Distribution of the neuronal gap junction protein Connexin36 in the spinal cord enlargements of developing and adult opossums, Monodelphis domestica.

We use opossums Monodelphis domestica to study the development of mammalian motor systems. The immature forelimbs of the newborn perform rhythmic and alternating movements that are likely under spinal control. The hindlimbs start moving in the second week. Chemical synapses are scant in the spinal enlargements of neonatal opossums and the presence of electrochemical synapses has not been evaluated in this species or in other marsupials. As a first step aiming at evaluating the existence of such synapses in the neonatal spinal cord, we have investigated the presence of the exclusively neuronal gap junction protein connexin36 (Cx36) by immunohistochemistry in light microscopy. At birth, Cx36 immunoreactivity is moderate in the presumptive gray matter in both enlargements. Thereafter, it decreases gradually, except in the superficial dorsal horn where it increases to a plateau between P10 and P20. Cx36 labeling is detected in the presumptive white matter at birth, but then decreases except in the dorsal part of the lateral funiculus, where it is dense between P10 and P20. Cx36 has become virtually undetectable by P52. The presence of Cx36 in the spinal enlargements of postnatal opossums suggests that neurons might be linked by gap junctions at a time when chemical synapses are only beginning to form. The greater abundance of Cx36 observed transiently in the superficial dorsal horn suggests a stronger involvement of this protein in spinal sensory systems than in direct motor control of the limbs.