Fear-enhanced acoustic startle is not attenuated by acute or chronic imipramine treatment in rats

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Abstract
The effect of acute or chronic administration of imipramine on fear-enhanced startle (potentiated startle) in rats was investigated. Thirty male albino rats were initially given preliminary startle testing, assigned to one of three matched groups, and trained for potentiated startle by presenting ten light-shock pairings on each of 2 days. Subsequent startle testing following a single injection of 0, 5 or 10 mg/kg imipramine revealed that the degree of startle potentiation (increased responding in the presence of the light previously paired with shock) was similar across treatment conditions. A significant and comparable potentiation of startle was observed in animals treated chronically with saline or imipramine (10 mg/kg/day) for 21 days between training and testing. Potentiated startle was also observed in these animals on the next (22 nd) day after injection of an additional dose of the drug (10 mg/kg) 5 min prior to testing. Plasma levels of imipramine and its metabolite, desipramine, were relatively high after each of these treatments. Since previous studies have shown that potentiated startle is decreased by diazepam, the present findings suggest that the potnetiated startle paradigm is a valid model for studying simple fear or anxiety rather than panic disorder.