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. 2002 Apr 1;540(Pt 1):129–138. doi: 10.1113/jphysiol.2001.013084

Effects of daily spontaneous running on the electrophysiological properties of hindlimb motoneurones in rats

Eric Beaumont 1, Phillip Gardiner 1
PMCID: PMC2290217  PMID: 11927675

Abstract

No evidence currently exists that motoneurone adaptations in electrophysiological properties can result from changes in the chronic level of neuromuscular activity. We examined, in anaesthetized (ketamine/xylazine) rats, the properties of motoneurones with axons in the tibial nerve, from rats performing daily spontaneous running exercise for 12 weeks in exercise wheels (‘runners’) and from rats confined to plastic cages (‘controls’). Motoneurones innervating the hindlimb via the tibial nerve were impaled with sharp glass microelectrodes, and the properties of resting membrane potential, spike threshold, rheobase, input resistance, and the amplitude and time-course of the afterhyperpolarization (AHP) were measured. AHP half-decay time was used to separate motoneurones into ‘fast’ (AHP half-decay time < 20 ms) and ‘slow’ (AHP half-decay time ≥ 20 ms), the proportions of which were not significantly different between controls (58 % fast) and runners (65 % fast). Two-way ANOVA and ANCOVA revealed differences between motoneurones of runners and controls which were confined to the ‘slow’ motoneurones. Specifically, runners had slow motoneurones with more negative resting membrane potentials and spike thresholds, larger rheobasic spike amplitudes, and larger amplitude AHPs compared to slow motoneurones of controls. These adaptations were not evident in comparing fast motoneurones from runners and controls. This is the first demonstration that physiological modifications in neuromuscular activity can influence basic motoneurone biophysical properties. The results suggest that adaptations occur in the density, localization, and/or modulation of ionic membrane channels that control these properties. These changes might help offset the depolarization of spike threshold that occurs during rhythmic firing.

Evidence is accumulating that α-motoneurones respond to chronic alterations in neuromuscular activity by changing their properties. The evidence to date is primarily biochemical. For example the motoneurones from endurance-trained rats exhibit an increase in fast axon transport of proteins towards the nerve terminals (Jasmin et al. 1988; Kang et al. 1995; Gharakhanlou et al. 1999), as well as an increased content of mitochondrial enzymes (Suzuki et al. 1991) and of trophic substances such as calcitonin gene-related peptide (CGRP) (Gharakhanlou et al. 1999) and brain-derived neurotrophic factor (BDNF) (Gómez-Pinilla et al. 2001). The functional correlates of these changes are currently unknown.

What is less clear is the extent, if any, to which motoneurones adapt in their physiological properties to chronic changes in neuromuscular activity. There are several bases upon which to propose such a change. For example since the electrophysiological properties of motoneurones and the contractile and histochemical properties of their innervated muscle fibres show a high degree of co-variation (Zengel et al. 1985; Gardiner, 1993), it is reasonable to postulate that chronic changes in activity which result in muscle changes should also alter motoneurone properties, in order that this co-variation be maintained. Indeed, the re-establishment of the normal relationship of nerve-muscle properties appears to occur following reinnervation of muscles by regenerating motor axons (Foehring et al. 1986a, 1987), as well as following spinal cord transection (Cope et al. 1986; Munson et al. 1986). Such changes could result from altered synaptic activity at the motoneuronal soma or from the influence of altered amounts or types of ‘trophic’ factors secreted from the target muscle (Czeh et al. 1978; Wolpaw & Carp, 1993; Mendell et al. 1994; Munson et al. 1997; Gonzalez & Collins, 1997), as well as a combination of both of these factors.

Properties which distinguish motoneurone ‘types’ are dictated more by ionic conductance channels than by morphological characteristics (Zengel et al. 1985; Binder et al. 1996). Although several examples of short-term (Brownstone et al. 1994; Krawitz et al. 2001) and long-term (Turrigiano et al. 1994; Halter et al. 1995; Desai et al. 1999) activity-dependent plasticity of neural conductances have been reported for several neurone types in response to various models of altered activity, no such evidence has been presented for motoneurones subjected to different levels of voluntary neuromuscular activity. The latter model, especially for rats, is of particular interest in light of the chronic changes in muscle properties (Rodnick et al. 1989; Lambert & Noakes, 1990; Seburn & Gardiner, 1995), and in specific central nervous system nuclei (Elam et al. 1987; Dluzen et al. 1995; Neeper et al. 1995), which can result from an increase in daily spontaneous locomotor activity. Our purpose, therefore, was to determine if rats allowed access to increased daily spontaneous running for 12 weeks would possess hindlimb motoneurones with different electrophysiological properties than their control counterparts, which would suggest activity-dependent plasticity of motoneurones never previously demonstrated.

METHODS

Animal care and treatment

Experiments were conducted on female Sprague-Dawley rats (150 g upon receipt), obtained from Charles River (St-Constant, Québec, Canada). Upon arrival, rats were assigned to either a control group (n = 28) or a daily spontaneous runner group (n = 18). Control group rats were confined to plastic cages (21 cm × 25 cm × 47 cm), three to a cage, until the terminal experiment 12 weeks later. Spontaneous runners were placed for 12 weeks in live-in voluntary exercise wheels (dimensions 21 cm wide, 46 cm in diameter, with a running surface of 4 mm stainless steel mesh (Gisiger et al. 1994)), in which the number of revolutions was monitored by computer. For both groups, food (Purina rat chow) and water were made freely available, and the housing facility was temperature-controlled, with a controlled 12 h light-dark cycle. All procedures were according to the recommendations of the Canadian Council for Animal Care, and were approved by the animal ethics committee of the Université de Montréal.

