Thyroxin dependency of the developing locus coeruleus. Evidence from intraocular grafting experiments

Area of Science:

• Neuroscience
• Endocrinology
• Developmental Biology

Background:

• Thyroid hormones are essential for normal development and function of the central nervous system (CNS).
• Noradrenaline neurons of the locus coeruleus play a role in various physiological processes.
• The influence of thyroid hormones on the development of neuronal projections, specifically from the locus coeruleus, remains an area of active investigation.

Purpose of the Study:

• To investigate the role of thyroid hormones in the fiber formation of immature noradrenaline neurons from the locus coeruleus grafted to the eye.
• To determine if thyroidectomy affects the distribution and morphology of nerve fibers originating from locus coeruleus neurons.

Main Methods:

• Surgical thyroidectomy was performed on adult recipients to alter thyroid hormone levels.
• Whole-mount preparations of irides were analyzed using fluorescence histochemistry (Falck-Hillarp method).
• Fiber number, distribution, and morphology in the iris dilator plate were subjectively assessed in a 'blind' manner.

Main Results:

• Locus coeruleus neurons formed nerve fibers in the irides of both control and thyroidectomized recipients.
• Thyroidectomy led to a marked difference in fiber distribution and morphology, with a lack of thick axon bundles and abundant peripheral accumulations of fluorescent material.
• Daily injections of l-thyroxine reversed these observed deficits in fiber formation.

Conclusions:

• Thyroid hormones, specifically thyroxine, play a critical role in the perinatal development of nerve fibers originating from locus coeruleus neurons.
• Thyroxine is likely involved in tubulin assembly, influencing the ability of neurons to form processes during CNS development.
• These findings support the crucial role of thyroxine in neuronal process formation during early development.