Structural changes of the soleus and the tibialis anterior motoneuron pool during development in the rat

Massive Loss of Proprioceptive Ia Synapses in Rat Spinal Motoneurons after Nerve Crush Injuries in the Postnatal Period

Peripheral nerve injuries (PNIs) induce the retraction from the ventral horn of the synaptic collaterals of Ia afferents injured in the nerve, effectively removing Ia synapses from α-motoneurons. The loss of Ia input impairs functional recovery and could explain, in part, better recovery after PNIs with better Ia synaptic preservation. Synaptic losses correlate with injury severity, speed, and efficiency of muscle reinnervation and requires ventral microglia activation. It is unknown whether this plasticity is age dependent. In neonates, axotomized motoneurons and sensory neurons undergo apoptosis, but after postnatal day 10 most survive. The goal of this study was to analyze vesicular glutamate transporter 1 (VGluT1)-labeled Ia synapses (which also include II afferents) after nerve crush in 10 day old rats, a PNI causing little Ia/II synapse loss in adult rats. We confirmed fast and efficient reinnervation of leg muscles; however, a massive number of VGluT1/Ia/II synapses were permanently lost. This synapse loss was similar to that after more severe nerve injuries involving full transection in adults. In adults, disappearance of ventrally directed Ia/II collaterals targeting α-motoneurons was associated with a prolonged microglia reaction and a CCR2 mechanism that included infiltration of CCR2 blood immune cells. By contrast, microgliosis after P10 injuries was fast, resolved in about a week, and there was no evidence of peripheral immune cell infiltration. We conclude that VGluT1/Ia/II synapse loss in young animals differs in mechanism, perhaps associated with higher microglia synaptic pruning activity at this age and results in larger losses after milder nerve injuries.

Neonatal Mice Spinal Cord Interneurons Send Axons through the Dorsal Roots

Spontaneous interneuron activity plays a critical role in developing neuronal networks. Discharges conducted antidromically along the dorsal root (DR) precede those from the ventral root’s (VR) motoneurons. This work studied whether spinal interneurons project axons into the neonate’s dorsal roots. Experiments were carried out in postnatal Swiss-Webster mice. We utilized a staining technique and found that interneurons in the spinal cord’s dorsal horn send axons through the dorsal roots. In vitro electrophysiological recordings showed antidromic action potentials (dorsal root reflex; DRR) produced by depolarizing the primary afferent terminals. These reflexes appeared by stimulating the adjacent dorsal roots. We found that bicuculline reduced the DRR evoked by L5 dorsal root stimulation when recording from the L4 dorsal root. Simultaneously, the monosynaptic reflex (MR) in the L5 ventral root was not affected; nevertheless, a long-lasting after-discharge appeared. The addition of 2-amino-5 phosphonovaleric acid (AP5), an NMDA receptor antagonist, abolished the MR without changing the after-discharge. The absence of DRR and MR facilitated single action potentials in the dorsal and ventral roots that persisted even in low Ca2+ concentrations. The results suggest that firing interneurons could send their axons through the dorsal roots. These interneurons could activate motoneurons producing individual spikes recorded in the ventral roots. Identifying these interneurons and the persistence of their neuronal connectivity in adulthood remains to be established.

Reevaluation of motoneuron morphology: diversity and regularity among motoneurons innervating different arm muscles along a proximal-distal axis

Dendritic fields of spinal motoneurons (MNs) are popularly believed to be stellate; however, variation in dendritic arborization, especially concerning the innervated muscle groups, has not been systematically studied. We addressed this problem by injecting Neurobiotin through patch-pipettes into single MNs (rat cervical cord slices) that had been retrogradely labelled from innervated muscle groups situated along a proximal (here named “scapular” including serratus anterior) to distal (forearm) axis. MNs had fairly straight dendrites with small numbers of mainly proximal branches that exhibited acute branch angles, leaving large areas around the cells with no dendrites. MNs in the same group of pools showed similar morphologies, but there were clear differences among groups. Forearm MNs (n = 35) showed hemidirectionally-extended multipolar dendrites, whereas scapular MNs (n = 15) had bipolar dendrites, and pectoralis MNs (n = 20) had an intermediate morphology. MNs thus showed a spectrum of morphologic characteristics along the axis. This may be because more distally-located forearm muscles are involved in finer movements and need a wider variety of inputs for neural control than proximally-located muscles. We also devised a quantitative method for evaluating the degree to which a cell’s dendritic field displays a symmetric spherical shape; only 20% of MNs tested reached this criterion.

Serotonergic modulation of sacral dorsal root stimulation-induced locomotor output in newborn rat

Descending neuromodulators from the brainstem play a major role in the development and regulation of spinal sensorimotor functions. Here, the contribution of serotonergic signaling in the lumbar spinal cord was investigated in the context of the generation of locomotor activity. Experiments were performed on in vitro spinal cord preparations from newborn rats (0-5 days). Rhythmic locomotor episodes (fictive locomotion) triggered by tonic electrical stimulations (2Hz, 30s) of a single sacral dorsal root were recorded from bilateral flexor-dominated (L2) and extensor-dominated (L5) ventral roots. We found that the activity pattern induced by sacral stimulation evolves over the 5 post-natal (P) day period. Although alternating rhythmic flexor-like motor bursts were expressed at all ages, the locomotor pattern of extensor-like bursting was progressively lost from P1 to P5. At later stages, serotonin (5-HT) and quipazine (5-HT2A receptor agonist) at concentrations sub-threshold for direct locomotor network activation promoted sacral stimulation-induced fictive locomotion. The 5-HT2A receptor antagonist ketanserin could reverse the agonist's action but was ineffective when fictive locomotion was already expressed in the absence of 5-HT (mainly before P2). Although inhibiting 5-HT7 receptors with SB266990 did not affect locomotor pattern organization, activating 5-HT1A receptors with 8-OH-DPAT specifically deteriorated extensor phase motor burst activity. We conclude that during the first 5 post-natal days in rat, serotonergic signaling in the lumbar cord becomes increasingly critical for the expression of fictive locomotion. Our findings therefore further underline the importance of both descending serotonergic and sensory afferent pathways in shaping locomotor activity during postnatal development. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Kinematics of treadmill locomotion in mice raised in hypergravity

The study compared the motor performance of adult C57Bl/6J mice previously exposed to a 2G gravity environment during different periods of their development. 12 mice were housed in a large diameter centrifuge from the conception to Postnatal day 10 (P10). Another group of 10 mice was centrifuged form P10 to P30, and a third group of 9 mice was centrifuged from conception to P30. Their gait parameters, and kinematics of joint excursions were compared with 11 control mice, at the age of 2 months using a video-radiographic apparatus connected to a motorized treadmill. The mice that returned to Earth gravity level at the age of P10 showed a motor pattern similar to control mice. At variance the two groups that were centrifuged from P10 to P30 showed a different motor pattern with smaller and faster strides to walk at the same velocity as controls. On the other hand all the centrifuged mice showed significant postural changes, particularly with a more extended ankle joint, but the mice centrifuged during the whole experimental period differed even more. Our results showed that the exposure to hypergravity before P10 sufficed to modify the posture, suggesting that postural control starts before the onset of locomotion, whereas the gravity constraint perceived between P10 and P30 conditioned the tuning of quadruped locomotion with long term consequences. These results support the existence of a critical period in the acquisition of locomotion in mice.

