Unilateral electrical stimulation of the inferior colliculus of rats modifies the prepulse modulation of the startle response (PPI): effects of ketamine and diazepam

The magnitude of an acoustic startle response can be reduced by a weak stimulus presented immediately before the startle-eliciting noise. This phenomenon has been termed prepulse inhibition of the startle reaction (PPI). Previous studies indicated that the primary neural pathways mediating PPI belong to the brain stem and that the inferior colliculus (IC) was crucial. Its destruction reduced PPI. Stimulations applied to brain areas may be as deleterious as lesions. Therefore, we looked for the possibility of a brain stimulation applied to the IC during a PPI test to reduce also PPI. Rats were implanted with chronic electrodes, their tips being aimed at the IC. They were located within or close to the inter-colliculus nucleus. A train of stimulations was applied and PPI was tested alternately during and between periods of stimulation. As the most common method used to attenuate PPI consists in administrating drugs, for example ketamine, we also tested the effect of this drug. Another drug was also tested, diazepam, since it alters the functioning of the IC without any known effect on PPI. This allowed a comparative analysis of the neurobiological and the pharmacological effects. It appeared that the stimulation decreased PPI quantitatively as much as ketamine (6 mg/kg) without an effect of the basic startle reaction. These effects did not interfere with each other. Diazepam (1 mg/kg) did not modify PPI, neither under stimulation nor per se. Only for a very high dose (4 mg/kg), a sedative and myo-relaxant one the basic startle and PPI were altered.