Intraventricular insulin suppresses the acoustic startle response in rats

We and others have previously reported that the hormone insulin alters brain noradrenergic function at the synaptic and molecular levels. In the present study, we examined the in vivo effect of insulin (administered chronically via osmotic minipumps at a dose of 5 mU/day into the third cerebral ventricle) on the acoustic startle response. Rats receiving chronic intraventricular insulin had a significantly reduced startle response relative to vehicle-treated controls (i.e., 47 +/- 21% of baseline control startle response). Because our previous findings suggest that on an acute basis, insulin may enhance endogenous noradrenergic activity by inhibiting norepinephrine reuptake, we speculate here that the chronic effect of insulin is similar to that of the noradrenergic reuptake blocker, desipramine, which has been reported to decrease baseline startle performance.