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1 Anatomy & Cell Biology, The Hebrew University Medical School, Jerusalem, Israel
* To whom correspondence should be addressed. E-mail: aharony{at}md.huji.ac.il.
Noradrenaline, a potent activator of rhythmogenic networks in adult mammals has not been reported to produce functional rhythmic patterns in isolated spinal cords of newborn rats. We now show that a "fast" (cycle time 1-4s) transient rhythm was induced in sacrococcygeal (SC) and rostral-lumbar spinal segments of the neonatal rat by bath-applied noradrenaline. The "fast" rhythm was blocked by 1µM of the 
1-adrenoceptor antagonist prazosin but not by 1-20µM of the 2-adrenoceptor blocker yohimbine, it could be initiated and maintained by 1-adrenoceptor agonists, and was accompanied by a slow non-locomotor rhythm. Transection at the lumbosacral junction abolished the "fast"-thoracolumbar (TL) rhythm while the "fast"-SC and slow-TL rhythms were unaffected. The NMDA-receptor antagonist AP5 abolished the slow- and did not interrupt the "fast"-rhythm. Thus, 1-adrenoceptor agonists induce an NMDA receptor-independent rhythm in the SC cord, and modulate NMDA receptor-dependent rhythmicity in TL segments. Injection of current steps into S2 and flexor-dominated L2 motoneurons during the "fast" rhythm revealed a 20-30% decrease in input-resistance (RN)), coinciding with contralateral bursting. The RN of extensor-dominated L5 motoneurons did not vary with the "fast" rhythm. The rhythmic fluctuations of RN in L2 motoneurons were abolished, but the alternating left-right pattern of the "fast" rhythm was unchanged in midsagittally-split TL cords. We suggest that the locomotor generators were not activated during the "fast" rhythm, that crossed-inhibitory pathways activated by SC projections controlled the rhythmic decrease in RN in L2 motoneurons, and that the alternating pattern of the split TL cord was maintained by excitatory SC projections.
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