Effects of intracerebroventricular 6-hydroxydopamine on catecholaminergic fibers in the rat hypothalamus.

Regeneration in the central nervous system has been claimed to be very limited and abortive, although functional regeneration of some of its pathways after destruction has been observed. The exact mechanisms by which axons regenerate fully or fail to have functional regeneration remain to be studied further. We explored whether or not there is a regional difference in regeneration of central catecholaminergic (CA) neurons in the hypothalamus of young adult rats after 6-hydroxydopamine (6-OH-DA) treatment. Four days after treatment, the numbers of CA terminals and axons were significantly reduced in the paraventricular hypothalamic nucleus, periventricular hypothalamic nucleus, supraoptic commissure (SOC), and dorsomedial hypothalamic nucleus as assessed by a morphometric quantitation on fluorescence microscopy micrographs; CA axons were gradually increased in numbers after the treatment. The number of CA varicosities in the supraoptic commissure was restored to 96% of control 180 days after the 6-OH-DA lesion, whereas the actual numbers of CA varicosities in the paraventricular, periventricular, and dorsomedial hypothalamic nuclei were attained at 79, 79, and 68% of control values, respectively. Our results indicate that CA fibers in the supraoptic commissure possess more regenerative potential than the three other hypothalamic regions studied, suggesting a regional difference in CA nerve sprouting during neuroplasticity within the hypothalamus. The favorable regeneration of CA axons in the supraoptic commissure implies to us that some trophic features along that pathway, particularly near the third ventricle, may have been stimulated after chemical lesion using 6-OH-DA, and gradually released in the distal field of the supraoptic commissure to attract CA stumps to sprout. These factors may thus induce both regenerative sprouting and collateral sprouting resulting in vigorous regrowth of CA fibers in the supraoptic commissure.