Preparations for motoneurone intracellular recordings

These procedures used in this laboratory have been described in previous reports (Gardiner, 1993; Gardiner & Seburn, 1997; Cormery et al. 2000). On the morning of the terminal experiment, the rat was anaesthetized with an intraperitoneal injection of ketamine/xylazine (80/10 mg kg−1). A jugular vein catheter was put in place, through which a mixture of anaesthetic (8/1 mg h−1) and 70 kD neutral polymer of sucrose used as plasma expander (Ficoll70, Amersham Pharmacia, 40 mg h−1) in physiological saline, was infused continually throughout the remainder of the experiment. A bolus intraperitoneal injection of 2 ml saline containing 0.05 mg kg−1 atropine and 100 mg dextrose was also given immediately following the onset of anaesthesia. A tracheal tube was installed, and the animal was ventilated with a mixture of room air and pure oxygen (2 strokes s−1, approximately 2 ml tidal volume). The ankle extensor muscles of the left hindlimb were exposed, and all lower hindlimb extensors except the gastrocnemius were denervated. The calcaneus with the gastrocnemius tendon attached was separated from the foot, and silk suture material (2–0) was tied to it for the purpose of later attachment to a force transducer. The sciatic nerve was gently exposed and cleared, and the peroneal and sural branches were separated from the tibial branch as rostral as possible, and cut. A mid-line dorsal incision was then performed, and back muscles removed to expose the vertebral column from T 11 to S2.

The rat was then transferred to a stereotaxic system which permitted stabilization of the head, thoracic to sacral vertebrae, hips, left knee, and left foot, with the animal lying prone. A laminectomy was performed to expose the spinal cord from low-thoracic to high-sacral areas. A mineral-oil bath was made using the skin edges of the back incision, and the bath filled with light mineral oil. The dura was removed, and the large lumbar dorsal roots at the lumbar enlargement were cut and reflected over the right side of the cord, exposing the cord surface. The edges of the hindlimb incision were also used to make a bath around the hindlimb muscles and sciatic nerve, which was filled with mineral oil. The silk ligature around the calcaneus was attached to a force transducer (Grass Pt-03). A bipolar silver electrode was placed on the tibial branch of the sciatic nerve, and the stimulation voltage, polarity and muscle optimum length were determined. Body temperature was maintained at 37 °C by a temperature-controlled heating blanket in contact with the ventral surface of the animal. The muscle bath was maintained at 36 to 37 °C by recirculating warmed mineral oil through the muscle bath via a peristaltic pump. Ventilatory volume was adjusted so that expired CO2 was 3 to 4 %.

Motoneurone recording

Intracellular recordings were made using glass microelectrodes (o.d. 1 mm) with tip diameters of approximately 1 μm containing 2 m potassium citrate (resistance 14 to 18 MΩ). Tibial motoneurones were sought using the antidromic field potential resulting from tibial nerve stimulation (once every 2 s). Successful penetration of a tibial α-motoneurone was evident by a change in potential from zero to resting membrane potential (at least −50 mV), an antidromic action potential of short (less than 3 ms) and reproducible latency from the stimulation artefact, and of an amplitude of at least 60 mV with a positive over-shoot.

The following were then recorded: (1) the average of two to three antidromic action potentials; (2) two to four spikes at rheobase current, the latter defined as the amplitude of a 50 ms depolarizing current pulse evoking an action potential 50 % of the time; (3) two to four spikes resulting from short (0.1 to 0.8 ms) supramaximal intensity current pulses; (4) the mean of 40 AHPs following a spike evoked by 1 ms supramaximal current pulses; (5) 15 to 20 antidromic spikes during sustained current injections of various small (less than 2 nA) intensities (for measurement of input resistance using the spike height method (Frank & Fuortes, 1956)); and (6) average of 100 responses to 150 ms current injections of +1 and −1 nA (for measurement of membrane time constant (Zengel et al. 1985)).

If spike generation via current injection also produced a muscle unit twitch response, specifying the motoneurone as innervating gastrocnemius, several muscle properties were also recorded. Since these were few in number, those data are not reported here.

During recording from each motoneurone, membrane potential was recorded continuously on a DC recorder, and was noted in the laboratory note-book during the recording of each property. At the end of recording from the motoneurone, the electrode was backed out of the cell in 5 μm steps, and the stabilized ‘zero’ was recorded. Searching for cells continued until 6 to 8 h following the introduction of the first electrode into the spinal cord. All data were recorded onto PC-based computer using custom-made software. At the end of the experiment, the rat was killed with an overdose of anaesthetic.

Motoneurone properties

The following properties were determined from the recorded data: (1) antidromic spike height (in mV); (2) spike latency, measured as the time difference in ms between the stimulation artefact and the beginning of the antidromic spike; (3) spike trigger level (in mV), which was determined by subtracting the spike height during rheobasic current injection from that during a short, supramaximal current pulse, and adding this value to the resting membrane potential; (3) AHP amplitude (in mV) and half-decay time (in ms); (4) cell input resistance, using the spike height method of Frank & Fuortes (1956); and, (5) membrane time constant, from a 150 ms hyperpolarizing current pulse of 1 nA, using the curve-peeling method (Ito & Oshima, 1965; Zengel et al. 1985).

