Enhancement of electrically elicited startle by amygdaloid stimulation

Previous experiments showed that acoustic startle amplitude can be enhanced by electrical stimulation of the amygdala. Because the acoustic startle pathway is organized in a serial fashion, startle can be elicited electrically with progressively shorter latencies by stimulating different points along this pathway [i.e., ventral cochlear nucleus (VCN), paralemniscal zone (PLZ), nucleus reticularis pontis caudalis (RPC) or medial longitudinal fasciculus (MLF)]. The present study evaluated the temporal characteristics of the facilitatory effect of amygdaloid stimulation on startle elicited electrically from different points along the acoustic startle pathway. A single 0.1-msec pulse was delivered to the central nucleus of the amygdala at various times before the onset of a 1-msec pulse in various sites in the startle pathway. The shortest amygdaloid stimulation-startle onset interval to significantly enhance startle was 0 msec for the VCN, 2 msec for the PLZ, 3 msec for the RPC and 7 msec for the MLF. These results indicate that amygdaloid stimulation enhances electrically elicited startle in a temporal manner that is complementary to facilitation of acoustic startle. The similarity of amygdala-stimulated enhancement and fear potentiation of electrically elicited startle is also discussed.