The effect of medial forebrain bundle lesion on thyrotropin secretion in the rat

The medial forebrain bundle (MFB) was partially lesioned with 6-hydroxydopamine (6-OHDA) in order to investigate the effect of deficient central noradrenergic regulation on thyrotropin (TSH) secretion in the rat. 6-OHDA injection into the MFB significantly reduced the noradrenaline (NA), dopamine (DA) and serotonin (5-HT) content of the whole hypothalamus. NA and 5-HT concentrations were also significantly decreased in the paraventricular nucleus (PVN). The MFB lesion did not affect the clonidine (250 micrograms/kg, i.p.) induced stimulation of TSH release or the isoproterenol (1 mg/kg i.p.) induced decrease in TSH levels. Thyrotropin releasing hormone (TRH, 5 micrograms/kg i.v.) caused a similar significant stimulation of TSH secretion in lesioned and non-lesioned rats. The present results do not support the hypothesis that the blunted TSH response to TRH observed in depressed patients results from a deficiency in noradrenergic neurotransmission.