Statistical analysis

The data were subjected to a two-way ANOVA procedure on the factors of activity level (controls vs. runners) and motoneurone ‘type’. For the latter, motoneurones were designated as ‘slow’ or ‘fast’ motoneurones based on the half-decay time of the AHP (fast < 20 ms, slow ≥ 20 ms, Gardiner, 1993). This was done to order to subgroup motoneurones on an intrinsic property which co-varies with muscle fibre type and excitability, and which remains relatively stable in a variety of conditions which evoke changes in other properties (Zengel et al. 1985; Gardiner, 1993; Gardiner & Seburn, 1997; Munson et al. 1997; Cormery et al., 2000). Where a significant interaction term was present, data were subjected to a Newman-Keuls post hoc comparisons test, to determine significant differences among individual means. All data are expressed as means ± 1 s.d.

RESULTS

The data set is summarized in Table 1, and includes 74 motoneurones from 18 rats given access to daily spontaneous wheel activity for 12 weeks, and 122 motoneurones from 28 rats confined to standard plastic cages for the same period of time. Rats in voluntary wheel cages performed an average of 13.6 km (range 6.8 to 18.5 km) of wheel activity per day during the last week before the terminal experiment. In all terminal experiments, records were take from four to eight motoneurones. The number of muscle-identified motoneurones was too small to allow for adequate statistical analysis; for this reason, data are expressed simply as tibial motoneurones.

Table 1.

Properties of motoneurones of controls and runners

Property ‘Fast’ motoneurones ‘Slow’ motoneurones
Controls Runners Controls Runners
Antidromic action potential amplitude (mV) 73.5 ± 8.9 74.9 ± 11.1 78.8 ± 14.0 85.5 ± 10.3*
 n 66 45 47 24
Cell input resistance (MΩ) 2.0 ± 1.2 1.6 ± 1.0 2.6 ± 1.7 2.5 ± 1.4*
 n 55 42 48 21
Rheobase (nA) 7.3 ± 3.9 7.7 ± 3.9 5.6 ± 3.6 5.2 ± 3.3*
 n 68 48 53 26
Rheobasic action potential amplitude (mV) 61.7 ± 7.3 61.0 ± 8.2 65.3 ± 8.5 73.4 ± 6.8 †
 n 70 48 52 26
Membrane time constant, τm (ms) 6.2 ± 2.2 4.9 ± 1.8 6.4 ± 1.9 6.5 ± 1.5*
 n 38 34 25 22
Resting membrane potential (mV) −60.4 ± 7.4 −61.7 ± 7.4 −60.9 ± 8.2 −66.7 ± 6.6 †
 n 69 48 53 26
Spike trigger level (mV) −41.8 ± 8.3 −43.2 ± 7.3 −40.4 ± 10.3 −47.3 ± 5.9 †
 n 68 48 52 26
Vth measured (mV) 18.4 ± 9.7 18.5 ± 9.2 21.0 ± 10.1 19.8 ± 7.3
 n 70 48 52 26
AHP half decay time (ms) 14.2 ± 4.2 15.6 ± 3.6 29.5 ± 5.5 29.2 ± 6.5
 n 58 46 53 26
AHP amplitude (mV) 1.8 ± 0.8 1.5 ± 0.7 2.8 ± 1.7 4.0 ± 2.0†
 n 58 46 50 26
Antidromic spike latency (ms) 1.6 ± 0.4 1.9 ± 0.3 2.5 ± 0.3 2.8 ± 0.2*
 n 58 46 50 26
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Values are means ± s.d., with numbers of motoneurones below.

*

Significant main effects of motoneurone ‘type’ or

†

significant interaction effects of motoneurone ‘type’ and activity (P < 0.01).

In an attempt to determine if significant effects were consistent across motoneurone ‘types’, we separated motoneurones into ‘fast’ and ‘slow’, using a half-decay time of the AHP. This is based on previous reports (Gardiner, 1993; Comery et al. 2000) that rat hindlimb motoneurones with AHP half-decay times equal to or greater than 20 ms innervate slow-twitch muscle fibres. χ2 analysis demonstrated no differences (P > 0.05) in the proportions of fast and slow motoneurones in the samples from controls (58 % fast, 42 % slow) and runners (65 % fast, 35 % slow), suggesting that the difference in neuromuscular activity between these two groups was of insufficient intensity to result in changes in motoneurone or muscle unit types of the kind reported, for example, following chronic electrical stimulation of cat gastrocnemius (Gordon et al. 1997), or high-intensity endurance treadmill training in rats (Green et al. 1984).

Motoneurone ‘type'differences

The two-way ANOVA procedure allowed the determination of differences between fast and slow motoneurones, differences due to spontaneous wheel activity, and interaction effects (i.e. whether fast and slow motoneurones were influenced differently by the wheel activity). The properties demonstrating significant motoneurone ‘type’ differences are summarized in Fig. 1 (where data from controls and runners are combined). As would be expected, significant main effects (with no interaction effect) of fast vs. slow were found for cell input resistance (1.8 ± 1.1 MΩ and 2.6 ± 1.6 MΩ for fast and slow, respectively) and rheobase (7.5 ± 3.9 nA and 5.7 ± 3.5 nA for fast and slow, respectively). Antidromic spike latency, which reflects (inversely) axonal conduction velocity, was also significantly larger in slow motoneurones (1.7 ± 0.4 ms and 2.6 ± 0.3 ms for fast and slow, respectively). Membrane time constant was significantly longer for slow than for fast motoneurones (5.6 ± 2.1 ms and 6.5 ± 1.7 ms for fast and slow, respectively). Antidromic spike height also showed a significant main effect of motoneurone type, with slow motoneurones being slightly but significantly larger than fast (74 ± 13 mV and 81 ± 10 mV for fast and slow, respectively) (Fig. 1).

