Influence of thyrotropin-releasing hormone (TRH) dialyzed into the hippocampus on memory processes in rabbit

There are conflicting reports on the influence of thyrotropin-releasing hormone (TRH) on the process of learning. We decided to study this problem using the paradigm of classical eyeblink conditioning as the animal model of learning processes. During the extinction training TRH in its natural form of pG1uTRH was applied into the rabbit hippocampus through a chronically implanted microdialysis probe. A Glu(1)TRH, analog with less biological potency than TRH as the control of specificity for TRH and a 0.9% NaCl solution as the control for both substances were applied by the same way as pGluTRH. We found that pGluTRH extended the process of extinction and enhanced the further acquisition of the reflex. The analog of TRH, Glu(1)TRH, was ineffective. Finally, it may be stated that TRH acting in the hippocampus prolonged process of forgetting and improved succeeding learning. The effect was specific and long lasting.