Development in neonatal rats of the sensory resetting of the locomotor rhythm induced by NMDA and 5-HT

 Developmental changes in the effects of quadriceps (Q) nerve stimulation on the locomotor rhythm induced by a mixture of N-methyl-d-aspartic acid and 5-hydroxytryptamine were examined using in vitro preparations from neonatal rats at postnatal days (P) 1–6. The effects of such stimulation on the rhythm were dependent both on stimulus strength and on the age of the animal. Low-intensity stimulation (≤3.0×T, where T=threshold for the monosynaptic reflex) during the flexor phase reset the rhythm via a prolongation of the flexor burst in most rats at P1–3, but via flexor burst truncation at P4–6. At any age, low-intensity stimulation during the extensor phase had no consistent effect on the ongoing rhythm. Activation of muscle afferents evoked via isometric contraction of the Q muscle caused effects similar to those obtained on low-intensity electrical stimulation in all age groups. In all age groups, high-intensity stimulation (≥5.0×T) caused resetting when delivered during the flexor phase via a prolongation of the flexor burst and during the extensor phase via a truncation of the extensor burst. These results suggest that the type of resetting evoked from low-threshold muscle afferents changes drastically during postnatal week1, while effects evoked from high-threshold afferents remain unchanged.