Acceleration of the regeneration of the crushed hypoglossal nerve by testosterone.

The effect of testosterone propionate (TP) on regeneration of the crushed hypoglossal nerve was studied in young adult rats. The TP treatment promoted axonal outgrowth as measured by an increase in the proportion of hypoglossal neurons labeled by retrograde transport of horseradish peroxidase injected into the tongue at selected postlesion periods compared with that in controls. The result indicated that TP exerts its effect on regeneration primarily by increasing the metabolic activity in hormone target neurons. The somatotopic organization of the hypoglossal nucleus and its projection into the major nerve branches were unaltered subsequent to crush injury. The maintenance of specificity was attributed to the intact perineurial and endoneurial sheaths which guided regenerating fibers to their original targets.