The effects of 5,7-dihydroxytryptamine and p-chlorophenylalanine on thyrotrophin-releasing hormone in regions of the brain and spinal cord of the rat.

The distribution of thyrotrophin-releasing hormone (TRH) and 5-hydroxytryptamine (5-HT) were compared in ten regions of the rat brain and in lumbar spinal cord. After dissection, using a cutting box and tissue punches, TRH was measured by radioimmunoassay and 5-HT by HPLC with electrochemical detection. Within the brain the highest levels of TRH were found in the median eminence and the remaining hypothalamus. There were also relatively high levels in the suprachiasmatic nucleus, septal nuclei and nucleus accumbens. Highest levels of 5-HT were found in the raphe nuclei, hypothalamic nuclei, nucleus accumbens, stria terminalis, septal nuclei and hippocampus. 5,7-Dihydroxytryptamine (5,7-DHT; 200 micrograms, i.c.v.) markedly reduced levels of 5-HT in brain and spinal cord. In the ventral lumbar cord there was a comparable decrease of TRH and 5-HT (-80%) and a smaller but significant decrease in the nucleus accumbens (-55%) and septal nuclei (-38%). p-Chlorophenylalanine (PCPA; 250 mg/kg X 2) reduced levels of 5-HT (-80%), without significantly altering those of noradrenaline. p-Chlorophenylalanine also significantly reduced levels of TRH in the nucleus accumbens (-72%) but not in the other regions of brain or spinal cord taken. The results are discussed in relation to the previously described co-existence of TRH and 5-HT in the spinal cord and the possible alternative forms of interactions between amine and peptide in the nucleus accumbens.