Peripheral Afferent Stimulation Research Articles

Low-Frequency Stimulation of Trpv1-Lineage Peripheral Afferents Potentiates the Excitability of Spino-Periaqueductal Gray Projection Neurons.

High-threshold dorsal root ganglion (HT DRG) neurons fire at low frequencies during inflammatory injury, and low-frequency stimulation (LFS) of HT DRG neurons selectively potentiates excitatory synapses onto spinal neurons projecting to the periaqueductal gray (spino-PAG). Here, in male and female mice, we have identified an underlying peripheral sensory population driving this plasticity and its effects on the output of spino-PAG neurons. We provide the first evidence that Trpv1-lineage sensory neurons predominantly induce burst firing, a unique mode of neuronal activity, in lamina I spino-PAG projection neurons. We modeled inflammatory injury by optogenetically stimulating Trpv1+ primary afferents at 2 Hz for 2 min (LFS), as peripheral inflammation induces 1-2 Hz firing in high-threshold C fibers. LFS of Trpv1+ afferents enhanced the synaptically evoked and intrinsic excitability of spino-PAG projection neurons, eliciting a stable increase in the number of action potentials (APs) within a Trpv1+ fiber-induced burst, while decreasing the intrinsic AP threshold and increasing the membrane resistance. Further experiments revealed that this plasticity required Trpv1+ afferent input, postsynaptic G protein-coupled signaling, and NMDA receptor activation. Intriguingly, an inflammatory injury and heat exposure in vivo also increased APs per burst, in vitro These results suggest that inflammatory injury-mediated plasticity is driven though Trpv1+ DRG neurons and amplifies the spino-PAG pathway. Spinal inputs to the PAG could play an integral role in its modulation of heat sensation during peripheral inflammation, warranting further exploration of the organization and function of these neural pathways.

Effects of different modalities of afferent stimuli of the lumbo-sacral area on control of lumbar paravertebral muscles.

Somatosensory feedback to the central nervous system is essential to plan, perform and refine spine motor control. However, the influence of somatosensory afferent input from the trunk on the motor output to trunk muscles has received little attention. The objective was to compare the effects of distinct modalities of afferent stimulation on the net motoneuron and corticomotor excitability of paravertebral muscles. Fourteen individuals were recruited. Modulation of corticospinal excitability (motor-evoked potential [MEP]) of paravertebral muscles was measured when afferent stimuli (cutaneous noxious and non-noxious, muscle contraction) were delivered to the trunk at 10 intervals prior to transcranial magnetic stimulation. Each peripheral stimulation was applied alone, and subsequent electromyography (EMG) modulation was measured to control for net motoneuron excitability. MEP modulation and MEP/EMG ratio were used as measures of corticospinal excitability with and without control of net motoneuron excitability, respectively. MEP and EMG modulation were smaller after evoked muscle contraction than after cutaneous noxious and non-noxious stimuli. MEP/EMG ratio was not different between stimulation types. Both MEP and EMG amplitudes were reduced after evoked muscle contraction, but not when expressed as MEP/EMG ratio. Noxious and non-noxious stimulation had limited impact on all variables. Distinct modalities of peripheral afferent stimulation of the lumbo-sacral area differently modulated responses of paravertebral muscles, but without an influence on corticospinal excitability with control of net motoneuron excitability. Muscle stimulation reduced paravertebral activity and was best explained by spinal mechanisms. The impact of afferent stimulation on back muscles differs from the effects reported for limb muscles.

TRPV1-Expressing Sensory Neuron Stimulation-based Model of Inflammatory Injury Enhances the Excitability of Spinal Neurons Targeting the Periaqueductal Gray

