Possible role of the brainstem in the mediation of prepulse inhibition in the rat.

Bilateral stimulation of electrodes aimed at the cuneiform nucleus produced significant inhibition of the startle response produced by presentation of an 8-kHz, 110-dB tone. Stimulation of electrodes aimed at the deep mesencephalic nucleus also reduced the magnitude of the startle response, but the effect was less than that following stimulation sites near the cuneiform nucleus. Histological reconstruction of the electrode tip locations revealed a significant negative correlation between the maximum magnitude reduction of the acoustic startle response following an electrical prepulse stimulus and the distance from the cuneiform nucleus. Histological examination also indicated that some electrodes aimed at the cuneiform nucleus were located in or near the inferior colliculus or parabrachial nucleus, all of which are thought to be part of an inhibitory circuit parallel to the acoustic startle reflex arc. These experiments support the view that the prepulse inhibition of the acoustic startle reflex originates in the brainstem.