Functional recovery of stepping in rats after a complete neonatal spinal cord transection is not due to regrowth across the lesion site

Rats receiving a complete spinal cord transection (ST) at a neonatal stage spontaneously can recover significant stepping ability, whereas minimal recovery is attained in rats transected as adults. In addition, neonatally spinal cord transected rats trained to step more readily improve their locomotor ability. We hypothesized that recovery of stepping in rats receiving a complete spinal cord transection at postnatal day 5 (P5) is attributable to changes in the lumbosacral neural circuitry and not to regeneration of axons across the lesion. As expected, stepping performance measured by several kinematics parameters was significantly better in ST (at P5) trained (treadmill stepping for 8 weeks) than age-matched non-trained spinal rats. Anterograde tracing with biotinylated dextran amine showed an absence of labeling of corticospinal or rubrospinal tract axons below the transection. Retrograde tracing with Fast Blue from the spinal cord below the transection showed no labeled neurons in the somatosensory motor cortex of the hindlimb area, red nucleus, spinal vestibular nucleus, and medullary reticular nucleus. Retrograde labeling transsynaptically via injection of pseudorabies virus (Bartha) into the soleus and tibialis anterior muscles showed no labeling in the same brain nuclei. Furthermore, re-transection of the spinal cord at or rostral to the original transection did not affect stepping ability. Combined, these results clearly indicate that there was no regeneration across the lesion after a complete spinal cord transection in neonatal rats and suggest that this is an important model to understand the higher level of locomotor recovery in rats attributable to lumbosacral mechanisms after receiving a complete ST at a neonatal compared to an adult stage.

The development of skilled walking in the rat

The rat is an altricial species and consequently undergoes considerable postnatal development. Careful analysis of the emergence and disappearance of motor behaviours is essential to gain insight into the temporal pattern of maturation of motor system structures. This study presents a qualitative analysis of the developmental progression of skilled movement in the rat by using a skilled walking task. A new rung bridge task was used to expose rat pups to a novel environment in order to reveal their potential capabilities. Ten rat pups were filmed daily from postnatal day 7 through postnatal day 30 as they explored the rung bridge task. Discrete changes in skilled and non-skilled walking in fore- and hind-limbs were evaluated by scoring seven categories and 24 subcategories of motor behaviour, including limb flexion and extension, coordination, posture, sensorimotor responses, distal control, and tail use in rat pups. Frame-by-frame analysis of ambulatory movement revealed six distinct stages of locomotor development. The most significant transformation to mature gait patterns was found between postnatal days 15 and 19, and maturation of all motor behaviour was completed by postnatal day 27. The findings are discussed in relation to the maturation of underlying structures and their relevance to studies of brain damage.

Crossing dendrites of the hypoglossal motoneurons: possible morphological substrate of coordinated and synchronized tongue movements of the frog, Rana esculenta

Application of different fluorescent tracers to the right and left hypoglossal nerve of the frog revealed the extent of dendrites crossing the midline into the territory of contralateral hypoglossal motoneurons. By using confocal microscopy, a large number of close appositions were detected between hypoglossal motoneurons bilaterally, which formed dendrodendritic and dendrosomatic contacts. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. Application of neurobiotin to one hypoglossal nerve resulted in labeling of perikarya exclusively on the ipsilateral side of tracer application, suggesting the absence of dye-coupled connections with contralateral hypoglossal motoneurons. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide electrotonic interactions between the neighboring profiles. We propose that dendrites of hypoglossal motoneurons that cross the midline subserve one of the morphological substrates of co-activation, synchronization and timing of bilateral activity of tongue muscles during prey-catching behavior of the frog.

Developmental up-regulation of the potassium-chloride cotransporter type 2 in the rat lumbar spinal cord

The classical GABA/glycine hyperpolarizing inhibition is not observed in the immature spinal cord. GABA(A) and glycine receptors are anions channels and the efficacy of inhibitory transmission in the spinal cord is largely determined by the gradient between intracellular and extracellular chloride concentrations. The concentration of intracellular chloride in neurons is mainly regulated by two cation-chloride cotransporters, the potassium-chloride cotransporter 2 (KCC2) and the sodium-potassium-chloride co-transporter 1 (NKCC1). In this study, we measured the reversal potential of IPSPs (E(IPSP)) of lumbar motoneurons during the first postnatal week and we investigated the expression of KCC2 and NKCC1 in the ventral horn of the spinal cord from the embryonic day 17 to the postnatal day 20 in the rat. Our results suggest that the negative shift of E(IPSP) from above to below the resting membrane potential occurs during the first postnatal week when the expression of KCC2 increases significantly and the expression of NKCC1 decreases. KCC2 immunolabeling surrounded motoneurons, presumably in the plasma membrane and NKCC1 immunolabeling appeared outside this KCC2-labeled fine strip. Taken together, the present results indicate that maturation of chloride homeostasis is not completed at birth in the rat and that the upregulation of KCC2 plays a key role in the shift from depolarizing to hyperpolarizing IPSPs.

Role of GluR1 in activity-dependent motor system development

Activity-dependent specification of neuronal architecture during early postnatal life is essential for refining the precision of communication between neurons. In the spinal cord under normal circumstances, the AMPA receptor subunit GluR1 is expressed at high levels by motor neurons and surrounding interneurons during this critical developmental period, although the role it plays in circuit formation and locomotor behavior is unknown. Here, we show that GluR1 promotes dendrite growth in a non-cell-autonomous manner in vitro and in vivo. The mal-development of motor neuron dendrites is associated with changes in the pattern of interneuronal connectivity within the segmental spinal cord and defects in strength and endurance. Transgenic expression of GluR1 in adult motor neurons leads to dendrite remodeling and supernormal locomotor function. GluR1 expression by neurons within the segmental spinal cord plays an essential role in formation of the neural network that underlies normal motor behavior.