Modeling inflammatory injury via high-intensity stimulation of peripheral afferents at low frequencies (LFS) potentiates excitatory input onto spinal neurons projecting (PNs) to the midbrain periaqueductal gray (PAG), a region critical to pain processing. Therefore, we sought to identify the underlying afferent population driving this potentiation and how this enhanced synaptic drive might affect spino-PAG PN output to supraspinal pain circuits. We used 3–6-week-old male and female mice expressing Cre recombinase driven by expression of the TRPV1 gene, which is expressed in the lineage of virtually all C fibers, and Cre-dependent channelrhodopsin to stimulate C fibers. Spinal PNs were retrogradely labeled from the PAG using diI. We performed whole-cell patch-clamp recordings in spino-PAG PNs using a semi-intact spinal cord preparation. Stimulation of TRPV1-expressing (TRPV1+) peripheral afferents induced burst firing in virtually all spino-PAG PNs sampled. Inflammation increases the firing rates of C fibers to 1-2 Hz; therefore, we used LFS (1 ms, 470 nm LED pulses at 2 Hz for 2 mins) to model inflammatory injury. LFS induced a persistent (≥20 min) increase in the number of action potentials (APs) within a C fiber-induced burst (n=10, p=0.029), rise in the frequency of synaptically evoked after discharge (n=10, p=0.0004), and decrease in intraburst afterhyperpolarization (n=10, p=0.037). Furthermore, LFS induced a transient membrane depolarization (Mean=6.796 mV, n=12, p<0.0001). Additional experiments suggested the enhanced synaptic excitability relied on postsynaptic G protein-coupled signaling, NMDA receptors, and TRPV1+ afferent input. Finally, LFS may increase the intrinsic excitability of spino-PAG PNs—by decreasing AP threshold (n=14, p=0.015) and increasing membrane resistance (n=14, p=0.0003). LFS had no effect on the membrane potential nor the firing frequency of spino-PAG PNs. In summary, this work suggests that LFS of TRPV1+ fibers persistently enhances spino-PAG projection neuron output, likely leading to a lasting increase in the activation of PAG neural circuits. Grant support from F32NS123008 NINDS; Brewer, C. B. T32DA035165 NIDA; Mackey, S. C. R01DA011289 NIDA; Kauer, J. A.

A Bayesian approach to analysing cortico-cortical associative stimulation induced increases in the excitability of corticospinal projections in humans

Repeated pairing of transcranial magnetic stimulation (TMS) over left and right primary motor cortex (M1), at intensities sufficient to generate descending volleys, produces sustained increases in corticospinal excitability. In other paired associative stimulation (PAS) protocols, in which peripheral afferent stimulation is the first element, changes in corticospinal excitability achieved when the second stimulus consists of brief bursts of transcranial alternating current stimulation (tACS), are comparable to those obtained if TMS is used instead (McNickle and Carson 2015). The present aim was to determine whether associative effects are induced when the first stimulus of a cortico-cortical pair is tACS, or alternatively subthreshold TMS. Bursts of tACS (500 ms; 140 Hz; 1 mA) were associated (180 stimulus pairs) with single magnetic stimuli (120% resting motor threshold rMT) delivered over the opposite (left) M1. The tACS ended 6 ms prior to the TMS. In a separate condition, TMS (55% rMT) was delivered to right M1 6 ms before (120% rMT) TMS was applied over left M1. In a sham condition, TMS (120% rMT) was delivered to left M1 only. The limitations of null hypothesis significance testing are well documented. We therefore employed Bayes factors to assess evidence in support of experimental hypotheses—defined precisely in terms of predicted effect sizes, that these two novel variants of PAS increase corticospinal excitability. Although both interventions induced sustained (~ 20–30 min) increases in corticospinal excitability, the evidence in support of the experimental hypotheses (over specified alternatives) was generally greater for the paired TMS-TMS than the tACS-TMS conditions.

Corticospinal excitability modulation by pairing peripheral nerve stimulation with cortical states of movement initiation.

