Variations in oxidative enzyme type profiles among prenatal rat lumbar motoneurons

Retrograde influence of muscle fibers on their innervation revealed by a novel marker for slow motoneurons

Mammalian limb and trunk skeletal muscles are composed of muscle fibers that differ in contractile and molecular properties. They are commonly divided into four categories according to the myosin heavy chain that they express: I, IIA, IIX and IIB, ranging from slowest to fastest. Individual motor axons innervate tens of muscle fibers, nearly all of which are of the same type. The mechanisms accounting for this striking specificity, termed motor unit homogeneity, remain incompletely understood, in part because there have been no markers for motoneuron types. Here we show in mice that the synaptic vesicle protein SV2A is selectively localized in motor nerve terminals on slow (type I and small type IIA) muscle fibers; its close relatives, SV2B and SV2C, are present in all motor nerve terminals. SV2A is broadly expressed at birth; fast motoneurons downregulate its expression during the first postnatal week. An inducible transgene incorporating regulatory elements from the Sv2a gene permits selective labeling of slow motor units and reveals their composition. Overexpression of the transcriptional co-regulator PGC1α in muscle fibers, which converts them to a slow phenotype, leads to an increased frequency of SV2A-positive motor nerve terminals, indicating a fiber type-specific retrograde influence of muscle fibers on their innervation. This retrograde influence must be integrated with known anterograde influences in order to understand how motor units become homogeneous.

Postnatal emergence of mature release properties in terminals of rat fast- and slow-twitch muscles

Motor nerve terminals in adult mammalian slow-twitch muscles have lower levels of spontaneous and evoked neurotransmitter release than terminals in fast-twitch muscles. These reflect adaptive differences, allowing terminals in slow (postural) muscles to sustain release during the prolonged firing trains experienced in vivo. Here we ask whether these differences in terminal release properties in Sprague-Dawley rat extensor digitorum longus (EDL, fast) and soleus (slow) muscles reflect their early cytodifferentiation in the embryo or whether they might be adaptations to their distinct mature activity patterns, which emerge around two weeks postnatally. We find that the mature pattern of differences in release arise through co-ordinated increases in presynaptically dependent release properties (quantal content, spontaneous release frequency and evoked potential amplitude), beginning at three weeks, which are particularly substantial in EDL. In contrast, other synaptic properties are either consistently greater in the same muscle throughout development (evoked potential kinetics, muscle fibre diameter) or display no systematic muscle type-dependent differences (terminal area, input resistance, spontaneous release amplitude). Thus, the emergence of adaptive differences in terminal release properties correlates with the differentiation of locomotor activity patterns in postnatal rat hindlimb muscles.

Enhanced metabolic activity coincides with survival and differentiation of cultured rat retinal ganglion cells exposed to glutamate

Neurotransmitters are prominent candidates for trans-cellular signals that influence the development of the CNS. The present study has examined the effect of glutamate on survival, differentiation and metabolic activity of cultured rat retinal ganglion cells at 3 days in vitro. Retinal cultures from neonatal Wistar rats were treated with glutamate for 48 h. The metabolic activity was markedly increased in the retinal ganglion cells exposed to 20 microM glutamate. This was accompanied by an enhanced survival of these neurons. The number of differentiated retinal ganglion cells as determined by microtubule-associated protein-2 labeling was significantly increased following exposure to low but not higher doses of glutamate. The effect of glutamate on the metabolic activity and differentiation was blocked by tetrodotoxin. The results of the present study shows that glutamate has a significant effect on survival, differentiation and metabolic activity. An increase in the metabolic activity indicates an enhancement in the electrical activity. Thus, our results are consistent with the hypothesis that glutamate is critically involved in the regulation of electrical activity in developing rat retinal ganglion cells.

Cytochrome oxidase activity in rat retinal ganglion cells during postnatal development

In this study, the metabolic activity of rat retinal ganglion cells during postnatal development has been examined in vivo using cytochrome oxidase histochemistry. The intensity of staining was measured by optical densitometry. The activity of cytochrome oxidase in retinal ganglion cells progressively increased from postnatal day 0 (P0) and reached a peak during the second week of postnatal development (P10-P14) and declined thereafter. Our data show that the increased levels of cytochrome oxidase seen in developing retinal ganglion cells occur at the same time, when neuronal maturity and synaptogenesis reach their peaks.