Adrenalectomy increases sprouting in a peptidergic neurosecretory system.

The neurons of the paraventricular nucleus of the hypothalamus have a substantial vasopressin-containing projection to the zona externa of the median eminence. Several parameters of neuronal function are increased in these cells following adrenalectomy; these increases can be prevented by glucocorticoid treatment. We have now used this system to study the effect of adrenal removal on the sprouting of axons in response to a unilateral lesion of the paraventricular nucleus. Results obtained from both immunocytochemical and anterograde tracing experiments indicate that the capacity of the paraventricular nucleus axons to sprout is enhanced in adrenalectomized animals. Anterograde tracing experiments indicate that the projection from the paraventricular nucleus to the zona externa is predominately ipsilateral confirming the findings of Alonso & Assenmacher and that sprouting does not occur in intact animals with a unilateral paraventricular nucleus lesion. However, the zona externa of adrenalectomized animals with a unilateral paraventricular nucleus lesion is filled bilaterally with neurophysin terminals: the volume occupied by such fibers is the same as in non-lesioned, adrenalectomized animals. These findings are interpreted to indicate that sprouting can occur in this system if the animal is adrenalectomized. To estimate the time course of sprouting, animals were first adrenalectomized for 2 weeks and then a unilateral lesion placed in the paraventricular nucleus. Animals were killed 1-26 days following lesion placement and the brains processed for immunocytochemistry. Initial denervation in the zona externa due to the lesion can be detected by day 6 post-lesions while reinnervation (sprouting) is seen by day 26. The results suggest that the increase in metabolic activity induced by adrenalectomy in the vasopressin neurons under study may influence the ability of the axons of these cells to produce new terminals.