Contextual fear conditioning depresses infralimbic excitability
Introduction
The increased fear responses in patients with posttraumatic stress disorder (PTSD) are associated with reduced ventromedial prefrontal cortex (vmPFC) activity (Milad et al., 2009, Rougemont-Bücking et al., 2011). However, it is unclear if this vmPFC hypo-activity is caused by the traumatic experience or is present prior to the traumatic experience. Either mechanism could lead to the development of PTSD, since low vmPFC activity is associated with decreased inhibition of the amygdala resulting in hyperactivation of the amygdala and subsequent increased fearful behavior (Milad et al., 2009, Rougemont-Bücking et al., 2011).
Studies done in the rodent homologue to the human vmPFC, the infralimbic cortex (IL) (Koenigs and Grafman, 2009, Milad and Quirk, 2012, Milad et al., 2006), found that auditory fear conditioning depresses the excitability of IL neurons (Cruz et al., 2014, Santini et al., 2008). This mechanism mimics the depressed vmPFC observed in patients with PTSD and demonstrates that aversive learning can depress vmPFC neurons. Interestingly, fear conditioning does not induce synaptic depression in IL (Pattwell et al., 2012, Sepulveda-Orengo et al., 2013) indicating that intrinsic rather than synaptic plasticity is the key determinate of IL excitability after aversive learning. Furthermore, pharmacological manipulation of IL intrinsic excitability is sufficient to reduce conditioned-fear expression (Santini and Porter, 2010, Santini et al., 2012) indicating that the depression is functionally important.
Since our previous studies used auditory fear conditioning (Cruz et al., 2014, Santini et al., 2008), we could not determine whether contextual or cued information was depressing IL excitability. Although IL is more known for its role in the extinction of fear memory (Burgos-Robles et al., 2007, Milad and Quirk, 2002, Vidal-Gonzalez et al., 2006), a recent study suggests that IL contributes to the contextual discrimination of fear conditioning memory (Zelikowsky et al., 2013). The depression of IL excitability after fear conditioning could convey contextual information which is key to determining which cues signal danger (Bouton, 2004, Bouton and Bolles, 1979). To examine this possibility, we investigated whether contextual information alone could depress IL excitability by combining a contextual fear conditioning paradigm with whole-cell patch-clamp recordings of IL neurons.
Section snippets
Contextual fear conditioning
Male Sprague Dawley rats (postnatal day 30 to 45) were group housed on a 12 h light/dark schedule with free access to food and water. All procedures were approved by the Institutional Animal Care and Use Committee of the Ponce Health Sciences University. On day 1, the contextual fear conditioned group (COND) was exposed to contextual fear conditioning consisting of a three minute exploration phase followed by three 0.7 mA scrambled footshocks (0.5 s in duration) with two minutes between shocks. A
Results
Three experimental groups were designed to test whether contextual fear conditioning affects IL intrinsic excitability (Fig. 1A). On day 1 the COND group (n = 5) received contextual fear conditioning, the EXPOSURE group (n = 7) received contextual exposure with no shock presentations, and the PSEUDO group (n = 3) received 3 consecutive shocks and was immediately removed from the conditioning chamber. All animals were tested for contextual fear on day 2 and immediately sacrificed. As expected (Fig. 1
Discussion
The main finding presented in this paper is that contextual fear conditioning depresses the intrinsic excitability of IL pyramidal neurons. The significant decrease in the number of action potentials and increase in sAHP suggest a similar mechanism of IL depression as that observed after auditory fear conditioning (Cruz et al., 2014, Santini et al., 2008). Furthermore, both parameters, number of spikes and sAHP, strongly correlated with the freezing behavior of the animals suggesting that IL
Acknowledgements
Special thanks to Ana López and María Colón for their technical assistance. This work was supported by R36 1R36MH102080-01 to EC, MBRS-RISE 1R25 GM 082046 to OSC and MCM, NIMHHD G12MD007579-27 and R15 MH101700 to JTP.
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