Noradrenergic Components of Locomotor Recovery Induced by Intraspinal Grafting of the Embryonic Brainstem in Adult Paraplegic Rats

Intraspinal grafting of serotonergic (5-HT) neurons was shown to restore plantar stepping in paraplegic rats. Here we asked whether neurons of other phenotypes contribute to the recovery. The experiments were performed on adult rats after spinal cord total transection. Grafts were injected into the sub-lesional spinal cord. Two months later, locomotor performance was tested with electromyographic recordings from hindlimb muscles. The role of noradrenergic (NA) innervation was investigated during locomotor performance of spinal grafted and non-grafted rats using intraperitoneal application of α₂ adrenergic receptor agonist (clonidine) or antagonist (yohimbine). Morphological analysis of the host spinal cords demonstrated the presence of tyrosine hydroxylase positive (NA) neurons in addition to 5-HT neurons. 5-HT fibers innervated caudal spinal cord areas in the dorsal and ventral horns, central canal, and intermediolateral zone, while the NA fiber distribution was limited to the central canal and intermediolateral zone. 5-HT and NA neurons were surrounded by each other’s axons. Locomotor abilities of the spinal grafted rats, but not in control spinal rats, were facilitated by yohimbine and suppressed by clonidine. Thus, noradrenergic innervation, in addition to 5-HT innervation, plays a potent role in hindlimb movement enhanced by intraspinal grafting of brainstem embryonic tissue in paraplegic rats.

Contribution of 5-HT2 Receptors to the Control of the Spinal Locomotor System in Intact Rats

Applying serotonergic (5-HT) agonists or grafting of fetal serotonergic cells into the spinal cord improves locomotion after spinal cord injury. Little is known about the role of 5-HT receptors in the control of voluntary locomotion, so we administered inverse agonists of 5-HT₂ (Cyproheptadine; Cypr), 5-HT_2A neutral antagonist (Volinanserin; Volin), 5-HT_2C neutral antagonist (SB 242084), and 5-HT_2B/2C inverse agonist (SB 206553) receptors intrathecally in intact rats and monitored their effects on unrestrained locomotion. An intrathecal cannula was introduced at the low thoracic level and pushed caudally until the tip reached the L2/L3 or L5/L6 spinal segments. Locomotor performance was evaluated using EMG activity of hindlimb muscles during locomotion on a 2 m long runway. Motoneuron excitability was estimated using EMG recordings during dorsi- and plantar flexion at the ankle. Locomotion was dramatically impaired after the blockage of 5-HT_2A receptors. The effect of Cypr was more pronounced than that of Volin since in the L5/L6 rats Cypr (but not Volin) induced significant alteration of the strength of interlimb coordination followed by total paralysis. These agents significantly decreased locomotor EMG amplitude and abolished or substantially decreased stretch reflexes. Blocking 5-HT_2B/2C receptors had no effect either on locomotion or reflexes. We suggest that in intact rats serotonin controls timing and amplitude of muscle activity by acting on 5-HT_2A receptors on both CPG interneurons and motoneurons, while 5-HT_2B/2C receptors are not involved in control of the locomotor pattern in lumbar spinal cord.

Activation of Brainstem Neurons During Mesencephalic Locomotor Region-Evoked Locomotion in the Cat

The distribution of locomotor-activated neurons in the brainstem of the cat was studied by c-Fos immunohistochemistry in combination with antibody-based cellular phenotyping following electrical stimulation of the mesencephalic locomotor region (MLR) – the anatomical constituents of which remain debated today, primarily between the cuneiform (CnF) and the pedunculopontine tegmental nuclei (PPT). Effective MLR sites were co-extensive with the CnF nucleus. Animals subject to the locomotor task showed abundant Fos labeling in the CnF, parabrachial nuclei of the subcuneiform region, periaqueductal gray, locus ceruleus (LC)/subceruleus (SubC), Kölliker–Fuse, magnocellular and lateral tegmental fields, raphe, and the parapyramidal region. Labeled neurons were more abundant on the side of stimulation. In some animals, Fos-labeled cells were also observed in the ventral tegmental area, medial and intermediate vestibular nuclei, dorsal motor nucleus of the vagus, n. tractus solitarii, and retrofacial nucleus in the ventrolateral medulla. Many neurons in the reticular formation were innervated by serotonergic fibers. Numerous locomotor-activated neurons in the parabrachial nuclei and LC/SubC/Kölliker–Fuse were noradrenergic. Few cholinergic neurons within the PPT stained for Fos. In the medulla, serotonergic neurons within the parapyramidal region and the nucleus raphe magnus were positive for Fos. Control animals, not subject to locomotion, showed few Fos-labeled neurons in these areas. The current study provides positive evidence for a role for the CnF in the initiation of locomotion while providing little evidence for the participation of the PPT. The results also show that MLR-evoked locomotion involves the parallel activation of reticular and monoaminergic neurons in the pons/medulla, and provides the anatomical and functional basis for spinal monoamine release during evoked locomotion. Lastly, the results indicate that vestibular, cardiovascular, and respiratory centers are centrally activated during MLR-evoked locomotion. Altogether, the results show a complex pattern of neuromodulatory influences of brainstem neurons by electrical activation of the MLR.

Motoneuron output regulated by ionic channels: a modeling study of motoneuron frequency-current relationships during fictive locomotion

Cat lumbar motoneurons display changes in membrane properties during fictive locomotion. These changes include reduction of input resistance and afterhyperpolarization, hyperpolarization of voltage threshold, and voltage-dependent excitation of the motoneurons. The state-dependent alteration of membrane properties leads to dramatic changes in frequency-current (F-I) relationship. The mechanism underlying these changes remains unknown. Using a motoneuron model combined with electrophysiological data, we investigated the channel mechanisms underlying the regulation of motoneuronal excitability and motor output. Simulation results showed that upregulation of transient sodium, persistent sodium, or Ca_v1.3 calcium conductances or downregulation of calcium-activated potassium or KCNQ/K_v7 potassium conductances could increase motoneuronal excitability and motor output through hyperpolarizing (left shifting) the F-I relationships or increasing the F-I slopes, whereas downregulation of input resistance or upregulation of potassium-mediated leak conductance produced the opposite effects. The excitatory phase of locomotor drive potentials (LDPs) also substantially hyperpolarized the F-I relationships and increased the F-I slopes, whereas the inhibitory phase of the LDPs had opposite effects to a similar extent. The simulation results also showed that none of the individual channel modulations could produce all the changes in the F-I relationships. The effects of modulation of Ca_v1.3 and KCNQ/K_v7 on F-I relationships were supported by slice experiments with the Ca_v1.3 agonist Bay K8644 and the KCNQ/K_v7 antagonist XE-991. The conclusion is that the varying changes in F-I relationships during fictive locomotion could be regulated by multichannel modulations. This study provides insight into the ionic basis for control of motor output in walking. NEW & NOTEWORTHY Mammalian spinal motoneurons have their excitability adapted to facilitate recruitment and firing during locomotion. Cat lumbar motoneurons display dramatic changes in membrane properties during fictive locomotion. These changes lead to a varying alteration of frequency-current relationship. The mechanisms underlying the changes remain unknown. In particular, little is known about the ionic basis for regulation of motoneuronal excitability and thus control of the motor output for walking by the spinal motor system.

