A PRIMARY ACOUSTIC STARTLE PATHWAY - OBLIGATORY ROLE OF COCHLEAR ROOTNEURONS AND THE NUCLEUS-RETICULARIS PONTIS CAUDALIS

Davis et al. (1982) proposed a primary acoustic startle circuit in rat s consisting of the auditory nerve, posteroventral cochlear nucleus, a n area near the ventrolateral lemniscus (VLL), nucleus reticularis pon tis caudalis (PnC), and spinal motoneurons. Using fiber-sparing lesion s, the present study reevaluated these and other structures together w ith the role of neurons embedded in the auditory nerve [cochlear root neurons (CRNs)], recently hypothesized to be involved in acoustic star tle. Small electrolytic lesions of the VLL or ventrolateral tegmental nucleus (VLTg) failed to eliminate startle. Large electrolytic lesions including the rostral ventral nucleus of the trapezoid body (rVNTB) a nd ventrolateral parts of PnC or lesions of the entire PnC blocked sta rtle. However, small NMDA-induced lesions of the rVNTB failed to block startle, making it unlikely that the rVNTB itself is part of the star tle pathway. In contrast, NMDA lesions of the full extension of the ve ntrolateral part of the PnC blocked startle completely, suggesting tha t the ventrolateral part of the PnC is critically involved. Bilateral kainic acid lesions of CRNs also blocked the startle reflex completely , providing the first direct evidence for an involvement of CRNs in st artle. This blockade probably was not caused by damage to the auditory nerve, because the lesioned animals showed intact compound action pot entials recorded from the ventral cochlear nucleus. Hence, a primary a coustic startle pathway may involve three synapses onto (1) CRNs, (2) neurons in PnC, and (3) spinal motoneurons.