Contributions of the amygdala central nucleus and ventrolateral periaqueductal grey to freezing and instrumental suppression in Pavlovian fear conditioning

Abstract

In Pavlovian fear conditioning animals receive pairings of a neutral cue and an aversive stimulus such as an electric foot-shock. Through such pairings, the cue will come to elicit a central state of fear that produces a variety of autonomic and behavioral responses, among which are conditioned freezing and suppression of instrumental responding, termed conditioned suppression. The central nucleus of the amygdala (CeA) and the ventrolateral periaqueductal grey (vlPAG) has been strongly implicated in the acquisition and expression of conditioned fear. However, previous work suggests different roles for the CeA and vlPAG in fear learning maybe revealed when fear is assessed with conditioned freezing or conditioned suppression. To further explore this possibility we gave rats selective lesions of either the CeA or vlPAG and trained them in Pavlovian first-order fear conditioning as well as Pavlovian second-order fear conditioning. We concurrently assessed the acquisition of conditioned freezing and conditioned suppression. We found that vlPAG and CeA lesions impaired both first- and second-order conditioned freezing. VlPAG lesions did not impair, and CeA lesions only transiently impaired, first-order conditioned suppression. However, both vlPAG and CeA lesions impaired second-order conditioned suppression. These results suggest that the CeA and vlPAG are critically important to expressing fear through conditioned freezing but play different and less critical roles in expressing fear through conditioned suppression.

Figure 1

Photomicrographs show representative Nissl sections of (A) Sham and (B) Neurotoxic lesions the CeA at Swanson level 26 (−2.12 mm posterior from bregma). High magnification images centered on medial CeA for (C) Sham and (D) Neurotoxic lesions are also shown. Note in (C) the abundance of healthy cells with little sign of gliosis and contrast with (D) where the number of healthy cells as decreased and gliosis (small black specks) is apparent. Finally, fiber staining for (E) Sham and (F) Neurotoxic CeA lesions is shown. For all acceptable lesions there was no damage to fibers of passage through CeA.

Figure 2

Photomicrographs show representative Nissl sections of (A) Sham and (B) Electrolytic lesions the vlPAG at Swanson level 51 (−8.30 mm posterior from bregma). Note the extensive damage ventrolateral to the periaqueductal grey and the enlargement of the central aqueduct.

Figure 3

Freezing and instrumental suppression in first-order Pavlovian fear conditioning. (A–C) Mean (± SEM) % time freezing during CS+ (closed shapes) and CS− (open shapes) over the course of first-order Pavlovian fear conditioning. (D–F) Mean (± SEM) lever-press rates during CS+ and CS− from the same sessions. Data points 1–3 are CS+ only sessions; 4–9 are two-session blocks of first-order discrimination. Sham (circles), vlPAG (triangles) and CeA (diamonds) rats are shown in three separate columns.

Figure 4

Freezing and instrumental suppression in second-order Pavlovian fear conditioning. (A–C) Mean (± SEM) % time freezing during SOCS+ (closed shapes) and SOCS− (open shapes) over the course of second-order Pavlovian fear conditioning. (D–F) Mean (± SEM) lever-press rates during SOCS+ and SOCS− from the same sessions. Data points 1–5 are the five sessions of second-order conditioning. Sham (circles), vlPAG (triangles) and CeA (diamonds) rats are shown in three separate columns.