Prefrontal circuits signaling active avoidance retrieval and extinction

Abstract

Objective: Neurons in PL and IL project densely to two areas implicated in active avoidance: the basolateral amygdala (BLA) and the ventral striatum (VS). We therefore combined c-Fos immunohistochemistry with retrograde tracers to characterize signaling in platform-mediated active avoidance.

Methods: Male rats were infused with retrograde tracers (CTB, FB) into basolateral amygdala and ventral striatum and conditioned to avoid tone-signaled footshocks by stepping onto a nearby platform. In a subsequent test session, rats received either 2 unreinforced tones (avoidance retrieval) or 15 unreinforced tones (avoidance extinction) followed by analysis of c-Fos combined with fluorescent imaging of retrograde tracers.

Results: Retrieval of avoidance did not activate IL neurons, but did activate PL neurons projecting to BLA, and to a lesser extent VS. Extinction of avoidance activated IL neurons projecting to both BLA and VS, as well as PL neurons projecting to VS.

Conclusions: Our observation that avoidance retrieval is signaled by PL projections to BLA suggests that PL may modulate VS indirectly via BLA, and agrees with other findings implicating BLA in active avoidance. Less expected was the signaling of extinction via PL inputs to VS, which may converge with IL inputs to VS to inhibit expression of avoidance.

Keywords: Amygdala; Infralimbic; Prelimbic; Retrograde tracers; Ventral striatum; c-Fos.

Fig. 1

Retrieval and extinction of platform-mediated avoidance. a Rats were first infused with retrograde tracers in BLA and VS and then conditioned in platform-mediated avoidance for 10 days. Twenty-four hours later, rats were tested for avoidance retrieval (2 unreinforced tones, Retr) or avoidance extinction (15 unreinforced tone, Ext), followed 60 min later by collection of brains and c-Fos immunohistochemistry. Unconditioned controls (Ctl) received tones but were never shocked. b Right panel: all conditioned rats showed high levels of avoidance (% time on platform; blocks of three) when compared with unconditioned controls. Left panel: subsequently, in rats receiving extinction training, a decrease in avoidance was observed by the end of extinction. Ctl: N = 8; Retr: N = 22; Ext: N = 21. Data shown as mean ± SEM. Repeated measures ANOVA. **p < 0.01

Fig. 2

Retrograde labeling and c-Fos immunohistochemistry of prefrontal neurons projecting to either BLA or VS. a Infusion sites of retrograde tracers in BLA and VS. b, c Upper panels show drawing of prelimbic and infralimbic cortices. Lower panels show representative images of retrogradely labeled, c-Fos-positive, and co-labeled neurons in PL and IL. d, e Density of retrogradely labeled neurons in different experimental groups. CTB PL: Ctl: N = 5; Retr: N = 16; Ext: N = 14. CTB IL: Ctl: N = 5; Retr: N = 17; Ext: N = 16. FB PL: Ctl: N = 5; Retr: N = 13; Ext: N = 13. FB IL: Ctl: N = 5; Retr: N = 13; Ext: N = 11. Ac, anterior commissure; BLA, basolateral amygdala; Cc corpus callosum; CeA, central amygdala; Core, nucleus accumbens core; LV, lateral ventricle; OT, optic track; Shell, nucleus accumbens shell. Data shown as mean ± SEM

Fig. 3

Density of c-Fos/tracer co-labeling. a Both retrieval and extinction of avoidance increased c-Fos immunohistochemistry in PL (upper panel), but retrieval only significantly increased activity in BLA-projecting neurons of PL (lower panels). Extinction resulted in significant activation of PL neurons projecting to VS. b IL was significantly activated only during avoidance extinction (upper panel), by neurons projecting to both BLA and VS (lower panels). c-Fos PL: Ctl: N = 5; Retr: N = 18; Ext: N = 18. c-Fos IL: Ctl: N = 5; Retr: N = 18; Ext: N = 18. c-Fos/CTB PL: Ctl: N = 5; Retr: N = 16; Ext: N = 14. c-Fos/CTB IL: Ctl: N = 5; Retr: N = 17; Ext: N = 16. c-Fos/FB PL: Ctl: N = 5; Retr: N = 13; Ext: N = 13. c-Fos/FB IL: Ctl: N = 5; Retr: N = 13; Ext: N = 11. Data shown as mean ± SEM. One-way ANOVAs. **p < 0.01, *p < 0.05, ^#p = 0.052, and ns = non-significant

Fig. 4

Neuronal circuits signaling avoidance retrieval and extinction. Based on the present data together with other avoidance studies, the proposed model suggests that retrieval activates PL neurons projecting mainly to BLA (upper panel), whereas avoidance extinction activates IL neurons projecting to both BLA and VS (lower panel). Our data are also consistent with a possible role of PL projections to VS in both retrieval and extinction