Monoamine neurotoxins: selective and delayed effects on behavior in colonies of laboratory rats

Male laboratory rats were raised in two colonies, each of 27 rats, and then given intraventricular injections of the norepinephrine neurotoxin 6-hydroxydopamine (6-OHDA) or the serotonin neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) or saline. They were then returned to their enclosure and behavior during the next 50 days was observed. Shortly after neurotoxin injections the 6-OHDA rats spent more time in the burrows than controls and when out were inactive. The 5,6-DHT rats in contrast spent more time in the open than controls, ran more in activity wheels, approached humans, and fought more. Fighting, mounting, and hoarding in the colony gradually increased for 25 days; during this time the status of the 6-OHDA animals fell progressively whereas the 5,6-DHT animals increased in dominance. Social behavior returned to more normal levels after 50 days. Several successive stages of behavioral alterations occur following neurotoxin injections.