Dendrodendritic and dendrosomatic contacts between oculomotor and trochlear motoneurons of the frog, Rana esculenta

Gaze fixation requires very fast movements of the eye during body displacement. The morphological and physiological background of the very fine and continuous tuning of gaze fixation is not yet fully understood. In a previous study we have shown that the dendrites of oculomotor neurons form bundles which invade the trochlear nucleus, and vice versa, trochlear dendritic bundles invade the oculomotor nucleus. Earlier physiological observations demonstrating electrotonic coupling between dendrites of spinal motoneurons in the frog suggest a similar mechanism between the oculomotor and trochlear motoneurons. We studied a possible morphological basis of gaze fixation. The experiments were carried out on common water frogs, Rana esculenta. The trochlear and oculomotor nerves were cut, and their proximal stumps were labeled simultaneously with different retrograde fluorescent tracers. Using confocal laser scanning microscope we detected a large number of close contacts in both nuclei, the majority of them were dendrodendritic apposition. The distance between the adjacent profiles suggested close membrane appositions without intercalating glial or neuronal elements. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide ephaptic interactions between the neighboring profiles. This electrotonic coupling between the oculomotor and trochlear nerve motoneurons may promote the co-activation of the muscles responsible for vertical eye movements.

A new stochastic tridimensional model of neonatal rat spinal motoneuron for investigating compartmentalization of neuronal conductances and their influence on firing

During postnatal development spinal motoneurons play a major role in expressing basic behaviours like reflex reactions and in allowing the onset of the locomotor programme. For this purpose it is useful to clarify how various inputs are integrated at the level of the motoneuron soma to generate phasic or rhythmic firing. Although existing models of motoneurons have indicated the distributed role of certain conductances in regulating firing, it is unclear how the spatial distribution of certain currents is ultimately shaping motoneuron output. Thus, it would be helpful to build a bridge between histological and electrophysiological data. The present report is based on the construction of a 3D motoneuron model based on available parameters applicable to the neonatal spinal cord. The presented algorithm allows building up a complex, variable dendrogram which, together with the somatic and axonic compartments, enables strategic location of certain voltage or ligand gated conductances and simulation of resulting electrical behaviour. One application of the present model has been exploring the functional location of the recently reported cystic fibrosis transmembrane conductance regulator (CFTR) which controls Cl(-) homeostasis of postnatal motoneurons. The 3D model is made available for free, user friendly use via the dedicated web site http://www.mn-morphology.org.

Developmental changes in distribution of γ-aminobutyric acid- and glycine-immunoreactive boutons on rat trigeminal motoneurons. I. Jaw-closing motoneurons

We have previously described the distribution pattern of inhibitory synapses on rat jaw-closing (JC) alpha- and gamma-motoneurons. In the present study, we investigated developmental changes in inhibitory synapses on JC motoneurons. We performed a quantitative ultrastructural analysis of putative inhibitory synaptic boutons on JC motoneuron somata by using postembedding immunogold labeling for GABA and glycine. In total, 206, 350, and 497 boutons contacting JC motoneuron somata were analyzed at postnatal days 2 (P2), 11 (P11) and 31 (P31), respectively. The size of the somata increased significantly during postnatal development. The size distribution was bimodal at P31. Mean length of the boutons and percentage of synaptic covering also increased during postnatal development, whereas bouton density did not differ significantly among the three age groups. Synaptic boutons on the somata of JC alpha-motoneurons could be classified into four types: boutons immunoreactive for 1) GABA only, 2) glycine only, 3) both GABA and glycine, and 4) neither GABA nor glycine. There was no developmental change in the proportion of putative inhibitory boutons to the total number of studied boutons. However, the glycine-only boutons increased significantly (15.1% to 27.3%), and the GABA-only boutons decreased significantly (17.7% to 2.6%) during the period from P11 to P31. Our ultrastructural data indicate that the inhibitory synaptic input to JC motoneurons is developmentally regulated and that there is a postnatal switch from GABA to glycine. The postnatal changes revealed in the present study could play an important role in the maturation of the oral motor system.

Motor learning changes GABAergic terminals on spinal motoneurons in normal rats

The role of spinal cord plasticity in motor learning is largely unknown. This study explored the effects of H-reflex operant conditioning, a simple model of motor learning, on GABAergic input to spinal motoneurons in rats. Soleus motoneurons were labeled by retrograde transport of a fluorescent tracer and GABAergic terminals on them were identified by glutamic acid decarboxylase (GAD)67 immunoreactivity. Three groups were studied: (i) rats in which down-conditioning had reduced the H-reflex (successful HRdown rats); (ii) rats in which down-conditioning had not reduced the H-reflex (unsuccessful HRdown rats) and (iii) unconditioned (naive) rats. The number, size and GAD density of GABAergic terminals, and their coverage of the motoneuron, were significantly greater in successful HRdown rats than in unsuccessful HRdown or naive rats. It is likely that these differences are due to modifications in terminals from spinal interneurons in lamina VI-VII and that the increased terminal number, size, GAD density and coverage in successful HRdown rats reflect and convey a corticospinal tract influence that changes motoneuron firing threshold and thereby decreases the H-reflex. GABAergic terminals in spinal cord change after spinal cord transection. The present results demonstrate that such spinal cord plasticity also occurs in intact rats in the course of motor learning and suggest that this plasticity contributes to skill acquisition.

Daily activity of the rabbit jaw muscles during early postnatal development

Early postnatal development of the jaw muscles is characterized by the transition from suckling to chewing behavior. As chewing develops the jaw closing muscles become more powerful compared with the jaw openers. These changes are likely to affect the amount of daily muscle activity. Therefore, the purpose of this study was to characterize for a jaw opener (digastric) and jaw closer (masseter) the total duration of daily muscle activity (i.e. the duty time), and the daily burst numbers and lengths during early postnatal development. Using radiotelemetry the activity of these muscles was recorded in 10 young New Zealand White rabbits between three and eight weeks of age. Fiber-type composition was analyzed at eight weeks of age by determining the myosin heavy chain content of the fibers. During postnatal development both muscles showed no significant decrease or increase in their daily activity. However, the interindividual variation of the duty time and burst number significantly decreased. There were no significant differences between the digastric and masseter except for the most powerful activities at eight weeks of age, where the masseter showed a significantly higher duty time and burst number than the digastric. The masseter contained a higher number of slow-type fibers expressing myosin heavy chain-I and myosin heavy chain-cardiac alpha than the digastric. The present results suggest that the amount of jaw muscle activation is already established early during postnatal development, before the transition from suckling to chewing behavior. This amount of activation seems to be related to the number of slow-type fibers.