We compare the effects on corticospinal excitability of repeatedly delivering peripheral nerve stimulation at three time points (-30ms, 0ms, +50ms) relative to muscle onset in a cue-guided task. Plastic changes in excitability are only observed when stimuli are delivered immediately before the time when muscles activate, while stimuli delivered at muscle onset or shortly later (0, +50ms) have no effect. Plastic effects are abolished if there is ongoing volitional electromyogram activity in the muscles prior to the onset of the phasic contraction. The plastic effects induced by timing peripheral stimulation relative to electromyographic markers of muscle activation are as effective as those that occur if stimulation is timed relative to electroencephalographic markers of motor cortical activation. We provide a simple alternative protocol to induce plasticity in people in whom electroencephalogram recording is difficult. Plastic changes in corticospinal excitability (CSE) and motor function can be induced in a targeted and long-term manner if afferent volleys evoked by peripheral nerve stimulation are repeatedly associated with the peak of premovement brain activity assessed with an electroencephalogram (EEG). The present study investigated whether other factors might also characterize this optimal brain state for plasticity induction. In healthy human volunteers (n=24), we found that the same reliable changes in CSE can be induced by timing peripheral afferent stimulation relative to the onset of electromyogram (EMG) activity rather than using the EEG peak. Specifically, we observed an increase in CSE when peripheral stimulation activated the cortex just before movement initiation. By contrast, there was no effect on CSE if the afferent input reached the cortex at the same time or after EMG onset, consistent with the idea that the temporal order of synaptic activation from afferent input and voluntary movement is important for production of plasticity. Finally, in 14 volunteers, we found that background voluntary muscle activity prior to movement also abolished the effect on CSE. One possible explanation is that the intervention strengthens synapses that are inactive at rest but change their activity in anticipation of movement, and that the intervention fails when the synapses are tonically active during background EMG activity. Overall, we demonstrate that, in individuals with voluntary control of muscles targeted by our intervention, EMG signals are a suitable alternative to an EEG for inducing plasticity by coupling movement-related brain states with peripheral afferent input.

Modulation of TARP γ8-Containing AMPA Receptors as a Novel Therapeutic Approach for Chronic Pain.

Nonselective glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists are efficacious in chronic pain but have significant tolerability issues, likely arising from the ubiquitous expression of AMPA receptors in the central nervous system (CNS). Recently, LY3130481 has been shown to selectively block AMPA receptors coassembled with the auxiliary protein, transmembrane AMPA receptor regulatory protein (TARP) γ8, which is highly expressed in the hippocampus but also in pain pathways, including anterior cingulate (ACC) and somatosensory cortices and the spinal cord, suggesting that selective blockade of γ8/AMPA receptors may suppress nociceptive signaling with fewer CNS side effects. The potency of LY3130481 on recombinant γ8-containing AMPA receptors was modulated by coexpression with other TARPs; γ2 subunits affected activity more than γ3 subunits. Consistent with these findings, LY3130481 had decreasing potency on receptors from rat hippocampal, cortical, spinal cord, and cerebellar neurons that was replicated in tissue from human brain. LY3130481 partially suppressed, whereas the nonselective AMPA antagonist GYKI53784 completely blocked, AMPA receptor-dependent excitatory postsynaptic potentials in ACC and spinal neurons in vitro. Similarly, LY3130481 attenuated short-term synaptic plasticity in spinal sensory neurons in vivo in response to stimulation of peripheral afferents. LY3130481 also significantly reduced nocifensive behaviors after intraplantar formalin that was correlated with occupancy of CNS γ8-containing AMPA receptors. In addition, LY3130481 dose-dependently attenuated established gait impairment after joint damage and tactile allodynia after spinal nerve ligation, all in the absence of motor side effects. Collectively, these data demonstrate that LY3130481 can suppress excitatory synaptic transmission and plasticity in pain pathways containing γ8/AMPA receptors and significantly reduce nocifensive behaviors, suggesting a novel, effective, and safer therapy for chronic pain conditions.

Long-term effects of direct current are reproduced by intermittent depolarization of myelinated nerve fibers.

