Growth hormone, insulin-like growth factor I, and motoneuron size.

In this study we asked whether growth hormone (GH) and one of its key mediators, insulin-like growth factor I (IGF-I), influence spinal motoneuron size in conjunction with whole body size. We present evidence that GH has such a role, possibly without the mediation of IGF-I. Both lumbar motoneuron and body size were found to be increased relative to littermate controls in transgenic mice overexpressing GH, while body size, but not motoneuron size, was increased in mice overexpressing IGF-I. GH overexpression coordinately increased nucleolar, nuclear, and cell body size in lumbar spinal motoneurons, so that their normal size relationships were preserved in the transgenic mice. In addition, spinal cord and brain weights were significantly increased in both types of transgenic animal. We conclude that GH can regulate motoneuron, central nervous system, and body size in the same animal, and that IGF-I can mimic the effects of GH on at least two of these three parameters.

Synchronous, intersegmental responses in motoneurons to lumbar ventral root injury.

Unilateral transection of lumbar ventral roots in the grass frog, Rana pipiens, causes large, multiphasic transcriptional responses within non-injured motoneurons as far away as the cervical enlargement. The responses, which are not seen after sham surgery, are remarkably similar in their form and timing to those which appear in the injured motoneurons, implying that both axotomized and spared motoneurons are responding to the same external signals. Elucidation of the source, pathway and properties of those signals will help to explain why spared neurons reflect some, but not all, of the features seen within injured motoneurons after motor axon damage.

Transcription and motoneuron size.

Nuclear size and total RNA synthesis were compared in single lumbar motoneurons isolated from the grass frog. Transcription was found to correlate significantly, but not exclusively, with nuclear area or volume over a wide range of nuclear size, the largest nuclei having the highest mean transcriptional activity. Flow cytometric analysis of propidium iodide-stained nuclei excluded polyploidy or polyteny as an explanation for the increased transcription, but left open the possibility of a small increase in DNA with increasing nuclear size. Alternatively, motoneurons may increase transcription and nuclear size without increasing their DNA content, possibly by increasing the proportion of dispersed chromatin (euchromatin). These two mechanisms for size-related changes in RNA synthesis in motoneurons present an interesting contrast to mechanisms used by many other large animal cells.