LFP Oscillations in the Mesencephalic Locomotor Region during Voluntary Locomotion

Oscillatory rhythms in local field potentials (LFPs) are thought to coherently bind cooperating neuronal ensembles to produce behaviors, including locomotion. LFPs recorded from sites that trigger locomotion have been used as a basis for identification of appropriate targets for deep brain stimulation (DBS) to enhance locomotor recovery in patients with gait disorders. Theta band activity (6–12 Hz) is associated with locomotor activity in locomotion-inducing sites in the hypothalamus and in the hippocampus, but the LFPs that occur in the functionally defined mesencephalic locomotor region (MLR) during locomotion have not been determined. Here we record the oscillatory activity during treadmill locomotion in MLR sites effective for inducing locomotion with electrical stimulation in rats. The results show the presence of oscillatory theta rhythms in the LFPs recorded from the most effective MLR stimulus sites (at threshold ≤60 μA). Theta activity increased at the onset of locomotion, and its power was correlated with the speed of locomotion. In animals with higher thresholds (>60 μA), the correlation between locomotor speed and theta LFP oscillations was less robust. Changes in the gamma band (previously recorded in vitro in the pedunculopontine nucleus (PPN), thought to be a part of the MLR) were relatively small. Controlled locomotion was best achieved at 10–20 Hz frequencies of MLR stimulation. Our results indicate that theta and not delta or gamma band oscillation is a suitable biomarker for identifying the functional MLR sites.

Serotonin controls initiation of locomotion and afferent modulation of coordination via 5-HT7 receptors in adult rats

KEY POINTS

Experiments on neonatal rodent spinal cord showed that serotonin (5-HT), acting via 5-HT₇ receptors, is required for initiation of locomotion and for controlling the action of interneurons responsible for inter- and intralimb coordination, but the importance of the 5-HT system in adult locomotion is not clear. Blockade of spinal 5-HT₇ receptors interfered with voluntary locomotion in adult rats and fictive locomotion in paralysed decerebrate rats with no afferent feedback, consistent with a requirement for activation of descending 5-HT neurons for production of locomotion. The direct control of coordinating interneurons by 5-HT₇ receptors observed in neonatal animals was not found during fictive locomotion, revealing a developmental shift from direct control of locomotor interneurons in neonates to control of afferent input from the moving limb in adults. An understanding of the afferents controlled by 5-HT during locomotion is required for optimal use of rehabilitation therapies involving the use of serotonergic drugs.

ABSTRACT

Serotonergic pathways to the spinal cord are implicated in the control of locomotion based on studies using serotonin type 7 (5-HT₇ ) receptor agonists and antagonists and 5-HT₇ receptor knockout mice. Blockade of these receptors is thought to interfere with the activity of coordinating interneurons, a conclusion derived primarily from in vitro studies on isolated spinal cord of neonatal rats and mice. Developmental changes in the effects of serotonin (5-HT) on spinal neurons have recently been described, and there is increasing data on control of sensory input by 5-HT₇ receptors on dorsal root ganglion cells and/or dorsal horn neurons, leading us to determine the effects of 5-HT₇ receptor blockade on voluntary overground locomotion and on locomotion without afferent input from the moving limb (fictive locomotion) in adult animals. Intrathecal injections of the selective 5-HT₇ antagonist SB269970 in adult intact rats suppressed locomotion by partial paralysis of hindlimbs. This occurred without a direct effect on motoneurons as revealed by an investigation of reflex activity. The antagonist disrupted intra- and interlimb coordination during locomotion in all intact animals but not during fictive locomotion induced by stimulation of the mesencephalic locomotor region (MLR). MLR-evoked fictive locomotion was transiently blocked, then the amplitude and frequency of rhythmic activity were reduced by SB269970, consistent with the notion that the MLR activates 5-HT neurons, leading to excitation of central pattern generator neurons with 5-HT₇ receptors. Effects on coordination in adults required the presence of afferent input, suggesting a switch to 5-HT₇ receptor-mediated control of sensory pathways during development.

Cholinergic mechanisms in spinal locomotion-potential target for rehabilitation approaches

Previous experiments implicate cholinergic brainstem and spinal systems in the control of locomotion. Our results demonstrate that the endogenous cholinergic propriospinal system, acting via M₂ and M₃ muscarinic receptors, is capable of consistently producing well-coordinated locomotor activity in the in vitro neonatal preparation, placing it in a position to contribute to normal locomotion and to provide a basis for recovery of locomotor capability in the absence of descending pathways. Tests of these suggestions, however, reveal that the spinal cholinergic system plays little if any role in the induction of locomotion, because MLR-evoked locomotion in decerebrate cats is not prevented by cholinergic antagonists. Furthermore, it is not required for the development of stepping movements after spinal cord injury, because cholinergic agonists do not facilitate the appearance of locomotion after spinal cord injury, unlike the dramatic locomotion-promoting effects of clonidine, a noradrenergic α-2 agonist. Furthermore, cholinergic antagonists actually improve locomotor activity after spinal cord injury, suggesting that plastic changes in the spinal cholinergic system interfere with locomotion rather than facilitating it. Changes that have been observed in the cholinergic innervation of motoneurons after spinal cord injury do not decrease motoneuron excitability, as expected. Instead, the development of a “hyper-cholinergic” state after spinal cord injury appears to enhance motoneuron output and suppress locomotion. A cholinergic suppression of afferent input from the limb after spinal cord injury is also evident from our data, and this may contribute to the ability of cholinergic antagonists to improve locomotion. Not only is a role for the spinal cholinergic system in suppressing locomotion after SCI suggested by our results, but an obligatory contribution of a brainstem cholinergic relay to reticulospinal locomotor command systems is not confirmed by our experiments.

5-HT₂ and 5-HT₇ receptor agonists facilitate plantar stepping in chronic spinal rats through actions on different populations of spinal neurons

There is considerable evidence from research in neonatal and adult rat and mouse preparations to warrant the conclusion that activation of 5-HT₂ and 5-HT_1A/7 receptors leads to activation of the spinal cord circuitry for locomotion. These receptors are involved in control of locomotor movements, but it is not clear how they are implicated in the responses to 5-HT agonists observed after spinal cord injury. Here we used agonists that are efficient in promoting locomotor recovery in paraplegic rats, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) (acting on 5-HT_1A/7 receptors) and quipazine (acting on 5-HT₂ receptors), to examine this issue. Analysis of intra- and interlimb coordination confirmed that the locomotor performance was significantly improved by either drug, but the data revealed marked differences in their mode of action. Interlimb coordination was significantly better after 8-OHDPAT application, and the activity of the extensor soleus muscle was significantly longer during the stance phase of locomotor movements enhanced by quipazine. Our results show that activation of both receptors facilitates locomotion, but their effects are likely exerted on different populations of spinal neurons. Activation of 5-HT₂ receptors facilitates the output stage of the locomotor system, in part by directly activating motoneurons, and also through activation of interneurons of the locomotor central pattern generator (CPG). Activation of 5-HT_7/1A receptors facilitates the activity of the locomotor CPG, without direct actions on the output components of the locomotor system, including motoneurons. Although our findings show that the combined use of these two drugs results in production of well-coordinated weight supported locomotion with a reduced need for exteroceptive stimulation, they also indicate that there might be some limitations to the utility of combined treatment. Sensory feedback and some intraspinal circuitry recruited by the drugs can conflict with the locomotor activation.