Morphological changes of the soleus motoneuron pool in chronic midthoracic contused rats

This study investigated the morphological features of the soleus motoneuron pool in rats with chronic (4 months), midthoracic (T8) contusions of moderate severity. Motoneurons were retrogradely labeled using unconjugated cholera toxin B (CTB) subunit solution injected directly into the soleus muscle of 10 contused and 6 age- and sex-matched, normal controls. Morphometric studies compared somal area, perimeter, diameter, dendritic length, and size distribution of labeled cells in normal and postcontusion animals. In normal animals, motoneurons with a mean of 110.4 +/- 5.2 were labeled on the toxin-injected side of the cord (left). By comparison, labeled cells with a mean of 93.0 +/- 8.4 (a 16% decrease, P = 0.006) were observed in the chronic spinal-injured animals. A significantly smaller frequency of very small (area, approximately 100 microm2) and medium (area, 545-914 microm2) neurons, and a significantly higher frequency of larger (area, >914 microm2) neurons was observed in the labeled soleus motoneuron pools of injured animals compared with the normal controls. Dendritic bundles in the contused animals were composed of thicker dendrites, were arranged in more closely aggregated bundles, and were organized in a longitudinal axis (rostrocaudal axis). Changes in soleus motoneuron dendritic morphology also included significant decrease of total number of dendrites, increased staining, hypertrophy of primary dendrites, and significant decreased primary, secondary, and tertiary branching. The changes in size distribution and dendritic morphology in the postcontusion animals possibly resulted from cell loss and transformation of medium cells to larger cells and/or injury-associated failure of medium cells to transport the immunolabel.

Effects of sensorimotor restriction and anoxia on gait and motor cortex organization: implications for a rodent model of cerebral palsy

Chronic or acute perinatal asphyxia (PA) has been correlated with the subsequent development of cerebral palsy (CP), a developmental neurological disorder characterized by spasticity and motor abnormalities often associated with cognitive deficits. Despite the prevalence of CP, an animal model that mimics the lifetime hypertonic motor deficits is still not available. In the present study, the consequences of PA on motor behavior, gait and organization of the primary motor cortex were examined in rats, and compared with the behavioral and neurological consequences of early postnatal movement-restriction with or without oxygen deprivation. Rats subjected to PA had mild increases in muscular tone accompanied by subtle differences in walking patterns, paralleled by significantly altered but relatively modest disorganization of their primary motor cortices. Movement-restricted rats, suffering PA or not, had reduced body growth rate, markedly increased muscular tone at rest and with active flexion and extension around movement-restricted joints that resulted in abnormal walking patterns and in a profoundly distorted representation of the hind limbs in the primary motor cortex. Within the sensorimotor-restricted groups, non-anoxic rats presented the most abnormal pattern and the greatest cortical representational degradation. This outcome further supports the argument that PA per se may represent a substrate for subtle altered motor behaviors, and that PA alone is sufficient to alter the organization of the primary motor cortex. At the same time, they also show that early experience-dependent movements play a crucial role in shaping normal behavioral motor abilities, and can make a powerful contribution to the genesis of aberrant movement abilities.

Growth-related changes in cell body size and succinate dehydrogenase activity of spinal motoneurons innervating the rat soleus muscle

Cell body sizes and oxidative enzyme (succinate dehydrogenase) activities of spinal motoneurons innervating the soleus muscle were determined in rats ranging in postnatal age from 3 to 13 weeks. The soleus motoneurons were labeled by a retrograde neuronal tracer, nuclear yellow. The mean cell body sizes of motoneurons increased from 3 to 7 weeks of age, while the mean succinate dehydrogenase activities of motoneurons decreased from 3 to 7 weeks of age. There were no changes in mean cell body size or mean succinate dehydrogenase activity of motoneurons from 7 to 13 weeks of age. An inverse relationship between cell body size and succinate dehydrogenase activity of motoneurons was observed, irrespective of age. These results indicate that motoneurons innervating the rat soleus muscle show the adult pattern of cell body size and succinate dehydrogenase activity at an earlier stage of postnatal growth, 7 weeks of age.

Relationship between morphoelectrotonic properties of motoneuron dendrites and their trajectory

The distribution and geometry of the dendritic trees of spinal motoneurons obey several well-established rules. Some of these rules are based on systematic relationships between quantitative geometrical features (e.g., total dendritic length) and the three-dimensional trajectory followed by dendrites from their origin to their termination. Because dendritic geometry partially determines the transmission of current and voltage signals generated by synapses on the dendritic tree, our goal was to compare the efficacy of signal transmission by dendritic trajectories that followed different directions. To achieve this goal, we constructed detailed compartmental models of the dendritic trees of three intracellularly stained biventer cervicis/complexus (BCCM) motoneurons and calculated the electronic properties of 361 dendritic paths. Each trajectory was classified according to its orientation, e.g., rostral, rostral-dorsal-lateral. The attenuation of current and voltage signals en route to the soma was strongly related to trajectory orientation. Trajectories with similar attenuation factors formed functional subunits that were arranged in distinct domains within the ventral horn. Changes in R(m) or R(i) had little effect on which trajectories belonged to each functional subunit. However, differences in the efficacy of signal transmission between subunits increased during high network activity (mimicked by decreases in R(m)). The most efficient subunit delivered two times more current and four times more voltage to the soma than the least efficient subunit. These results indicate that the input-output properties of motoneurons depend on the direction of the path taken by dendrites from their origin at the cell body to their terminals.

Increased electrotonic coupling in spinal motoneurons after transient botulinum neurotoxin paralysis in the neonatal rat

The effect of early postnatal blockade of neuromuscular transmission using botulinum neurotoxin (BoNT) type A on motoneuron gap junctional coupling was studied by means of intracellular recordings and biocytin labeling using the in vitro hemisected spinal cord preparation of neonatal rats. The somata of tibialis anterior (TA) motoneurons were retrogradely labeled at birth (P0) by intramuscular injection of fluorescent tracers. Two days later, BoNT was injected unilaterally into the TA muscle. The toxin blocked neuromuscular transmission for the period studied (P4-P7) as shown by tension recordings of the TA muscle. Retrograde horseradish peroxidase tracing in animals reared to adulthood demonstrated no significant cell death or changes in the soma size of BoNT-treated TA motoneurons. Intracellular recordings were carried out in prelabeled control and BoNT-treated TA motoneurons from P4 to P7. Graded stimulation of the ventral root at subthreshold intensities elicited short-latency depolarizing (SLD) potentials that consisted of several discrete components reflecting electrotonic coupling between two or more motoneurons. BoNT treatment produced a significant increase (67%) in the maximum amplitude of the SLD and in the number of SLD components as compared with control (3.1 +/- 1.7 vs. 1.4 +/- 0.7; means +/- SD). The morphological correlates of electrotonic coupling were investigated at the light microscope level by studying the transfer of biocytin to other motoneurons and the putative sites of gap junctional interaction. The dye-coupled neurons clustered around the injected cell with close somato-somatic, dendro-somatic and -dendritic appositions that might represent the sites of electrotonic coupling. The size of the motoneuron cluster was, on average, 2.2 times larger after BoNT treatment. Our findings demonstrate that a short-lasting functional disconnection of motoneurons from their target muscle delays motoneuron maturation by halting the elimination of gap junctional coupling that normally occurs during early postnatal development.

