Rats were given 10 light-shock pairings on 2 successive days. At 24-48 hr following training, groups of rats received bilateral transection of the cerebellar peduncles, bilateral lesions of the red nucleus (which receives most of the cerebellar efferents), or bilateral lesions of the central nucleus of the amygdala. Control rats were sham operated. At 3-4 days after surgery, the rats were tested for potentiated startle (increased acoustic startle in the presence of the light previously paired with shock). Potentiated startle was blocked by lesions of the central nucleus of the amygdala. Transection of the cerebellar peduncles or lesions of the red nucleus did not block potentiated startle. A second experiment in which a visual prepulse test was used indicated that the blockade of potentiated startle observed in the animals with amygdala lesions could not be attributed to optic tract damage. A third experiment demonstrated that the absence of potentiation in the animals with amygdala lesions was not simply due to a lowered startle level ceiling, because these animals could show increased startle with increased stimulus intensity and with administration of strychnine, a drug that increases startle. Taken together, the results are consistent with the hypothesis that the amygdala is involved in fear conditioning, because potentiated startle is a measure of conditioned fear.

Code availabilityAll code for data cleaning and analysis associated with the current submission is available upon request to the corresponding author and is provided as part of the replication package.

No abstract

Fear-potentiated startle in the rat is a measure of conditioned fear that is blocked by lesions of the central nucleus of the amygdala. In a companion study, Rosen, Hitchcock, Sananes, Miserendino, and Davis (1991) demonstrated a direct anatomical projection from the central nucleus to the brainstem startle reflex circuit. In the present study, fear-potentiated startle was blocked by lesions that interrupted this pathway at 3 different levels or by a crossed lesion that interrupted the pathway at its source on one side and at a more caudal level on the other side. Although synaptic relays have not been ruled out entirely, the data suggest that the direct projection from the central nucleus of the amygdala to the startle circuit mediates the expression of fear-potentiated startle. These findings are consistent with the literature indicating that efferent projections from the central nucleus to various brainstem structures are involved in the expression of several conditioned fear responses.

Previous work has shown that lesions of the central nucleus of the amygdala block fear-potentiated acoustic startle and that electrical simulation of the central nucleus enhances acoustic startle in rats. In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to identify and delineate the course of a direct projection from the central nucleus of the amygdala to the nucleus reticularis pontis caudalis, a nucleus in the acoustic startle circuit. Experiments using the retrograde tracer Fluoro-Gold confirmed this and indicated that the rostral part of the medial subdivision of the central nucleus of the amygdala contains the cells that project to the startle circuit. With this information, lesion studies (see companion article Hitchcock & Davis, 1991) may be used to determine whether this projection plays a role in fear-potentiated startle.

The present study is part of an ongoing series of experiments aimed at delineation of the neural pathways that mediate fear-potentiated startle, a model of conditioned fear in which the acoustic startle reflex is enhanced when elicited in the presence of a light previously paired with shock. A number of cortical areas that might be involved in relaying information about the visual conditioned stimulus (the light) in fear-potentiated startle were investigated. One hundred thirty-five rats were given 10 light-shock pairings on each of 2 consecutive days, and 1-2 d later electrolytic or aspiration lesions in various cortical areas were performed. One week later, the magnitude of fear-potentiated startle was measured. Complete removal of the visual cortex, medial prefrontal cortex, insular cortex, or posterior perirhinal cortex had no significant effect on the magnitude of fear-potentiated startle. Lesions of the frontal cortex attenuated fear-potentiated startle by approximately 50%. However, lesions of the anterior perirhinal cortex completely eliminated fear-potentiated startle. The effective lesions included parts of the cortex both dorsal and ventral to the rhinal sulcus and extended from approximately 1.8 to 3.8 mm posterior to bregma. Lesions slightly more posterior (2.3-4.8 mm posterior to bregma) or lesions that included only the perirhinal cortex dorsal to the rhinal sulcus had no effect. The region of the perirhinal cortex in which lesions blocked fear-potentiated startle projects to the amygdala, and thus may be part of the pathway that relays the visual conditioned stimulus information to the amygdala, a structure that is also critical for fear-potentiated startle. In addition, the present findings are in agreement with numerous studies in primates suggesting that the perirhinal cortex may play a more general role in memory.

Bilateral electrolytic lesions of the central, but not the lateral, nucleus of the amgydala blocked shock sensitization of startle (the increase in startle produced by presentation of ten 0.6-mA footshocks in rapid succession). Lesions of the central nucleus also decreased reactivity to shock (jumping and flinching) during shock presentation. However, this decrease in reactivity cannot account for the blockade of shock sensitization, because when a higher shock intensity (1.0 mA) was used, producing equivalent reactivity to that of controls at 0.6 mA, central nucleus lesions still blocked shock sensitization. Moreover, lesions of the caudal part of the ventral amygdalofugal pathway, which carries central nucleus efferents to the startle reflex pathway, also blocked shock sensitization. It is hypothesized that shock activates the central nucleus of the amygdala, which increases startle through modulation of the startle pathway. Activation of the amygdala by shock may be the unconditioned response relevant for fear conditioning.

No abstract