Peptidergic neuromodulation of the lumbar locomotor network in the neonatal rat spinal cord
Introduction
For many years, it has been known from simple nervous systems that central pattern generators (CPG) are the targets for numerous modulatory inputs (for review see [32]). However, it is only recently that comparable conclusions have been drawn from mammals in which several neurotransmitters have been shown to initiate and/or modulate the CPGs for locomotion ([1], [3], [6], [7], [11], [12], [13], [23], [24], for reviews see [41], [44]). In past years, these approaches have been largely stimulated by research aimed at identifying neuroactive substances that are able to activate motor networks following spinal cord injury (for reviews see [2], [42]).
Neuropeptides are known to act as hormones, neurotransmitters or trophic factors. Morphologic studies have revealed, in rats, that many peptides and peptide receptors are present in both the dorsal and ventral horn of the cord (for example see [19], [30], [31]). To date, most of the studies that have explored the role of peptides in the physiology of the spinal cord have focused on the control of sensory pathways [50] and autonomic functions (for examples see [43], [46]). In contrast, few studies are available regarding their role in modulation of spinal motor circuits. Substance P has been the only peptide tested on respiratory [35] as well as locomotor network activity in the rat [4]. More recently, it has been shown in the neonatal mouse spinal cord that vasopressin (AVP) and oxytocin could facilitate the expression of the motor pattern either independently or in combination with serotonin [40].
The present study was undertaken to test the effects of neuropeptides on fictive locomotor activity recorded in an isolated spinal cord of newborn rat. The neuropeptides tested here were chosen according to different criteria. All these peptides, or close analogs, and their related receptors have been detected in all ventral motor areas of the spinal cord (see Section 4). Furthermore, some of the peptides described in the present study were previously shown to control motoneuron excitability [27], [28], [37], [38], [49] or to modulate rhythmic motor networks in other preparations [5], [16], [32]. According to Hökfelt [20], the peptides belong to different peptide subfamilies and include: vasopressin and oxytocin (hypothalamic hormones), somatostatin and thyrotropin releasing hormone (hypothalamic releasing and inhibiting hormone family), methionine–enkephalin (opioid peptides), neurotensin (neurotensin family), bombesin (bombesin-like family), bradykinin (kinin peptides), angiotensine II (angiotensin peptides), FMRFamide (RFamide peptides) and proctolin (proctolin-like peptides). The aim of this study is to see whether peptides from different subfamilies could differentially modulate the spinal motor networks. We have observed (1) the ability of peptides to trigger a motor activity by themselves, (2) the potency of peptides in producing locomotor activity in the presence of subthreshold concentrations of the excitatory amino acid N-methyl-d,l-aspartate (NMA), and (3) finally, modulation of ongoing fictive locomotion during NMA and serotonin (5HT) perfusion.
Section snippets
Dissection and pharmacologic procedures
Experiments were performed on isolated spinal cord preparations (n = 232) obtained from newborn Wistar rats (P0–P4) bred in our laboratory. All experiments were carried out in accordance with the guidelines of our Institutional Animal Care and Use Committee. Prior to decapitation, the animals were anesthetized with ether. The spinal cord was exposed by a dorsal laminectomy, isolated, transferred to a recording chamber and continuously superfused with a 95% O2/5% CO2 oxygenated saline solution
Peptidergic activation of lumbar neurons
In the first part of this study, we examined the capability of 11 neuropeptides to directly stimulate the lumbar motor networks. Each peptide was bath-applied alone onto the lumbo-sacral segments of the spinal cord. The effects of the different peptides were systematically screened at concentrations ranging from 10−8 to 10−6 M. Angiotensin II (n = 13 experiments), bradykinin (n = 18), FMRFa (n = 8), met-enkephalin (n = 8), neurotensin (n = 7), proctolin (n = 7) and somatostatin (n = 5) failed to induce any
Discussion
In the present study, we showed that different peptide families can neuromodulate the motor networks of the lumbar spinal cord and that peptides from the same family could present different neuromodulatory profiles and potency. These data extend some previous reports that studied the actions of substance P, AVP and oxytocin on the lumbar locomotor network [4], [40]. In the neonatal rat spinal cord preparation, substance P has been shown to trigger a slow bursting activity that was recorded from
Acknowledgements
The authors wish to thank Dr. Jammes Einum and Dr. John Simmers for correcting the English. This work was funded in part by the Fondation pour la Recherche Médicale (INE2000407008; FDT20030627270; CIC20030926057), by the Institut International de Recherche en Paraplégie, Genève (IRP P58/01). Grégory Barrière is a fellowship of the Institut pour la Recherche sur la Moelle Epinière (IRME).
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