Fast axonal transport rates are unchanged in 6- and 24-month F344 rats

Novel method for the labeling of distant neuromuscular junctions

Essential to understanding the roles proteins and structural elements play at the synapse is to understand the development, remodeling and reinnervation of peripheral neuromuscular junctions. It has, however, been a challenging task to label and visualize neuromuscular junctions. In this paper we demonstrate how adenovirus technology can be combined with intraspinal microinjection techniques to follow both the development and the reinnervation of a distant peripheral neuromuscular junction in the rat. A recombinant adenovirus containing VAMP-2 (synaptobrevin-2) was fused to the green fluorescent protein (GFP) and microinjected into the region of the lumbar motor neurons. We were able to follow the neuronal incorporation, axonal transport and synaptic localization of the GFP-VAMP-2 using fluorescence microscopy. GFP-VAMP-2 was found in neuronal cell bodies, selected sciatic nerve axons and was concentrated in the presynaptic nerve terminal. During reinnervation of the neuromuscular junction, GFP-VAMP-2 allows us to follow the time course of junctional reinnervation. Thus, the microinjection of microliter amounts of labeled recombinant virus into locations far distant from target regions can be used to efficiently study the formation of neuromuscular junctions with a minimum of trauma to the animal.

Aging in peripheral nerves: regulation of myelin protein genes by steroid hormones

The process of aging deeply influences morphological and functional parameters of the peripheral nerves. Interestingly, recent observations performed in our laboratory on the rat sciatic nerves have indicated that the deterioration of myelin occurring in the peripheral nerves during aging may be explained by the fall of the messenger levels of the major peripheral myelin proteins (glycoprotein Po, myelin basic protein and peripheral myelin protein 22). At least in the case of the Po, the low levels of its messengers and of the protein itself found in aged animals are increased by the treatment with a physiological progesterone derivative like dihydroprogesterone. It has also been found that in normal adult male rats the levels of the messengers for Po in the sciatic nerve are increased by progesterone, dihydroprogesterone and tetrahydroprogesterone; surprisingly, the gene expression of peripheral myelin protein 22 is stimulated only by tetrahydroprogesterone. These observations have been confirmed in parallel studies performed on Schwann cell cultures. Since tetrahydroprogesterone does not bind to the progesterone receptor but is a ligand for the GABAA receptor, the hypothesis has been put forward that part of the steroidal effects reported might occur not through the classical progesterone receptor, but rather via an interaction with the GABAA receptor. In other experiments it has been found that the gene expression of Po may be decreased by orchidectomy and restored by treatment with the androgen dihydrotestosterone. Altogether, these observations suggest the future use of physiological and/ or synthetic steroid hormones as a possible therapeutic approach for some pathological situations occurring in peripheral nerves during aging and demyelinating diseases.

The effect of age and injury on the expression of inducible nitric oxide synthase in facial motor neurons in F344 rats

Nitric oxide has been implicated in both normal neuronal aging as well as nerve repair events because of its known roles in synaptic plasticity, synaptogenesis and neuroplathologic processes. In this study, we have determined the effect of aging, by comparing brainstem facial motor neurons (FMNs) as well as blood vessels, from adult F344 rats to those in old animals. Inducible nitric oxide synthase (iNOS) expression was determined both by immunohistochemistry using an antibody to iNOS on tissue sections and slot blots. In adult rats, iNOS expression was detectable only in FMNs and not in blood vessels. In old rats, there were robust levels of iNOS protein in blood vessels, while iNOS protein was not detectable in FMNs from old rats. There was also a 12-fold increase in iNOS expression in isolated blood vessels from old rats compared to vessels from adult animals. To determine the effect of injury on iNOS expression, the facial nerve was transected and immunocytochemistry performed as above. After nerve transection in adult rats, iNOS was demonstrable only in blood vessels after 1 day, but by 7 days iNOS protein immunoreactivity was robust in FMNs. In old animals, iNOS protein expression was observed only in FMNs at 1 day, but by 7 days after injury, protein immunoreactivity was localized to the blood vessels. These data suggest that aging and injury differentially affect the expression of iNOS and that the up-regulation of iNOS may be important for the availability of nitric oxide in the aged or injured nervous system.

Lumbar motor neuron size and number is affected by age in male F344 rats

Motor neurons in the spinal cord of old rats appear similar in size but less numerous compared with those in mature rats; they also contain a large amount of lipofuscin, the lipid peroxidation by-product whose function is largely unknown. The object of this study was to morphometrically characterize motor neurons found in the L4/L5 lumbar spinal cord of mature (6-month) and old (22-month) rats. Paraformaldehyde-fixed, lumbar spinal cords from six rats at each age were embedded in paraffin, sectioned at 6 microm and stained with 0.1% toluidine blue. The nucleolar diameter and area from a minimum of 34 motor neurons per spinal cord were measured. Motor neuron number was calculated using Abercrombie's (Abercrombie, 1946) formula after correcting for tissue shrinkage. Motor neuron number was decreased with age while the neuronal area increased with age. Nucleolar diameter also increased in old rats. Frequency distributions of motor neuron area revealed unimodal distributions of motor neurons rats of both ages. We suggest that larger nucleolar diameter reflects more metabolically active neurons in old rats while larger neuron area is a reflection of the presence of lipofuscin in old motor neurons.

Acceleration of axonal outgrowth in motor axons from mature and old F344 rats after a conditioning lesion

The conditioning lesion paradigm has proven to be a very useful model in which to examine the mechanisms of axonal outgrowth after injury. In the present study, we have used the conditioning lesion model to examine the ability of motor axons from mature (6-8 months) and old (22-24 months) Fischer 344 rats to form new axonal sprouts. We show that after a single lesion (sham-conditioned axons followed by a testing lesion), axonal outgrowth rates are slower at earlier vs longer postlesion times in mature rats: between 4 and 8 days postlesion, outgrowth rates are 2.4 +/- 0.4 mm/day, whereas between 8 and 11 days postlesion outgrowth rates are 4.6 +/- 0.7 mm/day. After a single lesion in the old rat, at early postlesion times, the axonal outgrowth rate is 1.9 +/- 0.4 mm/day but with increasing time after injury, outgrowth rates slow down to 1.1 +/- 0.8 mm/day. In conditioned motor axons from mature rats, outgrowth rates increase from 3.1 +/- 0.4 mm/day at early postlesion times to 5.2 +/- 0.6 mm/day at longer postlesion times. An even more dramatic increase in outgrowth rate is seen in conditioned axons from old rats: 2.4 +/- 0. 4 mm/day at early postlesion times to 6.3 +/- 1.0 mm/day at later times after lesion. There is no change in the initial delay before sprouting under any condition. These data support the hypothesis that axons from old animals can be stimulated to repair themselves at rates comparable to those seen in younger animals and suggest that there may be an absolute maximum outgrowth rate attainable by newly forming axon sprouts.