Regional changes in neuropeptide levels after 5,7-dihydroxytryptamine-induced serotonin depletion in the rat brain.

The levels of five neuropeptides (substance-P, somatostatin, cholecystokinin octapeptide, methionine-enkephalin and dynorphin) were examined in the brain and the spinal cord of rats 2 weeks after intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT). 5,7-DHT injection caused a significant reduction of the serotonin level in all regions of the brain. The level of each neuropeptide except dynorphin significantly increased in specific regions of the brain after 5,7-DHT treatment without any decrease in their levels in any region. Since, coexistence and interaction between classical neurotransmitters and neuropeptides in the same neurons have been reported, both are indispensable for evaluating pathophysiological state of the brain function associated with abnormal neural transmission. The present findings together with previous reports suggest that neuropeptides act as neurotransmitters and compensate for the impaired function of the serotonergic systems.