Direct current (DC) potently increases the excitability of myelinated afferent fibers in the dorsal columns, both during DC polarization of these fibers and during a considerable (>1 h) postpolarization period. The aim of the present study was to investigate whether similarly long-lasting changes in the excitability of myelinated nerve fibers in the dorsal columns may be evoked by field potentials following stimulation of peripheral afferents and by subthreshold epidurally applied current pulses. The experiments were performed in deeply anesthetized rats. The effects were monitored by changes in nerve volleys evoked in epidurally stimulated hindlimb afferents and in the synaptic actions of these afferents. Both were found to be facilitated during as well as following stimulation of a skin nerve and during as well as following epidurally applied current pulses of 5- to 10-ms duration. The facilitation occurring ≤2 min after skin nerve stimulation could be linked to both primary afferent depolarization and large dorsal horn field potentials, whereas the subsequent changes (up to 1 h) were attributable to effects of the field potentials. The findings lead to the conclusion that the modulation of spinal activity evoked by DC does not require long-lasting polarization and that relatively short current pulses and intrinsic field potentials may contribute to plasticity in spinal activity. These results suggest the possibility of enhancing the effects of epidural stimulation in human subjects by combining it with polarizing current pulses and peripheral afferent stimulation and not only with continuous DC. NEW & NOTEWORTHY The aim of this study was to define conditions under which a long-term increase is evoked in the excitability of myelinated nerve fibers. The results demonstrate that a potent and long-lasting increase in the excitability of afferent fibers traversing the dorsal columns may be induced by synaptically evoked intrinsic field as well as by epidurally applied intermittent current pulses. They thus provide a new means for the facilitation of the effects of epidural stimulation.

23. Effects of visual paired associative stimulation on visual evoked potentials in healthy volunteers

Associative stimulation of peripheral afferents paired with low-frequency transcranial magnetic stimulation (LF-TMS) over the motor area (PAS) at an interstimulus interval (ISI) slightly shorter or longer than the time needed for the afferent inputs to reach the cerebral cortex, can decrease or increase the excitability of the motor system. None has proven whether the same time-dependent plasticity rules may also be valid for the visual system. In 12 healthy subjects, we performed a visual PAS (vPAS) protocol by coupling 90 black-and-white checkerboard reversals with LF-TMS pulses over the occipital cortex at 4 ISIs in separate sessions chosen by subtracting or adding 25/50 ms to the visual evoked potential (VEP) P1 latency. We recorded VEPs (600 sweeps) before, after, and 10-min later each vPAS session. VEPs were partitioned in 6 blocks of 100 sweeps. We analyzed VEP N1-P1 first amplitude and delayed habituation. VEP amplitude of the first block was reduced immediately after PAS+25, while it remained unchanged after the other vPAS protocols. Delayed VEP habituation significantly increased after PAS−25, and decreased after PAS+25. Associative bidirectional synaptic plasticity is not confined to the corticomotor system, but can also be a feature of the visual system.

Stimulation is never quite as simple as it seems.

Stimulation is never quite as simple as it seems.

Paired associative transcranial alternating current stimulation increases the excitability of corticospinal projections in humans.

Many types of non-invasive brain stimulation alter corticospinal excitability (CSE). Paired associative stimulation (PAS) has attracted particular attention as its effects ostensibly adhere to Hebbian principles of neural plasticity. In prototypical form, a single electrical stimulus is directed to a peripheral nerve in close temporal contiguity with transcranial magnetic stimulation delivered to the contralateral primary motor cortex (M1). Repeated pairing of the two discrete stimulus events (i.e. association) over an extended period either increases or decreases the excitability of corticospinal projections from M1, contingent on the interstimulus interval. We studied a novel form of associative stimulation, consisting of brief trains of peripheral afferent stimulation paired with short bursts of high frequency (≥80 Hz) transcranial alternating current stimulation (tACS) over contralateral M1. Elevations in the excitability of corticospinal projections to the forearm were observed for a range of tACS frequency (80, 140 and 250 Hz), current (1, 2 and 3 mA) and duration (500 and 1000 ms) parameters. The effects were at least as reliable as those brought about by PAS or transcranial direct current stimulation. When paired with tACS, muscle tendon vibration also induced elevations of CSE. No such changes were brought about by the tACS or peripheral afferent stimulation alone. In demonstrating that associative effects are expressed when the timing of the peripheral and cortical events is not precisely circumscribed, these findings suggest that multiple cellular pathways may contribute to a long term potentiation-type response. Their relative contributions will differ depending on the nature of the induction protocol that is used.

Effects of somatosensory stimulation on corticomotor excitability in patients with unilateral cerebellar infarcts and healthy subjects - preliminary results.

