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-hydroxytryptamine (serotonin, 5-HT) and thyrotropin-releasing hormone (TRH) act synergistically to stimulate gastric function in vivo. Previous in vitro experiments have shown that this synergism does not occur at the level of the dorsal motor nucleus of the vagus (DMV) motoneurone. 2. In order to determine the mechanism of this action, whole cell patch clamp recordings were made from identified gastric-projecting rat DMV neurones to investigate the effects of 5-HT and TRH on GABAergic inhibitory postsynaptic 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 subsequent applications of 5-HT to decrease IPSC amplitude by 28 +/- 3%. Such TRH-induced 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 isobutylmethylxanthine (IBMX; 10 muM), the non-hydrolysable cAMP analogue 8-bromo-cAMP (1 mM), or the adenylate cyclase activator forskolin (10 muM), like TRH, uncovered 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 unresponsive to 5-MT. Conversely, the adenylate cyclase inhibitor, dideoxyadenosine (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 initiates an intracellular signalling cascade that raises the levels of cAMP sufficient to uncover previously silent 5-HT1A receptors on presynaptic nerve terminals within the dorsal vagal complex.