast to sham, LI-rTMS induced CF reinnervation to the denervated hemicerebellum. To examine potential mechanisms underlying the LI-rTMS effect, we verified that BHFS could induce CF reinnervation using our in vitro olivocerebellar explants in which denervated cerebellar tissue is co-cultured adjacent to intact cerebella and treated with brain-derived neurotrophic factor (BDNF) (as a positive control), sham or LI-rTMS for 2 weeks. Compared with sham, BDNF and BHFS LI-rTMS significantly increased CF reinnervation, without additive effect. To identify potential underlying mechanisms, we examined intracellular calcium flux during the 10-min stimulation. Complex high-frequency stimulation increased intracellular calcium by release from intracellular stores. Thus, even at low intensity, rTMS modifies PC structure and induces CF reinnervation.

Acute morphogenic and chemotropic effects of neurotrophins on cultured embryonic Xenopus spinal neurons

Neurotrophins constitute a family of trophic factors with profound effects on the survival and differentiation of the nervous system. Addition of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), but not nerve growth factor (NGF), increased the survival of embryonic Xenopus spinal neurons in culture, although all three neurotrophins enhanced neurite outgrowth. Here we report that neurotrophins also exert acute actions on the morphology and motility of 1-day-old cultured Xenopus spinal neurons. Bath application of BDNF induced extensive formation of lamellipodia simultaneously at multiple sites along the neurite shaft as well as at the growth cone. The BDNF-induced lamellipodia appeared within minutes, rapidly protruded to their greatest extent in about 10 min, and gradually disappeared thereafter, leaving behind newly formed thin lateral processes. When applied as microscopic concentration gradients, both BDNF and NT-3, but not NGF, induced the growth cone to grow toward the neurotrophin source. Our results suggest that neurotrophic factors, when delivered to responsive neurons, may serve as morphogenic and chemotropic agents during neuronal development.

Synapse elimination in the central nervous system: functional significance and cellular mechanisms

Recent research into the developmental elimination of supernumerary synapses has increased understanding of this process. In this review we discuss synapse elimination both at the neuromuscular junction and in the central nervous system, considering some possible underlying mechanisms suggested by recent studies. In addition a well-described example of central nervous system synapse elimination, the climbing fiber-Purkinje cell synapse of the cerebellum, is used to explore the functional significance of synaptic regression during brain development.

Potentiation of developing neuromuscular synapses by the neurotrophins NT-3 and BDNF

The neurotrophins are a family of neurotrophic factors that promote survival and differentiation of various neuronal populations. Although the long-term effects of neurotrophins on neuronal survival and differentiation have been intensively studied, nothing is known about their effects on synaptic function. Here we report that acute exposure to neurotrophin-3 (NT-3) or brain-derived neurotrophic factor (BDNF), but not nerve growth factor (NGF), rapidly potentiates the spontaneous and impulse-evoked synaptic activity of developing neuromuscular synapses in culture. The effect appears to be presynaptic in origin and to be mediated by the Trk family of receptor tyrosine kinases. These results provide evidence for the regulation of the function of developing synapses by neurotrophins.

Asymmetric modulation of cytosolic cAMP activity induces growth cone turning

The possible role of cyclic nucleotides as second messengers mediating growth cone turning was studied by producing an asymmetric distribution of cyclic nucleotides across the growth cone. A repetitive pulse application method was developed to produce microscopic chemical gradients near the growth cone of embryonic Xenopus neurons in cell culture. When picoliters of a solution containing 20 mM dibutyryl cAMP (dB-cAMP), a membrane-permeable analog of cAMP, were repetitively ejected from a micropipette near the growth cone, neurite growth was consistently directed toward the pipette. Theoretical analysis of the diffusion gradient showed that the neurite is capable of detecting a 10% difference in dB-cAMP concentration across the growth cone. Similar responses were also observed using gradients of the phosphodiesterase inhibitor isobutylmethylxanthine, or of forskolin, which activates adenylate cyclase. Dibutyryl cGMP, however, produced no significant turning. These results suggest that a cytoplasmic gradient of cAMP across the growth cone is sufficient to initiate its turning response, and that cAMP in the growth cone could serve as a second messenger in mediating the action of extracellular guidance cues.

Characterization of stimulation-produced analgesia from the nucleus tractus solitarius in the rat

Electrical stimulation of the commissural region of the nucleus tractus solitarius (NTS) inhibits the tail-flick reflex evoked by noxious heat. This antinociception can be measured in the awake or pentobarbital anesthetized rat at current intensities that do not induce overt behavioral side effects. Glutamate microinjections into the NTS, but not immediately surrounding the NTS, also inhibit the tail-flick reflex, demonstrating that activation of NTS cell bodies, and not fibers of passage, mediates antinociception from this region. In contrast, morphine microinjections into the NTS have no effect on the tail-flick reflex in anesthetized rats. These findings provide further evidence that the NTS is involved in the modulation of nociception.

Dopamine-independent motor behavior following microinjection of rimorphin in the substantia nigra

The motor-activating effects of rimorphin, an opioid peptide derived from prodynorphin, were examined in the substantia nigra pars reticulata of rats. Unilateral microinjections of rimorphin produced dose-dependent contralateral rotational behavior that was antagonized by naloxone, suggesting that these effects were mediated by opiate receptors. Lesions of midbrain dopamine cells with 6-hydroxydopamine (6-OHDA) produced a 95% or greater depletion of tyrosine hydroxylase in the striatum ipsilateral to the lesion, but failed to reduce the number of circles made by the rats. In addition to an overall preservation of rimorphin-induced circling in animals with 6-OHDA lesions, 50% of these rats exhibited circling that was at least 2 standard deviations above the mean of animals without lesions. The motor activating effects of rimorphin, thus, appear to occur independently of the nigrostriatal dopamine system; these effects may instead be mediated by GABAergic efferents in the pars reticulata.