[Age-related changes in sensory neurons containing calcitonin gene-related peptide under conditions of afferentation deficit in rats].

Morphological features of calcitonin gene-related peptide (CGRP)-immunoreactive neurons were studied in the sensory ganglia of the vagus and thoracic nerves in 3-, 10-, 20-, 30-, 60-, 90-, and 120-day-old rats under conditions of chemically-induced deafferentation. We found that, in rats, CGRP-containing neurons appeared in both ganglia immediately after they were born and their number decreased with aging. Most of CGRP-immunoreactive neurons were small in size, i.e., up to 600 microm2. Administration of capsaicin modified age-related changes in the number of CGRP-immunopositive neurons. In the thoracic nerve ganglion, the mean square of these cells and their number substantially decreased, whereas, in the vagus nerve ganglion, positive cells were not observed.

Regenerating motor bridge axons refine connections and synapse on lumbar motoneurons to bypass chronic spinal cord injury.

To restore motor control after spinal cord injury requires reconnecting the brain with spinal motor circuits below the lesion. A bridge around the injury is an important alternative to promoting axon regeneration through the injury. Previously, we reported a novel motor bridge in rats. The thirteenth thoracic nerve was detached from the muscle it innervates and the cut end implanted caudally into the lumbar gray matter where motor bridge axons regenerate. In this study, we first determined that regenerating bridge axons project to spinal motor circuits. Stable projections were present in ventral motor laminae of the cord, including putative synapses directly on motoneurons, 2 months after insertion in the intact cord. At this time, earlier-forming dorsal horn projections were mostly eliminated. Regenerating axons were effective in evoking leg motor activity as early as 2 weeks. We next determined that bridge axons could regenerate caudal to a chronic injury. We hemisected the spinal cord at L2 and inserted the bridge nerve 1 month later at L5 and found ventral laminae projections similar to those in intact animals, including onto motoneurons directly. Finally, we determined that the bridge circuit could be activated by neural pathways rostral to its origin. For spinally hemisected animals, we electrically stimulated the rostral spinal cord and recorded evoked potentials from the bridge and, in turn, motor responses in the sciatic nerve. Our findings suggests that bridge motoneurons could be used by descending motor pathways as premotor interneurons to transmit neural signals to bypass a chronic spinal injury.

The critical period for peripheral specification of dorsal root ganglion neurons is related to the period of sensory neurogenesis.

Thoracic sensory neurons in bullfrog tadpoles can be induced to form connections typical of brachial sensory neurons by transplanting thoracic ganglia to the branchial level at stages when some thoracic sensory neurons already have formed connections. In order to find out how many postmitotic sensory neurons survive transplantation, [3H]thymidine was administered to tadpoles in which thoracic ganglia were transplanted to the brachial level unilaterally at stages VII to IX. Between 16 and 37% of the neurons in transplanted ganglia were unlabeled, as compared to 46 to 60% in unoperated ganglia. Transplanted ganglia contained fewer unlabeled neurons than corresponding unoperated ganglia, indicating that transplantation caused degeneration of postmitotic neurons. Therefore, a large fraction of the neurons that formed connections typical of brachial sensory neurons probably differentiated while they were at the brachial level.

An HRP study of the cat's spinal respiratory motoneurones during postnatal development.

The localization and morphology of spinal respiratory motoneurones (phrenic and intercostal) were studied in the cat by retrograde labelling using Horseradish Peroxidase (HRP), at different stages of postnatal development. At birth, the distribution of the phrenic and intercostal motoneurones in the cervical and thoracic ventral horn, respectively, is similar to that observed in adult animals. At birth, the phrenic and intercartilaginous motoneurone somata have respectively 60% and 40% of their adult volume, appearing much more developed than the motoneurones involved in the motor control of limbs. During postnatal development, the phrenic and intercartilaginous motoneurones undergo a characteristic sagittal elongation without evident modification along their transverse axes. From birth, the ratio of the somatic volume to that of its corresponding motor column markedly decreases inside of the phrenic column compared to the data obtained in the limb's muscle motor columns by other authors. Similar determinations in intercostal motor columns give intermediate values between those obtained from the phrenic column and from the motor system. These results indicate that the motoneurones innervating the respiratory muscles have some specific features of development.

Quantitative analysis of growth and myelination of cutaneous nerve fibers in the chick.

Cutaneous thoracic nerves from 16-day embryos to 6-month chickens were studied. Special attention was given to the increase in caliber of developing axons, number of lamellae in the myelin sheath and distribution of group sizes. Myelination begins at 14 days of incubation but occurs mainly after hatching: 5% of fibers are myelinated in the 3-day chickens, 20% at 16 days and 63% at 6 months. During development, a rectilinear positive correlation between the axon diameter and the number of myelin lamellae was found at all stages, except in 16-day embryos, and the slope of the calculated regression lines decreased with age, as mean diameter and sheath thickness increased. The rate of daily myelin initiation was maximum between 3 and 16 days after hatching while the speed of myelin formation decreased with development. For nonmyelinated fibers, a regular reduction of the number of fibers per Schwann cell takes place during nerve maturation.