The role of serotonin in the control of locomotor movements and strategies for restoring locomotion after spinal cord injury

In this review we will discuss different ways for re-establishing serotonergic activity that can enhance recovery of coordinated plantar stepping after spinal cord injury in adult rats. It is well known that serotoninergic neurons located in the medulla are able to initiate locomotor activity. This effect is exerted by actions on motoneurons and on neurons of the locomotor CPG (Central Pattern Generator). Motoneuron and interneuron excitability is increased, and putative CPG interneurons display oscillatory behaviour in response to serotonin receptor activation. The medullary serotonergic nuclei play multiple roles in the control of locomotion, and they terminate on specific target neurons with different types of serotonergic receptors in the spinal cord. Activation of these serotonergic receptors can restore locomotor movements after spinal cord injury. Specifically, using defined serotonergic agonists the 5-HT2 receptors can be stimulated to control CPG activation as well as motoneuron output, while 5-HT7 receptors to control activity of the locomotor CPG. These results are consistent with the roles for these receptors during locomotion in intact rodents and in rodent brainstem-spinal cord in vitro preparations. The other possibility to encourage the remaining spinal cord circuitry below the total transection to control recovery of plantar hindlimb stepping is restoration of serotonergic innervation by intraspinal grafting of embryonic 5-HT neurons. Our data show that grafting of different populations of 5-HT neurons dissected from embryonic brainstem provides differential control over multiple components of the spinal locomotor circuitry through specific serotonin receptors. Moreover, we demonstrated that the best effect of motor recovery is obtained after grafting of neurons destined to form the B1, B2 and B3 descending 5-HT systems. Using only one of the subpopulations for intraspinal grafting, for example, B3 or the lateral group of 5-HT neurons, induces only partial recovery of plantar stepping with a clear lack of proper interlimb coordination. This confirms the hypothesis that transplantation of 5-HT neurons from specific embryonic sources is necessary to obtain optimal recovery of locomotor hindlimb movement.

Comment on "Restoring voluntary control of locomotion after paralyzing spinal cord injury"

Van den Brand et al. (Reports, 1 June 2012, p. 1182) claim to have restored voluntary control of locomotion after paralyzing spinal cord injury. They have not considered recent findings that their upright posture paradigm contributes to locomotor capability after such injuries. We propose that postural adjustments that activate the locomotor central pattern generator in the upright posture, rather than direct voluntary control of locomotion, account for their results.

Comment on "Restoring voluntary control of locomotion after paralyzing spinal cord injury"

Van den Brand et al. (Reports, 1 June 2012, p. 1182) claim to have restored voluntary control of locomotion after paralyzing spinal cord injury. They have not considered recent findings that their upright posture paradigm contributes to locomotor capability after such injuries. We propose that postural adjustments that activate the locomotor central pattern generator in the upright posture, rather than direct voluntary control of locomotion, account for their results.

The upright posture improves plantar stepping and alters responses to serotonergic drugs in spinal rats

Recent studies on the restoration of locomotion after spinal cord injury have employed robotic means of positioning rats above a treadmill such that the animals are held in an upright posture and engage in bipedal locomotor activity. However, the impact of the upright posture alone, which alters hindlimb loading, an important variable in locomotor control, has not been examined. Here we compared the locomotor capabilities of chronic spinal rats when placed in the horizontal and upright postures. Hindlimb locomotor movements induced by exteroceptive stimulation (tail pinching) were monitored with video and EMG recordings. We found that the upright posture alone significantly improved plantar stepping. Locomotor trials using anaesthesia of the paws and air stepping demonstrated that the cutaneous receptors of the paws are responsible for the improved plantar stepping observed when the animals are placed in the upright posture.We also tested the effectiveness of serotonergic drugs that facilitate locomotor activity in spinal rats in both the horizontal and upright postures. Quipazine and (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) improved locomotion in the horizontal posture but in the upright posture either interfered with or had no effect on plantar walking. Combined treatment with quipazine and 8-OH-DPAT at lower doses dramatically improved locomotor activity in both postures and mitigated the need to activate the locomotor CPG with exteroceptive stimulation. Our results suggest that afferent input from the paw facilitates the spinal CPG for locomotion. These potent effects of afferent input from the paw should be taken into account when interpreting the results obtained with rats in an upright posture and when designing interventions for restoration of locomotion after spinal cord injury.

Tetrodotoxin-, dihydropyridine-, and riluzole-resistant persistent inward current: novel sodium channels in rodent spinal neurons

Recently, we reported the tetrodotoxin (TTX)- and dihydropyridine (DHP)-resistant (TDR) inward currents in neonatal mouse spinal neurons. In this study, we further characterized these currents in the presence of 1-5 μM TTX and 20-30 μM DHP (nifedipine, nimodipine, or isradipine). TDR inward currents were recorded by voltage ramp (persistent inward current, TDR-PIC) and step (TDR-I(p)) protocols. TDR-PIC and TDR-I(p) were found in 80.2% of recorded neurons (101/126) crossing laminae I to X from T12 to L6. TDR-PIC activated at -28.6 ± 13 mV with an amplitude of 80.6 ± 75 pA and time constant of 470.6 ± 240 ms (n = 75). TDR-I(p) had an amplitude of 151.2 ± 151 pA and a voltage threshold of -17.0 ± 9 mV (n = 54) with a wide range of kinetics parameters. The half-maximal activation was -21.5 ± 8 mV (-37 to -12 mV, n = 29) with a time constant of 5.2 ± 2 ms (1.2-11.2 ms, n = 19), whereas the half-maximal inactivation was -26.9 ± 9 mV (-39 to -18 mV, n = 14) with a time constant of 1.4 ± 0.4 s (0.5-2.2 s, n = 19). TDR-PIC and TDR-I(p) could be reduced by 60% in zero calcium and completely removed in zero sodium solutions, suggesting that they were mediated by sodium ions. Furthermore, the reversal potential of TDR-I(p) was estimated as 56.6 ± 3 mV (n = 10). TDR-PIC and TDR-I(p) persisted in 1-205 μM TTX, 20-100 μM DHP, 3-30 μM riluzole, 50-300 μM flufenamic acid, and 2-30 mM intracellular BAPTA. They also persisted with T-, N-, P/Q-, and R-type calcium channel blockers. In conclusion, we demonstrated novel TTX-, DHP-, and riluzole-resistant sodium channels in neonatal rodent spinal neurons. The unique pharmacological and electrophysiological properties would allow these channels to play a functional role in spinal motor system.

Reversal of the late phase of spike frequency adaptation in cat spinal motoneurons during fictive locomotion

In spinal motoneurons, late spike frequency adaptation (SFA) is defined as the slowing of the firing rate over tens of seconds and can be seen during sustained or intermittent current injection. Although the function of late SFA is not known, it may result in a decrease in force production over time, or muscle fatigue. Because locomotion can persist for long periods of time without fatigue, late SFA was studied using intracellular recordings from adult cat motoneurons during fictive locomotion. Of eight lumbar motoneurons studied, all showed late adaptation during control conditions, but none demonstrated late adaptation during locomotor activity. The most consistent properties that correlated with the presence or absence of late SFA were those related to availability of fast, inactivating sodium channels, particularly action potential rate of rise. Evidence of the reversal of late SFA during locomotion was present for several minutes following locomotor trials, consistent with the suggestion that SFA is modulated through slow metabotropic pathways. The abolition of late adaptation in spinal motoneurons during fictive locomotion is an example of a state-dependent change in the "intrinsic" properties of mammalian motoneurons. This change contributes to increased excitability of motoneurons during locomotion and results in robust firing during sustained locomotion.

