Contextual fear conditioning depresses infralimbic excitability

Abstract

Patients with posttraumatic stress disorder (PTSD) show hypo-active ventromedial prefrontal cortices (vmPFC) that correlate with their impaired ability to discriminate between safe and dangerous contexts and cues. Previously, we found that auditory fear conditioning depresses the excitability of neurons populating the homologous structure in rodents, the infralimbic cortex (IL). However, it is undetermined if IL depression was mediated by the cued or contextual information. The objective of this study was to examine whether contextual information was sufficient to depress IL neuronal excitability. After exposing rats to context-alone, pseudoconditioning, or contextual fear conditioning, we used whole-cell current-clamp recordings to examine the excitability of IL neurons in prefrontal brain slices. We found that contextual fear conditioning reduced IL neuronal firing in response to depolarizing current steps. In addition, neurons from contextual fear conditioned animals showed increased slow afterhyperpolarization potentials (sAHPs). Moreover, the observed changes in IL excitability correlated with contextual fear expression, suggesting that IL depression may contribute to the encoding of contextual fear.

Keywords: Afterhyperpolarization; Brain slices; Contextual fear conditioning; Electrophysiology; Intrinsic excitability; Prefrontal cortex.

Figure 1

Behavioral experiments design. A, Three experimental groups were studied to assess the effect of contextual fear conditioning on IL intrinsic excitability. In the exposure control group (EXPOSURE; n = 7), rats received contextual exposure on day 1 and were tested for contextual fear on day 2. The pseudoconditioned control group (PSEUDO; n = 3) received 3 consecutive shocks and were removed immediately from the conditioning chamber on day 1. PSEUDO rats were tested for contextual fear on day 2. On day 1, the contextual fear conditioned group (COND; n = 5) received three shocks within 2 min intervals. On day 2, COND rats were tested for contextual fear. B, Average percent freezing of all the experimental groups during the two days of behavioral procedures. As expected, COND rats showed significant higher freezing levels to the conditioning context when compared to EXPOSURE and COND rats. *p < 0.05

Figure 2

Contextual fear conditioning depresses the intrinsic excitability of IL pyramidal neurons. A, Representative traces of the response to a 310 pA current pulse in a single neuron from each experimental group. The neuron in the conditioned group fired fewer spikes. B, Input-output curve shows that depolarizing steps evoked fewer spikes in neurons from the COND group. C, The maximum number of evoked spikes at any current step was significantly reduced in the COND group. D, Cumulative histogram of the maximum number of spikes fired by neurons from each group. E, The COND group had more cells with a maximum of ≤ 3 spikes compared to cells in the EXPOSURE and PSEUDO groups. * p < 0.05

Figure 3

Contextual fear conditioning depressed IL excitability by increasing the sAHP. A, Sample traces showing the time interval where sAHPs were measured (green box; 50 ms interval starting 280 ms after the end of the 800 ms depolarizing pulse). B, Neurons from the COND group had larger sAHPs compared to neurons from the EXPOSURE and PSEUDO groups. C, Cumulative histogram showing that neurons from the COND group had larger sAHPs. D, The COND group had more cells with a sAHP > 2 mV. *p < 0.05

Figure 4

IL intrinsic excitability inversely correlates with contextual fear. A, Higher contextual fear correlates with fewer spikes in IL pyramidal neurons. B, Higher contextual fear correlates with larger sAHPs in IL pyramidal neurons. C, Larger sAHPs are associated with fewer spikes in IL pyramidal neurons.