Pierre A. Guertin

The mammalian central pattern generator for locomotion

At the beginning of the 20th century, Thomas Graham Brown conducted experiments that after a long hiatus changed views on the neural control of locomotion. His seminal work supported by subsequent evidence generated largely from the 1960s onwards showed that across species walking, flying, and swimming are controlled largely by a neuronal network that has been referred to as the central pattern generator (CPG) for locomotion. In mammals, this caudally localized spinal cord network was found to generate the basic command signals sent to muscles of the limbs for locomotor rhythm and pattern generation. This article constitutes a comprehensive review summarizing key findings on the organization and properties of this network.

Contribution of spinal 5-HT1A and 5-HT7 receptors to locomotor-like movement induced by 8-OH-DPAT in spinal cord-transected mice

Growing evidence from in vitro studies suggests that spinal serotonin (5‐HT) receptor subtypes 5‐HTR1A and 5‐HTR7 are associated with an induction of central pattern generator activity. However, the possibility of a specific role for these receptor subtypes in locomotor rhythmogenesis in vivo remains unclear. Here, we studied the effects of a single dose (1 mg/kg, i.p.) of 8‐hydroxy‐2‐(di‐N‐propylamino)‐tetralin (8‐OH‐DPAT), a potent and selective 5‐HTR1A/7 agonist, in mice spinal cord transected at the low‐thoracic level (Th9/10). The results show that 8‐OH‐DPAT acutely induced, within 15 min, hindlimb movements that share some characteristics with normal locomotion. Paraplegic mice pretreated with the selective 5‐HTR1A antagonists, WAY100,135 or WAY100,635, displayed significantly less 8‐OH‐DPAT‐induced movement. A similar reduction of 8‐OH‐DPAT‐induced movements was found in animals pretreated with SB269970, a selective 5‐HTR7 antagonist. Moreover, a near complete blockade of 8‐OH‐DPAT‐induced movement was obtained in wild‐type mice pretreated with 5‐HTR1A and 5‐HTR7 antagonists, and in 5‐HTR7–/– mice pretreated with 5‐HTR1A antagonists. Overall, these results clearly demonstrate that 8‐OH‐DPAT potently induces locomotor‐like movement in the previously paralysed hindlimbs of low‐thoracic‐transected mice. The results, with selective antagonists and knockout animals, provide compelling evidence of a specific contribution of both receptor subtypes to spinal locomotor rhythmogenesis in vivo.

Central pattern generator for locomotion: anatomical, physiological, and pathophysiological considerations.

This article provides a perspective on major innovations over the past century in research on the spinal cord and, specifically, on specialized spinal circuits involved in the control of rhythmic locomotor pattern generation and modulation. Pioneers such as Charles Sherrington and Thomas Graham Brown have conducted experiments in the early twentieth century that changed our views of the neural control of locomotion. Their seminal work supported subsequently by several decades of evidence has led to the conclusion that walking, flying, and swimming are largely controlled by a network of spinal neurons generally referred to as the central pattern generator (CPG) for locomotion. It has been subsequently demonstrated across all vertebrate species examined, from lampreys to humans, that this CPG is capable, under some conditions, to self-produce, even in absence of descending or peripheral inputs, basic rhythmic, and coordinated locomotor movements. Recent evidence suggests, in turn, that plasticity changes of some CPG elements may contribute to the development of specific pathophysiological conditions associated with impaired locomotion or spontaneous locomotor-like movements. This article constitutes a comprehensive review summarizing key findings on the CPG as well as on its potential role in Restless Leg Syndrome, Periodic Leg Movement, and Alternating Leg Muscle Activation. Special attention will be paid to the role of the CPG in a recently identified, and uniquely different neurological disorder, called the Uner Tan Syndrome.

Differential effects of 5-HT1 and 5-HT2 receptor agonists on hindlimb movements in paraplegic mice

The effects induced by serotonergic (5-HT) agonists of the 5-HT1 and 5-HT2 subclasses were examined on hindlimb movement generation in adult mice completely spinal cord transected at the low thoracic level. One week postspinalization, intraperitoneal injection (0.5-10 mg/kg) of meta-chlorophenylpiperazine (m-CPP; 5-HT(2B/2C) agonist) or trifluoromethylpiperazine (TFMPP; 5-HT(1B) agonist) failed to induce locomotor-like movements. However, dose-dependent nonlocomotor movements were induced in air-stepping condition or on a motor-driven treadmill. In contrast, hindlimb locomotor-like movements were found after the injection of quipazine (5-HT(2A/2C) agonist; 1-2 mg/kg). Combined with L-DOPA (50 mg/kg, i.p.), low doses of quipazine but not of m-CPP and TFMPP produced locomotor-like and nonlocomotor movements in air-stepping condition or on the treadmill. Subsequent administration of m-CPP or TFMPP significantly reduced and often completely abolished the hindlimb movements induced by quipazine and L-DOPA. Altogether, these results demonstrate that 5-HT(2A/2C) receptor agonists promote locomotion while 5-HT(1B) and 5-HT(2B/2C) receptor agonists interfere with locomotor genesis in the hindlimbs of complete paraplegic mice. These results suggest that only subsets of spinal 5-HT receptors are specific to locomotor rhythmogenesis and should be activated to successfully induce stepping movements after spinal cord injury.

