Thyroid Hormone Level Positively Regulates NOS and cGMP in the Developing Rat Cerebellum

Abstract

Thyroid hormones and nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling play a significant role in the structural development of the cerebellum, respectively. In the present study, the possible contribution of neuronal NO synthase (NOS) and cGMP in the thyroid hormone-induced structural changes was investigated in the cerebella of postnatal rats at different hormone levels. Animals were treated from postnatal day 4 until days 7, 14, and 21, by i.p. injection of 1 μg thyroxine (T₄)/10 g body weight/4 days, or p.o. with 100 μg 6-n-propyl-2-thyouracil (PTU)/10 g body weight/day. Control groups consisted of i.p. and p.o. vehicle controls and PTU/T₄-treated animals. Measurement of serum fT₄, TSH as well as total T₃ and T₄ concentration of the cerebellar tissue indicated the changed thyroid status. nNOS extensively expressed in growing parallel fibers revealed by quantitative Western blot and layer analysis of immunohistochemically labeled coronal sections. Simultaneously, the cGMP concentration increased and the distribution of cGMP-immunoreactive (cGMP-IR) material in Purkinje cell perikarya and in the molecular layer expanded during cerebellar development. T₄ increased nNOS and cGMP level, and accelerated the development of nNOS-IR parallel fibers and cGMP-IR molecular layer. In contrast, PTU retarded the development by decreasing nNOS and cGMP concentration and slowing down layer formation. A single dose of T₄ could rescue the PTU-induced changes. Results suggest that the contribution of nNOS- NO/cGMP signaling in thyroid hormone regulated structural maturation of the cerebellum. The possible involvement of neurotrophins, calcium, and apoptotic events in this process was discussed.