Chapter 12--modulation of rhythmic movement: control of coordination

Three rhythmic movements, breathing, walking, and chewing, are considered from the perspective of the emerging factors that control their coordination. This takes us beyond the concept of a core excitatory kernel and into the common principles that govern the interaction between components of the neural networks that must be orchestrated properly to produce meaningful movement beyond the production of the basic rhythm. We focus on the role of neuromodulators, especially 5-hydroxytryptamine (5-HT), in the production of coordinated breathing, walking, and chewing, and we review the evidence that at least in the case of breathing and walking, 5-HT input to the CPGs acts through the selection of inhibitory interneurons that are essential for coordination. We review data from recently developed mouse models that offer insight into the contributions of inhibitory coordinating neurons, including the development of a new model that has allowed the revelation that there are glycinergic pacemaker neurons that likely contribute to the production of the respiratory rhythm. Perhaps walking and chewing will not be far behind.

Multiple Effects of Serotonin and Acetylcholine on Hyperpolarization-Activated Inward Current in Locomotor Activity-Related Neurons in Cfos-EGFP Mice

Hyperpolarization-activated inward current ( Ih) has been shown to be involved in production of bursting during various forms of rhythmic activity. However, details of Ih in spinal interneurons related to locomotion remain unknown. Using Cfos-EGFP transgenic mice (P6–P12) we are able to target the spinal interneurons activated by locomotion. Following a locomotor task, whole cell patch-clamp recordings were obtained from ventral EGFP+ neurons in spinal cord slices (T13–L4, 200–250 μm). Ih was found in 51% of EGFP+ neurons ( n = 149) with almost even distribution in lamina VII (51%), VIII (47%), and X (55%). Ih could be blocked by ZD7288 (10–20 μM) or cesium (1–1.5 mM) but was insensitive to barium (2–2.5 mM). Ih activated at −80.1 ± 9.2 mV with half-maximal activation −95.5 ± 13.3 mV, activation rate 10.0 ± 3.2 mV, time constant 745 ± 501 ms, maximal conductance 1.0 ± 0.7 nS, and reversal potential −34.3 ± 3.6 mV. 5-HT (15–20 μM) and ACh (20–30 μM) produced variable effects on Ih. 5-HT increased Ih in 43% of EGFP+ neurons ( n = 37), decreased Ih in 24%, and had no effect on Ih in 33% of the neurons. ACh decreased Ih in 67% of EGFP+ neurons ( n = 18) with unchanged Ih in 33% of the neurons. This study characterizes the Ih in locomotor-related interneurons and is the first to demonstrate the variable effects of 5-HT and ACh on Ih in rodent spinal interneurons. The finding of 5-HT and ACh-induced reduction of Ih in EGFP+ neurons suggests a novel mechanism that the motor system could use to limit the participation of certain neurons in locomotion.

Multiple Patterns and Components of Persistent Inward Current With Serotonergic Modulation in Locomotor Activity–Related Neurons in Cfos-EGFP Mice

Using CFos-EGFP transgenic mice (P6–P12), we targeted persistent inward current (PIC) in the spinal interneurons activated by locomotion. Following a locomotor task, whole cell patch-clamp recordings were obtained from ventral EGFP+ neurons in spinal cord slices (200–250 μm from T13–L4). PIC was recorded by a family of 10 s voltage bi-ramps starting from −70 mV with 30 mV steps. PIC could be classified as ascending and descending forms based on the rising and falling phases of the bi-ramps. Multiple patterns of PIC with various hystereses were found in EGFP+ neurons. A novel form of PIC, single PIC crossing both phases of the bi-ramps, was described in this study. PIC was found in 82% of EGFP+ neurons ( n = 129) with no significant difference in laminar distribution. PIC activated at −56.7 ± 8 mV with an amplitude of 85.3 ± 59 pA and time constant of 657.0 ± 272 ms ( n = 63). PIC in lamina VIII neurons activated significantly lower (−60.2 ± 7 mV) than in lamina VII (−54.8 ± 6 mV) and lamina X (−55.8 ± 9 mV) neurons. PIC could be differentiated as calcium dependent (Ca-PIC) by bath application of 1–5 μM TTX or sodium dependent (Na-PIC) by administration of 20–30 μM dihydropyridine. Ca-PIC activated at −40.2 ± 19 mV ( n = 49), whereas Na-PIC activated at −46.8 ± 16 mV ( n = 17). Composite-, Ca-, and Na-PICs were significantly different in activation but not amplitude and time constant. Bath application of 5-HT (10–30 μM) enhanced PIC ( n = 32) by hyperpolarizing onset (4.2 ± 6 mV) and increasing amplitude (16%). 5-HT–increased amplitude seemed to be significantly larger in lamina VII neurons (32%) than VIII (6%) and X (14%) neurons. 5-HT enhancement of Ca-PIC ( n = 6) and Na-PICs ( n = 4) was also observed in EGFP+ neurons. This study unveiled unique properties of PICs in EGFP+ neurons. The lamina-related PIC activation and variable effects of 5-HT on PIC amplitude provides insight into the ionic basis on which locomotion could be generated.

Electrophysiological and pharmacological properties of locomotor activity-related neurons in cfos-EGFP mice

Although locomotion is known to be generated by networks of spinal neurons, knowledge of the properties of these neurons is limited. Using neonatal transgenic mice that express enhanced green fluorescent protein (EGFP) driven by the c-fos promoter, we visualized EGFP-positive neurons in spinal cord slices from animals that were subjected to a locomotor task or drug cocktail [N-methyl-D-aspartate, serotonin (5-HT), dopamine, and acetylcholine (ACh)]. The activity-dependent expression of EGFP was also induced in dorsal root ganglion neurons with electrical stimulation of the neurons. Following 60-90 min of swimming, whole cell patch-clamp recordings were made from EGFP+ neurons in laminae VII, VIII, and X from slices of segments T(12) to L(4). The EGFP+ neurons (n = 55) could be classified into three types based on their responses to depolarizing step currents: single spike, phasic firing, and tonic firing. Membrane properties observed in these neurons include hyperpolarization-activated inward currents (29/55), postinhibitory rebound (11/55), and persistent-inward currents (31/55). Bath application of 10-40 microM 5-HT and/or ACh increased neuronal excitability or output with hyperpolarization of voltage threshold and changes in membrane potential. 5-HT also increased input resistance, reduced the afterhyperpolarization (AHP), and induced membrane oscillations, whereas ACh reduced the input resistance and increased the AHP. In this study, we demonstrate a new way of identifying neurons active in locomotion. Our results suggest that the EGFP+ neurons are a heterogeneous population of interneurons. The actions of 5-HT and ACh on these neurons provide insights into the neuronal properties modulated by these transmitters for generation of locomotion.