BackgroundIn healthy humans, somatosensory stimulation in the form of 2 h-repetitive peripheral afferent nerve stimulation (SS) increases excitability of the contralateral motor cortex. In this preliminary study, we explored effects of SS on excitability to transcranial magnetic stimulation (TMS) in patients with unilateral cerebellar infarcts and age-matched controls.MethodsTen patients with infarcts in one cerebellar hemisphere and six age-matched controls participated in the study. Each subject participated in one session of active, and one session of sham SS delivered to the median nerve ipsilateral to the cerebellar infarct in patients, and to the homologous nerve in controls. Before and after each session, the following TMS measures were performed: resting motor threshold (rMT), motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF). Amplitudes of motor evoked potentials were normalized to amplitudes of supramaximal M responses (MEP/M ratios).ResultsIn the control group, there was a significant increase in rMT, and a significant increase in MEP/M ratios after active, but not after sham SS. There were no significant differences in rMT or MEP/M ratios in the group of patients after active or sham SS. There were no significant differences in SICI or SICF after active or sham SS in either group.ConclusionConsistent with results reported in rodents, these preliminary findings suggest for the first time in humans, that normal cerebellar activity is required so that SS can modulate excitability of the sensorimotor cortex.

The pressor response to activation of the central command pathway is attenuated by stimulation of peripheral sensory afferents in normotensive but not hypertensive rats

Recent evidence suggests that central command (CC; a neural drive originating in higher brain centers) partially mediates the exaggerated BP response to exercise in hypertension. The mechanisms underlying this CC overactivity remain unclear. It has been suggested that CC is modulated by input from peripheral sensory afferents (PSA). A change in this modulatory relationship could potentially play a role in the generation CC overactivity in hypertension. To address this issue, the BP response to activation of the CC pathway before and during concurrent stimulation of PSA was examined in normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Rats were decerebrated and paralyzed. Activation of the CC pathway was evoked by electrical stimulation of the mesencephalic locomotor region (MLR; 20–50 μA in 10‐μA steps). Electrical stimulation of the sciatic nerve (3, 5 and 10 × motor threshold; MT) was used to activate PSA. The BP response to MLR activation was significantly attenuated by sciatic nerve stimulation at 5 and 10 × MT in WKY but not in SHR. This finding suggests that PSA input maintains the potential to inhibit CC activity in normal rats. The data further indicate that this inhibitory influence is reduced in hypertension. This change in the modulatory capacity of PSA may contribute, in part, to the development of CC overactivity in this disease. Support: NIH HL‐088422

Evidence of Spinal Cord Hyperexcitability as Measured With Nociceptive Flexion Reflex (NFR) Threshold in Chronic Lateral Epicondylalgia With or Without a Positive Neurodynamic Test

There is emerging evidence of altered pain signal processing as a likely underlying mechanism in chronic lateral epicondylalgia (LE), yet this remains to be assessed. Furthermore, it has been proposed that neurodynamic tests reflect nociceptive withdrawal responses. Therefore, the objective was to improve our understanding of spinal cord excitability as measured by nociceptive flexion reflex (NFR) threshold in chronic LE with and without a positive neurodynamic test. NFR threshold, pain-free grip, and pressure pain threshold were measured in 30 LE participants and 31 healthy controls. Test of neural tissue involvement (using upper limb neural tension, radial bias) was used to differentiate LE participants with or without a positive neurodynamic test. There were significant differences in NFR threshold between the control and LE with or without a positive neurodynamic test (F[2,54] = 5.68, P = .006), after adjusting for age, sex, pain rating at NFR threshold, and reflex size (NFR interval peak z score). The mean differences (95% confidence interval) in NFR threshold between the control and LE with or without a positive neurodynamic test were 3.74 mA (.637, 6.84) and 3.38 mA (.0245, 6.74) respectively. PerspectiveThe results suggest evidence of spinal cord hyperexcitability, particularly sensory hypersensitivity, in LE with or without a positive neurodynamic test. Our data appear to support the hypothesis that continued peripheral afferent stimulation results in facilitation of nociceptive pathways in this patient population.

Abnormal sensory-motor integration in a patient with anti-NMDA-receptor encephalitis.

