Comparative behavioural and biochemical effects of repeated intrathecal administration of thyrotrophin-releasing hormone (TRH) or two analogues of trh in adult rats

Abstract

The effect of repeated intrathecal injection of thyrotrophin-releasing hormone (TRH) and two analogues of TRH, C-terminally modified RX 77638 and N-terminally modified CG 3509, were examined on behavioural (wet-dog shakes and forepaw licking) and biochemical markers for spinal motoneurones, bulbospinal raphe nerve terminals and the pituitary-thyroid axis in rats. Saline (10 mu;l washed in with 15 μl), TRH (20 μg), RX 77368 (2 μg) or CG 3509 (2 μg) were administered intrathecally (twice daily for 3 or 5 days), after which levels of plasma-free thyroxine and thyroid-stimulating hormone (TSH) were measured and the dorsal and ventral portions of the thoracolumbar spinal cord, brainstem and hypothalamus were assayed for TRH- and calcitonin gene-related peptide (CGRP)-like immunoreactivity, levels of indoleamines and the activity of choline acetyltransferase (ChAT). Behavioural tolerance developed rapidly with consecutive injections of RX 77368, such that wet-dog shakes were significantly reduced and forepaw-licking tended to be decreased by the third intrathecal injection. Five, but not 3, days of administration of RX 77368 selectively elevated levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid in the ventral spinal cord, where these substances are principally located in bulbospinal raphe nerve terminals. The time course of the change in indoleamines suggests that administration of TRH peptides elevated the synthesis, rather than the release, of 5-HT from these nerve terminals. When compared with saline, only CG 3509 and not TRH or RX 77368, elevated the activity of ChAT in the ventral horn while none of the TRH peptides altered CGRP-like immunoreactivity in the ventral horn, two markers for spinal motoneurones. Plasma-free thyroxine was elevated and plasma TSH reduced by treatment with TRH and RX 77368, compared with levels in saline-treated controls, while CG 3509 was apparently without effect on these parameters. Thus, N-terminal modification of the TRH peptide appears to be important in determining any longer-term efiects on motoneurones but not the neuroendocrine actions of the TRH peptides following intrathecal administration.