Fear conditioning is disrupted by damage to the postsubiculum

Abstract

The hippocampus plays a central role in spatial and contextual learning and memory, however relatively little is known about the specific contributions of parahippocampal structures that interface with the hippocampus. The postsubiculum (PoSub) is reciprocally connected with a number of hippocampal, parahippocampal and subcortical structures that are involved in spatial learning and memory. In addition, behavioral data suggest that PoSub is needed for optimal performance during tests of spatial memory. Together, these data suggest that PoSub plays a prominent role in spatial navigation. Currently it is unknown whether the PoSub is needed for other forms of learning and memory that also require the formation of associations among multiple environmental stimuli. To address this gap in the literature we investigated the role of PoSub in Pavlovian fear conditioning. In Experiment 1 male rats received either lesions of PoSub or Sham surgery prior to training in a classical fear conditioning procedure. On the training day a tone was paired with foot shock three times. Conditioned fear to the training context was evaluated 24 hr later by placing rats back into theconditioning chamber without presenting any tones or shocks. Auditory fear was assessed on the third day by presenting the auditory stimulus in a novel environment (no shock). PoSub-lesioned rats exhibited impaired acquisition of the conditioned fear response as well as impaired expression of contextual and auditory fear conditioning. In Experiment 2, PoSub lesions were made 1 day after training to specifically assess the role of PoSub in fear memory. No deficits in the expression of contextual fear were observed, but freezing to the tone was significantly reduced in PoSub-lesioned rats compared to shams. Together, these results indicate that PoSub is necessary for normal acquisition of conditioned fear, and that PoSub contributes to the expression of auditory but not contextual fear memory.

Figure 1

Schematic of corticohippocampal circuitry depicting multiple cortical and subcortical connections between PoSub and brain regions involved in contextual and auditory fear conditioning. Thicker arrows denote more dense connections. PoSub, postsubiculum; RSP, retrosplenial cortex; ENTO, entorhinal cortex; PER, perirhinal cortex; HIPPO, hippocampus.

Figure 2

(A) Photomicrograph of a postsubiculum (PoSub) lesion. Arrows delineate the dorsal and ventral borders of the PoSub. Arrowhead denotes the ventral portion of the PoSub that was spared in some rats. (B) Schematic diagram indicating the largest (black) and smallest (gray) lesions of the PoSub in Experiment 1. A range of damage to primary visual cortex (V1) was observed in all PoSub lesioned rats. The stippled area represents the extent of damage to the lesioned rat with the most V1 damage) (C) Schematic diagram showing coronal sections 6.84, 7.32 and 7.80 mm posterior to bregma. The shaded area in each diagram depicts the boundaries of PoSub. RSP_A, retrosplenial cortex; SUB, subiculum; PFCC, posterior forceps of the corpus callosum.

Figure 3

Experiment 1 – Freezing behavior and locomotor activity. Freezing behavior: Effects of pre-training postsubiculum (PoSub) lesions on freezing behavior of lesioned (n = 14) and Sham-operated (n = 20) groups during the Acquisition session (A) and during the Context (B) and Tone (C) tests. BL, baseline; PS1 – 3, postshock 1 – 3. Bilateral lesions of the PoSub significantly decreased postshock freezing during training as well as freezing behavior during the Context and Tone tests (ps < 0.001). Locomotor activity: Average open field activity depicting that PoSub lesioned-rats (n = 11) were hyperactive compared to Sham-operated rats (n = 12) and that both groups habituated to the open-field over time (D). Relationship between freezing behavior and locomotor activity. Postshock freezing data from PoSub-lesioned rats was divided into normo-active (PoSub-Low, n = 5) and hyperactive (PoSub-High, n = 6) groups and replotted for the Acquisition session (E) and the Context (F) and Tone (G) tests. Replotted locomotor activity of normo-active and hyperactive groups (H). Data represent means ± standard errors. **p <05 compared to Sham-operated and PoSub-Low groups. # indicates that PoSub-lesioned rats showed a trend toward less freezing compared with Sham-operated rats (ps=0.06).

Figure 4

Experiment 2 – Freezing behavior and locomotor activity. Freezing behavior: Effects of post-training postsubiculum (PoSub) lesions on freezing behavior of lesioned (n = 14) and Sham-operated (n = 15) groups during the Acquisition session (A) and during the Context (B) and Tone (C) tests. BL, baseline; PS1 – 3, postshock 1 – 3. Bilateral lesions of the PoSub significantly decreased freezing during the Tone test, but had no effect on freezing during the Context test. Locomotor activity: Average open field activity demonstrating that PoSub lesioned-rats (n = 14) were hyperactive compared to Sham-operated rats (n = 15) and that both groups habituated to the open-field over time (D). Relationship between freezing behavior and locomotor activity: Postshock freezing data from PoSub-lesioned rats was divided into normo-active (PoSub-Low, n = 8) and hyperactive (PoSub-High, n = 6) groups and replotted for the Acquisition session (E) and the Context (F) and Tone (G) tests. Replotted locomotor activity of sham, normo-active and hyperactive groups (H). Data represent means ± standard errors. *p <05 compared to Sham-operated group. **p <05 compared to Sham-operated and PoSub-Low groups.