Postural modifications and neuronal excitability changes induced by a short-term serotonin depletion during neonatal development in the rat

Serotonin (5-HT) plays an important role both in the development and in the recovery of locomotion after spinalization in vertebrates. We investigated the contribution of the serotonergic system to the maturation of the lumbar motoneurons and networks in the neonatal rat. A 5-HT synthesis inhibitor, p-chlorophenylalanine (PCPA), was administered daily from the first postnatal day (P0) onward. This protocol depleted serotonin in the spinal cord within 3-4 d, as demonstrated by immunohistochemistry. PCPA-treated rats exhibited postural changes characterized by lesser flexion at the knee and ankle levels and lesser extension of the hip. Posture was asymmetric, suggesting possible deficits in the interlimb coordination. Intracellular recordings were made at P3-5 from motoneurons innervating different hindlimb muscles, using the in vitro brainstem-spinal cord-nerve-attached preparation. In PCPA-treated rats, the conduction velocity of motoneurons was increased, and their excitability was decreased (because of higher rehobase and input conductance) compared with sham animals. In accordance with postural observations, changes were more pronounced in hip extensor/knee flexor than in ankle extensor motoneurons. The maturation of repetitive firing properties was stopped by PCPA treatment, although PCPA, applied in vitro, had no effect on membrane properties. The spontaneous endogenously generated activity, which is a characteristic of immature networks, was increased in PCPA-treated rats, suggesting that developing lumbar networks are sensitive to 5-HT levels. Serotonin may play a critical role during development in regulating the balance between the excitability of motoneurons and that of interneurons. Interneuronal excitability is crucial for the activity-dependent development of spinal cord networks.

Cell body size and succinate dehydrogenase activity of spinal motoneurons innervating the soleus muscle in mice, rats, and cats

The cell body sizes and succinate dehydrogenase (SDH) activities of motoneurons in the retrodorsolateral region of the ventral horn in the spinal cord innervating the soleus muscle in mice, rats, and cats were compared using quantitative enzyme histochemistry. There was an inverse relationship between cell body size and SDH activity of motoneurons in the three species. The mean cell body sizes of both gamma and alpha motoneuron pools were in the rank order of mice < rats < cats, while the mean SDH activities of both gamma and alpha motoneuron pools were in the rank order of mice > rats > cats. It is concluded that smaller motoneurons innervating the soleus muscle have higher SDH activities than larger motoneurons, irrespective of the species, and that motoneuron pools innervating the soleus muscle in smaller animals have smaller mean cell body sizes and higher mean SDH activities than those in larger animals.

Early ontogeny of locomotor behaviour: a comparison between altricial and precocial animals

The focus of this review is to examine the physiological and behavioural differences between the early ontogeny of locomotion in precocial and altricial species. Both groups of animals are capable of performing alternating stepping movements upon birth or hatching, indicating that the basic elements underlying locomotor synergy are present prior to expression of mature overground gait. Nevertheless, the notable difference between precocial and altricial animals is the ability of the former to walk and run soon after birth or hatching. The weight of experimental evidence suggests that postural constraints play an important role in preventing early expression of locomotor behaviour in altricial species. Even some precocial animals, however, need time to develop sufficient stability and balance to walk as an adult. Therefore, components of locomotor behaviour involving the maintenance of equilibrium need a period of maturation in both precocial and altricial species, possibly requiring locomotor experience to become fully mature.

Perinatal development of lumbar motoneurons and their inputs in the rat

The rat is quite immature at birth and a rapid maturation of motor behavior takes place during the first 2 postnatal weeks. Lumbar motoneurons undergo a rapid development during this period. The last week before birth represents the initial stages of motoneuron differentiation, including regulation of the number of cells and the arrival of segmental and first supraspinal afferents. At birth, motoneurons are electrically coupled and receive both appropriate and inappropriate connections from the periphery; the control from supraspinal structures is weak and exerted mainly through polysynaptic connections. During the 1st postnatal week, inappropriate sensori-motor contacts and electrical coupling disappear, the supraspinal control increases gradually and myelin formation is responsible for an increased conduction velocity in both descending and motor axons. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are transiently overexpressed in the neonatal spinal cord. The contribution of non-NMDA receptors to excitatory amino acid transmission increases with age. Activation of gamma-aminobutyric acid(A) and glycine receptors leads to membrane depolarization in embryonic motoneurons but to hyperpolarization in older motoneurons. The firing properties of motoneurons change with development: they are capable of more repetitive firing at the end of the 1st postnatal week than before birth. However, maturation does not proceed simultaneously in the motor pools innervating antagonistic muscles; for instance, the development of repetitive firing of ankle extensor motoneurons lags behind that of flexor motoneurons. The spontaneous embryonic and neonatal network-driven activity, detected at the levels of motoneurons and primary afferent terminals, may play a role in neuronal maturation and in the formation and refinement of sensorimotor connections.

Gap junctional coupling and patterns of connexin expression among neonatal rat lumbar spinal motor neurons

Interneuronal gap junctional coupling is a hallmark of neural development whose functional significance is poorly understood. We have characterized the extent of electrical coupling and dye coupling and patterns of gap junction protein expression in lumbar spinal motor neurons of neonatal rats. Intracellular recordings showed that neonatal motor neurons are transiently electrically coupled and that electrical coupling is reversibly abolished by halothane, a gap junction blocker. Iontophoretic injection of Neurobiotin, a low molecular weight compound that passes across most gap junctions, into single motor neurons resulted in clusters of many labeled motor neurons at postnatal day 0 (P0)-P2, and single labeled motor neurons after P7. The compact distribution of dye-labeled motor neurons suggested that, after birth, gap junctional coupling is spatially restricted. RT-PCR, in situ hybridization, and immunostaining showed that motor neurons express five connexins, Cx36, Cx37, Cx40, Cx43, and Cx45, a repertoire distinct from that expressed by other neurons or glia. Although all five connexins are widely expressed among motor neurons in embryonic and neonatal life, Cx36, Cx37, and Cx43 continue to be expressed in many adult motor neurons, and expression of Cx45, and in particular Cx40, decreases after birth. The disappearance of electrical and dye coupling despite the persistent expression of several gap junction proteins suggests that gap junctional communication among motor neurons may be modulated by mechanisms that affect gap junction assembly, permeability, or open state.