Role of spinal 5-HT2 receptor subtypes in quipazine-induced hindlimb movements after a low-thoracic spinal cord transection

A role of serotonin receptors (5‐HTRs) in spinal rhythmogenesis has been proposed several years ago based mainly upon data showing that bath‐applied 5‐HT could elicit locomotor‐like rhythms in in vitro isolated spinal cord preparations. Such a role was partially confirmed in vivo after revealing that systemically administered 5‐HTR2 agonists, such as quipazine, could induce some locomotor‐like movements (LM) in completely spinal cord‐transected (Tx) rodents. However, given the limited binding selectivity of currently available 5‐HTR2 agonists, it has remained difficult to determine clearly if one receptor subtype is specifically associated with LM induction. In situ hybridization, data using tissues from L1–L2 spinal cord segments, where critical locomotor network elements have been identified in mice, revealed greater 5‐HTR2A mRNA levels in low‐thoracic Tx than non‐Tx animals. This expression level remained elevated for several days, specifically in the lateral intermediate zone, where peak values were detected at 1 week post‐Tx and returned to normal at 3 weeks post‐Tx. Behavioral and kinematic analyses revealed quipazine‐induced LM in 1‐week Tx mice either non‐pretreated or pretreated with selective 5‐HTR2B and/or 5‐HTR2C antagonists. In contrast, LM completely failed to be induced by quipazine in animals pretreated with selective 5‐HTR2A antagonists. Altogether, these results provide strong evidence suggesting that 5‐HTR2A are specifically associated with spinal locomotor network activation and LM generation induced by quipazine in Tx animals. These findings may contribute to design drug treatments aimed at promoting locomotor function recovery in chronic spinal cord‐injured patients.

Synergistic activation of the central pattern generator for locomotion by l-beta-3,4-dihydroxyphenylalanine and quipazine in adult paraplegic mice

L-beta-3,4-Dihydroxyphenylalanine (L-DOPA) and quipazine, respectively dopamine/noradrenaline precursor and serotonergic (5-HT(2)) receptor agonist, were injected intraperitoneally in low-thoracic spinal mice at 7 days post-spinalization. In mice pre-treated with decarboxylase and monoamine oxydase inhibitors, L-DOPA (30-100 mg/kg) was found not to induce air-stepping. On the other hand, L-DOPA (40 mg/kg) consistently triggered locomotor-like movements if combined with low doses of quipazine (0.4-0.7 mg/kg) or if mice were placed on a motor-driven treadmill running at low speed. However, twitches, spasms, and other non-locomotor movements were also induced, especially on the treadmill. These results suggest that (1) spinal catecholaminergic and serotonergic receptors interact synergistically to generate locomotor-like movements in chronic spinal mice, and that (2) hindlimb afferent inputs associated with the treadmill conditions contribute to the genesis of locomotor-like and non-locomotor movements induced by these drugs.

Synergistic Effects of D1/5 and 5-HT1A/7 Receptor Agonists on Locomotor Movement Induction in Complete Spinal Cord–Transected Mice

Monoamines are well known to modulate locomotion in several vertebrate species. Coapplication of dopamine (DA) and serotonin (5-HT) has also been shown to potently induce fictive locomotor rhythms in isolated spinal cord preparations. However, a synergistic contribution of these monoamines to locomotor rhythmogenesis in vivo has never been examined. Here, we characterized the effects induced by selective DA and 5-HT receptor agonists on hindlimb movement induction in completely spinal cord transected (adult) mice. Administration of the lowest effective doses of SKF-81297 (D 1/5 agonist, 1-2 mg/kg, ip) or 8-OH-DPAT (5-HT 1A/7 agonist, 0.5 mg/kg, ip) acutely elicited some locomotor-like movements (LM) (5.85 +/- 1.22 and 3.67 +/- 1.44 LM/min, respectively). Coadministration of the same doses of SKF-81297 and 8-OH-DPAT led to a significant increase (7- to 10-fold) of LM (37.70 +/- 5.01 LM/min). Weight-bearing and plantar foot placement capabilities were also found with the combination treatment only (i.e., with no assistance or other forms of stimulation). These results clearly show that D 1/5 and 5-HT 1A/7 receptor agonists can synergistically activate spinal locomotor networks and thus generate powerful basic stepping movements in complete paraplegic animals. Although previous work from this laboratory has reported the partial rhythmogenic potential of monoamines in vivo, the present study shows that drug combinations such as SKF-81297 and 8-OH-DPAT can elicit weight-bearing stepping.

