Differential inhibition of postnatal brain, spinal cord and body growth by a growth hormone antagonist.

BACKGROUND: Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH. RESULTS: Before 50 days of postnatal age, GHA reduces spinal cord weight more than brain weight, but less than body weight. Thereafter, GHA ceases to inhibit the increase in body weight, which approaches control levels by day 150. In contrast, GHA continues to act on the CNS after day 50, reducing spinal cord growth to a greater extent and for a longer duration than brain growth. CONCLUSIONS: Judging from its inhibition by GHA, GH differentially affects the magnitude, velocity and duration of postnatal growth of the brain, spinal cord and body. GH promotes body enlargement more than CNS growth early in postnatal life. Later, its CNS effects are most obvious in the spinal cord, which continues to exhibit GH dependence well into adulthood. As normal CNS growth slows, so does its inhibition by GHA, suggesting that reduced trophic effects of GH contribute to the postnatal slowing of CNS growth. GHA is a highly useful tool for studying the role of endogenous GH on organ-specific growth during aging.

Magnitude, laterality, and uniformity of swelling in axotomized spinal motoneurons: lack of evidence for influence by the distal stump.

Injury to frog lumbar motor axons produces a coordinated, allometric enlargement of the nucleolus, nucleus, and cell body of the injured neuron. The mechanisms by which swelling is initiated and sustained are not known. In this study, we have sought evidence for a role of the severed distal stump in the magnitude, laterality, and uniformity of the swelling response in frog spinal motoneurons. We find that swelling of motoneuron nucleoli, nuclei, and perikarya after unilateral spinal nerve transection is exclusively ipsilateral and uniform among motoneurons of different sizes. Removal of the severed distal stump does not affect the magnitude, unilaterality, or uniformity of the swelling responses. Thus, the distal stump appears to play no role in initiating swelling following spinal nerve transection.

Radiolabeling motoneuron proteins in the isolated frog spinal cord preparation.

An in vitro method for radiolabeling protein in adult frog spinal motoneurons is described, with per cell incorporations which are 2-3 orders of magnitude higher than previously reported for mammalian brain neurons. In the procedure, isolated lumbar spinal cord preparations from Rana pipiens are labeled with 3H-L-leucine, motoneuron cell bodies are recovered and TCA-precipitated protein is analyzed by scintillation counting. The higher levels of labeling (> 90 cpm/cell body) allow one to quantify newly synthesized protein within individual or small groups of identified nerve cell bodies. Motoneuronal labeling correlates directly with cell body size, and other sources of variation in labeling and their control are identified and discussed.

Isolation of motoneuron cell bodies from spinal cord stored at -70 degrees C in ethylene glycol.

Pretreatment of spinal cord with ethylene glycol permits long-term storage of the tissue at -70 degrees C prior to isolation and biochemical analysis of the cell bodies of spinal motoneurons. The method is useful for storing spinal tissue from laboratory animals, as well as from human post mortem specimens, where aliquots of tissue may then be used for motoneuron isolation over an indefinitely long period. In addition to inhibiting the loss of soluble proteins from the neurons during freezing and thawing, cryoprotection increases the yield and improves the appearance of the isolated cell bodies. The method should aid biochemical studies of many kinds of neuronal subpopulations isolated from small amounts of starting material.

Allocation of newly synthesized proteins in spinal motoneurons.

When vinblastine sulfate is used in vitro to block slow and fast transport of proteins within frog lumbar spinal motoneurons, the amounts of both newly synthesized protein and total protein increase in motoneuronal perikarya. Analyses of motoneurons isolated from control and vinblastine-treated spinal cords show that 1) about 55 p. 100 of the newly synthesized protein is exported from motoneuron cell bodies during a 4 h incubation period; 2) only about 5 p. 100 of the total perikaryal protein is exported during the same period; and 3) less than 10 p. 100 of the labeled protein is exported by fast axonal transport. Thus, a substantial amount of the newly synthesized protein is quickly and preferentially exported from the cell body. It is not known how much of this exported protein reaches the axon by slow transport. However, when interpreted in conjunction with the studies of Schubert, Kreutzberg and Lux (Brain Res. 47, 331-343, 1972), the above data strengthen the possibility that substantial amounts of the new protein made by a motoneuron may be committed to its dendrites.