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We have compared the effects of thyroidectomy with those of thyroxine (T₄) replacement and excess T₄ treatment on neuropeptide Y (NPY) in the rat anterior pituitary, and compared these with the effects on substance P (SP) and vasoactive intestinal peptide (VIP). Thyroidectomy produced large increases in the peptide content of NPY (335 ± 58 fmol/gland vs 15 ± 4 fmol/gland in controls), SP (581 ±90 vs 199 ±32 fmol/gland) and VIP (1386 ± 395 vs 417 ± 77 fmol/gland) together with large increases in the respective prohormone encoding mRNAs, NPY 21 760%±1290%, preprotachykinin-A (PPT-A; which encodes the substance P precursor) 1744%± 190% and VIP 680% ± 129%. Thyroidectomy together with T₄ replacement produced an increase in both NPY peptide (426 ±72 vs 15 ±4 fmol/gland) and mRNA content 970%±156% of controls). The peptide contents of SP and VIP were not significantly different from controls. PPT-A and VIP mRNA levels were decreased relative to controls (31%±8% and 23%± 10% respectively). In intact animals treated with excess T₄ (hyperthyroid animals), SP and VIP peptide contents were significantly reduced (55 ±13 vs 199±32 fmol/gland and 226 ± 24 vs 417± fmol/gland respectively) and the SP and VIP encoding mRNAs were also decreased (8% ±3% and 11%±4% respectively). In this group there was no detectable alteration in either the peptide or mRNA content of NPY. Thus, the response of pituitary NPY to thyroid hormone manipulations cannot be explained in terms of negative feedback physiology and is different from those of SP and VIP. The results suggest that the regulation of locally produced NPY in the rat anterior pituitary is complex and may be influenced by thyroidal factors in addition to T₄.

Journal of Endocrinology (1994) 143, 393–397

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