Early inhibition and changes in diurnal rhythmicity of the pituitary-thyroid axis after superior cervical ganglionectomy of rats

Abstract

The effect of superior cervical ganglionectomy (SCGx) on the pituitary-thyroid axis was examined in rats. SCGx decreased serum thyrotropin (TSH) and thyroxine (T 4) levels for up to 4 days after surgery, during and immediately after completion of anterograde degeneration of regional sympathetic terminals. At later times TSH levels in control and SCGx rats did not differ, but a significant increase of serum T 4 was found two weeks after SCGx. A diurnal rhythm in serum TSH and T 4 levels with maxima at 11.00 h (TSH) and at 14.00 and 22.00 h (T 4) was found in sham-operated rats 3 days after surgery. At this time SCGx evoked a general depression of TSH levels as well as a shift of 3 h in their maximum. A similar shift of the afternoon peak and abolition of the nocturnal peak in serum T 4 were detectable in SCGx rats. In SCGx animals examined during anterograde nerve degeneration, i.e. 14 h after surgery, injection of the α 1-adrenoceptor blocker phenoxybenzamine negated denervation-induced changes of TSH and counteracted partially T 4 effects. The β-adrenergic blocker propranolol did not modify serum TSH levels in SCGx rats but further decreased serum T 4 concentration. Treatment with both drugs simultaneously did no affect TSH release compared to SCGx, phenoxybenzamine-treated rats but effectively decreased serum T 4. These results further support the involvement of superior cervical ganglion neurons in the control of thyroid function.