Target organ regulation of sympathetic neuron development

The role of target organs in the morphological and biochemical development of sympathetic neurons was examined in the neonatal rat. The superior cervical ganglion (SCG) and its end organs, the salivary glands and iris were employed as a model system. Unilateral sialectomy and iridectomy prevented the normal developmental increase in ipsilateral ganglion tyrosine hydroxylase (T-OH) activity, a marker for adrenergic maturation. Enzyme activity remained depressed by approximately 30% for at least 6 months, the longest time tested. Ganglion morphometry was performed to investigate the basis of the abnormal biochemical ontogeny. Target organ removal significantly decreased the number of adrenergic neurons in the Scg by approximately 30%. Total ganglion volume was reduced in a parallel fashion. Thus, end organ extirpation may prevent the biochemical maturation of the SCG by decreasing adrenergic neuron survival. Sialectomy without iridectomy prevented the normal postnatal increase in ganglion T-OH activity, but did not alter iris activity. These observations suggest that target removal prevents the development of only those neurons destined to innervate that organ. In addition to preventing normal adrenergic neuron ontogeny, target extirpation also prevented the normal development of presynaptic choline acetyltransferase activity. Presynaptic ganglion terminal may have failed to mature normally secondary to adrenergic destruction, or may have responded in some other manner to target organ extirpation.