Compensatory sprouting of uninjured magnocellular neurosecretory axons in the rat neural lobe following unilateral hypothalamic lesion.

Axonal sprouting of intact neurons of the magnocellular neurosecretory system was investigated using a unilateral hypothalamic knife cut of the hypothalamoneurohypophysial tract to partially denervate the rat neural lobe (NL). Densitometric, morphometric, ultrastructural, and metabolic measures were utilized to demonstrate the compensatory response to denervation in this system. Densitometric analysis revealed a transient reduction in the intensity of vasopressin staining in the NL at 10 days postsurgery (PS) with a subsequent recovery by 20 days PS. There was a comparable initial reduction in the cross-sectional area of the NL followed by a more gradual recovery to normal by 90 days PS. Ultrastructural investigation revealed a reduction in total axon number in the NL at 10 days PS similar to the declines in vasopressin immunoreactivity and size of the NL. A subsequent partial recovery of axon number occurred, paralleling the return to normal NL size between 30 and 90 days PS. Hypertrophy of both somata and cell nuclei of magnocellular neurons in the supraoptic and paraventricular hypothalamic nuclei contralateral to the lesion was also apparent during this period. Daily measurements of urine osmolality revealed an initial transient hypoosmolality followed by a chronic hyperosmolality which persisted throughout the 90 day postsurgical period. There was a concomitant chronic decrease in both daily drinking and urine excretion volumes which began immediately following surgery. These results suggest that intact, contralateral magnocellular vasopressinergic efferents undergo compensatory sprouting as a result of partial denervation of the NL in the absence of a functional deficit in vasopressin.