Figure 1. Properties of slow (AHP half-decay time ≥ 20 ms) and fast (AHP half-decay time < 20 ms) motoneurones.

Figure 1

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Data from controls and runners are combined. All properties show significant differences (* P < 0.01).

Effects of activity

Activity level had an overall effect (independent of motoneurone type) only on antidromic spike amplitude, in that runners had antidromic spikes which were slightly but significantly larger than those of controls (by 3 to 4 mV). All other properties showing a significant main effect of wheel activity level also demonstrated a significant interaction term, indicating that the treatment effect on that property depended on motoneurone type. These interactions are illustrated in Figs 2 to 4, and demonstrate quite clearly that activity effects were most evident on slow motoneurones. Figure 2 shows the mean and standard deviations, while Fig. 3 and Fig. 4 show the cumulative percentiles. In the case of resting membrane potential and spike trigger level, slow motoneurones of runners showed significantly hyperpolarized values (by 6 to 7 mV) compared to fast motoneurones of runners, and to slow and fast motoneurones of controls. With regards to AHP amplitude, statistical analysis revealed that the slow motoneurones of runners had the highest amplitudes, followed by the slow motoneurones of controls, and finally the fast motoneurones, in which AHP amplitude was not different between controls and runners (Fig. 2 and Fig. 3). In all cases, the distributions of data for fast motoneurones were very similar for controls and runners (Fig. 3 and Fig. 4).

Figure 2. Activity effects on fast and slow motoneurones.

Figure 2

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Asterisks indicate significantly different from all other means (* P < 0.025, ** P < 0.01).

Figure 4. Percentile distributions for resting membrane potential (A and B) and spike trigger level (C and D), in slow and fast motoneurones (A, C and B, D, respectively).

Figure 4

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Arrows indicate mean values. □ controls and ▪ runners.

Figure 3. Percentile distributions for rheobasic spike (A and B) and AHP amplitude (C and D), in slow and fast motoneurones (A, C and B, D, respectively).

Figure 3

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Arrows indicate mean values. □ controls and ▪ runners.

To measure spike trigger level, we added the difference in voltage between orthodromic spikes evoked by short intense vs. rheobasic currents to the resting membrane potential. While this absolute value is dependent upon an accurate measurement of resting membrane potential, an estimate of the activity effect on this property can be obtained, which is independent of resting membrane potential, by simply comparing the amplitudes of the rheobasic spikes. Differences found using this property are independent of differences among motoneurones in antidromic spike height or resting potential. For this property, also included in Table 1, the statistical results were the same as for spike trigger level, resting membrane potential, and AHP amplitude, in that slow motoneurones from runners showed a higher mean value than the other groups (Fig. 2). As was the case for other properties described above, data distributions for the fast motoneurones were similar between controls and runners (Fig. 3 and Fig. 4).

A summary of relationships among several properties is presented in Table 2. Across the entire sample, antidromic spike height was modestly, but significantly, correlated with resting membrane potential but not with spike trigger level or AHP amplitude (Table 2). Similarly, relationships of resting membrane potential with spike trigger level and AHP amplitude were poor. Thus, systematic differences in properties among groups could not be explained by corresponding variations in either antidromic spike height or resting membrane potential. Analysis of covariance was performed to determine if using antidromic spike height as a covariate would influence the degree of significant difference among the groups in resting membrane potential, rheobasic action potential, spike trigger level or AHP amplitude. In fact, these differences persisted following this procedure, indicating that the activity-related effects on these three properties were not related to a common index of the quality of cell penetration. Similarly, effects of activity on rheobasic action potential, spike trigger level and AHP amplitude persisted when resting membrane was used as a covariate

Table 2.

Relationships (r2 values) among antidromic spike amplitude (mV), resting membrane potential (mV), spike trigger level (mV) and AHP amplitude (mV), for all motoneurones combined

Antidromic action potential amplitude Resting membrane potential Spike trigger level AHP amplitude
Antidromic action potential amplitude — 0.17* 0.01 0.08
Resting membrane potential — — 0.09* 0.01
Spike trigger level — — — 0.02
AHP amplitude — — — —
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*

Significant relationship (P < 0.01).

DISCUSSION

The most significant finding of this study is that basic motoneurone properties, which are important in determining their functional characteristics, are modified by physiological modifications in voluntary neuromuscular activity levels. Thus, the motoneurone is a target for increased spontaneous-activity-modulated neuromuscular plasticity. The most striking effects of increased activity seen in the present study were in spike trigger level, resting membrane potential (both more hyperpolarized in runners) and AHP amplitude (larger in runners). Furthermore, these effects were restricted to the subpopulation of motoneurones that one would expect to be recruited relatively more frequently during the voluntary activity (slow motoneurones).

It might be argued that the findings of alterations in resting membrane potential, spike trigger level and AHP amplitude could be explained by differences in the quality of motoneurone penetrations. This is highly unlikely for several reasons. First, it is not reasonable to expect that the best motoneurone penetrations would occur systematically for the slow motoneurones of the runners, especially since these motoneurones are smaller (i.e. have larger input resistances) than the fast motoneurones of both groups. In addition, the significant activity effects persisted when the analysis of variance was conducted using antidromic spike height and resting membrane potential as covariates, a procedure which effectively ‘corrects’ for variations in these properties. Finally, and consistent with this, correlations among spike trigger level, resting membrane potential, action potential amplitude and AHP amplitude were poor (Table 2).

