Properties of Rhythmic Activity Generated by the Isolated Spinal Cord of the Neonatal Mouse

Abstract

We examined the ability of the isolated lumbosacral spinal cord of the neonatal mouse (P0-7) to generate rhythmic motor activity under several different conditions. In the absence of electrical or pharmacological stimulation, we recorded several patterns of spontaneous ventral root depolarization and discharge. Spontaneous, alternating discharge between contralateral ventral roots could occur two to three times over a 10-min interval. We also observed other patterns, including left-right synchrony and rhythmic activity restricted to one side of the cord. Trains of stimuli delivered to the lumbar/coccygeal dorsal roots or the sural nerve reliably evoked episodes of rhythmic activity. During these evoked episodes, rhythmic ventral root discharges could occur on one side of the cord or could alternate from side to side. Bath application of a combination of N-methyl-D,L-aspartate (NMA), serotonin, and dopamine produced rhythmic activity that could last for several hours. Under these conditions, the discharge recorded from the left and right L(1)-L(3) ventral roots alternated. In the L(4)-L(5) segments, the discharge had two peaks in each cycle, coincident with discharge of the ipsilateral and contralateral L(1)-L(3) roots. The L(6) ventral root discharge alternated with that recorded from the ipsilateral L(1)-L(3) roots. We established that the drug-induced rhythm was locomotor-like by recording an alternating pattern of discharge between ipsilateral flexor and extensor hindlimb muscle nerves. In addition, by recording simultaneously from ventral roots and muscle nerves, we established that ankle flexor discharge was in phase with ipsilateral L(1)/L(2) ventral root discharge, while extensor discharge was in phase with ipsilateral L(6) ventral root discharge. Rhythmic patterns of ventral root discharge were preserved following mid-sagittal section of the spinal cord, demonstrating that reciprocal inhibitory connections between the left and right sides of the cord are not essential for rhythmogenesis in the neonatal mouse cord. Blocking N-methyl-D-aspartate receptors, in both the intact and the hemisected preparation, revealed that these receptors contribute to but are not essential for rhythmogenesis. In contrast, the rhythm was abolished following blockade of kainate/AMPA receptors with 6-cyano-7-nitroquinoxalene-2,3-dione. These findings demonstrate that the isolated mouse spinal cord can produce a variety of coordinated activities, including locomotor-like activity. The ability to study these behaviors under a variety of different conditions offers promise for future studies of rhythmogenic mechanisms in this preparation.

Role of NMDA receptor activation in serotonin agonist-induced air-stepping in paraplegic mice

“…Indeed, although NMDA can trigger rhythmic ventral root activity in in vitro preparations, stable and well-coordinated locomotorlike patterns are known to be best induced by combining NMDA and 5-HT in the neonatal rat 8,9,19 and mice preparations. 10,11,20 5-HT 2 agonists have also been shown to be involved in pattern generation since reversal of disorganized NMDA-induced rhythms was found in in vitro preparations obtained from early spinal neonatal rats. 3 Furthermore, appearance of NMDAinduced fictive locomotion is delayed in MAOAdeficient mice, suggesting an important role for 5-HT in spinal network maturation and pattern formation.…”

“…26 These results are in apparent contrast with those observed in the neonatal mouse-isolated spinal cord showing that AP5 or MK-801 do not abolish but only reduce fictive locomotion induced by dopamine and 5-HT. 10 However, nearly complete blocking effects of MK-801 have been reported on slow locomotor rhythms induced by bicuculline. 12 In vivo, MK-801 and AP5 can interfere with L-DOPA-induced fictive locomotion in curarized spinal rabbits 27 and block MLR-induced fictive activity in decerebrate curarized cats.…”

“…7 However, stable and wellorganized fictive locomotor rhythms are best induced when NMDA and 5-HT receptor agonists are combined at least in in vitro preparations obtained from neonatal rats 3,8,9 and mice. [10][11][12] Based on results reported in vitro and because no studies have tested this combination of drugs in vivo, the aim of the present study was to examine whether NMDA and 5-HT receptors synergistically interact also in vivo to generate basic spinal stepping movements in awake behaving paraplegic mice. Part of this work has been published in abstract form.…”

Interaction Between Developing Spinal Locomotor Networks in the Neonatal Mouse

“…To address this issue, we build on data that examined the rhythmogenic capability of thoracosacral segments of the mouse spinal cord (Whelan et al 2000). In the present work, we demonstrate that brain stem stimulation can reliably elicit coordinated cervical-lumbar locomotor-like patterns.…”

“…These networks are termed central pattern generators (CPGs). Studies from diverse species including turtle, cat, and rodent show that these networks can produce a variety of rhythmic outputs (Cowley and Schmidt 1994;Mortin and Stein 1989;Pearson and Rossignol 1991;Whelan et al 2000). In the case of mammals, most studies have concentrated on hindlimb CPG function.…”