Spinal 5-HT7 receptors are critical for alternating activity during locomotion: in vitro neonatal and in vivo adult studies using 5-HT7 receptor knockout mice

5-HT7 receptors have been implicated in the control of locomotion. Here we use 5-HT7 receptor knockout mice to rigorously test whether 5-HT acts at the 5-HT7 receptor to control locomotor-like activity in the neonatal mouse spinal cord in vitro and voluntary locomotion in adult mice. We found that 5-HT applied onto in vitro spinal cords of 5-HT7+/+ mice produced locomotor-like activity that was disrupted and subsequently blocked by the 5-HT7 receptor antagonist SB-269970. In spinal cords isolated from 5-HT7-/- mice, 5-HT produced either uncoordinated rhythmic activity or resulted in synchronous discharges of the ventral roots. SB-269970 had no effect on 5-HT-induced rhythmic activity in the 5-HT7-/- mice. In adult in vivo experiments, SB-269970 applied directly to the spinal cord consistently disrupted locomotion and produced prolonged-extension of the hindlimbs in 5-HT7+/+ but not 5-HT7-/- mice. Disrupted EMG activity produced by SB-269970 in vivo was similar to the uncoordinated rhythmic activity produced by the drug in vitro. Moreover, 5-HT7-/- mice displayed greater maximal extension at the hip and ankle joints than 5-HT7+/+ animals during voluntary locomotion. These results suggest that spinal 5-HT7 receptors are required for the production and coordination of 5-HT-induced locomotor-like activity in the neonatal mouse and are important for the coordination of voluntary locomotion in adult mice. We conclude that spinal 5-HT7 receptors are critical for alternating activity during locomotion.

Modulation of transient and persistent inward currents by activation of protein kinase C in spinal ventral neurons of the neonatal rat

Neuronal excitability can be regulated through modulation of voltage threshold (Vth). Previous studies suggested that this modulation could be mediated by modulation of transient sodium currents (I(T)) and/or persistent inward current (PIC). Modulation of I(T) and PIC through activation of protein kinase C (PKC) has previously been described as a mechanism controlling neuronal excitability. We investigated modulation of I(T) and PIC by PKC in neonatal rat spinal ventral neurons. In whole cell voltage clamp, activation of PKC by application of 1-oleoyl-2-acetyl-sn-glycerol (OAG, 10-30 microM) resulted in 1) a reduction of I(T) amplitude by 33% accompanied an increase in half-width and a decrease in the maximal rise and decay rates of the I(T); 2) a reduction of PIC amplitude by 49%, with a depolarization of PIC onset by 4.5 mV. Activation of PKC caused varied effects on Vth for eliciting I(T), with an unchanged Vth or depolarized Vth being the most common effects. In current-clamp recordings, PKC activation produced a small but significant depolarization (2.0 mV) of Vth for action potential generation with an increase in half-width and a decrease in amplitude and the maximal rise and decay rates of action potentials. Inclusion of PKCI19-36 (10-30 microM), a PKC inhibitor, in the recording pipette could block the OAG effects on I(T) and PIC. The ability of serotonin to hyperpolarize Vth was not altered by PKC activation or inhibition. This study demonstrates that activation of PKC decreases the excitability of spinal ventral neurons and that Vth can be modulated by multiple mechanisms.

Cholinergic and serotonergic excitation of ascending commissural neurons in the thoraco-lumbar spinal cord of the neonatal mouse

Locomotion requires the coordination of the two sides of the spinal cord-a function fulfilled by commissural neurons. Ascending commissural neurons (aCNs) are known to be rhythmically active during locomotion, and mice lacking a population of aCNs display uncoupling between the left and right hemicords during locomotion. Acetylcholine (ACh) applied to the isolated spinal cord commonly produces left-right alternation, with co-contraction of ipsilateral flexor and extensor motoneuron groups. In this study, aCNs were examined in the neonatal mouse spinal cord after retrograde labeling with a fluorescent dextran. The axons of these cells crossed in the ventral commissure with many crossing in the same transverse plane as the cell body. For cells located in lamina VII and VIII, ACh (10-50 microM) depolarized 92% (13/14) of the cells tested. ACh depolarized and increased the excitability of aCNs in the presence of a decrease in input resistance. ACh was without significant effect on afterhyperpolarization amplitude or voltage threshold of action potential initiation. In those cells sensitive to application of ACh, 90% (9/10 cells) were also depolarized by 5HT (10-50 microM). Application of 5HT significantly increased the input resistance of these cells, and this effect was likely responsible for the observed increase in excitability, because significant effects on the afterhyperpolarization and voltage threshold were again not detected. The high proportion of aCNs excited by both ACh and 5HT suggests that direct activation of aCNs by these two neurotransmitters contributes to the production of a bilaterally coordinated locomotor-like rhythm in the isolated spinal cord.

Stimulation of the parapyramidal region of the neonatal rat brain stem produces locomotor-like activity involving spinal 5-HT7 and 5-HT2A receptors

Locomotion can be induced in rodents by direct application 5-hydroxytryptamine (5-HT) onto the spinal cord. Previous studies suggest important roles for 5-HT7 and 5-HT2A receptors in the locomotor effects of 5-HT. Here we show for the first time that activation of a discrete population of 5-HT neurons in the rodent brain stem produces locomotion and that the evoked locomotion requires 5-HT7 and 5-HT2A receptors. Cells localized in the parapyramidal region (PPR) of the mid-medulla produced locomotor-like activity as a result of either electrical or chemical stimulation, and PPR-evoked locomotor-like activity was blocked by antagonists to 5-HT2A and 5-HT7 receptors located on separate populations of neurons concentrated in different rostro-caudal regions. 5-HT7 receptor antagonists blocked locomotor-like activity when applied above the L3 segment; 5-HT2A receptor antagonists blocked locomotor-like activity only when applied below the L2 segment. 5-HT7 receptor antagonists decreased step cycle duration, consistent with an action on neurons involved in the rhythm-generating function of the central pattern generator (CPG) for locomotion. 5-HT2A antagonists reduced the amplitude of ventral root activity with only small effects on step cycle duration, suggesting an action directly on cells involved in the output stage of the pattern generator for locomotion, including motoneurons and premotor cells. Experiments with selective antagonists show that dopaminergic (D1, D2) and noradrenergic (alpha1, alpha2) receptors are not critical for PPR-evoked locomotor-like activity.

Localization of spinal neurons activated during locomotion using the c-fos immunohistochemical method

The c-fos immunohistochemical method of activity-dependent labeling was used to localize locomotor-activated neurons in the adult cat spinal cord. In decerebrate cats, treadmill locomotion was evoked by electrical stimulation of the mesencephalic locomotor region (MLR). Spontaneous or MLR-evoked fictive locomotion was produced in decerebrate animals paralyzed with a neuromuscular blocking agent. After bouts of locomotion during a 7- to 9-h time period, the animals were perfused and the L3-S1 spinal cord segments removed for immunohistochemistry. Control animals were subjected to the same surgical procedures but no locomotor task. Labeled cells were concentrated in Rexed's laminae III and IV of the dorsal horn and laminae VII, VIII, and X of the intermediate zone/ventral horn after treadmill locomotion. Cells in laminae VII, VIII, and X were labeled after fictive locomotion, but labeling in the dorsal horn was much reduced. In control animals, c-fos labeling was a small fraction of that observed in the locomotor animals. The results suggest that labeled cells in laminae VII, VIII, and X are premotor interneurons involved in the production of locomotion, whereas the laminae III and IV cells are those activated during locomotion due to afferent feedback from the moving limb. c-fos-labeled cells were most numerous in the L5-L7 segments, consistent with the distribution of locomotor activated neurons detected through the use of MLR-evoked field potentials.