There is evidence that motoneurone membrane properties are mutable with various interventions. Besides the changes that occur in motoneurones with development (Ziskind-Conhaim, 1988; Cameron et al. 2000), the most documented are the adaptations that occur in the properties of motoneurones which are axotomized (Eccles et al. 1958; Foehring et al. 1986b) or deprived of supraspinal influence via spinal cord transection (Munson et al. 1986; Hochman & McCrea, 1994). Very little direct evidence is available, however, which suggests that altered chronic voluntary activity alone may change the properties of motoneurones. In the cat, chronic stimulation of the medial gastrocnemius nerve for 6 months to alter its muscle fibres to slow-twitch also causes subtle changes in biophysical properties of the innervating motoneurones, such that they begin to resemble slow-twitch motoneurones (lower rheobase, higher input resistance, longer AHP) (Munson et al. 1986). Whether these changes are due to antidromic action potential generation, concurrent afferent stimulation, or the influence of muscle-derived trophic influences on nerve properties, is unknown. In muscles of the dominant hands of human subjects, motor units have lower force thresholds, lower initial and average firing rates and lower discharge variability than in non-dominant hands, all of which would be expected to occur if motoneurones became slightly more like slow motoneurones with increased chronic activity (assuming that handedness differences in motor unit recruitment behaviour are due to difference in chronic usage) (Adam et al. 1998).

The differences seen in motoneurones between controls and runners in the present study were more subtle, and perhaps different, than one might expect from a traditional, chronic, endurance-type training programme. In the latter, there is an increase in the transcription and translation of type I and IIa myosin heavy chains, at the expense of IIx (and in the rat, IIb) in the training muscles (Green et al. 1984; Demirel et al. 1999). Since motoneurone and muscle fibre properties appear to covary systematically under a variety of conditions, one might expect to see motoneurones change in properties towards higher input resistance, lower rheobase and longer AHP values, as suggested by the results summarized in the previous paragraph. In fact, in the present study, the proportions of slow and fast motoneurones were almost identical in the two groups. Even in the absence of overt changes in motoneurone type from fast to slow, a tendency towards a change from fast to slow motoneurones might be expressed as a slightly higher mean input resistance, slightly lower mean rheobase or slightly longer AHP half-decay time, for the fast motoneurones of runners. For example increased running activity can result in changes in the fast fibre population towards higher proportions of IIA, without changes in type I (Ishihara et al. 1991), as well as an increase in mitochondrial enzyme activity (Rodnick et al. 1989). In spite of this, however, none of the properties was significantly altered in the fast motoneurones (in fact, input resistance and rheobase tended to decrease and increase, respectively, in the fast motoneurones, see Table 1).

The importance of muscle adaptations in promoting motoneurone changes in the present study is not known. During reinnervation of cat soleus muscles by medial gastrocnemius motoneurones, a proportion of the latter eventually become more like slow than the original fast motoneurones, although the same motoneurones innervating the original muscle resume their original fast properties (Foehring et al. 1986a). Low-frequency, chronic stimulation of cat medial gastrocnemius to convert its fibres to slow-twitch, also converts a proportion of its innervating motoneurones to those with more slow-like properties (Munson et al. 1997). In addition, muscle paralysis via pharmacological blockade of axon spike transmission shortens the duration of the AHP and increases the rheobase current of the most-affected, slow motoneurones (Cormery et al. 2000). Finally, chronic infusion of BDNF into the gastrocnemius in the rat decreases motoneuronal rheobase after 5 days (Gonzalez & Collins, 1997). These results, taken together, suggest that motoneurone adaptations might be evoked as a result of trophic substances secreted by active muscle fibres and taken up by nerve terminals. In our model of increased spontaneous running, classic adaptations normally seen in endurance-trained muscles, such as increase in the proportion of type I fibres and increased mitochondrial enzyme content, are not seen except in those rats running 15 km or more per day (Rodnick et al. 1989). This was the case for less than half of the rats in the present study. Nonetheless, it is conceivable that trophic substances from active muscle fibres (or by inactive muscle fibres, in the case of the control rats), are implicated in the motoneuronal adaptations found in this study.

Interestingly, several of the changes seen in motoneurones in this study are reminiscent of changes reported to occur in motoneurones of Aplysia californica following long-term sensitization training of the tail-siphon withdrawal reflex. These include a significantly more hyperpolarized resting membrane potential and spike trigger level, and a tendency for a decreased cell input resistance (Cleary et al. 1998). Other evidence of activity-dependent regulation of neuronal ion conductances, but involving decreases in activity, has been presented in the research literature. For example when stomatogastric ganglion neurones of the lobster are isolated from synaptic drive (removed and put into culture), they gained the ability to fire bursts endogenously, and when rhythmic drive was restored, this ability was reduced or lost. An increase in intracellular calcium was necessary to produce these changes (Turrigiano et al. 1994). In addition, there is some suggestion from studies of inactive rat pyramidal cells in culture that activity-induced changes in neuronal sodium channel function occur (Desai et al. 1999).

It may be that the differences noted in motoneurone properties between controls and runners is an effect of reduced spontaneous locomotor activity, in the case of the cage-confined rats. For example spinal cord transection results in motoneurones below the lesion which are less excitable and have a slightly shorter AHP duration (Hochman & McCrea, 1994). This might indicate that the spontaneous running condition is the true ‘control’ condition, and that cage-confined rats are subjected to forced reduced activity which produces adaptations that are more or less severe depending on their original propensity for spontaneous locomotor activity. It should be pointed out that we have no measurement of the relative activity levels of the control rats, which might have been significantly higher when housed in groups of three than would be the case if, for example, they had been housed individually. Nonetheless, it appears that the daily, high-intensity, intermittent running activity which rats perform in these exercise wheels (Seburn & Gardiner, 1995) provides the stimulus that either evokes the adaptations noted in this as well as previous studies (Rodnick et al. 1989; Dluzen et al. 1995; Neeper et al. 1995; among others), or prevents decreased use-induced adaptations.