Sciatic nerve transection in the adult rat: abnormal EMG patterns during locomotion by aberrant innervation of hindleg muscles

The effects of lesions in the sciatic nerve were studied in adult rats. In the left hindleg, a segment 12 mm long was resected from the proximal part of the nerve, before the bifurcation into the peroneal and tibial nerves. This segment in a reversed orientation was used as a nerve graft. EMG patterns in the tibialis anterior and the gastrocnemius muscles at both sides were recorded during locomotion in six rats after recovery periods varying from 15 to 21 weeks. The specificity of axonal outgrowth was studied in nine rats by retrogradely labeling the motoneurons with unconjugated Cholera Toxin subunit B (CTB) after injections into the gastrocnemius, the soleus, and the tibialis anterior muscles at both sides. EMG patterns at the operated side were irregular and we often observed coactivation of the gastrocnemius and tibialis anterior muscle. Moreover, burst activity was badly adjusted to the phases of the stepcycle. Retrogradely labeling indicated that the pools of motoneurons innervating the respective muscles at the left side had increased in volume. Neuronal diameters were slightly decreased but a considerable decrease was observed in dendritic branching and dendrite bundles in the pools of the SOL and in the GC were absent. No consistent trends in neuronal numbers at the affected side in comparison to the right side were detected. We conclude that axons, sprouting from the proximal stump of the sciatic nerve, innervate the muscles aselectively and that the motoneurons of origin maintain their original activation pattern.

Persistence of somatic and dendritic growth associated processes and induction of dendritic sprouting in motoneurones after neonatal axotomy in the rat

The effect of neonatal axotomy on the maturation of motoneurone somadendritic morphology was studied in identified motoneurones innervating the ankle dorsiflexor muscles tibialis anterior (TA) and extensor digitorum longus (EDL) of the rat by intracellular injection of Lucifer Yellow and confocal microscopy. At birth, the entire somatodendritic surface is covered with fine filopodial growth-associated processes. These are eliminated from the soma and proximal dendrites during the first postnatal week as part of a somatofugal process of dendritic maturation. Following neonatal axotomy, the postnatal elimination of growth associated processes was halted and new, axonal-like processes were seen to sprout from the soma and proximal dendrites in some of the axotomized motoneurones. These results indicate that synaptic interaction with the target muscle during the early postnatal period is essential for the maturation of the somatodendritic receptive surface of the motoneurone.

Dendro-dendritic connections between motoneurons in the rat spinal cord: an electron microscopic investigation

The ultrastructural characteristics of identified dendrite bundles in the rat spinal cord were analyzed following retrograde tracing from the soleus muscles. Dendrite bundles are arranged in networks that are heavily interconnected by means of gap junctions. The bundles are formed by at least 10 crossing dendrites travelling in different focal planes. In between dendrites, elongated gap junctional complexes are frequently found. Dendrite lamellar bodies, recently described to occur in relation with gap junctions in the central nervous system were not observed in the present study.

Localization of the calcium/calmodulin-dependent protein phosphatase, calcineurin, in the hindbrain and spinal cord of the rat

The calcium/calmodulin-dependent protein phosphatase calcineurin was localized at the light microscopic level in the rat hindbrain and spinal cord by using an antibody against the alpha-isoform of the catalytic subunit. Calcineurin was highly concentrated in axons, dendrites, and cell bodies of a subpopulation of alpha-motoneurons in hindbrain motor nuclei and the lateral motor column along the length of the spinal cord. These calcineurin-positive alpha-motoneurons appeared to be randomly distributed and represented approximately 25% of the total alpha-motoneuron pool in the motor trigeminal nucleus and the spinal cord lateral motor column. Within the facial nucleus, calcineurin-containing motoneurons were present in the medial and dorsal subdivision but not in the lateral and intermediate subdivision. In addition to the enrichment in motoneurons, calcineurin was enriched in cells of the superficial laminae of the spinal cord dorsal horn and its extension into the medulla, the caudal spinal trigeminal nucleus. Axonal staining in the white matter of the spinal cord was generally weak, except in the dorsolateral funiculus, where strongly calcineurin-positive axons formed a putative ascending tract that appeared to terminate uncrossed in the caudal lateral reticular nucleus of the medulla. This tract may originate from calcineurin-positive cells in the dorsolateral funiculus. We also compared the distribution of calcineurin with calcium/calmodulin-dependent kinase II in the spinal cord and found that the kinase is more widely expressed. Thus, calcineurin is highly restricted to a few locations in the hindbrain and spinal cord. Selective staining in facial subnuclei that innervate phasically active muscles suggests that calcineurin-positive motoneurons represent a subset of alpha-motoneurons innervating a metabolic subtype of muscle fibers, possibly fast-twitch fibers.

Comparison of the topology and growth rules of motoneuronal dendrites

The complexity, shape, and branching modes of the dendrites of spinal motoneurons were compared in cat, rat, and frog using topological analysis and growth models. The complexity of motoneuronal dendrites, measured as the mean number of terminal segments, varied significantly among samples and was related to contractile properties of innervated motor units. Despite this variation, all mature motoneurons having a mean number of terminal segments per dendrite greater than ten (up to 24.3) exhibited a narrow range of values of coefficients describing the symmetry of tree shapes (0.42-0.47). This implies low variability in the topological shape of motoneuronal dendrites of different animals. This similarity of tree shapes proved to be a result of the similarity of growth rules. The growth of the dendrites could be described to a first approximation by a two-parameter (Q and S) model called the QS model and by a multitype Markovian model. The estimation of parameters of the QS model, in which parameter Q is related to the probability of branching of intermediate segments, revealed that Q was equal or close to 0, implying that branching of dendrites is restricted to terminal segments. The estimates of the parameter S, which describes whether the probability of branching increases (S < 0) or decreases (S > 0) exponentially with segment order, were positive. This was in agreement with the results of estimation of probabilities of branching provided by the Markovian model, which showed that the branching probabilities decreased with segment order in an exponential manner in most of the neurons studied. The QS and Markovian models involve different assumptions about the sequence and timing of branching events, and selection of the best model can provide insight into details of dendritic outgrowth. Extensive simulation of tree outgrowth using a Markovian model revealed significant differences between stimulated trees and real dendrites, particularly with regard to variability of the number of terminals and to symmetry. In contrast, the QS model provided a good fit to the mean values and standard deviations of basic topological parameters. This model is adequate to describe the shape of mature motoneuronal dendrites. It implies that dendritic branches have many opportunities to bifurcate during the whole time of development and that bifurcating potency of a branch is a function of the number and position of other branches of that dendrite. Combined with analysis of metrical properties such as lengths of segments, the QS model can assist in a quantitative analysis of development and plasticity.

