Intraocular fibre growth of grafted locus coeruleus neurons: sensory denervation counteracts alterations induced by thyroid hormone deficiency.

Immature central noradrenaline neurons from the nucleus locus coeruleus were grafted to surgically thyroidectomized adult rats. As previously reported, fibre production from locus coeruleus in the host iris is hampered by thyroid hormone deficiency. We wanted to investigate if sensory denervation of the target iris tissue, a procedure known to enhance fibre production from locus coeruleus neurons, would interact with and possibly reverse the morphological signs of thyroid hormone deficiency. We found that a sensory denervation of the host iris reversed all morphological signs of the endocrine manipulation. Thus the number of axons found in bundles increased 30-fold and the density of accumulations intermingled with the fluorescent nerve plexus fell by half. Furthermore, the distribution of fibres in the host iris changed, spreading to more than double the surface area of the control thyroidectomized group. In all these respects the fibre growth of locus coeruleus in the sensorily denervated group mimicked what has been reported before in similarly denervated animals with intact endocrine status. These results show that the hampered nerve fibre growth induced by a thyroid hormone deficiency can be counteracted by a specific sensory denervation of the target tissue restituting the endocrine imbalance.