Correlation of functional activation in the rat spinal cord with neuronal activation detected by immunohistochemistry

The relationship between neuronal activity in the rat cervical and lumbar spinal cord was examined using functional magnetic resonance imaging (fMRI) and immunohistochemistry. Neuronal activity determined by c-fos staining was greatest between L4 and L6, and C5 to C7 spinal cord segments during noxious electrical stimulation of the rat hindpaw and forepaw, respectively. Areas of activity determined by fMRI are consistent with spinal cord physiology, and are predominantly found in regions of the spinal cord associated with pain, namely the dorsal horn. Activity in the ventral region of the cord was also observed, as expected. Combined results from repeated experiments demonstrated consistent areas of activity in response to stimulation, and show a high degree of reproducibility. Good correspondence was observed between functional MRI and sites of neuronal activity determined by c-fos labeling.

Mechanism for activation of locomotor centers in the spinal cord by stimulation of the mesencephalic locomotor region

The synaptic pathways of mesencephalic locomotor region (MLR)-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) recorded from lumbar motoneurons of unanesthetized decerebrate cats during fictive locomotion were analyzed prior to, during, and after cold block of the medial reticular formation (MedRF) or the low thoracic ventral funiculus (VF). As others have shown, electrical stimulation of the MLR typically evoked short-latency excitatory or mixed excitatory/inhibitory PSPs in flexor and extensor motoneurons. The bulbospinal conduction velocities averaged approximately 88 m/s (range: 62-145 m/s) and segmental latencies for EPSPs ranged from 1.2 to 10.9 ms. The histogram of segmental latencies showed three peaks, suggesting di-, tri-, and polysynaptic linkages. Segmental latencies for IPSPs suggested trisynaptic or polysynaptic transmission. Most EPSPs (69/77) were significantly larger during the depolarized phase of the intracellular locomotor drive potential (LDP), and most IPSPs (35/46) were larger during the corresponding hyperpolarized phase. Bilateral cooling of the MedRF reversibly abolished locomotion of both hindlimbs as measured from the electroneurogram (ENG) activity of muscle nerves and simultaneously abolished or diminished the motoneuron PSPs and LDPs. Unilateral cooling of the VF blocked locomotion ipsilaterally and diminished it contralaterally with concomitant loss or decrease the motoneuron PSPs and LDPs. Relative to the side of motoneuron recording, cooling of the ipsilateral VF sometimes uncovered longer-latency EPSPs, whereas cooling of the contralateral VF abolished longer-latency EPSPs. It is concluded that MLR stimulation activates a pathway that relays in the MedRF and descends bilaterally in the VF to contact spinal interneurons that project to motoneurons. Local segmental pathways that activate or inhibit motoneurons during MLR-evoked fictive locomotion appear to be both ipsilateral and contralateral.

Functional motor neurons differentiating from mouse multipotent spinal cord precursor cells in culture and after transplantation into transected sciatic nerve

OBJECT

One of the current challenges in neurobiology is to ensure that neural precursor cells differentiate into specific neuron types, so that they can be used for transplantation purposes in patients with neuron loss. The goal of this study was to determine if spinal cord precursor cells could differentiate into motor neurons both in culture and following transplantation into a transected sciatic nerve.

METHODS

In cultures with trophic factors, neurons differentiate from embryonic precursor cells and express motor neuronal markers such as choline acetyltransferase (ChAT), Islet-1, and REG2. Reverse transcription-polymerase chain reaction analysis has also demonstrated the expression of Islet-1 in differentiated cultures. A coculture preparation of neurospheres and skeletal myocytes was used to show the formation of neuromuscular connections between precursor cell-derived neurons and myocytes both immunohistochemically and electrophysiologically. Following various survival intervals, precursor cells transplanted distal to a transection of the sciatic nerve differentiated into neurons expressing the motor neuron markers ChAT and the alpha1 1.2 (class C, L-type) voltage-sensitive Ca++ channel subunit. These cells extended axons into the muscle, where they formed cholinergic terminals.

CONCLUSIONS

These results demonstrate that motor neurons can differentiate from spinal cord neural precursor cells grown in culture as well as following transplantation into a transected peripheral nerve.

A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneurones

During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of approximately 6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na(+) current, three Ca(2+) currents, the "h" mixed cation current, the "A" potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action.

A population of oligodendrocytes derived from multipotent neural precursor cells expresses a cholinergic phenotype in culture and responds to ciliary neurotrophic factor

Because oligodendrocytes and their precursors possess receptors for classical transmitters, and neurotransmitters such as glutamate and noradrenaline can mediate oligodendroglial proliferation and differentiation, it is possible that other neurotransmitters can also exert regulatory roles in oligodendrocyte function. We used mitogen-proliferated multipotent neuroepithelial precursors (neurospheres) and identified oligodendroglia that expressed markers traditionally found in cholinergic neurons. Regardless of culture conditions, there existed a large population of cells that resembled oligodendrocytes morphologically and coexpressed the oligodendrocyte-specific marker galactocerebroside (GalC) and the acetylcholine (ACh)-synthesizing enzyme choline acetyltransferase (ChAT). These cells did not express neuronal markers, and whole-cell recordings from cells with similar morphology displayed only outward currents in response to depolarizing voltage steps, further supporting their oligodendroglial identity. Another cholinergic marker, the vesicular ACh transporter, was also detected in GalC(+) oligodendrocytes. Furthermore, neurospheres cultured in the presence of the cholinergic receptor antagonist atropine showed a decrease in the number of GalC(+) spheres, implicating the muscarinic ACh receptor in oligodendrocyte development. The actions of neurotrophins and ciliary neurotrophic factor (CNTF) on these ChAT(+) oligodendrocytes were examined. Among these, CNTF treatment significantly increased oligodendrocytic process outgrowth. These results demonstrate classical cholinergic neuronal markers in oligodendrocytes as well as an effect of muscarinic receptor blockade on oligodendrocyte differentiation.

Neurogenesis in postnatal mouse dorsal root ganglia

Neurogenesis continues in various regions of the central nervous system (CNS) throughout life. As the mitogen basic fibroblast growth factor (bFGF) can proliferate neuronal precursors of CNS neurons in culture, and is also upregulated within adult dorsal root ganglia following axotomy, it is possible that the postnatal dorsal root ganglia contain bFGF-responsive neuronal precursors. We undertook cell culture of postnatal mouse dorsal root ganglia to demonstrate neurogenesis. Basic FGF induced a cellular proliferative response in dorsal root ganglia cell culture. After 2 weeks in serum-free medium containing bFGF, neurons were rarely observed. However, following removal of bFGF and addition of trophic factors, many cells were observed that morphologically resembled dorsal root ganglia neurons, stained for neuronal markers, and generated action potentials. Furthermore, bromodeoxyuridine, used as a marker of cytogenesis, was detected in neurofilament-160(+) and/or microtubule-associated protein-2(+) cells that morphologically resembled neurons. In addition to bFGF, epidermal growth factor, nerve growth factor, and sonic hedgehog were also capable of generating spherical cell clusters that contained cells that stained for neuronal markers following the addition of trophic factors. These results suggest that early postnatal dorsal root ganglia contain neural precursors that appear to proliferate in response to various factors and can then be induced to differentiate into neurons. In conclusion, the existence of neural precursors and the possibility of neurogenesis in postnatal dorsal root ganglia may provide a greater range of plasticity available to somatosensory systems during growth or following injury, perhaps to replace ineffectual or dying neurons.