Effect of transient neonatal muscle paralysis on the growth of soleus motoneurones in the rat

The postnatal growth of soleus motoneurones was studied during normal development and following transient paralysis of the soleus muscle in neonatal rats. Paralysis was achieved by implanting a silicon strip containing alpha-bungarotoxin alongside the soleus muscle in rat pups within 3-6 h of birth. The soleus muscle was completely paralysed for at least 24 h, and by 9 days neuromuscular transmission was fully restored. The soma size of normal and target-deprived soleus motoneurones was compared at intervals during the first 3 postnatal weeks and in adults, using the retrograde horseradish peroxidase technique. There was a four-fold increase in the soma area of normal motoneurons during the first 3 postnatal weeks, with the greater part of the increase occurring between 7 and 14 days. At 3 days, the distribution of soma areas was unimodal and became bimodal by 21 days. Paralysis during the first postnatal week did not significantly affect the developmental changes in motoneurone soma area or their distribution up to 3 weeks of age. Thus, motoneurones deprived of functional neuromuscular contact appear to grow normally during the early postnatal period, although previous results show that at later stages (2-3 months of age), many of these motoneurones die and the remaining cells are smaller than normal.

Postnatal changes in cell body size and oxidative enzyme activity of spinal motoneurons innervating the rat tibialis anterior muscle

The cell body size and succinate dehydrogenase (SDH) activity of spinal motoneurons innervating the superficial and deep regions of the tibialis anterior muscle were studied in rats ranging in postnatal age from 3 to 11 weeks, by retrograde neuronal labeling using fluorescent neuronal tracers. The motoneurons innervating the tibialis anterior muscle were located primarily at the L4 spinal cord segment and those innervating the superficial and deep regions of the muscle were distributed throughout the entire extent of the motoneuron pool. The distribution of the motoneurons during postnatal development was similar to that observed in the adult animal. The mean cell body size of the motoneurons innervating the superficial region of the muscle in rats from 5 to 11 weeks of age was greater than that innervating the deep region at corresponding ages. The mean SDH activity of the motoneurons innervating the deep region of the muscle increased during postnatal development, while there were no changes in the mean SDH activity of those innervating the superficial region during this period. At 11 weeks of age, the motoneurons innervating the deep region of the muscle had a higher mean SDH activity than those innervating the superficial region. An inverse relationship between cell body size and SDH activity of motoneurons innervating both the superficial and deep regions of the muscle was observed, independent of age. These results indicate that motoneurons innervating the superficial and deep regions of the rat tibialis anterior muscle have different developmental patterns with regard to cell body size and SDH activity.

Development of the EMG of the soleus muscle in the rat

The EMG of the soleus muscle was recorded with bipolar electrodes chronically implanted in rats aged 10-30 days. Changes in the activity pattern were studied in relation to motor development. The firing pattern and shape of motor units were studied at higher resolution. EMG activity was closely related to motor behaviour and posture. At P11, soleus was only phasically active during movements. Tonic EMG activity appeared with age, and by P16 the activity pattern was similar to that of the adult. The activity level also increased markedly with age. This was paralleled by a change in hindlimb posture. The development of tonic activity precedes the development of dendrite bundles in the motoneuron pool of the soleus muscle, which does not support the hypothesis that bundle formation is causally related to tonic activity in postural muscles. At young ages, the tonic EMG was characterized by a clear oscillation pattern with a frequency ranging from 7 to 20 Hz, the higher frequencies occurring at older ages. This oscillation disappeared around P16, which suggests that it is related to functional development of the stretch reflex. Periods of high frequency firing were observed in the EMG; the highest firing frequencies occurred during locomotion. The maximum firing frequency increased steeply to about 80 Hz at P18. It is suggested that bursts of high frequency firing are related to the development of monoaminergic innervation of the spinal cord. During the first three weeks, motor unit action potentials often showed complex shapes of long duration and considerable spike-to-spike variability. Computer simulation of the summation process of fibre potentials at the electrode showed that the small fibre diameters and high variability in diameter at young ages are major factors with respect to the generation of complex potentials.

Early postnatal changes in the somatodendritic morphology of ankle flexor motoneurons in the rat

The development of locomotor function in the rat spans the first 3 postnatal weeks. We have studied morphological features of the soma and dendrites of motoneurons innervating the physiological flexor muscles of the ankle, tibialis anterior and extensor digitorum longus, by intracellular injection in vitro between the first and ninth postnatal days. We obtained serial optical sections of 96 adequately filled motoneurons in whole-mounted hemisected spinal cords by confocal microscopy, projected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such motoneurons were covered in growth-associated spiny, thorny or hair-like appendages. These had disappeared from the soma by the fourth postnatal day and from most proximal dendrites by day 7, but were still common distally on day 9. During this period there was little or no net growth of either the soma (which was still much smaller than in the adult) or the dendritic tree. A dorsal dendritic bias was present and 'sprays' of long, loosely bundled dorsal dendrites were often seen. The mean number of primary dendrites remained constant at about eight, and their combined diameter was already significantly correlated with mean soma diameter, as in the adult cat. Thus, the critical neonatal period during which these ankle flexor motoneurons are known to change their electrophysiological properties and to be particularly sensitive to interference with neuromuscular interaction is characterized by major changes in the neuronal surface, presumably linked to synaptogenesis.