In this report, we describe the case of patient who showed abnormal corticomotor responses after peripheral afferent stimulation, namely sensory afferent inhibition (SAI) and sensory afferent facilitation (SAF), and its reversibility after treatment in a patient with anti-N-methyl D-aspartate receptor (NMDAR) encephalitis. A 26-year-old woman was admitted to a hospital after experiencing fever and headache for 11 days. Comprehensive serological studies, a brain magnetic resonance imaging scan, and cerebrospinal fluid (CSF) analysis were unremarkable. In the following days, she developed hallucinations and generalized tonic seizures. A repeat CSF analysis showed lymphocytic pleocytosis (261 white blood cells/ mm; 97% lymphocytes) and a normal protein concentration (63 mg/dl). She continued to have hyperkinetic movements and hyper-pyrexia despite receiving antibacterial and antiviral treatments. Electroencephalography showed diffuse slow waves with unremarkable somatosensory evoked potentials and brainstem auditory evoked potentials. After 30 days, she became unresponsive and had respiratory failure, and was then transferred to our hospital. On examination, she presented with dystonic posturing, oro-lingual-facial dyskinesia, and myoclonic movements. A computed tomography scan of the chest and abdomen and a trans-vaginal sonogram disclosed no abnormalities. Her serum and CSF studies were positive for anti-NMDAR antibodies. Transcranial magnetic stimulation (TMS) demonstrated intact corticospinal tracts with normal latencies and thresholds for motor-evoked potentials (MEPs). However, the conditioned peripheral afferent stimulation failed to show an SAI response at inter-stimulus intervals (ISIs) in the range of 15–25 ms, while exaggerated SAF responses were observed at ISIs between 30 and 70 ms. These data are contrasted to data from a healthy control subject in Fig. 1a, b and Table 1. The patient’s condition showed a little improvement in that she started to have some slow responses to verbal stimulation after steroid pulse therapy and plasmapheresis. Intravenous immunoglobulin was then started, and her condition improved dramatically. In a 3-week follow-up SAI/SAF study, the SAF had made notable progress toward normalization, but a normal SAI response was still absent (Fig. 1c; Table 1). NMDARs are expressed at high levels in the central nervous system, and play an important role in excitatory synaptic transmission and synaptic plasticity. Patients with anti-NMDAR antibodies usually present with various neurological symptoms presumably resulting from an AMPA receptor-mediated hyperglutamatergic state [1]. The MEP amplitude and latency of the patient described herein were similar to normal values. Impaired SAI and exaggerated SAF, such as observed in the presently described case, are suggestive of abnormal motor-sensory Y.-H. Dou K.-L. Lai K.-K. Liao S.-P. Chen (&) Department of Neurology, Neurological Institute, Taipei Veterans General Hospital, 112 Taipei, Taiwan e-mail: chensp1977@gmail.com

Spinal inhibition of descending command to soleus motoneurons is removed prior to dorsiflexion

It has recently been demonstrated that soleus motor-evoked potentials (MEPs) are facilitated prior to the onset of dorsiflexion. The purpose of this study was to examine if this could be explained by removal of spinal inhibition of the descending command to soleus motoneurons. To test this, we investigated how afferent inputs from the tibialis anterior muscle modulate the corticospinal activation of soleus spinal motoneurons at rest, during static contraction and prior to movement. MEPs activated by transcranial magnetic stimulation (TMS) and Hoffmann reflexes (H-reflexes), activated by electrical stimulation of the posterior tibial nerve (PTN), were conditioned by prior stimulation of the common peroneal nerve (CPN) at a variety of conditioning-test (CT) intervals. MEPs in the precontracted soleus muscle were inhibited when the TMS pulse was preceded by CPN stimulation with a CT interval of 35 ms, and they were facilitated for CT intervals of 50-55 ms. A similar inhibition of the soleus H-reflex was not observed. To investigate which descending pathways might be responsible for the afferent-evoked inhibition and facilitation, we examined the effect of CPN stimulation on short-latency facilitation (SLF) and long-latency facilitation (LLF) of the soleus H-reflex induced by a subthreshold TMS pulse at different CT intervals. SLF is known to reflect the excitability of the fastest conducting, corticomotoneuronal cells whereas LLF is believed to be caused by more indirect descending pathways. At CT intervals of 40-45 ms, the LLF was significantly more inhibited compared to the SLF when taking the effect on the H-reflex into account. Finally, we investigated how the CPN-induced inhibition and facilitation of the soleus MEP were modulated prior to dorsiflexion. Whereas the late facilitation (CT interval: 55 ms) was similar prior to dorsiflexion and at rest, no inhibition could be evoked at the earlier latency (CT interval: 35 ms) prior to onset of dorsiflexion. The observation that the CPN-induced inhibition of soleus MEPs disappears prior to onset of dorsiflexion may explain why soleus MEPs are facilitated prior to onset of dorsiflexion contraction. A possible mechanism involves the removal of inhibition of the descending command to the motoneurons at a spinal interneuronal level because the inhibition was seen in LLF and not in SLF, and the MEP inhibition was not observed in the H-reflex. The data illustrate that spinal interneuronal pathways modify descending commands to human spinal motoneurons and influence the size of MEPs elicited by TMS.