Data-driven practice improvement: the AANA Foundation closed malpractice claims study

Research in anesthesia risk management has focused primarily on adverse patient outcomes. Most risk management studies have evaluated the practices of the physician anesthesiologist, while minimal research has been conducted to examine anesthesia care provided by Certified Registered Nurse Anesthetists (CRNAs). For this reason, the American Association of Nurse Anesthetists Foundation supported an examination of closed malpractice claim files from St Paul Fire and Marine Insurance Company that involved insured CRNAs. A team of 8 CRNA researchers investigated 223 closed claim files that involved incidents from 1989 to 1997. The results indicated that preoperative physical status, patient age, surgical procedure, type of anesthetic, age of anesthesia provider, and the type of anesthesia providers, (e.g., CRNA alone vs CRNA and anesthesiologist working together) did not have a statistically significant relationship with adverse anesthetic outcomes. However, providing appropriate care, being vigilant, encountering a less severe adverse outcome, and not being able to prevent the outcome were associated with smaller monetary awards. The findings of this study support those of similar studies.

State-dependent hyperpolarization of voltage threshold enhances motoneurone excitability during fictive locomotion in the cat

Experiments were conducted on decerebrate adult cats to examine the effect of brainstem-evoked fictive locomotion on the threshold voltage (Vth) at which action potentials were initiated in hindlimb motoneurones. Measurements of the voltage threshold of the first spike evoked by intracellular injection of depolarizing ramp currents or square pulses were compared during control and fictive locomotor conditions. The sample of motoneurones included flexor and extensor motoneurones, and motoneurones with low and high rheobase currents. In all 38 motoneurones examined, action potentials were initiated at more hyperpolarized membrane potentials during fictive locomotion than in control conditions (mean hyperpolarization -8.0 +/- 5.5 mV; range -1.8 to -26.6 mV). Hyperpolarization of Vth occurred immediately at the onset of fictive locomotion and recovered in seconds (typically < 60 s) following the termination of locomotor activity. The Vth of spikes occurring spontaneously without intracellular current injection was also reduced during locomotion. Superimposition of rhythmic depolarizing current pulses on current ramps in the absence of locomotion did not lower Vth to the extent seen during fictive locomotion. We suggest that Vth hyperpolarization results from an as yet undetermined neuromodulatory process operating during locomotion and is not simply the result of the oscillations in membrane potential occurring during locomotion.The hyperpolarization of Vth for action potential initiation during locomotion is a state-dependent increase in motoneurone excitability. This Vth hyperpolarization may be a fundamental process in the generation of motoneurone activity during locomotion and perhaps other motor tasks.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

A variable resolution x-ray detector for computed tomography: II. Imaging theory and performance

A computed tomography (CT) imaging technique called variable resolution x-ray (VRX) detection provides variable image resolution ranging from that of clinical body scanning (1 cy/mm) to that of microscopy (100 cy/mm). In this paper, an experimental VRX CT scanner based on a rotating subject table and an angulated storage phosphor screen detector is described and tested. The measured projection resolution of the scanner is > or = 20 lp/mm. Using this scanner, 4.8-s CT scans are made of specimens of human extremities and of in vivo hamsters. In addition, the system's projected spatial resolution is calculated to exceed 100 cy/mm for a future on-line CT scanner incorporating smaller focal spots (0.1 mm) than those currently used and a 1008-channel VRX detector with 0.6-mm cell spacing.

Spinal cholinergic neurons activated during locomotion: localization and electrophysiological characterization

The objective of the present study was to determine the location of the cholinergic neurons activated in the spinal cord of decerebrate cats during fictive locomotion. Locomotion was induced by stimulation of the mesencephalic locomotor region (MLR). After bouts of locomotion during a 7-9 h period, the animals were perfused and the L(3)-S(1) spinal cord segments removed. Cats in the control group were subjected to the same surgical procedures but no locomotor task. The tissues were sectioned and then stained by immunohistochemical methods for detection of the c-fos protein and choline acetyltransferase (ChAT) enzyme. The resultant c-fos labeling in the lumbar spinal cord was similar to that induced by fictive locomotion in the cat. ChAT-positive cells also clearly exhibited fictive locomotion induced c-fos labeling. Double labeling with c-fos and ChAT was observed in cells within ventral lamina VII, VIII, and possibly IX. Most of them were concentrated in the medial portion of lamina VII close to lamina X, similar in location to the partition and central canal cells found by Barber and collaborators. The number of ChAT and c-fos-labeled neurons was increased following fictive locomotion and was greatest in the intermediate gray, compared with dorsal and ventral regions. The results are consistent with the suggestion that cholinergic interneurons in the lumbar spinal cord are involved in the production of fictive locomotion. Cells in the regions positive for double-labeled cells were targeted for electrophysiological study during locomotion, intracellular filling, and subsequent processing for ChAT immunohistochemistry. Three cells identified in this way were vigorously active during locomotion in phase with ipsilateral extension, and they projected to the contralateral side of the spinal cord. Thus a new population of spinal cord cells can be defined: cholinergic partition cells with commissural projections that are active during the extension phase of locomotion.

Dendritic L-type calcium currents in mouse spinal motoneurons: implications for bistability

The intrinsic properties of mammalian spinal motoneurons provide them with the capability to produce high rates of sustained firing in response to transient inputs (bistability). Even though it has been suggested that a persistent dendritic calcium current is responsible for the depolarizing drive underlying this firing property, such a current has not been demonstrated in these cells. In this study, calcium currents are recorded from functionally mature mouse spinal motoneurons using somatic whole-cell patch-clamp techniques. Under these conditions a component of the current demonstrated kinetics consistent with a current originating at a site spatially segregated from the soma. In response to step commands this component was seen as a late-onset, low amplitude persistent current whilst in response to depolarizing-repolarizing ramp commands a low voltage clockwise current hysteresis was recorded. Simulations using a neuromorphic motoneuron model could reproduce these currents only if a noninactivating calcium conductance was placed in the dendritic compartments. Pharmacological studies demonstrated that both the late-onset and hysteretic currents demonstrated sensitivity to both dihydropyridines and the L-channel activator FPL-64176. Furthermore, the alpha1D subunits of L-type calcium channels were immunohistochemically demonstrated on motoneuronal dendrites. It is concluded that there are dendritically located L-type channels in mammalian motoneurons capable of mediating a persistent depolarizing drive to the soma and which probably mediate the bistable behaviour of these cells.

Critical decisions in the management of endoscopic perforations of the colon

The ideal management of suspected colon perforation following colonoscopy remains elusive because the incidence is only 0.1 to 2.0 per cent. The patient with obvious perforation deserves immediate exploration, but the patient with equivocal findings poses a diagnostic dilemma. We propose an algorithm based on the results of water-soluble contrast enema that allows for rapid, definitive surgical decision-making. If perforation is confirmed, early operation allows for primary repair without resection or colostomy, or if no perforation is identified, medical management can be undertaken with confidence. This algorithm should ensure that the surgical management of this potentially lethal complication is not unnecessarily delayed.

