Increased behavioural response to intrathecal serotonin after lesion of serotonergic pathways with 5,7‐dihydroxytryptamine seems not to be due to depletion of serotonin

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Abstract
The hehavioural response to intrarhecal i.th. serotonin (5‐HT) was examined in mice pretreated with the neurotoxin 5,7‐dihydroxytryptamine (5,7‐DIIT) or the 5‐HT synthesis inhibitorp‐chlorophenylalanine (PCPA) which produce similar extensive depletion of central 5‐HT levels. Intrathecal 5‐HT (0.4 γg) elicited a hehavioural response consisting of reciprocal hindlimb scratching and biting or licking of the hindquarters indicative of nociceptive stimulation. The response to i.th. 5‐HT was markedly increased 5 days after intracerebroventricular (i.c.v.) injection of 5,7‐DHT (80 γgbase per mouse). On the other hand, 24 h after the last pretreatment injection of PCPA (400 mg kg‐1for 6 consecutive days), the response to i.th. 5‐HT was unaltered. These results indicate that i.c.v. 5,7‐DHT produces supersensitivity to 5‐HT. Since PCPA failed to alter the effect of 5‐HT, the supersensitivity seems not to be due to depletion of 5‐HT levels after the lesion.

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