Motoneuron morphology in the dorsolateral nucleus of the rat spinal cord: normal development and androgenic regulation

The rat lumbar spinal cord contains two sexually dimorphic motor nuclei, the spinal nucleus of the bulbocavernosus (SNB), and the dorsolateral nucleus (DLN). These motor nuclei innervate anatomically distinct perineal muscles that are involved in functionally distinct copulatory reflexes. The motoneurons in the SNB and DLN have different dendritic morphologies. The dendrites of motoneurons in the medially positioned SNB have a radial, overlapping arrangement, whereas the dendrites of the laterally positioned DLN have a bipolar and strictly unilateral organization. During development, SNB motoneuron dendrites grow exuberantly and then retract to their mature lengths. In this experiment we determined whether the adult difference in SNB and DLN motoneuron morphology was reflected in different patterns of dendritic growth during normal development. Furthermore, the development of both these nuclei is under androgenic control. In the absence of androgens, SNB dendrites fail to grow; testosterone replacement supports normal dendritic growth. Thus, we also examined the development of DLN dendrites for similar evidence of androgenic regulation. By using cholera toxin-horseradish peroxidase (BHRP) to label motoneurons retrogradely, we measured the morphology of DLN motoneurons in normal males, and in castrates treated with testosterone or oil/blank implants at postnatal day (P) 7, P28, P49, and P70. Our results demonstrate that in contrast to the biphasic pattern of dendritic development in the SNB, dendritic growth in the DLN was monotonic; the dendritic length of motoneurons increased more than 500% between P7 and P70. However, as in the SNB, development of DLN motoneuron morphology is androgen-dependent. In castrates treated with oil/blank implants, DLN somal and dendritic growth were greatly attenuated compared to those of normal or testosterone-treated males. Thus, while androgens are clearly necessary for the growth of motoneurons in both the SNB and DLN, their different developmental patterns suggest that other factors must be involved in regulating this growth.

Development of locomotion in the rat: the significance of early movements

The development of the nervous system is determined by an interaction between genetic and epigenetic factors. We investigated the possible role of proprioceptive afferent input in the development of locomotion in the rat. Kinematic analysis of locomotion in normal rats reveals a marked transition from immature overground locomotion into the adult pattern around the 15th postnatal day. Around this age, the timing of EMG activity in the tibialis anterior and the gastrocnemius muscle in the hindpaw gains accuracy and the intensity of the EMG increases. In the soleus muscle we observed an increased regularity in the shape of individual motor unit potentials. Neuroanatomical research revealed the occurrence of dendrite bundles in the motoneuronal pool of the soleus muscle pool from the 16th day as the result of a reorganisation. Immobilisation of one hindlimb in an extended position by casts around the leg from the first till the 20th postnatal day does not interfere with the development of dendrite bundles in the soleus motoneuronal pool. However, we demonstrated long lasting abnormalities in the timing of the EMG activation patterns in the gastrocnemius and the tibialis anterior muscles. It is conceivable that the experimental interference with afferent feed back is the causative factor of these functional abnormalities.

Neonatal nerve injury causes long-term changes in growth and distribution of motoneuron dendrites in the rat

Disruption of neuromuscular contact by nerve-crush during the early postnatal period results in the death of a large proportion of affected motoneurons. Increased activity and abnormal reflex responses are evident in those that survive. We have studied the aberrant dendritic morphology of surviving cells and have attempted to correlate the observed alterations in morphology with the above experimental findings. Motoneurons supplying the extensor hallucis longus muscles of the rat were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase. The dendritic tree of labelled cells was analysed in adult animals having undergone unilateral sciatic nerve-crush at birth. Unoperated control animals were also examined. Following nerve-crush at birth, total visible dendritic length was more than 30% smaller than control cells in the transverse plane. This decrease was confined largely to the medially directed segments of the dendritic field and appeared to be due to a reduction in dendritic branching combined with a failure to achieve the correct branch length. There was no overall change in total visible dendritic length in the longitudinal plane, but a reorientation of dendrites in favour of rostrodorsal regions was observed. There was no alteration in dendritic length in cells contralateral to the nerve injury. These results show that nerve injury during early postnatal development produces lasting changes in the distribution of motoneuron dendrites. The localized nature of these changes may explain the altered activity and induced death of motoneurons seen after neonatal nerve-crush.

Consequences of cerebellar lesions at early and later ages: clinical relevance of animal experiments

Animal experiments demonstrated that reactions of the brain after early lesions differ from those after lesions at adult age. Detailed knowledge on the neuroanatomical and neurophysiological consequences of brain lesions was obtained in humans and will be gained from lesion experiments in animals. Prerequisites for extrapolating animal data to the clinical situation are discussed: knowledge on the maturational stage at which the lesion occurs and the behavioral expression of the damaged neural system. The extensive remodelling after early unilateral cerebellar hemispherectomy and its consequences for behavioural development in the rat are presented and discussed.

Early cerebellar hemispherectomy in the rat. Effects on the maturation of two hindlimb muscles and on lumbar motoneurones

Cerebellar hemispherectomy before the 10th day in rats leads to extensive neuronal remodelling. In the present study the problem was studied whether such early lesions also have effects on the maturation of the soleus and the extensor digitorum longus muscles in the hindleg as well as on the formation of dendrite bundles from motoneurons innervating the soleus muscle. Results indicate consistent left-to-right differences in the numbers of muscle fibres but no differences in muscle differentiation. Dendritic bundles of soleus motoneurons, at the side ipsilateral to the cerebellar lesion are absent or less conspicuous in comparison to the contralateral side or to those bundles in normal rats. Cerebellar lesioning at the 30th day does not affect dendritic bundles.

Postnatal maturation of the dendritic fields of motoneuron pools supplying flexor and extensor muscles of the distal forelimb in the rat

In the rat cervical spinal cord the corticospinal projection on motoneurons either direct or indirect (via interneurons) comes about postnatally making it accessible for experimental research. Therefore, the postnatal developmental changes of motoneurons and in particular their dendritic fields were examined. Motoneurons innervating the two antagonistic muscles in the distal forepaw, the m. flexor digitorum profundus and the m. extensor digitorum communis, were retrogradely labelled by intramuscular injections of cholera toxin subunit B conjugated with horseradish peroxidase in rats of various postnatal ages. Following a 48–72 hour survival period the motoneurons and their dendritic fields were studied in the seventh and eighth cervical spinal cord segments. Both the number and the position of motoneurons were found to remain constant throughout postnatal development. Extensor motoneurons were positioned dorsolaterally in the ventral horn at the border of grey and white matter, flexor motoneurons were in general medial to extensor motoneurons. The results on the dendritic field demonstrate firstly, that during postnatal development the extension of the dendrites of both flexor and extensor motoneurons changes from spreading out in all directions at postnatal day 2 to spreading in only a few, specific directions from postnatal day 21 onwards, with the restriction that both motoneuron pools follow a different time scale to achieve this. Secondly, both pools have a temporal dendritic component extending into the white matter of the lateral funiculus.(ABSTRACT TRUNCATED AT 250 WORDS)