The mechanisms for the differences seen in resting membrane potential and the spike trigger level might include the density and/or modulation of the inward-rectifier potassium channels, which are known to influence these properties (Cameron et al. 2000; Oliver et al. 2000), and which change during development (Cameron et al. 2000). The involvement of potassium channels is also implied by the effect on the AHP amplitude. Other mechanisms might include alterations in the maximum conductance and gating of the neuronal sodium channels by a number of mechanisms which include phosphorylation of the channel via protein kinase A and C, and glycosylation of the subunits (Marban et al. 1998). For example during induction of long-term potentiation rat hippocampal cultures, the half-activation voltage of sodium channels changed by 5 mV in the negative direction, an effect which depended upon protein kinase C activation via calcium entry into the cell (Ganguly et al. 2000).

The implications for these changes on the function of the neuromuscular system are not known. The changes noted, however, might be seen as consistent with a slowing of contractile speed and increased fatigue resistance of the muscles with increased activity. For example all of the changes noted (increased AHP amplitude, more hyperpolarized resting membrane potential and spike trigger level) might work together to delay the onset of membrane accommodation and late adaptation that would occur with repetitive firing (Granit et al. 1963; Schwindt & Crill, 1982).

In conclusion, increased daily spontaneous activity in rats results in a more hyperpolarized mean resting membrane potential and spike trigger level, and a larger mean amplitude of the AHP, in ‘slow’ (AHP≥ 20 ms) motoneurones, with no effects noted in ‘fast’ motoneurones. Thus, the adaptation was restricted to those motoneurones that one would expect to be most extensively recruited during this activity. The specific underlying conductance channel changes associated with and explaining these adaptations, as well as their functional significance, remain to be elucidated in future studies.

Acknowledgments

This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada to P.G. The authors wish to thank Gérard Ouellet, Paul Martin, Simon Doucet, and Amélie Boisclair for their technical assistance.

