The peptide TRH uncovers the presence of presynaptic 5-HT1A receptors via activation of a second messenger pathway in the rat dorsal vagal complex

1. It is well recognized that brainstem microinjections of 5-hydroxytryptam ine (serotonin, 5-HT) and thyrotropin-releasing hormone (TRH) act synergist ically to stimulate gastric function in vivo. Previous in vitro experiments have shown that this synergism does not occur at the level of the dorsal m otor nucleus of the vagus (DMV) motoneurone. 2. In order to determine the mechanism of this action, whole cell patch cla mp recordings were made from identified gastric-projecting rat DMV neurones to investigate the effects of 5-HT and TRH on GABAergic inhibitory postsyn aptic currents (IPSCs) evoked by stimulation of the nucleus of the tractus solitarius (NTS). 3. 5-HT (30 pM) decreased IPSC amplitude by 26 +/- 2.5% in approximately 43 % of DMV neurones. In the remaining neurones in which 5-HT had no effect on IPSC amplitude, exposure to TRH (1 muM) uncovered the ability of subsequen t applications of 5-HT to decrease IPSC amplitude by 28 +/- 3%. Such TRH-in duced 5-HT responses were prevented by the 5-HT1A antagonist NAN-190 (1 muM ) and mimicked by the 5-HT1A agonist 8-OH-DPAT (1 muM). 4. Increasing cAMP levels using the phosphodiesterase inhibitor isobutylmet hylxanthine (IBMX; 10 muM), the non-hydrolysable cAMP analogue 8-bromo-cAMP (1 mM), or the adenylate cyclase activator forskolin (10 muM), like TRH, u ncovered the ability of 5-HT to decrease evoked IPSC amplitude (17 +/- 2.2% , 28.5 +/- 5.3% and 30 +/- 4.8%, respectively), in neurones previously unre sponsive to 5-MT. Conversely, the adenylate cyclase inhibitor, dideoxyadeno sine (10 muM) and the protein kinase A inhibitor, Rp-cAMP (10 muM), blocked the ability of TRH to uncover the presynaptic inhibitory actions of 5-MT. 5. These results suggest that activation of presynaptic TRH receptors initi ates an intracellular signalling cascade that raises the levels of cAMP suf ficient to uncover previously silent 5-HT1A receptors on presynaptic nerve terminals within the dorsal vagal complex.