The effect of p-chloroamphetamine and p-chlorophenylalanine on the level of thyrotropin-releasing hormone and its receptors in some brain structures and lumbar spinal cord of the rat.

The concentration of thyrotropin-releasing hormone (TRH) and the density and affinity of TRH receptors were examined in the ventral and dorsal lumbar spinal cord, nucleus accumbens and striatum of rats with the 5-hydroxytryptamine (5-HT) nerve terminal destroyed with p-chloroamphetamine (PCA), or in animals treated with the inhibitor of 5-HT synthesis p-chlorophenylalanine (PCPA). PCA (2 x 10 mg/kg i.p., 9 and 8 d before killing) and PCPA (3 x 300 mg/kg i.p., 72, 48 and 24 h before killing)--either of them dramatically diminishing the 5-HT and 5-HIAA concentrations in all the examined structures--reduced the TRH level and increased the density of TRH receptors in the ventral lumbar spinal cord. PCPA also reduced the TRH content in the nucleus accumbens. The PCA-induced reduction in the TRH level and increase in the density of TRH receptors in the ventral lumbar spinal cord were significantly attenuated by citalopram (2 x 20 mg/kg i.p., 30 min before PCA), a selective inhibitor of 5-HT uptake. Our results constitute a further proof that coexistence of TRH and 5-HT takes place in the ventral lumbar spinal cord and then indicate that other form(s) of relationship between 5-HT and TRH may exist in some parts of the central nervous system. They also suggest that an up-regulation of TRH receptors occurs in the spinal cord as a result of TRH depletion.