Initiation of locomotion in mammals

Several "locomotor regions" of the mammalian brain stem can be stimulated, either electrically or chemically, to induce locomotion. Active cells labeled with c-fos within the mesencephalic locomotor region (MLR) have been found in the periaqueductal gray, the cuneiform nucleus, the pedunculopontine nucleus, and the locus coeruleus. Different subsets of these nuclei appear to be activated during locomotion produced in different behavioral contexts. The locomotor nuclei can be classified into areas associated with exploratory, appetitive, and defensive locomotion, in accordance with the proposal of Sinnamon (1993, Prog. Neurobiol. 41: 323-344). The interpretation of lesion studies designed to reveal areas of the brain essential for locomotion must be based on knowledge of the nuclei which become active in the specific locomotor task being tested. An argument is put forward in favor of the continued use of the term "mesencephalic locomotor region."

Line spread function study of kinestatic charge detectors operating at high gas pressures

A systematic study of the line spread function (LSF) in the drift direction of a high-pressure ionization chamber for x-ray detection and imaging is presented. Experimental results, obtained by operating a KCD krypton-filled detector at pressures up to 60 atm and constant electric field-to-gas pressure ratio, indicate that the width of the LSF increases with the drift distance and decreases with increasing pressure, both effects being quite large. The hypothesis of this paper is that, at sufficiently high pressures, formation of clusters of molecular ions with a unique or narrowed mobility distribution take place by means of energy exchange mechanisms. Therefore, the LSF of the ionic signal becomes narrower and the FWHM of the ionic signal improves significantly with increasing gas pressure. This research is aimed at investigating methods to improve the spatial resolution as part of the development of a large field-of-view prototype digital radiographic scanner operating on kinestatic charge detection principles.

Enhanced X-Ray Detectors Using Polar Dopants for KCD Digital Radiography

The goal of this study is to develop high resolution imaging detectors with applications in digital radiography and computed tomography. A physical treatment aimed at a better understanding of the line-spread function response of kinestatic charge detector (KCD) gas media, using dopants with permanent electric dipoles, is presented. Experimental results were obtained by operating a KCD krypton-filled detector at pressures up to 60 atm and constant electric field-to-gas density ratio doped with small amounts of polar or nonpolar polyatomic molecules with low or high ionization potential. The results clearly indicate that the addition of dopants having both low ionization potential and high dipole moment significantly enhance the imaging signal quality. An analysis of the experimental results aimed at providing a plausible interpretation of the reported observations is offered.

C-terminals on motoneurons: electron microscope localization of cholinergic markers in adult rats and antibody-induced depletion in neonates

C-terminals on motoneurons are defined as those accompanied by characteristic postsynaptic specializations termed subsurface cisterns. We have previously shown, by light microscope immunolabelling methods, that subsurface cisterns occur regularly beneath choline acetyltransferase- and acetylcholinesterase-containing boutons on motoneurons. In the present study, the cholinergic nature of C-terminals suggested by these results was further investigated by immunohistochemistry and electron microscopy in adult rats and in neonates treated with a murine monoclonal acetylcholinesterase antibody which was previously shown to cause immunological lesions of central cholinergic systems. In both the facial nucleus and lumbar segment of spinal cord of adult rats, C-terminals were seen intensely immunostained for the cholinergic markers choline acetyltransferase and acetylcholinesterase. Immunolabelled terminals made contact with either neuronal somata or large calibre dendrites, which were positive for the cholinergic markers, and exhibited club-shaped or thin elongated morphologies suggestive of terminal or en passant type synaptic interactions. The close relationship found between cholinergic markers and immunolabelled subsurface cisterns in adults was maintained on motoneurons of eight-day-old rats. While subcutaneous treatment of newborn rat with acetylcholinesterase antibody appeared to have no effect on the distribution of immunopositive subsurface cisterns in motoneurons when examined on postnatal day 8, the density of labelling for the two cholinergic markers around these neurons was reduced. Areas of neuropil immediately surrounding motoneurons in treated animals often showed signs of extensive swelling and deterioration indicative of a lesion event, and these motoneurons frequently displayed subsurface cisterns unapposed to C-terminals. These results support our earlier conclusion, based on light microscope investigation, that the majority if not all C-terminals are cholinergic in the areas investigated and demonstrate the potential utility of immunolesion methods in the study of C-terminal function.

Field potential mapping of neurons in the lumbar spinal cord activated following stimulation of the mesencephalic locomotor region

The spinal neurons involved in the control of locomotion in mammals have not been identified, and a major step that is necessary for this purpose is to determine where these cells are likely to be located. The principal objective of this study was to localize lumbar spinal interneurons activated by stimulation of the mesencephalic locomotor region (MLR) of the cat. For this purpose, extracellular recordings of MLR-evoked cord dorsum and intraspinal field potentials were obtained from the lumbosacral enlargement during fictive locomotion in the precollicular-postmammillary decerebrate cat preparation. Potentials recorded from the dorsal surface of the cord between the third lumbar (L3) and first sacral (S1) segments typically showed four short-latency positive waves (P1-P4). These P-waves were largest between the L4-L6 segments. The amplitude of the P2-4 waves increased with the appearance of locomotion and displayed rhythmic modulation during the locomotor step cycle. Microelectrode recordings from the L4-L7 spinal segments during fictive locomotion revealed the presence of both positive and negative short-latency MLR-evoked intraspinal field potentials, and were used to construct isopotential maps of the evoked potentials. Positive field potentials were observed throughout the dorsal horn of the L4-L7 spinal segments with the largest amplitude potentials occurring in laminae III-VI. Negative field potentials were found in laminae VI-X of the lumbar cord. The shortest latency negative field potentials were observed in lamina VII and at the border between laminae VI and VII and were considered to be evoked monosynaptically from the arrival of the descending volley. Short-latency mono- and disynaptic negative field potentials were also observed in lamina VIII. Longer latency, tri- and polysynaptic field potentials were observed in laminae VII and VIII. Many of the longer latency negative waves observed in laminae VII and VIII followed shorter latency negative potentials recorded from the same location. Laminae VII and VIII negative field potentials were largest in the L5-6 and L4-5 spinal segments, respectively. Negative field potentials were also evoked in the motor nuclei of the L4-7 spinal segments. The segmental latencies for these potentials indicate that they were evoked di- and trisynaptically.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformational study of biotinylated DNA oligonucleotides utilizing two-dimensional nuclear magnetic resonance and molecular dynamics

The solution structure of DNA oligonucleotides containing a biotin group covalently attached through a linker arm to the 5' end has been studied by two-dimensional NMR and molecular modeling, in an attempt to determine whether the biotin end group is accessible to avidin binding. Such experiments are useful in suggesting that purification of synthetic DNA oligonucleotides by avidin-biotin affinity chromatography is carried out with a minimum amount of intramolecular association of biotin with potential binding sites within the DNA. Two DNA oligonucleotides that form hairpin structures have been proposed as possible worst case scenarios where biotin would not be available for avidin binding. These structures are d(XGCGCGTTTTCGCGC) and d(XGCGCGTTTTCGCGCAAAAA), where X represents the biotin and linker structures. No NOEs were detected between the biotin/linker portion of the molecule and the DNA hairpin. In addition, the magnitude of the NOEs between neighboring groups in the biotin/linker was approximately an order of magnitude smaller than that of the neighbors at a comparable distance within the DNA hairpin, suggesting that the biotin/linker undergoes considerable additional motion compared to the DNA hairpin. Molecular dynamics calculations also show that the biotin/linker undergoes a considerable range of motion. Thus, all data indicate that the biotin is not immediately associated with the hairpin and should be available for binding to avidin under chromatographically relevant solvent conditions.