Identification of Minimal Neuronal Networks Involved in Flexor-Extensor Alternation in the Mammalian Spinal Cord

Neural networks in the spinal cord control two basic features of locomotor movements: rhythm generation and pattern generation. Rhythm generation is generally considered to be dependent on glutamatergic excitatory neurons. Pattern generation involves neural circuits controlling left-right alternation, which has been described in great detail, and flexor-extensor alternation, which remains poorly understood. Here, we use a mouse model in which glutamatergic neurotransmission has been ablated in the locomotor region of the spinal cord. The isolated invitro spinal cord from these mice produces locomotor-like activity-when stimulated with neuroactive substances-with prominent flexor-extensor alternation. Under these conditions, unlike in control mice, networks of inhibitory interneurons generate the rhythmic activity. In the absence of glutamatergic synaptic transmission, the flexor-extensor alternation appears to be generated by Ia inhibitory interneurons, which mediate reciprocal inhibition from muscle proprioceptors to antagonist motor neurons. Our study defines a minimal inhibitory network that is needed to produce flexor-extensor alternation during locomotion.

Endogenous descending modulation: spatiotemporal effect of dynamic imbalance between descending facilitation and inhibition of nociception

In conscious rats, we investigated the change of nociceptive paw withdrawal reflexes elicited by mechanical and heat stimuli during intramuscular (i.m.) 5.8% hypertonic (HT) saline elicited muscle nociception. i.m. injection of HT saline caused rapid onset, long lasting (around 7 days), bilateral mechanical hyperalgesia, while it induced bilateral, slower onset (1 day after the HT saline injection), long-term (about 1-2 weeks) heat hypoalgesia. Ipsilateral topical pre-treatment of the sciatic nerve with 1% capsaicin significantly prevented the occurrence of both the bilateral mechanical hyperalgesia and the contralateral heat hypoalgesia. Intrathecal administration of either 6-hydroxydopamine hydrobromide (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT), and intraperitoneal injection of naloxone all markedly attenuated the HT saline induced bilateral heat hypoalgesia, but not the mechanical hyperalgesia. Combined with experiments with lesioning of the rostroventral medulla with kainic acid, the present data indicate that unilateral i.m. injection of HT saline elicits time-dependent bilateral long-term mechanical hyperalgesia and heat hypoalgesia, which were modulated by descending facilitatory and inhibitory controls, respectively. We hypothesize that supraspinal structures may function to discriminate between afferent noxious inputs mediated by Aδ- and C-fibres, either facilitating Aδ-fibre mediated responses or inhibiting C-fibre mediated activities. However, this discriminative function is physiologically silent or inactive, and can be triggered by stimulation of peripheral C-fibre afferents. Importantly, in contrast to the rapid onset of descending facilitation, the late occurrence of descending inhibition suggests a requirement of continuous C-fibre input and temporal summation. Thus, a reduction of C-fibre input using exogenous analgesic agents, i.e. opioids, may counteract the endogenous descending inhibition.

