Increased insulin-like growth factor-I (IGF-I) expression during early postnatal development differentially increases neuron number and growth in medullary nuclei of the mouse

Morphometric analyses of the medulla were performed in transgenic mice that overexpress insulin-like growth factor-I (IGF-I) postnatally and in non-transgenic littermates. The total volume of the medulla was increased in transgenic mice at all postnatal ages studied: 14 days (18%), 21 days (23%), 28 days (23%), and 35 days (27%). By 35 days of age, the volumes of individual medullary nuclei were also increased: nucleus of the tractus solitarius (NTS, 59%), dorsal motor nucleus of the vagus (DMV, 84%), hypoglossal nucleus (HN, 29%) and facial nucleus (FN, 21%). Neuron number in transgenic mice was significantly greater in NTS (50%) and DMV (53%), but not in the HN or the FN. Motor neurons in DMV, HN and FN of transgenic mice exhibited increases in mean profile areas of the soma and decreased numerical densities, suggesting increases in neuritic outgrowth. These results point to IGF-I actions in promoting neuron survival and growth, and suggest that IGF-I has differential effects on distinct neuron populations, possibly depending upon its time of expression.