Adrenergic neurons in the C1 region in the ventrolateral medulla oblongata send descending axons into spinal cord which terminate in thoracic and upper lumbar segments, overlapping the distribution of sympathetic preganglionic neurons. The present study was undertaken to determine whether adrenergic fibers synapse directly on preganglionic neurons which innervate the adrenal medulla and to examine the ultrastructure of these fibers during development. The ultrastructure and synaptology of adrenergic axons in the intennediolateral nucleus of mid-thoracic spinal cord were studied in 7-, 9-, 24-, 30-, 60-, and 90-day-old rats using immunocytochemical staining for phenylethanolamine N-methyltransferase, the epinephrine-synthesizing enzyme. Phenylethanolamine N-methyltransferase-immunoreactivity was observed in the cytoplasm of unmyelinated axonal varicosities and intervaricose segments in the neuropil of intemediolateral nucleus. Phenylethanolamine N-methyltransferase-immunoreactive synaptic boutons were filled with spherical electron-lucent vesicles and occasional larger dense-core vesicles. These boutons were observed to form symmetrical synaptic contacts with dendritic processes at all ages examined. Asymmetrical synapses on dendrites were also observed in adult rats. Axosomatic synaptic contacts were frequently observed in immature rats, but were never observed in adult rats. To determine whether adrenergic axons synapse on preganglionic neurons which project to the adrenal medulla, adrenal preganglionic neurons were retrogradely labeled with horseradish peroxidase and adrenergic axons were stained for phenylethanolamine N-methyltransferase-immunoreactivity. In young rats, phenylethanolamine N-methyltransferase-immunoreactive boutons were observed to form symmetrical axosomatic and axodendritic synaptic contacts with adrenal preganglionic neurons in intermediolateral nucleus. These contacts had already formed by postnatal day 7, the youngest age studied. In constrast, it was not possible to verify that adrenal preganglionic neurons receive adrenergic innervation in adult rats, since phenylethanolamine N-methyltransferase-immunoreactive boutons were only observed in contact with small diameter dendrites that were not retrogradely labeled by horseradish peroxidase. These studies demonstrate that adrenal preganglionic neurons receive adrenergic synapses prior to the first postnatal week. The initial synapses which form on preganglionic somata and proximal dendrites appear to reorganize late in development. It is suggested that these become more distally located as the dendritic tree matures. More generally, these observations suggest that adrenergic bulbospinal neurons are involved in central regulation of adrenal development and function.
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