REFERENCES

  1. Adam A, De Luca CJ, Erim Z. Hand dominance and motor unit firing behavior. Journal of Neurophysiology. 1998;80:1373–1382. doi: 10.1152/jn.1998.80.3.1373. [DOI] [PubMed] [Google Scholar]
  2. Binder MD, Heckman CJ, Powers RK. The physiological control of motoneuron activity. In: Rowell LB, Shepherd JT, editors. Handbook of Physiology 12:Exercise, Regulation and Integration of Multiple Systems. New York: Oxford University Press; 1996. pp. 3–53. [Google Scholar]
  3. Brownstone RM, Gossard J-P, Hultborn H. Voltage-dependent excitation of motoneurones from spinal locomotor centres in the cat. Experimental Brain Research. 1994;102:34–44. doi: 10.1007/BF00232436. [DOI] [PubMed] [Google Scholar]
  4. Cameron WE, Núñez-Abades PA, Kerman IA, Hodgson TM. Role of potassium conductances in determining input resistance of developing brain stem motoneurons. Journal of Neurophysiology. 2000;84:2330–2339. doi: 10.1152/jn.2000.84.5.2330. [DOI] [PubMed] [Google Scholar]
  5. Cleary LJ, Lee WL, Byrne JH. Cellular correlates of long-term sensitization in Aplysia. Journal of Neuroscience. 1998;18:5988–5998. doi: 10.1523/JNEUROSCI.18-15-05988.1998. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Cope TC, Bodine SC, Fournier M, Edgerton VR. Soleus motor units in chronic spinal transected cats: physiological and morphological alterations. Journal of Neurophysiology. 1986;55:1202–1220. doi: 10.1152/jn.1986.55.6.1202. [DOI] [PubMed] [Google Scholar]
  7. Cormery B, Marini JF, Gardiner PF. Changes in electrophysiological properties of tibial motoneurones in the rat following 4 weeks of tetrodotoxin-induced paralysis. Neuroscience Letters. 2000;287:21–24. doi: 10.1016/s0304-3940(00)01110-1. [DOI] [PubMed] [Google Scholar]
  8. Czeh G, Gallego R, Kudo N, Kuno M. Evidence for the maintenance of motoneurone properties by muscle activity. Journal of Physiology. 1978;281:239–252. doi: 10.1113/jphysiol.1978.sp012419. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Demirel HA, Powers SK, Naito H, Hughes M, Coombes JS. Exercise-induced alterations in skeletal muscle myosin heavy chain phenotype: dose-response relationship. Journal of Applied Physiology. 1999;86:1002–1008. doi: 10.1152/jappl.1999.86.3.1002. [DOI] [PubMed] [Google Scholar]
  10. Desai NS, Rutherford LC, Turrigiano GG. Plasticity in the intrinsic excitability of cortical pyramidal neurons. Nature Neuroscience. 1999;2:515–520. doi: 10.1038/9165. [DOI] [PubMed] [Google Scholar]
  11. Dluzen D, Liu B, Chen C, Dicarlo S. Daily spontaneous running alters behavioral and neurochemical indexes of nigrostriatal function. Journal of Applied Physiology. 1995;78:1219–1224. doi: 10.1152/jappl.1995.78.4.1219. [DOI] [PubMed] [Google Scholar]
  12. Eccles JC, Libet B, Young RR. The behaviour of chromatolysed motoneurones studied by intracellular recording. Journal of Physiology. 1958;143:11–40. doi: 10.1113/jphysiol.1958.sp006041. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Elam M, Svensson T, Thoren P. Brain monoamine metabolism is altered in rats following spontaneous, long-distance running. Acta Physiological Scandinavica. 1987;130:313–316. doi: 10.1111/j.1748-1716.1987.tb08142.x. [DOI] [PubMed] [Google Scholar]
  14. Foehring R, Sypert G, Munson J. Properties of self-reinnervated motor units of medial gastrocnemius of cat. I. Long-term reinnervation. Journal of Neurophysiology. 1986a;55:931–946. doi: 10.1152/jn.1986.55.5.931. [DOI] [PubMed] [Google Scholar]
  15. Foehring R, Sypert G, Munson J. Properties of self-reinnervated motor units of medial gastrocnemius of cat. II. Axotomized motoneurons and time course of recovery. Journal of Neurophysiology. 1986b;55:947–965. doi: 10.1152/jn.1986.55.5.947. [DOI] [PubMed] [Google Scholar]
  16. Foehring R, Sypert G, Munson J. Motor-unit properties following cross-reinnervation of cat lateral gastrocnemius and soleus muscles with medial gastrocnemius nerve. II. Influence of muscle on motoneurons. Journal of Neurophysiology. 1987;57:1227–1245. doi: 10.1152/jn.1987.57.4.1227. [DOI] [PubMed] [Google Scholar]
  17. Frank K, Fuortes G. Stimulation of spinal motoneurones with intracellular electrodes. Journal of Physiology. 1956;134:451–470. doi: 10.1113/jphysiol.1956.sp005657. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Ganguly K, Kiss L, Poo MM. Enhancement of presynaptic neuronal excitability by correlated presynaptic and postsynaptic spiking. Nature Neuroscience. 2000;3:1018–1026. doi: 10.1038/79838. [DOI] [PubMed] [Google Scholar]
  19. Gardiner P. Physiological properties of motoneurons innervating different muscle unit types in rat gastrocnemius. Journal of Neurophysiology. 1993;69:1160–1170. doi: 10.1152/jn.1993.69.4.1160. [DOI] [PubMed] [Google Scholar]
  20. Gardiner PF, Seburn KL. The effects of tetrodotoxin-induced muscle paralysis on the physiological properties of muscle units and their innervating motoneurons in rat. Journal of Physiology. 1997;499:207–216. doi: 10.1113/jphysiol.1997.sp021921. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Gharakhanlou R, Chadan S, Gardiner P. Increased activity in the form of endurance training increases calcitonin gene-related peptide content in lumbar motoneuron cell bodies and in sciatic nerve in the rat. Neuroscience. 1999;89:1229–1239. doi: 10.1016/s0306-4522(98)00406-0. [DOI] [PubMed] [Google Scholar]
  22. Gisiger V, Bélisle M, Gardiner PF. Acetylcholinesterase adaptation to voluntary wheel running is proportional to the volume of activity in fast, but not slow, rat hindlimb muscles. European Journal of Neuroscience. 1994;6:673–680. doi: 10.1111/j.1460-9568.1994.tb00979.x. [DOI] [PubMed] [Google Scholar]
  23. Gómez-Pinilla F, Ying Z, Opazo P, Roy RR, Edgerton VR. Differential regulation by exercise of BDNF and NT-3 in rat spinal cord and skeletal muscle. European Journal of Neuroscience. 2001;13:1078–1084. doi: 10.1046/j.0953-816x.2001.01484.x. [DOI] [PubMed] [Google Scholar]
  24. Gonzalez M, Collins WF., III Modulation of motoneuron excitability by brain-derived neurotrophic factor. Journal of Neurophysiology. 1997;77:502–506. doi: 10.1152/jn.1997.77.1.502. [DOI] [PubMed] [Google Scholar]
