Antagonism of pentobarbital-induced hormonal changes by TRH in rats

In adult male rats, injection of TRH into a lateral ventricle of the brain 5 min prior to pentobarbital (PB) administration caused a significant dose-related inhibition of prolactin (PRL) release, in doses ranging from 500 to 5 ng. Among 8 TRH analogues devoid of thyrotropin-releasing activity, 6 were found to significantly suppress PB-induced PRL secretion at an intraventricular dose level of 10 microgram, and the 3 most effective in this respect were also able to counteract growth hormone (GH) release elicited by PB. The derivative [1,3'-DCM2]TRH was still potent enough to block PB-induced PRL secretion at an intraventricular dosage of 50 ng. The peptide ACTH 4--10 was ineffective, whereas another ACTH derivative H-Met(O2)-Glu-His-Phe-D-Lys-Phe-OH (Org 2766) reduced PRL release. TRH did not affect the increase of plasma PRL induced by acute stress. alpha-Methyl-p-tyrosine (alpha-MT) failed to influence the inhibiting effect of TRH on GH secretion but significantly reduced that on PRL release. p-Chlorophenylalanine (PCPA) completely blocked the antagonistic effect of TRH on all PB-induced hormonal changes, suggesting that serotoninergic mechanisms may be involved in the extra-pituitary effect of TRH.