Clonidine (10-40 μg/kg) produced a dosedependent reduction of fear as measured by the potentiated startle effect (increased acoustic startle in the presence of a cue which had been previously paired with shock). The reduction of potentiated startle could not be accounted for entirely by a general depressant effect of clonidine on startle nor by an acceleration of extinction. Piperoxane and yohimbine, which are associated with anxiety in humans, increased potentiated startle, whereas propranolol and WB-4101 did not. These results provide further evidence that the potentiated startle paradigm in the rat is sensitive to drugs that alter anxiety in humans. Moreover, they support the hypothesis that norepinephrine transmission is important for the expression of fear or anxiety.
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