Cerebellar Golgi cells in the rat receive convergent peripheral inputs via a lateral reticular nucleus relay

Golgi cells are important players in the function of the cerebellar cortex, controlling the flow of incoming information from mossy fibres to the granule cells, which excite other cortical neurons. We recently showed that in anaesthetized rats most Golgi cells respond to stimulation of afferents from a very wide peripheral receptive field with a long-lasting depression of firing. These responses are mediated via a crossed ascending afferent pathway but the supraspinal part of this pathway is unknown. Here we have examined the hypothesis that the lateral reticular nucleus, a brainstem nucleus with known broad afferent convergence that projects mossy fibres to much of the cerebellum, is involved. First, we showed that single-pulse electrical microstimulation within the lateral reticular nucleus can elicit long-lasting depressions in Golgi cells, which are qualitatively similar to those evoked by peripheral afferent stimulation. Second, we showed that the amplitude of the depressions of Golgi cell firing evoked by peripheral stimulation can be reduced by pharmacological manipulation of the lateral reticular nucleus, either ipsilateral or contralateral to the stimulus site, with local injections of either the GABA(A) receptor agonist muscimol or the AMPA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione. This evidence suggests that the lateral reticular nucleus is a relay nucleus in the brainstem for peripheral afferent information in a pathway that generates Golgi cell long-lasting depression responses.

Repetitive paired stimulation of nasotrigeminal and peripheral chemoreceptor afferents cause progressive potentiation of the diving bradycardia

Hallmarks of the mammalian diving response are protective apnea and bradycardia. These cardiorespiratory adaptations can be mimicked by stimulation of the trigeminal ethmoidal nerve (EN5) and reflect oxygen-conserving mechanisms during breath-hold dives. Increasing drive from peripheral chemoreceptors during sustained dives was reported to enhance the diving bradycardia. The underlying neuronal mechanisms, however, are unknown. In the present study, expression and plasticity of EN5-bradycardias after paired stimulation of the EN5 and peripheral chemoreceptors was investigated in the in situ working heart-brain stem preparation. Paired stimulations enhanced significantly the bradycardic responses compared with EN5-evoked bradycardia using submaximal stimulation intensity. Alternating stimulations of the EN5 followed by paired stimulation of the EN5 and chemoreceptors (10 trials, 3-min interval) caused a progressive and significant potentiation of EN5-evoked diving bradycardia. In contrast, bradycardias during paired stimulation remained unchanged during repetitive stimulation. The progressive potentiation of EN5-bradycardias was significantly enhanced after microinjection of the 5-HT(3) receptor agonist (CPBG hydrochloride) into the nucleus tractus solitarii (NTS), while the 5-HT(3) receptor antagonist (zacopride hydrochloride) attenuated the progressive potentiation. These results suggest an integrative function of the NTS for the multimodal mediation of the diving response. The potentiation or training of a submaximal diving bradycardia requires peripheral chemoreceptor drive and involves neurotransmission via 5-HT(3) receptor within the NTS.

Climbing fibre‐dependent changes in Golgi cell responses to peripheral stimulation

Golgi cells are important elements of the cerebellar cortex, controlling the flow of mossy fibre information to other cells via granule cells. Several anatomical reports suggest that climbing fibre afferents contact Golgi cells, and electrophysiological studies suggest that they depress Golgi cell firing. We reinvestigated this issue and, given that climbing fibres mediate synaptic plasticity in the cerebellar cortex, we have examined the effects of conjunctive stimulation of peripheral afferents and climbing fibres on Golgi cell responses. The results confirm that climbing fibre stimulation depresses Golgi cell firing at short latency. Golgi cells responded to stimulation of peripheral afferents with longer latency depressions of firing and after conjunctive stimulation with climbing fibres these were significantly reduced. The reductions developed progressively over 20 min of conjunctive stimulation and were persistent (up to 84 min). Temporal conjunction of the inputs was important because non-synchronous stimulation of climbing fibres and peripheral afferents failed to alter the peripheral afferent-evoked response in Golgi cells. In control experiments using either the same climbing fibre stimulation alone, or peripheral afferent stimulation paired with brainstem stimulation that did not activate climbing fibres, responses were not depressed. The results thus show that conjunctive stimulation of climbing fibres with other inputs to Golgi cells can induce long-term changes in Golgi cell responses in vivo. This raises the possibility that changes in Golgi cell peripheral responses mediated by climbing fibres can potentially contribute to cerebellar motor learning.