Physiological identification and infralimbic responsiveness of rat intercalated amygdala neurons

Abstract

Intercalated (ITC) amygdala neurons are thought to play a critical role in the extinction of conditioned fear. However, several factors hinder progress in studying ITC contributions to extinction. First, although extinction is usually studied in rats and mice, most ITC investigations were performed in guinea pigs or cats. Thus it is unclear whether their connectivity is similar across species. Second, we lack criteria to identify ITC cells on the basis of their discharge pattern. As a result, key predictions of ITC extinction models remain untested. Among these, ITC cells were predicted to be strongly excited by infralimbic inputs, explaining why infralimbic inhibition interferes with extinction. To study the connectivity of ITC cells, we labeled them with neurobiotin during patch recordings in slices of the rat amygdala. This revealed that medially located ITC cells project topographically to the central nucleus and to other ITC clusters located more ventrally. To study the infralimbic responsiveness of ITC cells, we performed juxtacellular recording and labeling of amygdala cells with neurobiotin in anesthetized rats. All ITC cells were orthodromically responsive to infralimbic stimuli, and their responses usually consisted of high-frequency (~350 Hz) trains of four to six spikes, a response pattern never seen in neighboring amygdala nuclei. Overall, our results suggest that the connectivity of ITC cells is conserved across species and that ITC cells are strongly responsive to infralimbic stimuli, as predicted by extinction models. The unique response pattern of ITC cells to infralimbic stimuli can now be used to identify them in fear conditioning experiments.

Neural Structures Mediating Expression and Extinction of Platform-Mediated Avoidance

“…Importantly, we found that IL is necessary for forming a memory of extinction of avoidance, consistent with its role in extinction of fear conditioning (Burgos-Robles et al, 2007;Laurent and Westbrook, 2009b;Amir et al, 2011;Fontanez-Nuin et al, 2011;Sierra-Mercado et al, 2011). Previous studies have linked extinction of active avoidance with the amygdala (Jiao et al, 2011), medial septum (Pang et al, 2011), and anterior thalamus (Gabriel et al, 1983), but not to IL.…”

“…Previous studies have linked IL to fear extinction (Mueller et al, 2010;Peters et al, 2010;Santini and Porter, 2010;Thompson et al, 2010;Fontanez-Nuin et al, 2011;Holmes et al, 2012), proposing that IL inhibits freezing via feed-forward inhibition of the amygdala (Quirk et al, 2003;Amano et al, 2010;Amir et al, 2011). In particular, inactivating IL immediately before fear extinction training leaves extinction intact but impairs retrieval of extinction the next day (Laurent and Westbrook, 2009b;Sierra-Mercado et al, 2011;Holmes et al, 2012).…”

“…BLA drives conditioned freezing through its projections to the central nucleus of the amygdala (LeDoux et al, 1988;Muller et al, 1997). Expression of amygdala-dependent fear memories is regulated by prelimbic (PL) and infralimbic (IL) prefrontal cortices, which increase and decrease freezing, respectively, through projections to the amygdala (Vidal-Gonzalez et al, 2006;Sotres-Bayon and Quirk, 2010;Amir et al, 2011;Cho et al, 2013).…”

Hunger promotes fear extinction by activation of an amygdala microcircuit

“…The hippocampus modulates fear behavior by direct connections to the amygdala or indirectly via activation of the prefrontal cortex (Maren, 2005). Interestingly, the infralimbic region of the prefrontal cortex activates inhibitory intercalated neurons (Amir et al, 2011;Pinard et al, 2012), a population of neurons that were activated in our experiments by shortterm fasting and consequently reduced CEm activation by feed-forward inhibition (Supplementary Figure 8).…”

The effects of muscimol and AMN082 injections into the medial prefrontal cortex on the expression and extinction of conditioned fear in mice

“…Furthermore, stimulation of the IL at the CS onset (Milad and Quirk, 2002), but not prior or after the CS (VidalGonzalez et al, 2006) has been shown to result in extinction of fear memory, suggesting its role in the gating of downstream structures involved in fear memory. The IL is thought to inhibit the amygdala, especially its basolateral complex (BLA) (Grace and Rosenkranz, 2002), and the effect of the IL on suppression of the BLA is mediated by inhibitory interneurons (Amir et al, 2011;Pare and Smith, 1993).…”