Specific role of dopamine D1 receptors in spinal network activation and rhythmic movement induction in vertebrates

Dopamine (DA) is well‐recognized for its determinant role in the modulation of various brain functions. DA was also found in in vitro isolated invertebrate preparations to activate per se the central pattern generator for locomotion. However, it is less clear whether such a role as an activator of central neural circuitries exists in vertebrate species. Here, we studied in vivo the effects induced by selective DA receptor agonists and antagonists on hindlimb movement generation in mice completely spinal cord‐transected (Tx) at the low‐thoracic level (Th9/10). Administration of D1/D5 receptor agonists (0.5–2.5 mg kg−1, i.p.) was found to acutely elicit rhythmic locomotor‐like movements (LMs) and non‐locomotor movements (NLMs) in untrained and non‐sensory stimulated animals. Comparable effects were found in mice lacking the D5 receptor (D5KO) whereas D1/D5 receptor antagonist‐pretreated animals (wild‐type or D5KO) failed to display D1/D5 agonist‐induced LMs. In contrast, administration of broad spectrum or selective D2, D3 or D4 agonists consistently failed to elicit significant hindlimb movements. Overall, the results clearly show in mice the existence of a role for D1 receptors in spinal network activation and corresponding rhythmic movement generation.

Spontaneous recovery of hindlimb movement in completely spinal cord transected mice: a comparison of assessment methods and conditions

Study design:To compare results obtained with a variety of locomotor rating scales in Th9/10 spinal cord transected (Tx) mice.Objectives:To assess spontaneous recovery with a variety of rating scales to find the most sensitive methods for assessing recovery levels in Tx mice and differences associated with gender and condition.Setting:Laval University Medical Center, Neuroscience Unit & Laval University, Department of Anatomy and Physiology, Quebec City, Quebec, Canada.Methods:Scales including the Basso, Beattie and Bresnahan (BBB), the Basso Mouse Score (BMS), the Antri, Orsal and Barthe (AOB), the Motor Function Score (MFS) and the Averaged Combined Score (ACOS) were used to assess, in open-field and treadmill conditions, spontaneous locomotor recovery in male and female Tx mice.Results:The ACOS scale revealed a progressive increase of spontaneous recovery during 5-weeks post-Tx. The other methods detected a progressive increase for the first 2–3 weeks post-Tx without any significant progress in weeks 4 and 5. Generally, scores obtained with each method were nonsignificantly different between males and females or between open-field and treadmill conditions.Conclusion:These results further confirm the existence of a limited but significant increase of locomotor function recovery, occurring without intervention, in Tx animals. Although each method could detect small levels of recovery, the ACOS method was discriminative enough to detect progressive changes up to 5 weeks post-Tx. In conclusion, the ACOS rating scale was the most discriminative method for assessing the spontaneous return of hindlimb movements found in Tx mice, both in open-field and treadmill conditions.

Body weight, limb size, and muscular properties of early paraplegic mice

Patients with spinal cord injury (SCI) typically experience body weight loss, motor function deficits, and a general decline of physical fitness. Animal models with these characteristics can serve to study the detailed adaptive changes following SCI. In the present study, we report the use of an adult paraplegic mouse model to study SCI-induced changes. We characterized the early effects of complete thoracic spinal cord transection on (1) whole body weight, (2) forelimb and hindlimb weight and volume, and (3) contractile properties of hindlimb extensor muscle. Drastic changes were found at 7 days post-spinal cord transection. These included a 24% loss in whole body weight accompanied by a large decrease of weight and volume in the forelimbs and the hindlimbs. We also observed in the soleus muscle, a 32% decrease in mass and maximal tetanic tension (Po) as well as a 21% and 48% increase in time-to-peak tension (TPT) and half-relaxation time (1/2 RT) respectively. After 28 days, all of the changes remained, except for 1/2 RT and TPT which nearly returned to control levels. Altogether, the results reveal that large changes in body weight, limb size and musculoskeletal properties occur within only one week after complete spinal cord transection. The use of paraplegic mouse models may provide new therapeutic approaches to restore motor and locomotor functions after SCI.