Brainstem serotonergic hyperinnervation modifies behavioral supersensitivity to 5-hydroxytryptophan in the rat

Rat pups were injected intracisternally (i.c.) or intraperitoneally (i.p.) with 5,7-dihydroxytryptamine (5,7-DHT) or saline and challenged 2 and 14 weeks later with the 5-HT precursor 5-hydroxytryptophan (5-HTP), which evokes behavioral supersensitivity in adult rats, 5,7-DHT induced transient postinjection convulsions in rats injected i.c. but not i.p. Rats with either type of 5,7-DHT lesions displayed supersensitive behavioral responses to 5-HTP. However, rats lesioned by i.p. injections exhibited significantly greater shaking behavior (+1445%) in response to 5-HTP than their i.c. counterparts, who instead showed more forepaw myoclonus (+250%) and head weaving (+270%), the core features of the 5-HT syndrome. Differences in 5-HT syndrome behaviors were already present 2 weeks after lesioning, whereas the difference in shaking behavior was not. After 14 weeks, 5-HT was selectively depleted (-43 to -92%) in hippocampus, spinal cord, and frontal cortex, and differences between i.c. and i.p. 5,7-DHT routes were insignificant except in frontal cortex. Brainstem 5-HT concentrations were significantly increased (+35%) after i.p. 5,7-DHT injections in contrast to reduction (-89%) after i.c. 5,7-DHT; 5-hydroxyindole acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) ratios were decreased (-20%) with either route. These data suggest that brainstem 5-HT hyperinnervation following i.p. 5,7-DHT injection modifies the functional consequences of injury in abating the 5-HT syndrome, but does not result in complete recovery since shaking behavior is enhanced. Loss of presynaptically mediated autoregulation or receptor dysregulation may play a major role in behavioral supersensitivity induced by 5-HTP in rats with 5,7-DHT lesions. To the extent that the 5-HT syndrome is mediated by 5-HT1A receptors and shaking behavior by 5-HT2 sites, differential responses to injury of 5-HT1A and 5-HT2 receptors may contribute to these behavioral differences.