Endocrine effects of blinding in male Syrian hamsters are associated with increased hypothalamic 5-hydroxyindoleacetic acid/serotonin ratios.

The antigonadal, antithyroid, and antiadrenal effects of blinding were studied in male Syrian hamsters receiving propylthiouracil (PTU) or PTU plus thyroxine (T4) replacement. Ten weeks after blinding, the expected gonadal involution and reduction of circulating levels of T4, as well as a reduction of circulating levels of corticosterone, were observed. T4 treatment significantly increased testicular weights, but it did not prevent gonadal involution. Neither PTU nor T4 administration significantly influenced the inhibition of serum corticosterone levels observed in blinded hamsters. The increase in serum thyroid-stimulating hormone (TSH) observed in hypothyroid (PTU-treated) hamsters was significantly greater in intact hamsters than in blind hamsters. Blinding was associated with a small but highly significant increase in ratios of 5-hydroxyindoleacetic acid (5HIAA) to serotonin (5HT) in extracts of the medial basal hypothalamus. Based on the assumption that 5HIAA represents primarily intraneuronal metabolism of excess 5HT, the present results are consistent with reduced 5HT release in blinded hamsters. Daily evening melatonin administration also increased the 5HIAA/5HT ratios of the mediobasal hypothalamus concurrently with gonadal involution, reduction of circulating levels of T4, and reduction of circulating levels of corticosterone. These results are consistent with the view that serotonergic neurons entering the hypothalamus are components of the photoneuroendocrine system of the hamster.