  25. Gordon T, Tyreman N, Rafuse VF, Munson JB. Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. 1. Muscle and motor unit properties. Journal of Neurophysiology. 1997;77:2585–2604. doi: 10.1152/jn.1997.77.5.2585. [DOI] [PubMed] [Google Scholar]
  26. Granit R, Kernell D, Shortess GK. Quantitative aspects of repetitive firing of mammalian motoneurones, caused by injected currents. Journal of Physiology. 1963;168:911–931. doi: 10.1113/jphysiol.1963.sp007230. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Green H, Klug G, Richmann H, Seedorf U, Wiehrer W, Pette D. Exercise-induced fibre type transitions with regard to myosin, parvalbumin, and sarcoplasmic reticulum in muscles of the rat. Pflugers Archiv European Journal of Physiology. 1984;400:432–438. doi: 10.1007/BF00587545. [DOI] [PubMed] [Google Scholar]
  28. Halter JA, Carp JS, Wolpaw JR. Operantly conditioned motoneuron plasticity: Possible role of sodium channels. Journal of Neurophysiology. 1995;73:867–871. doi: 10.1152/jn.1995.73.2.867. [DOI] [PubMed] [Google Scholar]
  29. Hochman S, McCrea DA. Effects of chronic spinalization on ankle extensor motoneurons. II. Motoneuron electrical properties. Journal of Neurophysiology. 1994;71:1468–1479. doi: 10.1152/jn.1994.71.4.1468. [DOI] [PubMed] [Google Scholar]
  30. Ishihara A, Inoue N, Katsuta S. The relationship of voluntary running to fibre type composition, fibre area and capillary supply in rat soleus and plantaris muscles. European Journal of Applied Physiology. 1991;62:211–215. doi: 10.1007/BF00643744. [DOI] [PubMed] [Google Scholar]
  31. Ito M, Oshima T. Electrical behaviour of the motoneurone membrane during intracellularly applied current steps. Journal of Physiology. 1965;180:607–635. doi: 10.1113/jphysiol.1965.sp007720. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Jasmin B, Lavoie P, Gardiner P. Fast axonal transport of labeled proteins in motoneurons of exercise-trained rats. American Journal of Physiology: Cell Physiology. 1988;255:C731–736. doi: 10.1152/ajpcell.1988.255.6.C731. [DOI] [PubMed] [Google Scholar]
  33. Kang C-M, Lavoie P-A, Gardiner PF. Chronic exercise increases SNAP-25 abundance in fast-transported proteins of rat motoneurones. NeuroReport. 1995;6:549–553. doi: 10.1097/00001756-199502000-00035. [DOI] [PubMed] [Google Scholar]
  34. Krawitz S, Fedirchuk B, Dai Y, Jordan LM, McCrea DA. State-dependent hyperpolarization of voltage threshsold enhances motoneurone excitability during fictive locomotion in the cat. Journal of Physiology. 2001;532:271–278. doi: 10.1111/j.1469-7793.2001.0271g.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  35. Lambert MI, Noakes TD. Spontaneous running increases VO2max and running performance in rats. Journal of Applied Physiology. 1990;68:400–403. doi: 10.1152/jappl.1990.68.1.400. [DOI] [PubMed] [Google Scholar]
  36. Marban E, Yamagishi T, Tomaselli GF. Structure and function of voltage-gated sodium channels. Journal of Physiology. 1998;508:647–657. doi: 10.1111/j.1469-7793.1998.647bp.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Mendell LM, Collins WF, III, Munson JB. Retrograde determination of motoneuron properties and their synaptic input. Journal of Neurobiology. 1994;25:707–721. doi: 10.1002/neu.480250610. [DOI] [PubMed] [Google Scholar]
  38. Munson JB, Foehring RC, Lofton SA, Zengel JE, Sypert GW. Plasticity of medial gastrocnemius motor units following cordotomy in the cat. Journal of Neurophysiology. 1986;55:619–634. doi: 10.1152/jn.1986.55.4.619. [DOI] [PubMed] [Google Scholar]
  39. Munson JB, Foehring RC, Mendell LM, Gordon T. Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. 2. Motoneuron properties. Journal of Neurophysiology. 1997;77:2605–2615. doi: 10.1152/jn.1997.77.5.2605. [DOI] [PubMed] [Google Scholar]
  40. Neeper S, Gómez-Pinilla F, Choi J, Cotman C. Exercise and brain neurotrophins. Nature. 1995;373:109. doi: 10.1038/373109a0. [DOI] [PubMed] [Google Scholar]
  41. Oliver D, Baukrowitz T, Fakler B. Polyamines a gaging molecules of inward-rectifier K+ channles. European Journal of Biochemistry. 2000;267:5824, 5829. doi: 10.1046/j.1432-1327.2000.01669.x. [DOI] [PubMed] [Google Scholar]
  42. Rodnick K, Reaven G, Haskell W, Sims C, Mondon C. Variations in running activity and enzymatic adaptations in voluntary running rats. Journal of Applied Physiology. 1989;66:1250–1257. doi: 10.1152/jappl.1989.66.3.1250. [DOI] [PubMed] [Google Scholar]
  43. Schwindt PC, Crill WE. Factors influencing motoneuron rhythmic firing:results from a voltage-clamp study. Journal of Neurophysiology. 1982;4:875–890. doi: 10.1152/jn.1982.48.4.875. [DOI] [PubMed] [Google Scholar]
  44. Seburn KL, Gardiner P. Adaptations of rat lateral gastrocnemius motor units in response to voluntary running. Journal of Applied Physiology. 1995;78:1673–1678. doi: 10.1152/jappl.1995.78.5.1673. [DOI] [PubMed] [Google Scholar]
  45. Suzuki H, Tsuzimoto H, Ishiko T, Kasuga N, Taguchi S, Ishihara A. Effect of endurance training on the oxidative enzyme activity of soleus motoneurons in rats. Acta Physiologica Scandinavica. 1991;143:127–128. doi: 10.1111/j.1748-1716.1991.tb09207.x. [DOI] [PubMed] [Google Scholar]
  46. Turrigiano G, Abbott LF, Marder E. Activity-dependent changes in the intrinsic properties of cultured neurons. Science. 1994;264:974–977. doi: 10.1126/science.8178157. [DOI] [PubMed] [Google Scholar]
  47. Wolpaw J, Carp J. Adaptive plasticity in spinal cord. Advances in Neurology. 1993;59:163–174. [PubMed] [Google Scholar]
  48. Zengel J, Reid S, Sypert G, Munson J. Membrane electrical properties and prediction of motor-unit type of medial gastrocnemius motoneurons in the cat. Journal of Neurophysiology. 1985;53:1323–1344. doi: 10.1152/jn.1985.53.5.1323. [DOI] [PubMed] [Google Scholar]
  49. Ziskind-Conhaim L. Electrical properties of motoneurons in the spinal cord of rat embryos. Developmental Biology. 1988;128:21–29. doi: 10.1016/0012-1606(88)90262-x. [DOI] [PubMed] [Google Scholar]

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