Sensitivity of spinal reflexes to TRH and 5-HT in 5,6-dihydroxytryptamine-treated rats

Destruction of descending serotonergic nerve terminals containing thyrotropin-releasing hormone (TRH) was affected in rats by the intracisternal injection of 5,6-dihydroxytryptamine (5,6-DHT) two weeks before subsequent experiments. Although the level of TRH in the lumbar enlargement was significantly reduced in 5,6-DHT-treated rats, the effects of TRH on the monosynaptic reflex (MSR) and the polysynaptic reflex (PSR) in these rats were no different from those in control rats. MSR inhibition by 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was attenuated by 5,6-DHT treatment although there was no obvious difference in the effects of 5-MeODMT on the PSR between 5,6-DHT-treated and control rats. In 5,6-DHT-treated rats, L-5-hydroxytryptophan (5-HTP) markedly decreased the MSR and increased the PSR although the same doses of 5-HTP did not produce any effects on either the MSR or the PSR in control rats. In control rats, after administration of imipramine or clorgyline, 5-HTP produced effects similar to those observed in 5,6-DHT-treated rats. These results suggest that the supersensitivity to 5-HTP in 5,6-DHT-treated rats is due to a lack of 5-hydroxytryptamine (5-HT) uptake into 5-HT-containing nerve terminals rather than to a change in 5-HT receptors.