SEROTONERGIC INVOLVEMENT IN THE CARDIOVASCULAR STIMULATION BY THYROTROPIN-RELEASING-HORMONE (TRH) IN ANESTHETIZED RATS

The cardiovascular effects of intracerebroventricularly administered thyrotropin-releasing hormone (TRH) were studied in anesthetized rats in the presence of serotonin (5-HT) depletion induced by pretreatments with p-chloroamphetamine (PCA) or p-chlorophenylalanine (PCPA). After PCA the reduction of the whole brain 5-HT and 5-HIAA (5-hydroxy-indoleacetic acid) was 53% and 32% of control, respectively. PCPA reduced the brain 5-HT and 5-HIAA levels even to a greater extent, corresponding levels were 9% and 17% of control. TRH 1-100 nmol/kg increased dose dependently blood pressure and heart rate. PCPA pretreatment significantly attenuated the pressor effect and the tachycardia induced by TRH, whereas PCA did not modify the effects of TRH, which may be related to its weaker capacity to deplete 5-HT in TRH sensitive brain areas. These results suggest the involvement of the central serotonergic system in the TRH-induced cardiovascular stimulation.