NADPH diaphorase expression in superior colliculus of developing, aging and visually deafferented rats.

We have studied the development of NADPH-diaphorase activity in the retinorecipient layers of the superior colliculus (SC) in rats from embryonic day 17 to adulthood, during aging, and following neonatal tetrodotoxin injection or unilateral eye removal in the neonatal or in the adult animal. In the superficial SC, NADPH-d activity is first seen in neurons on postnatal day (P) 4; over the next two weeks, enzyme expression increases gradually, in cells as well as in the neuropil. By P12-14, around the time of eye opening, NADPH-d reactivity increases dramatically. In parallel, the dendrites of many NADPH-d-positive neurons in the superficial gray layer, more or less randomly distributed at first, gradually align their orientation relative to the dorsoventral axis. The pattern of NADPH-d activity in the superficial layers of the SC (i.e. stratum griseum superficiale and stratum opticum) is adult-like by the fourth week of age. Deafferentation of the superficial SC, both in the neonatal and adult rat, and block of retinal activity lead to reduction in the size of the SC and changes in NADPH-d-positive neurons, including dendrite misorientation, decreased cell size and reduced number. Some of these changes are seen also in the aging animal. These results document a protracted and progressive increase in the development of NADPH-d expression in the SC. Our results suggest a strong influence of retinal afferents and activity on the development and maintenance of NAPHD-positive neurons in the retinorecipient layers of the SC, where NO can act as a retrograde signal to carve the terminal arbors of retinal axons.

Ablation of de novo DNA methyltransferase Dnmt3a in the nervous system leads to neuromuscular defects and shortened lifespan.

DNA methylation is an epigenetic mechanism involved in gene regulation and implicated in the functioning of the nervous system. The de novo DNA methyltransferase Dnmt3a is expressed in neurons, but its specific role has not been clarified. Dnmt3a is activated around embryonic day 10.5 in mouse neuronal precursor cells and remains active in postmitotic neurons in the adult. We assessed the role of neuronal Dnmt3a by conditional gene targeting. Mice lacking functional Dnmt3a in the nervous system were born healthy, but degenerated in adulthood and died prematurely. Mutant mice were hypoactive, walked abnormally, and underperformed on tests of neuromuscular function and motor coordination. Loss of Dnmt3a also led to fewer motor neurons in the hypoglossal nucleus and more fragmented endplates in neuromuscular junctions of the diaphragm muscle. Our results implicate a role for Dnmt3a in the neuromuscular control of motor movement.

Diverse dependencies of developing Merkel innervation on the trkA and both full-length and truncated isoforms of trkC.

This study demonstrates that innervation dependent on two different neurotrophin tyrosine kinase (trk) receptors can form the same types of sensory endings (Merkel endings) in the same target (Merkel cells of vibrissa follicles). Some endings transiently express trkA during their initial development, whereas others express trkC throughout their development. Consequently, elimination of kinase domains of either trkA or trkC each result in a partial loss of Merkel endings, whereas absence of kinase domains of both receptors results in a total loss. At the onset of Merkel ending development, at least one kinase-lacking trkC isoform is transiently expressed on all the follicle cells, while neurotrophin 3 is transiently expressed only in the cells at the middle third of the follicle where the Merkel endings and cells develop. This transient non-neuronal expression of truncated trkC is essential for development of any Merkel endings, whereas some Merkel endings and cells still begin to develop in the absence of neurotrophin 3. Therefore, truncated trkC plays a more important role in the development of this innervation than kinase forms of trkA or trkC or of NT3, the only known ligand for trkC receptors.