Time relationships and reversal by two indoleamines of the pinealectomy-inhibited thyrotropin secretion in male rats.

Time relationships and mediators of the inhibitory effect of pinealectomy on cold-induced TSH secretion were studied in male Sprague-Dawley rats. Pinealectomy but not sham operation significantly reduced the TSH cold-response (30 min at +4 degrees C) in 6-8 week-old rats on the 3rd postoperative day. Longer postoperative periods resulted in gradual attenuation of the effect of pinealectomy. Pinealectomy did not modify basal TSH levels on the 3rd or 7th postoperative day. Melatonin, arginine vasotocin and crude pineal extracts failed to antagonise the suppressed cold-response in pinealectomised rats. 5-Hydroxytryptamine and 6-methoxy-tetrahydro-beta-carboline, however, antagonised partially but significantly the pinealectomy-induced reduction in TSH cold-response. Although transient, the suppression of cold-stimulated TSH secretion by pinealectomy may be caused by lack of two pineal indoleamines 5-hydroxytryptamine and/or 6-methoxy-tetrahydro-beta-carboline.