In our experiments, spinal interneurons send axons through dorsal roots. We localized most of these interneurons close to the intermediate nucleus. They have several shapes that differ from motoneurons.
In our study, we did not study dendritic arborizations nor their changes with age, as assessed in previous studies. In previous studies, Westerga & Gramsbergen observed a considerable increase in motoneuron soma size in rats, but with different distribution and arborizations patterns in a developing stage, which are longer and more extensive at first in cervical than in the lumbar region (10). This temporal and spatial differences may influence the motor development in a rostrocaudal manner (11). Dendrite bundles appeared relatively late in the Soleus’ motoneuron compared to the Tibial anterior; this is related to the fine-tuning of neuronal activity, rather than patterning of motor activity (10). These observations will be studied in neonatal mice.
Developing serotoninergic motoneuron innervation is related to the postnatal development of motor function already recognized in the second postnatal week (11). In our study, we found a significant neuronal soma size increase at a similar postnatal age. Marked neurons are not of the same type or from a specific neuron group. That could be related to a different organization of the activation pattern.
We found some cells traveling in the spinal cord dorsal surface. We did not know if these cells are neurons or glia. In a developmental study of kittens, the volume of the lateral cervical nucleus and the glial cells increased sixfold during 120-day observation, as did both the volumes of myelinated axons (12).
As we noticed cells traveling in rafts in the dorsal horn surface of the spinal cord in the mouse spinal cord, further immunohistological studies could reveal the type of cells and clarify if some of them are progenitor neurons (13–15).
We cannot confirm whether the recorded interneurons produce activity (action potentials) traveling antidromically in dorsal roots. However, we found antidromic activity in dorsal roots, even in bicuculline, AP5, and low calcium. In another study, 2–4 postnatal day mice presented depression curves unexplained by presynaptic activation failure (suppressed by AP5). Low calcium concentration reduced average amplitude and depression, and a higher calcium concentration increased average amplitude and depression. Increasing the bath temperature from 24 to 32 Celsius produced little change in amplitudes, but the depression was noticeably reduced at most frequencies (16). Therefore, these AP could be generated by these interneurons when their axons are sufficiently depolarized.
5HT, DA, and NA produced no change in the compound antidromic potentials evoked by intraspinal microstimulation, indicating that DRP depression is unrelated to direct changes in the excitability of intraspinal afferent fibers (17). Thus, antidromic activity could have an origin other than PAD, and consequently, other functions. Ephaptic interaction in afferent fibers could also produce antidromic firing (18).
Antidromic spike function in dorsal roots could participate in regulating activity in the afferent inflow of information related to inflammation and pain. DRR in afferent fiber raises the hypothesis that mediated antidromic activity contributes to neurogenic inflammation (19). Sectioning the sciatic nerve of neonatal rat's triggers growth of afferent fiber in VR, and stimulation in the L5 spinal cord evoked long latency antidromic potentials in the L5 ventral root. However, in normal rats, such potentials rarely appeared (20). Several experimental conditions, such as axotomy of sensory afferents, produced ectopic antidromic activity in their respective DRG, due to branched sensory afferents fiber (3).
In our experiments, the antidromic activity in DR, even in low calcium concentration, is indicative of axons in dorsal roots. We cannot assert their functional significance or action in the neonatal mouse. It would be essential to find out whether these antidromic potentials in dorsal afferent fibers are favoring some spinal circuit formation which remain in adulthood or are only part of a development process.
Sympathetic preganglionic neurons (PGNs) in the neonatal rat's isolated spinal cord could be synaptically activated either by the dorsal root or spinal pathway stimulation. Dorsal root projections already appeared mature in the neonatal rat, and primary afferents did not appear to project directly to PGNs (21).