Behavioral assessment of norepinephrine and serotonin function and interaction in the hippocampal formation

Abstract

Norepinephrine (NE) and serotonin (5-HT) were injected either into the dorsal or ventral hippocampal formation of rats in doses ranging from 0.005 μg/μl to 5.0 μg/μl. Behavioral reactivity was assessed by recording latency to paw lick when placed on a hot plate and magnitude of force displaced in a vertical direction to a footshock. In addition open field activity was measured. NE injections resulted in a dose-dependent increase in behavioral reactivity to the hot plate and footshock; 5-HT injections resulted in a dose-dependent decrease in behavioral reactivity to hot plate and footshock. Both NE and 5-HT injections resulted in a dose-dependent increase in open field activity. NE injections were more effective in increasing reactivity when injected into the dorsal hippocampus while 5-HT injections were more effective in decreasing behavioral reactivity when injected into the ventral hippocampus. Both NE and 5-HT were most effective in increasing open field behavior, however, when injected into the dorsal hippocampus. When NE and 5-HT were injected simultaneously they resulted in no change in bevavioral reactivity as compared to saline injections. Simultaneous injections of NE and 5-HT neither enhanced nor antagonized the increase in open field activity of each amine injected alone. The results are discussed in terms of the functional significance of NE and 5-HT in the hippocampus, their modes of action and significance for understanding dorsal-ventral hippocampal differences.