Prenatal development of cerebrorubral and cerebellorubral projections in cats

Development of functional topography in the corticorubral projection: An in vivo assessment using synaptic potentials recorded from fetal and newborn cats

In mammals, topographic maps emerge from initially diffuse projections during development. To gain insight into the mechanisms governing the transition from a diffuse projection to a topographic map, we studied topographic specificity of functional connections during development, using the cat corticorubral system as a model. In the adult cat, rubrospinal neurons in the dorsomedial part of the red nucleus (RN) receive input primarily from the forelimb area of the sensorimotor cortex, whereas those in the ventrolateral part receive input primarily from the hindlimb area. During development, axons from the sensorimotor cortex arrive in the RN at embryonic day 50 (E50) (Song et al., 1995a) and are diffusely distributed in the RN until postnatal day 13 (P13) (Higashi et al., 1990). Here, we studied the development of the pattern of functional cortical inputs to individual rubrospinal neurons, using synaptic potentials recorded in vivo. The functional topography in each rubrospinal neuron in developing cats was examined and classified either as adult-like or nonadult-like by comparison with the adult pattern. In preterm kittens from E61 to E65, only about half of the recorded neurons (41%; n = 22) showed adult-like functional topography. This percentage, however, increased to 82% (n = 56) in P1-P8 kittens and to 93% (n = 42) in P13-P28 kittens. These results, in conjunction with the above mentioned anatomical observations, suggest that corticorubral axons make functional synapses nonselectively with rubrospinal neurons before birth. Furthermore, the functional topographic map developed earlier than the anatomical map (<P8 vs >P13), suggesting that there is a developmental step of selective promotion of synapse formation and/or selective enhancement of synaptic efficacy in topographically appropriate regions in the RN, before the emergence of the mature anatomical map.

Preferential termination of corticorubral axons on spine-like dendritic protrusions in developing cat

The formation of synaptic contacts is a crucial event during neural development and is thought to be achieved by complex interactions between incoming axons and the neurons in the target. We have focused on spine-like dendritic protrusions (SLDPs), which are transient pleomorphic protrusive structures seen in developing brains. Although the functional significance of SLDPs remains unknown, accumulating in vitro evidence suggests that the SLDP plays an important role in synaptogenetic interactions with axons. As a test of this idea, the present study was performed to examine whether the SLDPs are the preferential sites of synapse formation in vivo. The ultrastructure of biocytin-labeled corticorubral (CR) terminals was examined in serial thin sections during the period of synaptogenesis in newborn cats. We found that a major proportion (86%) of the CR synapses was formed on SLDPs. The presynaptic terminals were often invaginated by fine processes extending from the tips of SLDPs. Synaptic structures presumably of cortical origin were also found on SLDPs of HRP-labeled rubrospinal cells, suggesting that SLDPs postsynaptic to labeled CR terminals originate at least in part from rubrospinal cells. Taken together, these results indicate that SLDPs may represent preferred sites of synapse formation and support the notion that SLDPs play a role in synaptogenic interactions during brain development.

Innervation of the caudate nucleus, thalamus and red nucleus by the remaining sensorimotor cortex in cats with fetal or neonatal unilateral frontal cortex removal

We studied the projections to the caudate nuclei, thalami and red nuclei from the remaining sensorimotor cortex in adult cats that had sustained a unilateral frontal cortex resection prenatally or neonatally. Four cats had the lesion at age E 50-55 and six animals sustained the ablation at age P 8-14 (seven cats were intact controls). All cats grew to young adulthood and then received injections of tritiated leucine-proline in the remaining sensorimotor cortex. Injection sites and axon terminal fields were reconstructed using autoradiography-processed tissue. In all cats the label filled a similar extent of the right pericruciate cortex. Terminal field densities in the subcortical nuclei were estimated using computer-based video software. Three medial-lateral sectors at five coronal levels were examined in the caudate nucleus. Three nuclear groups were analyzed in the thalamus (intralaminary, ventralis lateralis and ventrobasal complex). For the red nucleus, the four quadrants were examined at four coronal levels. The main goal of the study was to assess possible changes in the cortical innervation of the nuclei ipsilateral to the lesion. Therefore, the mean particle counts per nucleus (and per area or sector of nuclei) and per animal group were used to calculate percentage values for the decussated (crossed, or contralateral to the injection site) as a function of the non-decussated (uncrossed, or ipsilateral to the injection site) innervation. The percentage values for the crossed projections were: (a) for the entire caudate nucleus, 61.3% for the intact. 56.7% for the fetal-lesioned and 42.7% for the neonatal-lesioned cats, with no statistical differences between groups; (b) for the thalamus the proportion of crossed projections was minimal fluctuating between a low 0.06-0.16% for the nucleus ventralis lateralis and a high of 2.01-3.46% for the intralaminary nuclei, with the highest values belonging to the lesioned groups but with no significant differences between groups: (c) for the entire red nucleus, 1.98%, 12.74% (P < 0.05) and 6.76% for the intact, fetal- and neonatal-lesioned cats respectively. In the lesioned cats, the topography of the distribution of the axon terminals was bilaterally the same as in the controls. In conclusion, only the red nucleus of the frontal-lesioned cats showed an increased crossed innervation from the remaining sensorimotor cortex but this was relatively weak and statistically significant only for the fetal-lesioned animals. These results as well as the literature suggest that: (a) the crossed corticorubral projections in fetal cats may represent true reinnervation (i.e., newly originated, no preexisting terminals); (b) the relative paucity of the crossed projections in the present cats as compared to the extensive reorganization of subcortical terminals seen after cerebral hemispherectomy (our original postnatal lesion model) may be due to the much smaller size of the present cortical lesion which presumably induced only a limited amount of subcortical nuclear deafferentation.